Aero Engine Miscellineus

8/3/2019 Aero Engine Miscellineus

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Pre-Ignition Characteristics of Ethanol andE85 in a Spark Ignition Engine

1. L. J. Hamilton,2. M. G. Rostedt,3. P. A. Catonand4. J. S. Cowart

Ethanol based fuels have seen increased use in recent years due to their renewable nature as well as

increased governmental regulatory mandates. While offering performance advantages over

gasoline, especially at high compression ratios, these fuels are more sensitive to pre-ignition (PI).

Pre-ignition experiments using ethanol (E100) and E85 were performed in a CFR spark ignition

engine using a diesel glow plug hot spot to induce PI. PI is found to occur over a specific air-fuel

ratio range based on hot spot temperature. Additionally, increasing ethanol content or compressionratio (CR) decreases glow plug temperature thresholds for PI. A kinetics-based model was used to

simulate pre-ignition of E100 and to elucidate sensitivities of pre-ignition to various operating

parameters, including initial charge temperature, air dilution, and residual dilution. The model shows

that the most violent cases of PI can be mitigated by switching to either lean or rich operation.

DEFINITIONS, ACRONYMS, ABBREVIATIONS

AFR:

air-fuel ratio

ATC:

after top center

:

equivalence ratio,

imepg :

gross indicated mean effective pressure

KI:

knock intensity

:

lambda,
http://saefuel.saejournals.org/search?author1=L.+J.+Hamilton&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=L.+J.+Hamilton&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=M.+G.+Rostedt&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=M.+G.+Rostedt&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=P.+A.+Caton&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=P.+A.+Caton&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=J.+S.+Cowart&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=J.+S.+Cowart&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=J.+S.+Cowart&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=P.+A.+Caton&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=M.+G.+Rostedt&sortspec=date&submit=Submithttp://saefuel.saejournals.org/search?author1=L.+J.+Hamilton&sortspec=date&submit=Submit


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m:

mass

MAP:

manifold absolute pressure

MON:

motoring method octane number

PI:

pre-ignition

qhot-spot:

thermal energy gain from hot-spot

qloss:

thermal energy loss

nd :

non-dimensional thermal flux

RF:

residual gas mass fraction

RON:

research method octane number

Ti:

initial charge temperature

tPI:

time to pre-ignition

TC:

top center

u:

specific internal energy

UEGO:


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universal exhaust gas oxygen

:

engine rotational speed

WOT:

wide open throttle

BTC:

before top center

cV:

specific heat at constant volume

CAD:

crank angle degrees

CFR:

Cooperative Fuels Research

CR:

compression ratio

Glow plug (model engine)

A glow plug (alternatively spelled glowplug or glow-plug) is a device, similar to asparkplug, used to help ignite thefuelin thevery small internal combustion enginestypically used

inmodel aircraft, model cars and similar applications. The ignition is accomplished by a

combination of heating from compression and heating from the glow plug. The glow plug is a

durable, mostlyplatinum, helical wire filament recessed into the plug's tip. When an electric

current runs through the plug, or when exposed to the heat of the combustion chamber, the

filament glows, enabling it to help ignite the special fuel used by these engines. Power can be

applied using a special connector attaching to the outside of the engine, and may use a

rechargeable battery or DC power source.

Glow fuelgenerally consists ofmethanolwith varying degrees ofnitromethanecontent as an

oxidizerfor greater power, generally between 5% and 30% of the total blend. These volatiles

are suspended in a base oil ofcastor oil,synthetic oilor a blend of both for lubrication and

heat control, again, in varying degrees of overall content. The lubrication system is a "total

loss" type, meaning that the oil is expelled from the exhaust after circulating through the

engine. The fuel ignites when it comes in contact with the heating element of the glow plug.Between strokes of the engine, the wire remains hot, continuing to glow partly due to thermal
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inertia, but largely due to thecatalyticcombustion reaction of methanol remaining on the

platinum filament. This keeps the filament hot, allowing it to ignite the next charge, thus

sustaining the power cycle.

Some aircraft engines are designed to run on fuel with no nitromethane content whatsoever.

Glow fuel of this type is referred to as"FAI fuel"after the aeronautical governing body of thesame name.

To start a glow engine, a direct current (around 3 amps and 1.25 to 2 volts, often provided by

a single, high current capacity rechargeableNiCd,NiMHorlead-acid batterycell, or a

purpose-built "power panel" running on a 12VDC source) is applied to the glow plug,

initially heating the filament. (The name 'glow plug' comes from the fact that the plug's

filament glows red hot.) The engine is then spun from the outside using a manual crank, built-

in rope-based recoil starter, spring-loaded motor or purpose-builtelectric motor, or by hand,

to introduce fuel to the chamber. Once the fuel has ignited and the engine is running, the

electrical connection is no longer needed and can be removed. Each combustion keeps the

glow plug filament glowing red hot, allowing it to ignite the next charge, thus sustaining thepower cycle.

Lead-acid battery cells that are used to ignite a model engine glow plug, due to their two volt

output when freshly charged, usually cause a regular 1.5 volt glow plug to burn out

instantaneously, and either aresistorof the proper value and wattage, or a high-power

germanium transistor's base/emitter junction (in a series connection with one of the plug's

terminals) can reduce the lead-acid cell's voltage to a suitable 1.5 volt level for engine

starting.

Technically a glow plug engine is fairly similar to adiesel engineandhot bulb enginein that

it uses internal heat to ignite the fuel, but since the ignition timing is not controlled by fuel

injection (as in an ordinary diesel engine), or electrically (as in a spark ignition engine), it

must be adjusted by changing fuel/air mixture and plug/coil design (usually through adjusting

various inlets and controls on the engine itself.) A richer mixture will tend to cool the

filament and so retard ignition, slowing the engine. This "configuration" can also be adjusted

by using varying plug designs for a more exact thermal control. Of all internal combustion

engine types, the glow plug engine resembles most thehot bulb engine, since on both types

the ignition occurs due to a "hot spot" within the engine combustion chamber.

Glow plug engines can be designed for two-cycle operation (ignition every rotation) or four-

cycle operation (ignition every two rotations). The two-cycle (or two-stroke) versionproduces more power, but the four-cycle engines have more low-end torque, are less noisy

and have a lower-pitched, more realistic sound.[1]

Considerations when using glow plugs

Depending on engine type, usage of a turbo plug may be required. For turbo enginesuse turbo plugs. Never install a turbo plug in a standard engine or vice versa.

Big engines have more mass and retain heat better. Smaller, lighter engines don't, and

need the help a hotter plug can offer.

The "right" plug for an engine can change with the temperature. The hotter the day,

the colder the plug.
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Hot plugs promote better idling and acceleration. If an engine runs rough or

accelerates sluggishly, a hotter plug will help.

Cold plugs produce more power and may improve performance if an engine runs hot.

The downside is rougher idling and more difficulty in tuning.

For cars: If the track/course has a lot of twists and turns, a hot plug is fine. If the

track/course has long straights where you'll reach maximum rpm, a colder plug isbest.

Over-leaning an engine can harm it, by raising operating temperatures; "burning up" a

plug inside itsproduct lifetime.

Higher nitro means hotter fuel: needs colder plugs, and vice versa.

If the engine sags when the battery is disconnected, the plug is too cold or more nitro

is needed (or the plug is at the end of its life), and if the engine bites back or backfires

when hand cranking, the plug is too hot or less nitro is needed.

Glow plugs get very hot, enough to glow the filament red or white hot, and removing

a glow plug while power is applied can cause burning if appropriate care is not taken.

Special caution must be taken while near fuel sources.

Some connectors for glow plugs can short-circuit and damage batteries, or cause themto explode. Batteries may get hot during the use of a glow plug. This especially

applies to home-made or nonstandard connectors.

Glow plugs have a limited lifespan. Always keep three or four of them on hand.

