Holding fire

Lotus Engineering is to use its Active Valve Train technology (AVT) to develop an auto-ignition engine that it believes could allow car makers to almost eliminate emissions of nitrogen oxides.

In auto-ignition the presence of a certain amount of hot exhaust gas remaining in the cylinder from the previous cycle allows the fuel/air mixture in the cylinder to ignite without a spark. The mixture burns more slowly than under spark ignition and burning starts simultaneously at several points in the combustion chamber. This avoids the pressure and temperature peaks of normal spark ignition which encourage NOx formation. NOx is typically reduced by over 90 per cent, said David Blundell of Lotus’s powertrain research group.

Auto-ignition was discovered accidentally in two-stroke engines where, at anything other than full load, exhaust gases are always present during combustion. A conventional four-stroke engine will not run with more than 20 per cent exhaust gas recirculation. Lotus has been able to run engines with around 50 per cent recirculation by ‘controlled auto-ignition’, in which the timing of the spark is retarded, so that it acts as one of the simultaneous ignition sites.

Auto-ignition is also promoted by varying the valve timing so both inlet and exhaust valves are closed for significantly longer, to increase the pressure in the cylinder. Blundell says the high degree of control over valve opening and closing provided by its AVT technology is needed to make this work.

Lotus Engineering is on the verge of signing up a tier 1 supplier and a major car maker to put the electro-hydraulic AVT system into production. The full potential of the system, which allows valve lift, as well as opening and closing, to be varied at will, is only just becoming apparent.

Conventionally the extent of valve timing and lift is determined by the need to get the required flow of air in and exhaust gases out at the top of the engine’s rev range. At lower loads and engine speeds there is capacity to draw in far more air than is necessary. This leads to pumping losses, in which the engine is simply pumping gases in and out, which can amount to 50 per cent of the engine output at low loads. ‘Because AVT allows control of valve lift as well as opening you can fully optimise pumping at all speeds and loads,’ said Blundell.

It is expected to be at least five years before AVT is available on the market, though Lotus plans to show a demonstrator vehicle using a production-ready system by the latter half of next year.

Blundell said that the new system opens up the prospect of improving fuel economy by 25 to 50 per cent by running without a throttle and changing the exhaust valve openings. ‘We’re still scratching the surface of what’s possible with everything infinitely variable,’ he said. ‘Once you have full control of the gas exchange process you can do virtually anything.’

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