1. Two Stroke Engines
Roger Bortignon

2. Applications for 2 stroke engines
chain saws
dirt bikes
mowers & trimmers
personal watercraft
Mopeds & Scooters
cylinder Saab
outboard motors
R/C model planes
augers
snow blowers
others???

3. Tid Bits
both the largest and smallest engines in the world are 2 stroke engines
2 stroke engines existed before the invention of the 4 stroke engine (1860)

4. 2 Stroke Theory of Operation
intake, compression, power & exhaust strokes requires 1 revolution of the crankshaft to complete
How does this compare to a 4 stroke engine?
crankcase is used for part of the intake phase (crankcase compression)
poppet type intake & exhaust valves aren’t needed
piston movement controls AFM flow in & out of the cylinder through ports
AFM: air/fuel mixture

5. Intake & compression phase
piston moves up
AFM above the piston is compressed
low pressure (vacuum) created below piston
this low pressure draws fresh AFM into the crankcase

6. Ignition / Power Phase spark (via spark plug) ignites AFM
expanding gases force piston downward (torque is created)
AFM below the piston in the crankcase is compressed (≈7 - 20psi) as the piston moves down the cylinder
this is called (crankcase compression or pre-compression)

7. Exhaust Stroke as piston moves downward, exhaust port is uncovered
most of the burnt gases leave the cylinder
this is called exhaust blowdown 1

8. Exhaust & Fuel Transfer Phase
the piston continues to move downward uncovering the transfer port
fresh AFM begins filling the cylinder while simultaneously forcing out any remaining exhaust gases
the process of using the intake charge to help clear out the burnt gases is called scavenging
scavenging occurs when both the exhaust & transfer ports are open
note: the exhaust port is always higher than the transfer port
scavenging causes fresh AFM to be pushed out the exhaust port
called short-circuiting

9. Intake & compression phase
as piston moves up, transfer port is closed off
low pressure below piston creates a vacuum which draws in fresh AFM for the next transfer phase
piston continues to move up and closes off the exhaust port – compression now begins

11. Scavenging Designs 1. Cross Flow older design, not as common
employs domed shaped “deflector piston”
shape of the piston directs incoming charge up & over
easy starting
lower compression
smooth idle & good low RPM performance

12. Cross Scavenging Continued
not an efficient design
at high rpm exhaust gases tend to get left behind here
lower manufacturing costs
the edges of the piston can become overheated
this can lead to pre-ignition so compression ratio must be limited

13. Scavenging Designs 2. Loop scavenged
a series of ports (2,3, or more) directs the AFM into the cylinder
superior power throughout rpm range
(better scavenging) = >hp/weight
better fuel economy
scavenges more completely than cross flow design
piston has slots in its’ skirt

14. Controlling Crankcase Compression AFM Flow
3 different systems have been used to prevent fresh AFM from being pushed back out the carburetor during the power stroke
AFM = Air-Fuel Mixture

15. 1. Piston Port piston skirt controls port opening/closing
piston skirt is longer – more weight & inertia
usually used with loop scavenged engines
simple, inexpensive design

16. 2. Reed Valve Engine most common type of crankcase valve used
a pressure sensitive one-way valve
thin spring steel reeds (or other material) flex during the intake stroke to allow AFM to enter cylinder during the intake stroke
during the power stroke the positive pressure in the crankcase closes and prevents fresh AFM from being pushed out of the crankcase

17. 2. Simplified Reed Valves in Operation

18. 2. Reed Valve Engine
reduces volumetric efficiency (VE) because atmospheric pressure must overcome the tension of the reeds before AFM can enter the crankcase
performance reeds use reeds that open easier to improve VE – this creates a “snappier“ throttle response

19. 3. Rotary Disc Valve Engine
not as common – SeaDoos & snowmobiles employ this type of valve
uses a rotating disc driven off of the crankshaft
during the intake stroke, AFM is admitted into the crankcase only when the opening in the rotary valve is in alignment with the crankcase port
The advantage of a disk rotary valve is that it enables the two-stroke engine's intake timing to be asymmetrical which is not possible with two-stroke piston port type engines.
The two-stroke piston port type engine's intake timing opens and closes before and after top dead center at the same crank angle making it symmetrical whereas the disk rotary valve allows the opening to begin earlier and close earlier.