Glow Plugs do not need to be very tight. Just seat them, then another 1/4 turn.

Technical specs

Turbo Glow Plug

Overall Length: 17mm (.67")

Diameter: .35" (9mm) Thread size: M8x.75mm

[2]

Normal Glow Plug

Length: .8"

Diameter: .35"

Threads: 1/4-32 UNEF[3]

All about GLOW PLUGS

Written by Brian Gardiner, and Central Coast Model Aero Club Inc.Submitted by Wayne Beasley

_________

How Does A Glow Plug Work?

Contrary to what many have previously been lead to believe the following is anexplanation of how a glow plugfunctions in a motor. The plug is initially heated by

applying a voltage (typically 1.5 volts) to it. This is to cause it to glow so as to ignite thefuel at compression and start the internal combustion cycle.
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Once the cycle has started, the power source can be disconnected, as with the heatgenerated at combustion the CATALYTIC Reaction generated between the methanol

and platinum in the plug coils becomes sufficient to keep the process going. The

catalytic reaction is a reaction whereby platinum will glow in the presence of methylalcohol vapour. This will happen without any external power source being applied.

How do you select the correct PLUG for your application, and why ?

To do this you need to understand a little more of the theory behind the process. In glow

fuel the catalytic reaction is generated between the methanol and platinum only. Castoroil, synthetic oil, nitro methane, etc do not generate a catalytic reaction with the

platinum.

Next you need to understand that a certain surface area of platinum is required togenerate a sufficient catalytic reaction to keep the internal combustion process going.

Also it is necessary to allow extra surface area for the reaction to be great enough when

it diminishes with the available methanol dropping as in the case at motor idle. Simplyput, cold plugs are manufactured using a thicker wire to give greater surface area to

facilitate a greater reaction and thus the required catalytic reaction where less methanolis present in the fuel mixture.

So! More nitro means less methanol which in turn means a greater surface area toplatinum will be required to generate a sufficient catalytic reaction. Suddenly it all

makes sense! To work out which temperature plug to use, you need to know how much

methanol is in your fuel, not how much nitro or oil.

As a rough rule of thumb;

80% methanol or above, use a hot plug.

70%-75% use a medium plug.

60%-75% use a cold plug. 65% or less use a very cold plug.

Idle Bars and Other Stuff

Again, contrary to what many believe, the idle bar on a glow plug is not necessarilywhat its name would suggest. It is in fact to stop any fuel not vaporized from dousing

the platinum coil of the glow plug by dispersing it away from the coil.

Why are plated coils not as good as platinum alloy coils?

Plated coils suffer from very quick degeneration as the plating breaks down under

operating conditions. As bits of plating come off, the plug is effectively becoming ahotter and hotter unit until in a comparatively short time it is

no longer able to perform its function. Conversely, a platinum alloy coil will still

degenerate, but as it is platinum alloy throughout, the surface remains as platinum alloyand the plug continues giving much the same characteristics for quite a long time.


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Plated coils are very poor value when compared to platinum alloy coiled glow plugs.

GLOW PLUG REFERENCE LISTING

Submitted by Brian CooperCooper's Custom Blended Fuels LLC - www.cooperfuels.com

MFG Number / name Use Remarks

Hot Glow Plugs- for low nitro fuels

Mc Coy MC 55- R/C Long sport hot Similar to K&B 1L

Mc Coy MC 59 sport hot low nitro

Mc Coy MC 14 very hot heli/ inverted 4-strokeFireball Hot 1.2-3.0V sport hot red

Fireball S-20 R/C Long sport hot 1.5-2.0V

SonicTronics Glowdevil #300 sport med-hot

Rossi R/C Hot, R-1&R-2 sport hot low nitro

Sig R/C Long` sport hot

Enya #3 Sport hot low nitro

Thunderbolt R/C Long` sport med-hot

K&B 1L sport hot

Fox Miracle sport hot low notro or 4-stroke

Fox Standard sport hot

Fox R/C Long (2V) sport hot

Standard (med) glow plugs-for 10%-15% fuels


Fireball Standard 1.2-2V standard sport yellow

Sonic Tronics Glowdevil standard standard sport

Rossi Medium and R-3 standard sport

Enya #4 standard sport med-hot

Enya #5 standard sport med-cool

Mc Coy MC 50 all purpose sport w/idle bar

Mc Coy MC 8 sport med-cool med to high nitro

Hangar 9 Sport Long standard sport MC 50 packaged for Hangar 9

Tower Performance plug standard sport w/o idle bar

Fox R/C Long (1.2-1.5V) standard sport med-cool

Fox Gold standard sport med-cool
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Cold Glow Plugs-High nitro, 25% and over or racing/ fans


Mc Coy MC 9 hi-perf Ducted Fan / racing / high nitro

Fireball Cool 1.2-1.5V hi-perf Blue

Enya #6 hi-perf extremely hot engines/high compression/highnitro

K&B Long & Short hi-perf cold standard plug

K&B Hi Perf Nitro Plug hi-perf high nitro / high perf

Rossi R/C Cold hi-perf R-4 & R-5

Rossi Extra Cold hi-perf R-6 & R-7

Sig Cool 1.5V hi-perf

OS R-5

Fox R/C 1.2V

Fox #8 hi-perf high nitro / high perf

Four Stroke plugs- hot


Fox Miracle 4-stroke 2-cyl low nitro or 4-cyl

Mc Coy MC 14 very hot heli and inverted 4 strokes

OS Type F 4-stroke

Sig GP-001 4-stroke

Sonic Tronics Glowdevil St 301/302 4-stroke

Diagnosis or Symptoms:

High Nitro = Hot Fuel: needs colder plugs

Low Nitro = Cold Fuel: needs hotter plugs

High Nitro + High rpm's: needs colder plug ie:ducted fans

If engine sags when the battery is disconnected, the plug is too cold or more nitro is needed

If the engine bites back or backfires when hand cranking, the plug is too hot or less nitro is needed

Most HOT plugs can take 2 volts

Most COLD plugs can take 1.2 - 1.5 volts

Most 4-strokes need very hot plugs or high nitro

Note: Mc Coy 4-cycle plugs (not the MC 14) are "Saito Original Equipment" Hangar 9 plugs are Original Mc Coy plugs packaged for Horizion


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5, 6 & 7. Other Considerations

Here are a few other things you should know.

Hotplugs promote better idling and acceleration. If your engine runs rough oraccelerates sluggishly, a hotter plug will help.

Cold plugs produce more power and may improve performance if your engine runshot. The downside is rougher idling and more difficulty in tuning.

Whereyou run also plays a part. If the track/course has a lot of twists and turns, ahot plug is fine. If the track/course has long straights where you'll reach maximumrpm, a colder plug is best.

Fuel-air mix not only affects how your engine performs; it can also have an impact onhow long your plug lasts. If you run rich, it means that you're using more fuel thannecessary for top performance. Modelers are often advised to run rich during enginebreak-in, because it helps cool the engine. However, running too rich can also causean engine to "bog down" or quit entirely. In addition, it also means that the glowelement is being exposed to more contaminants than necessary, which shortensplug life.

Running leanmeans that you're using less fuel. "Leaning down" an engine has apositive effect on performance. However, care is needed here, because over-leaningan engine can harm it, by raising operating temperatures, "burn up" a plug before itstime.

Final Thoughts

Choosing the right glow plug not only improves performance, but can also extend the life of

your engine and the glow plug itself. With the guidelines above and the tips below, you're

well on your way to achieving both.

1. Buy qualityplugs. You're protecting your investment.2. Store plugs where it's dry. Moisture can ruin them.3. Use the rightglow plug. Follow the guidelines above.4. Follow proper break-in procedures.5. Tune your engine carefully. Running too lean will make your engine "blow" plugs

more often. Proper tuning helps extend plug life.6. Nevertouch the filament of a glow plug. Doing so can break the filament and ruin a

plug.7. Don't over-tightenyour plug. Tighten it until it's just snug.8. Be sure to shimyour engine correctly. A plug that's too close to the piston can

cause detonation, which will quickly damage a glow plug.9. Use onlya glow starter or 1.5V battery to heat your plug. Otherwise, your plug may

burn out ahead of its time.

