Diffusion- and Homogeneous-Charge Combustion of Volatile Ethers in a Compression Ignition Engine

摘要

The combustion characteristics of several volatile ether molecules were studied under diffusion and homogeneous charge combustion modes in a compression ignition engine. Volatile ethers of low molecular mass are organic molecules that could be used as fuel for compression ignition engines. The physical and chemical characteristics of such ethers comprise high oxygen content, high volatility, and low viscosity, all of which are conducive to high thermal efficiencies and low pollutant emission during engine operation. It is thought that sootless combustion may be achieved in diffusion flames of high-speed direct injection diesel engines with some volatile ethers, due to their high oxygen content. The formation of oxides of nitrogen from the combustion of ethers may be almost eliminated by the use of lean, homogeneous charge compression ignition combustion, for which these fuel molecules are particularly suitable due to their high volatility. The first part of the experiments examines the combustion characteristics of dimethyl ether under diesel engine diffusion combustion conditions. Heat release of combustion, engine efficiency, gaseous pollutant, and nanoparticle emissions were measured and results were compared with those of conventional diesel fuel at comparable engine operating conditions. It was observed that within the accuracy of the instruments, soot-free combustion could be achieved with pure dimethyl ether in diffusion combustion. The addition of an alkyl ester lubricity-improving additive to dimethyl ether was observed to result in the emission of small nucleation mode nanoparticles (5-40 nm diameter) in the exhaust gas under certain conditions. The experiments suggested that these particles may consist of droplets of the fuel lubricity additive and engine oil forming through condensation. The second part of the experiments investigates the potential of using volatile ethers in direct-injected homogeneous charge compression ignition combustion. The molecular structure of the fuel was observed to have an important influence on the ignition process and heat release characteristics combustion. Several straight-chained and branched ether molecules were used for homogeneous charge compression ignition combustion. The experiments showed that intentional blending of the fuel molecules could be used to achieve homogeneous charge compression ignition combustion at high thermal efficiency, and that the ignition timing of the homogeneous ether-air charge within the cycle could be controlled via the molecular composition of the fuel.