Using engine valves to find TDC is a common method but requires a lot of techniques. You need to understand the valve operating system and be familiar with the valve train system.
I will use layperson’s terms so that you can understand the explanation.
You need to know the engine’s direction. Some engines are working clockwise, while some are anti-clockwise.
As soon you ascertain the engine direction, you will need to turn the engine. In this example, I will use a six-cylinder engine as an example.
In the six-cylinder engine, cylinder piston number 1 and 6 are always at the same position but at a different strokes.
While cylinder piston number 2 and 5 are always at the same position but at a different strokes. Lastly, numbers 3 and 4 are always at similar positions but at different strokes.
You need to manually rotate the engine to use the valve to detect the TDC. Please don’t use the ignition switch to rotate it so you can get what you are looking for
You will keep turning your engine until the number one cylinder valve, both intake and exhaust valves, is closed.
At the same time, you need to check the number six cylinder; the intake and exhaust valves must remain open.
At the point where the number one cylinder valve, both intake and exhaust closed, and number six-cylinder, both intake and exhaust valves are open is your TDC.
A similar method can also be used in a four-cylinder engine. In the four-cylinder engine, piston numbers 1 and 4 are used as you use pistons 1 and 6 in the above example.
So whenever the number 1 valve both intake and exhaust closed and valves number 4 both the intake and exhaust are opened, your engine is at TDC
That is how to use the valve to find the engine TDC.
Click here to read how TDC sensor work.
What Is Valve Position At TDC
At TDC, both valve intake and exhaust are closed. For more details explanations of this question, please read below.
There are two TDC, one at the end of the compression stroke and the second at the end of the exhaust stroke.
In this essay, I’ll concentrate on the TDC at the conclusion of the compression stroke.
Both valves close during the compression stroke, and the intake and exhaust valves remain closed after the compression stroke. They are shut while the injectors atomize.
In a four-cylinder engine, when the number one piston is at the fullest, and both the exhaust and intake valves on the number 4 cylinder are open, that is the TDC.
At that point, the engine is at the end of the compression stroke.
But in a six-cylinder, the position of the valve is quite different from the four-cylinder engine.
In 6 cylinder engine, the Valve position at the TDC is when the intake and exhaust valves are closed on cylinder number 1, while in the number 6 cylinder, both the intake and exhaust valves are open.
So the valve position during the TDC on the four-cylinder engine differs from that of the six-cylinder engine during the TDC.
The similarity is that: the number one cylinder on a four-cylinder engine
How Do You Set Valve In TDC
When the intake valve opens, the air is sucked in (intake stroke). The intake valve closes at BDC (bottom dead centre), and the compression stroke begins with the valves closed.
Depending on the engine type, it is injected into the compressed and thus heated air at approx. 20° before TDC (top dead centre) and ignition are initiated.
In petrol engines, ignition is initiated by a spark from the spark plug. Ignition occurs faster here than in a diesel engine, and the ignition point is later or closer to TDC.
TDC is now the most favourable condition for adjusting the valve clearance because both valves are reliably closed.
After the ignition has unfolded, the working cycle begins as soon as possible after TDC and ends at BDC.
If the ignition is set too early, the piston gets ignition pressure before TDC, leading to engine knocking and overheating. The ignition that is too late gambles away the full power development of the ignition. At BDC, the exhaust valve opens, and the exhaust stroke begins.
A correct valve clearance enables the valves to close reliably and the engine’s opening times to be set specifically.
Excessive valve clearance reduces engine performance. A valve clearance that is too small prevents the valve from closing reliably.
Hot combustion air then flows past the valves and damages the valve seats, leaving them permanently leaky.
The process of adjusting the valve is enumerated below:
1. Each cylinder is adjusted individually when the engine is cold.
2. With the valve cover removed, the crankshaft is turned in the normal direction of rotation (from the front to the right) using the starting crank or pushing with a high gear engaged.
The outlet valve must be observed. The inlet valve opens immediately as soon as it has opened and closed again. When the inlet valve has closed again, turn about half a turn further.
3. In this position of the crankshaft, both valves are reliably relieved for another half-turn.
The rocker arms must be easy to move by hand.
4. To check, a feeler gauge corresponding to the required dimension is pushed between the rocker arm and the valve stem. When set correctly, the valve can be pulled back and forth horizontally with a very slight clamping resistance. Then the lock nut is tightened again and checked again. This is necessary because the backlash changes when tightening the lock nut, requiring patience or routine.
5. The valve clearance for both valves is 0.25mm. (depending on the engine designs)
6. Only when both valves have been carefully adjusted should the second cylinder be adjusted as previously described.
For this purpose, the favourable point for setting after closing the outlet valve must first be determined again.
From the above explanation, you can now see how to use the valve to find TDC.