Two Stroke Engines Roger Bortignon

Applications for 2 stroke engines chain saws dirt bikes mowers & trimmers personal watercraft Mopeds & Scooters 1960 3 cylinder Saab outboard motors R/C model planes augers snow blowers others???

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)

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

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

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)

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

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

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

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

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

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

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

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

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

2. Simplified Reed Valves in Operation

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

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.

3. Rotary Disc Valve Engine in Action

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

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.

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

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

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

envirofit.org

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

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

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