Dugald Clark (1878) invented the two-stroke engine. Theoretically the power output of the engine can be doubled for the same speed compared to a four-stroke engine. In two-stroke engines the cycle is completed in one revolution of the crankshaft. The main difference between 2-stroke and 4-stroke engines is in the method of filling the fresh charge and removing the burnt gases from the cylinder.
In the four-stroke engine these operations are performed by the engine piston during the suction and exhaust strokes. But in a two stroke engine, the filling process is accomplished by the charge compressed in crankcase. The induction of the compressed charge moves out the product of combustion through exhaust ports. Therefore, no separate piston stokes are required for these two operations. Two strokes are sufficient to complete the cycle, one for compressing the fresh charge and the other for expansion stroke. It is to be noted that the effective stroke is reduced. The air-fuel charge is inducted into the crankcase through the spring loaded inlet valve when the pressure in the crankcase is reduced due to upward motion of the piston during compression stroke. After the compression and ignition, expansion takes place in the usual way.
During the expansion stroke the charge in the crankcase is compressed. Near the end of the expansion stroke, the piston uncovers the exhaust ports and the cylinder pressure drops to atmospheric pressure as the combustion products leaves the cylinder. Further movement of the piston uncovers the transfer ports, permitting the sightly compressed charge in the crankcase to enter the engine cylinder. The piston top usually has a projection to deflect the fresh charge towards the top of the cylinder preventing the flow through the exhaust ports. This serves the double purpose of scavenging the combustion products from the upper part of the cylinder and preventing the fresh charge from the flowing out directly through the exhaust ports.
The same objective can be achieved without piston deflector by proper shaping of the transfer port. During the upward motion of the piston from BDC the transfer ports close first and then the exhaust ports, thereby the effective compression of the charge begins and the cycle is repeated.
- They are lighter in weight and they can also produce a higher power-to-weight ratio.
- Two-cycle engines are useful in applications such as chainsaws, Weedeaters, outboards, lawnmowers and motorcycles etc.,
- Two-cycle engines are also easier to start in cold temperatures.