Closed-Loop Controller Evaluation

The closed-loop controller presented in Subsection 6.5.2 is evaluated in this subsection. At first, the performance is evaluated using only a PID controller for throttle and waste gate control without a feed-forward model. After that, the performance is evaluated including a feed-forward model to highlight the advantages.

a) Closed-Loop PID control

The PID controllers for throttle position and waste gate open area were tuned using the following ad-hoc values:

• Throttle Controller: ; s 0.001, Ä s 0.01, ë s 0 • Waste Gate Controller: ; s 0.002, Ä s 0.03, ë s 0

Figure 7.9 shows the intake manifold pressure response for a duel-ramp MAP sweep with a ramp time of 30 seconds at 2000 rpm engine speed.

Figure 7.9: Closed-loop controller: intake manifold pressure response for 30 seconds ramp time

Figure 7.10: Closed-loop controller: Intercooler outlet pressure response for 30 seconds ramp time

Figure 7.9 and Figure 7.10 show that the PID controllers manage to control the intake manifold pressure and intercooler outlet pressure with high accuracy to the desired values _{; _H8} and _{; _H8} .

To investigate if the controller achieves a similar accuracy for fast ramps, the simulation was repeated with a ramp time of 3 seconds. Figure 7.11 and Figure 7.12 show the response of intake manifold pressure and intercooler outlet pressure respectively.

Figure 7.11: Closed-loop controller: Intake manifold pressure response for 3 seconds ramp time

Figure 7.12: Closed-loop controller: Intercooler outlet pressure response for 3 seconds ramp time

Figure 7.11 and Figure 7.12 highlight that the accuracy of the closed loop controller using only a PID controller has its limitations. The results show that the tracking accuracy is significantly reduced compared to the simulation results with a ramp time of 30 seconds. It might be possible to improve the tracking accuracy by tuning the PID controllers with a more advanced method. However, higher gains in the controllers will result in overshoots and fluctuations, which are not desirable since this causes excitation of the gas dynamics.

As mentioned in Subsection 6.5.2, it is possible to improve the tracking accuracy of the controller by combining the PID controllers with a feed-forward model for throttle and waste gate position.

b) Closed-Loop PID control with Feed-Forward Model

P re ssu re [ P a ] P re ssu re [ P a ]

were identified using steady-state simulation data. Throttle position and waste gate open area are modelled as a function of intake manifold pressure and engine speed. The use of perfect models eliminates the need for the PID controllers. Therefore, the steady-state data used to generate the models were simulated with a delta pressure of 0kPa. The simulation results to follow were generated with a desired delta pressure of 10kPa to ensure that the PID controllers have some work to do in order to correct for the errors in the feed-forward models. The models are finally implemented as 2D lookup tables. The reader is referred to Figure 6.9 and Figure 6.11 in Subsection 6.5.2 for an example of the implemented feed-forward models.

Figure 7.13 and Figure 7.14 show the response of intake manifold pressure and intercooler outlet pressure for a ramp time of 3 seconds.

Figure 7.13: Closed-loop controller with feed-forward model: Intake manifold pressure response for 3 seconds ramp time

Figure 7.14: Closed-loop controller with feed-forward model: Intercooler outlet pressure response for 3 seconds ramp time

P re ssu re [ P a ] P re ss u re [ P a ]

The results show a significant improvement in the tracking accuracy of _{; _H8} and _{; _H8} . Dual-ramp MAP sweeps with a ramp time smaller than 3 seconds are definitely not required. Therefore, it can be concluded that the closed-loop controller with feed-forward models for throttle position and waste gate open area is a suitable control solution.

7.4 Summary

A virtual engine test bed was presented in Section 7.1. The core of the simulation platform is a 1D crank angle resolved engine model which was developed in Ricardo WAVE software. A co-simulation between the engine model in MATLAB Simulink permits implementation and switching between different engine air-path controllers. Additionally, the implementation in Simulink allows an easy and direct use of the simulation data in MATLAB. The prediction accuracy of the model for air charge was validated in Section 7.2 and achieved a _{±5% accuracy in} volumetric efficiency for 80% of the validation points. The maximum prediction error was 8%. The evaluation of the different air-path controllers in Section 7.3 clarified that the open-loop solution is not the ideal control solution for a dual- ramp MAP sweep. Due to the strongly nonlinear response of intake manifold pressure to throttle angle and waste gate open area, over 30% of the total test time is wasted with this control method. Furthermore, the applicability is limited to engine control strategies, which do not use a delta pressure across the throttle in the boosted operating range. The closed-loop controller provides a much more advanced control solution for a dual-ramp MAP sweep. Only using a PID controller to control intake manifold pressure and intercooler outlet pressure is suitable for ramp times down to 30 seconds. For ramp times faster than 30 seconds, the PID controllers have to be combined with a feed-forward model for throttle position and waste gate open area to achieve a good tracking of the target intake manifold pressure and target boost pressure.