Variable valves -- an open and shut case

By Barry Lake

IN heavy traffic you need a car that will happily plod along at low engine speeds; but when the road is clear, you want some sporty top-end performance. It's all in the valve timing.

Most people with even a modicum of knowledge of how a motor car works will know that the inlet valve opens at the beginning of the intake stroke and closes at the end of it to allow the fuel/air mixture to enter the engine as the piston is going down the cylinder.

They will also know that both valves are closed for the compression and power strokes and that the exhaust valve then opens for the burned gases to be expelled during the exhaust (upward) piston stroke.

It sound straight forward enough and in the earliest days of motoring, when engines ran at only a couple of hundred revolutions per minute, it was fairly simple.

But, when engine speeds rose progressively into thousands of revolutions per minute -- with today's engines commonly capable of 6000rpm, things became more complicated.

The incoming air and fuel need time to accelerate up to the speed required by an engine operating at high rpm.

This means that, in order to arrive within the combustion chamber at the desired time in a fast running engine, the air/fuel mixture needs to be on the move early.

The way this is done is to open the inlet valve before the piston gets to top-dead-center at the top of its exhaust stroke, which is just before beginning the intake stroke.

At the same time, the exhaust valve is kept open for a short while after top-dead-center. So, there is a considerable overlap between when the inlet valve opens and the exhaust valve closes.

High gas velocities generated by an engine running at high rpm creates considerable momentum in both the incoming and outgoing gases and this is harnessed by timing the valve opening and closing in such a way that each helps the other.

For example, there is a low pressure area left behind the departing exhaust gases, just before the piston finishes rising, that helps to draw in the incoming charge.

And the momentum of the exhaust gases leaving the cylinder allows it to continue exiting even after the piston passes top- dead-center.

Also, there is value in opening the exhaust valve early, before the piston reaches bottom-dead-center at the end of the power stroke, because there is no longer sufficient pressure to add to the engine's performance, but there is enough pressure to give the exhaust gases a head start in leaving the engine.

So, both inlet and exhaust valves open early and close late.

During the considerable overlap period at the end of the exhaust stroke and the beginning of the intake stroke, both valves are only partly open -- because the exhaust is in the act of closing and the inlet is in the process of opening.

The timing of the opening and closing of the valves is governed by the shape -- or profile -- of the cams on the camshaft, and this is usually different for the inlet and exhaust cams.

Changing the timing of the valve operation is a matter of grinding the cams in a way that the valve starts to lift earlier and closes later, by giving the cam a "fatter" profile.

But there is a catch. The efficiency of this process depends on high gas velocities generated by high engine speeds.

So, what happens when you're toddling along in a traffic jam? Your car that normally runs like a turbine at high engine speeds can be sluggish and inefficient at low speeds.

Slow-moving gases aren't sure whether they are coming or going when both valves are open.

For efficient running at low engine speeds, the valves need to open later and close earlier -- still before and after the end of the piston's cycle, but by a lesser amount.

In the past, cars have had cam grinds designed to suit the car's expected use. Sports cars had large overlaps and what was termed "wild" timing, while "shopping" cars had small overlap periods and what was known as "mild" timing.

But it was always a compromise. And that is the reason for the increasing interest in variable valve timing systems in the last couple of decades, something that is only of value in engines with separate inlet and exhaust camshafts.

Alfa Romeo, for example, had a hydraulic system back in the 1980s which rotated the camshafts back and forth through a few degrees to increase or decrease the timing and the overlap.

Porsche, BMW, Mercedes Benz and others now have more sophisticated methods of advancing or retarding the camshafts -- computer controlled, of course.

Where the original Alfa design, and others like it, had just one advanced and one retarded position and switched back and forth between the two, the latest versions are infinitely variable between maximum advance and maximum retard.

The disadvantage of these systems, though, is that any change at one end of the valve's cycle gives an equal -- and usually undesirable -- change at the other end of the cycle because the cam profile and, therefore, the duration of the valve opening can not be altered.

Honda takes a different route with its VTEC engines by providing two different-profile cam lobes for each valve.

At low engine speeds, the low-speed cam operates the valves.

As the engine speed and load increases the engine computer operates a hydraulic system that causes a pin to engage the separate rocker of the high speed cam and this then operates the valve.

Another computer-controlled system varies the length of the intake system to smooth the transition period from one cam profile to the other.

It is a classic example of engineering design that highlights Honda's commitment to technical excellence.