Simulation Based Optimization of a Mechatronic Pressure Controller for CNG Powered Engines

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1 Simulation Based Optimization of a Mechatronic Pressure Controller for CNG Powered Engines Dirk H. Hübner Department of Mechanical Engineering Trier University of Applied Sciences P.O. Box 1, 5 Trier and Harald H. Ortwig Department of Mechanical Engineering Trier University of Applied Sciences P.O. Box 1, 5 Trier ABSTRACT One of the essential parts of Compressed Natural Gas (CNG) powered engines is the pressure controller which reduces the storage pressure (up to 5 bar) from the tank system to a near constant outlet pressure of approximately up to 1 bars within the fuel injection system. To generate fundamental data for the scientific project described in this paper a standard mechanical pressure controller for CNG-powered engines was investigated theoretically and practically. Thereto the particular components of such a system were modeled, simulated and verified by comparison of measured and simulated results. In the next step the modeled state of the art mechanical pressure controller was substituted by new developed mechatronical devices and the space of improvement for different versions of those pointed out based on simulation. Subsequently the realized mechatronical pressure controllers with the major potential for further development were practically investigated on a test rig at Trier University of Applied Sciences. Experiments and Simulations are aligned here to validate a new device. The main advantages that accure by using a mechatronical pressure controller instead of an ordinary mechanical one are demonstrated. Keywords: Compressed Natural Gas, Fuel System, Mechatronical Pressure Controller, Modelling, Simulation 1. INTRODUCTION Alternative fuel types currently favored by political decisionmakers, automotive manufacturers and customers chiefly include natural gas (CNG or LNG), butane/propane mixtures (LPG) or hydrogen (CH or LH ). Hydrogen-powered engines have not yet found their way into high-volume manufacturing due to a variety of technical obstacles. On the other hand, natural gas (CNG) or butane/propane mixtures (LPG) have been around for years as a fuel for converted spark-ignition engines originally optimized for gasoline operation. Conversion kits are marketed by various manufacturers but usually fail to meet OEM equipment standards in terms of system integration and performance characteristics. The central module of any CNG fuel system is the pressure controller, which reduces the gas storage pressure to a level acceptable for injection into the intake pipe of the respective cylinder. The mechanical controllers used to date are but conditionally suitable for automotive use, given their poor dynamic response and inadequate accuracy. System-inherent disadvantages associated with the decreasing supply pressure and variable flow demand have given rise to the development of a mechatronic pressure controller. In this unit, the mechanical operating principle is supplemented by a sensor system detecting the outlet pressure and by an actuator system operating the slide valve. The system thus formed is controlled by a processor integrated into the engine control system.. MECHANICAL PRESSURE CONTROLLER In the fuel system of a CNG-powered engine, the pressure controller is a critical component. Its purpose is to reduce the pressure prevailing in the fuel storage system to a controller outlet pressure of maximum constancy. The disturbance factors whose impact must be minimized are fluctuating flow demand on the one hand and the declining supply pressure on the other. A high level of control dynamics is also desirable since this is a key determinant for engine responsiveness. Moreover, the system must meet exacting safety standards the pressure controller needs to be highly leak-proof and has to comply with adequate fail-safe criteria. The structure and operation of mechanical controllers are presented below. In deriving the model, all aspects known to date were duly taken into account. The model was created on the basis of the inter-relationships given in the following figure: Figure 1: Mechanical Pressure Controller Outlet pressure Assuming an isentropic change of state in the system's lowpressure volume, the pressure change will follow the equation given: V d p& + p V dv& p V m dm& = (1) m m

