Low temperature combustion technologies like RCCI are gaining popularity because of their potential for good performance while promoting the use of renewable energy sources. Conventionally in a biogas diesel RCCI engine biogas is inducted and compressed along with air while diesel is injected very early in the compression stroke. The early injection of diesel using the conventional wide angle fuel injector that is suitable for conventional diesel combustion leads to wall wetting and hence elevates the HC emissions and also lowers the efficiency. This seminar discusses a method to address these issues with the use of a novel twin injector concept for the biogas diesel RCCI mode of operation. The conventional Wide angle injector which is located at the center of the combustion chamber needs to be retained because the engine has to be operated in the conventional mode at low and high loads. Thus, the Narrow angle injector is an additional injector which has to be located at the available space in an offset manner from the Wide injector. The role of the Narrow angle injector is to ensure that the fuel hits the piston surface rather than the liner during early injections in the RCCI mode. A CFD model of the engine that was validated using experimental data was used for the parametric studies reported in this work. The Narrow injector has been first located and oriented in the combustion geometry based on the space constraints in the cylinder head. The number of holes in the injector were fixed as three and their orientations were then determined based on the simulations taking into consideration the benefits of spray targeting. The importance of spray targeting the piston bowl periphery was first established by a comparative study. The Narrow injector was then manufactured with the spray configuration based on the simulation results and experiments were conducted after installing this on the engine. Even though there was improvement in efficiency, the use of NI alone resulted in higher emissions due to the accumulation of fuel on specific regions inside the piston bowl. Considering the difficulties encountered while using the narrow angle injector alone for RCCI operation, splitting the injection between narrow and wide angle injectors seems to be logically having more potential to overcome them. The use of narrow angle injector for early injections helps in mitigating the wall wetting issue and the reduced quantity due to split injection makes it less susceptible to fuel film formation on the piston bowl surface. On the other hand, second injection using wide angle injector with more number of holes and at timings close to TDC facilitates better distribution of fuel which enhances the combustion process.
The effectiveness of this novel twin injector dual fuel RCCI concept has been experimentally assessed at different biogas energy shares at 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. Thus the use of two injectors with wide and narrow injection spray angles for diesel gave enhanced performance in the case of a biogas diesel RCCI engine.