Ignition Delays of Different Homogeneous Fuel-air Mixtures in a Rapid Compression Expansion Machine and Comparison with a 3-Stage-ignition Model Parameterized on Shock Tube Data 2013-01-2625

An optically accessible Rapid Compression Expansion Machine (RCEM) has been used to investigate the homogeneous auto-ignition of five candidate fuels for Homogenous Charge Compression Ignition (HCCI) combustion. Two technical fuels (Naphthas) and three primary reference fuels (PRF), (n-heptane, PRF25 and PRF50) were examined. The Cetane Numbers (CN) of the fuels range from 35 to 56. The PRF25 and PRF50 were selected in order to approximately match the CN of the two Naphthas. Variation of the operating parameters has been performed, in regard to initial charge temperature of 383, 408, and 433K, exhaust gas recirculation (EGR) rate of 0%, 25%, and 50%, and equivalence ratio of 0.29, 0.38, 0.4, 0.53, 0.57, and 0.8. Pressure indication measurements, OH-chemiluminescence imaging, and passive spectroscopy were simultaneously implemented.

Experiments with n-heptane have also been examined against homogeneous, variable-volume perfectly stirred reactor (PSR) simulations with various chemical mechanisms. The experimental ignition delay has been compared to the computations of an empirical three-stage, Arrhenius-type ignition delay model. The model has been parameterized on shock tube data for the n-heptane and Naphtha fuels. For the PRF25 and RPF50 the parameterizations of the Naphtha 2 and Naphtha 1 respectively have been used.

Very good agreement has been observed between the 3-Arrhenius empirical model and the experimental ignition delay data, for the fuels that have been parameterized (n-heptane and Naphthas) based on shock-tube data. Fuels that have not been explicitly parameterized, exhibit fairly good agreement as well. The empirical model, utilized through a knock integral, is found to be applicable in the transient environment of the RCEM. The potential of the RCEM as a device for fast evaluation of the homogeneous fuel-air mixtures auto-ignition delay is discussed.