Four stroke petrol engine
Works on the
principle of theoretical Otto or constant volume cycle consist of a cylinder
with one end fitted with cover and other left open .cover is provided with
inlet and outlet apertures (opened and closed by mechanically operated valves).
Valve operating inlet is inlet valve and one operating exhaust is exhaust
valve. Spark plug is at top of the cover which ignites the petrol a freely
reciprocating piston inside cylinder, connecting rod and crank converts
reciprocated motion of piston into rotational motion.
1. 1. . Suction stroke
Inlet valve opens, piston moves from
T.D.C to B.D.C. energy required for this stroke is provided by cranking at time
of storing and while running fly wheel provide mechanical energy. Pressure
initially is atmospheric, but volume increases as piston moves and pressure
decreases.
Pressure difference between
atmospheric and in cylinder provides suction of petrol- air mixture.
2. 2. Compression stroke
Piston moves B.D.C to T.D.C initially
energy provided by cranking at starting and then by flywheel at time of running.
At this stroke air-petrol mix is
compressed at cylinder (to 1:7 to 1:11). This process is theoretically
reversible adiabatic.
At the end of this stroke air-petrol
mix ignited by spark provided by spark plug. Here, hot gases are released which
increases pressure at constant volume
3. 3. Working / Power stroke
Piston moves from T.D.C to B.D.C by
force exerted by hot gases linear motion of piston causes it to produce the
mechanical power during the strike which is transmitted to crank shaft by
connecting rod and crank ,
As piston moves pressure gradually decreases,
at or near the end of this stroke exhaust valve opens which causes lowering in
pressure at constant volume.
4. 4. Exhaust stroke
Piston moves from B.D.C to T.D.C. Energy
required for this stroke is provided flywheel
As this stroke being performed burnt
gases will be expelled out of cylinder at atmospheric pressure.
