Reactivity enhancement of natural gas/diesel RCCI engine by adding ozone species

Abstract

Reactivity controlled compression ignition (RCCI) is an alternative combustion strategy with the potential of significant advantages in terms of increasing thermal efficiency together with reduction of NOx and soot emissions. The RCCI engines have been encountered not to be applicable broadly since they suffer from some difficulties mainly contributed to the controllability of combustion and limited operating ranges. The difficulties become even worse where low-reactivity fuels like natural gas (NG) are applied. Ozone gas, a chemical species with an extreme level of reactivity, can improve the combustion efficiency along with the advancement of combustion phasing in the RCCI engine. In this study a multidimensional computational fluid dynamic (CFD) which is coupled with proper detailed chemical kinetic mechanisms is employed to investigate the influence of ozone addition (i.e., 10, 100 and 1000 ppm) to air-fuel mixture with different initial conditions (e.g., intake temperature and equivalence ratio), on the performance and emissions characteristics of the RCCI engine. The results imply that the addition of even low concentrations of ozone (10 ppm) have considerable influences on the RCCI combustion characteristics. Addition of ozone by a distinct quantity not only can enhance the combustion phasing controllability, but also can extend the operating range of the RCCI engine in both lower intake air temperature and the lower fraction of the high-reactive fuel. From the results it is conceived that by adding 1000 ppm ozone into the air-fuel mixture, it is possible to reduce the diesel fuel fraction from 20% to 10% and intake air temperature from 355 K to 335 K, respectively.