Lecture 8.pptx

7/24/2019 Lecture 8.pptx

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AUTE3450U

Combustion and EnginAir and Fuel Induction

Anand Joshi

Academic Associate, UOIT


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Intae !"stem

The aimof the intake system is to deliver the proper amoand fuel accurately and equally to all cylinders at the propthe engine cycle.

Flow into an engine is pulsed as the intake valves open

but can generally be modeled as quasi-steady state ow.

The intake system consists of an intake manifold, a throt

valves, and either fuel injectors or a carburetor to add fuel

Fuel injectors can be mountedby the intake valves of eacmultipoint port injection!, at the inlet of the manifold thrinjection!, or in the cylinder head"# engines and modern cycle and some four-stroke cycle $# automobile engines!.


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Intae !"stem

Throttle body injection %ultipoint fuel inject$ource& https&''www.google.ca'


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Intae #ani$oldThe intake manifold is a system designed to deliver

engine through pipes to each cylinder, called runners.

The inside diameter of the runnersmust be large enouga high ow resistance and the resulting low volumetric do not occur.

)n the other hand, the diameter must be small enoughhigh air velocity and turbulence, which enhances its ca

carrying fuel droplets and increases evaporation anmi*ing.

The length of a runner and its diameter should be si+edto equali+e, as much as possible, the amount of air andis delivered to each separate cylinder.

$ome engines have active intake manifolds with the cachanging runner length and diameter for dierentengin


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t high engine speeds, shorter, larger diameter runnerswhich minimi+es ow resistance but still enhances prope

The amount of air and fuel in one runner length is amount that gets delivered to one cylinder each cycle.

To minimi+e ow resistance, runners should have no sha

and the interior wall surface should be smooth protrusions such as the edge of a gasket.

$ome intake manifolds are heatedto accelerate the evof the fuel droplets in the air-fuel mi*ture ow.

This is done by heating the walls with hot engine coolandesigning the intake manifold to be in close thermal cothe hot e*haust manifold, or sometimes with electrical h


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#n $# engines, air ow rate through the intake mcontrolled by a throttle plate buttery valve!usually lthe upstream end.

The throttleis incorporated into the carburetorfor thosso equipped.

Fuel is added to inlet air somewhere in the intake systethe manifold, in the manifold, or directly into each cylind

The further upstream the fuel is added, the more time tevaporate the fuel droplets and to get proper mi*ing and fuel vapor.

owever, this also reduces engine volumetric e(cdisplacing incoming air with fuel vapor. /arly fuel add


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#t is found that when fuel is added early in the intake syow of fuel through the manifold occurs in threemanners.

Fuel vapor mi*es with the air and ows with it. 0ery smfuel droplets are carried by the air ow, smaller dropletsthe streamlines better than larger droplets.

1ith a higher mass inertia, liquid particles will not alwathe same velocity as the air and will not ow around creadily, larger droplets deviating more than smaller one

The third way fuel ows through the manifold is in a t2lm along the walls.

This 2lm occurs because gravity separates some drop


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These latter two types of liquid fuel ow make it di(cult to deliver thfuel ratio to each of the cylinders.

The length of a runner to a given cylinder and the bends in it will inamount of fuel that gets carried by a given air ow rate.

The liquid 2lm on the manifold walls also makes it di(cult to have precontrol.

1hen the throttle position is changed quickly and the air ow rate c

time rate of change of fuel ow will be slower due to this liquid wall 2l

3asoline components evaporate at dierent temperatures and at die

4ecause of this, the composition of vapor in the air ow will not be same as that of the fuel droplets carried by the air or the liquid manifold walls.


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The air-fuel mi*ture which is then delivered to each cybe quite dierent, both in composition and in air-fuel result of this is that the possibility of knock problemdierent in each cylinder.

The minimum fuel octane number that can be used in tis dictated by the worst cylinder i.e., the cylinder

greatest knock problem!.

This problem is further complicated by the fact that theoperated over a range of throttle settings.

t part throttle there is a lower total pressure in tmanifold, and this changes the evaporation rate of th

fuel components.


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%olumetric E&cienc" o$ !IEngines

#t is desirable

ma*imum volumetrin the intake of any

This will vary wspeed, and Fig. 5-6the e(ciency curveengine.

