CFD Study of Combustion Chambers for Lower Engine Exhaust Emissions from Diesel Engines Operated in HCCI and Conventional Diesel Mode 2009-26-0027

The exact shape of the combustion chamber will not have a major effect in case of homogeneous type of combustion. However, presently engine needs to be operated in diesel mode during start-up and outside HCCI operating range to meet stringent emission norms. Hence, the combustion chamber is therefore optimized for conventional diesel mode operation for minimum emissions as well as benefit to HCCI mode operation with better swirl. When operated in pure HCCI mode, the exact shape of the combustion chamber is of little relevance-mainly because the fuel jets do not interact with the bowl at early injection conditions. However, at some operating conditions it is necessary to operate with partially premixed conditions where fuel is injected between 20 to 30° BTDC. Then bowl geometry plays significant role to promote homogeneous mixture of air and fuel. There is no need for late cycle turbulence generation in HCCI mode, since it is not a mixing controlled process. The lean burn homogeneous mode is purely chemically controlled.

This study is mainly concentrated on finalization of bowl shape suitable to minimize CO, HC, NOx and PM emissions from diesel combustion, when operated on both convention mode as well as HCCI mode. Due to limitations of HCCI combustion expansion to very low loads as well as high loads, it is necessary to run on conventional mode. Keeping this view, four combustion chambers including base combustion chamber have been studied to improve emissions by using 3D-CFD software and ECFM-3Z combustion model. Performance characteristics and emissions of modified bowls are compared with the base bowl at full load and 50% (part load) load point. Interestingly, performance characteristics of all the bowls are closely matched irrespective of the bowl geometry; however significant difference in emission levels were observed. At full load, bowl option-B produces lower soot, CO and HC emissions, but 25% higher NOx levels as compared to bowl option-C. At 50% load conditions bowl option-C producing almost 2.87 time lower soot, 23.43% lower CO and 7.8% lower NOx emissions as compared to bowl option-B which is best at full load.