Engine downsizing has become a primary concern in Europe as the commitment made by the ACEA, the European automobile manufacturers association, to reduce average carbon dioxide emissions to 140 gm/km for passenger cars sold in Europe by 2008 looms ever closer. Initial reductions in European CO2 levels were largely achieved by the increasing popularity of diesel engines, and while the diesel market share is still growing in Europe, it is likely to peak soon, if for no other reason than that the oil refineries will not be able to cope with the demand. So the onus is going to come back to the gasoline engine. "I am very sure that over the next years we will see further development in gasoline engines," says Prof. Wilfried Bockelmann, main board director at Volkswagen. "We already have direct-injection FSI engines and there will be more. The diesel has made a big leap forward with direct injection, special turbochargers with variable nozzles and so on, and I am very sure the petrol engine will follow over the next few years."
An option being seriously considered by most vehicle manufacturers is downsizing the engine as that offers significant advantages with respect to fuel consumption and emissions. The downside is that the torque produced by a small engine is markedly less than that of a large one, and while the end consumer might accept a reduced displacement, he still demands the same driving performance and comfort of a large-displacement engine. It is trying to address this conundrum that a variety of solutions are currently being offered. Turbochargers—which have been principally absent from European gasoline engines for the last decade—are one of them. In fact, they are now set to make a spectacular return, not due to consumer demand this time, but as a weapon by the automakers to meet emissions regulations. BorgWarner Turbo Systems is in the vanguard of this movement and has developed the eBooster, a new charging system that makes small-displacement engines without turbo lag possible. The system is based on the use of a flow compressor driven by an electric motor which has been designed to be placed either before or after the standard turbocharger. Due to its electric drive, it is completely independent from the turbocharger and the thermal energy of the exhaust gasses.
The SuperGen system is designed to allow smaller engines to provide performance that consumers want to have.
In contrast to an electrically assisted turbocharger, the system works in two stages, with two flow machines connected in series so that the pressure ratios of both charging devices are then multiplied. The main advantage of this over single-stage units is that two different sized compressors can be connected in series so that an optimized map is available for each flow-rate range. This results in an increase in the intermediate pressure curve regardless of how much exhaust gas is available. One drawback, though, is that it was developed with a 42-volt architecture in mind, which at the outset of the program looked to be imminent. Events, though, have shown otherwise so BorgWarner is now developing it to be compatible with a 12-volt solution, meaning that it is unlikely to enter the market until 2008 or 2009.
This problem does not afflict another innovative solution that has been presented by Integral Powertrain, an independent consultancy based in the UK that provides powertrain engineering services, and DriveTec, another UK company that has been established to develop and exploit a portfolio of power transmission technologies. Their answer is SuperGen, a variable-speed electrically controlled supercharger design that takes most of its power from a physical link to the vehicle’s engine. "Turbochargers, which get their power from the flowing exhaust gases, are a thermodynamically efficient boosting system," says Luke Barker, technical director at Integral Powertrain, "but under some conditions they suffer from lag as the exhaust flow builds to the point where effective boost can be delivered. As specific outputs increase, this effect is magnified, limiting the downsizing and CO2 reduction potential from conventional turbocharging. Vehicle manufacturers commonly adapt ‘shorter’ ratios in the lower gears to mitigate this effect, but this has the opposite effect to downsizing on CO2 emissions performance. In order to get its full benefit, performance and drive feel must encourage the driver to operate in the same speed range as a much larger engine."
Some manufacturers, says Barker, have tried to solve the driveability problem by adopt-ing positive displacement superchargers, which are mechanically linked to the engine’s crankshaft. These have lower compressor efficiency than turbochargers, are inherently noisy, and cause significant parasitic losses when boost is not required, harming fuel economy and CO2 emissions. Complex and bulky clutches, by-pass valves and noise attenuation systems are required to alleviate these problems.
There are a range of different compressor designs for mechanically driven superchargers but it is the centrifugal compressor, the same design as most turbochargers, that is the most efficient when the engine is at full load. It also has the lowest part load losses. Unfortunately, as the centrifugal compressor delivers its boost roughly in proportion to the square of its rotational speed, it produces very poor low speed torque, says Barker. The SuperGen breaks the fixed link between engine speed and compressor speed in a centrifugal supercharger, allowing the compressor to run at the optimum speed. "When the driver depresses the throttle, the SuperGen system can spin up the compressor extremely rapidly—within 300 milliseconds—to deliver near instant power and response," says Barker. The result enables the use of a much smaller spark-ignition engine that can deliver fuel economy and CO2 performance approaching today’s high-performance diesels, in a lower cost package and with better driveability, claims Barker.
The key to the SuperGen technology lies in the combination of a patented epicyclic traction transmission and electric gear ratio control. Rather than a conventional epicyclic gear with meshing teeth, a Rotrex traction-drive system that uses oil viscosity to transfer motion between the sun, planets and annulus, has been adopted. The design makes use of advanced lubricant—traction fluid—that changes its characteristics under load in order to transmit shear forces. Compared to a conventional toothed gear system, the traction-drive device can transmit more power for its size and weight at very high transmission ratio with much less noise. Its self-centering design also allows it to rotate at extremely high speeds without vibration problems or excessive bearing wear. Used to vary the compressor’s speed between zero and up to 150 times crankshaft speed, this SuperGen transmission allows the compressor to spin at up to 225,000 rpm with speed control being provided by a small electric motor which imparts extra motion to the planet hubs of the epicyclic transmission. The approach needs few special components and provides a highly efficient mechanical link between the engine and the compressor. SuperGen can therefore deliver strong boost at all engine speeds and near-instantaneous high boost at low speed when needed, but negligible losses at part load when no boost is required. According to Barker, it can respond so quickly, the limiting factor is the vehicle driveline with the rate of torque needing to be controlled to avoid unpleasant driveline "shunt".
According to Barker, SuperGen will enhance vehicle driveability by improving mid-range torque by up to 22% on a 1.4-liter engine compared to a baseline 2.0-liter engine. This allows the smaller engine to have a 10% taller gearing and still comfortably exceed the tractive force provided by a 2.0-liter baseline engine.
The main elements of SuperGen are the control motor, the power electronics and the traction drive. "The system has been designed to work effectively at 12 volts, minimizing impact on the vehicle electrical infrastructure," says Barker. "The traction drive is already in production for conventional mechanical supercharging applications and has completed extensive durability trials in automotive, industrial and aerospace applications." While designed principally for the gasoline engine, Barker says that SuperGen could work for diesels, as well. "Integral Powertrain engineers have developed concepts for the use of this system in conjunction with a conventional turbocharger as a more effective alternative to the smaller turbo in a twin sequential turbocharging setup for large diesels," says Barker. This SuperGen "pre-booster" works effectively with a control motor of only 1.0 to 1.5 kW and does not need to incorporate the starter/generator.