Integrated Simulation of a Truck Diesel Engine with a Hydraulic Engine Braking System N. Brinkert, K. Kanning GT-Suite Users Conference 2008 I want to give you a short presentation about a project we work on at Daimler Trucks, that deals with a hydro-mechanic engine braking system. 1
Outline Engine Braking Systems (EBS) Decompression Braking EBS with Hydraulic Valve Actuation Simulation Simulation of Hydraulic System Calibration / Comparison with Measurements Integrated Simulation of Hydraulic and Thermodynamic System Motivation for Integrated Model Results / Comparison with Measurements Benefit of Integrated Approach Summary / Future Prospects 2 First, I d like to give you a short overview. As a start, I want to give those of you who aren t in Truck business a short introduction in engine braking systems and describe the system we investigate. Then I come to the simulation topics: after some information about the hydromechanic simulation and its calibration and results, I will switch to the integrated simulation with the thermodynamic engine model. I will give you a short motivation for the use of this integrated modeling and show its results and benefits. Finally there will be a brief summary, and I will give you an idea of what we plan for the future with this tool. 2
Engine Braking Systems Legal Regulation A sustained-action brake is required by law for CV with a gross vehicle weight over 5t (depending on application). This brake must work wear-free and independently from the standard (wheel) brake. Costs The use of the wear-free brake system leads to lower load on the standard brake, therefore costs for maintenance and repair are reduced. Safety Gain As the load on the standard brake is reduced, in case of emergency its full performance can be used. Sustained-action Brake Exhaust Brake Flap Fixed Pressure Controlled Engine Braking Systems Decompression Brake Exhaust Valve(s) Additional Valve(s) Constant Clocked Boosting Systems Turbobrake VGT Add-on Braking Systems Retarder,... 3 Let me give you some information about engine braking systems. There are three main reasons for special brake systems: legislation, safety and costs. For heavy duty commercial vehicles, a sustained-action brake is required by law; the use of this wear-free brake leads to lower load on the standard brake and can therefore reduce costs for maintenance and repair and increase the safety margin in case of emergency. There are several systems on the market, engine brake systems and add-on systems, like retarders, as well. In our project we deal with an EBS, that uses the thermodynamical principle of a decompression brake by opening the exhaust valves for a second time. 3
Principle of Decompression Braking Engine Working Process: Cylinder Pressure Cylinder Pressure Without Combustion Valve Lift 4 Something about the thermodynamical priciple of a decompression brake: On the left hand side you can see the pressure and valve lift curves of a motored engine, on the right hand side there is the corresponding pressure-volumediagram. 4
Principle of Decompression Braking Engine Working Process: Compression Release (CR): The exhaust valves open during TDC, allowing the cylinder charge to escape, reducing the cylinder pressure Cylinder Pressure Cylinder Pressure Without Combustion + Compression Release Valve Lift 5 When the brake system is activated, the exhaust valves are opened for a second time near TDC to release the cylinder pressure - this increases the negative work of the engine process. Since the pressure increases quite drastically just bevor the TDC, you can imagine that the timing of the CR EV lift will have a significant influence on the maximum cylinder pressure. 5
Principle of Decompression Braking Engine Working Process: BGR Compression Release (CR): The exhaust valves open during TDC, allowing the cylinder charge to escape, reducing the cylinder pressure Back Gas Recirculation (BGR): Due to the increased exhaust back pressure, an additional charge is realized through the exhaust valves at the end of the intake and before compression Cylinder Pressure Cylinder Pressure Without Combustion + Compression Release + Back Gas Recirculation Valve Lift 6 As the engine is turbocharged, from mid to high engine speed the exhaust pressure will become higher than the boost pressure. To make use of this high pressure in the exhaust manifold, the exhaust valves can be opened for a third time to increase the cylinder filling. This so-called BGR-lift is located between the end of the intake and the start of compression. 6
Engine Braking System (EBS) with Hydraulic Valve Actuation Principle of engine brake system Rocker Exh. Valve EBS unit EV Spring Fire Cam Roller Follower Swivel Foot Brake Cam Firing Cam Other Engine Brake System Units 7 So how are the exhaust valves activated for the EBS? On this slide you can see some details about the system. The camshaft has two cams: the standard cam, that is used all the time and the braking cam, that can be activated by the EBS. The rocker, that converts the rotational movement of the camshaft into a linear movement of the exhaust valves, has two contacts on the cam side one directly at the standard cam and one at the so-called slave piston. This slave piston is located in one of the two chambers of the main EBS unit, which are connected by a drilling. In the second chamber, there is the so-called master piston. When the engine brake is activated, the two chambers are filled with oil through the control valve. The master piston moves out until it hits the braking cam. When the cam induces a lift, the control valve is closed and the force is directed via the master piston, the oil in the chambers and the slave piston to the second contact of the roller. Thus the exhaust valves open. The EBS is therefore a hydraulic load transmitter, transmitting from the braking cam to the rocker. Naturally there is some leakage in the system, which is compensated after each cycle by a small check valve through which the system is refilled with oil. For safety reasons, there is an additional pressure relief valve to avoid too high pressures in the system. 7
