New emission legislations applicable in the near future to sea-going vessels, off-road and off-highway vehicles require drastic nitric oxides emission reduction. A promising approach to achieve part of this decrease is charge air temperature reduction using Miller timing. However, it has been shown in literature that the reduction potential is limited, achieving a minimum in NOx emissions at a certain end-of-compression temperature. Further temperature reduction has shown to increase NOx emissions again. Some studies have shown that this increase is correlated to an increased amount of premixed combustion.
In this work, the effects of pilot injection on engine out NOx emissions for very early intake valve closure (i.e. extreme Miller), high boost pressures and cold end-of-compression in-cylinder conditions are investigated. The experiments are carried out on a 3.96L single cylinder heavy-duty common-rail Diesel engine operating at 1000 rpm and at constant global air-to-fuel ratio. Engine operating conditions with single and pilot injection are compared for a wide range of intake air temperatures and injection timings.
From the experiments, it is shown that at cold TDC conditions, further decreasing intake temperatures for single injection events leads to an increase in exhaust NOx emissions at constant specific fuel consumption. For these points longer ignition delays and greater premixed proportions are observed. Splitting the injection event shows to counteracts this effect. This approach leads to a decrease of NOx emissions with decreasing intake temperatures. Thus, the applicability of extreme Miller valve timing is extended, allowing its use for further reduction of NOx emissions.
