| Hybrid powertrains and forced induction are two approaches to the demands placed on OEMs to reduce fuel consumption and CO2 emissions, enabling downsized IC engines to meet performance and driveability criteria.
Developed by automotive consultancy Integral Powertain in partnership with NexxtDrive, the Supergen i-Hybrid system combines the advantages of both, and is claimed to allow a 1.4 litre gasoline engine C-segment car to produce 170bhp with CO2 emissions of only 120g/km. Transient response is 300ms, much quicker than most exhaust driven turbochargers and the key to proving the performance associated with a larger engine.
Volkswagen’s 1.4 litre TSI engine installed in the Golf GT combines both super- and turbo-charging to develop 168bhp, 0-100km/h in 7.9s and CO2 of 175g/km: a very successful application of downsizing.
The new Supergen concept differs significantly in that the compressor is electrically driven, the electrical machines also serve as starter motor/generator, providing stop/start functionality, and regenerative braking charges a bank of capacitors that provide additional energy for launch assist.
Supergen layout
Compact design
Designed for use with 12V architectures, the system is housed in a single compact unit which provides straightforward packaging on to existing gasoline powertrains, I4 units in particular, and is claimed to be highly cost effective. The system is scalable and can be applied to engines from 1.2 to 4.5 litres. It is also suitable for diesel engines as an interbooster.
The unit comprises a belt driven generator, which replaces the alternator and starter motor, connected via an epicyclic traction drive to a second, smaller, electric motor. A centrifugal aluminium compressor wheel is mounted on the end of the motor shaft.
The Rotrex traction drive allows a continuously variable transmission ratio to be achieved, allowing the compressor to operate at up to 150 times crankshaft rotation, spinning to 180,000rpm and developing peak boost pressure of 1 bar for the 1.4 litre application. The fluid drive was chosen over conventional toothed gears because of cost, packaging and noise benefits.
A peak efficiency of 98 per cent has been claimed and the fluid – originally developed by Nasa for rocket pump drives – does not need replacing over the life of the vehicle. An oil cooler is incorporated by having a water jacket encapsulating the sump.
At low speeds, almost two thirds of the power to the compressor is supplied electrically. Under full load, almost all of the power is supplied mechanically via the crankshaft pulley. Under part load conditions requiring low boost pressure, the transmission keeps mechanical losses to a minimum. When not boosting, the torque input to the smaller motor is removed.
Under these conditions, the larger motor acts as a generator, producing up to 15kW of power. This provides regenerative braking and charges a bank of capacitors. These are packaged into an enclosure “no larger than a biscuit tin” and when discharged into both electrical machines provide up to 10kW of power for additional torque assistance for launch and transient response at low speeds.
The first stage effectively substitutes electrical energy for mechanical to drive the compressor, reducing the load on the crankshaft. Under full torque assist, power is transmitted to the crankshaft via the belt drive. This lifts tractability to a level that the makers say is indistinguishable from a larger capacity engine.
John Martin, business development manager for Integral Powertrain, said: “For downsizing to be successful, you have to make the small engine genuinely like a large engine, and this isn’t the case with a turbocharged solution.”
Capacitor sizing is governed by the regenerative capacity of the system. Given the relatively small size of the electrical machines, this is not great, but the energy requirements of the system are correspondingly low. “Typically we can provide 20s of additional boosting. We see that as an enabler for launch feel – typical first and second gear behaviour. This is an area where downsized engines typically perform badly; this concept will perform far better,” said Luke Barker, technical director for Integral Powertrain.
“The amount of electrical energy we need to circulate is far lower than for hybrids. We need approximately 300kJ; a Prius needs about 4,000kJ. The steady state power consumption is 2-2.5kW. The system is cost-effective because energy storage capacity is lower: the additional cost of hybrids is proportional to the energy and power requirements."
Self-sustaining system
Martin was also keen to point out that unlike some gasoline/battery hybrids, the supergen system is self-sustaining: “When not using the capacitors, the energy comes from the crankshaft, so the battery is not involved. A car fitted equipped with supergen could pull a trailer uphill all day long with no loss of power. This is not true of some hybrids.”
For Europe, diesel applications are important as specific outputs continue to rise with the demand for better performance while maintaining fuel economy. The supergen has not been designed to provide the full charge air requirements for a diesel but would function very well as an interbooster.
Barker explained: “Under part load the engine runs with a lot of boost so that you can operate high levels of exhaust gas recirculation (EGR); the system needs to be able to do this efficiently. Supergen isn’t really a replacement for the turbocharger in that scenario. What it can do is allow you to use a bigger turbo and extend the range while still providing the response and EGR capability you get with a smaller unit. Supergen has the capacity to raise diesel engine output beyond 80kW/litre (107bhp/litre).”
Here, as with gasoline engines, the system would be of maximum benefit in transient conditions. High output diesel engines can still suffer from pronounced turbo lag at low speeds because of the inertia of the relatively large turbine wheels but Supergen’s near-instant response would compensate. Barker said: “There may be only a few hundred watts of power at the turbocharger when tipping-in. This booster provides ten times more power.”
Supergen test rig
So far, the system is in development. Undergoing rig testing, dynamometer tests begin in November. Vehicle manufacturers are studying progress closely. Martin said: “There is a lot of interest from OEMs in Europe, Japan, Korea and North America. It is fair to say that several respected OEMs are likely to take development units for pre-development purposes.”
The project has made good progress to date, and the 2010 model year has been slated for production in “significant” volume.
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