10. Don'tbe afraid to ask for help. Experienced modelers have already "been there, donethat." Their experience can save you time and money - and most are glad to help.


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11.Glow Plug Selection & Engine Tuning.12.Safety Note The propellor of a model aircraft engine is essentially a circular saw

revolving at 20,000 rpm and can cause horrendous injuries. Ideally you should wear

goggles, mesh gloves and full body armour but extreme care and common sense are

usually sufficient. Never try to adjust an idle screw with the engine running, it usually

needs a small screwdriver and if this flicks into the rotating propellor the results donot bear thinking about. Always stand BEHIND the rotating propellor disc and read

all your club rules regarding engine operation and safety. Fully read the manufactures

instructions on engine safety, running in and operation. Most clubs do not like lone

flyers for reasons of safety. See also the BMFA guidelines on this subject. You treat

engine safety lightly at your peril. Always use a proper model restraint and ensure the

model is always pulled forward into the restraint. For safety reasons many model

engine designers are now fitting the carburetor at the engines rear which makes

adjustments a little safer. Bear this in mind when purchasing a new engine. As a final

warning Methanol can KILL or cause BLINDNESS if swallowed. If you have

decided to stick with the hobby, read on:-

13.1) Glow Plug Selection14.If you are like me, you tend to use any old glow plug and then wonder why your

engine will not start or does not perform as well as it should. A quick look at any

model aircraft engine catalogue will show you there is quite a range of glow plugs to

choose from, so the engine manufactures must feel the need to supply different grades

and types.As a first step go to the engine manufactures web site and see what is

recommended for your engine. For best results this is a good starting point. Make a

note of your fuel type and the methanol content, as this has a major influence on plug

selection. With a new engine care should be taken to follow the running in guide and

let the engine settle down before any serious attempt is made to tune for peak revs.

You will need to run at least 4 full tanks of fuel through a new engine with frequent

stops to prevent overheating, low revs and the mixture set slightly on the rich side

giving plenty of oil for lubrication. Before the engine starts to loosen up or gets run in

the golden rule is not to overheat or let the engine "labour". Some engines are very

tight at top dead center (TDC) when new but this is all part of the design, as the

bore/piston is very slightly tapered. Careful running in will ensure a perfect fit. A bit

of castor oil (2%) will aid running in, as it is a far better lubricant than any synthetic

oil and will not fail at high temperatures. Links are given below to the major engine

manufactures web sites where you will find plug grades for your motor. Failing this,

the following guide will help, or your local model shop will usually supply the

information required.

Methanol %Plug

Type

Enya

GradeFire Power

OS

Grade

OS Wankel F

4 Strokes & Multi Cylinder F

Above 80% Hot No.3 F7 A3

70 - 80% Medium No.4 F5 #8

65 - 70% Cool No.5 F4 A5

Below 65% Cold No.6 F3 P8

High Reving Ducted Fans -High Nitro ExtraCold F2 F


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15.Note:- Glow plugs are very often unmarked, once removed from the package theidentity is lost (OS Plugs are an exception) Leave in the bubble pack until required as

this also prevents damage to the filament.. Keep used plugs wrapped in tissue in

marked tins. A plug in good condition should give a bright orange glow. If it is dull

on 1.5 volts there is little chance of a good glow with the engine running and the

battery removed. Do not use plated plugs, the platinum flakes off in time andperformance falls, it is false economy. When you buy cheap, you buy twice. Replace

plugs every 12 months if you fly regularly. If the revs fall when you remove the glow

battery the plug is too cold or more nitro is needed. If it backfires when hand cranking

the plug is too hot or less nitro is needed. Hot plugs need 2 volts, cold plugs need 1.2 -

1.5 volts. Ducted Fan Engines with high nitro and high revs need cold plugs. Four

Strokes ** generally run with less oil (10%) and no nitro, although modern Four

Strokes seem to have moved to the use of Nitro, and the plug needs to be selected

with this in mind. A high methanol percentage usually means a low nitro percentage

as the oil content remains more or less constant. For example an 80% methanol fuel

will have no nitro as the rest will be oil. (20%) The basic 80/20 mix. Most club flyers

use between 5% and 10% nitro so a No. 4, F5 or #8 Plug is probably most suitable.See Fuel Mixing Section for more detail.

16.** This may have been true with early Four Strokes, modern engines with muchhigher outputs have gradully moved to the use of Nitro. For normal club flying I use a

5% Nitro mix with 20% Synthetic Oil in all my engines. Saves a lot of trouble.

Always bear in mind that a Four Stroke only fires on every other revolution (4

strokes) and the plug needs to remain hot between cold strokes. When landing on

tickover a sudden input of cold fuel/air can cool the plug and the engine cuts. A Glow

Booster may be useful in such circumstances. This is not such a prblem with Two

Strokes where the engine fires on every revolution.

17.Idle Bars. These are designed to prevent un-vapourized fuel hitting the glow plug coiland cooling it. Useful for motors that tend to run wet or old motors. Generally best

avoided as rather than preventing oiling up can make the situation worse.

18.2) Tuning the Glow Motor19.The carburetors on most model glow engines are, by automobile standards very crude,

but the do exactly the same job in converting the liquid fuel/oil mix into a very fine

mist or vapour. For the engine to run well they need to do this over a wide RPM range

with instant pickup. An engine that cuts out as you open the throttle quickly is not

much use. You will soon get fed up shouting "Dead Stick". However very few

engines react well to the throttle being slammed fully open in a split second, allow at

least a second from closed to fully open. As these are simple carburetors they only

have two adjustments, a needle valve for top end mixture and a simple screw for thelow idle range. With the correct plug and some half decent fuel close the needle valve

fully then open two full turns. Connect the glow plug battery and with the throttle

about 2/3rds open try to start the engine. Slowly open the needle valve and the engine

should start to run after a fashion. Once the engine is running allow 30 - 40 seconds

running to warm up.

20.Peak Revs or Top End adjustment.21.1) Slowly close the needle valve a couple of clicks at a time until the engine revs peak

and then start to fall off. This is usually done with the engine running so take extreme

care.

22.2) Back off for peak revs.
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23.3) Squeeze the fuel line gently between each adjustment and note if the revs rise. Letgo of the fuel line as soon as the revs alter. If the revs rise, continue to turn needle

slowly clockwise.

24.4) When the engine is properly adjusted the RPM will increase a little if you gentlysqueeze the fuel line. If the engine cuts out when the fuel line is squeezed, the engine

is too lean.25.5) Finally, hold aircraft with the nose pointing up 45 degrees. If the engine cuts out

then it is too lean. If it starts to misfire or splutter it is too rich.

26.6) The mixture on some models tends to alter in flight, I always set my mixture acouple of clicks rich, but only experience can help here.

27.Idle or Bottom End adjustment.28.See Safety note on adjusting this screw.29.1) Close the slow running screw and then open by approximately 23 full turns.30.2) Start engine and run at full throttle for about 20 seconds. Bring engine to idle

(2,000 rpm) for about 20 seconds.

31.3) Squeeze the fuel line and note any change in revs. It the revs increase and then the

motor stops, the bottom end is too rich. Turn the needle 1/4 turn clockwise. If the revsdecrease and the motor stops, the bottom end is too lean. (turn the needle 1/4 turn

anti-clockwise)

32.4) Repeat this process until the engine does not gain or lose revs before stoppingwhen the fuel line is squeezed. When you are getting close to perfection it will be

necessary to make very small adjustments, perhaps only 1/8th of turn at a time.

33.5) Run engine at full throttle and ensure that all is well, making any small adjustmentsthat may be necessary. Ensure that the top end is properly adjusted before attempting

to make any bottom end adjustments.

