Assessment of air-management strategies to improve the transient performance of a gasoline engine under high EGR conditions during load-decrease operation

Abstract

Advanced gasoline engines in hybrid powertrains are currently regarded as the most widespread solution for passenger cars in the upcoming years, due to the strong commitment of many countries to reduce greenhouse gas emissions and the limitations of pure electric vehicles at present. At the same time, it is widely proven that exhaust gas recirculation (EGR) through the low pressure (LP) configuration allows reducing fuel consumption and, consequently, CO2 emissions in gasoline engines. However, it is challenging to extract the full EGR potential, since it means operating close to the EGR tolerance, and this can compromise the engine transient response because of the long transport delays associated with LP EGR systems. Hence the paper presents an analysis on the capability of three strategies to accelerate the transient response of a gasoline engine operating with high LP EGR levels during tip-out maneuvers, while guaranteeing the combustion stability. Such strategies are the optimization of valve synchronization (based on applying different delays between the throttle actuation and pedal demand), the integration of a secondary air-path which bypasses the compressor, and the use of a pressurized air tank directly connected to the intake manifold. Both experimental and modeling data were utilized to assess the advantages and drawbacks of these strategies: engine tests for the validation of 1D model transient performance and the evaluation of combustion stability and torque evolution, while the fluid-dynamics and transport phenomena were analyzed by simulation.