Modeling the Performance of a Turbo-Charged S.I. Natural Gas Engine With Cooled EGR

A variety of gaseous fuels and wide range of cooled EGR could be used in turbocharged S.I. gas engines. This makes experimental investigation of knocking behavior both unwieldy and uneconomical. Accordingly, it would be attractive to develop suitable effective predictive model that can be used to improve understanding the role of various design and operating parameters and achieve a more optimized turbo-charged engine operation. A two-zone predictive model developed mainly for naturally aspirated S.I. engine applications of natural gas and validated earlier, was extended to consider applications employing turbochargers, after-coolers and cooled EGR. A suitably detailed kinetic scheme involving 155 reaction steps and 39 species for the oxidation of natural gas is employed to examine the pre-ignition reactions of the unburned natural gas-air mixtures that can lead to knock before being fully consumed by the propagating flame. The model predicts the onset of knock and its intensity once end gas auto-ignition occurs and considers the effects of turbo-charging and cooled EGR on the total energy to be released through auto-ignition and its effect on the intensity of the resulting knock. The consequences of changes in the effectiveness of after- and EGR-coolers when fitted, lean operation and reductions in the compression ratio on engine performance parameters, especially the incidence of knock were examined. The benefits, limitations and possible penalties of the application of fuel lean operation combined with cooled EGR are also examined and discussed.