Past experimental studies conducted by the current authors on a 13 liter 16.7:1 compression ratio heavy-duty diesel engine have shown that diesel-Compressed Natural Gas (CNG) Reactivity Controlled Compression Ignition (RCCI) combustion targeting low NOx emissions becomes progressively difficult to control as the engine load is increased. This is mainly due to difficulty in controlling reactivity levels at higher loads. For the current study, CFD investigations were conducted in CONVERGE using the SAGE combustion solver with the application of the Rahimi mechanism. Studies were conducted at a load of 5 bar BMEP to validate the simulation results against RCCI experimental data. In the low load study, it was found that the Rahimi mechanism was not able to predict the RCCI combustion behavior for diesel injection timings advanced beyond 30 degCA bTDC. This poor prediction was found at multiple engine speed and load points. To resolve this, multiple reaction mechanisms were evaluated and a new reaction mechanism, that combines the GRI Mech 3.0 mechanism with the Chalmers mechanism, was proposed. This mechanism was shown to accurately predict the ignition delay and combustion behavior with early diesel injection timings (> 30 degCA bTDC), which is representative of the timing applied with low temperature RCCI combustion to achieve simultaneous low-NOx and PM emissions.
With the new combined mechanism, a number of simulation studies were conducted to quantify the in-cylinder conditions that are needed at 12 bar BMEP to effectively control a low-NOx RCCI combustion. A number of design parameters were examined in this study, including; exhaust gas recirculation rate, CNG substitution, fuel injection pressure, injector nozzle included angle and compression ratio. The study revealed that lowering the compression ratio was most effective in controlling a low NOx RCCI combustion. By lowering the base compression ratio by 4 points, to 12.7:1, a low-NOx RCCI combustion was achieved at 12 bar BMEP. Specifically, compared to the baseline diesel case, NOx and PM emissions were reduced by 70% and 67% respectively, while fuel consumption was improved by 5.5%.
As a next step, CFD studies were conducted at 20 bar BMEP and 1500 rpm using the combined mechanism along with the application of lower compression ratio. At this load point it was found that the peak cylinder pressures were much higher (>250 bar) with the lower compression ratio when compared to the baseline hardware. In order to limit the peak cylinder pressures, a spilt injection strategy was then investigated at 20 bar BMEP. Application of a spilt injection strategy along with lower compression ratio was successful in achieving the target of a low-NOx RCCI combustion at 20 bar BMEP and 1500 rpm. The final simulation results showed a 2% improvement in ISFC compared to the baseline diesel case, while NOx and PM emission were simultaneously reduced, by 87.5% and 95% respectively, compared to the baseline diesel case.