Effect of crevice mass transfer in a rapid compression machine

Rapid Compression Machines (RCMs) often employ creviced pistons to suppress the formation of the roll-up vortex. However, the use of a creviced piston promotes mass transfer into the crevice when heat release takes place in the main combustion chamber. This multi-dimensional effect is not accounted for in the prevalent volumetric expansion approach for modeling RCMs. The method of crevice containment avoids post-compression mass transfer into the crevice. In order to assess the effect of the crevice mass transfer on ignition in a RCM, experiments are conducted for autoignition of isooctane in a RCM with creviced piston in the temperature range of 680–940 K and at compressed pressures of ∼15.5 and 20.5 bar in two ways. In one situation, post-compression mass transfer to the crevice is avoided by crevice containment and in other it is allowed. Experiments show that the crevice mass transfer can lead to significantly longer ignition delays. Experimental data from both scenarios is modeled using adiabatic volumetric expansion approach and an available kinetic mechanism. The simulated results show less pronounced effect of crevice mass transfer on ignition delay and highlight the deficiency of the volumetric expansion method owing to its inability to describe coupled physico-chemical processes in the presence of heat release. Results indicate that it is important to include crevice mass transfer in the physical model for improved modeling of experimental data from RCMs without crevice containment for consistent interpretation of chemical kinetics. The use of crevice containment, however, avoids the issue of mass transfer altogether and offers an alternative and sound approach.