The Influence of Multiple Injections on Combustion : an Experimental Investigation

Doctoral thesis, 2007

The high fuel efficiency and torque of the diesel engine has made it the obvious choice for heavy duty applications for a long time. In addition, these characteristics of the modern super-charged HSDI diesel engine also make it very suitable and popular for car applications. However, since all engines emit combustion products that can be harmful to the environment, and the transport sector makes a major contribution to total anthropogenic emissions of potentially harmful substances, increasing legislative demands are being placed on manufacturers of diesel (and other) engines to minimise emissions. One of several possible strategies to reduce pollutant formation is to use multiple injections, i.e. to divide injections into two or more parts. In the studies underlying this thesis the effects of multiple injections on diesel combustion were investigated experimentally, in both spray chamber experiments and engine tests. The main objectives of the work were to investigate ways in which multiple injections could be used to reduce engine-out emissions from diesel engines and to increase the efficiency of diesel engines. Split injections consisting of two injection pulses were shown to have significant effects on the combustion processes in both a heavy duty and a light duty engine, although the effects differed somewhat between the two types of engine. In the light duty engine splitting injections into two pulses lowered the engineout particulate and CO emissions and increased the fuel efficiency. However, they also resulted in increased NOX emissions. Increased air-fuel mixing is believed to be the main reason for this. In the heavy duty engine a split with a very short dwell time, resulting in a rate-shaped injection rather than a true split injection, resulted in shortened combustion durations, which simultaneously reduced fuel consumption and NOX emissions. Use of multiple injection regimes alone will not meet the tough emissions legislation of the future. However, they seem likely to play a significant role in the continuing efforts to develop cleaner diesel engines.