High Power Output Operation of RCCI Combustion

Lim, J. H. High Power Output Operation of RCCI Combustion. University of Wisconsin-Madison, 2014.

A computational investigation of methods to extend the upper limit of power output of reactivity controlled compression ignition (RCCI) engines was performed. The study utilized two approaches. The first approach is to increase the engine speed while maintaining a medium load. The second approach is to operate at higher loads without changing the engine speed. Iso-octane and n-heptane were used to represent the low-reactivity fuel and high-reactivity fuel, respectively.

A light-duty diesel engine was modeled for the high speed dual-fuel RCCI combustion study. With high-speed operation several benefits were identified. Firstly, the peak pressure rise rates (PPRR), both crank angle-based and time-based, were reduced compared to those with low-speed operation. Secondly, at high speed the NO formation residence time became short, leading to reduced NOx emissions. Lastly, a frictional penalty analysis of high-speed operation using the Chen-Flynn model was conducted, which showed only 0.5 bar FMEP increase compared to that at low-speed. These findings indicate that high-speed RCCI is a very promising path for high-power output operation.

For the high-load operation study use of dual direct-injectors was explored in order to direct-inject both fuels. Analysis of the optimum injection strategy revealed two main physical mechanisms enabling high-load operation with dual direct-injectors. The first exploited local evaporative cooling from the iso-octane injection, which delayed the iso-octane ignition. The second mechanism was related to the shorter chemical residence time of the iso-octane due to its late delivery into the cylinder. It was also noted that n-heptane’s role as an ignition source could not be achieved with just iso-octane.

Finally, the co-axial injector location assumption was removed by using an actual dual-injector layout. Unlike results with the co-axial injector design, the actual dual-injector layout exhibited soot and CO emission problems. In order to attempt to accommodate off-center injector locations, various injector hole patterns were tested. Although these unconventional injector hole patterns improved the emissions, it is concluded that the development of a co-axial dual-fuel injector is imperative in order to achieve clean RCCI combustion at high load.