A computational fluid dynamics (CFD) guided combustion system optimization was
conducted for a heavy-duty diesel engine running with a gasoline fuel that has a
research octane number (RON) of 80. The goal was to optimize the gasoline
compression ignition (GCI) combustion recipe (piston bowl geometry, injector
spray pattern, in-cylinder swirl motion, and thermal boundary conditions) for
improved fuel efficiency while maintaining engine-out NOx within a
1-1.5 g/kW-hr window. The numerical model was developed using the
multi-dimensional CFD software CONVERGE. A two-stage design of experiments (DoE)
approach was employed with the first stage focusing on the piston bowl shape
optimization and the second addressing refinement of the combustion recipe. For
optimizing the piston bowl geometry, a software tool, CAESES, was utilized to
automatically perturb key bowl design parameters. This led to the generation of
256 combustion chamber designs evaluated at several engine operating conditions.
The second DoE campaign was conducted to optimize injector spray patterns, fuel
injection strategies and in-cylinder swirl motion for the best performing piston
bowl designs from the first DoE campaign. This comprehensive optimization study
was performed on a supercomputer, Mira, to accelerate the development of an
optimized fuel-efficiency focused design. Compared to the production combustion
system in the baseline engine, the new combustion recipe from this study showed
significantly improved closed-cycle fuel efficiency across key engine operating
points while meeting the engine-out NOx targets. Optimized piston
bowl designs and injector spray patterns were predicted to provide enhanced
in-cylinder air utilization and more rapid mixing-controlled combustion, thereby
leading to a fuel efficiency improvement. In addition, shifting the engine
thermal boundary conditions toward leaner operation was also key to the improved
fuel efficiency.
