Calculation Methods and Tasks
AVL CRUISE includes a number of different calculation tasks, which were specifically designed for applications in the automotive industry. Depending on the task, different calculation methods can be used.
Different calculation methods are used for different applications:
The Stationary Calculation is used when no time dependencies have to be considered. At the Stationary Calculation, the conditions for steady state are determined for every calculation step i.e. all accelerations equal zero. So you can, for example, acquire the maximum ascending gradient that can be surmounted at each engine speed and transmission step (this results in the vehicle speed by considering influencing variables, such as slip).
This calculation method is used for time dependent calculations such as the Cycle Run. In the Quasi-stationary Calculation (also known as “Reverse Engineering”) targets are defined exactly (velocity, acceleration), and the required drive operating point can be calculated.
At the Quasi-stationary Computation, the conditions for the steady state are equal to the stationary calculation. In contrast to the stationary calculation, accelerations are allowed, but they have to be fixed at a constant value. Thus, cycles or full load accelerations can also be calculated. The motion variables of the vehicle (for instance, the acceleration from the driving profile) are defined. Because no controller is necessary for these computations, they are very time efficient.
In contrast to the Quasi-stationary computation, Simulation models the real behavior if the overall model is harmonized correspondingly. This leads, for the simulation of real driving tasks, automatically to a control loop in which the driver (modeled as Driver Module in the program) acts as a controller. In this case, the connection of the modules may be rigid or elastic.
The calculation tasks are organized in folders. These folders are used for the definition of general calculation data, such as the accuracy or the time step width.
CRUISE calculation tasks, organized in folders
The folders, as well as the calculation tasks, can be switched off to save calculation time, if already defined tasks are not needed temporarily.
Most of the calculation tasks can be computed either for the roadway or the chassis dynamometer. The main difference is the calculation of the resistances of the vehicle. For running on a chassis dynamometer the inertia roller masses can be defined.
Additionally, the calculation with or without slip is possible to see what could theoretically be possible with the delivered engine power.
In addition to the main results of each calculation task, detailed information for every vehicle's component (torque, speed, control signals) is calculated unless the user turns off the output for specific elements.
The main goal of this task is the calculation of fuel consumption and emissions on driving cycles (e.g. NEDC, FTP72). The most common driving cycles are already pre-defined. If any additional cycle is needed, it can easily be defined through the use of the graphical profile editor. The profile can be defined either as a time-based or as a distance-based cycle.
This task is designed for the calculation of the maximum ascending gradients the vehicle can surmount. The maximum gradients for each gear are determined. Additionally, measuring points can be defined to derive output data for special points e.g. for defined vehicle velocities.
This task serves for the determination of fuel consumption and emissions at constant velocities. The calculation is performed for the whole engine speed range in all gears. An additional feature is the calculation of the actual and the theoretical top speed. This is made by a variation of the gear ratio. The multiplication factor for the gear ratio is printed out as well.
Definition of the driving cycle (Profile) with the profile editor
Full Load Acceleration
This task can be separated into three sub-tasks:
- Calculation of the maximum accelerations: The maximum accelerations of the vehicle are computed for the whole engine speed range in all gears.
- Acceleration from Rest: The vehicle is accelerated from rest up to top speed with shifting gears. The standard outputs, e.g. the time for the acceleration from 0 to 100 kph or for the mile with dead start, can be defined by measuring points. These measuring points can be defined separately as velocity, distance, and time measuring points. For each measuring point, special outputs such as the time to reach it, the covered distance, the actual gear, or the actual engine speed, are printed out.
- Elasticity: Starting with a definable velocity and a definable gear the vehicle is accelerated up to the defined final velocity limit. The final velocity is defined as a measuring point, so that different elasticity calculations can be performed within one calculation (as long as the starting conditions are the same)
Definition of the driving cycle (Profile) with the profile editor
Maximum Traction Force
This task can be used for the generation of the tractive-power diagram. This calculation is similar to the Climbing Performance. The calculation is performed for the engine speed range in all gears.
Cruising is very similar to the Cycle Run. The main differences are that the defined profile is a distance-based speed limit, which must not be exceeded. The definition in the Cycle Run is done as target velocity. An additional difference is that for Cruising, maximum acceleration and deceleration values can be defined which must not be exceeded during the calculation. With this calculation the track between two cities can be defined (with speed limit, altitude, etc.) and the overall fuel consumption can be computed.
