Under pressure

Automotive technology specialist Prodrive has thrown its weight behind a variable compression ratio engine concept that could improve fuel economy by 30-40 per cent.

Prodrive exhibited details of its evaluation and simulation work on the Gomecsys GoEngine at this week’s Engine Expo/Vehicle Dynamics Expo 2004 in Stuttgart.

Finding a practical way of varying the compression ratio – the extent to which the fuel and air mixture is compressed in the cylinder of an engine before combustion – is seen as the key to effective engine ‘downsizing’ or combining the power of a large engine with the economy of a small one.

Prodrive Automotive Technology’s chief engine technology specialist David Hemming said: ‘A lot of VCR concepts have been put forward which are quite impractical mechanically. But the more we worked on the Gomecsys engine the more we thought it was one of the most practical VCR solutions we’d seen.’

The GoEngine is the brainchild of Dutch automotive engineer Bert de Gooijer. In his system each cylinder’s connecting rod is linked to the crankshaft via an eccentric pivot. The pivot is geared using an epicyclic gearset (one gear rotating inside another) so that it rotates at half the speed of the crankshaft.

There are two benefits. The eccentric pivot results in over-expansion – that is, on the power and exhaust strokes of the four-stroke engine cycle, the piston travel is longer than on the induction and compression strokes. The longer power stroke allows more energy to be extracted from the burnt fuel, instead of being dissipated down the exhaust pipe. On its own 25 per cent over-expansion improves fuel economy by six per cent.

The second benefit is that rotating the outer ring gear varies the position of the piston at the top of its stroke, and hence the compression ratio.

This comes into its own in highly boosted (turbocharged or supercharged) engines. One of the strategies being pursued by automotive engineers to improve fuel consumption is downsizing. This effectively means replacing a larger engine with a smaller one, using boosting to keep the power output the same.

Engineers would like to be able to downsize engines by 50 per cent – or replace a three-litre engine with a 1500cc one. The problem is that for a boosted engine the compression ratio is limited, to prevent problems such as detonation of the fuel or ‘pinking’, to about 8:1 or 9:1 compared to a typical naturally aspirated engine’s 11:1. But at low throttle openings the turbo is inactive, so the engine behaves like a small and not very efficient naturally aspirated power plant.

‘With VCR you can combine the wide-open throttle performance of a turbo and the part-load efficiency of a properly optimised naturally aspirated engine,’ said Hemming.

He added that work so far suggested that the system worked well with a relatively small amount of eccentricity giving 25 per cent over-expansion, and a compression ratio variable between 8:1 and 17:1.

Simulations suggest that replacing a 120kW two-litre engine with a one-litre supercharged GoEngine of the same power would improve economy by 30 per cent.

Multi-fuel engines could also be made more efficient. Current dual-fuel conversions are unable to take advantage of the fact that a higher compression ratio could be used when running on LPG rather than on petrol.

Prodrive was originally brought in by Gomecsys to carry out simulation work on the gear system because the car makers to whom de Gooijer presented the idea did not believe it would work. ‘We found the gears are quite lightly loaded,’ said Hemming. ‘We improved the design of the gearset to reduce noise, but we couldn’t suggest any improvements to the concept.’

The relationship developed into a partnership in which Prodrive will help to market the engine. The companies are currently seeking funding to produce a production-representative prototype. ‘We’re in talks with several car makers and a couple of tier-one suppliers,’ said Hemming.

The VCR engine is likely to come into favour as manufacturers seek to meet the 2012 proposed fleet average fuel economy target of 120g of carbon dioxide per kilometre.

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