Variable Valve Timing

Press, Issue 32333, 26 June 1970, Page 16

Variable Valve Timing

(By A. R.. BATEY) If the reciprocating internal combustion engine is to survive against the challenge of the Wankel, and the gas turbine, all possible avenues of refinement and increased efficiency must be explored.

In recent years, overhead camshafts have been much more generally used, and current practice generally demands a main bearing between each cylinder. The next refinement could well be variable valve timing. Cars are full of compromises—the rear axle ratio that is right for the city is usually wrong for the highway; the steering ratio that is ideal for parking is too low-geared for general motoring; springing that is right for a full load is very hard when driving solo. As a result, we now find overdrives becoming common, also power steering, and very sophisticated suspension systems. The whole automotive picture shows more refinement and complication every year. One compromise which is left alone is that of valve timing. If you are plodding at 1500 r.p.m. in a traffic jam, or drifting along a motorway at 6000 r.p.m., the valves are fixed at the same timing, which is obviously not right for both speeds.

To take an obvious example, let us compare a single-cylinder engine of the type which powers a concrete mixer, with a racing motor. The single-cylinder motor spends nearly all its life at a constant speed of 1000 to 1200 revs. Its inlet valve opens about top dead centre of the piston stroke (T.D.C.) and closes at 8.D.C., the exhaust valve opening at B.D.C. and closing at T.D.C. A no-overlap no-nonsense arrangement. This motor is quite efficient when worked A< its normal speed range. At the other end of the scale is the racing motor with valve timing suited for 9000 to 12,000 r.p.m. This motor is likely to have the inlet valves opening about 55 degrees before T.D.C. and closing at 75 degrees

after 8.D.C., and the exhaust valves opening at 80 degrees before B.D.C. and closing 55 degrees after T.D.C. This timing will result in terrific power, but only over a small range of r.p.m. at the top of the scale, and with very heavy fuel consumption.

Between these two extremes of valve timing lies the normal saloon car engine with inlet valve opening about 10 degrees before T.D.C. and closing 50 degrees after 8.D.C., and its exhaust valves opening about 40 degrees before B.D.C, and closing about 10 degrees after T.D.C. As well as determining at what r.p.m. maximum power is developed, the valve timing also dictates at what r.p.m. maximum torque is developed. The use of a “hot” camshaft makes a vast difference to how and where a motor develops its power.

The intriguing possibilities of a motor which advanced its valve timing as its speed increased interested me some years ago, and I worked out a number of possible systems. I decided to put the operating device in the pinion or sprocket which drove the camshaft. In my schematic system, an independent oil pump supplies pressure to a needle valve operated by a governor. The pressure from this valve actuated two pistons which propelled an internally and externally splined helix sleeve through the boss of the pinion. This would advance the camshaft in relationship to its driving sprocket. As the r.p.m. increase, the governor controlled needle valve allows a greater pressure to force the operating pistons out further, thus advancing the valve timing. When the r.p.m. fall, the oil pressure is reduced, and return springs force back the helix sleeve and retard the valve timing. Alternatively, pressure is supplied in the reverse direction to produce the same result. Various other systems are capable of producing a similar result. The torque curve of a motor with variable valve timing could be expected to be higher at low and high r.p.m. than a motor with fixed valve timing. The curves should coincide at the figure where maximum torque occurs in the latter engine. The b.h.p. curve should be reshaped in a similar manner. The over-all effect would be to give low-speed pulling and torque like a steam engine, with top-end zip like a sports motor. The volumetric efficiency at all speeds should be improved

by the valve timing being “spot on” at all speeds. The thermal efficiency would be improved, and therefore pollution would be greatly reduced.

Some years ago I wrote to Jaguars, and received a reply from Sir William Lyons and the then Mr W.

Heynes, the man behind the famous XK engine. They could see no reason why the device would not work and produce the desired effects, but considered it doubtful if it could be manufactured at an economic price. But perhaps the situation has now changed.