Twin injector biogas diesel RCCI mode-an effective means to reduce NOx emissions without penalty in fuel consumption

Abstract

The RCCI (Reactivity Controlled Charge Compression Ignition) mode of operation of an IC engine using biogas and diesel as fuels has the potential to offer high efficiencies and low NOx and soot emissions. One of the drawbacks while converting conventional diesel engines to run in this mode is the wall wetting of the diesel that has to be injected early in the compression stroke. This leads to high THC emissions and lubricating oil dilution. In this work a twin cylinder turbocharged, common rail diesel engine was modified to accommodate a newly developed Narrow angle Injector (NI) for early injection of diesel without wall wetting, along with the existing Wide angle Injector (WI). Biogas was also injected into the intake manifold and the engine was run in the RCCI mode. The effectiveness of this novel twin injector dual fuel RCCI concept has been experimentally assessed at different biogas energy shares under a BMEP of 3 bar and at a constant engine speed of 1500 rpm. The different biogas – diesel modes considered were the conventional dual fuel mode (CDF), RCCI with the WI alone, RCCI with the NI alone and finally RCCI with the combined NI-WI. In the biogas diesel RCCI mode the use of only the WI resulted in low efficiency, high THC and smoke emissions due to cylinder wall wetting of diesel. Though the use of only NI in this mode could eliminate cylinder wall wetting, the diesel directly hitting the piston bowl resulted in poor performance. However, the combination of NI and WI resulted in high BTE and extremely low levels of NOx emissions and lower THC as compared to the CDF mode. The range of injection timings for the NI was 90° to 45° bTDC while for the WI it was 42° to 4° bTDC when the energy share of biogas was changed. Thus the use of two injectors with wide and narrow injection spray angles for diesel gave enhanced flexibility in operating at widely different injection timings as required in biogas diesel RCCI operation.