There will be a cerspeed at which thee(ciency is decreasing at both lower speeds.

There are many poperating variables


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Fuel

#n a naturally aspirated engine, volumetric e(ciency wbe less than 6778 because fuel is also being addedvolume of fuel vapor will displace some incoming air.

The type of fuel and how and when it is added will determuch the volumetric e(ciency is aected.

$ystems with carburetors or throttle body injection add in the intake ow and generally have lower overall ve(ciency.

This is because the fuel will immediately start to evapfuel vapor will displace incoming air.


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%ultipoint injectors which add fuel at the intake valve ports

better e(ciencybecause no air is displaced until after the intak

Fuel evaporation does not occur until the ow is entering the

the intake valve.

Those engines that inject fuel directly into the cylinders after

valve is closed will e*perience no volumetric e(ciency loss d

evaporation. %anifolds with late fuel addition can be designed to furthe

volumetric e(ciencyby having larger diameter runners.

igh velocity and turbulence to promote evaporation are not ne

They can also be operated cooler, which results in a denser inle


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Those fuels with a smaller air-fuel ratio, like alce*perience a greater loss in volumetric e(ciency.

Fuels with high heat of vapori+ation will regain some oe(ciency due to the greater evaporation cooling that with these fuels.

This cooling will create a denser air-fuel ow for a givenallowing for more air to enter the system.

lcohol has high heat of vapori+ation, so some e(ciencto F is gained back again.


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3aseous fuels like hydrogen and methane displace more incomiliquid fuels, which are only partially evaporated in the intake syst

This must be considered when trying to modify engines made ffuel to operate on these gaseous fuels.

#t can be assumed that fuel vapor pressure in the intake system 68 and 678 of total pressure when gasoline-type liquid fuel is be

1hen gaseous fuels or alcohol is being used, the fuel vapor often greater than 678 of the total.

#ntake manifolds can be operated much cooler when gaseous fuas no vapori+ation is required.

This will gain back some lost volumetric e(ciency.


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The later that fuel vapori+es in the intake system, the bevolumetric e(ciency.

)n the other hand, the earlier that fuel vapori+es, the better areprocess and cylinder-to-cylinder distribution consistency.

#n older carbureted automobile engines, somewhere aroevaporation of the fuel in the intake manifold was consideredwith the rest of the evaporation taking place during the compresand combustion process.

#f fuel is evaporated too late in the cycle, a small percent omolecular-weight components may not vapori+e.

$ome of this un-vapori+ed fuel ends up on the cylinder walls, whby the piston rings and dilutes the lubricating oil in the crankcase


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'eat Trans$er('igh Tem)era

ll intake systems are hotter than the surrountemperature and will consequently heat the incoming ai

This lowers the density of the air, which reduces ve(ciency. #ntake manifolds for carbureted systems obody injection systems are purposely heated to enhevaporation.

t lower engine speeds, the air ow rate is slower anremains in the intake system for a longer time.

#t thus gets heated to higher temperatures at low speelowers the volumetric e(ciency curve in Fig. 5-6 at the end.


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%al*e O*erla)

t T:" at the end of the e*haust stroke and the beginning of the inboth intake and e*haust valves are open simultaneously for a brief m

1hen this happens, some e*haust gas can get pushed through the valve back into the intake system.

The e*haust then gets carried back into the cylinder with the incharge, displacing some of the incoming air and lowering volumetric

This problem is greatest at low engine speeds, when the real tim

overlap is greater.

This eect lowers the e(ciency curve in Fig. 5-6 at the low engine sp

)ther factors that aect this problem are the intake and e*haust vaand engine compression ratio.


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Fluid Friction +osses

ir moving through any ow passage or past any ow undergoes a pressure drop.

For this reason, the pressure of the air entering the cyless than the surrounding atmospheric air pressure,amount of air entering the cylinder is subsequently redu

The viscous ow friction that aects the air as it passethe air 2lter, carburetor, throttle plate, intake manintake valve reduces the volumetric e(ciency of thintake system.

0iscous drag, which causes the pressure loss, increasessquare of ow velocity.

