The influence of residual gas on ozone-assisted combustion in an HCCI reciprocating engine

This work is an investigation on the influence of both residual gas and ozone on the combustion process in an HCCI engine fuelled with iso-octane/air/ozone mixtures and on how ozone reacts with residual gases. It is well known that ozone releases oxygen atoms mainly through the O3 + M <-> O2 + O + M reaction and this may lead to an improvement of the engine combustion. Nevertheless, the chemical interaction between ozone and the species in the residual gas has not yet been investigated in the literature. CFD simulations of a closed-valve engine cycle have been carried out by using an axisymmetric domain. The model has been validated against experimental data and a parametric analysis has been carried out to assess different compositions and amounts of residual gases on the engine performance. The results show that during compression ozone enables Low-Temperature Combustion, with a small amount of heat released before the occurrence of auto-ignition. The combustion is faster and advanced with a reduction of UHCs and an increase of NOx. The results show that the influence of ozone is more effective as the mass of residual gases increases. With an ozone concentration of 35 ppm at IVC the gross work per cycle increases by 2.52 %, 3.58 % and 5.7 %, compared to the case without ozone, with 0 %, 5 % and 10 % by mass of residual gases, respectively. It can be concluded that ozone is more beneficial for cases where combustion is slower due to the higher amount of residual gases.