The importance of double bond position and cis–trans isomerisation in diesel combustion and emissions

Future fuels will be developed from a variety of biomass and fossil sources, and this presents an opportunity to design new fuels that carry a much reduced environmental burden. To this end, understanding how the molecular structure of a fuel impacts on the processes of combustion and emissions production is critical in selecting suitable feed-stocks and conversion methods. This paper presents experimental studies carried out on a diesel engine supplied with a range of single-molecule fuels to investigate the effect of fuel molecular structure on combustion and emissions. Four isomers of octene, 1, trans-2, cis-3 and trans-3, were investigated so as to ascertain the way in which the position and arrangement of a double bond within an alkene molecule affects diesel combustion and emissions. The engine tests were carried out at constant injection timing and they were repeated at constant ignition timing and at constant ignition delay, the latter being achieved through the addition of small quantities of ignition improver (2-ethylhexyl nitrate) to the various fuels. The order of ignition delay (shortest first) was found to be 1-octene, cis-3-octene, trans-3-octene and trans-2-octene. The higher reactivity of the cis isomer was attributed to the need of trans isomers to adopt the cis conformation prior to undergoing certain low temperature radical branching reactions. The longer ignition delay of trans-2-octene relative to trans-3-octene suggests a greater net reactivity of the saturated alkyl portions of the latter molecule. At constant injection and constant ignition timings, the combustion phasing and the exhaust level of NOx emitted by each fuel were dictated by the duration of ignition delay. With ignition delay equalised, an effect of increasing particulate emissions with the movement of the double bond towards the centre of the molecule was observed.