Two Stroke Engines

Two Stroke Engines

2

Everything a 4 stroke engine does in 2 revolutions a 2 stroke engine does in 1 revolution of the crankshaft

11/2001

2 Stoke Engines

311/2001

2 Stoke Engines part names

Piston

Cylinder

CrankshaftConnectingRod

Still uses a flywheel(not shown)Combustion

chamber

Intake port

Exhaust port

Reed valve

Transfer port

Crankcase

4

2 Stroke EnginesLets start with the strokes

511/2001

Intake Stroke

TDC BDC

Piston moves fromBDC to TDCAir/Fuel/Oil mixture is sucked into crankcase

Reed ValveIs sucked

open

6

Two Stroke◦ Piston BDC to TDC◦ A/F/O mixture

sucked into crankcase

Four Stroke◦ Piston TDC to BDC◦ A/F mixture sucked

into cylinder

11/2001

Compare intake strokes

711/2001

Down to bottom of page

TDC BDC

Piston gets toTDCAir/Fuel/Oil mixture is now trapped in crankcase

Reed ValveShuts

811/2001

Crankcase compression (primary compression)

TDC BDC

Piston moves backTo BDCAir/Fuel/Oil mixture is now pressurized in crankcase

Reed Valvesealing

9

Is only a few pounds of pressure per square inch (psi) [very weak]

Cylinder compression in a four stroke engine was several psi [very strong]

The crankcase in a two stroke engine has to be very small so we can build some pressure when the piston is moving to BDC

11/2001

Crankcase compression

1011/2001

What is going to happen when the top of piston gets to here?

CrankcaseCompression

1111/2001

A/F/O mixture squirts into cylinder because of crankcase compression

1211/2001

Piston reaches BDC

Cylinder fills with A/F/O mixture

1311/2001

Intake/________ Stroke

Another A/F/O mixture is sucked into crankcase whileFirst one is compressed in cylinder

Compression

1411/2001

Down to bottom of page

TDC BDC

Piston gets toTDCAir/Fuel/Oil mixture is ignited in cylinder

Piston is poundedDown the cylinder

1511/2001

Blow-by adds to crankcase compression

CrankcaseCompression

What is going to happen when the piston gets here?

1611/2001

Power/Exhaust Stroke (TDC-BDC)

TDC BDC

Exhaust port is uncovered and exhaust starts leaving

Still closed

1711/2001

What’s going to happen when the piston gets back to TDC

ANOTHER POWER STROKE

OK, I get it !

1811/2001

Every time the piston reaches TDC,

there is another power stroke !

1911/2001

Let’s make a chart

Stroke Piston A/F/O MixtureIntake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder

Power/Exhaust TDC-BDC Blows-up and leaves, another squeezed in crankcase

2011/2001




2111/2001




2211/2001




2311/2001




2411/2001




2511/2001




2611/2001




2711/2001

Reed valve on the side of crankcase

2811/2001

2911/2001

3011/2001

3111/2001

3211/2001

Tuned Exhaust

3311/2001

Tuned Exhaust

3411/2001

Tuned Exhaust

3511/2001

Tuned Exhaust

3611/2001

Power stroke every revolution

3711/2001

Let’s watch CDX Disk 4-1

Look for two stroke with no reed valve

39

Pre-mix◦ Have to mix before

filling tank

Oil injection◦ Has separate fuel and

oil tank and carburetor to mix proper amounts

11/2001

Oil mixing

40

2 Stroke◦ Smaller/lighter◦ Cheaper◦ Less friction◦ No oil to change◦ Quicker acceleration◦ Operate at different

angles

4 Stroke◦ More efficient◦ No oil to mix◦ Less emissions◦ Less exhaust smoke◦ More low RPM power

11/2001

Advantages

41

Fuel to Oil mixtures

16 to 1 25 to 1 32 to 1

40 to 1 50 to 1 75 to 1

100 to 1

11/2001

Look in owners manual for exact ratio for your engine

42

32 parts of gasoline to 1 part of oil◦ 32 ounces of gas to 1 ounce of oil◦ 32 pints of gas to 1 pint of oil◦ 32 quarts of gas to 1 quart of oil◦ 5 gallon gas can = How many ounces of oil

11/2001

32 to 1 Mix

1 pint = 16 oz2 pints = 1 qt

1 quart = 32 oz4 quarts in a gal

32 X 4 = 1281 gal = 128 oz

How am I going to figure

this out?

Think . . Think . . Think . .

Think . . Think

5 gals X 128 ozs would give

me . . . 640 ozs of fuel

640 ozs of fuel divided by 32 .

. . .

128X 5640

640322

64

0

20 oz of oil for 640 oz of fuel or 5 gallons

43

Fuel to Oil mixtures

16 to 1 25 to 1 32 to 1

40 to 1 50 to 1 75 to 1

100 to 1

11/2001

Let’s say we are going snowmobiling for theWeekend and you have 1 quart of two stroke oil

You figure it out

4411/2001

How many gallons of fuel will be needed to mix with one quart of oil with a 50 to 1 ratio



32 X 4 = 1281 gal = 128 oz

4511/2001

There is always more than one way to figure something out

46

32 oz times 50 = 1600 I will need 1600 oz of fuel for 32 oz of oil

with a 50 to 1 ratio Since there are 128 oz in a gallon, divide

1600 by 128

11/2001

Here is how I figured it out



32 X 4 = 1281 gal = 128 oz

47

If you forget to add oil for one tank of gas, you will destroy your engine

If you mix too much oil with the gas◦ Your engine will smoke a lot◦ It will be hard to start◦ It will have less power◦ It might foul the spark plug (not start)

If you don’t mix enough oil with the gas◦ You will cause lots of engine wear

11/2001

Mixing oil is critical

Measure, don’t guess!

4811/2001

Two stroke pistonNotice the piston skirt scars

(lack of oil)Notice the 2 ring

lands(only have compression

rings)Notice the ring pins(prevents rings from

rotatingAnd catching on cylinder

ports)

49

Two stroke◦ Exhaust port in

center of cylinder◦ Small crankcase

Four stroke◦ Exhaust port in end

of cylinder◦ Large crankcase

11/2001

How to identify from outside

50

Because it is light, can be used at any angle, operates at high RPM◦ Weed eaters◦ Chain saws◦ Boats◦ Snowmobiles◦ Dirt bikes

11/2001

Good uses of two stroke engines

51

Because of pollution, efficiency, lack of low RPM power, and convenience of not mixing oil◦ Cars◦ Trucks◦ Road bikes◦ Lawn mowers◦ Generators

11/2001

Usually don’t see 2 strokes in

52

The crankcase below each piston has to be sealed to create crankcase compression for that cylinder

11/2001

Multiple cylinder two strokes

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Two Stroke Engines

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