Engine downsizing can sometimes include a distinct lack of torque at low revs and slow transient response, even when subtle turbocharging techniques are applied. A potential solution to these issues is variable-drive supercharging, currently under investigation by Torotrak and the University of Bath. The joint research project, supported by the U.K. government, aims for productionization of the technology.
The project is examining how a state-of-the-art downsized gasoline engine combined with a variable supercharger performs at a system level. The research is studying interactions with Torotrak’s V-Charge variable-drive supercharger unit to deliver far higher levels of low-end torque, fast transient response, and reduced fuel consumption.
V-Charge has been designed and developed to provide what the company describes as “near instant” response at any engine speed through the use of a supercharger with a mechanical variable speed drive, and to create the performance “feel” of a larger, naturally-aspirated unit.
Torotrak’s Chief Technical Officer, Doug Cross, said: “For more aggressive downsizing strategies to be implemented, we need solutions that will improve drivability without adding cost and complexity. And part of the challenge is that this cost and complexity is not just in the pressure charging system. The growing sophistication of exhaust aftertreatment, with its need for high and stable temperatures, is also compromised by having turbochargers in the exhaust stream.”
The optimum solution
There's also the fundamental challenge of insufficient airflow in the intake system at low engine speeds and throttle positions, making it difficult to drive even the smaller turbo in a two-stage system. To provide instant throttle response from idle on a downsized engine requires a supercharger, Cross maintains. This also mitigates many of the aftertreatment challenges as superchargers operate on the cold side of the engine without interrupting the exhaust stream.
Traditional mechanical blowers bring their own limitations, of course. A unit sized for low speed engine response is unable to deliver the volume of air required at higher speeds, while a larger unit — if suitably geared for low speed response — either requires a bypass to avoid over-delivery at higher speeds (thus wasting energy), or must be clutched. Adding a clutch can create loading challenges for the accessory drive as well as introducing an additional subsystem.
Some vehicle manufacturers, notably VW, have elected to use a small supercharger to enhance low speed torque and response, combined with a turbocharger to provide high power at the upper end of the engine range.
Another technology demonstrated as a potential solution is electric supercharging, but this is acutely power-limited in terms of the air it can deliver, restricting its contribution at low engine speeds, Cross noted. A 12-v system provides an extra 2-3 kW (2.6 to 4 hp) to compress the intake air, while even a 48-v system only produces some 6 kW (8 hp).
“The optimum solution is a means of boosting that responds quickly to transient changes, even at low engine speeds, and keeps pace with engine demand throughout the speed range,” he told Automotive Engineering. It would not introduce inefficiencies through throttling at times of partial demand or wasting surplus energy, and would minimize cost and complexity through simplicity of installation.
The Bath research project also involves Ford, whose 1.0-L 3-cylinder Ecoboost range of engines is available across several model ranges including the C/D segment Mondeo (see http://articles.sae.org/14204/).
Having evolved to a pre-production design level, V-Charge will be evaluated against current boosting solutions, initially through extensive simulation, then via a downsized gasoline engine. The concept has already been demonstrated by Torotrak fitted in in a 1.1-L car to potential customers.
“A conventional turbocharger is a highly effective device for optimizing steady-state fuel economy but as the specific output of engines climbs to 150 kW/L and beyond, no conventional single-stage solution can deliver the required low-speed drivability,” Cross explained.
He said that as future emissions regulations take effect, the combination of smaller engines and drive cycles closer to real-world use patterns will make engine operation under transient boost conditions increasingly important. This is the region where V-Charge (applicable to both gasoline and diesel engines) is particularly effective, addressing one of the major constraints that presently limits engine downsizing.
The system operates by connecting a conventional centrifugal supercharger to the engine through a compact variable-speed drive. This allows air delivery to be altered independently of engine speed or exhaust energy to match the engine’s requirement. It is designed to be installed on the front end accessory drive (FEAD) of an engine and provides a significant boost capacity, with a continuous rating of 20 kW (27 hp).
The gearless mechanical traction drive provides a 10:1 ratio spread, giving a wide speed range that allows a much greater compressor operating envelope than would be possible with a fixed speed drive.
Cooler intake air is added benefit
The unit is able to spin up quickly like an e-supercharger at low speed, then carry on to deliver the required air mass flow at higher engine rpm, within the limits of compressor performance. Using V-Charge, engine torque output can increase from 0-95% in less than 400 ms, cutting the time-to-torque by up to 70% compared with the latest state-of-the-art single turbocharger technologies, claimed Cross.
Because ratios are changed by a 10W actuator using electro-mechanical control, and no power is required to hold the unit at a given ratio, the system offers much higher efficiency than a conventional supercharger drive, according to Cross.
“We have minimized parasitic losses, not just when the unit is boosting, but also when off-boost," he reported. "We also have the potential to disconnect the drive at small throttle openings. This provides a big advantage because superchargers normally generate a huge inertial shock on the FEAD when re-engaging. But our variable drive can reduce the ratio and the referred inertia from the supercharger at the moment of re-clutching.”
A further efficiency gain, via the centrifugal blower, allows the V-Charge to deliver cooler intake air than a screw-type supercharger. This helps to overcome a further fundamental constraint on downsized engines: high combustion temperatures. His prediction for diesel emissions control is that SCR (selective catalytic reduction) aftertreatment will become the preferred option for reducing NOx, because moving the pressure charger to the cold side "will allow smaller, lower cost SCR systems.”
Cooler intake air also benefits other strategies for improving fuel economy, such as Miller cycle operation (recently adopted by Toyota for non-hybrids as well as by Audi on its A4), which relies on cooler induction temperatures and higher induction pressures throughout the rev range.