Mixed message

A new generation of internal combustion engines that can use different mixes of fuels according to driving conditions is being developed by researchers at Birmingham University.

Supported by Jaguar, Land Rover, Johnson Matthey and Shell, the project is working to improve homogenous charge compression ignition (HCCI) engines by focusing on in-cylinder mixture preparation.

Birmingham research group Future Power Systems is also researching HCCI in its recently opened, purpose-built laboratory.

Dr Hongming Xu, reader in Automotive Engineering at Birmingham, believes all internal combustion engines should be able to operate in multi-fuel and multi- combustion modes. Currently, only petrol engines in countries such as Brazil have demonstrated the ‘Flex-fuel’ engine technology and Xu believes diesel engines will need a similar system called ‘Flex-diesel’ — which the researchers have dubbed ‘dieseline’.

Xu — who is working with project leader Prof Miroslaw Wyszynski and Dr Athanosios Tsolakis — compares the development of the multi-fuel technology to a colour printer, combining ‘multiple-colours’ (fuel properties) to produce just the right mix for the engine’s needs.

The future engine/vehicle management system will use technology powered by a central processor unit (CPU) to provide the best control strategies, allowing the fuel mixture preparation to optimise the way the car runs.

‘The differences between modern diesel and petrol engine configurations are now becoming smaller and smaller, and will be even smaller in the near future,’ said Xu.

‘They will all use moderately high compression ratios and complex direct injection strategies. HCCI combustion is likely to lead to the merging of petrol and diesel engine technologies. This will help to handle the challenges faced by the development of the fuels, engine control and after-treatment, and could mean that by using improved three-way catalysts most stringent emissions legislation could be met.’

HCCI relies on a chemical reaction, rather than conventional spark ignition and flame propagation with turbulent in-cylinder flow, to control and complete the combustion process.

It is claimed to be able to provide improved fuel economy by cutting down pumping loss and the loss of engine power due to a reduction in pressure. It does this using a negative valve overlap which increases airflow to the pistons by overlapping the time when the intake and exhaust valves are closed, creating a high-vacuum.

This extra airflow helps mix the air and fuel and provides better atomisation and vaporisation, which in turn gets more from the fuel.

The engine can operate in this way with a wide open throttle and it allows a high exhaust gas recirculation (EGR) rate and air-fuel ratios to dilute the mixture.

The absence of local high temperature regions associated with flame front in the ignition process, and the presence of a very high level of EGR also make it possible to achieve much lower NOx emissions.

Petrol, diesel and alternative fuels can all potentially run in these engines but none of them alone provide a satisfactory operating range over speed and engine brake torque.

By combining the fuels into dieseline, the research sought to expand this range, while at the same time optimising performance, by using the complementary properties of petrol and diesel, such as the high volatility of petrol with the higher ignitability of diesel. The use of dieseline is claimed to reduce HCCI dependence on exhaust gas recirculation trapping and cut the requirement of intake heating.

Xu believes this fuel merging is the future of engine technology. There is also evidence that petrol or dieseline can be burnt cleanly in HCCI diesel engines, with reduced fuel injection. This is a significant part of the cost of the diesel engine, and some benefits are also observed in running dieseline in unmodified petrol engines.

‘Future downsized engines should all be boosted as standard,’ added Xu. ‘They will possibly operate with petrol-type fuel in spark ignition mode at high speeds and full loads to provide full power, and with diesel-type fuels for lower and medium speeds for high torque.

‘Variable mixing ratios for dieseline, including the HCCI mode, should be used for high fuel economy and clean emissions. This will lead to a new concept in combustion engines — only one type, not limited by the constraints of current petrol and diesel versions.’

Future Power Systems has completed two UK Foresight Vehicle LINK projects — CHARGE and CHASE (2002-2007) — into HCCI-related projects, and work is about to start on advanced biodiesel engine technology at Birmingham University’s School of Engineering.