34.An engine correctly set up should idle reliably and then pick up cleanly as the throttleis opened reasonably quickly. Very rapid opening "gags" the motor. If it is a bit rich it

will hesitate for a second before the revs rise. If it is a bit lean it is liable to cut out.

When opening up from a long idle a lot of smoke indicates a rich bottom end setting.

35.Always ensure that the fuel pickup in the tank is 1/2" above or below the carburetorscenterline and the tank is insulated to isolate it from vibration. This can cause the fuel

to foam. Bubbles in the fuel line always indicate a leak or possible foaming. For

reliable running this MUST be cured. Some fuels have an anti-foaming addative.

36.Glow Plugs & Platinium37.All glow plugs work on the same principal. The Platinum Element will continue to

glow from the retained heat of the last firing stroke coupled with a catalytic reaction

between the Platinum & the Methanol. The Glow Coil is an iron core with a thin

coating of very expensive Platinum, cheaper plugs have a thinner coating. This is whyan engine will run with the Glow Booster connected but stops as soon as the battery is

removed, the Platinum has all gone, it also explains why a motor will run perfectly

but will not re-start. The element has probably broken (a short circuit) and will

continue to glow as usual. Once the motor stops and the battery is re- connected, as

the filament is broken it refuses to glow. Checking a plug for a good glow is NOT a

good ideahttp://www.freeinfosociety.com/electronics/schematics/sensor-

based/modelengineglowplugdriver.pdf as it burns off the Platinum coat. Look at the

plug with an eye glass, the coating should be bright & shiny. Always wet a plug

before attaching the Glow Battery.

38.Glow Plug Driver (Power Panel)

39.There are a number of Glow Plug Driver Circuits on the web. Just Google "glowplugdriver" Due to advances in electronics most are out of date and over complicated but


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the one using an LM350 Voltage Regulator covers most requirements. You can buy

ready built MOSFET panels for 20.00, making your own may be a waste of time and

money !!

Aircraft Diesel Engines

The last couple of years development in aircraft engines has been more or less concentrating

on diesel engines. We have seen one off installations to fully developed engine production

lines.

A number of companies are active on this market primarily due to major concern of long term

availability and the relative high price of AVgas (Europe). Jet fuel is available worldwide, even in

places where AVgas is not and has to be flown in.

Diesel engines are able to use JET fuel (avtur). This fuel is available everywhere and can also be made

of renewable sources (biomass) which will contribute to a cleaner environment.

The engines weigh a little more compared to a gasoline engine, but advantages are found in a much

longer service life and higher MTBO.

Last but not least: they also have an excellent specific fuel consumption compared to their AVgas

cousins and as fuel is denser too, range of a diesel powered aircraft is improved. Or with equal range

payload will increase.

Diesels in General

A diesel engine is a compression ignition engine and it draws in air and compresses it. Fuel is

then injected into the combustion chamber (either direct or indirect) with injection pressures

up to 2000 bar. Due to compression of air by the piston in the cylinder (compression ratios

are in the range of 14:1 to 24:1), temperatures are very high (700 - 900 C) and the fuel

ignites almost instantly when injected.

There is no need for a carburettor, a throttle valve (no carb ice!) or a separate ignition system.Starting a diesel in a cold environment can be difficult, a form of preheat should be used. To

implement this, diesel engines use a glow plug per cylinder to preheat the cold air before and

after starting and thus help the combustion the first couple of minutes after a cold start.

Two principles

Diesel engines are either two or four stroke models. And in aviation you will find both types.

Automotive diesels are almost exclusively four strokes but in marine applications the large

propulsion engines are two strokes. Where in aviation you will find both types.


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High torque at low RPM

Diesel fuel burns slower than gasoline therefore restricting the maximum RPM of the engine

to around 4500 RPM. On the contrary, diesels deliver a very high torque at low RPM. This is

ideal for propeller driven aircraft. One drawback is that due to higher compression and actingforces in the engine, these engines tend to be a bit heavier than a comparable gasoline engine.

Two stroke diesels overcome this problem, because they have a power stroke for every

revolution per cylinder, compared to a four stroke diesel (every other revolution per

cylinder).

Aircraft diesel engines are usually the in line or flat four type but BMW and Packard (among

others) once developed a radial diesel engine, see image.

Reliable design

Diesels are (compared to their gasoline types) basically simpler: they have no ignition

system, are more reliable, durable, have more torque, use less fuel and have a higher thermal


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efficiency and denser fuel which gives us more range (about 9%) for the same volume of

fuel. Diesels have been used in aircraft before WW-II (the JUMO series come to mind), but

since then, development was very slow due to the development of the JET engine.

Until now.

Fuel system

As already said, due to the fuel injection method used there is no carburettor or associated

throttle valve. Power is controlled by the amount of fuel injected by the high pressure fuel

pump. This is a very reliable but also a very delicate piece of hardware, fuel must be filtered

(below 70 micron) and fed through a water/fuel separator sometimes combined with a electric

heater so that any water is dissolved in the fuel and can cause no blocking of filters due to ice

formation.

With the high pressure fuel system and injector there is more fuel fed to the engine than it

uses, this extra fuel is heated by the engine and return to the tank in use. Advantage is that

warm fuel reduces the possibility of ice formation.

Turbo and Intercooler

As diesels will be heavier in construction compared to a

gasoline engine, it pays to increase efficiency and power to

weight ratio by adding aturbo or supercharger combined

with an intercooler. Air compressed by the turbochargerheats up and is cooled to lower its density by an intercooler

and the extra compressed air is then able to burn more fuel for

the same cylinder volume. Increased power is the result.

Diesel knock

Older type of diesel engines (tractor type) have a characteristic sound: diesel knock.Especially when the engine was cold and at low rpm. This knock is basically the same as

detonation in a gasoline engine, unstable combustion and high peak pressures. This knocking

was one of the reasons that these older diesels were build quite heavy.

Conclusion

Modern light weight diesel engines have dealt with this typical diesel knock. They useindirect injection, where fuel is injected into a prechamber; two stage injectors, which

prolong combustion; electronic engine control (FADEC), for exact timing considering engine

conditions; compression ratio's that are not over 20:1 and better fuel air mixing all have led to

almost no knock at all.

The engine may be somewhat heavier but this will translate into reliability, durability and

piece of mind when flying over inhospitable terrain or large bodies of water.

The future will be bright for the aircraft diesel engine!
http://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.phphttp://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.phphttp://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.phphttp://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.phphttp://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.phphttp://www.experimentalaircraft.info/homebuilt-aircraft/supercharging-engines.php


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Spark Plug

Background

The purpose of a spark plug is to provide a place for an electric spark that is hot enough to

ignite the air/fuel mixture inside the combustion chamber of an internal combustion engine.

This is done by a high voltage current arcing across a gap on the spark plug.

A spark plug is made of a center electrode, an insulator, a metal casing or shell, and a side

electrode (also called a ground electrode). The center electrode is a thick metal wire that lies

lengthwise within the plug and conducts electricity from the ignition cable hooked to one end

of the plug to the electrode gap at the other end. The insulator is a ceramic casing that

surrounds much of the center electrode; both the upper and lower portions of the center

electrode remain exposed. The metal casing or shell is a hexagon-shaped shell with threads,

which allow the spark plug to be installed into a tapped socket in the engine cylinder head.The side electrode is a short, thick wire made of nickel alloy that is connected to the metal

shell and extends toward the center electrode. The tips of the side and center electrodes are

about 0.020 - 0.080 inch apart from each other (depending on the type of engine), creating the

gap for the spark to jump across.

The several hundred types of spark plugs available cover a variety of internal-combustion

engine-driven transportation, work, and pleasure vehicles. Spark plugs are used in

automobiles, trucks, buses, tractors, boats (inboard and outboard), aircraft, motorcycles,

scooters, industrial and oil field engines, oil burners, power mowers and chain saws. Turbine

igniters, a type of spark plug, help power the jet engines in most large commercial aircraft

today while glow-plugs are used in diesel engine applications.

