The experimental tests showed the engine performances with both the configurations in terms of engine power and combustion stability. Although the experimental findings have been useful for the evaluation of the benefits for a transition from a production SPI CNG heavy-duty engine to a MPI one, alternative engine controls could be evaluated with numerical codes. Therefore, the actual engine has been modeled in GT-Power v7.4 environment. The numerical code used is briefly described. Flow fluid dynamic is characterized by 1D Navier-Stokes equations, thus requiring a refined implementation of the geometric characteristics of all the elements of the internal combustion engine. The correct implementation of the material, of the fluid and of the valves’ characteristics taken into consideration is mandatory. The pressure drop and heat transfer coefficients depend indeed primarily on the correct implementation of the technical characteristics of the experimental test bench. The combustion model used for the preliminary tests is a non-predictive combustion one. A non-predictive combustion model sets the burn rate as a function of crank angle [17]. It is appropriate when experimental data are provided and if the model is used to study variable that are minor affected by
the burn rate. Predictive combustion models are in turn slower, more complex and difficult to be calibrated. They are mandatory when the variable to be studied has a direct and significant effect on the burn rate. The ensemble in- cylinder pressure cycles collected for each cylinder at each steady state operating condition have been used for the implementation of the non- predictive Three-Pressure Analysis (TPA) tool. The TPA tool imposes the combustion rate to guarantee the experimental one (the heat coefficients have been properly tuned in order to guarantee the correct time history and look-up tables have been implemented). Each cylinder presents its own combustion rate. More details can be found in [17]. The maps of the turbine and compressor have been used for their characterization. The west gate diameters have been set by targeting the boost level and the turbine efficiency coefficients determined depending on the errors of the simulated pressure at the turbine inlet with respect to the experimental one. Finally, both the injection system configurations have been modeled and the two models have been calibrated considering the experimental data shown in Figure 4.3.
The SPI and MPI systems have been modeled considering the actual layouts. The single-point system is composed by four injectors that deliver the fuel in a steel pipe connected to a plastic robber bending pipe. The plastic pipe discharges the fuel in the intake duct through the mixer. The injection events have been reproduced by introducing the actual delivery rate of the CNG injector (for the given experimental rail pressure), and the injection durations have been corrected depending on the experimental relative air-to-fuel ratios. The based SPI injection strategy provides six injection events (produced by the four injectors) equally spaced in the engine cycle (120 deg). The 24 holes of the mixer have been substituted with an orifice. The diameter of the orifice has been calibrated in order to reproduce the experimental delivery rate and the proper dynamic response in the steel pipe. In the MPI model the single-point injection system has been neglected and six CNG injectors have been introduced in the GT-Power map in the proper positions. Finally, the injection phasing has been properly reproduced.
4.4.1 Validation of the numerical models
The GT-Power models have been set to simulate the engine map, consisting of the 32 operating conditions. The comparison between the experimental data (light blue dots) and simulation results (red squares) is displayed in Figure 4.10. Both the configurations have been analyzed in terms of air mass flow rate
Engine numerical models 75
(Figure 4.10a and c for SPI and MPI respectively) and IMEP (Figure 4.10b and d for SPI and MPI respectively). The results of the full load and 50% load conditions are highlighted. The numerical data approach the experimental results with negligible variances and the accuracy of the models is guaranteed by two major factors. The good agreement of the simulated air mass flow rate with the experimental one confirms the correct implementation of the engine layout (the models work with the same intake manifold and turbine inlet), whereas the goodness of the simulated IMEP certifies the appropriate application of the TPA tool. Moreover, the effectiveness of the numerical models is shown in Figure 4.11. For the sake of conciseness, the average in- cylinder pressure cycles of the 2nd cylinder running at WOT and 50% load at 1000 rpm and 2000 rpm are represented. A satisfying correlation is observed: the simulation data (dashed red lines) of the SPI and MPI models are in-line with the experimental readings (solid light blue lines). The negligible differences in the compression phases are correlated with the slight minor discrepancies of the induce air mass flow rates. The models have been hence correctly validated, thus leading to a detailed investigation into the effects of the injection phasing of the SPI system and specific advanced control of the MPI one (cylinder deactivation) on the engine performances.
Figure 4.10: Comparison between experimental data and numerical results considering the air mass flow rate (a, c) and IMEP (b, d) [14]
Figure 4.11: Experimental and simulated in-cylinder pressure cycle for 2nd cylinder at
1000 rpm (a, c) and 2000 rpm [14]