The simultaneous reduction of engine-out nitrogen oxide (NOx) and particulate emissions via low-temperature combustion (LTC) strategies for compression-ignition engines is generally achieved via the use of high levels of exhaust gas recirculation (EGR). High EGR rates not only result in a drastic reduction of combustion temperatures to mitigate thermal NOx formation but also increases the level of pre-mixing thereby limiting particulate (soot) formation. However, highly pre-mixed combustion strategies such as LTC are usually limited at higher loads by excessively high heat release rates leading to unacceptable levels of combustion noise and particulate emissions. Further increasing the level of charge dilution (via EGR) can help to reduce combustion noise but maximum EGR rates are ultimately restricted by turbocharger and EGR path technologies. Furthermore, at low loads, the low combustion temperatures lead to high levels of unburned hydrocarbon (HC) and carbon monoxide (CO) emissions. There is therefore a significant interest in trying to limit the EGR rates to the minimum necessary levels in order to achieve LTC (
i.e.
low NOx and particulate emissions) by using alternative strategies to control combustion to address the HC/CO emissions at low load and the noise/particulates trade-off at higher loads. The present study investigates the potential of controlling combustion directly via fuel injection with the use of advanced injection strategies such as rate-shaping and/or closely-spaced double injections.
Experiments were conducted on a single-cylinder all-metal engine and an equivalent optically-accessible version of the same engine. The optical engine allowed for the application of several laser-based, optical diagnostic techniques in order to study the in-cylinder processes of mixture preparation, combustion and emissions. Overall, the results of this study highlight the potential of controlling LTC directly via fuel injection using advanced injection strategies which could ultimately reduce the dependency on excessively high EGR rates in order to address the HC/CO emissions at low load and the emissions/noise/fuel economy trade-off at higher load LTC operating conditions without penalising fuel consumption.
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