Mini driver

Researchers at CambridgeUniversity are developing a 1mm- scale internal combustion engine that could be used to power miniature spy planes or ‘smart dust’, it is claimed.

The research is a joint project between the university’s Combustion Research Centre and the Centre for Micro-Engineering and Nanotechnology at BirminghamUniversity.

The miniature combustion engine will have autoignition, with a compression engine that uses a pre-mixed mixture that ignites upon compression.

According to Prof Simone Hochgreb who is leading Cambridge’s side of the project, the technology would be similar in design to the much larger mini-engines that are used to power model aeroplanes. However, whereas mini-engines have a 1cm3 combustion chamber, the chamber for the new micro-engines would be no bigger than 1mm3 in volume.

Spy planes

‘One immediate application that comes to mind for the devices would be in tiny spy planes or smart dust,’ said Hochgreb.

The fuel used in the engines is likely to be a mixture of hydrogen and methanol, which is clean and easily ignited. As most liquid hydrocarbon fuels hold over 300 times more energy than nickel-cadmium batteries, a micro combustion engine would have the potential to release far more energy for its size.

Researchers on the project hope the technology could replace batteries in some devices, as the new engines would be able to last almost 20 times as long, and the fuel capsule could be easily replaced.

Unlike her colleagues at Birmingham, however, Hochgreb is not so convinced that the technology could one day replace batteries in devices such as mobile phones and PDAs. ‘Being able to power small mechanical devices would be quite an achievement but I believe it would take a long time to solve the noise issues for devices such as mobiles,’ she said.

The original idea for the device was proposed by researchers at MIT in the mid-1990s. But the research was taken on by BirminghamUniversity which patented the fabrication process for prototype micro-engines in 1999.

One of the major problems that researchers have to overcome is that the silicon-based components in micro-engines have difficulty in coping with the heat of combustion. To overcome this obstacle the project team proposes to make the micro-components out of ceramic materials that can withstand these high temperatures.

A second problem is to produce a sustained combustion reaction in small dimensions, as heat transfer affects the system. To deal with this issue the engine will have to be operated at extremely high speeds using the autoignition process and to produce around 11W of power the engine will have to run at a speed of 50,000rpm.

Heat transfer

Hochgreb said that handling heat transfer would be a further obstacle. ‘If you want to reduce the heat lost by an order of magnitude we find we have to increase the speed at which it runs by an order of magnitude too,’ she said.

To maintain a high level of efficiency inside the engine it must also be completely sealed, posing manufacturing problems as the 1mm surfaces will have to be completely sealed in three dimensions.

For Hochgreb one of the benefits of this miniaturisation process is the possibilities it could open up for further research. ‘When you are trying to break barriers as we are, it opens up the door for other people to find new ways of using the technology,’ she said.

However, she added that the engineering problems were such that it would be at least a couple of years before the micro-engines could be successfully commercialised.