Variations In Combustion Process Research Articles

Influence of Diesel Fuel Injection Characteristics on Dual-Fuel Combustion Modes in a Large-Bore, Medium-Speed Engine

Abstract Experiments were conducted on a large bore, medium speed, single cylinder, diesel engine to investigate operation with substitution ratio of natural gas (NG) varying from 0% to 93% by energy. In a previous study by the same group, these data were used to validate an analytical methodology for predicting performance and emissions under a broad spectrum of energy substitution ratios. For this paper, these experimental data are further analyzed to better understand the performance and combustion behavior under NG substitution ratios of 0%, 60%, and 93%. These results show that by transitioning from diesel-only to 60% dual-fuel (DF) (60% NG substitution ratio), an improvement in the NOx-efficiency trade-off was observed that represented a ∼3% improvement in indicated efficiency at constant NOx. Further, the transition from 60% DF to 93% DF (93% NG substitution ratio) resulted in additional efficiency improvement with a simultaneous reduction in NOx emissions. The data suggest that this improvement can be attributed to the premixed nature of the high substitution ratio case. Furthermore, the results show that high cycle-to-cycle variation was observed for some 93% DF combustion tests. Further analysis, along with diesel injection rate measurements, shows that the observed extreme sensitivity of the combustion event can be attributed to critical parameters such as diesel fuel quantity and injection timing. These results suggest a better understanding of the relative importance of combustion system components and operating conditions in controlling cycle-to-cycle variation of combustion process.

Experimental investigation of the cycle-to-cycle variations in combustion process of a hydraulic free-piston engine

The piston motion of HFPE (hydraulic free-piston engine) was determined by the forces acting on the mover rather than on the crankshaft. This makes the complex interaction coupling between the piston motions and the combustion process. These factors lead to the combustion in the HFPE unstable rather easily. In this study, cycle-to-cycle variations in an HFPE combustion process and their correlations were investigated. Fifty consecutive cycles under steady-state operations were selected from the experimental data for this study. To evaluate the cyclic variation in the HFPE combustion process, the CoV (coefficient of variation) and the parameter correlations were redefined because the working principles of an HFPE differ from those of a conventional engine. Therefore, cyclic variations in the combustion parameters were investigated, and the CoV and correlation coefficients were calculated. Strong correlations were observed both between the piston velocity and the ignition delay period of combustion, and between the IMEP (indicated mean effective pressure) and the rapid combustion phase. The results show that the cyclic variations in an HFPE are primarily due to the “free” piston. These results provide theoretical basis for controlling the cyclic variations in an HFPE.