Diesel engine heat release analysis by using newly defined dimensionless parameters

Abstract

Diesel engine combustion has been studied during the last decades by researchers in terms of improving the performance of the engine. In order to improve the analysis of the diesel engine combustion, dimensionless parameters were used in this study. It was concluded that the newly introduced dimensionless parameters developed in this study facilitate understanding of diesel engine combustion process. A new method has been proposed to determine the values of the form factor (m) and efficiency factors (a) of the Wiebe equation. This is achieved by developing a modified form of Wiebe equation with only one constant. The modified version of Wiebe equation facilitates the determination of constants accurately, which enhances the accuracy of evaluating the burn fraction. The error induced on the burn fraction f with respect to the values of constants a and m obtained through different methods is discussed and compared. The form factor affects the burn fraction significantly compared to the efficiency factor. A new non-dimensional parameter ‘combustion burn factor (Ci)’ has been identified in the modified Wiebe equation. The burn fraction f was found to be a function of Ci only, thus the benefits of expressing heat release rate with respect to Ci have been presented. The errors associated with the determination of apparent heat release rate (AHRR) and the cumulative heat release (Cum.Hrr) from the measured cylinder pressure data and the assumed specific heat ratio (γ) was determined and compared. The γ affected the calculated AHRR more than the cylinder pressure. Overestimation of γ resulted in an underestimation of the peak value of the AHRR and vice versa, this occurred without any shift in the combustion phasing. A new methodology has been proposed to determine the instantaneous and mean value of γ for a given combustion. A two litre Ford puma Zetec diesel engine, four cylinder and 16 valves was employed to carry out this investigation. This new methodology has been applied to determine γ for a wide range of injection pressure (800 bar to 1200 bar), injection timing (9 deg BTDC to -2 deg BTDC) and engine loads at 2.7 BMEP and 5 BMEP. Standard ultra-low sulphur diesel fuel and two bio-diesels (Rapeseed Methyl Ester and Jatropha Methyl Ester) were studied in this investigation. Ignition delay is one the most important parameter that characterises the combustion and performance of diesel engines. The relation between ignition delay and combustion performance in terms of efficiency and emission was revealed by researchers. Ignition delay period measurements in diesel engine combustion along with the most used correlation for calculating ignition delay are discussed in this work. The effect of constants on accuracy in the correlation were discussed, and induced error on calculated ignition delay periods with respect to constants were calculated and compared. New techniques were proposed to calculate the constant values directly by using the experimental data. It was found that the calculated values for ignition delay using the new techniques matched well with the experimental data. These techniques can improve the accuracy of the ignition delay correlation. Also a new correlation without any constants was introduced in this work. This correlation can be used to predict ignition delay directly by using engine parameters only. The introduced correlation provides better results compared to Arrhenius type correlation presented by Wolfer. This new correlation can be used for feedback control engine combustion process.