A blow for lower CO2 emissions
3 min read
An integrated air management system offers more torque from smaller engines while consuming no more fuel. Roger Bishop describes how it works
Visteon’s torque enhancement system (VTES) is intended to help car-makers to meet the tough 2008 targets set out in the ACEA (Association des Constructeurs Européens d’Automobiles) agreement to cut carbon dioxide output from new cars. Only greater fuel economy can reduce the amount of CO2 released to the atmosphere by vehicles. This should not be confused with emissions of the poisonous gases CO and NOx. In their efforts to meet the ‘fleet’ fuel economy targets agreed by ACEA, car-makers have gone about as far as they can in adding small, fuel efficient—but very expensive—cars to their ranges to balance the fuel greedy but highly profitable luxury models at the top end. Now they are having to consider radical fuel-saving measures in the mass production sector. And that means engine downsizing while still meeting market demands for power and performance. Visteon’s development, which can be applied to both naturally-aspirated and turbocharged petrol and diesel engines, addresses these apparently conflicting requirements. VTES is a fully integrated engine air management system at the centre of which is a electrically powered supercharger. It delivers additional torque as soon as the throttle demands it, without turbo lag, and is particularly effective at low engine speeds. Furthermore, it is a production-ready technology that can be applied to existing engines with little or no modification. VTES compresses air and delivers it to the engine to increase volumetric efficiency by allowing more air to be inducted at each intake stroke. The effect is higher torque output. It is a transient device, working only at (or near) wide open throttle. In a naturally aspirated engine it operates for around 7 per cent of the time but in a turbocharged engine this figure might be only 1 per cent. VTES is necessarily positioned very close to some of the hottest parts of the engine, requiring good thermal management. In addition, spinning up the supercharger impeller to some 50,000rev/min in around 300ms puts a big demand on the 14V electrical system which also needs careful control and close integration with the engine and its management systems. The company holds at least 12 patents on the technology, with others pending, which indicates the work that has gone into the system. There are differences in the way VTES operates with spark ignition and diesel engines but the effect is the same—higher engine torque with an overall improved torque curve profile. In the case of diesels, manufacturers have long wanted to be able to widen the usable range of the engine’s slow revving characteristic and that means delivering significant torque at lower engine speeds. Visteon says VTES has replotted that figure at around 1000rev/min. The four major components are: the air induction system (AIS), which houses the main elements of VTES, including the air cleaner, battery, cooling box, power electronics and air bypass mechanics; an electrically driven on-demand inlet air compressor; a VTES controller; and a smart charge alternator. Under light engine load, when no boost is required, most of the inducted air passes through an air bypass and non-return valve and out to the engine. When boost is called for, the compressor is rotated at high speed by the electric motor producing an airflow up to 0.4 bar above atmospheric pressure. This closes the non-return valve and prevents air from flowing back through the bypass. The result is that all the filtered air passes through the compressor and out to the engine. Smart charge is used to alter the voltage set point of the alternator and therefore the output current. This allows VTES to take an intelligent approach to charge balance enabling it, for example, to charge at maximum rate when the engine is at closed throttle and the vehicle is decelerating. It also allows VTES to make big power demands—up to 2kW in a 14V system—without compromising other vital systems. Applied to a spark ignition engine, the powertrain control module (PCM) would sense accelerator pedal position and use it to calculate a desired engine torque. From this it would calculate desired manifold absolute pressure (MAP) or mass air flow (MAF) and open the throttle accordingly. When the throttle is sufficiently open for manifold pressure to have reached atmospheric pressure, the PCM would send a command to VTES demanding boost and the required value of MAP or MAF. On a 1.2 litre engine, torque has increased by up to 40 per cent in some areas, giving the host vehicle a performance approaching that of 1.8 litre version. Fuel economy was identical to the smaller engine without VTES. This could be expressed as a 27 per cent improvement in fuel economy for roughly equal performance. The trend in diesels is towards turbocharging, where the function of VTES is to blow air into the turbo until it is spinning fast enough to take over. In this way VTES overcomes the turbo lag effect. Visteon says the smooth management of the transition from VTES to turbo was one of the main challenges for such applications. Applying VTES to a 1.9 litre turbo diesel produced an improvement in steady state torque of around 30Nm and a transient boost response time reduction of around 50 per cent in a third gear acceleration. VTES can also be applied to petrol driven turbocharged engines. Testing on a vehicle equipped with a 2.0 litre turbo petrol engine resulted in improved transient response for a third gear acceleration with turbo lag reduced from around 3s to less than 1s.