F2008-06-113
The Fuel Economy Potential of the Naturally Aspirated Gasoline Engine - Synergies between Technologies
Future powertrains are required to generate high torque and power levels, ultra-low emissions, and provide great fuel economy. Today, the diesel engine sets the standard for low-speed torque and fuel economy, but faces significant challenges for meeting future emission standards. The gasoline engine generates good power along with the potential to meet future emission standards, but is limited in terms low-speed torque and fuel economy performance. Fundamentally, the gasoline engine is knock-limited during low speed and high load operation, keeping the torque levels below that of the diesel engine. Also, current high-volume gasoline engines are throttled under normal driving conditions, which limit the potential fuel economy as compared to that of the diesel engine. GM continues to advance both the diesel and gasoline engine, focusing on further improving the emission performance of the diesel engine and improving the fuel economy and low-speed torque performance of the gasoline engine.
Numerous engine technologies are routinely investigated with the intention to improve the fuel economy of the gasoline engine. Current high-volume production gasoline engines are port-fuel-injected (PFI) and calibrated with internal or external exhaust-gas-recirculation (EGR) to provide better fuel economy and lower NOx emission levels. The PFI engine is being replaced with the spark-ignition-direct-injection (SIDI) engine to provide some additional fuel economy along with better idle quality, emissions, and wide-open-throttle (WOT) performance. These gasoline engine concepts operate at stoichiometry which requires throttled operation during normal driving conditions; limiting the potential fuel economy. More complicated valvetrain technologies, such as the BMW Valvetronic system [1], allows the stoichiometric gasoline engine to be unthrottled lowing the pumping work penalty and improving fuel economy. The homogeneous-charge gasoline engine can also be calibrated lean of stoichiometry to improve fuel economy with the additional challenge of managing the lean NOx emissions. The stratified-charge and homogeneous-charge-compression-ignition (HCCI) gasoline engine technologies offers higher fuel economy as these engines are calibrated at higher dilution levels with added emissions risks and more complicated controls. This paper describes each of these gasoline engine technologies and the associated fuel economy potential during normal driving conditions. A breakdown of the individual mechanisms of reduced pumping work and improved indicated efficiency is provided with the intention to explore synergies between the various technologies. To illustrate the feasibility of successfully transferring fuel economy technologies from a single-cylinder engine to the multi-cylinder engine and finally to a vehicle, a fuel economy analysis along with measured vehicle data is summarized for the stratified-charge engine technology.
Session: Fuel Efficient Powertrains II