Exploring the Potential of Miller Cycle with and without EGR for Maximum Efficiency and Minimum Exhaust Emissions in a Heavy-Duty Diesel Engine

In order to improve the fuel conversion efficiency and meet more stringent exhaust emissions regulations, Miller cycle and exhaust gas recirculation (EGR) have been researched as separate means to reduce carbon dioxide (CO₂) and pollutant emissions from the internal combustion engines. In this article, an experimental work was carried out to explore the potential benefits of Miller cycle operation via late intake valve closing (LIVC) with and without EGR in a single-cylinder heavy-duty (HD) diesel engine equipped with a variable valve actuation (VVA) system. The overall engine-out emissions, fuel conversion efficiency, and estimated urea consumption in the selective catalytic reduction (SCR) aftertreatment were analysed and compared over the World Harmonized Stationary Cycle (WHSC) for different combustion control strategies. Additionally, the potential of Miller cycle with and without EGR based on the “SCR-only” and “SCR + EGR” technical routes to meet the Euro VI nitrogen oxides (NO_x) limit of 0.4 g/kWh was assessed at different NO_x aftertreatment efficiencies.

When considering the urea consumption in the SCR, the results showed that the introduction of EGR allowed for an engine operation with higher corrected net indicated efficiency (NIE_corr.) or lower specific total fluid consumption than the baseline cases without EGR due to the relatively lower engine-out NO_x emissions. However, the use of EGR adversely affected soot and carbon monoxide (CO) emissions when operating with constant intake pressure (P_int) of the baseline case. The application of Miller cycle with and without EGR strategies decreased the NIE and NIE_corr. when operating with the same P_int of the baseline operation. The use of higher P_int helped to improve upon the NIE and NIE_corr. of the Miller cycle cases. The WHSC cycle-averaged analysis showed that different combustion and engine control technologies can be adopted with and without EGR to meet Euro VI NO_x limit. A conventional baseline engine operation without EGR would require a high SCR efficiency of 96% in order to curb a cycle-averaged NO_x emissions level of 10 g/kWh. Miller cycle operation without EGR achieved the optimum NIE_corr. at the cycle-averaged NO_x level of 8 g/kWh. When increasing the P_int, this strategy enabled an increase of 2.6% in the NIE_corr. and reduced the required SCR efficiency to 93.5%, but with a penalty on the NIE of 3.3% when controlling the cycle-averaged NO_x level at 6.5 g/kWh. Alternatively, Miller cycle operation with EGR and higher P_int allowed for cycle-averaged NO_x levels of 4.0 g/kWh, decreased the total fluid consumption by 8%, and minimised the required SCR efficiency to 90%. Therefore, this study has presented promising cost-effective emission control and fuel efficiency technologies that could be suitable for the “SCR-only” and “SCR + EGR” technical routes for the future HD diesel engines.