Pressure gauge

Probing a car engine’s combustion chambers with lasers could lead the way to more efficient and environmentally-friendly cars.

An Oxford University team has developed a technique to accurately measure the temperature and pressure anywhere inside a combustion chamber and at any time in the engine cycle. Since conventional pressure sensors on the cylinder walls can only take readings at certain points in the cycle, the Oxford method could help engineers design better engines.

Prof Paul Ewart, of Oxford’s combustion physics and non-linear optics group, said the team had already developed a technique whereby a hologram effect is created inside a combustion chamber by combining two laser beams. This enabled them to study molecular behaviour during the engine cycle.

Now the lasers are used to create a comb-like fringe pattern which creates a ‘grating’ of slightly higher and lower density gas. Another laser is fired towards the grating and is scattered. The oscillations of the scattered laser light are measured by a photo-electric cell to determine the temperature inside the cylinder. Meanwhile the rate at which the oscillations decay indicates pressure.

The foremost application of this technique, said Ewart, would be designing a way of eliminating engine knock, caused when the fuel-air mixture spontaneously ignites, The sudden shockwave can damage the engine. Before the advent of clean fuels, this was prevented by lead additives.

‘There’s a premium on designing an engine that will run on the best fuel-air mixture for efficiency but that will not knock,’ said Ewart. Of particular interest is the drive to design a workable Homogeneous Charge Compression Ignition engine (HCCI), where a fuel-air mixture is automatically ignited by heat and pressure without a spark plug. Ewart said that while HCCIs could improve efficiency by around 10 per cent, the physics were not fully understood, a problem that his technique aims to address.

Other applications could include designing re-entry protection for space vehicles. The high temperatures involved in this can only be recreated in the laboratory by making shockwaves in special combustion chambers, in which the same laser process could be used. Ewart is seeking government funding, and hopes to integrate his system into a ‘cutaway’ engine to demonstrate its applications.