Fuel Economy Improvement Potential of a PFI Gasoline Engine Using VVT/VCR

Abstract

It is desired to optimize a spark ignition PFI (port fuel injected) engine for various regimes within the operating ranges of a vehicle. The goal of this work is to identify the set of technologies that complement each other and offer the optimum performance and fuel economy. For an ideal powertrain system, the engine should be optimized for best fuel economy during the typical drive cycles and best performance during high load acceleration. A typical PFI 1.8L four-cylinder engine is baselined at cycle representative speed/load points. The engine is supercharged and intercooled to later quantify the efficiency benefits from replacing a larger engine with a smaller boosted engine that offers similar performance. Then the effects of different operating regimes and the effect of different proposed technologies are studied. The fuel economy enablers considered include variable valve timing (VVT) and variable compression ratios (VCR). The effects of VVT was studied to see which valve event scenarios afford the best operating efficiency and fuel economy during part load operation. VVT can also be a source of performance improvement if implemented appropriately. VCR operation is studied to see if the efficiency gains from VCR are additive with VVT or if they overlap to some degree. Typically, the fuel efficiency potential of a production engine is limited by spark knock. The engine studied here uses the geometrical and virtual compression ratio reductions offered by the VVT and VCR systems to give knock limit relief and allows the knock-limited BMEP curve to be pushed up. The results showed that the fuel economy gain with the above mentioned technologies is somewhat additive throughout the typical driving cycle but is highly dependent on proper optimization of the many system variables.