Brake / Coast / Thrust
This task serves for the determination of the braking performance of the vehicle. Gear shifting and actual brake force can be defined. By changing some parameters, a coast-down analysis can be performed to check whether the resistances in the vehicle model are set up correctly.
Some input data can be varied. The data for which variations are allowed are limited to the following:
- Vehicle mass
- Drag coefficient
- Engine displacement
- Multiplication factor for the transmission ratio
The components of the drive-train link the engine to the wheel and brakes. In general these components are grouped in clutches and transmissions. Control devices are provided to build, for example, vehicles with automatic transmissions.
The maximum transferable torque is the primary quantity of the friction clutch. It can be defined entering geometric data or specifying the maximum transferable torque directly. The Clutch can be controlled either by the driver via the clutch pedal travel or a clutch control or clutch program.
- Outer/inner radius of the clutch liners
- Number of the pairs of friction surfaces
- Coefficients of static and sliding friction and gradient of approx. function
- Maximum transferable torque
- Axial pressure as a function of the clutch pedal travel
- Mass moments of inertia on the drive and power take-off sides
The torque is transmitted depending on the differential speed by considering the drive and power take-off mass moments of inertia.
- Mass moment of inertia on the drive and power take-off sides
- Idle friction torque, factor and exponent for the Viscous-Clutch characteristic
Automatic vehicle with a detailed gear box model
This self-operating clutch acts independently from the driver or a controller.
- Reference speed
- Reference torque as function of the ratio between drive and power take–off speed
- Ratio between clutch torque and reference torque as function of the ratio between drive and reference speed
- Mass moments of inertia on the drive and power take-off sides
The component Torque Converter, which contains also a lockup clutch, is used in vehicles with automatic transmissions.
For computing the operating state, various characteristic curves of the Torque Converter are used.
- Reference speed
- Pump torque and ratio between turbine and pump torque as function of ratio between turbine and pump speed
- Mass moments of inertia of the pump and turbine sides with oil share
- Maximum transferable torque of the lockup clutch
The component is used to switch the lockup clutch of a torque converter between open and closed state. The shifting points are defined for each gear step as speed curves as function of the load signal.
- Opening curve
- Closing curve
The clutch program is similar to the clutch control. The difference is that the Clutch Program controls a clutch under three operating conditions:
- Open clutch (no clutch torque)
- Closed clutch (same drive and power take-off speed)
- Controlled clutch (fixed value for slip speed between input and output side)
Under controlled clutch conditions the Clutch Program works as a PID-controller to reach the required slip speed.
- Shifting map for different gear steps and load conditions (open, closed, controlled)
- Speed difference between drive and power take-off side for different gear steps as function of speed and load condition
Shaft connections in the drive train can be modeled to be rigid or elastic under torsion. The elastic modeling of the shafts makes it possible to study low-frequency dynamic effects in the drive train, which are caused by shifting gears, engaging or disengaging clutches, etc.
- Kind of shaft (rigid, elastic at torsion, last with or without clearance)
- Mass moment of inertia
- Torsion stiffness
- Rotational clearance
In this component, the input power is turned into a output power by considering the gear ratio, the mass moments of inertia, and the torque of loss.
- Number of gears
- Mass moments of inertia on the drive and the take–off sides for each gear step
- Transmission ratio or number of teeth of the gears for each gear step
- Losses of the transmission step depending on drive speed and drive torque
- Temperature dependent torque loss
Gear Box Control
The component Gear Box Control provides a simple definition of the gear shifting points for automatic gear boxes. The shifting points can be defined either as function of speed or as function of velocity. In addition, the Gear Box Control is used to link a Gear Box Program to the gear box.
- Up- and downshifting speeds for single gear steps
- Up- and downshifting velocities for single gear steps
Gear Box Program
The component Gear Box Program allows a more sophisticated version of defining the shifting points for automatic gear boxes. Here, the shifting speeds do not only depend on the gear step but also on the actual load signal.
- Up- and downshifting speeds in dependence of the gear and the load signal
By means of this component, two kinds of CVT's, i. e. infinitely variable or gradual, can be simulated. The gradual CVT works with a kinematic coupling, where the transmission ratio is only changed when the difference between actual and target transmission exceeds a threshold. The infinitely variable CVT works with a dynamic coupling where the belt slip can be considered.
- Kind of the CVT (infinitely variable, gradual)
- Minimum/maximum transmission
- Mass moments of inertia at the drive and power take-off sides
- Maximum adjusting speed
- Torque losses depending on drive speed and drive torque
- Temperature dependent torque loss
- Reference torque, reference slip, and reference transmission ratio
- Characteristic curve of the belt slip
The CVT and a starting clutch are controlled by the component CVT Control.
