Taking the spark out of ignition

Compression technology could mean more cost-effective hydrogen engines.

UK researchers are to investigate an auto-ignition combustion engine running on hydrogen, which could challenge the fuel cell, but at a fraction of the cost.

Hydrogen will be combined with homogenous charge compression ignition (HCCI) technology, in which the fuel-air mix is ignited automatically through heat and pressure rather than by a spark. This could create an emission-free, fuel-efficient engine, according to Dr Pavlos Aleiferis, lecturer in mechanical engineering at University College London.

In an EPSRC-funded project due to begin next week, Aleiferis will study the processes involved in HCCI combustion of hydrogen, and investigate whether these can easily be incorporated into existing engine designs to significantly reduce the cost of introducing emission-free cars. The researchers will be the first to attempt to combine hydrogen with HCCI combustion, said Aleiferis. ‘Nobody has tried it before. We don’t even know if it will work, but if it does it could be very significant,’ he said.

Aleiferis will convert an optical single-cylinder direct-injection petrol engine to run initially in petrol HCCI mode and then eventually in hydrogen HCCI mode, using existing low-cost technologies. While working towards full hydrogen mode he will also investigate using various mixtures of hydrogen and petrol, which could offer the best compromise between reducing emissions and maintaining car performance in the short term.

As The Engineer has previously reported, in an HCCI (also known as controlled auto-ignition) engine the air-fuel mixture is pre-heated. The mixture burns more slowly than with spark ignition, and burning begins at the same time at various ‘hot spots’ in the cylinder. This is claimed to eliminate the high temperature combustion zones found in conventional engines and slash nitrogen oxide (NOx) emissions. HCCI combustion is also more fuel efficient, as it minimises throttle use at part-load, reducing energy wastage through pumping losses, where more air is drawn into the chamber than needed.

As part of the three-year project air-preheating methods will be investigated with a variety of different air-fuel ratios. A number of exhaust gas recirculation (EGR) techniques, which are used to reduce the combustion temperature, will also be studied.

To control the speed of auto-ignition in an HCCI engine the air-fuel ratios must be very lean. So the project will include an investigation of just how lean a hydrogen HCCI engine would need to be to prevent knocking and back-fire.

Aleiferis is in discussions with an engine manufacturer to secure additional support for the project.