Optimized heat release rate for enhanced thermal efficiency under NOx, noise and peak firing pressure constraints in light-duty Diesel engines

Abstract

Engine manufacturers have to face stringent emission regulations worldwide while maintaining performance and drivability targets as high as possible. However, some strong paradigm shifts are still necessary to further improve the typical trade-off between fuel consumption and tailpipe emissions of Diesel engines.

Split-of-losses analyses have shown that improvements in fuel efficiency could be obtained by optimizing the combustion timing and duration. More generally, this article aims at showing that a fully optimized heat release rate could help in increasing the efficiency while complying with NOx, noise and peak firing pressure constraints.

The reference engine for this study is a 2.3L Euro-6 compliant engine used for light commercial vehicles and passenger cars. First, experimental heat release rates have been modeled for different operating points by spline functions using a reduced number of relevant parameters. Then, based on these parameters, numerous heat release rates have been defined thanks to a design of experiments approach. Engine performance such as fuel consumption, noise, NOx emissions and peak firing pressure have then been quantified on a 0D single cylinder engine simulation platform. To do so, a new two-zones combustion model has been developed, allowing the user to specify the heat release rate due to combustion while quantifying the NOx emissions.

All these steps define a complete workflow which allows the optimization of the heat release rate in different operating conditions. Thanks to this approach, it has been shown that the fuel consumption could be reduced by 5.5% at 3000 rpm full load and by 2% at 2000 rpm middle load. Finally, these works do not only confirm that the NOx emissions and noise can be lowered by controlling the very beginning of combustion, but they also show that the fuel consumption strongly depends on the duration of the second half of combustion.