2 Stroke Engine
Principles of operation of a two stroke engine
It takes two movements of the piston (or strokes) to complete the combustion cycle. Every time the piston moves away from the combustion chamber, it is a power stroke. Every time the piston moves toward the combustion chamber, it is a compression stroke.
Two-Stroke
Means it takes two movements of the piston (strokes) to complete the combustion cycle.
Timing
Movement of engine parts and pattern of combustion-cycle events
Exhaust Port
Opening in cylinder wall that lets exhaust gases out
Intake Port
Opening that allows fuel mixture to enter engine
Intake Event
Piston completes its movement toward the crankcase. This opens the transfer port and allows fuel-air mix to enter combustion chamber.
Scavenging
Process of removing exhaust gases
Scavenging
Process of removing exhaust gases. Uses part of fresh fuel air mixture to push exhaust through the exhaust port.
Transfer Port
Allows movement of fuel-air mixture from the crankcase into the combustion chamber
Types of pistons used in 2 stroke engines
1. Flat-head piston 2. Deflector Head piston
Loop-scavenged
1. Intake and exhaust ports are 90 degrees from each other causing combustion-chamber gases to swirl. 2. Used second intake port to improve the looping effect of combustion chamber gases. 3. Intake control is provided by read valve or piston.
Cross-scavenged
1. Intake and exhaust ports are opposite each other. 2. The piston head includes deflector to separate intake and exhaust gases. 3. Intake control is provided by read valve or rotary valve.
Factors that determine timing in 2 stroke engines
1. Piston -shape, length of piston skirt, and length of stroke. 2. Ports-number of ports, location of ports, size of ports. 3. Valves-type of valve, size of valve opening, sensitivity to pressure changes
Power Event
1. Piston completes movement toward the combustion chamber and intake port closes. This seals the fuel-air mix in the crankcase. 2. A spark in the spark plug ignites the compressed fuel-air mix.
Exhaust Event
1. Piston is forced away from the combustion chamber and causes the crankshaft to turn. 2. As the piston moves away from the combustion chamber it opens the transfer port.
Compression Event
1. Piston moves toward the combustion chamber, which closes the transfer and exhaust ports. It compresses the fuel-air mix. 2. Creates a suction on the intake port, which opens the intake port and allows fresh fuel-air mix to enter and lubricate the crankcase.
Valves used in 2 stroke engines
1. Reed Valve (leaf valve)-spring-steel valve. allows fuel-air mix to flow one direction only. 2. Rotary Valve-flat circular plate with part of plate cut away. Operates between carburetor and crankcase. 3. Piston controlled port-design where port placement and piston open and close the port, so no valve is needed.
Components used to control scavenging
1. Size of exhaust port 2. Location of exhaust port 3. Design of piston. 4. Design of exhaust pipe.
Valve
Device for opening or closing a port
Power Stroke
When the piston moves away from the combustion chamber.
Compression Stroke
When the piston moves toward the combustion chamber.