The cross section of a single cylinder spark ignition internal combustion engine as shown in Figure. The cylinder is supported in position by the cylinder block at the top and is covered by cylinder head. In the cylinder a piston travels in reciprocating motion. The space endorsed between the upper part of the cylinder and the top of the piston during the combustion process is called the combustion chamber.
A mixture of air and fuel enters the cylinder through the carburetor in spark ignition engine, via the Inlet manifold (the pipe which connects the inlet part of the engine to the air intake.). In carburetor throttle is provided to control the mass of mixture entering the combustion chamber. In the cylinder head are inlet valves for taking the charge in the cylinder and exhaust valves for discharging the products of combustion. A Spark plug near the top of the cylinder initiates the combustion. The energy of the expanding gas is transmitted by the piston (having piston rings to prevent leakage) through the gudgeon pin to the connecting rod. The connecting rod and the crank arm of the crankshaft translate the reciprocating motion of piston into rotational motion of the crank shaft as shown in Figure. The crankshaft is supported in bearings attached to the crankcase. The crankcase is the main body of the engine to which the cylinder is attached. The products of combustion leave through exhaust port and exhaust manifold. Both the intake and exhaust valves are operated by the valve mechanism. A cum shaft is driven by the crank shaft through timing gears Lobed cams on the camshaft actuate the push rods and rocker arms for opening the valves against the force of valve springs .
In the Compression ignition (CI) engine, the fuel is injected directly into the combustion chamber through a fuel injection nozzle with the help of fuel pump There is no carburetor ignition system, and spark plug in the CI engine) Since the power stroke exists for only a part of the total time a flywheel used to smooth out the power pulses and thus obtain a uniforms rotation of the crankshaft as shown in figure. For control of speed under varying load condition a governor is provided.