Abstract
In order to meet customer needs the automotive industry is more and more focussing on the development of engines with a very low fuel consumption. At the same time especially the european customer does not accept restrictions in driveability and engine performance. Gasoline direct injection seems to be the ideal measure to fulfill both requirements. On the one hand gasoline direct injection offers the biggest fuel economy potential, on the other hand engine torque and power can be increased significantly through the charge-cooling effect of the direct injection which leads to higher volumetric efficiency and increased knock resistance. In addition the lack of wall film build-up should improve the dynamic response of the engine.
In the course of the development of the FSI it became obvious that fulfilling both objectives, high performance and low fuel consumption, is a stressing challenge. In order to operate the engine in the fuel-saving stratified combustion mode special inlet ports or other incylinder-flow devices, complex combustion chamber geometries and injector configurations have to be developed, which not necessarily are the best solution for the best full load performance at the same time. The complex combustion chamber geometry and the demand for a compact fuel spray might result in insufficient homogenisation which again might lead to increased knock sensitivity or high exothermic reactions in the exhaust system. Since the NOx storage catalyst needed for the exhaust gas aftertreatment of a lean burn FSI engine has still a higher temperature sensitivity compared to conventional 3way catalysts, special focus has to be laid on this point.
VW´s measurements in the concept phase of the FSI development showed that at first sight it is easier to use a swirl concept for good stratified combustion. However, further investigations proved that swirl concepts are not suitable to fulfill VW´s high demands at WOT operation. Consequently VW decided for a combustion concept with switchable tumble which on the one hand shows the potential for more than 55 kW/l and 100 Nm/l respectively, on the other hand proved to have a fuel-saving potential of 10 to 15 % compared to state-of-the-art PFI engines.
In addition measurements of VW FSI engines with combustion chamber geometries and injector configurations optimized for homogeneous combustion show that the potential at WOT operation is not fully exploited yet and that gasoline direct injection is the ideal basis for future high performance naturally aspirated engines and turbo-charged engines used for downsizing concepts. Consequently VW will focus on FSI technology for future SI engine concepts.