4 Stroke Diesel Engine Valve Timing Diagram

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4-stroke diesel engine is a type of engine that has 4 processes in one cycle. In the previous article we discussed how it work both 4 stroke and 2 stroke diesel engines.

On a four stroke diesel engine, we will find a valve mechanism where this mechanism will regulate the opening of the suction valve and exhaust valve.

The question is, when is this valve open? what is the angle of opening of the two valves?

This is what we will explain in detail

Four Stroke Diesel Valve Timming Diagram


We know that a 4-step diesel engine has 4 processes, namely;

  • Intake stroke
  • compression stroke
  • combustion stroke
  • exhaust stroke


Intake stroke is a step where the air is inserted into the combustion chamber, in this case the suction valve must be open.

Compression strokes and combustion strokes do not require valve opening, because both processes compress air and combust material, so both valves must be tightly closed.

While in the exhaust stroke, the exhaust valve must be open to move the remaining residual gases out.

From this brief statement, we understand that the valve on a diesel engine is only open in two processes, namely the intake and exhaust processes.

1. Diagram of opening the suction valve



The suction valve is a valve that uses to entrance fresh air into the combustion chamber. This valve opens when the piston moves from TDC to BDC at the intake stroke.

Then when the piston is still at TDC, the suction valve must begin to open.

And from the picture above, shows that the suction valve starts to open about 10-15 degrees before the piston reaches TDC. And the suction valve will close tightly around 25 degrees after the piston reaches BDC.

That’s mean, before the piston reaches TDC the suction valve has begun to open. And when the piston moves down to reach the BDC the valve is also still open, then when the compression stroke starts the valve is still open about 25 degrees.

2. Diagram of opening the exhaust valve



The exhaust valve is a valve that uses as exit gate for the remaining residual gas to the mufler. The procesess is take out the residual gasses from the combustion chamber, that’s make the valve must open when the piston moves up (from BDC to TDC) in the exhaust stroke.

From the picture above, after combustion step the piston moves from TDC to BDC. At the end of the combustion step (before reaching BDC) the exhaust valve has begun to open.

That is true, the exhaust valve will open about 25 degrees before the piston reaches BDC. And just like the suction valve, the exhaust valve will remain open even though the piston has reached TDC.

The exhaust valve will be tightly closed at the beginning of the suction step when the piston moves from TDC to BDC around 15 degrees.

3. Valve overlap



Valve overlap is a condition where both valves (suction valves and exhaust valves) are both open.

This condition occurs at the end of the exhaust stroke until the beginning of the intake stroke. As explained above, the exhaust valve opens when the piston moves to TDC in the end of combustion stroke. But the exhaust valve is still open when the piston moves down in the beginning of intake stroke.

On the other hand, the suction valve also starts to open even before the piston reaches TDC in the end of exhaust stroke. This will cause both valves to open together with a duration of about 25 degrees.

What is the purpose of the valve overlap?

The purpose of the overlap is to flush the exhaust gasses using fresh air. When both valves are open together it will allow fresh air to enter and then push the exhaust gas so that in the next cycle, the fresh air content becomes more maximal.

Why is the valve open prematurely?

From the explanation above we can conclude that the valves on diesel engines open and closed always exceed the piston movement.

The reason, is because it anticipates the vacuum in the cylinder chamber when the engine run in high RPM. For example is in the intake stroke, when the engine RPM is high, the up and down motion of the piston will also be high.

And this gives little time for the air suction process, so that if the valve is not opened earlier there will be a vacuum which will inhibit the engine RPM.

It also same during the exhaust process, the exhaust valve opens early to avoid the effects of compression when removing. Because when the engine RPM is high, there is also little time to discharge air.