The heat range or rating of a spark plug refers to its thermal characteristics. It is the measure

of how long it takes heat to be removed from the tip of the plug, the firing end, and

transferred to the engine cylinder head. At the time of the spark, if the plug tip temperature is

too cold, carbon, oil, and combustion products can cause the plug to "foul out" or fail. If the

plug tip temperature is too hot, preignition occurs, the center electrode burns, and the piston

may be damaged. Heat range is changed by altering the length of the insulator nose,

depending on the type of engine, the load on the engine, the type of fuel, and other factors.

For a "hot" plug, an insulator with a long conical nose is used; for a "cold" plug, a short-

nosed insulator is used.

Spark plugs are under constant chemical, thermal, physical, and electrical attack by corrosive

gases at 4,500 degrees Fahrenheit, crushing pressures of 2,000 pounds per square inch (PSI),

and electrical discharges of up to 18,000 volts. This unrelenting assault under the hood of a

typical automobile occurs dozens of times per second and over a million times in a day's

worth of driving.

History

The spark plug evolved with the internal combustion engine, but the earliest demonstration of

the use of an electric spark to ignite a fuel-air mixture was in 1777. In that year, Alessandro


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Volta loaded a toy pistol with a mixture of marsh gas and air, corked the muzzle, and ignited

the charge with a spark from a Ley den jar.

In 1860, French engineer Jean Lenoir created what most closely resembles the spark plug of

today. He combined an insulator, electrodes, and spark gap in a single unit. As part of his

patent application for the internal combustion engine that year, he devoted one sentence todescribing the spark plug. He refined this spark plug in 1885.

In the early 1900s, Robert and Frank Stranahan, brothers and partners in an automobile parts

importing business, set out to produce a more efficient and durable spark plug. They added

gaskets between the metal shell andporcelaininsulator, made manufacturing easier, and

reduced the possibility of gas leakage past the gaskets. In 1909, Robert Stranahan sold the

plug to one automobile manufacturer and went into the spark plug manufacturing business,

cornering the market at that time.

The industry exploded as the age of the automobile opened. Eventually, variations in ignition

systems, fuel, and performance requirements placed new demands on spark plugs. Althoughthe basic design and function of the plug has changed little since its inception, a staggering

variety and number of electrode and insulator materials have been tried.

Raw Materials

The electrodes in a spark plug typically consist of high-nickel alloys, while the insulator is

generally made of aluminum oxide ceramic and the shell is made of steel wire.

Selection of materials for both the electrodes and the insulator have consumed much research

and development time and cost. One major spark plug manufacturer claims to have tested2,000 electrode materials and over 25,000 insulator combinations. As electrodes erode, the

gap between them widens, and it takes more voltage than the ignition system can provide to

fire them. High-nickel alloys have been improved and thicker electrodes have been used to

reduce engine performance loss. In addition, precious and exotic metals are increasingly

being used by manufacturers. Many modern plugs feature silver, gold, and platinum in the

electrodes, not to mention center electrodes with copper cores. Silver has superior thermal

conductivity over other electrode metals, while platinum has excellent corrosion resistance.

Insulator material also can have a dramatic effect on spark plug performance. Research

continues to find a material that better reduces flashover, or electrical leakage, from the plug's

terminal to the shell. The breakthrough use of Sillimanite, a material that is found in a naturalstate and also produced artificially, has been succeeded by the use of more heat-resistant

aluminum oxide ceramics, the composition of which are manufacturers' secrets.

One major manufacturer's process for making the insulator involves wet grinding batches of

ceramic pellets in ball mills, under carefully controlled conditions. Definite size and shape of

the pellets produce the free-flowing substance needed to make a quality insulator. The pellets

are obtained through a rigid spray-drying operation that removes the water from the ceramic

mixture, until it is ready for pouring into molds.
http://www.enotes.com/how-products-encyclopedia/porcelainhttp://www.enotes.com/how-products-encyclopedia/porcelainhttp://www.enotes.com/how-products-encyclopedia/porcelainhttp://www.enotes.com/how-products-encyclopedia/porcelain


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The Manufacturing Process

Each major element of the spark plugthe center electrode, the side electrode, the insulator,

and the shellis manufactured in a continuous in-line assembly process. Then, the side

electrode is attached to the shell and the center electrode is fitted inside the insulator. Finally,

the major parts are assembled into a single unit.

Shell

1 The one-piece spark plug shells can be made in several ways. When solid steel wire is used,

the steel can be cold-formed, whereby coils of steel are formed and molded at relatively low

temperatures. Or, the steel can be extruded, a process in which the metal is heated and then

pushed through a shaped orifice (called a die) to produce the proper hollow shape. Shells

can also be made from bars of steel that are fed into automatic screw machines. These

machines completely form the shell, drill the hole through it, and ream ita process that

improves the finish of the drilled hole and makes the size of the hole more exact.

2 The formed or extruded shellscalled blanks until they're molded into their final shapes

require secondary operations to be performed on them, such as machining and knurling.

Knurling a shell blank involves passing it through hard, patterned rollers, which form a series

of ridges on the outside of the blank. Similarly, machining-in which machine tools cut into

the exterior of the shell blankgenerates shapes and contours on the outside of the shell.

The shells are now in their final shape and are complete except for threads and side

electrodes.

Side electrode

3 The side electrode is made of a nickel alloy wire, which is fed from rolls into an electric

welder, straightened, and welded to the shell. It is then cut to the proper length. Finally, the

side electrode is given a partial bend; it is given its final bend after the rest of the plug

assembly is in place.

4 The threads are then rolled on the shells. Now complete, the shells are usually given a

permanent and protective silvery finish by an electrolytic process. In this process, the shell is

placed in a solution of acids, salts, or alkalis, and an electrical current is passed through the

solution. The result is a thin metal coating applied evenly over the shell.

Insulator

5 Insulators are supplied from stock storage. Ceramic material for the insulator in liquid formis first poured into rubber molds. Special presses automatically apply hydraulic pressure to

produce unfired insulator blanks. The dimensions of the borethe hollow part of the

insulatorinto which the center electrodes will be pressed are rigidly controlled.

6 Special contour grinding machines give the pressed insulator blanks their final exterior

shape before the insulators are fired in a tunnel kiln to temperatures in excess of 2,700

degrees Fahrenheit. The computer-controlled process produces insulators that are uniformly

strong, dense, and resistive to moisture. The insulators may be fired again after identifying

marks and a glaze are applied.


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Center electrode

7 The nickel alloy center electrode is first electrically welded to the basic steel terminal stud,

a narrow metal wire that runs from the middle of the plug to the lower end (the opposite

end from the electrode gap). The terminal stud is attached to a nut, which in turn is attached

to the ignition cable that supplies the electric current to the plug. 8 The center electrode/terminal stud assembly is sealed into the insulator and tamped

under extreme pressure. Insulator assemblies are then sealed in the metal shell under 6,000

pounds pressure. After reaming to correct depth and angle, the rim or edge of the shell

called theflangeis bent or crimped to complete a gas-tight seal. Spark plug gaskets from

stock are crimped over the plug body so that they won't fall off.

9 To form the proper gap between the two electrodes, the center electrode of the now

completely assembled spark plug is machine-trimmed to specifications, and the ground

electrode is given a final bend.

Packaging

10 After a final inspection, the spark plugs are placed in open cartons that have been

automatically formed. The plugs are generally wrapped in plastic film, placed first in a

carton, and then prepared for shipping in quantity to users.

Quality Control

Inspections and measurements are performed throughout the manufacturing and assembly

operations. Both incoming parts and tooling are inspected for accuracy. New gauges are set

up for use in production while other gauges are changed and calibrated.

