Upcoming emission limits such as Euro VII will make it necessary to further reduce the NOx emission level of internal combustion engines while stricter CO2 limits demand lower fuel consumption. Early closing of the intake valves (Miller timing) leads to reduced combustion temperatures due to lower effective compression ratio, and therefore lower formation and emission of nitrogen oxides. Miller timing is frequently used in gasoline engines, while in Diesel engines it competes with exhaust gas recirculation (EGR). When both measures are applied simultaneously, this may lead to increased emission of soot using standard Diesel fuel, as combustion temperature and oxygen content of the charge become too low. This work shows the investigation of different intake valve timings on an externally supercharged single-cylinder heavy-duty Diesel engine, stationary operated with hydrogenated vegetable oil (HVO), oxymethylene ether (OME), and standard Diesel fuel (DF). The synthetic fuels have a higher cetane number than DF, which supports ignition at lower temperatures. Moreover, OME has a soot-free combustion, which allows an extension of the operating limits without increased emissions. The results show that especially with Miller timing a high-performance turbocharging system is crucial, since higher boost pressure is required to compensate for the filling losses due to the earlier intake closing. The application of a high EGR rate is limited in this case, leading to a trade-off between Miller timing and EGR. All fuels show a reduction in nitrogen oxides of up to 40% with an improved efficiency of more than 3% at a typical road-load point. Measures to reduce ignition delay were found to be necessary, especially for DF. For OME, increased soot formation does not occur when combining Miller timing with low rail pressure, reduced boost pressure or EGR, which promotes simultaneous application of the measures resulting in minimized emissions of nitrogen oxides.

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The use of CO2-neutral synthetic fuels in internal combustion engines contributes to achieving climate targets. Since their combustion is still associated with pollutant emissions, current research is focusing on minimizing harmful emissions, as well as on improving the efficiency of the combustion. The soot-free combustion of the synthetic diesel fuel oxymethylene ether (OME) opens up new scopes for reducing the remaining pollutants, particularly nitrogen oxides (NOX). Catalytic aftertreatment of NOX is costly and may lead to the generation of further emissions such as nitrous oxide, making it necessary to further investigate engine-internal approaches of NOX reduction. This work shows the influence of various in-engine measures on emissions and efficiency for OME and hydrogenated vegetable oil (HVO) as paraffinic diesel fuel, performed on a 1.75 l single-cylinder research engine. An injector variation was carried out for OME to compensate for the reduced lower heating value by a higher nozzle flow rate, which increases efficiency, but also NOX emissions. The examination of the measures of increasing exhaust gas recirculation, lowering rail pressure, Miller valve timing and high compression ratio shows a significant reduction in nitrogen oxide emissions in each case. At the same time, there is an improvement in indicated efficiency for Miller valve timing and high compression ratio, and with OME, in contrast to HVO, also by reducing the rail pressure. With HVO, each measure increases particulate number by up to several orders of magnitude. For OME, none of the measures resulted in a deterioration of the low particulate emissions, which allows an intensified application and combination of the measures. For example, the simultaneous application of higher compression, early intake closing and decreased injection pressure reduces nitrogen oxide emissions by more than two-thirds and improves efficiency by 5% without increasing particulate emissions.