Abstract
Lignocellulosic materials have been identified as potential carbon–neutral sources of sustainable power production. Catalytic conversion of lignocellulosic biomass results in liquid fuels with a variety of aromatic molecules. This paper investigates the combustion characteristics and exhaust emissions of a series of alkylbenzenes, of varying number of methyl branches on the monocyclic aromatic ring, when combusted in a direct injection, single cylinder, compression-ignition engine. In addition, benzaldehyde (an aldehyde (-CHO) branch on the monocyclic ring) was also tested. All the molecules were blended with heptane in different proportions, up to 60% wt/wt. The tests were conducted at a constant engine speed of 1200 rpm, a fixed engine load 4 bar IMEP, and at two injection modes: constant start of fuel injection at 10 CAD BTDC, and varying fuel injection timing to maintain constant start of fuel combustion at TDC.The results showed that the ignition delay period increased with increasing number of methyl branches on the ring, due to the rapid consumption of OH radicals by the alkylbenzenes for oxidation to stable benzyl radicals. Peak heat release rates, and concurrently NOx emissions, initially increased with increasing methyl branches, but subsequently both decreased as the bulk of heat release occurred further into the expansion stroke with significant thermal energy losses. With the exception of toluene, the number of particles in the engine exhaust increased as the number of methyl branches on the aromatic ring increased, attributable to the formation of thermally stable benzyl radicals.
Highlights
There is international consensus on the adverse changes in global climate arising from anthropogenic emissions of carbon (CO2, CH4, particulates) from fossil fuel source [1,2]
The same trend of increasing ignition delay with increasing aromatic proportion can be seen in Fig. 4b when the combustion phasing is altered so that start of combustion ignition (SOC) occurs at engine TDC
It can be concluded that the ignition delay increases when the number of methyl branches in the binary mixture increases, whether that is by increasing the proportion of a given alkylbenzene in the binary mixture, or by having a alkylbenzene with a higher number of methyl branches
Summary
There is international consensus on the adverse changes in global climate arising from anthropogenic emissions of carbon (CO2, CH4, particulates) from fossil fuel source [1,2]. This concern has resulted in legislative targets in many countries for the use of renewable alternatives in road transport fuels [3,4,5]. There is growing unease regarding the use of fuels primarily derived from food crops. There is a need to develop future fuels from sustainable non-food crop sources, which produce lower toxic pollutant emissions
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