A novel twin injector concept for the effective utilization of biogas along with diesel in the RCCI mode, its implementation and its potential benefits were discussed in Seminar-1. The effectiveness of this concept was experimentally assessed at different biogas energy shares at a BMEP of 3 bar and at a constant engine speed of 1500 rpm. In this second seminar, the performance and emissions of the same engine at higher loads corresponding to BMEPs of 5 bar and 7 bar are first discussed. Comparisons are also made with respect to Conventional Dual Fuel (CDF) and Reactivity Controlled Compression Ignition (RCCI) modes as regards performance, emissions and combustion.
Though, biogas is a renewable fuel it is generally used in combination with diesel in the CDF or the RCCI modes for stationary power generation. This seminar further discusses the experimental work done to run an RCCI engine mainly on biogas but along with another renewable fuel instead of diesel namely DME. For this the engine was run on biogas inducted along with air while Dimethyl Ether (DME) that has a high cetane number and can also be produced from biogas was injected along with air and used instead of diesel in order to enhance the reactivity of the charge. This led to operation of the RCCI engine on completely renewable fuels. The effect of varying the concentration of methane from a low level of 45% (representing low quality biogas) to about 95% (representing natural gas) was studied experimentally. Comparisons of performance, combustion and emission parameters have been made on the same engine while being operated in the DME-Biogas-RCCI, Diesel-Biogas-CDF and Diesel-Biogas-RCCI modes. At both the BMEPs of 3 and 5 bar the DME Biogas-RCCI mode resulted in the highest brake thermal efficiency (BTE) while also exhibiting ultra-low NOx and zero soot emissions at all the biogas compositions. Two stage heat release that resulted in proper combustion phasing was observed in the DME-biogas-RCCI mode. In the Diesel-biogas-RCCI mode the combustion was quite advanced but the rate was low which resulted in lower BTE. The CH4 emissions were the lowest in the DME-Biogas-RCCI mode and decreased further with increase in the concentration of methane in biogas. All the three modes showed good combustion stability and the BTEs were generally insensitive to the concentration of methane in biogas. On the whole the DME-biogas-RCCI mode is a sustainable option for decentralized power generation using completely renewable fuels with ultra-low NOx and soot, high thermal efficiency and low THC and CH4 emissions.
Finally, a control strategy is discussed for operating the engine in three modes namely Conventional Diesel Ignition (CDI), CDF and RCCI with biogas as the main fuel and diesel as the reactivity enhancer. In all the cases twin injectors (Narrow Injector and Wide Injector) for diesel as this mode was the most suitable for the RCCI method which is the main focus here on account of its high efficiency and low NOx emissions. The other two modes namely CDI and CDF are used to ensure that the load range can be extended and smooth transition between loads can be achieved in severe transients. The mode change is made by controlling the fuel parameters with the help of an ECU in accordance with the changes in the engine operating conditions like load, rate of change of load and temperature. The ECU has two blocks; the master controller block and the slave controller block. The master controller block constantly monitors the operating conditions and sends signals to the slave controller as per the control logic being fed into it. The slave controller modifies the engine operating parameters in response to the signals it receives from the master controller. It basically makes use of different maps to adjust the fuel injection timings and duration. The ECU responds to the changes in the operating parameters in real time so that seamless transition of modes take place and the engine operates at the optimal mode suited for a particular operating condition. The control logic has been robust enough to operate the engine over a wide load range without any appreciable fluctuations in the BMEP when subjected to different load conditions.