This results in decreasing the e(ciency on the high-spe


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%uch development work has been done to reduce pressure lointake systems.

$mooth walls in the intake manifold, the avoidance of sharp cbends, elimination of the carburetor, and close-2tting parts aligno gasket protrusions all contribute to decreasing intake pressure

)ne of the greatest ow restrictions is the ow through the intake

To reduce this restriction, the intake valve ow area has been inbuilding multivalve engines having two or even three intake cylinder.

ir-fuel ow into the cylinders is usually diverted into a rotapattern within the cylinder. This is done to enhance evaporatiand ame speed.


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This ow pattern is accomplished by shaping the intakand contouring the surface of the valves and valve ports

This increases inlet ow restriction and decreases ve(ciency.

#f the diameter of the intake manifold runners is increavelocity will be decreased and pressure losses will be de

owever, a decrease in velocity will result in poorer mi*air and fuel and less accurate cylinder-to-cylinder dis"ompromises in design must be made.

#n some low-performance, high fuel-e(cient engines, ththe intake manifold are made rough to enhance turbule

better air-fuel mi*ing. igh volumetric e(ciency i


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Choed Flo

The e*treme case of ow restriction is when choked ow occu

location in the intake system.

s air ow is increased to higher velocities, it eventually rea

velocity at some point in the system.

This choked fow condition is the ma*imum ow rate that can b

in the intake system regardless of how controlling conditions ar

The result of this is a lowering of the e(ciency curve on the end in Fig. 5-6.

"hoked ow occurs in the most restricted passage of the systeat the intake valve or in the carburetor throat on those en

carburetors.


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Closing Intae %al*e A$ter -

The timing of the closure of the intake valve aects howends up in the cylinder.

;ear the end of the intake stroke, the intake valve is the piston is moving from T:" towards 4:".

ir is pushed into the cylinder through the open intake to the vacuum created by the additional volume being by the piston.

There is a pressure drop in the air as it passes through tvalve, and the pressure inside the cylinder is less pressure outside the cylinder in the intake manifold.

This pressure dierential still e*ists the instant the pisto

4:" and air is still entering the cylinder.


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This is why the closing of the intake valve is timed to occur a4:".

1hen the piston reaches 4:", it starts back towards T:" and starts to compress the air in the cylinder.


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This valve-closing point in the engine cycle, at which theinside the cylinder is the same as the pressure in tmanifold, is highly dependent on engine speed.

t high engine speeds, there is a greater pressure drthe intake valve due to the higher ow rate of air.

This, plus the less real cycle time at high speed, wou

close the intake valve at a later cycle position.

)n the other hand, at low engine speeds the pressure dacross the intake valve is less, and pressure equali+atoccur earlier after 4:".

#deally, the valve should close at an earlier position in

at low engine speeds.


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The position where the intake valve closes on most econtrolled by a camshaft and cannot change with engine

Thus, the closing cycle position is designed for one engidepending on the use for which the engine is designed.

This is no problem for a single-speed industrial enginecompromise for an automobile engine that operates ovspeed range.

The result of this single position valve timing is to revolumetric e(ciency of the engine at both high and low

This is a strong argument for variable valve-timing contr


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Intae Tuning

1hen gas ows in a pulsed manner, as in the intake man engine, pressure waves are created that travel length of the ow passage.

The wavelength of these waves is dependent on pulse and air ow rate or velocity.

1hen these waves reach the end of the runner or an o

in the runner, they create a reected pressure wave bthe runner.

The pressure pulses of the primary waves and the waves can reinforce or cancel each other, depending onthey are in or out of phase.

#f the length of the intake manifold runner and the ow


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1hen this happens, the system is tuned and ve(ciency is increased.

owever, when the ow rate of air is such that thepressure pulses are out of phase with the primary ppressure pushing air into the cylinder is slightly redvolumetric e(ciency is lower.

ll older engines and many modern engines haveconstant-length intake runner systems that can be tuneengine speed i.e., length of runner designed for one cow rate and pulse timing!.

t other speeds the system will be out of tune, and v

e(ciency will be less at both higher and lower engine sp


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This is done by changing the length of the intake rmatch the air ow rate at various engine operating cond

0arious methods are used to accomplish this.