Detailed inspections of shells from each machine are constantly made for visible flaws. The

ceramic insulator contour can be checked by projecting its silhouette onto a screen at a

magnification of 20 times actual size and matching the silhouette to tolerance lines. In

addition, regular statistical inspections can be made on insulators coming off the production

line.

During spark plug assembly, a random sampling are pressure tested to check that the center

electrode is properly sealed inside the insulator. Visual inspections assure that assembly is in

accordance with design specifications.

Where To Learn More

Books

Heywood, John.Internal Combustion Engine Fundamentals. McGraw-Hill, 1988.

Schwaller, Anthony.Motor Automotive Mechanics. Delmar Publishers, 1988.

Periodicals

Davis, Marlan. "Fire in the Hole: Spark-plug Design Heats up with New High-tech Materials

and Design Concepts."Hot Rod. February, 1990.


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"Spark Plug 'Sees' Inside Engines."Design News. October 17, 1989.

"Hot Spark Basics." Popular Mechanics. May, 1989.

How Ignition Systems Work

Before you can figure out how to make something work better you firstneed to understand how it works. An ignition system builds up a chargethen releases it at the right moment to ignite the air fuel mixture in thecylinder. There are two ways to improve an ignition system, increase the

amount of energy that is thrown at the spark plug to increase the chancesof lighting the mixture, and better time the spark delivery to get the mostfrom the burn. The timing issue is covered on theIgnition Curvepage. Onthis page I'll do my best to explain how ignition systems work. But first...

What the ignition system does

The internal combustion engine is the most popular motivator of vehiclesbig and small. Internal combustion engines convert the energy of burningfuel into mechanical motion. An engine ingests fuel, along with air so the

fuel will burn, compresses it then burns it. The burning mixture expandsrapidly and pushes a piston, rotor, or turbine. Gasoline is the fuel of choicefor most road going vehicles because it packs a lot of power per gallon (youcan go farther with fewer and quicker fill-ups). The biggest problem withusing gasoline is that it must be vaporized and mixed with the proper ratioof air or it will not burn. The flammability range is 1.4% to 7.6% (by massnot volume), so for every pound of air the motor pulls in you must mix in0.224 to 1.216 grams of vaporized gasoline. Any more or less and it will notburn. The duty of the carburetor or fuel injection is to monitor the air inputand meter the appropriate amount of gasoline. The engine thencompresses the volatile air fuel mixture. The more its compresses the morepower will be extracted from it when burned. Compressing the mixtureheats it, if you compress it too much it will ignite on its own. That is howdiesel engines work, however gasoline tends to explode rather than simplyignite which will tear an engine apart. The goal is to compress it as muchas you can without it self igniting. Then apply more heat (from an externalsource) to a point in the cylinder to initiate combustion. Early engines useda glow plug. Sometimes that was as simple as a copper rod threaded intothe head and heated with a torch. Eventually, they figured out the same

could be done by creating a spark inside the cylinder. The first sparkignition systems made a constant spark so they functioned the same as a
http://www.gofastforless.com/ignition/advance.htmhttp://www.gofastforless.com/ignition/advance.htmhttp://www.gofastforless.com/ignition/advance.htmhttp://www.gofastforless.com/ignition/advance.htm


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glow plug. The revelation that changed gasoline engines forever was thetimed spark ignition. Instead of creating a constant spark that would lightthe mixture at any random time, a single spark is delivered with precisetiming to most efficiently burn the fuel.

There are several challenges with timed spark ignitions. The spark onlylasts about a millisecond. If the conditions are not just right for that onemillisecond then the fuel will not be ignited. The carburetor or fuel injectiondoesn't always add the right amount of fuel. Even if the exact amount ofgas was mixed with the air it doesn't mean that every cubic centimeter ofthe cylinder has the perfect air fuel ratio. There will always be pockets ofrich and lean. On top of that, not all the gasoline will be completelyvaporized. Better fuel systems, intake manifolds, and heads are constantlybeing developed so that the mixture inside the combustion chamber staysas accurate and consistent as possible. But even with all the

advancements and technology put into modern engines the ideal conditionsfor combustion are not always met. A good ignition system can notcompensate for these bad conditions but it will increase your chances oflighting a less than perfect mixture. One trick is to increase the size of thespark gap. Making the spark travel through more air fuel mixture increasesthe odds that it will hit a pocket that is combustible. Another trick is to makethe spark last longer. The air fuel mixture is swirling around in the chamber,the longer the spark is present the better the odds are that a good pocket offuel will run into it and burn. To get a larger and/or longer spark you need tothrow more energy at the spark plug. To do that you need to know...

How ignition systems work

If you want to make a spark you need a spark gap. The electricity has tojump from somewhere to somewhere. Since the spark gap is subject toerosion and fouling it is made to be easily removed and replaced. There isa threaded hole in the head that leads to the combustion chamber. Thespark gap threads into this hole and plugs it so the chamber is sealed.That's as good a reason as any to call it a spark plug, I guess. A spark plug

has three main parts, the center electrode, the porcelain insulator, and thebody. The center electrode is what carries the electricity into thecombustion chamber. The porcelain insulator keeps the electricity in thecenter electrode from grounding out to the head before it has a chance to

jump the gap. The body of the spark plug is what threads into the head andit also has a ground electrode connected to it which catches the spark fromthe center electrode and grounds it to the head.

On many engines the spark plug is fed electricity through a spark plugwire. Spark plug wires need to contain very high voltage so the insulation isextremely thick. The conductor is often a resistive or inductive material to

reduce electro magnetic interference which can interfere with electroniccomponents. On older multi-cylinder engines the spark plug wires were


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connected to a distributor. A distributor is a rotary "switch" that is used tofeed the spark to the appropriate cylinder. The distributor is fed a sparkthrough a coil lead, which is the same thing as a spark plug wire. The otherend of the coil lead is hooked to the coil. The coil is what produces the high

voltage necessary to generate a spark. In an effort to simplify systems andincrease service life, the coil lead was eliminated and the coil was mountedinside the distributor. The next step was to eliminate the distributor andinstead hook several coils straight to the spark plug wires, often referred toas a Distributorless Ignition System or DIS. The next inevitable step was toeliminate the spark plug wires and mount the coils right to the end of thespark plugs, called Coil On Plug or COP.

The ignition coil is what produces the several thousand volts needed tomake a spark across the spark plug gap. Surprisingly the basic design ofignition coils hasn't changed at all in the last century. What we call a coil is

actually two separate coils wrapped around a common metal core. In anyother electrical system this would be called a transformer. The coil you putelectricity into is called the primary and the coil that sends the electricity outis the secondary. Transformers are used to step up or step down voltage.The voltage is changed according to the ratio of the turns. Say you have atransformer with 50 turns of wire on the primary and 100 turns on thesecondary, a 1:2 ratio. If you put 10 volts into the primary you will get 20volts out of the secondary. The trick to transformers is that to see anythingfrom the secondary the current of the primary must be changing. If you puta constant DC current through the primary you will have no current flowthrough the secondary. Transformers are usually used in AC systems sincethe current is constantly changing. If you put 120 volts AC through atransformer with a turns ratio of 10:1 then you will get 12 volts AC out of thesecondary. An ignition coil isn't fed with a constant AC signal. All we needis one spark so all we are going to put into the primary is one voltage spike.An ignition coil typically has a 1:100 turns ratio, so if 10,000 volts isrequired to generate a spark then we need to feed the primary a 100 voltspike.

Notice I said if10,000 volts is required. The required voltage changes

constantly. Firing a spark plug outside the engine requires far less voltagethan it does when firing an engine at wide open throttle. Pressure plays amajor role, the higher the pressure in the chamber the more voltage isrequired. The size of the spark gap is the other big factor, the further thespark has to jump the more voltage is required. Firing a spark through anair fuel mixture requires more voltage than it would in pure air, especiallywith exotic fuels. There are many other small factors that determine howmuch voltage will be required to make a spark. You may see "highperformance" coils advertises as 50,000 volt coils but all that ismeaningless since under most conditions you will only need 5,000 - 10,000

volts. Now if you are running wide gap plugs in an alcohol burning,


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supercharged engine then you will likely need to step up to a system with ahigher voltage potential.

