A free piston, rapid compression expansion machine (RCEM) has been used previously to investigate the autoignition, or homogeneous charge compression ignition (HCCI) behavior of a wide range of gaseous and liquid hydrocarbon fuels [1]. Reported data were the efficiency and emissions characteristics of these fuels, where the compression ratios required to achieve autoignition ranged from 16:1 to 50:1. The purpose was to assess the potential of HCCI as a driver for a high efficiency, low emissions futuristic powerplant. In [1] no attempt was made to study the ignition temperatures of the fuel-air mixtures, or the rates and extent of combustion seen under the experimental conditions. These however are important metrics by which kinetic and other combustion models can be validated, and would also provide a means by which the RCEM data can be compared to rapid compression machine (RCM) data. The purpose of this work was to develop a method to calculate these parameters. A new crevice-ring model was employed for the RCEM ringpack. Calculations are presented for both uncombusted and combusted data where the ring model is demonstrated, and the autoignition characteristics of natural gas in this device are quantified.

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