A study on effect of engine operating parameters on NOx emissions and exhaust temperatures of a heavy-duty diesel engine during idling

Though NOx emissions from on-road heavy duty vehicles (HDVs) have reduced drastically over the past three decades, upcoming North American regulations may call for further reductions. Lowering NOx emissions below current levels is challenging, especially at lower operating loads such as idling and/or colder climates when the exhaust gas temperature is insufficient for high NOx conversion in the HDV’s exhaust after treatment (EAT) system. This paper investigates the impact of various engine operating parameters on the three-way trade-off between NOx emissions, exhaust temperature, and fuel consumption using a single-cylinder, heavy-duty diesel engine outfitted with a conventional EAT system and diesel exhaust fluid (DEF) injection. The engine parameters investigated include exhaust gas recirculation (EGR) ratio, intake pressure, intake temperature, injection pressure, multiple injections, and engine fluid temperatures. In order to simulate cold climate operation, the baseline intake air, coolant, and lubricating oil temperatures are maintained at 10°C, 40°C, and 50°C, respectively. Results show that using a moderate level of EGR has the dual benefit of reducing the engine-out NOx and increasing the intake charge temperature. Increasing the intake or injection pressure does not result in significant benefit in improving the three-way trade-off. However, using double injection with the second injection at the start of the expansion stroke can help reduce NOx emissions and increase exhaust temperature with marginal impact on fuel efficiency. Based on the parametric study, three particular engine operating conditions are chosen for further investigation with the EAT online. At an identical DEF dosing rate, the rates of NOx reduction in the SCR are similar for the three conditions (∼96%), and the lowest tail-pipe NOx observed for this study is 0.032 g/kWh.