- Desired transmission ratio
- Clutch position
Single Ratio Transmission
The Single Ratio Transmission is similar to the component Gear Box, but having only one transmission ratio.
- Mass moments of inertia on the drive and the take–off sides
- Transmission ratio
- Losses of the transmission depending on drive speed and drive torque
- Temperature dependent torque loss
The division of one drive torque into two power take-off torques or the summation of two drive torques to one power take-off torque is done by considering the mass moments of inertia. In addition, a differential lock is possible.
- Torque split factor
- Mass moments of inertia at the drive and power take-off sides
Combustion Engine, Engine Control and Exhaust System Components
The combustion engine is modeled by means of characteristic curves and maps. Ideally, fuel consumption and emission maps measured on a test bed are used. These maps apply to a certain operating temperature of the engine. Temperature and friction models are implemented to consider increased fuel consumption and emissions for a cold engine in a cold start test.
If additional characteristics for a turbocharger (boost pressure, charger inlet temperature, etc.) are input, the influence of the behavior of the turbocharger on full load acceleration times can be investigated.
The data of the engine can be scaled to a new engine displacement. All data, which are dependent on engine displacement, are multiplied by an engine displacement ratio.
Most of the listed input values are optional. The amount of data needed depends on the enabled properties of the engine:
- Number of cylinders, number of strokes, engine displacement
- Mass moment of inertia of the engine
- Reaction time for the engine
- Idle and maximum speed
- Full load and motoring curve (optionally as power, torque or BMEP)
- Full load Reduction:
- Reduced full load curves
- Reduction curve
- Reduction factor
- Fuel consumption:
- Fuel consumption map
- Fuel consumption at idle
- Fuel upgrading (start, run-up, acceleration upgrading)
The consumption map can be entered as specific or as absolute map, where the specific map can be converted into the absolute one.
- Emission maps (CO, NO x , HC, Soot)
- Charger characteristics (optional: boost pressure and charger inlet temperature, intercooler temperature, boost pressure build-up time)
- Data for engine temperature and friction model (according to the selected cold start model)
- Load signal map
Cold Start Models
The cold start models evaluate the engine temperature during the simulation cycle and consider the temperature to predict the fuel consumption and emissions.
For the determination of the engine temperature CRUISE offers two possibilities:
Engine Temperature Characteristic. The engine temperature can be entered as a curve versus time.
AVL Temperature Model. The engine temperature is computed.
By considering the fuel consumption, the heat produced by the combustion can be calculated. This heat will be distributed among the engine block and exhaust gas by a factor. By considering the temperature loss in the radiator, the heating of the engine can be established.
This technique has two advantages: First, the actual heat flow in the engine is represented. Second, the required data can be measured on the test bed with little effort. No transient maps, difficult to produce, are required.
Fuel consumption distribution for a FTP-75 cycle simulation generated automatically with CRUISE
Increased Fuel Consumption and Emissions
The lower temperature during a cold start causes an enrichment of the fuel consumption as well as higher emission values. The emissions are increased in the same ratio as the fuel consumption.
Fixed Increase of Fuel Consumption The additional fuel consumption as a function of the engine temperature is defined.
Calculated Increase Factor By means of the viscosity constants, the oil temperature, and the resulting viscosity, the mean friction pressures during the run-up stage and the instantaneous additional power dissipation can be computed. The total power required by the vehicle, which is increased in this stage due to the cold starting conditions, results in the required mean pressure, which also is higher than that in the hot stage.
The increase factor is now related to the mean pressure at current temperature and the mean pressure of the hot engine.
Transformed operating point This model corresponds to the Increase Factor model. The mean friction pressure for the cold start is calculated. This additional mean pressure causes a higher BMEP in the engine's operating map. For this new operating point, the actual fuel consumption, as well as the emissions, can be determined in the engine maps.
For the engine control, there are several variants, which can also be combined:
- Fuel cutoff in the overrun
- Engine shut-off
- Start-Stop automatic
- Cylinder cutoff
Fuel cutoff in the overrun
Activation of this option in the engine cuts off fuel injection while motoring for saving fuel.
With the component Engine shut-off it is possible to turn the engine off while the vehicle is in standstill. Some conditions like a minimum engine temperature or minimum battery charge have to be fulfilled for the engine shut-off to be activated.
