Effective engine technologies for optimum efficiency and emission control of the heavy-duty diesel engine

Abstract

The new emissions legislation for the heavy-duty (HD) diesel engine will require cutting NOx emissions by 90% to 0.02 g/bhp.hr, which is heavily relied upon the effective operation of the SCR in the aftertreatment systems (ATS). However, the low exhaust gas temperature (EGT) at low-load operation usually impedes the efficient exhaust emissions reduction by these aftertreatment systems, which require a minimum EGT of approximately 200°C to initiate the emission control operations. In this research, studies have been carried out on the effectiveness and trade-off of the advanced combustion control strategies, such as Miller cycle, internal (iEGR) and external exhaust gas recirculation (eEGR), on the engine efficiency, emissions, and EGT management at low-load operation. Experiments were performed on a single-cylinder HD diesel engine equipped with a high-pressure loop cooled eEGR and a variable valve actuation (VVA) system. The VVA system enables the Miller cycle operation with variable later intake valve closing (LIVC) and produces iEGR via a second intake valve opening (2IVO) event during the exhaust stroke. The results show that common techniques such as the retarded injection timing, intake throttling, iEGR combined with high exhaust back pressure could increase the EGT but at the expense of high fuel consumption and deteriorating the combustion process. In comparison, the Miller cycle operation could increase EGT to more than 200℃ with insignificant impact on the net indicated efficiency (NIE). However, the resulting lower effective compression ratio (ECR) decreased the combustion gas temperature, leading to higher hydrocarbon (HC) and carbon monoxide (CO) emissions. The combined Miller cycle with iEGR helped to reduce CO and HC emissions but this strategy demonstrated a limited NOx emissions reduction, particularly when the injection timing was optimised to achieve the maximum NIE. The introduction of 26%eEGR on the Miller cycle operating with iEGR decreased NOx emissions by 50%, on average, but presented insignificant impact on the NIE and EGT. When introducing a higher eEGR of 44%, the NOx emissions were substantially decreased while increasing the EGT to more than 200℃ with a higher NIE. However, these were attained with an increase in soot emissions. The additional results demonstrated that the optimised Miller cycle operating with iEGR and eEGR of 44% achieved the highest EGT of 225℃ and the lowest NOx emissions of 0.5 g/kWh but with a soot emissions of 0.026 g/kWh. Alternatively, Miller cycle operating with eEGR of 44% and with no iEGR achieved the highest NIE of 43.7% and the lowest total fluid (fuel and urea) consumption of 0.83 kg/h as well as increasing the EGT to 216℃. Meanwhile, the soot and NOx emissions were 2 decreased to below 0.01 g/kWh and 0.79 g/kWh, respectively. Thus, the Miller cycle operating with iEGR and eEGR have been identified as the most effective means of achieving simultaneous higher engine efficiency, lower emissions, and desired EGT, substantially improving the effectiveness of ATS at the low-load operation.