Numerical and Experimental Investigation into Brake Thermal Efficiency Optimum Heat Release Rate for a Diesel Engine

According to thermodynamic analysis of ideal engine cycles, Otto cycle thermal efficiency exceeds that of the Diesel and Sabathe (or Dual) cycles. However, zero-dimensional calculations indicated that the brake thermal efficiency (BTE) of an actual Otto or Diesel engine could be higher with a Sabathe (or Seilliger) type cycle, within a limited peak firing pressure (PFP). To confirm these results with an actual engine, a three-injector combustion system (center and two sides) was utilized to allow more flexibility in the heat release rate (HRR) profile than the conventional single injector system in the previous study. The experimental result was qualitatively consistent with the calculated results even though its HRR had less peak and longer duration than ideal. In this study, a new thermodynamic cycle with higher HRR in the expansion stroke than the ideal Sabathe cycle, was thus developed. The proposed (higher) HRR was achieved by overlapped fuel injection with the three injectors. Experimental results were qualitatively consistent with calculations, though the measured HRR had a lower peak value and longer duration. Three-dimensional analysis using the CONVERGE computational fluid dynamics (CFD) software package was also performed in order to obtain a complementary view of the test results. Numerical and experimental results were quite similar, showing improvement in BTE for increased overlap of center and side injections, but only up to a certain level. For near-simultaneous injection, the experimental BTE rapidly deteriorated, whereas the numerical BTE deteriorated only slightly and remained higher than measured values. Analysis of both direct in-cylinder combustion images and three-dimensional numerical simulations revealed that spray-to-spray interference could strongly affect mixture formation (local fuel/air equivalence ratio).