So all that's left is to explain where we get the large voltage spike for theprimary. The ignition coil may not have changed in the last 100 years but

the primary circuit sure has. There are two different types of ignitionsystems, inductive and CDI. Each system has its strengths andweaknesses. The ignition coil, spark plugs and such are the same betweenthem. The difference is how they generate the primary voltage spike thatdrives the coil.

Inductive Ignition

Inductive ignition systems have been around almost aslong as the internal combustion engine. It is a simple and

rugged design which is why it is still being used on newcars today. With an inductive ignition all that's needed tomake enough juice for a spark is the ignition coil, a switchand a low voltage power source. Wait a minute, how do you get a bigvoltage spike from a 12 volt battery with only a switch? The truth is, aninductive ignition doesn't actually throw a voltage spike at the primary.Instead, it makes the primary generate its own voltage spike. Oh, ok...wait... what? A coil of wire, also called an inductor, exhibits strangeproperties when you run electricity through it. You probably remember fromelementary school what happens when you wrap a bunch of wire around a

metal object then hook it to a battery. That's right, you get a magnet. Themore current you run through a coil the greater the magnetic field. Whatyou probably don't remember (unless you happened to be touching bothends of the wire when you disconnected the battery) is that the magneticfield is actually stored energy, and when the current flow through the coilstops the stored energy causes a voltage spike. Inductors resist currentchanges. Think of them like the electrical equivalent of a flywheel. Whencurrent is low and you try increasing it, the inductor will add resistance totry and keep the current low. When current is high and you try reducing it,

the inductor will increase voltage to try and keep the current high. Since thecurrent flow in the primary of our ignition coil goes from several amps tozero amps almost instantly, the voltage will rise very fast until it can get thecurrent flowing again. The goal is to prevent any more current flow in theprimary so the coil will start the current flowing again in the secondary,giving us a spark.

Inductive ignitions are self adjusting. If a 100 volt spike is needed toinitiate a spark the primary will only rise to 100 volts. The spark will thenburn until all the energy in the coil is used up. The lower the voltage thelonger the spark will last. Let's say you open up the spark plug gap and

now 150 volts is required to fire the coil. When the current is interrupted onthe primary, the magnetic field will cause the voltage to rise rapidly. The


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voltage will rise until it reaches 150 volts at which point the spark plug willfire. After the spark is started the voltage will actually drop since it takesless voltage to maintain a spark than it does to start one. The spark willcontinue to burn as long as there is energy left in the coil. Only now the

spark duration is shorter since the spark fired at a higher voltage thanbefore. Increasing the spark plug gaps without increasing the amount ofenergy stored in the coil can actually hurt performance. If most of theenergy is used to initiate the spark then there will be little left to maintain it.It's even possible there won't be enough energy to make a spark in the firstplace. To get the benefits of wider spark gaps you need to put more energyinto the coil. When you put more energy into the coil you will increase sparkduration. When you have longer spark duration you can afford to lose someby opening up the spark gaps.

Early inductive ignitions actually used a mechanical switch, commonly

referred to as points, to physically break the primary circuit. When thepoints close it closes the primary circuit and current flows through theprimary, when the points open the circuit is open and there is no currentflow. In the seventies points were phased out in favor of electronic ignitions.Electronic ignitions use a transistor to switch the current on and off insteadof a mechanical switch. There is no magic to electronic ignitions, they are

just a switch the same as points. It's the switch opening (stopping currentflow) that triggers the voltage spike that fires the coil.

When the switch closes current flows through the primary, but it takes awhile for the current to build up. That is the biggest issue with inductiveignitions. The inductance of the coil limits change in current. When theswitch is open there is zero amps flowing through the primary. When theswitch closes the current starts at zero and ramps up until it reaches itslimit. It takes time for the current to build up. You can calculate how long ittakes for a coil to charge with this formula, T = ( L * I ) / V where T is time inseconds, L is inductance in henries, I is amps, and V is volts. Say you havea 7 millihenry coil and want to charge it to 6 amps with 14 volts. T = ( .007 *6 ) / 14 It will take 3 milliseconds (0.003 sec.) for the coil to reach six amps.That is not much time at all, so what's the problem? Believe it or not you

don't always have that much time to charge the coil. For example, a V8with a single coil running at 5000 rpm fires every 3 milliseconds. If you takeaway one millisecond for the spark that only leaves two milliseconds tocharge the coil. In two milliseconds the coil will only reach four amps beforeit has to fire again. The energy stored in the coil is determined by theinductance and the current. The formula is, J = 0.5 * L * I * I where J is theenergy in joules, L is henries and I is amps. Notice amps is squared whichmeans small changes in amps will greatly effect the amount of energystored. Our 7 millihenry coil at 6 amps holds 126 millijoules (0.126 joules),the same coil at 4 amps only has 56 millijoules of energy available to fire

the spark. So more amps mean more energy but it also means more heat.


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The current needs to be limited or the coil will go up in smoke.One strategy for controlling current is to add resistance to the primary

circuit. Ohm's law says that current equals volts divided by resistance, I = V/ R. If you run 12 volts through a coil with a primary resistance of 3 ohms

then the peak current will be 4 amps. Running a coil with a high primaryresistance is the simplest way to limit current but far from the best. Whenvoltage drops, like when the starter motor is engaged, the peak currentdrops and as a result you're spark output drops. A band aid fix is to use alower resistance coil and add a ballast resistor. On many old cars you willsee a 1.5 ohm coil and a 1.5 ohm ballast resistor, total series resistance isstill 3 ohms so the peak current is the same. The trick is that the ballastresistor is bypassed when the starter motor is engaged so the circuitconsists of only the 1.5 ohm coil. Now if the voltage drops to 9 volts whenthe starter is engaged your peak current will be 6 amps. That means you

will have a more powerful spark when starting than you do when the engineis running which is good since it's harder to initiate combustion in a coldengine. The coil can handle the increased current because it is only donefor a short period of time. If you bypassed the ballast resistor permanentlyyou would likely burn up the coil. Early electronic ignitions still used aballast resistor but they soon figured out the same task could beaccomplished with the switching transistor. This makes the circuit simplersince there is no longer a ballast resistor or bypass circuit. But the bigadvantage is that it can automatically adjust resistance to keep the peakcurrent consistent. If you have a 1.5 ohm coil it will add 1.5 ohms to keeppeak current at 4 amps. If voltage drops to 10 volts it will only add 1 ohm sopeak current will remain 4 amps. If you swap to a coil with a 0.5 ohmprimary it will add 2.5 ohms. An automatic current limited works well butputs a greater load on the ignition module. A big heat sink is required todissipate the heat from the switching transistor since it is still usingresistance to limit current. With computer controlled dwell came the "ramp-and-fire" dwell strategy. Since the current ramps up gradually and at aknown rate, the computer starts charging the coil late enough that therewon't be enough time for it to go over current. If it takes three milliseconds

for the coil to reach 7 amps then the computer will start charging the coilexactly three milliseconds before it has to fire. With this strategy you canrun higher peak current without over heating the coil or module.