2 If we further surmise that the volume change remains negligible, the following equations are obtained: Pressure change over time dm& R T d p& = p = dm& () m V From the analytical work an action diagram (shown in figure ) was derived and simulation results were achieved for different working points. To verify the model statical and dynamical measurements were carried out on the mechanical pressure controller and are to be compared with simulation results. Mass flow differential d m& = m& in m& out (3) Mass flow m& = μ A ρ1 p1 + 1 p p () 1 p1 p1 Cross-sectional area of flow A = A + k y (5) Figure : Action Diagram Mechanical Pressure Controller Valve gate travel y = y& dt = & y d () df Fy Acceleration & y = = (7) m m Sum of forces F = Fpressure + Fspring + Ffriction Fstream () ΔT 73 Temperature = α (9) Δp T h For the static behaviour Fig. a shows the measured outlet pressure vs. inlet pressure while Fig. 3a plots the piston position over the inlet pressure for the respective cross-sections of the pressure controllers low-pressure system. Figures b and 3b give the corresponding simulation results. A high coincidence quality is evident from these results. Existing deviations are attributable to slight non-linearities remaining in the model. p outlet 1 < Q p outlet 1 < Q Figure a: Measured Outlet vs. Inlet Pressure Figure b: Simulated Outlet vs. Inlet Pressure y (mm).1. y (mm) < Q.5 < Q Figure 3a: Measured Valve Position vs. Inlet Pressure Figure 3b: Simulated Valve Position vs. Inlet Pressure

3 The dynamic measurements were conducted with a nearconstant inlet pressure but with abrupt variations of the crosssectional area of flow in the low-pressure section. Figures and 5 show the measured step responses of the outlet pressure and piston position at an inlet pressure of 5 bars. The corresponding dynamic simulation results appear directly alongside these diagrams on the left. As we see now mechanical pressure controllers definitely do not adequately meet existing demands on dynamic control response and control accuracy. A further inherent drawback is their invariable (preset) output pressure setpoint. Thus, for instance, an all-mechanical governor cannot adapt to abrupt engine load changes. A sudden rise in the disturbance variable will result in a major permanent deviation from the setpoint. Compensation for these inadequacies can only be provided via a suitable adjustment of control parameters. This is achievable through the use of a mechatronic pressure controller p sim 1. p mess 1. y sim (mm). y mess (mm) Figure : Simulated/Measured Outlet Pressure vs. Time Figure 5: Simulated/Measured Valve Position vs. Time 3. MECHATRONIC PRESSURE CONTROLLER The objectives underlying the use of mechatronical controller can be clearly stated. One is to improve on the control dynamics and control accuracy of mechanical controllers. Another is to open up new potentials via an expanded control capability. The permanent deviation from the set point encountered with mechanical controllers can be compensated for by employing computer-based control algorithms. The outlet pressure of a mechatronical controller ceases to be a fixed parameter determined by design but is variable within a certain range. This permits the use of new control strategies, such as increasing the outlet pressure upon a rise in engine torque demand to compensate for influences of the disturbance variable. Overall, it becomes possible in this manner to control the injected amount of gas not merely by injector cycling but also via the dynamically adjustable rail pressure. An additional parameter can thus be made available for adaptive control, as has been state-of-the-art for some time on direct-injection spark ignition engines. At the present time the development of the basic structure and operating principle of a mechatronical pressure controller with electro-pneumatic or electro-hydraulic pilot control has been done in further investigations. The basic principle of the concept is to generate an additional force also acting in the point of the sum of forces which influences the sum of forces (df 1 ) and consequently the outlet pressure. To implement a mechatronical control system, it is necessary to detect the prevailing pressure in the volumes downstream of the low-pressure chamber. An appropriate computer-based controller can use this information to influence the actuators of the pilot-control system, thus controlling the outlet pressure to a desired level. Two systems (a pneumatically controlled and a hydraulic controlled one) were parameterized according to design before being simulation based examined for their potential and stability. The following figures and 7 show the associated simulation results regarding pressure obtained with an abrupt set point change. On principle, both controllers exhibit a stable behaviour which is indicative of a favourable design rating of the relevant system components. It is also evident from the results that the hydraulic system exhibits a more dynamic response and hence, constitutes a superior alternative. In the further course of the project, prototypes of both variants were built and subjected to functional validation within the entire fuel rail system. Figure shows the step response of the control loop regarding the outlet pressure versus time obtained with the pneumatic system; figure 9 reflects that of its hydraulic counterpart.

