A Computational Investigation of Stepped-Bowl Piston Geometry for a Light Duty Engine Operating at Low Load 2010-01-1263

The objective of this investigation is to optimize a light-duty diesel engine in order to minimize soot, NOx, carbon monoxide (CO), unburned hydrocarbon (UHC) emissions and peak pressure rise rate (PPRR) while improving fuel economy in a low oxygen environment. Variables considered are the injection timings, fractional amount of fuel per injection, half included spray angle, swirl, and stepped-bowl piston geometry. The KIVA-CHEMKIN code, a multi-dimensional computational fluid dynamics (CFD) program with detailed chemistry is used and is coupled to a multi-objective genetic algorithm (MOGA) along with an automated grid generator. The stepped-piston bowl allows more options for spray targeting and improved charge preparation. Results show that optimal combinations of the above variables exist to simultaneously reduce emissions and fuel consumption. Details of the spray targeting were found to have a major impact on the combustion process. With the stepped-bowl and split injection strategy, combustion can occur in two distinctly different regions of the bowl. When the first injection targeted the top portion and second injection targeted the bottom portion of the stepped-bowl a low emission combustion process with improved fuel economy resulted. This is because combustion takes place in two different areas, allowing better use of the oxygen, thus limiting the production of soot. The stepped-bowl has less surface area than the conventional reentrant bowl, resulting in less heat transfer out causing the fuel consumption to be lower in the stepped-bowl. Swirl, and spray angle, along with the bowl geometry details, affected the pre-combustion mixing process. Second injection timing and amount of exhaust gas recirculation (EGR) affected the objectives as well. These parameters were varied and showed the stepped-bowl piston allowed better use of the oxygen. The optimized stepped and conventional bowls were also investigated at mid-load conditions, both at low and high engine speeds. The stepped-bowl continued to show reduced soot and CO emissions and improved fuel economy over the conventional bowl. This investigation shows the importance of the bowl geometry, spray targeting, injection timing, split fuel amounts and swirl on emissions and fuel efficiency in direct injection diesel engines.