Two Stroke Engines Roger Bortignon Applications for 2
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? ? ? 2
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) 3
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 4
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 5
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 - 20 psi) as the piston moves down the cylinder Ø this is called (crankcase compression or pre-compression) 6
Exhaust Stroke • as piston moves downward, exhaust port is uncovered Ø most of the burnt gases leave the cylinder • this is called exhaust blowdown 1 7
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 Ø called short-circuiting 8
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 9
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Scavenging Designs 1. Cross Flow Ø older design, not as common Ø employs domed shaped “deflector piston” Ø shape of the piston directs incoming charge up & over Ø easy starting v lower compression Ø smooth idle & good low RPM performance 11
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 Ø this can lead to pre-ignition so compression ratio must be limited 12
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 13
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 14
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 15
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 16
2. Simplified Reed Valves in Operation 17
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 18
3. Rotary Disc Valve Engine § not as common – Sea. Doos & 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 19
3. Rotary Disc Valve Engine in Action 20
2 Stroke Engine Lubrication • two ways a 2 stroke engine can be lubricated… Ø 1. gas-oil mix - can be between 25: 1 to 50: 1 (total loss lubrication) v ratio is very important! n too much, too little? speed-sports. com/more/oil. html 21
2 Stroke Engine Lubrication Ø 2. oil injection – oil is placed in a separate reservoir v oil is injected into the engine based on engine rpm & throttle position v uses a pump driven by the crankshaft v common on scooters, motorcycles, outboard motors v supplies oil as demand increases or decreases n less at light load, more at higher loads 22
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 (2 X power strokes) Ø a heavy flywheel isn’t needed 23
Disadvantages: 2 stroke engines • shorter life span (2 X power strokes + poorer lube system) • additional cost of 2 stroke oil (≈4 oz/gal)(20 ml/1 L) • higher fuel consumption Ø why? • higher pollution levels Ø why? • piston scuffing at ports • noisier – 2 X power strokes • ½ as much time to cool • 2 X power strokes but only a 40%-50% increase in power Ø a result of poor filling of fresh AFM & shorter power stroke 24
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 25
• envirofit. org 26
Advances in 2 stroke technology - exhaust port valve Ø varies exhaust port height to change power stroke length (beginning of “exhaust blow-down” is altered) Ø valve is closed at low rpm – this raises compression ratio and lengthens power stroke length Ø at higher rpm, the valve opens to help the engine breathe v >volumetric efficiency Ø this helps spread the power curve of the engine over a wider rpm range Ø exhaust control valve can be cable operated via the throttle or computer controlled Ø in use since early 1980’s 27
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) Ø loose/stripped fasteners Ø broken/worn gaskets or seals • worn piston rings Ø can lead to broken piston & cylinder wall damage 28
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 29
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