In four stroke cycle engine,there is only one working stroke in each cycle.It means,two revolutions of crank-shaft are required to produce work in each cycle.The desire to have one working stroke per cylinder for every revolution of the crank-shaft has led to development of two stroke cycle engines.
In 1838,Barnett an Englishman describe the mechanism for supplying a charge to the cylinder by means of separate pumps,the fresh charge of fuel,displacing the product of the previous charge.Dugald Clerk also made a lot of contributions in this direction in 1878.
Two stroke cycle are very widely employed where small power required for motor cycle,auto rickshaw,scooters.This type of engines are compact in size,easy for manufacturing and simple in operations.One drawback is there,Specific Fuel Consumption (S.F.C) is more.(means fuel per Break Horse Power (b.h.p.) per hour is more).
There are no inlet and exhaust valves as in four stroke engine but we have inlet and exhaust ports only,due to which suction and exhaust stroke are eliminated in two stroke cycle engine.Here the burnt exhaust gases are forced out through the exhaust port by a fresh charge of fuel which enters the cylinder nearly at the end of working stroke through inlet port.This process is called as "Scavenging".Details about Scavenging will be covered in another post.
As I told above,it has no valves but consists of the inlet port(IP),exhaust port(EP) and transfer port(TP).The ignition starts due to the spark given by spark plug when the piston be nearing the completion of its compression stroke.As a result,piston is pushed down performing the working stroke and in doing so,the air-fuel mixture already drawn from the inlet port in the previous stroke is compressed to a pressure of about 1.4 kilogram/centimeter square.
When 80% of this stroke is completed the exhaust port is uncovered slightly and some of the charge of burnt gases escape to the atmosphere.As the exhaust port is uncovered by the further downward movement of the piston,the transfer port,which is slightly lower than exhaust port,is also uncovered and a charge of compressed air-fuel mixture enters the cylinder and further pushes out the burnt gases out of the exhaust port.
To facilitate the deflection of the fresh charge upward and to avoid its escape along with exhaust gases,the top of the piston is made of a particular shape.
From bottom dead centre,when the piston moves up,it first closes the transfer port and then exhaust port.The charge of fuel which previously entered in the cylinder is now compressed.When the piston is nearing the upward movement the inlet port opens and afresh charge of air-fuel mixture from the carburettor enters the crankcase.After the ignition of charge takes place the piston moves down for the power stroke and thus the cycle is repeated.
It can be fairly assumed that the crankcase and the bottom of the piston form a compressor assembly.In which the work used up in compressing the charge is negative.The top of the piston is considered as positive and crankcase is as negative.The difference between the positive and negative loop gives us the net work available from engine.
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