4 p p 1 1 Figure : Simulated Pressure Electro-Pneumatic System Figure 7: Simulated Pressure Electro-Hydraulic System p 1 p Figure : Step Response Electro-Pneumatic System Figure 9: Step Response Electro-Hydraulic System The results show that neither system tolerates any major permanent deviation from the set point; both will quickly correct any deviation caused by a disturbance variable. With the uncompensated actuating devices employed here, this is a system-inherent feature. It emerges that the hydraulic variant has a broad hysteresis band in terms of its recovery behaviour. This is due to the unfavourable volumetric flow characteristic of the proportional-type hydraulic valve employed. Since very small input signals on the volumetric flow valve will not give rise to a manipulated variable value, the controlled variable must differ substantially from the set point before the control loop becomes active again. In terms of this property, the pneumatic system is more advantageous. On the other hand, the hydraulic system is superior in terms of actuating speed and duration of the recovery cycle. It is evident from these results that the simulation reflected the control behaviour very well; even with the rudimentary equation set used for modelling, and indeed proved a helpful support tool in the design phase. 7. CONCLUSION The objective of this study was a simulation based optimization of the behavior of a CNG-fuel system with mechanical and mechatronical pressure controllers. Using the equations developed, it is possible to describe the control behavior of mechanical single- or multiple-stage pressure controllers (whether mechanical or mechatronical), to simulate such devices, and to optimize their performance by means of computer-based parameter variations, all without having to resort to time-consuming and costly development tests. The first investigations were carried out for a CNG-fuel system with a mechanical single-stage piston-type pressure controller. This device was selected in view of its widespread use in vehicles powered by CNG-powered spark-ignition engines. All particular components of such a system were modelled, simulated and verified by comparison of measured and simulated results. A comparison of the measurements and simulation results of the system behaviour by use of the mechanical piston pressure controller reveals a very good

5 coincidence in the pressure profiles. The model of the fuel system and the mechanical pressure controller may thus be deemed successfully validated. In the next step the modelled state of the art mechanical pressure controller was substituted by a model of a new developed mechatronical device and the space of improvement for different versions of those was pointed out based on simulation. By using the developed model it is possible to simulate, validate and optimize new devices for CNG-fuel systems, especially mechatronical pressure regulation devices. Here two different types of Investigations will permit comparisons and evaluations of different versions. The improvement potentials residing in this technology could thus be fathomed, and significant improvements in the operating reliability and efficiency of CNG-powered spark ignition engines would become achievable.. REFERENCES Avramopoulos, Sprysch, Holthaus, What demands does serial development place on natural-gas-vehicles? Examples of solutions and a look at their cost-benefit characteristics, Gasfahrzeuge, Oliver Dingel Backé, W., Grundlagen der Pneumatik [Fundamentals of Pneumatics], 7th edition, 19 Cerbe / Hoffmann, Einführung in die Thermodynamik [Introduction into Thermodynamics], 1th ed., Carl Hanser Verlag, Munich/Vienna, European Natural Gas Vehicle Association, Medard, L., Encyclopédie des gaz - l air liquide, Elsevier/l air liquide, (197) Murrenhoff, H. Grundlagen der Fluidtechnik - Pneumatik, [Fundamentals of Fluid Mechanics - Pneumatic], Wissenschaftsverlag Aachen, 9. CONTACT Harald Ortwig holds a Dipl.-Ing. (MSc) and Dr.-Ing. (PhD) degree in Mechanical Engineering from RWTH Aachen University of Technology, Germany. Currently, he is Professor at Trier University of Applied Sciences, Germany. He is acknowledged as a Publicly Certified Expert in Fluid Power and Failure Analysis and the Valuation of Machines, apart from being a member of the SAE Fluid Power Committee and Fluid Power Net International FPNI. Dirk Hübner holds a Dipl.-Ing. degree from Trier University of Applied Sciences, Germany. His scientific interests include modelling, simulation, and applied optimization of CNG injection systems. He was a member of the scientific staff in the Fluid Power Department of that university and is currently finishing his PhD thesis.