Supercharged engines exhibit fuel consumption comparable to a normally aspirated engine when they are operated with small to moderate engine speeds, but they develop power and torque outputs comparable to larger capacity engines when operating at higher speed. This is often a desirable feature for passenger vehicles, where good fuel economy is desirable in city and suburban traffic, and higher performance is required in freeway conditions.

There is a major drawback associated with turbocharging. The inlet pressure cannot rise until the engine develops an increase in exhaust pressure, which in turn accelerates the turbine. There exists a finite time between the opening of the throttle and the response of the engine. This delay is often termed turbo lag. Alternative supercharging schemes which employ direct mechanical compression are less prone to such effects. However, the major drawback with these schemes is that they are generally much more complex and costly in relation to turbocharging.

The invention described in this specification is a technique which provides an initial or pre-boost to the inlet pressure when the engine's throttle is opened. This preboost is supplied via a reservoir of compressed air, and a controlled release valve. The preboost provides an immediate response in engine output for the interval of time where the turbocharger increases its speed. The prebooster then enters a recharging cycle, in readiness for subsequent throttle changes.

Figure 1 (a) is a diagrammatic representation of a conventional turbocharged internal combustion engine. The air intake (i) is made via a turbine (ii) which is driven by exhaust flow from the engine (iv) as the engine vents to the atmosphere (v). As the exhaust pressure increases, the turbine accelerates and in turn it increases the inlet manifold pressure (iii), which provides a higher-than-atmospheric charge to the cylinders for their inlet cycles.

Figure 1 (b) shows the same engine with a storage prebooster. This comprises a storage cylinder (v), an inlet valve (iv) and an outlet valve (vi). The storage cylinder is charged from the inlet manifold whenever the inlet pressure is higher than that in the cylinder itself. Changes in the engine's throttle setting result in the outlet valve opening appropriately. For example, the outlet valve's position may be proportional to the rate of change in the position of the throttle, and as such may be operated via a simple PD controller.

The additional resulting flow of gas accelerates the turbocharger faster than would be the case if it relied on exhaust flow alone. This causes the inlet pressure to rise more rapidly than would be the case otherwise, i. e., the supercharging effects occur sooner than would be the case for the engine in Figure 1 (a).

The delivery of the preboost in Figure 1 (a) may also be achieved by releasing pressurised fluid into an ancillary drive chamber for the turbocharger. For example, the storage reservoir and the turbocharger's ancillary drive chamber may be part of a closed circuit comprising pressurised oil. In a passenger vehicle which also comprises power assisted steering, the existing oil pressurising system may be used to drive the turbocharger's preboost. Alternatively, the oil reservoir may be pressurised by the introduction of exhaust gas or compressed air from the inlet as before.

A variation for the scheme depicted in Figure 1 (b) is to charge the storage cylinder with exhaust gas from the outlet manifold, rather than with compressed air from the inlet manifold. Yet another variation is for the prebooster to drive the turbocharger separately, thus avoiding any interference with exhaust gas flow.

Figure 1 (c) shows another variation, where the inlet valve may be operated at the same time as the outlet valve. This provides a direct preboost to the engine, rather than solely via the turbocharger. This may be done in isolation, or in combination with the preboost flow to the turbocharger when air is used to charge the storage cylinder.

The above techniques may also be applied to other engine types, e. g., gas turbines, jet engines, although the utility in such applications may be limited, owing to the manner in which such engines are normally used. For example, turbines are generally operated at fairly constant throttle settings, as opposed to constantly varying settings as is the case for road vehicles.