Fuel Economy Improvement and Knock Tendency Reduction of a Downsized Turbocharged Engine at Full Load Operations through a Low-Pressure EGR System

摘要

It is well known that the downsizing philosophy allows the improvement of Brake Specific Fuel Consumption (BSFC) at part load operation for spark ignition engines. On the other hand, the BSFC is penalized at high/full load operation because of the knock occurrence and of further limitations on the Turbine Inlet Temperature (TIT). Knock control forces the adoption of a late combustion phasing, causing a deterioration of the thermodynamic efficiency, while TIT control requires enrichment of the Air-to-Fuel (A/F) ratio, with additional BSFC drawbacks. In this work, a promising technique, consisting of the introduction of a low-pressure cooled exhaust gas recirculation (EGR) system, is analyzed by means of a 1D numerical approach with reference to a downsized turbocharged SI engine. Proper “in-house developed” sub-models are used to describe the combustion process, turbulence phenomenon and the knock occurrence. The effects of cooled gas recirculation are studied for various EGR and A/F levels. The presented results highlight that the proposed solution involves significant BSFC improvements. In particular, it is able to reduce the engine knock tendency, resulting in the possibility to advance the combustion phasing. Moreover, it involves a TIT reduction that allows to limit the over-fuelling. The developed numerical procedure is able to take into account the complex interactions among different driving parameters affecting the fuel consumption. It can be hence very useful to define a numerical engine pre-calibration and to realistically predict the EGR-related BSFC advantages which can be gained at different speed and load conditions.