Four stroke engines are internal combustion engines that require four strokes of the piston to complete one cycle of operation. Strokes refer to the movement of the piston in upward and downward direction. The cycle of operation of the engine are; intake, compression, combustion, exhaust.
Intake: During this stage, air is sucked into the cylinder as the piston approaches bottom dead centre. The intake valves are opened and the movement of the piston downwards creates a vacuum which is filled with air. The piston then moves upwards as the intake valves are closed.
Compression: As the piston approaches top dead centre the air in the cylinder is compressed and raised to a very high temperature and pressure. For gasoline engines the fuel is mixed with air and compressed to a very high temperature and pressure, and for diesel engines fuel is injected into the compression chamber after the air has been compressed. During this stage both the intake and exhaust valves are closed.
Combustion: At this stage fuel is injected into the combustion chamber and is immediately ignited for diesel engines. For gasoline engines the air- fuel mixture is compressed to a very high temperature and pressure and ignited by a spark plug. The ignition produces huge amounts of energy which pushes the piston downward to drive the crankshaft. The motion of the crankshaft produces electrical power which powers the machine.
Exhaust: During the exhaust stroke, the piston moves from bottom dead centre to top dead centre. The exhaust valves are opened and exhaust gases from the entire combustion process are expelled onto the atmosphere or exhaust pipes where the exhaust gases are cleaned of all soot and reused for combustion in a process known as turbocharging.
Four stroke engines are used widely to power automobiles and small vessels where speed is required. In large ships four stroke engines are used as generators to supply power for starting equipment such as compressors, purifiers, boilers, pumps and fresh water generators. Since they run at higher speeds than two stroke engines, they are not suitable to power large ocean going vessels which do not require speed.