20. 3. Rotary Disc Valve Engine in Action

21. 2 Stroke Engine Lubrication
two ways a 2 stroke engine can be lubricated…
1. gas-oil mix - can be between 25:1 to 50:1
(total loss lubrication)
ratio is very important!
too much, too little?
speed-sports.com/more/oil.html

22. 2 Stroke Engine Lubrication
2. oil injection – oil is placed in a separate reservoir
oil is injected into the engine based on engine rpm & throttle position
uses a pump driven by the crankshaft
common on scooters, motorcycles, outboard motors
supplies oil as demand increases or decreases
less at light load, more at higher loads
These pumps are controlled by an extra throttle cable and deliver the oil either to the throat of the carburetor or directly to the main bearings.

23. Advantages: 2 stroke engine versus 4 stroke engines
simple: no valves needed
lighter: fewer parts
fewer parts to malfunction
lower manufacturing costs
higher power-weight ratio
will work at any angle with gas/oil mix
quick acceleration (2X power strokes)
a heavy flywheel isn’t needed
4 strokes coast for 3 strokes to create 1 power stroke
a heavy flywheel is necessary
this makes acceleration & throttle response slower

24. Disadvantages: 2 stroke engines
shorter life span (2X power strokes + poorer lube system)
additional cost of 2 stroke oil (≈4oz/gal)(20ml/1L)
higher fuel consumption
why?
higher pollution levels
piston scuffing at ports
noisier – 2X power strokes
½ as much time to cool
2X power strokes but only a 40%-50% increase in power
a result of poor filling of fresh AFM & shorter power stroke
2X power strokes does not mean 2X horsepower
2X the power strokes with only a 40%-60% increase in hp
power stroke is much shorter because of blowdown
lighter flywheels means can’t hang on to heavy engine loading
2X as many power strokes means cooling becomes more important
multicyclinder engines require sealing in the crankcase area
low VE

25. Advances in 2 stroke technology
direct fuel injection – eliminates “short circuiting” of fuel
EPA mandated a 75% reduction in outboard motor hydrocarbon and NOx emissions as of 2006 with tougher ones on tap as of 2008

26. envirofit.org

27. Advances in 2 stroke technology - exhaust port valve
varies exhaust port height to change power stroke length (beginning of “exhaust blow-down” is altered)
valve is closed at low rpm – this raises compression ratio and lengthens power stroke length
at higher rpm, the valve opens to help the engine breathe
>volumetric efficiency
this helps spread the power curve of the engine over a wider rpm range
exhaust control valve can be cable operated via the throttle or computer controlled
in use since early 1980’s
All power valve systems vary the duration of the exhaust port open time, which gives the engine usable low end power combined with excellent top end power
developed by Yamaha in 1977 on a race bike

28. Common 2 stroke issues in the high school shop…
incorrect gas–oil mix ratio
carburetor gummed up (off season)
heavy carbon deposits (ports, piston top, cylinder head)
sticking/stuck exhaust port power valve
crankcase not sealed (poor or no pre-compression)
loose/stripped fasteners
broken/worn gaskets or seals
worn piston rings
can lead to broken piston & cylinder wall damage

29. What’s with the goofy looking exhaust pipe?
called an expansion chamber
designed to use the exhaust sound waves to first suck the cylinder of burnt gasses and in the process, draw fresh AFM into the chamber
the charge is then crammed back into the cylinder, filling it to greater pressures than could be achieved by simply venting the exhaust port into the open atmosphere