Effect of fuel cetane number on a premixed diesel combustion mode A M Ickes*, S V Bohac, and D N Assanis Walter E. Lay Automotive Laboratory, Ann Arbor, Michigan, USA The manuscript was accepted after revision for publication on 27 May 2009. DOI: 10.1243/14680874JER03809 Abstract: 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. Keywords: premixed diesel combustion, cetane number, diesel, emissions, NO x 1 INTRODUCTION 1.1 Premixed diesel combustion implementation concerns The motivation for the development of premixed low-temperature diesel combustion is widely under- stood – this style of combustion yields low engine- out nitrogen oxides (NO x ) and particulate emissions. With the increasingly strict emissions regulations, minimizing engine-out emissions with premixed low-temperature combustion becomes highly attrac- tive. Implementing this combustion mode in future vehicles is therefore of great interest. Substantial research has been conducted on developing differ- ent strategies to achieve premixed low-temperature diesel combustion, and the results have been widely documented [1–3]. One of the constants in many prior premixed diesel combustion studies is the fuel. However, if premixed diesel combustion strategies are imple- mented into production vehicles, the fuel will certainly not be a constant: production vehicle engines have to operate on any and all diesel fuels available from the pump, and their properties can vary substantially. Surveys conducted in 2003 found that the cetane number of diesel fuels available at fuelling stations in the United States ranged any- where from around 40 to the mid 50s, with an average value of approximately 46 [4]. A 15-point variation in cetane number represents a significant variation in fuel ignition behaviour. With the wide cetane number range of diesel fuels available in the field, it becomes critical to under- stand how low-temperature premixed diesel com- bustion strategies respond to changes in fuel proper- ties, especially cetane number, for production implementation. Additionally, optimizing an engine for one fuel specification may not, and probably will not, give optimum performance when the fuel is *Corresponding author: 1090 Walter E. Lay Automotive Labora- tory, 1231 Beal Avenue, Ann Arbor, MI 48109, USA. email: aickes@umich.edu 251 JER03809 Int. J. Engine Res. Vol. 10