Model of Engine Braking System (EBS) in GT-Suite Leakage Leakage Rocker Arm Input: Cylinder Pressure Exhaust Port Pressure Output: Exhaust Valve Lift Hydraulic Pressure... Oil Supply Cutoff Oil Return Line 8 In a first step, this EBS was modeled in GT-Suite solely as a hydro-mechanical system. The valves, the pistons and the chambers are modeled as masses, pipes and volumes with several control elements to simulate the orifices in the system, that are opened and closed with the movement of the components. For the camshaft and the rocker arm, the appropriate GT elements are used. The input to the simulation are the cam profile on the one side and the pressures on both sides of the exhaust valves on the other side. The calculation then delivers the exhaust valve lift and the hydraulic pressure in the system as the main output. 8
Calibration of the EBS Model Event Duration Simulation results: Simulation results show overall good agreement with measurements Even the dynamical behavior of the system (hydraulic pressure fluctuations) can be reproduced The exhaust valve lift reveals the high pressure load in the valve train during the decompression event A comparison of the master and the slave piston displacement shows beside the geometrical ratio - the fluid compressibility and system leakage The check valve lift at the end of the decompression event represents the refilling of the system 9 The hydro-mechanical model was then calibrated with measurement data from the test bench. Unfortunately no valve lift measurement was available, so the only data we had were the pressures in the cylinder and in the EBS itself. On this slide we see measured and simulated data for one exemplary engine speed. On the left hand side, there are the pressure curves: the solid lines show the hydraulic pressure in the system (black=measured; red=simulated); the dotted lines represent the corresponding input to the simulation: the measured cylinder pressure and a calculated exhaust port pressure. This exhaust port pressure was generated with a GT-Power model of the engine with an assumed valve lift, as no measurement in the exhaust port directly behind the valves was possible. On the right hand side only simulation data is plotted: the diagram shows the movement of the master and slave pistons, the exhaust valves and the check valve. 9
Motivation for Integrated Model Previous Approach Integrated Approach Hydraulic Simulation Exhaust Port + Cyl. Pressure interdependency Valve Lift Thermodynamic Simulation Valve Lift and Corresponding Cylinder Pressure The integrated approach gives a direct interaction between the cylinder pressure and the valve lift No more iteration loops during development are necessary, which leads to faster results The integrated model enables: Optimization of the complete system Investigations of system transients Development of a control strategy 10 On the previous slide we already touched on the problem of simulating the EBS only: the hydraulic simulation needs the pressure on both sides of the exhaust valves as an input and delivers the exhaust valve lift as an output. A thermodynamic simulation of the engine process needs just this valve lift as an input to generate the pressure curves as an output. As there is a strong interdependency between the valve lift and especially the cylinder pressure, you have to make several iteration loops to get reasonable results. This can be avoided by using the integrated approach: combining the hydromechanical and the thermodynamical simulation leads to one calculation run, that delivers the valve lift and the corresponding pressure in one step. Furthermore, optimisation can be performed with the integrated approach, which is only partially possible with the individual simulation. Finally, the integrated approach is the only way to simulate transient events, i.e. switching operation. 10
Integrated Model of Engine with EBS Engine Model Integrated Model EBS Model 11 So, what we did is: we took the already existing engine model and the EBS model and put them together to one GT-Suite model. The EBS model was implemented in the engine model as a subassembly. Because of the special design of the exhaust manifold with an asymmetric turbine, the EGR supply from only one exhaust manifold and the turbine wastegate only on the other exhaust manifold, we used not only one EBS subassembly, but two: one for each side of the manifold. 11
