4-stroke cycle:
1. with intake valve open, the piston goes down, sucking air-fuel mixture into the cylinder
2. with both valves closed, the piston goes up compressing the air-fuel mixture.
3. with both valves closed, the spark plug fires, igniting the fuel, pushing piston downward.
4. with exhaust valve open, the piston goes up, pushing air-fuel mixture out of the cylinder.
2-stroke cycle:
1. with inlet port open, the piston moves upward, which compresses the air-fuel-oil mixture in the upper part of the cylinder while simultaneously drawing new air-fuel-oil mixture into the lower part of cylinder.
2. When the piston is at top, the spark fires, igniting the previous air-fuel-oil mixture, which drives the piston downward, during which the burned fuel leaves the exit port and the new air-fuel-oil mixture enters the transfer port to enter the upper part of the cylinder (to be compressed when piston moves upward).
Differences:
1. In the 4-stroke, the combustion is more complete before being exhausted from the engine, hence:
– less pollution emitted.
– gets more out of each input of gas – greater fuel efficiency.
2. In the 2-stroke, oil must be mixed in with the gas to lubricate the inner parts, hence more toxic gases are emitted.
3. The 2-stroke is louder, due to several factors including that the 2-stroke has twice as many ignitions for the same rpm, involves smaller surrounding structure to dampen the noise, and the gas exiting the cylinders are hotter.