Using Advanced Injection Timing and EGR to Improve DI Diesel Engine Efficiency at Acceptable NO and Soot Levels

The direct injection diesel engine is one of the most efficient thermal engines known to man. For this reason DI diesel engines are widely used for heavy-duty applications and especially for the propulsion of trucks. Even though the efficiency of these engines is currently at a high level there still exist possibilities for further improvement. One way to accomplish this is by increasing the injection timing which usually improves, depending on the operating conditions, the indicated efficiency of the engine. On the other hand advanced injection timing has a negative effect on peak pressure causing a serious increase of its value, a negative effect on NO emissions which are also seriously increased and a positive effect on Soot emissions which are reduced. In the present work a theoretical and experimental investigation is presented to determine the effect of more advanced injection timing on engine performance and pollutant emissions. The theoretical work is conducted using a multi-zone combustion model describing in detail the combustion mechanism and formation of pollutants inside the engine cylinder. The experimental investigation is conducted on a heavy-duty single cylinder truck test engine developed under a European Project. As revealed from the modeling and the experimental findings injection advance has a positive effect on engine efficiency which is greater at higher engine operating speeds and loads. On the other hand a serious increase of NO emissions is observed leading to values that are higher than the limits set by European legislation. For this reason it is examined the use of exhaust gas recirculation (EGR) to control NO emissions. From the analysis of theoretical and experimental findings it is revealed the required percentage of EGR at various engine operating conditions to maintain NO at acceptable levels. The use of EGR causes a sharp reduction of NO and an increase of soot emissions, which is partially compensated by its reduction due to the more advanced injection timing. On the other hand EGR results to a slight reduction of engine efficiency and maximum combustion pressure which in any case does not alter the benefits obtained from the high injection timing. As observed it is possible to increase brake efficiency considerably for the specific engine using a combination of more advanced injection timing and EGR while maintaining pollutants at acceptable levels. In this case the maximum combustion pressure rises to values around 220-230 bar, while a further increase does not result to significant improvement of efficiency. An important issue is that the findings of the theoretical investigation agree with the ones of the experimental one revealing the importance of modeling for similar applications since it can provide assistance during engine development, speeding up the entire process and also a better understanding of the experimental findings.