Free wheeling

UK researchers are attempting to develop a free-piston engine capable of running on a variety of fuels, which could ultimately become a competitor to the fuel cell.

Researchers at Sheffield and Loughborough universities will develop the crank-less engine, which could significantly improve fuel efficiency and reduce emissions. The design is for Lotus Engineering, in an EPSRC-funded project due to start this autumn. Lotus hopes ultimately to use it within a new hybrid vehicle, according to Dr Paul Stewart, lecturer in electronic and electrical engineering at Sheffield.

‘It’s going to be a new power plant and generator for a hybrid electric vehicle for Lotus. It will be using a new concept in piston trajectory and control. It should make it possible not only to increase the efficiency of the engine, but also to run it on a variety of fuels, so hopefully it will be able to run on hydrogen, standard petrol and everything in between,’ he said.

The single-cylinder, four-stroke engine has no crankshaft attached to the piston. Instead, the piston is combined with the rotating armature of an electrical generator, allowing it to convert combustion energy directly into electricity to power the hybrid car’s motor, as well as its own operation.

Production costs should be kept low thanks to the reduction in moving parts, which should also cut efficiency losses caused by friction. But more significantly, by freeing the piston from all external constraints, its movement can be precisely controlled throughout the combustion process. This allows elements such as the compression ratio to be varied, and optimised for use with different fuels including biofuels and even hydrogen.

Controlling the compression ratio should also make adoption of highly efficient combustion concepts such as controlled auto ignition (CAI) easier, said Dr Rui Chen, lecturer in automotive engineering at Loughborough.

In CAI the fuel-air mix is ignited automatically through heat and pressure rather than by a spark, improving fuel efficiency and slashing emissions of nitrogen oxide by up to 99 per cent, as well as reducing emissions of carbon monoxide and particulates. But a major barrier to its adoption in conventional engines is the difficulty of controlling the timing of ignition and the rate at which the energy is released.

‘Many car companies have been looking at the CAI concept, and we have been investigating it for a long time, but the problem with it is control. One way to control the combustion is to control the compression ratio, but with a conventional engine it is close to impossible to do that,’ said Chen.

‘If we free the piston completely we can achieve a compression ratio of whatever we are looking for. So we could create a fully variable compression ratio engine, and that type of engine could easily adopt the CAI concept,’ he added.

Although the free-piston research is at a very early stage it builds on work Loughborough (and many car makers) have been carrying out into CAI over the past few years. Chen believes they are the only researchers in the world attempting to combine free-piston technology with CAI combustion.

The research will also build on work the Sheffield team is carrying out into a two-stroke free-piston design, in an EC Framework VI project involving Volvo. In this design the piston moves between two combustion chambers at each end of the cylinder. But the engine compromises fuel efficiency and emissions.