Residual Gas Fraction Measurement and Estimation of the CFR Octane Rating Engine Operating Under HCCI Conditions

The autoignition chemistry of fuels depends on the pressure, temperature, and time history that the fuel-air mixture experiences during the compression stroke. While piezoelectric pressure transducers offer excellent means of pressure measurement, temperature measurements are not commonly available and must be estimated. Even if the pressure and temperature at the intake and exhaust ports are measured, the residual gas fraction (RGF) within the combustion chamber requires estimation and greatly impacts the temperature of the fresh charge at intake valve closing. This work replaced the standard D1 Detonation Pickup of a CFR engine with a rapid sampling valve to allow for in-cylinder gas sampling at defined crank-angle times during the compression stroke. The extracted cylinder contents were captured in an emissions sample bag and its composition was subsequently analyzed in an AVL i60 emissions bench. Carbon dioxide levels beyond atmospheric concentration directly identified the existing RGF within the combustion chamber during the compression stroke. The CFR engine was operated under homogeneous charge compression ignition using iso-octane/n-heptane Primary Reference Fuels with seven blends of 0-100% iso-octane to measure the RGF as a function of compression ratio over a range from 6:1 to 17.5:1. Furthermore, a modified compressed air intake manifold enabled evaluation of intake pressure and intake temperature effects on the RGF. The measurements were shown to compare correctly with expected trends as a function of compression ratio and clearance volume. A model was developed to successfully estimate the RGF of a fresh charge based on intake and exhaust pressures and temperatures, as well as the compression ratio. This model showed significantly improved correlations over literature RGF models, such as simple geometric compression ratio models based on clearance volume and displacement.