What are two-strokes and four-stoke engines? Explained

Two-stroke engine

The air-fuel mixture from the carburettor enters the crankcase through the inlet port during the upward movement of the piston. At the same time, the mixture in the cylinder is compressed. The combustion takes place and the piston moves imparting motion to be crankshaft. During the downward movement of the piston, the mixture in the crankcase is compressed and pushed into the cylinder through the transfer port, which pushes out the gases through the exhaust post, at the same time filling the cylinder with a new charge. Thus the whole cycle is completed in two strokes.

Four-stroke engine

The cycle of events that take place in a 4-stroke engine is having suction of air-fuel mixture in the cylinder during the downward movement of the piston,. The charge is compressed when the piston moves up at the end of which the mixture is ignited. The gases under high pressure then expand and push the piston down, thereby imparting the motion to the crankshaft. Finally, the exhaust gases are driven out of the cylinder. Thus the entire cycle is completed in four strokes.

Single cylinder engines

Single-cylinder engines are used generally for scooters and motorcycles. It is seen that the maximum size of the single-cylinder engine is restricted to about 500-600 c.c., because of the higher unbalance forces, which become difficult to be balanced. Further, the weight of the flywheel required becomes excessive for higher engine sizes.

Two cylinder engines

This type of engine, apart from providing more power, gives more uniform torque and balancing is also better as compared to single-cylinder engines. However, in practice, two-cylinder engines are rarely employed for automotive use.

Inline vertical type: In this, the two single cylinders are in effect, placed side by side vertically, so that their cranks are 180^o apart. This type provides good balancing, but the disadvantage is in unequal firing intervals: the spark takes place at 0^o, 180^o,720^o,900^o and so on.

V-type: In this, the two cylinders are placed with their centre line at the same angle to each other. Generally, this angle is kept at 60^o, but in some instances, angles from 40 to 90 have also been used. Its main advantage is that it can be made more compact.