VALVE TIMING DIAGRAM ON SECTIONAL MODEL OF 4-STROKE SINGLE
CYLINDER CI ENGINE
Objective: To draw the actual valve timing diagram of a four-stroke single-cylinder
CI engine.
Apparatus required:
Measuring tape,
Sectional four-stroke single-cylinder CI engine test rig
Theory:
In a four-stroke CI engine, the thermodynamic cycle of operations is completed in
two revolutions of the crankshaft or four strokes of the piston. During the four
strokes, there are five events to be completed, viz., suction, compression,
combustion, expansion and exhaust.
The suction stroke starts when the piston is at TDC and about to move downwards.
The inlet valve is open at this time and the exhaust valve is closed. Due to the
suction created by the motion of the piston towards the BDC, the air is drawn into
the cylinder. When the piston reaches the BDC the suction stroke ends and the
inlet valve closes. The air taken into the cylinder during the suction stroke is
compressed by the return stroke of the piston. During this stroke, both inlet and
exhaust valves are in a closed position. The air is now compressed to the clearance
volume. At the end of the compression stroke, a metered quantity of fuel (diesel) is
injected into the hot compressed air in fine sprays by the fuel injector and it starts
burning. During the burning process, the chemical energy of the fuel is converted
into heat energy. The high pressure of the burnt gases forces the piston towards
the BDC. Both the valves are in a closed position. Of the four strokes only during
this stroke, power is produced. Both pressure and temperature decrease during
expansion. At the end of the expansion stroke, the exhaust valve opens and the
inlet valve remains closed. The pressure falls to atmospheric level as a part of the
burnt gases escape. The piston starts moving from BDC to TDC and sweeps the
burnt gases out from the cylinder. The exhaust valve closes when the piston
reaches TDC.
Procedure:
1. TDC & BDC are identified and marked on the flywheel with respect to one fixed
point in the engine.
2. The circumference of the flywheel is measured using a measuring scale or
measuring tape.
3. By slowly rotating the flywheel in the direction of rotation, the opening of the
inlet valve is marked on the flywheel w.r.t. fixed point when the push rod of the
inlet valve starts to move.
4. Mark a point on the flywheel where the inlet valve is completely closed.
5. In the same way, mark the points where the exhaust valve open and close.
6. The distance of opening of inlet valve and closing of exhaust valve from TDC and
closing of inlet valve and opening of the exhaust valve from BDC is measured
using thread and scale.
