CAR CLINIC | Why valve timing is an intricate process you cannot ignore

Valve timing is the most important factor determining an engine's character.

A good illustration of this is provided by the following curious historical fact. In 1947 the Standard Motor Company of Britain introduced an inline four-cylinder petrol engine of 2,088cc.

This same engine was used in the Ferguson TE20 tractor, the Standard Vanguard sedan and the Triumph TR-X sports car. The essential thing that varied from one application to the other was the valve timing. By merely changing the valve timing the engine could be made into a slogger for the tractor, an all-rounder for the sedan or a sprinter for the sports car. That is what is meant by changing an engine's “character”.

To understand how this is achieved, we have to look at the meaning of the term valve timing. Recall that the valves are opened by the lobes on a rotating camshaft, driven from the crankshaft. The lobes either press directly on the valves to open them, or they do so through the intermediary of finger-type cam followers, or by means of pushrods and rockers. To simplify matters, we consider an engine with one inlet and one exhaust valve per cylinder. On the rotating camshaft there will be a particular lobe assigned to the inlet valve of No 1 cylinder and another lobe assigned to the exhaust valve of No  1 cylinder. This will be the case for each cylinder.  

Let us focus on one cylinder only. The purpose of valves is to enable the engine to breathe properly. During the intake stroke it must inhale enough air (on a diesel engine) or air/fuel mixture (on a petrol engine) to satisfy the power demand of the moment. This intake charge will then be compressed, and at a certain point combustion will be initiated. The heat generated will cause expansion of the charge, thus delivering the power stroke. Thereafter the spent charge has to be expelled during the exhaust stroke.  

The amateur engineer may regard this as an easy objective to achieve: Position the lobes on the camshaft so that at the beginning of the intake stroke the inlet valve is opened, and it remains open until the valve spring closes it (as the rotating lobe releases its pressure) right at the end of the intake stroke. The exhaust valve remains closed for this half rotation of the crankshaft. For the next full rotation, as the piston moves first upward on the compression stroke and then downward on the power stroke, both valves remain shut.

Finally, as the crankshaft starts the last half revolution of its two rotation cycle, the exhaust valve's lobe on the camshaft is in precisely the right position to open the exhaust valve, which is then kept open (while the inlet valve is still closed) until the piston reaches its topmost position, at which point the exhaust valve is allowed to close abruptly. The cycle then repeats itself.  

Unfortunately, mechanical engineering is not really the realm of amateurs. This engine will struggle to fill its cylinders adequately. In engineering jargon, it will have “poor volumetric efficiency”. Here's why. At 2,500 rpm the half revolution of the crankshaft during which the inlet valve is open lasts only 12 milliseconds. At 5,000 rpm we are looking at six milliseconds. The blink of an eye usually takes at least 100 milliseconds.

So before your eyelids have even started to move, the valve has opened and closed. During that time the intake charge has to be sucked from the inlet manifold into the combustion chamber, and then cut off again. There is simply not sufficient time to fill the cylinder properly, and the problem becomes more acute as the revs rise. 

To overcome this problem the lobes on the camshaft are located and shaped so the inlet valve is opened before the piston is at TDC (top dead centre) at the end of the exhaust stroke and the exhaust valve is kept open until after TDC. This tactic allows the last of the escaping exhaust gas to draw in the inlet charge faster. This means both valves are open simultaneously for a certain number of degrees of crankshaft rotation. This number of degrees is called valve overlap.

Similarly, at the end of the intake stroke, the inlet valve will remain open until after BDC (bottom dead centre) to extend the time available for the inrushing charge to fill the cylinder. Likewise the exhaust valve will open before the end of the power stroke to allow the still-expanding exhaust gas to start escaping.

In all cases the number of degrees of crankshaft rotation that should ideally be allowed for these extensions will depend on the revs. A slow-revving engine will need comparatively small numbers of degrees of extension — think of the tractor engine. A high-revving engine needs more degrees of extension to get proper filling of the cylinders — think of the sports car engine, which will perform well at high revs but idle poorly. The family sedan's “valve timing” must clearly be a compromise.

The ideal is obviously to make the valve timing adjustable by automatic control so at low revs it is tractor-like, and at high revs sportscar-like. This is where “Variable Valve Timing” (VVT) enters the picture. How this is achieved, continuously, without the driver even being aware of it, is a story for a different day.