The potential of dissociated methanol as a fuel for spark ignition engines

Abstract

This thesis examines the potential of dissociated methanol to increase the thermal efficiency or reduce the exhaust emissions of an internal combustion engine. It is assumed that liquid methanol will be dissociated onboard a vehicle using engine waste heat to produce a gas consisting of hydrogen and carbon monoxide in a molar ratio of 2:1. Tests were conducted on a single cylinder engine using liquid, vaporised and dissociated methanol fuels. The dissociated methanol was derived from bottled hydrogen and carbon monoxide. Indicated thermal efficiency together with methane, methanol and formaldehyde exhaust emissions were measured. The effect of the carbon monoxide in the dissociated methanol on efficiency was investigated by operating the engine on both hydrogen and carbon monoxide separately. The results for thermal efficiency showed that the presence of carbon monoxide resulted in a lower efficiency than for pure hydrogen. However, if the waste heat recovered in the dissociation reaction is not included in the calorific value of the fuel, then dissociated methanol offers a significant improvement in thermal efficiency compared to liquid methanol. Vaporised methanol offers efficiencies comparable to dissociated methanol for mixtures leaner than stoichiometric, again benefitting from the recovered heat. The results for exhaust emissions showed that methanol and formaldehyde emissions were effectively eliminated and methane emissions significantly reduced with dissociated methanol fueling. Vaporised methanol fueling reduced emissions of these species to approximately ⅓ of the value with liquid methanol fueling. NOx emissions may be expected to increase for both vaporised and dissociated methanol. Preliminary design considerations indicated that there is probably insufficient high temperature energy in the exhaust gas to dissociate all the engine's fuel requirement. As a result, it was concluded that while vaporised methanol could not match the increase in efficiency or the reduction in exhaust emissions of dissociated methanol, its greater practicability for onboard implementation probably makes it a better candidate for future development.