That segues us nicely into the next critical element of inductive ignitions,coil charge time also known as dwell. Most guys when they hear the worddwell immediately think of old breaker point systems where you would setthe dwell by adjusting point gap. Dwell readings were in degrees. Thirtydegrees dwell meant that the distributor rotated 30 degrees between thetime the points closed and when they opened again. When the points areclosed the coil is charging. A V8 fires every 45 degrees of distributor

rotation so the coil is charging for 30 degrees and is given 15 degrees to


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discharge. When the engine is running the coil is being charged 2/3 of thetime regardless of engine speed. This is not a good dwell strategy. To seewhy, you have to look at dwell as time and not some arbitrary distance. At1000 rpm it takes 15 milliseconds for the distributor to turn 45 degrees, so

the coil will be charging for 10 milliseconds and given 5 milliseconds todischarge. It only takes 2 milliseconds for the coil to reach 4 amps where itstays for 8 milliseconds before the points open and the coil fires. The coilhas reached its full power potential after 2 milliseconds, the additional 8milliseconds does nothing but heat up the coil and waste power. The dwellstrategy on early electronic ignitions wasn't much better than points (somewere considerably worse) but as they improved the dwell times got closerand closer to the actual charge time of the coil. This means less power wasbeing wasted. Coils were running cooler, so the peak current could beincreased. More amps into the coil mean more energy out of the coil. As

stated before, with more energy you will have longer spark duration andcan run larger spark gaps. When you improve both of those you increaseyour chances of lighting the fuel mixture which ultimately means morepower and better fuel economy.

To sum it all up, inductive ignitions are simple and make a nice longduration spark. To do so they need as much current run through theprimary as possible, which if not controlled can burn up the coil. It takestime to build up primary current and as rpm increases the time available tobuild up current decreases. When there is insufficient time to reach a highcurrent, the energy available to fire the spark plugs drops significantly. Thisleads to misfiring at higher rpms.

CDI (Capacitive Discharge Ignition)

CDI systems came out about the timeinductive ignitions were becoming electronic.Conventional electronic ignitions simplyreplaced the points with a transistor but CDIcompletely reinvented the way the spark is

generated. Instead of slowly charging the coil then relying on it to generateits own voltage spike, capacitive discharge ignitions charge a capacitor withhigh voltage which is discharged through the coil to make a spark. Thecapacitor can charge and discharge much faster than a coil so a CDI canoperate at a much higher speed than an inductive ignition. The basiccomponents of a CDI are a high voltage power supply, a capacitor, aswitch, and a coil. The construction of a CDI system is a bit morecomplicated but the principle is quite simple. The capacitor is connected tothe high voltage supply and charged. When it's time to fire, the capacitor isconnected to the coil. The high voltage applied to the primary causes

current to rise very rapidly and that's where the secondary get the power tomake the spark. So the coil is basically just used to step up the voltage that


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the CDI module produces. That, in a nutshell, is the difference between aconventional electronic ignition module and a CDI module. A CDI moduleputs out power where a conventional ignition module simply switches thepower to the coil on and off like a switch. Some guys refer to any electronic

ignition as a CDI but it's only a CDI if spark is achieved by discharging ahigh voltage capacitor through the coil.It all starts with the high voltage power supply. There are two common

approaches to generating the necessary high voltage. One is to have avoltage converter built into the module that converts the 12 volts from thevehicles electrical system into a high voltage. The converter consists of anoscillator to convert the 12 volt DC to 12 volt AC, a transformer to step upthe AC voltage to several hundred volts, and a rectifier to make the ACback into DC. This arrangement is what makes CDI modules more complexand costly than a conventional electronic ignition. You will see voltage

converters used mainly on cars and street bikes where there is a batteryand charging system to power it. The alternative is to use the alternator toproduce a high voltage AC signal. Then the CDI module only has to rectifyit before it can be used to charge the capacitor. This design is generallyused on dirt bikes and lawn equipment where there is no battery orcharging system. It's also used on many quads and enduros, even thosethat have a battery and charging system. In a setup like that the alternatorhas two outputs, a high voltage output to power the ignition and a 12 voltoutput to run the lights and charge the battery.

The high voltage supply is used to charge the capacitor. Thecharacteristics of a capacitor charging are the opposite of a coil. When youput power to a coil the current starts at zero then ramps up at a linear rateas high as it's allowed, and you need to keep the current flowing through itto keep it charged. The amount of time the power is applied to the coil iscritical. Too little time and the coil will not be charged. Too much timemeans that the current used to maintain the charge is being wasted heatingup the coil. CDI eliminates these concerns. When you put power to acapacitor the current starts extremely high then drops exponentially as thecapacitor charges. When the capacitor is charged the current flow is nearly

zero amps. You can leave the capacitor hooked to the power and almostno energy is wasted. You can even disconnect the power and the capacitorwill retain its charge. As you can see, with CDI there is no need for acurrent limiter or finely tuned dwell time. Generally, CDIs are more efficientsince very little energy is wasted charging the capacitor.

The energy stored in a capacitor is determined by the size of thecapacitor and the stored voltage. J = .5 * C * V * V Inductive ignitions take along time to charge because of the large coil required to hold a charge. ACDI can hold the same charge with a very small capacitor since it is beingcharged with several hundred volts. For example, an MSD 6 ignition

charges a 1 microfarad (.000001 farad) capacitor to 500 volts. Do the math


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and you see that it stores 125 millijoules (0.125 joules). That's the same asthe inductive ignition example in the previous segment. However, theinductive ignition took 3 milliseconds to charge where the CDI charges inonly a few microseconds.

OK, so the cap is charged. The only step left is to dump the charge intothe coil. For that we need a switch. The schematic above is an overlysimplified representation of a CDI circuit. An actual CDI circuit doesn't usea physical switch. Instead, an SCR is most commonly used as the switch.An SCR is basically a transistor that when turned on, stays on until currentstops flowing through it. That eliminates the need to control the switch time.All you have to do is trigger the SCR, it will stay on as long as the capacitoris discharging and shuts off automatically when the cap is empty.

There are a few variations to CDI circuits but the one shown is a verycommon scheme. It's a bit hard to see how it works when you first look at it.

I animated the drawing to make it a little clearer. The process starts whenthe switch is open. The high voltage supply charges the capacitor. Noticehow the current flows through the diode, bypassing the coil. [A diode is likean electric check valve, current can only flow through it one way.] Thediode is not mandatory, many circuits don't have this diode, I only added itto make it easier to see when the cap is charging and when it's dischargingthrough the coil. When the capacitor is fully charged the current flow stops.The fully charged capacitor sits waiting to be dumped. When the switchcloses it connects the positive side of the capacitor to ground. This in effectbypasses the power supply and hooks the capacitor straight to the coil.Notice the positive side of the capacitor is hooked to the negative side ofthe coil. This is why, on many CDIs, you see a negative voltage whenprobing the positive coil lead. Once the capacitor is drained the switch turnsitself off and the capacitor charges again.

An inductive ignition charges a coil then allows the primary voltage torise high enough to initiate a spark. CDIs don't wait for the coil to do thework. The high voltage surge from the capacitor causes the voltage on theprimary to rise much faster and higher than it would on its own. This

creates a very intense spark. [CDI is great for two strokes since the hotterspark can better fire an oily spark plug] Unfortunately to get the big hotspark you have to give up duration. A CDI spark may last only 50microseconds (0.00005 seconds) where the spark from an inductiveignition typically lasts about 1 millisecond (.001 seconds). The short sparkcan hurt both driveability and gas mileage. A common approach to coveringup this deficiency is to fire the CDI multiple times at lower rpm. While a fewshort sparks are better than one short spark, it's still not as effective as onelong duration spark.

Which is better


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Each system has it's strengths and weaknesses. The question isn't'Which is better' but rather 'Which is better for my application'. Inductiveignitions are rugged and simple. An inductive ignition with good currentcontrol and well timed dwell works excellent for most applications.

However, inductive ignitions can't keep up on a high revving, multi-cylinderengine running a single coil. Modern motorcycle engines spin fast enoughthat even with a distributorless setup there is not enough time to fullycharge the coils. That's why capacitive discharge ignitions are seen mostlyon race cars and motorcycles.

If you have any questions or comments please e-mail me [email protected].
mailto:[email protected]:[email protected]:[email protected]

Date post:	07-Apr-2018
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