Abstract
In today´s environment the development of vehicles is confronted with very high customer expectations and legislative restrictions, which can only be fulfilled with a high technological effort and profound know-how. These challenges are further increased due to the diversity of markets with regional preferences and increased cost and demand for energy.
The recent development in crude oil prices and the more and more stringent emission regulations exhibit the strongest drivers in the automotive industry nowadays. This is especially the case for commercial vehicles and buses, where the economy was always one of the most important factors in the development.
In that sense commercial vehicles of the future must be characterised by superior fuel economy, fulfilling all the strict emission regulations but at least maintaining the mobility level achieved today that can be summarised as efficient dynamics. This process may result in hybrid drive technologies for city and short haul transportation. But besides better utilisation of the already produced energy a key development aim is still the improvement of the raw energy utilisation i.e. better fuel energy utilisation of the prime movers, the Diesel or compression ignition engines.
In a strategy to meet future fleet CO2 emission limits downsizing of compression-ignition engines is an effective means for reducing fuel consumption but it requires supercharging to restore full load torque. A compact and cost effective solution is with an exhaust turbocharger, which however has the drawback of a slow response to fast load demands by the driver. When the engine is increasing in speed, a delay occurs between advancing the throttle pedal (which lets more fuel into the engine) and the time the exhaust pressure reaches a steady rate sufficient for the turbine. This delay is known as turbo lag. This response problem is causing driveability issues such as reduction in climbing power, resulting in burnt clutches, low accelerations levels, or critical overtaking manoeuvres that can be only reduced by more frequent gear shifting.
Besides downsizing efforts the new emission regulations make a similar need for increased charging level as well. Namely future regulations (Euro6 and further) seem to be able to be fulfilled only by the usage or combination of exhaust gas recirculation (EGR) technology that was not the applied at the commercial vehicles till some recent developments. To maintain the torque level with exhaust gas recirculation the fresh air portion of the cylinder charge, mixed with a certain ratio of exhaust gas even at full load, must be at least retained or increased. This can be achieved with the further increase of the turbo charging level only.
As the above mentioned downsizing and EGR technologies call an increase of the charger pressure levels this makes a consequence of the turbo lag increase as well. Here, an attractive alternative using the external compressed air to boost the turbocharger dynamics is considered since this medium is available on each commercial vehicle. First the possible competing concepts are summarized. Then the compressed air charging solution is further discussed in details by means of simulation studies, dynamometer and vehicle tests. Finally the conclusions are drawn.