The influence of fuel octane number on the potential for the reduction of fuel consumption in a spark-ignited engine

One of the measures for increasing the thermal efficiency and reducing the fuel consumption of spark-ignition engines is the increase of compression ratio of the engine. Due to the nature of combustion process in a spark-ignition engine, the occurrence of knock presents one of the biggest obstacles in increase of compression ratio. Modern SI engines apply high levels of boost in order to reduce the overall losses. To avoid knock in these conditions the engine manufacturers often have to reduce the compression ratio. Engine knock occurs after the start of main combustion do to auto ignition of hot-spots in the end gas. The main parameters which influence the onset of auto ignition (knock) are the in-cylinder pressure, end gas temperature, end gas composition and the chemical reactivity of the fuel. The tendency of a specific fuel to knock in a fuel-air mixture is expressed with the octane number of the fuel which is determined by a standard procedure in an experimental CFR (Cooperative Fuels Research) engine. Larger octane number of the fuel means that this fuel is less prone to knock. In this paper a potential for change in fuel consumption and thermal efficiency of an engine running with fuels that have different octane ratings is analyzed. The change of thermal efficiency comes from the possibility to run the engine with higher compression ratio when higher octane rating fuel is applied. In this work the following fuels are analyzed: methane, n-heptane and gasoline. For the analysis a 1-D/0-D model of the engine was made in the AVL Boost software package. In the calculations a newly developed multi-zone knock model and a physical combustion model which is able to simulate the cycle-to-cycle variations of combustion was used. The combustion model and the knock model were calibrated with the data from the experimental measurements on the engine.