Computational Analysis of a Diesel Engine Autoignition, Combustion, and Emissions Using Injection Rate Shapes

Abstract

Injection rate shaping is a method used to control the instantaneous mass flow rate of the fuel during an injection event. The rate at which the fuel is delivered affects the composition of the combustible mixture and its distribution in the combustion chamber, thereby has an impact on the combustion process in the diesel engine. This paper investigates the effects of five different types of injection rate shapes on diesel engine autoignition, combustion, and engine-out emission trends using a three-dimensional computational simulation approach. For this purpose, an n-heptane fuel model is utilized. Initially, a tophat rate-shape, characterized by the constant mass flow rate of the fuel, is assumed to represent the actual injection profile of an actual engine. Then, in order to develop sufficient confidence in the simulation predictions, this assumption together with the calibrated CFD models are validated by reproducing the cylinder gas pressure, the rate of heat release, and engine-out emissions trends for two sets of engine operating conditions. Later, using all the rate shapes the investigation is conducted for one test point considering two different cases of fuel injection: Case 1 - same SOI and duration of injection (DOI), and Case 2 - same combustion phasing and DOI. The results obtained from the computational analysis show that the injection rate shape affects the autoignition, combustion, and emissions of a diesel engine. It is observed that the rate shapes, characterized by high injection rates at the beginning of the injection event, enhance the formation of negative temperature coefficient (NTC) regime. Therefore, the mole fractions of different species are determined during the NTC regime in order to examine the processes relevant to the formation of the NTC regimes for these rate shapes. Further, for the same SOI and DOI case, significant differences in the ignition delays between each rate shapes are observed. The maximum deviation of the ignition delay from the reference tophat is found to be 37%. Furthermore, the paper highlights the differences in the cylinder gas pressure, gas temperature, and rate of heat release due to different fuel delivery rates of different rate shapes. Finally, the comparison of the engine-out emissions for different rate shapes for both the cases of injection are presented and discussed in detail.

Effects of Injection Rate Shape on Performance and Emissions of a Diesel Engine Fuelled by Diesel and Biodiesel B20

“…The values obtained when using diesel fuel are constantly greater than those achieved when using B20 due to the lower heating value, which is higher for diesel fuel than for B20 (41.87 MJ/kg compared to 40.59 MJ/kg). Similar results regarding the influence of the rate of injection shape on the peak in-cylinder temperature when using diesel fuel were reported in [14,15,22]. Figure 9 shows the dependence of peak in-cylinder pressure values on the ROI shape.…”

“…For both fuels, the maximum Soot values are obtained with the TH shape. Similar results on Soot emissions for diesel fuel usage have been reported in [9,10,13,15,22]. Nevertheless, some different results were presented in [7], where, for increased values of injection rate at the beginning of the injection process on certain boot shapes, higher Soot emissions were disclosed.…”

“…The maximum values are reached for these shapes, because the fuel amount is rapidly injected at the end of the compression stroke compared to the other shapes. Comparable results on the peak in-cylinder pressure for diesel fuel were reported in [7,15,18,20,21,23] for biodiesel usage. The increase in thermal efficiency when using triangular shapes is also reported in [18].…”

“…However, this effect is overpassed, and the trend is reversed in the case of the R1 test condition because of the elevated differences existing between the two fuels in terms of peak cylinder temperatures and combustion durations. Comparable results regarding the NO x emissions when using diesel fuel were reported in [9,10,12,13,15,18,21,22]. However, different results are presented in [7], where, for some increases applied at the beginning of the injection rate on certain boot shapes, the decrease of NO x emissions was registered.…”

“…Using computational fluid dynamics (CFD) software, Shrestha et al [15] observed notable differences in the ignition delays among the considered rate shapes with the same start and duration of injection. Using a right-angled triangle shape with greater pressure at the end of the injection event leads to the lowest NO x , but the highest Soot emissions.…”