Page 55 - Mechanic Diesel - TT

MECHANIC DIESEL - CITS

Valve timing diagram of all types of Engine

Objectives: At the end of this lesson you shall be able to
• explain valve timing diagram

The time taken for the opening and closing of the valve in the eternal combustion engine related to the movement
of the piston and the flywheel is called valve timing.
Functions of Valve Timing
Theoretically at the time of suction the inlet valve is at TDC of the piston. But Should open and similarly at the
time of exhaust stroke the exhaust valve B.D.C. But should open. In the same sequence, the inlet valve should
close at BDC after the end of the suction stroke and the exhaust valve should close at TDC after the end of the
exhaust stroke. But it should stop.

Thus, the inlet valve T.D.C. (TDC) to B.D.C. (BDC) opens 180° and similarly the exhaust valve opens up to B.D.C.
From T.D.C. Opens up to 180°. This diagram is only theoretical.
In reality this does not happen in the practical aspect of the engine. In fact, the inlet valve opens a few degrees
before the start of the suction stroke and remains open for a few degrees during the compression stroke also.
In this, the inlet valve opens 25° before the suction stroke and B.D.C. Remains open till after 30°C.
Similarly, the exhaust valve opens a few degrees before the start of the exhaust stroke, i.e. in the power stroke
itself and remains open for a few degrees in the suction stroke. The actual valve timing of a petrol engine is shown
here, but the actual valve timing of a diesel engine is slightly different from that of a petrol engine. Similarly, in the
exhaust valve, exhaust stroke, B.D.C. Opens 45° before T.D.C. Remains open till after 15°. In this way the inlet
valve opens up to 235° of frank station and the exhaust valve remains open up to 240°.

The injection timing is also shown in the above diagram. As per the picture, fuel injection starts 5° before the
power stroke and continues till 25° after. Thus the total fuel injection is 30°. It is clear from the above pictures that
the inlet valve is at T.D.C. Opens before and B.D.C. Closes after. Similarly, exhaust valve B.D.C. Opens before
T.D.C. Closes after.
Valve timing diagrams illustrate the timing of the opening and closing of the intake and exhaust valves in relation
to the movement of the piston within the cylinder of an engine.
For a typical four-stroke engine, the valve timing diagram shows the following key events:
1 Intake Stroke: The intake valve opens as the piston moves downward, allowing the air-fuel mixture to enter
the cylinder. The intake valve closes as the piston reaches the bottom of its stroke.
2 Compression Stroke: Both valves are closed as the piston moves upward, compressing the air-fuel mixture
in preparation for combustion.

3 Power Stroke: The compressed air-fuel mixture is ignited by the spark plug, causing rapid expansion and
pushing the piston downward. Both valves remain closed during this stroke.
4 Exhaust Stroke: The exhaust valve opens as the piston reaches the bottom of its stroke, allowing the burnt
gases to exit the cylinder. The exhaust valve closes as the piston moves upward, pushing out the remaining
exhaust gases.

The valve timing diagram visually represents these events, showing the position of the piston (usually in degrees
of crankshaft rotation) along with the opening and closing times of the intake and exhaust valves....
The specific timing of these events can vary based on engine design, performance requirements, and other
factors. Advanced engine designs may employ variable valve timing (VVT) systems to optimize performance and
efficiency across a range of operating conditions.

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CITS : Automotive - Mechanic Diesel - Lesson 08 - 10