Two Stroke Engines
Will need CDX Disk 4-1 on day 2
2 Stoke Engines
Everything a 4 stroke
engine does in 2
revolutions a 2 stroke
engine does in 1
revolution of the
crankshaft
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2
2 Stroke Engines
The next page in your notebooks will be
“2 stroke engines” page #_________
There will be a “2 Stroke Quiz” _______
Lets start with the names of the parts
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3
2 Stoke Engines part names
Cylinder
Combustion
Exhaust port
chamber
Still uses a flywheel
(not shown)
Crankcase
Piston
Connecting
Intake port
Rod
Transfer port
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Reed valve
Crankshaft
4
2 Stroke Engines
Turn your page over and lets start with the strokes
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Intake/________ Stroke
TDC
BDC
Reed Valve
Is sucked
open
Piston moves from
BDC to TDC
Air/Fuel/Oil mixture is sucked into crankcase
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Compare intake strokes
Two Stroke


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Piston BDC to TDC
A/F/O mixture
sucked into
crankcase
Four Stroke


Piston TDC to BDC
A/F mixture sucked
into cylinder
7
Down to bottom of page
TDC
BDC
Reed Valve
Shuts
Piston gets to
TDC
Air/Fuel/Oil mixture is now trapped in crankcase
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Crankcase compression
TDC
BDC
(primary compression)
Reed Valve
sealing
Piston moves back
To BDC
Air/Fuel/Oil mixture is now pressurized in crankcase
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Crankcase compression
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
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What is going to happen when
the top of piston gets to here?
Crankcase
Compression
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A/F/O mixture squirts into
cylinder because of crankcase compression
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Piston reaches BDC
Cylinder fills with
A/F/O mixture
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Compression Stroke
Intake/________
Another A/F/O mixture is sucked into crankcase while
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First one is compressed in cylinder
14
Down to bottom of page
TDC
BDC
Piston is pounded
Down the cylinder
Piston gets to
TDC
Air/Fuel/Oil mixture is ignited in cylinder
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Blow-by adds to crankcase
compression
Crankcase
Compression
What is going to happen when the piston gets here?
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Power/Exhaust Stroke
TDC
(TDC-BDC)
BDC
Still closed
Exhaust port is uncovered and exhaust starts leaving
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What’s going to happen when
the piston gets back to TDC
OK, I get it !
ANOTHER POWER
STROKE
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Every time the piston reaches TDC,
there is another power stroke !
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Let’s make a chart
Stroke
Piston
A/F/O Mixture
Intake/Compression BTC-TDC Sucked into crankcase, another squeezed in cylinder
Power/Exhaust
TDC-BDC Blows-up and leaves, another squeezed in crankcase
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Reed valve on the side of crankcase
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Tuned Exhaust
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Tuned Exhaust
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Tuned Exhaust
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Tuned Exhaust
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Power stroke every revolution
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Let’s watch CDX Disk 4-1
Look for two stroke with no reed valve
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Oil mixing
Pre-mix

Have to mix before
filling tank
Oil injection

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Has separate fuel
and oil tank and
carburetor to mix
proper amounts
40
Advantages
2 Stroke






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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
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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
Look in owners manual for exact ratio for your engine
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32 to 1 Mix
gals X 128toozs
would
32 parts of5gasoline
1 part
of oil
640
Think
How
20 oz
ozs
am
.of.ofThink
Ioilgoing
fuel
fordivided
.640
.toThink
figure
oz of
by
..
 32 ounces give
of gas
meto. .1. ounce
640 ozsofofoil
fuel
Think
this
or
32
5
.
.
out?
.
.
gallons
Think
.
.
 32 pints of gas to 1 pint of oil
fuel


32 quarts of gas to 1 quart of oil
5 gallon gas can = How many ounces of oil
1 pint = 16 oz
2 pints = 1 qt
1 quart = 32 oz
4 quarts in a gal
32 X 4 = 128
1 gal = 128 oz
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128
X 5
640
20
32 640
64
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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
You figure it out
Let’s say we are going snowmobiling for the
Weekend and you have 1 quart of two stroke oil
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How many gallons of fuel will
be needed to mix with one
quart of oil with a 50 to 1 ratio
1 pint = 16 oz
2 pints = 1 qt
1 quart = 32 oz
4 quarts in a gal
32 X 4 = 128
1 gal = 128 oz
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There is always more than
one way to figure something
out
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Here is how I figured it out
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
1 pint = 16 oz
2 pints = 1 qt
1 quart = 32 oz
4 quarts in a gal
32 X 4 = 128
1 gal = 128 oz
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Mixing oil is critical
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

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You will cause lots of engine wear
48
Two stroke piston
Notice the piston skirt scars
(lack of oil)
Notice the 2 ring lands
(only have compression rings)
Notice the ring pins
(prevents rings from rotating
And catching on cylinder ports)
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How to identify from outside
Two stroke

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Exhaust port in
center of cylinder
Small crankcase
Four stroke


Exhaust port in end
of cylinder
Large crankcase
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Good uses of two stroke engines
Because it is light, can be used at any
angle, operates at high RPM
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

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Weed eaters
Chain saws
Boats
Snowmobiles
Dirt bikes
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Usually don’t see 2 strokes in
Because of pollution, efficiency, lack of
low RPM power, and convenience of not
mixing oil

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Cars
Trucks
Road bikes
Lawn mowers
Generators
52
Multiple cylinder two strokes
The crankcase below each piston has to
be sealed to create crankcase
compression for that cylinder
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Two stroke engine questions?
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