Wouldn’t it be the biggest step forward in over a century of automotive engineering if a gasoline internal combustion engine (ICE) could finally be conceived that undercuts the diesel engine in terms of fuel consumption? Of course it would!
The diesel engine is expensive, emits high levels of particulate matter and NOx, and achieves a rather low power density. But it does consume less carburant while providing an identical amount of useful work at the wheels.
It’s well known that the diesel engine’s superior efficiency is due to two main reasons: at full load, the higher expansion ratio of the diesel engine and at low load, the absence of pumping work. The current mechanical concept of both engine types, diesel and petrol, are characterised by equal compression and expansion strokes. This is not really an issue for the diesel engine, but for the petrol engine it’s quite simply the key issue, because the knocking phenomenon forces the compression ratio, and thus also the expansion ratio, to not exceed 10 – 11:1. And if the engine designer intends to increase the specific power of the engine by turbocharging it, for example, then this ratio limit is even more demanding.
It’s well known that the diesel engine’s superior efficiency is due to two main reasons: at full load, the higher expansion ratio of the diesel engine and at low load, the absence of pumping work.
To advance the petrol engine, we need firstly a higher expansion ratio to improve the thermal efficiency, and secondly turbocharging to avoid the pumping work at lower loads. From all of this it can be derived that the real Gordian knot of the petrol engine is the aforementioned constraint demanding equality of compression and expansion strokes. Once this knot is untied, the compression ratio can be designed in order to avoid knocking, even at high intake air pressures, and the expansion ratio can be designed in order to maximise the thermal efficiency of such an engine.
This can be done with a five-stroke engine. My five-stroke engine design encompasses two small high pressure combustion cylinders and one large expansion cylinder. Both combustion cylinders accomplish the common four strokes, but instead of exhausting the burned gases to the surrounding area, they are forwarded alternatively to the bigger expansion cylinder, where they expand a second time. If we multiply the expansion ratio provided by the high pressure (HP) cylinders with the one provided by the low pressure (LP) expander, we obtain the global expansion ratio of the engine, which is obviously different from the compression ratio of the HP cylinders.
The real Gordian knot of the petrol engine is the constraint demanding equality of compression and expansion strokes.
The very first prototype built by Ilmor Ltd. in 2007 on behalf of Mercedes-Benz HPE, a subsidiary of Mercedes-Benz, with a global piston displacement of only 1478cc, achieved impressive dyno results only three months after fire-up: 130hp @ 7000rpm, 166Nm @ 5000rpm and a BSFC of only 226g/kWh.
Considering that this very first prototype exhibited rather poor combustion characteristics, these results are even more impressive. Indeed, further GT-Power simulations predict a BSFC of about 200 g/kWh. Conclusion: it’s time for that big step forward – a five-stroke gasoline ICE can undercut the diesel engine in terms of fuel consumption.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Gerhard Schmitz: www.5-stroke-engine.com
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