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Valve Timing and Engine Efficiency

In connection with the resonance phenomena, it is necessary to consider the valve opening times in relation to the rotation of the crankshaft the so-called valve timing. Fig.1 is the valve-timing diagram for a Porsche eight-cylinder Grand Prix racing engine. The long valve-opening periods are necessary in a high-speed engine to ensure efficient intake and exhaust of the cylinders.

Also, at high engine speeds the acceleration and deceleration of the valve movements must not become inadmissibly high. Opening and closing must therefore be performed at a relatively slow rate i.e., the valves must have a large travel, or lift.

The earliest variable valve timing systems came into existence in the nineteenth century on steam engines. Stephenson valve gear, as used on early steam locomotives supported variable cutoff, that is, changes to the time at which the admission of steam to the cylinders is cut off during the power stroke. Early approaches to variable cutoff coupled variations in admission cutoff with variations in exhaust cutoff. Admission and exhaust cutoff were decoupled with the development of the Corliss valve. These were widely used in constant speed variable load stationary engines, with admission cutoff, and therefore torque, mechanically controlled by a centrifugal governor.

As poppet valves came into use, simplified valve gear using a camshaft came into use. With such engines, variable cutoff could be achieved with variable profile cams that were shifted along the camshaft by the governor. The earliest Variable valve timing systems on internal combustion engines were on the Lycoming R-7755 hyper engine, which had cam profiles that were selectable by the pilot. This allowed the pilot to choose full take off and pursuit power or economical cruising speed, depending on what was needed.

The exhaust valve opens at 81 degrees before bottom dead center, when the power stroke is still only little more than half completed and the combustion gases have not yet fully expanded. At low engine speeds this early opening of the exhaust valve would cause a lowering of the mean effective pressure and of the torque – an acknowledged drawback of the racing engine.

The exhaust valve remains open until the crank has rotated to 51 degrees beyond top dead center. Although the piston on its way to bottom dead center has started the intake stroke, exhaust gas is nevertheless discharged from the cylinder in consequence of resonance phenomena in the exhaust duct.

The inlet valve begins to open at 81 degrees before top dead center, while the piston is forcing the exhaust gas out of the cylinder. This likewise reduces the volumetric efficiency and the torque at low speeds; but at high speeds efficient charging is achieved on account of the oscillation and resonance phenomena established in the inlet system.

Within the range shown hatched in Fig.1 the inlet valve and the exhaust valve are open at the same time. Because of the suction in the exhaust system, this over lapping of the valve-opening periods promotes the development of a low pressure in the cylinder and thus assists the intake of mixture and improves volumetric efficiency. The inlet valve closes at 71 degrees after bottom dead center, during the compression stroke. Thus the charging action due to the inertia of the flowing gas is utilized.

This kind of valve timing, while appropriate to a racing engine, is not suitable for an ordinary car engine because of the low torque at low and medium speeds, so that the engine would be deficient in flexibility of performance. The timing approximately suited for ordinary engines is also indicated in Fig.1 (points 1 to 4): the valve opening periods now to overlap much less (points 3 and 1), the exhaust valve does not open so far in advance of bottom dead center (2), and the inlet valve does not close so late (4). Fig.2 is a diagram showing the valve lift plotted against the angular rotation of the crankshaft.

It is seen that for equal valve timing it is nevertheless possible to have different amounts of lift and different cross-sectional flow areas through the opened valves (black and red curves respectively). The intake and exhaust can be improved by an increase in the valve lift. The valve movements are controlled by cams on the camshaft (Figs 3 and 4), which rotates at half the speed of the crankshaft and is driven from the latter by a chain drive or gearing.