Four stroke cycle Engine

1 - Intake stroke , which start when the piston at TC* and ends when it at BC **.

2 - Compression stroke , which start when the piston at BC and both valves are closed .

3 - Expanssion " power " stroke ,which start
    when the piston at TC and both valves are
    closed an ends when the piston at BC as
    the high pressure , high temperture gases
    push the piston down and force the crank
    to rotate.

4 - Exhaust stroke,it begin to start when the
    exhaut port opens and end when the piston
     reachs the TC where the Exhaust port close
    and the intake port open to repeat the cycle.

The inlet valve (IV) communicates through a throttle valve with the carburettor or vaporiser, from which a combustible mixture of fuel and air is drawn. The exhaust valve (EV) communicates with the silencer through which the burnt gases are discharged to the atmosphere. These valves are opened
and closed at suitable intervals by mechanisms, which will be described later.

The four strokes of the complete cycle are shown at flash above :

(1) Induction stroke – exhaust valve closed: inlet valve open

The momentum imparted to the flywheel during previous cycles or rotation by hand or by starter motor, causes the connecting rod to draw the piston outwards, setting up a partial vacuum which sucks in a new charge of combustible mixture from the carburettor . The pressure will be below atmospheric pressure by an amount which depends upon the speed of the engine and the throttle valve opening.

(2) Compression stroke – both valves closed

The piston returns, still driven by the momentum
of the flywheel, and compresses the charge into
the combustion head of the cylinder.

The pressure rises to an amount which depends
on the ‘compression ratio’, that is, the ratio of
the full volume of the cylinder when the piston
is at the outer end of its stroke to the volume
of the clearance space when the piston is at
the inner (or upper) end.In ordinary petrol engines
this ratio is usually between 6 and 9 and the
pressure at the end of compression is about 620
to 827.4 kN/m2, with full throttle opening.

(3) Combustion or working stroke – both valves closed

Just before the end of the compression stroke, ignition
of the charge is effected by means of an electric spark,
and a rapid rise of temperature and pressure occurs
inside the cylinder . Combustion is completed while the
piston is practically at rest, and is followed by the expansion of the hot gases as the piston moves
outwards.

The pressure of the gases drives the piston forward and turns the crankshaft thus propelling the car against the external resistances and restoring to the flywheel the momentum lost during the idle strokes. The pressure falls as the volume increases.

(4) Exhaust stroke – inlet valve closed: exhaust valve open

The piston returns,again driven by the momentum of the flywheel,and discharges the spent gases through the exhaust valve. The pressure will be slightly above atmospheric pressure by an amount depending on the resistance to flow offered by the exhaust valve and silencer.

It will thus be seen that there is only one working stroke for every four piston strokes, or every two revolutions of the crankshaft, the remaining three strokes being referred to as idle strokes, though
they form an indispensable part of the cycle . This has led engineers to search for a cycle which
would reduce the proportion of idle strokes,the various forms of the two-stroke engine being the result.

The correspondingly larger number of useful strokes per unit of time increases the power output relative
to size of engine, but increases thermal loading.