Midhat Talibi

An overview of the effects of fuel molecular structure on the combustion and emissions characteristics of compression ignition engines

Future fuels for compression ignition engines will be required both to reduce the anthropogenic carbon dioxide emissions from fossil sources and to contribute to the reductions in the exhaust levels of pollutants, such as nitrogen oxides and particulate matter. Via various processes of biological, chemical and physical conversion, feedstocks such as lignocellulosic biomass and photosynthetic micro-organisms will yield a wide variety of potential fuel molecules. Furthermore, modification of the production processes may allow the targeted manufacture of fuels of specific molecular structure. This paper therefore presents an overview of the effects of fuel molecular structure on the combustion and emissions characteristics of compression ignition engines, highlighting in particular the submolecular features common to a variety of potential fuels. An increase in the straight-chain length of the alkyl moiety reduces the duration of ignition delay, and the introduction of double bonds or branching to an alkyl moiety both increase ignition delay. The movement of a double bond towards the centre of an alkyl chain, or the addition of oxygen to a molecule, can both increase and decrease the duration of ignition delay dependent on the overall fuel structure. Nitrogen oxide emissions are primarily influenced by the duration of fuel ignition delay, but in the case of hydrogen and methane pilot-ignited premixed combustion arise only at flame temperatures sufficiently high for thermal production. An increase in aromatic ring number and physical properties such as the fuel boiling point increase particulate matter emissions at constant combustion phasing.

Development of a Fast-Acting, Time-Resolved Gas Sampling System for Combustion and Fuels Analysis

Development of new fuels and engine combustion strategies for future ultra-low emission engines requires a greater level of insight into the process of emissions formation than is afforded by the approach of engine exhaust measurement. The paper describes the development of an in-cylinder gas sampling system consisting of a fast-acting, percussion-based, poppet-type sampling valve, and a heated dilution tunnel; and the deployment of the system in a single cylinder engine. A control system was also developed for the sampling valve to allow gas samples to be extracted from the engine cylinder during combustion, at any desired crank angle in the engine cycle, while the valve motion was continuously monitored using a proximity sensor. The gas sampling system was utilised on a direct injection diesel engine co-combusting a range of hydrogen-diesel fuel and methane-diesel fuel mixtures. In-cylinder gas sample composition was investigated at two sampling locations; within the diesel fuel spray and between adjacent spray cones. Concentrations of NOx were found to be higher between the two diesel sprays relative to within the spray cone for both hydrogen and methane addition. In the case of hydrogen-diesel fuel co-combustion, the particulate levels were observed to be higher in the diesel fuel spray relative to between two sprays; however, in the case of methane-diesel fuel co-combustion, higher particulate levels were measured in the region between the two sprays. This was attributed to methane contributing significant quantities of particulates to the total particulate concentration produced from the methane-diesel fuel mixture in between two sprays.