Four and two strokes engines

Four strokes engine: is an internal combustion engine in which the piston completes four separate strokes which gives the name of the engine, constitute a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four separate strokes are termed:

1.   Intake: this stroke of the piston begins at top dead center. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. A mixture of fuel and air is forced by atmospheric (or greater by some form of air pump) pressure into the cylinder through the intake port.

2.   Compression: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the cylinder head.

3.   Power: this is the start of the second revolution of the cycle. While the piston is close to Top Dead Centre, the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward bottom dead centre.

4.   Exhaust: during the exhaust stroke, the piston once again returns to top dead centre while the exhaust valve is open. This action expels the spent fuel-air mixture through the exhaust valve(s).

Two strokes engine: a two-stroke, or two-cycle, engine is a type of internal combustion engine which completes a power cycle with two strokes (up and down movements) of the piston during only one crankshaft revolution. This is in contrast to a "four-stroke engine", which requires four strokes of the piston to complete a power cycle. In a two-stroke engine, the end of the combustion stroke and the beginning of the compression stroke happen simultaneously, with the intake and exhaust (or scavenging) functions occurring at the same time.

Two-stroke engines often have a high power-to-weight ratio, usually in a narrow range of rotational speeds called the "power band". Compared to four-stroke engines, two-stroke engines have a greatly reduced number of moving parts, and so can be more compact and significantly lighter.

Differences:

- Two-stroke engines do not have valves, simplifying their construction. 
- Two-stroke engines fire once every revolution (four-stroke engines fire once every other revolution). This gives two-stroke engines a significant power boost. But at the same time, they contaminate more
- Two-stroke engines are lighter, and cost less to manufacture. 
- Two-stroke engines have the potential for about twice the power in the same size because there are twice as many power strokes per revolution.

- Two-stroke engines don't live as long as four-stroke engines. The lack of a dedicated lubrication system means that the parts of a two-stroke engine wear-out faster. Two-stroke engines require a mix of oil in with the gas to lubricate the crankshaft, connecting rod and cylinder walls.
- Two-stroke oil can be expensive. Mixing ratio is about 4 ounces per gallon of gas (28.349g/3.8L) burning about a gallon of oil every 1,000 miles(0.62km).
- Two-stroke engines do not use fuel efficiently, yielding fewer miles per gallon.
- Two-stroke engines produce more pollution.
From:
-- The combustion of the oil in the gas. The oil makes all two-stroke engines smoky to some extent, and a badly worn two-stroke engine can emit more oily smoke.
-- Each time a new mix of air/fuel is loaded into the combustion chamber, part of it leaks out through the exhaust port.