The Influence of Crevice Flows and Blow-By on the Charge Motion and Temperature Profiles Within a Rapid Compression Expansion Machine Used for Chemical Kinetic (HCCI) Studies

The charge dynamics within a Rapid Compression Expansion Machine (RCEM) have been investigated using an integrated computational fluid dynamics / chemical kinetics code, KIVA3V/CHEMKIN. A 0D ring-dynamic model, first developed at MIT, and subsequently modified at UIUC to include circumferential flow past unlubricated rings, was added to the code in order to account for flow into, out of and past the piston's ringpack. Simulations were conducted using two different compression ratios (25:1 and 50:1) for an unreacting (‘motored’) charge and at 38:1 for a reacting (‘fired’) charge, in this case with a lean H₂/air mixture. A 19-step detailed kinetic mechanism was employed for the reacting simulation. The effects of various modeling parameters, including the mesh configuration, ring-dynamic parameters and turbulent/laminar assumptions were explored; the simulation results were compared to experimental data from the RCEM.

It was seen that the integrated model compares well with experimental results for the two ‘motored’ cases and for the ‘fired’ case. The crevice flows and blowby mainly seem to affect the in-cylinder dynamics on the expansion stroke. The most significant issue between the calculations however, seems to be the amount of heat transfer predicted in the simulation. The turbulent law-of-the-wall model results in much greater stratification at TDC, and this can lead to the development of strong radial pressure waves within the cylinder; experimental data shows the wave energy to be distributed more in circumferential and axial modes. The existence of pressure waves within the cylinder can lead to enhanced mixing of the reemerging crevice charge, and this may lead to greater oxidation of the unreacted gases.