HCCI Octane Number Scale in a Pressure-Temperature Diagram - A Tool for Investigating Combustion Behavior of Practical Fuels 2019-01-0965

Spark ignition (SI) engines have been encountering a dilemma between achieving high efficiencies and avoiding knock, which is a fuel-dependent phenomenon attributed to the autoignition of the unburned end gases ahead of flame propagation. Knock resistance is best represented by the Research Octane Number (RON) and Motor Octane Number (MON). However, since RON and MON represent a limited range of operating conditions, describing fuel combustion behavior out of this range requires additional metric tools to capture a whole spectrum of operating conditions. Recently, the pressure-temperature diagram was introduced as a powerful tool to widen the range of operating conditions represented by RON and MON. In the present study, pure isooctane and pure n-heptane together with Primary Reference Fuels (PRFs), binary mixtures of isooctane and n-heptane, were tested in Homogeneous Charge Compression Ignition (HCCI) mode using a variable compression ratio CFR engine operating in lean conditions. This study aims to find the autoignition pressure-temperature conditions with respect to the octane number. PRFs are common simple surrogates used to model gasoline combustion behavior as a much simpler fuel in research and experiments. However, a practical fuel can be represented by a PRF in only a specific range of intake conditions due to fuel sensitivity. Hence, it is necessary to represent the behavior of a given practical fuel with several PRFs and each of which shall represent only a range of intake conditions. The study aims to develop an octane number scale into the pressure-temperature diagram. To an end, the octane number of a given practical fuel will be provided by comparing its combustion behavior with that of PRFs into the same pressure-temperature diagram. To represent a wide range of operating conditions, sweeps of intake temperature for PRFs and selected practical fuel will be conducted.