The ability of premixed low-temperature diesel combustion to deliver low particulate matter (PM) and NO x emissions is dependent on achieving optimal combustion phasing. Small deviations in combustion phasing can shift the combustion to less optimal modes, yielding increased emissions, increased noise, and poor stability. This paper demonstrates how variations in fuel cetane number affect the detailed combustion behaviour of a direct-injection, diesel-fuelled, premixed combustion mode. Testing was conducted under light load conditions on a modern single-cylinder engine, fuelled with a range of ultra-low sulphur fuels with cetane numbers ranging from 42 to 53. Fuel cetane number is found to affect ignition delay and, accordingly, combustion phasing. Gaseous emissions are a function of combustion phasing and exhaust gas recirculation (EGR) quantity, but are not directly tied to fuel cetane number. Fuel cetane number is merely one of many different engine parameters that shift combustion phasing. Furthermore, the operating range is constrained by the changes in cetane number: no injection timings yield acceptable combustion across the whole spread of tested cetane numbers. However, in terms of combustion phasing, the operating range is consistent, independent of fuel cetane number.

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A study of high-efficiency premixed low-temperature combustion (LTC) with dieseline (a blend of diesel and gasoline) was conducted on a high-speed compression ignition engine and strategies to broaden the operational range of this dieseline LTC were investigated. Increased ignition delay and higher volatility of dieseline relative to diesel fuel contribute to produce a well-mixed charge, and a simultaneous reduction in NO x and smoke emissions with a moderate exhaust gas recirculation (EGR) rate can be achieved across a relatively narrow load window. An intake boost strategy was employed to broaden the LTC operational window. Higher intake pressure increases air charge and allows for higher fuelling, so the upper load limit of this LTC mode is extended. An increase in NO x emissions is observed at light to mid loads, but a slight increase in EGR rate can reduce NO x emissions to an acceptable level. Because dieseline requires only a relatively light use of EGR, combustion efficiency across the entire dieseline LTC operational range is maintained at a high level.

Production implementation of premixed low-temperature diesel combustion (LTC) is highly attractive because it can lead to low engine-out particulate matter (PM) and NO x emissions. It is important for LTC strategies to yield acceptable emissions while consuming commercially available fuel, whose properties vary substantially and which contains various additives. The chemical reactions of 2-ethylhexyl nitrate (2-EHN), an additive used to increase the cetane number of a diesel fuel, show potential to increase the NO x emissions of a diesel engine. While 2-EHN has been widely shown to not affect the emissions of conventional diesel combustion, few results are available for LTC, which features different combustion characteristics and significantly lower NO x emissions. This paper demonstrates the effect of using a fuel doped with 2-EHN cetane improver in a direct-injection, diesel-fueled LTC operating mode. Testing was conducted at a light-load condition on a modern single-cylinder engine, fueled with two sets of test fuels at matching cetane numbers (47 and 53), with some test fuels using 2-EHN cetane improver. The addition of 2-EHN to the fuel increases the engine-out NO x for the tested LTC operating condition. Decomposition of the nitrate cetane improver forms NO and NO 2 in significant quantities relative to the low engine-out NO x emissions from LTC.

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