Evaluating the Heat Losses from HCCI Combustion within a Rapid Compression Expansion Machine

A high pressure capable, free piston rapid compression expansion machine (RCEM) has been used to investigate the autoignition, or Homogeneous Charge Compression Ignition (HCCI) behavior of a wide range of fuels. Thermal efficiencies and emissions characteristics were reported previously, but the heat release rates (HRR) and mass fractions burned (χ) seen under the experimental conditions were not specifically determined. This work investigates the characteristic heat losses in this device for use in determination of the HRR and χ. The heat flux is derived from surface temperature thermocouple data; a spatially-uniform, global convection model is correlated to this. Data from lean n-pentane and n-hexane in air mixtures were used to calibrate the model.

The RCEM-calibrated model was compared to similar models that were calibrated to IC engines operating on HCCI, and to predictions from the CFD code KIVA3V. It was found that the overall heat flux in the RCEM is somewhat less than engine heat fluxes for similar charge strengths; the peak values however seem to correspond. This is probably due to the fact that the in-cylinder charge motion within the RCEM is less intense than in an operating engine due to the absence of the turbulent gas exchange process. Combustion in the RCEM is more uniform and much more rapid, and this initiates resonant acoustic waves which can enhance the wall heat flow. The KIVA3V simulations (of compression + expansion only) yielded significantly greater heat loss (+70%). The overall computed mass burned fractions were compared to emissions-derived combustion efficiencies; significant discrepancies exist between these parameters for the fuel-air charges used and this requires additional investigation.