CAR CLINIC | A deep dive into the intricacies of valve timing

In last month's article we saw t it would be highly desirable to make an engine's valve timing adjustable by automatic control to achieve strong torque at both low and high revs. This is easier said than done. How do you change valve timing without dropping a new camshaft into the cylinder head?

It is interesting that the steam engines of yesteryear used variable valve timing. The driver could manually change the length of time for which the valve admitting steam into the cylinder remained open. In steam parlance this was called “steam cut-off”. To pull away, they kept the valve open for almost the entire power stroke to get as much high-pressure steam into the cylinder as possible for maximum torque. For cruising they would cut off the steam much sooner, relying on expansion of the hot steam to do most of the work. They would still get enough torque to maintain cruising speed, but they would use less steam, thus reducing coal consumption.

In the automotive field of the post-war era, the Italians were the first to put into practice the pipe dream of automatically changing the valve timing. In the late 1960s Fiat patented a system of variable valve timing (and variable lift) for conventional automotive internal combustion engines. Alfa Romeo was the first manufacturer to use VVT in production cars when they fitted a mechanical VVT system to their 1980 fuel-injected Spider 2000. From 1983 on their Spider models used electronic VVT.

Honda was another pioneer of VVT. They released their VTEC (Variable Valve Timing and Lift Electronic Control) system in 1989, later to be superseded by i-VTEC (i for intelligent).

In the ensuing 33 years virtually every car maker in the world has come up with its own version of VVT. Different manufacturers use different acronyms to denote their systems. The mechanisms employed in the systems range from complicated to nightmarishly intricate. We can only discuss one of the more easily explained ones here.  

The VVT systems all operate on overhead cam engines. Bear in mind that in such an engine there is a sprocket assembly for driving the camshaft(s). The drive comes from the crankshaft by means of a chain or toothed belt. If a toothed belt is used, a notched pulley replaces the sprocket, but for the sake of simplicity we assume we are dealing with a sprocket assembly. On twin overhead cam engines there will be two sprocket assemblies, one for the intake camshaft and one for the exhaust camshaft.

The easiest way to vary valve timing is to rotate the camshaft relative to the spinning sprocket. If you rotate the camshaft so it “leads” its drive sprocket, valve timing will be advanced. If a sleeve with helical teeth on the outside can be made to move into and out of the camshaft extension where its teeth mesh with similar teeth on the inside of the extension, it will cause the camshaft to rotate clockwise or counter-clockwise relative to the sprocket. The sleeve is splined to the shaft of the sprocket with straight splines, so it is forced to spin with the sprocket, but it can slide lengthwise along the splines.

All you now need is a way to move the sleeve in or out as required to advance or retard the valve timing. This can be done by hydraulic pressure applied to the end cap of the sleeve, as directed by the engine computer.

I emphasise this is a greatly simplified picture of one of the more basic designs. Obviously it will be better if this can be done for both intake and exhaust camshafts independently. The cost will be greater computing capacity. It will be even better if you can vary the valve lift as well as the timing because that will allow you to open the inlet valves wider when the need arises to get more air into the cylinders. This is i done on several designs, but it clearly requires even more computing power. 

Most designs use hydraulic pressure (with engine oil as the hydraulic fluid) to operate components, and increasingly this is done in conjunction with electronic control units and sensors. Conversations with people in the automotive maintenance and repair industry have revealed the earlier more mechanically orientated systems  were bulletproof, with BMW's VANOS system receiving high praise.

There is one crucial proviso, though, which applies to all VVT designs, and that is the vitally important need for the correct engine oil and scrupulously clean oil. Use only oil recommended by the manufacturer, expensive though it might be. Verify that the service centre puts nothing else in the engine. Bear in mind that severe duty (which includes predominantly short trips and stop/start operation in congested traffic) calls for the change interval to be halved.

The consensus of opinion is that the latest heavily computerised systems work marvellously well until they fail, whereupon financial ruin looms large. This is especially important for buyers of used cars, where you never quite know how the servicing was done under the first owner, and you often don't have the protection of a factory warranty.