Similar to the Engine shut-off, the Start-Stop automatic turns the engine off while some conditions are fulfilled. In contrast to the Engine shut-off, the Start-Stop automatic is only activated while the neutral gear is shifted, the load signal is zero and the vehicle speed is below a specified border.
The Cylinder Cutoff Engine is directly connected to the main engine. The number of active cylinders sets the Cylinder Cutoff Engine apart from the main engine. Only data, which are not identical to the main engine, has to be input in the Cylinder Cutoff Engine (full load characteristic, fuel consumption map, etc.). Via the Cylinder Cutoff Control the decision is made which engine is activated, either the main engine or the Cylinder Cutoff Engine.
The component Exhaust System considers the aftertreatment of the engine's raw emissions in a catalytic converter. Conversion efficiencies for the single exhaust gas components on the basis of temperature dependent maps are used for computing the exhausted emissions. The temperature of the catalytic converter is determined by considering the amount of energy transmitted to the converter from the exhaust gases.
CRUISE model of an Automatic RWD vehicle with Cylinder Cutoff, Start-Stop Automatic and Exhaust System
- Weight of the catalytic converter
- Specific heat capacity
- Heat transfer coefficient between the catalytic converter and the air
- Reference surface between the catalytic converter and the air
- Heat transferred from exhaust gas to Exhaust System
- Conversion efficiencies for the exhaust gas components versus temperature
Driver, Cockpit and Course Components
The Driver component simulates the behavior of an actual driver. In order to do this, CRUISE receives information that is also available to a driver of a vehicle in real-life situations, such as the route data, the instantaneous driving state, the vehicle's state, etc. The input variables are put into action by considering the driver type with the characteristic properties for driving off, gear shifting and braking. The Cockpit component forms the interface between the Driver and the Vehicle. Road conditions like friction coefficient and ambient influences, such as air temperature or wind velocity, can be considered in the computation via the Course component.
The obtainable values for the fuel consumption and the vehicle performance in a driving test are mainly influenced by the quality of the driver's behavior. The driver's properties, characterizing the driver type, are the basis of decisions on reactions to input signals.
The modular concept of CRUISE makes it possible to combine different drivers with the same vehicle for performing a large variety of possible tasks, by changing only driver characteristics. In the reverse case, the same driver module can be used with different vehicles for the fuel consumption as well as for performance tests.
The vehicle can be started in different ways:
- Starting test-like with manually shifted vehicles: At a definable launch speed, the clutch is engaged following a parabolic curve.
- Starting test-like with automatically shifted vehicles.
- Starting customer-like with manually shifted vehicles: A definable launch speed is controlled during the starting process.
- Starting customer-like with automatically shifted vehicles: The vehicle is starting with a definable launch speed.
In CRUISE there are several possible ways to define when the driver has to shift gears:
- Shifting gears on points defined in the profile of the vehicle speed
- Shifting gears according to:
- vehicle speeds
- speeds of one of the components
- best acceleration performance
Accelerator and clutch pedal paths, and gear step for a shift from 2 nd into 3 rd gear
The user can also define the gear shifting process itself. Acceleration and clutch pedal paths, and the gear change position within the shift window are Driver properties used for attaining realistic behavior.
As in reality, the Vehicle, the Driver and the environment are completely separated within CRUISE. The Cockpit component serves as the link between the Driver, the Vehicle, and the environment. This component defines what data and information are available to the Driver, and what possible influences over the Vehicle the Driver has.
The Course component describes the environmental conditions for a calculation task. Among other things, the instantaneous atmospheric pressure and density, as well as the temperature are acquired to describe the ambient conditions.
- The following input data of the Course can be defined as function of distance or time:
- Ambient temperature
- Wind velocity in the driving direction
- Course signals, which can be used as external input signals to the vehicle model
The remaining input data of the Course are distance-dependent:
- Atmospheric density or pressure
- Friction coefficient
- Speed limit
Interface – AVL BOOST
Vehicle power train simulation is available in a great variety of application fields. The layout of the power train itself changes with new customer and legislator demands. To meet all the requirements of new innovative power train concepts, flexibility is important when arranging standard components with special tools for certain complex parts of a power train to maintain fast modeling for various applications. With regard to the engine, the link with AVL BOOST makes this possible.
The engine forms a central component in power train simulation. There are several ways to describe this element but mostly there is a compromise with respect to calculation speed and data availability. For vehicle performance analysis a full load characteristic and a motoring curve or load signal map obtained from measurement is satisfactory in most cases. Also, the effects of a supercharged engine are open for a sensible correction by a pressure build up characteristic.