$ome systems have single-path runners that can be chlength during operation by various mechanical methods

)ther systems have dual-path runners with controllinand'or secondary throttle plates.

s the engine speed changes, the air is directed througlength runners which best tune the ow for that speed.


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E/haust esidual

:uring the e*haust stroke, not all of the e*haust gases get pus

the cylinder by the piston, a small residual being trapped in thevolume.

The amount of this residual depends on the compression somewhat on the location of the valves and valve overlap.

#n addition to displace some incoming air, this e*haust gainteracts with the air in two other ways.

1hen the very hot gas mi*es with the incoming air it heats thethe air density, and decreases volumetric e(ciency.

This is counteracted slightly, however, by the partial vacuum creclearance volume when the hot e*haust gas is in turn cooincoming air.


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E1

#nall modern automobile engines and in many other engines, some e*

recycled /3=! into the intake system to dilute the incoming air.This reduces combustion temperatures in the engine, which results in l

o*ides in the e*haust.

78 of e*haust gases will be diverted back into the intadepending on how the engine is being operated.

;ot only does this e*haust gas displace some incoming air, but it alsincoming air and lowers its density.

4oth of these interactions lower the volumetric e(ciency of the engine.

#naddition, engine crankcases are vented into the intake systems, dispof the incoming air and lowering the volumetric e(ciency.

3ases forced through the crankcase can amount to about 68 of the tothrough the engine.


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Intae %al*es

#ntake valves of most #" engines are poppet valvesspring loaded closed and pushed open at the proper cby the engine camshaft, shown schematically in Fig. 6-6

%uch rarer are rotary valves or sleeve valves, found engines.

%ost valves and valve seats against which they close

of hard alloy steel or, in some rarer cases, ceramic.

They are connected by hydro-mechanical or mechanicto the camshaft.

#deally, they would open and close almost instantaneouproper times.

This is im ossible in a mechanical s stem and slower


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The lobes on a camshaft are designed to give quick buopening and closing without bounce at the mechanical i

This requires some compromise in the speed of valve ac

/arlier engines had camshafts mounted close to the cand the valves mounted in the engine block.

s combustion chamber technology progressed, valmoved to the cylinder head overhead valves!, anmechanical linkage system push rods, rocker arms, taprequired.


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This was improved by also mounting the camshaft in thead i.e., overhead cam engines!.

%ost modern automobile engines have one or two cmounted in the head of each bank of cylinders.

The closer the camshaft is mounted to the stems of ththe greater is the mechanical e(ciency of the system.

The distance which a valve opens dimension # in Ficalled valve lift and is generally on the order of a few mto more than a centimeter,


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Ai is the total inlet valve area for one cylinder, wheth

one, two, or three intake valves.

)n many newer engines with overhead valves and sburn combustion chambers, there is often not enough win the combustion chambers to 2t the spark plug andvalve and still have room for an intake valve large e

satisfy /q. 5-?!. For this reason, most engines are now built with more

intake valve per cylinder. Two or three smaller intake vamore ow area and less ow resistance than one largerwas used in older engines.

t the same time, these two or three intake valves, a


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%ultiple valves require a greater comple*ity of design with more

and mechanical linkages. #t is often necessary to have specially shaped cylinder heads an

piston faces just to avoid valve-to-valve or valve-to-piston contact

These designs would be di(cult if not impossible without computer- aided design ":!. 1hen two or more valves are usedone, the valves will be smaller and lighter.

This allows the use of lighter springs and reduces forces in the link

Bighter valves can also be opened and closed faster.

3reater volumetric e(ciency of multiple valves overshadows the of manufacturing and the added comple*ity and mechanical ine(


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E2A#+E O-+E# 5(

>.C-liter four-cylinder square engine bore D stroke!

intake valves per cylinder is designed to have a ma*imof E577 =%. #ntake temperature is 977o". "alculate&

6. #ntake valve area

>. :iameter of intake valve

A. 0alve lift


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!olution


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e$erence


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e$erence

/ngineering Fundamentals of the #nternal "ombustion

by 1. 1. ulkrabek, >nd edition, rentice all, 77?!.

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