Mitigating Knock in Heavy Duty Spark Ignition Engines : Experiments and simulations of diluted ethanol and methanol combustion

Sammanfattning: To effectively reduce fossil fuel dependence in the transport sector, an unprecedented increase in renewable fuel production is required. Short chain alcohols, such as ethanol and methanol, are well placed as they can be produced in a variety of renewable pathways from most carbon sources. Due to its high autoignition resistance, ethanol and methanol cannot be used as drop-in fuels in compression ignition engines that are prevalent in the heavy duty (HD) transport sector but can be an immense advantage when used in HD spark ignition (SI) engines.One crucial disadvantage experienced by HD SI engines is the end gas autoignition or knock which limits engine load, compression ratio and efficiency. It was not established if ethanol and methanol can in fact achieve the required load range in HD SI engines and if so, how efficient they would be. Diluting the air-fuel mixture with excess air or exhaust gas recirculation can add knock resistance by lowering in-cylinder temperature. Though dilution increases load and efficiency, it also increases instability and ultimately causes misfires. In this thesis, diluted combustion, knock limit and performance of ethanol and methanol was studied using a single cylinder heavy duty research engine. The required load was achieved with relatively good efficiency at lean operation and potential for improving efficiency further was investigated using 1D simulations. The modifications needed to utilize a semi-predictive combustion model in diluted operation were presented. Using simulations, the impact of turbulence on the performance of Miller valve timing and the effect of squish area on piston shapes to improve turbulence was discussed. With Miller timing and fast combustion using high squish pistons, lean burn ethanol and methanol can offer high efficiency, on par with compression ignition engines. If ethanol or methanol production can be scaled up, HD SI engines can provide good performance, low capital and operating cost for future transport.