The determination of fuel consumption and emissions is usually based on stationary measured maps which return the current rate of fuel used and emission produced respectively for any given set of speed and load of the engine. All the effects regarding quick variation of load signal and engine speed are not taken into account.
Co-Simulation CRUISE - BOOST
For a very detailed and accurate interaction between vehicle and engine, the co-simulation of CRUISE and BOOST is the best solution. BOOST is the simulation software used for s tate-of-the-art engine cycle analysis and simulation. Steady state and transient engine performance optimization are the main goals but also the prediction of intake and exhaust orifice noise is performed.
The “BOOST Engine” component makes the link between CRUISE and BOOST. It transmits the necessary information from CRUISE to BOOST (driver's load signal, speed of the power train) and vice versa (engine torque, fuel consumption). The interface component only needs the reference to the BOOST engine model file and the dynamic link library of the BOOST code, which is delivered with BOOST license.
- Transient on-line mode: CRUISE and BOOST run parallel in a co-simulation with the same time scale and both programs exchange data with each other for every time step. In this calculation mode, transient effects like the engine load response in acceleration tests can be investigated.
- Stationary off-line mode: At the beginning of the CRUISE calculation BOOST is called to compute stationary data (e.g. full load characteristic, fuel consumption map). The method of recording maps and characteristics is similar to a stationary measurement procedure of a real engine. The data logged by CRUISE in this initialization is used afterwards when BOOST is off-line.
- Consideration of transient behavior of the engine within drive train simulation
- Evaluation of new engine concepts with consideration of transient effects
- Accurate load response calculation
Interfaces – MATLAB / SIMULINK, Black Box
The variety of calculation models for modern drive train layouts is increasing continuously, especially when using vehicle simulation in the field of advanced product engineering. With new power train concepts, the need for new parts or further development of existing components arises. By using the CRUISE interface components Black Box, MATLAB DLL and MATLAB API, models can be designed and implemented appropriate to the demands of the engineer. For instance, it is possible to integrate complex control units for gearboxes as well as mechanical and electrical components developed by the user. Contractors are not required to disclose their know-how with the supplied parts for the Black Box and MATLAB DLL interfaces, as important information can be concealed by linking models to a dynamic link library. MATLAB a API is applied for the co-simulation between CRUISE and a MATLAB a or SIMULINK a model.
Controllers, as well as mechanical and electrical elements created using MATLAB / SIMULINK, can be integrated into the computational model of CRUISE. Vehicle parts designed with SIMULINK are exported, compiled and linked to a dynamic link library (DLL) by using Real-Time Workshop. This is part of the MATLAB software package. The CRUISE interface component MATLAB DLL only needs the reference to this dynamic link library. The communication to the linked element is done by SIMULINK vectors called inports and outports which receive signals from and pass signals to the Data Bus.
The advantage of this procedure is that once a DLL of a model is created with Real-Time Workshop , installation of MATLAB or one of its parts is not required for running the model with CRUISE. Saving technical know-how, e.g. CVT control units, is also an important topic for the supplier. This can be guaranteed by creating dynamic link libraries.
The interface component MATLAB API is similar to MATLAB DLL.The link from the MATLAB API to a MATLAB or SIMULINK model is done via co-simulation. Both CRUISE and MATLAB are loaded in parallel. The data exchange and calculation control are executed by using the MATLAB Engine, the communication environment of MATLAB / SIMULINK.
The transfer of the input and output values between CRUISE and MATLAB is done by vectors. CRUISE provides the input vector with data from the Data Bus. This input vector is written into the MATLAB workspace. The MATLAB or SIMULINK model receives the elements of the vector by “Constant” blocks. Otherwise, signals created by the model are written into the MATLAB workspace in vector format by using the “To Workspace” block. CRUISE passes the elements of this vector to the Data Bus where they can be used by other components.
The advantage of this type of interface is that every feature of MATLAB / SIMULINK, including graphical elements like Scopes or Plots, can be integrated into the model. It is also possible to do on-line parameter settings of the user model during the co-simulation. Finally, additional software like Real-Time Workshop a or a compiler is not required.
The program code implemented with C or FORTRAN programming languages can be included into the vehicle model of CRUISE without having to compile the entire CRUISE code. This is done by linking the user-defined subroutine to a dynamic link library (DLL), which is then referenced by the element Black Box. Input and output vectors as well as parameters are the communication elements between CRUISE and the user-defined DLL. This method is highly applicable for integrating control algorithms and other third party codes into CRUISE.