Just like sparking over

Ford and Jaguar are researching an engine that combines auto-ignition with direct-injection technology to improve the fuel efficiency of cars and dramatically reduce emissions.

The car makers are taking part in an EPSRC-funded project, led by Brunel University, to develop a direct-injection petrol engine capable of operating in Controlled Auto-Ignition (CAI) combustion in the part-load conditions that constitute around 80 per cent of driving time for passenger cars.

The combustion system, in which the fuel-air mix is ignited automatically through heat and pressure rather than by a spark, is an extremely promising technology and could lead to a new generation of highly efficient petrol engines, according to Prof Hua Zhao, director of automotive and motorsport engineering at Brunel.

However, one of the major barriers to the adoption of CAI, also known as Homogenous Charge Compression Ignition (HCCI), is the difficulty of controlling the timing of ignition and the rate at which the energy is released. So the team is combining the system with direct- injection technology to gain control of the timing and number of fuel injections, said Zhao.

‘With port fuel injection, the problem with CAI is how to control it. We believe the flexibility of direct-injection technology will help to expand the operational range of the new combustion process.’

The CAI system operates by retaining some of the burnt gas from the previous cycle within the cylinder and using it to heat up the fresh fuel-air mixture. The mixture burns more slowly than in spark ignition combustion and burning begins at the same time at various ‘hot-spots’ throughout the cylinder.

This eliminates the high-temperature combustion zones found in conventional engines, slashing nitrogen oxide emission by 95-99 per cent. The system also reduces carbon monoxide emissions and eliminates particulate emissions.

‘Controlled auto-ignition gives you much better fuel efficiency than spark ignition combustion because you can minimise the use of the throttle at part-load. With spark ignition you use the throttle to control the power output and in CAI you don’t need to do that,’ said Zhao.

Minimising throttle use would reduce pumping losses at slower engine speeds at which energy is wasted when more air is drawn in than is necessary, meaning the engine is simply pumping gases in and out.When operating at full load, the engine would switch to conventional spark ignition. This is because at full load the combustion pressure rise under CAI would be extremely fast, potentially damaging the engine.

The engine would switch between the two modes using hydraulics, although with the move towards camless engines, electro-hydraulic actuators such as those used on Lotus’ active valve train system (AVT) will ultimately be used to control the valve timing and duration.

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