Integrated Model of Engine with EBS Two EBS systems are calculated: cyl.1 and cyl.6 Exhaust port and cylinder pressure are fed into EBS subassembly Calculated braking valve lift is added to standard lift curve and fed back to cyl.1 and cyl.6, resp. Calculated lift of cyl.1 is used for cyl. 2 and 3 with appropriate shifting - lift of cyl.6 is used for cyl. 4 and 5 accordingly EBS subassemblies are connected by a 0mm dummy connection to generate one hydraulic calculation circuit Integrated Model 12 12
Integrated Model: Settings and Calculation Times Special attention must be given to the simulation settings: Use of Circuit Based Solution Maximum Ratio of Time Steps in Flow Circuits Mechanical Solver: Maximum Integration Time Step Mechanical Convergence Criteria The integrated simulation results in higher calculation times: Hydraulic simulation - 2.9 min calc. time/sec real time Thermodynamic simulation - 2.0 min calc. time/sec real time Integrated Simulation 1 EBS - 7.9 min calc. time/sec real time Integrated Simulation 2 EBS - 12.4 min calc. time/sec real time Nevertheless, as the need for iterative hydraulic and thermodynamic simulations is eliminated, the total time and especially the engineer s time to get reliable results is reduced by the integrated simulation! 13 The use of such an integrated model needs some special settings - and has of course some consequences on the simulation time. First of all, you have to switch the project type to GT-SUITE. If the sytem is uncoupled in terms of fluid interaction, in Run Setup you should use the Circuit Based Solution and set appropriate values for convergence and the time steps, especially for the ratio between the flow and the mechanical solver. These values are depending on your systems architecture, of course. Calculation time is always an issue, so we took a closer look and did some test runs. The run times are increasing significantly with the integrated approach, nevertheless, as the iteration loops between hydro-mechanic and thermodynamic simulations are omitted, the total time to get results is reduced. 13
Simulation Results: Comparison with Measurements Cylinder Pressure [logarithmic scale] Measurement at high RPM Measurement at medium RPM Measurement at low RPM Simulation at high RPM Simulation at medium RPM Simulation at low RPM Exhaust Valve Displacement Hydraulic Pressure High RPM Medium RPM Low RPM Energy Release Gradient Simulation results: The Integrated simulation results shows good agreement with pressure measurements in the hydraulic system and in the cylinders even during during gas exchange The exhaust valve lift shows a strong sensitivity to engine speed The hydraulic pressure indicates the increasing dynamical response with increasing speed The energy release gradient ( negative burn rate ) allows a detailed analysis of the effective mass flow 14 So, let s get to some results from the integrated simulation. On the left hand side, we see calculated and measured cylinder pressures for three different engine speeds. Please note, that the plot has a logarithmic scale. On the right hand side, the corresponding valve lifts, hydraulic pressures and the energy release gradients, comparable to the burn rate in standard engine operation, from the simulation are shown. From these plots, it is possible to see different system characteristics, such as clearance, mass flow through the exhaust valves or flow conditions at the valves. 14
Simulation Results: Benefit of Integrated Aproach Comparison of simulation results of an integrated model and a thermodynamic model with given valve lift from hydraulic simulation (one simulated valve lift at high engine speed for whole speed range) integrated model Thermod. model engine speed Predicted braking performance is similar Predicted max. cylinder pressures are quite different engine speed Cyl. 1 Cyl. 6 As the max. cylinder pressure is most important for the design of the system (load on valve train), it would have been necessary to make several iteration loops between hydraulic and thermodynamic simulation and this for different engine speeds. 15 On this last slide, I d like to show what benefit the integrated approach provides. In these diagrams, there is a comparison between simulations with the integrated model and with the thermodynamical model, that uses one exhaust valve lift for all engine speeds, i.e. we took the calculated valve lift for high speed from the integrated model, fed it into the thermodynamical model and simulated all engine speeds with this valve lift. In the upper diagram there is the normalized braking power, and below you see the normalized max. cylinder pressures in cylinder 1 and cylinder 6. 15
Summary and Future Prospects An integrated model of a truck diesel engine with a hydraulically activated decompression engine braking system (EBS) has been built. A good agreement between simulated and measured pressure traces in the engine cylinder and in the EBS has been achieved. The integrated model provides deeper understanding of the system s behaviour and its key parameters. Results can be generated faster, because otherwise necessary iteration loops between hydraulic and thermodynamic simulation can be omitted. Future work will include the following topics: o Modelling of the hydraulic system for all cylinders including the feeding line o Optimization of the total system o Investigation of system transients, i.e. switching operation 16 Finally, I want to summarize and give you an idea of what we plan for the future. 16
Thank You for Your Attention...and thanks for the great support from GTI, especially from Shawn Harnish Questions??? 17 17