Two vs Four Strokes
The internal combustion (IC) engines are classified as two and four strokes engines. The difference between the two is the number of times the piston moves up and down in the cylinder to complete one combustion cycle, named as Otto Cycle (Suck, Squeeze, Bang and Blow of the air and fuel mixer). In two strokes engine, there is one upward and downward stroke, whereas in four strokes it has two each giving a total of four strokes in its combustion cycle.
Two Strokes
The two strokes of the two strokes engine are named as compression stroke and return stroke. During the compression stroke, the compression of the sucked air-fuel-oil mixture (with petrol engine) or air (with diesel engines) is compressed and then followed by the explosion of the fuel. In the return stroke, the exhaust is forced out through the bypass port using the passage formed with the piston slots and simultaneously a new mixture is sucked into the cylinder.
The presence of only two strokes to complete the combustion cycle and the absence of valves to control suction and discharge of fuel mix give a simple engine construction. So, it is easier and less costly to manufacture. It also has a power stroke for each revolution of the crankshaft producing twice the power of a four stroke engine with the same size. The small size of the engine to a given power has given a wide range of applications such as in chain saws, lawn movers, motor bikes and large, high power marine ships and electric – diesel trains etc.
With the simple construction of the two strokes engine, it does not have a separate lubricating system. So, its spare parts could wear out much faster compared to the four strokes. The addition of oil to fuel and its combustion makes the two strokes engine produce much more pollution.
Four Strokes
In four strokes engines, there is one compression and one exhaust stroke, and they are followed by return stroke to complete the combustion cycle. Compression stroke compress the fuel mixture, and at the TDC (Top Dead Centre), the combustion takes place. Piston returns with the power and starts moving up again. The exhaust valve gets open during this second upward movement (Exhaust stroke) and allows the burnt fuel to exhaust from the cylinder. During the next return stroke of the engine with exhaust valve closed and intake valve open, the mixture is sucked into the cylinder.
With this combustion system, four strokes engine has to have a separate mechanism for controlling the valves and a proper lubricating mechanism. It also produces one power stroke for two revolutions of the crankshaft. So, for a given power, the construction of the engine is costly compared to two strokes engines.
Four strokes engines can have much higher compression ratios compared to two strokes engines, and thus, much more fuel efficient. It means, four strokes engines can do more mileage per a gallon of fuel. The four strokes to complete one combustion cycle give a smoother operation of the engine. Addition of no oil with fuel gives a much cleaner exhaust and less pollution of the environment.
Difference between two strokes and four strokes
The number of available strokes to complete a combustion cycle in an engine distinguishes it as a two or four stroke engine.
With the main similarity of the two engines as the “internal combustion”, they have distinct differences in its construction as well as advantages and disadvantages by having two strokes and four strokes. The main advantages of two strokes engines are less costly, simple construction together with high cycle (engine) efficiency. However, the fuel efficiency is bit lower compared to four strokes engine.
While the four strokes engine is complicated in its construction with the addition of puppet valves and a separate mechanism for lubrication, it gives a smoother, less polluted operation with high fuel efficiency. The above advantages of four strokes engines and the longer lasting of the engines have attracted the use of them in automobiles.