CPT says electric supercharging offers performance and driveability as yet unmatched by other air charging methods. Tests confirm that when applied to a radically downsized and down-speeded engine, the VTES system dramatically increases transient response, delivering significantly more torque thereby enhancing a car's driveability at low engine speeds.
For example, when applied to a radically downsized 1.2-litre turbocharged engine, VTES delivers in excess of a 50 per cent increase in torque at engine speeds below 3,000rpm, more than compensating for insufficient power from the exhaust turbine. Significantly, more than 90 per cent of the available torque is delivered in less than a second. Compared with a 1.6-litre naturally aspirated engine, the downsized engine with electric supercharger reduces the 70-100kph (44-63mph) top gear acceleration time from 18 to 11 seconds.
"Even though engines equipped with a conventional mechanical supercharger or exhaust driven turbocharger have seen significant improvements in their dynamic behavior at low engine speeds, they are fundamentally dependant on engine speed for their operation," says CPT senior engineering manager Mark Criddle. "Even state-of-the-art systems struggle to satisfy this critical customer requirement for driveability. And while series boosting solutions are gaining popularity because of their extended low speed capability, their dynamic performance is still linked to engine speed and air mass flow rates. The alternative method of boosting performance by integrating an electric motor between the engine and transmission to create a mild hybrid is a costly exercise and difficult to package in front wheel drive vehicles."
CPT's electric supercharger provides a viable low cost micro-hybrid solution, significantly increasing an engine's air charge density over the critical first 10 combustion cycles of a low speed transient. Fitted with a low inertia compressor, the supercharger accelerates from idle to its maximum speed of 70,000rpm in less than a third of a second enabling even a turbocharged engine to achieve full load torque within one second at very low engine speeds. This fast dynamic response and rapid air boosting enables the system to react instantly to high transient load conditions, delivering up to 25kW (33bhp) of additional power at the crankshaft. This is more than enough to compensate for any turbo lag and more cost effective than integrating a 25kW electric motor into the powertrain since only a 12-volt alternator and battery system is required. "Fast response air-boost systems are essential for delivering radically-downsized engines," says Morris. "Even the most dynamically optimized turbocharger cannot deliver an air-side response approaching the capability of the latest fuel injection technology. The consequence is `air limited' combustion, which imposes many compromises on the engine developer."