Model Description and Validation
4.3 Model Validation
The CFD model was validated using experimental data collected from the single-cylinder research engine with the specifications listed in Table 4.1. The model was validated for both single and multiple injections in order to make sure that fuel strategies would not affect the predicted results. From the results analysed in Sections 4.3.1 and 4.3.2, it can be concluded that simulation results nearly match the experimental results for single injection cases. At the same time, simulation results for multiple injections are also rather similar to actual values. Thus, the model used in this study can confidently simulate, with near-accuracy, the combustion process and emissions.
4.3.1 Single Injection
The model was validated for a single fuel injection at two different engine speeds and loads under the following engine conditions:
Table 4.1: Single injection engine conditions for model validation.
Inlet Valve Closure (IVC) 64°ATDC Exhaust Valve Opening (EVO) 69°BBDC
Engine speed 1,200rpm and 2,000 rpm
Engine load 18% and 81%
Fuel quantity 6 mg and 14 mg
Fuel injection pressure 1,600 bar Start of Injection (SOI) 10°BTDC
Figures 4.5 and 4.6 show the comparison between the predicted and measured in-cylinder pressure and heat release rate for low load at 1,200rpm and high load at 2,000rpm, respectively. The result is based on the assumption of uniform wall tem-perature 470K for the cylinder wall and 570K for the cylinder head and piston top.
Figure 4.5: Comparison of simulated and measured in-cylinder pressures and heat release rates for single injection at 1,200rpm, low load.
Figure 4.6: Comparison of simulated and measured in-cylinder pressures and heat release rates for single injection at 2,000rpm, high load.
The CFD simulation trend for the in-cylinder pressure seems to be reasonably close to the experimentally measured values for both operating conditions. There is only a slight pressure difference after the start of combustion, which might be related to experimental uncertainties in parameters in the computations such as the precise injection duration, start of injection and gas temperature at the IVC. On the other hand, the calculated heat release rate based on the experimental results seems to follow the same trend as in the simulation. However, the calculated HRR is slightly higher than the simulation experiments, and it seems to have a smoother drop after the end of combustion. Slight pressure and HRR variations between the experimental and simulation results will have a minimum impact on the results of the in-cylinder mixture homogeneity. A contingent small fuel injection variation will not significantly interfere with the air and fuel flow motions within the cylinder.
Figure 4.7: Comparison of simulated and measured NOx and soot emissions for single injection.
Figure 4.7 presents the comparison of NOx and soot emission formation for single injection cases with different starts of injection timing at 2,000rpm. It can be seen that simulation and experimental emission results are very similar for the cases where the start of injection occurs close to TDC. For cases with an earlier injection start, there is a very slight divergence that has possibly been caused by some air motion instabilities. In addition, the lower NOx levels predicted by the CFD model are due to the fact that the NOxformation model neglects the prompt NO formation by the attack of HC fragments on the air-nitrogen mixture, which could comprise up to 5% of total NOx formation.
4.3.2 Multiple Injections
The model was validated for adopting a pilot and a main injection strategy in order to make sure that multiple injections do not affect the accuracy of the predicted results.
The pilot injection was varied from 5% to 20% of the total fuel injected per cycle, while the start of the main injection was kept constant at 10°CA BTDC.
Figure 4.8 shows the NOx and soot results gathered for three operating conditions featuring a pilot injection at a pressure of 1,600 bar and at a dwell angle of 5°CA. The simulation and experimental results for operating conditions featuring pilot injection are not as similar as the results in the single injection cases. This could possibly be due to variations of the injected fuel mass compared to the simulation model. Injection mass variations can be caused by fuel dribbling after the pilot injection or by failure of the injector’s needle to reach its maximum lift due to the very small fuel amount injected during the first pulse. This can also be confirmed by the higher error in the soot and NOx
emissions in the cases with very low pilot fuel amount. Likewise in the single injection cases, the predicted NOx formation is lower than the actual NOx levels due to the lack of prompt NO formation in the simulation results.
Figure 4.8: Comparison of simulated and measured NOxand soot emissions for multiple injections.
4.4 Summary
This chapter has presented the CFD software used for simulating diesel engine com-bustion. A detailed analysis of the sub-models used in this research work has been performed. The chapter also included a grid independence analysis for obtaining mesh independent results and a model validation for single and multiple injection strategies using experimental data obtained from the Hydra engine described in Section 5.