DME Combustion in Heavy Duty Diesel Engines

Doktorsavhandling, 2011

Interest in alternative energy and fuels has grown considerably in recent years due to increasing concern about the adverse effects of global warming, the limited sources of non-renewable feedstocks, the insecurity of their supply and associated fluctuations in their prices. However, although there is an urgent, and widely recognized, need to address these issues, selecting suitable candidate feedstocks and fuels is far from straightforward, since numerous factors must be considered when selecting promising candidates for further evaluation and, ultimately, commercial use. This thesis addresses the potential use of one alternative fuel for Diesel engines that is renewable (when produced from biomass): dimethyl ether, DME, which is a promising candidate mainly because of its excellent life cycle efficiency and low carbon dioxide footprint (due to its renewability). A combustion system designed to use Diesel fuel is not suitable for combusting DME efficiently, especially not at high levels of exhaust gas recirculation (EGR), which is used to control emissions of nitrogen oxides (NOX). Hence, the aims of the work this thesis is based upon were to: (i) elucidate the requirements of a combustion system using DME that enables faster combustion, providing higher efficiency while yielding lower emissions, and (ii) implement appropriate modifications to the test engine. This was done by adapting the engine’s combustion system to the intrinsic characteristics of the fuel. Piston and nozzle geometries were selected as the most important and relevant hardware parameters for achieving this aim. These were re-designed and the effects of the re-configurations were evaluated in combustion computational fluid dynamics (CFD) analyses, heavy duty single-cylinder engine experiments and spray chamber studies. Combusting DME in a Diesel engine has faced challenges of poor efficiency, hardware considerations by peak cylinder pressures and exhaust gas temperatures, and high emissions of carbon monoxide (CO) as large amounts of EGR have been used to suppress NOX formation. However, these challenges can be solved by appropriate adjustments of the combustion system, which can be summarized as follows. First, the piston bowl geometry should enable greater tangential and radial spreading of the flames, thereby exposing a larger flame area to the surrounding air (and combustion products). Second, the included angle should allow for sufficiently large distances between the spray axis and piston bowl, and optimal spreading of the sprays/flames when interacting with the bowl. Third, the flow should be lower than flows historically used, to enhance mixing and thereby limit the extent of very rich zones impinging on the piston bowl. Fourth, the number of orifices should be correlated to piston bowl diameter to balance the advantages of increasing the total spray area and the disadvantage of reducing their spatial distribution in the bowl. Fifth, the injection pressure should be increased to promote mixing and thereby reduce CO emissions. In conclusion, it has been shown that a combustion system for DME can be considerably improved by using a piston design that promotes flame spread; a nozzle configuration that improves mixing late during the diffusion combustion; and raising the injection pressure. These modifications result in significantly reduced exhaust emissions and increased engine efficiency.