Closed cycle measures for thermal efficiency improvement of a heavy-duty ultra-high compression ratio combustion engine: A numerical and experimental analysis

Measures to improve the thermal efficiency of heavy- duty commercial vehicle engines with compression ignition continue to be an important topic in research and development. Increasing the compression ratio (CR) of the engine is a direct way to increase the process efficiency. However, to ensure an optimum combustion and emission behavior at very high compression ratio is challenging. In addition, the combustion and emission behavior of heavy-duty compression ignition (CI) engines with compression ratios beyond 21:1 has hardly been reported in the literature. In this study, a combination of the experimental and 3D-CFD based numerical methods were applied to a high compression ratio heavy duty engine to analyze the combustion process and emissions so as to evaluate the thermal efficiency improvement potential. Different fuel injection hydraulic flow rates (HFR) of the injector ranging from 1700 cc/min up to a high HFR of up to 3000cc/min were assessed for compression ratios from 21:1 to 24:1 and compared with the baseline configuration. Two types of the step piston bowl configurations namely the conventional step piston with fuel-spray targeting the bowl-bottom and an injection-split piston bowl, with fuel-spray targeting the bowl edge were investigated. The simulations predict that the injection- split piston bowl exhibits better thermal efficiency improvement potential than the conventional piston bowl. Furthermore, for this type of piston bowl increased HFR of up to 3000cc/min leads to improved thermal efficiency without worsening of mixture preparation, which can be attributed to late or minimal interaction of neighboring spray or flame fronts caused by shorter injection duration. The injection-split piston, with HFR of 2040cc/min, and a ultra-high CR of 23:1 and 24:1 yields an indicated thermal efficiency benefit of up to 2.1% - 2.7% points compared to the baseline CR18. The experimental investigation on this combustion layout shows similar thermal efficiency improvement as predicted by the numerical investigation.