In a whirl of its own

A cost effective way to improve compressor performance promises to enhance the power or driveability of cars with turbocharged and supercharged engines.

A cost effective way to improve compressor performance promises to enhance the power or driveability of cars with turbocharged and supercharged engines.

Integral Powertrain’s pre-whirl generator is expected to provide a way of extending the operating range of compressors – the key to ’downsizing’ or making a smaller engine do the job of a larger one – while preserving the economy of the small engine.

Its designers expect pre-whirl to be suited to substituting, say, a 1.4-litre engine for a 2-litre one in a Ford Mondeo-sized car, where much of the initial interest is expected to lie.

The generator system avoids a phenomenon called surge, which affects compressors when a high level of boost and low engine speed coincide. This typically happens if the engine is running at around 1500rpm in top gear and the driver wants to overtake, or entering and leaving roundabouts in third.

A mismatch between the speed and direction of the incoming air and the compressor vanes lead to poor compressor efficiency and reduced boost, perceived by the driver as roughness. As surge worsens, boost can be lost altogether, but compressors for production cars are sized so this never happens.

’Our device allows a compressor which would otherwise be affected by surge to operate with improved efficiency at lower flows,’ said Integral Powertrain technical director Luke Barker. The pre-whirl generator uses a system of helical guide vanes to impart a swirling motion to the air, keeping the relative velocity of air and compressor within acceptable limits.

The result is either more power with the same driveability, or better driveability with the same power. The efficient operating range of the compressor can be extended by 20 per cent, according to computational fluid dynamics simulations.

’This has been done before but only with complex mechanisms involving moving guide vanes. Our system is very cost-effective, with low pressure losses,’ said Barker.

The solution uses two coaxial air paths. A normal butterfly-type throttle valve switches between them, under control of the enginemanagement system. At high throttle openings the incoming air passes straight into the compressor as normal. At low throttle openings the inner flowpath is blocked and the air is forced down an outer path through the helical guide vanes.

The idea emerged from work the independent consultant was undertaking on a range of diesel boosting solutions. So far Integral Powertrain has optimised the device in CFD but has yet to build a physical prototype. It is now presenting the idea to customers with the aim of collaborating on developing a prototype, which could be done for ’a few thousand pounds’ according to Barker. The technology is simple enough to introduce ’in a model year upgrade’, Barker said. This would allow it to be brought to market within two or three years.

Though the principle could be applied to petrol or diesel engines, Barker said: ’The first application is more likely in a diesel because the power and driveability of diesels are quite limited by the operating range of the compressor.’

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