Centrifugal compressor technology – whether in turbochargers, superchargers or emerging electrically actuated air compressor systems – will be a key element in the next generation of ‘downsized’ internal combustion engines, claims engine technology specialist Integral Powertrain, which has invented a method which allows compressors to operate efficiently across a wider range of mass flow rates, improving fuel economy and power output in all driving conditions.
A turbo-compressor works by forcing air into the engine cylinder under pressure, allowing higher power densities and an improvement in engine efficiency. However, a conventional compressor only operates effectively over a limited range of air flow rates. When the air flow drops below a critical level a condition known as surge occurs: The air flow becomes unstable and the compressor stalls.
Solutions to this problem have been applied in motor sport and high performance road car applications, often making use of adjustable vanes to match compressor geometry to flow rate. Such devices are mechanically complex and expensive and can generate high pressure losses however, which has delayed their uptake in volume production applications.
Integral Powertrain’s innovation is a simplified yet effective “pre-whirl generator” system which uses guide vanes in the gas inlet path to introduce axial rotation to the flow. This rotation has the effect of changing the angle at which the incoming air strikes the compressor blades. By adding just the right amount of rotation, the system extends the range of efficient operating flow rates by as much as 20%.
To ensure that the right degree of pre-whirl is applied in all operating conditions, the IP system uses two coaxial air paths – an outer path with guide vanes and an inner one without. Each path is regulated by a simple valve mechanism on a common shaft, which can be controlled by a single actuator, splits the incoming air proportionally between the two paths. When the flow recombines, a predetermined amount of whirl is produced.
Sophisticated CFD simulations have demonstrated that the new system will be capable of extending the compressor operating range by 20% – which will translate into improved low speed torque and enhanced driveability. Alternatively, it can be employed to enable the engine to achieve 20% more power for a given driveability.
“We have managed to achieve a robust, easily controlled solution that can be manufactured from inexpensive materials,” said IP’s technical director Luke Barker. “And our analysis indicates that the system will operate with minimal pressure loss in the intake system, so maximum operating efficiency is maintained.”
The characteristics of the IP pre-whirl system make it particularly desirable for the new generation of high output diesel engines which are expected to power an increasingly significant proportion of European passenger vehicles over the next decade.