Studies have shown that premixed combustion concepts such as PCCI and RCCI can achieve high efficiencies while maintaining low NO_x and soot emissions. The RCCI (Reactivity Controlled Compression Ignition) concept use blending port-injected high-octane fuel with early direct injected high-cetane fuel to control auto-ignition. This paper describes studies on RCCI combustion using CNG and diesel as the high-octane and high-cetane fuels, respectively. The test was conducted on a heavy-duty single cylinder engine. The influence of injection timing and duration of the diesel injections was examined at 9 bar BMEP and1200 rpm. In addition, experiments were conducted using two different compression ratios, (14 and 17) with different loads and engine speeds. Results show both low NO_x and almost zero soot emissions can be achieved but at the expense of increasing of unburned hydrocarbon emissions which could potentially be removed by catalytic after-treatment. CA50 generally occurred before TDC when using a compression ratio of 17. While the CA50 could be shifted to slightly after TDC by increasing the amount of EGR, this would lead to excessive HC emissions. A lower compression ratio of 14 was therefore used to retard the CA50 while maintaining acceptable UHC emissions.
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