Walker, N., Dempsey, A., Andrie, M., and Reitz, R., "Use of Low-Pressure Direct-Injection for Reactivity Controlled Compression Ignition (RCCI) Light-Duty Engine Operation," SAE Int. J. Engines 6(2):1222-1237, 2013, doi:10.4271/2013-01-1605.
Reactivity-controlled compression ignition (RCCI) has been shown to be capable of providing improved engine efficiencies coupled with the benefit of low emissions via in-cylinder fuel blending. Much of the previous body of work has studied the benefits of RCCI operation using high injection pressures (e.g., 500 bar or greater) with common rail injection (CRI) hardware. However, low-pressure fueling technology is capable of providing significant cost savings. Due to the broad market adoption of gasoline direct injection (GDI) fueling systems, a market-type prototype GDI injector was selected for this study. Single-cylinder light-duty engine experiments were undertaken to examine the performance and emissions characteristics of the RCCI combustion strategy with low-pressure GDI technology and compared against high injection pressure RCCI operation. Gasoline and diesel were used as the low-reactivity and high-reactivity fuels, respectively. GDI injection pressures range from 150 to 200 bar, while the CRI pressures range from 250 to 500 bar. Start of injection (SOI) timings ranged from -35° aTDC and -115° aTDC. The experimental results show comparable engine performance and emissions output, but with slight reductions in overall combustion efficiency when using low-pressure fueling with the stock re-entrant piston. CFD simulations were also performed to aid in visualization of the in-cylinder fuel distributions, which are controlling factors for RCCI combustion. By utilizing an optimized RCCI piston geometry, equivalent RCCI combustion performance can be achieved under low-pressure fueling, at moderate and high loads. The optimized geometry also allows for significant increases in thermal efficiency, with peak efficiencies over 47% observed.