Innovations for C[O.sub.2] reduction

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 C[O.sub.2] 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.

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 C[O.sub.2] 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 C[O.sub.2] 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 adopting 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 C[O.sub.2] emissions. Complex and bulky clutches, by-pass valves and noise attenuation systems are required to alleviate these problems.