Abnormal combustion in spark ignition engines

Emissions from internal combustion engines are a major contributor to anthropogenic climate change. In order to decrease the amount of emissions, car manufacturers are investing in increasing the efficiency of spark ignition engines. Means for this include downsizing and turbocharging which come with an exacerbated risk of abnormal and harmful combustion phenomena, notably autoignition, knock and pre-ignition and thus pose a limit to the efficiency of the engines. Abnormal combustion depends on the engine geometry, the operating conditions and the fuel. Industrial standard classification systems are outlined to be insufficient, misleading or non-existent for modern engines and fuels. This thesis aims to improve the understanding of the abnormal combustion phenomena through an experimental project which can be utilised for improved classification systems. The vast majority of the experiments were conducted on a variable compression ratio engine which was fitted with modern control, measurement and data acquisition equipment to resemble an industrially-used test engine. In a first study, methods of finding the ideal engine operating point were investigated. Knock was induced in the engine, and knock indicators and limitations of knock are discussed here. Enhanced humidity was passed into the heated air-inlet stream by means of a custom-built humidifying unit. Results showed that both the power output of the engine and the severity of knock were reduced with increased humidity. This was explained by the exclusion of combustible air. A fuel-vaporization unit allowed for experiments with fully vaporized fuel. It could be shown that this had an adverse effect on knock as the cooling effect of the enthalpy of vaporization was removed. A second study employed a temperature-controlled glow plug to induce surface pre-ignition. A range of analysis techniques were tested and discussed which ranged from flame ionization detection to several in-cylinder pressure based methods. A cycle-by-cycle analysis with a maximum pressure method revealed an unexpected trend of surface pre-ignition tendency in sweeps of stoichiometry and fuels, with slightly weak of stoichiometric mixtures being the most susceptible to pre-ignition. Enhanced humidity had a negligible effect on surface pre-ignition under real world conditions. A third study concerned itself with the analysis of knock-induced heat flux, which is both a major cause for damage to the engine and trigger for surface pre-ignition. A heat flux probe was fitted to the engine and results linking heat flux to knock could be obtained on cycle-by-cycle basis and cycleaveraged basis. A linear trend between heat flux and knock intensity was found.