THE BENEFITS OF HIGHER ENGINE SPEED

In a nutshell, if we double an engine's speed, we get double the power, without having to increase its weight. In practice, though, other issues come into play.

In terms of fuel conversion efficiency, the ideal engine speed would seem to be the one that is 'matched' to the fuel 'burn' time such that the reaction concludes as close to the end of the 'driven' piston travel as possible. Under these conditions, the greatest amount of 'available' energy will be absorbed by the piston, with the least wasted to heat. However, the highest energy yield, per se, is 'available' under the highest compression (lowest entropy) conditions - when the piston is at TDC. Ergo, the best we can hope for will have to be a compromise between the low entropy and 'matched' speed. Having said that, the closer the engine speed is to the burn time, the more useful work we can extract from the engine. With spontaneous conversion, that is very high.

Clearly, the speed to weight ratio can be particularly significant for vehicles, on the road, on the sea and in the air.

The constant speed engine, with infinitely variable power-train, has for many years been a topic of speculation based around the fact that only one engine speed can give the best power output. The diesel engine, in particular, would be greatly enhanced by such a configuration, there being a single engine speed at which a diesel engine is extremely efficient with respect to fuel / realisable power conversion. Unfortunately, optimum efficency tends to be achieved at a rather low speed, all of the injection having to be post TDC, but completed before the required adiabatic pressure is lost to the expanding chamber.

In a conventional gasoline engine, the spark plug ignites the fuel / air mix, and the 'flame' initially moves through the mixture under high compression (low entropy) conditions, where the "free energy" yield is high. But as the piston descends, and the chamber expands, the conversion efficiency drops off, and the advance 'speed' of the flame front, relative to the mass of fuel, is considerably reduced. Again, with the gasoline engine, there will be an optimum engine speed for fuel conversion. However, with the 'bonfire' nature of this combustion system, and unlike with the diesel engine, there is no particularly good compromise between fuel conversion and power delivery at high engine speeds.

With any engine, to get the best conversion, the whole of the combustion process must be completed at the lowest entropy (highest compression) time. The VLB engine comes significantly closer to this than any other practical engine configuration. However, best conversion of fuel to energy, by no means ensures best conversion of the energy to realisable power output, the problem being that the low entropy 'period' is but a tiny fraction of the cycle time.

In the VLB chamber, spontaneous combustion takes good advantage of the low entropy period, and the excess of air goes some way to stretching the energy transfer period, although, just as with the other engines, there is an optimum engine speed. Or rather, there is a range of reasonably optimum speeds. But unlike the others, this range is effective for vehicle propulsion, and the losses incurred beyond this range are significantly smaller. Only at "tick-over" does the VLB engine lose its remarkable efficiency, which is why its operational mode reverts to a degree of "choke" controll, with a "not quite so very lean" air / fuel mixture.

Having said all that, it should be clear that the "best balance" between ensuring enough time in the adiabatic zone for combustion to be effected, and absorbing the most energy into the piston, is going to be effected at higher speeds. Exactly how high, depends to some extent on the fuel. But suffice it to say, 20,000 RPM was the VLB engine's bottom target (the limit set by the development control system) - and the prototype was entirely comfortable with that. My estimate is that 40,000 would be nearer the optimum, but to get there, gas transfer will have to be the next 'development' area - not that I anticipate there being any great difficulties.

Obviously, very high engine speeds require very slick control systems. For example, a camshaft operating at 40,000 RPM would have to be driven from both ends if it is not to yield to significant torque distortion. This is why the VLB engine does not have any camshafts, or steel springs, or any other such mechanical device which could generate timing errors. In practice, it would be virtually impossible to drive the VLB exhaust valves via a camshaft, since valve timing and duration have to be controlled almost to the millisecond. In reality, 40,000 RPM only 'sounds' fast. Expressing it as 667 RP Second, and with an ECU running at 12,000,000 cycles per second, tends to put this into perspective.

Finally, having mentioned the 'constantly variable drive train' above, it would be silly not to point out that the most fertile development area for such, is an hydrautic system. The VLB engine is, by far, the best IC engine configuration for pumping fluid, and the raised engine speed is best harnessed by such a load.

John Allen