In most machines, including engines some part of work done is expended in over coming friction or resistance of that nature; and the remaining portion is described as useful work. The ratio of useful work to the whole work done is called the efficiency of the machine.
The degree of heat utilization in internal combustion engines is determined by different efficiencies: thermal, overall, mechanical, etc.
The thermal efficiency of the working cycle of an engine characterizes the degree of perfection with which heat is converted in to work, without account taken of losses involved in friction, cooling, heat transfer, etc. This cycle accounts only for the heat lost with the exhaust gases.
Two-stroke engines do not have valves. Simple in construction.It generate a power pulse in each revolution of the crank gear. So that it's having a higher power to weight ratio.
During the working cycle, the crankshaft starts driving the piston toward the spark plug for the compression stroke. While the air-fuel mixture in the cylinder is compressed, a vacuum is created in the crankcase. The crankcase, is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
This vacuum opens the reed valve and sucks air & fuel from the carburetor. Once the piston leads to the end of the compression stroke, the spark plug fires to generate combustion pressure to drive the piston. The sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports communicating into the side of the cylinder wall.
Two stroke engines lighter, simpler and less expensive to manufacture.They have a greater power to weight ratio.But they are lesser in efficiency and it require lubrication oil to be fed with fuel.
It is evident when investigating the cycle of internal combustion engine, only 40% or lesser amount of heat energy is converted in to useful work and the major part of energy is dissipated as losses.Since the ideal power requirement is in rotary manner, the linear motion has to convert to rotary motion, and it is far from ideal. The performance of the reciprocating piston engine is limited due to its inherent characteristics of the crank gear