## Using WAVE: Sub-Models: Simple Load

#### Description:

The simple load sub-model represents a compliant system with lumped inertias and a single gear ratio.  It is intended to represent a generic load system and could be adapted to represent a generator, propeller, pump, or even a simple vehicle model.  The sub-model consists of four inertias representing an engine, gears, and load inertias.  These inertias are coupled by flexible drive shafts having universal joints at both ends of each shaft.  The gear inertias are directly coupled.

Each compliant shaft has a stiffness and damping rate associated with it.  These compliances allow the simulation to capture some of the important torsional vibrations present.  The universal joints are capable of providing additional torsional excitations when operated through a nonzero angle.  For rigid shafts, the universal joint angles are zero.

The model also allows damping between the engine and ground, gearbox and ground, and load and ground.  These represent external load torques on the system caused by bearing and windage losses.

The externally supplied torques acting on the drive line are the engine torque and load torque.  The engine torque will be supplied by the WAVE calculation, which depends upon the instantaneous engine speed and fuel rate.  Engine friction losses are also accounted for, using either the Chen-Flynn or IRIS friction sub-model.  The load torque is determined from user inputs.  Three possible specifications of load torque are allowed:

The first method for describing load torque is a table of arbitrary load torques as a function of time.  The second method is to specify the load torque versus angular velocity and another independent load input in a table.  This independent input could represent generator current, propeller pitch or pump pressure ratio, for example.  It is assumed that the user can supply such a correlation.  The independent load input is then specified in the form of a table of arbitrary values as a function of time.  The angular velocity as a function of time is developed from the load model itself.  The third method is to attach a torque actuator on the load side of the model.

The net torque on each inertia is used to determine the acceleration of the inertia.  These accelerations are integrated to develop the angular velocity and position of each element.

The speeds of each load component are initialized to correspond with the initial engine speed specified by the user with the gearbox ratio taken into consideration.  The initial shaft twists are set to zero.

#### Inputs:

The lumped engine rotational inertia.  The engine inertia value needed by the sub-model is the summation of the inertia of all the rotating parts.  Typically this will be dominated by the flywheel and crankshaft values.

The lumped rotational inertia of the load.

Engine viscous damping coefficient.  Normally this variable is set to zero since it represents friction losses due to engine rotation, which has already been accounted for in the engine friction model.  However, the friction model is only updated on a cyclic basis, while this variable responds to instantaneous speed fluctuations.

Switch to specify how loading is to be applied.  It may have one of three values:

 Load Type Option Description Time The load torque is specified as a function of time.  Two variable arrays, time and load, are required. Table An external file is read which contains a two-dimensional array of load torque vs. load speed and an independent control variable.  A schedule of the independent control variable vs. time is also read.  To set the control variable to a constant value, set the value at a time of 0 in the control variable profile. Actuator Load is applied via an existing torque or power actuator.

Name or torque or power actuator attached to the engine which is used to represent the load.  Only selectable if the load type is set to "Actuator".

File tag or <pathname> of the external load characteristics file containing tabulated values of load rotational speed (rpm), the independent input variable values at which the load torque is specified, and values of the load torque (N*m, N*m, lbf*ft) at those speed and independent variable breakpoints.  If a file tag is used to load the external load characteristics file, it must be defined as type "LOAD" in the active.tags file.

Switch to specify type of shaft to be modeled.  It may have one of two values:

 Model Option Description Rigid Shaft is rigid.  The inertia of the gear in the gearbox will be lumped with the engine/load side inertia and the relevant joints are removed from the model. Compliant Shaft is compliant.  The inertia of the gear is treated as a separate mass and further shaft and joint definitions are required.

Shaft stiffness.  Not required if the shaft type is "Rigid".

Shaft viscous damping coefficient.  Not required if the shaft type is "Rigid".

Angles of the joints where the shafts attach to the engine, gearbox, and load masses.

Phasing of the gears in the gearbox and the load mass in respect to the previous joint (from engine to load, directionally).  The first joint (engine-side gear in the gearbox) is measured relative to top dead center of cylinder #1 on the engine.

Number of teeth of the engine-side and load-side gears.  This determines the transmission speed ratio of the gearbox.

Inertia of the engine-side and load-side gears.  If the attached shaft is "Rigid", this inertia is lumped with the engine or load inertia, respectively.

Gearbox viscous damping coefficient.

#### Outputs:

The table below lists all of the available time plots from the simple engine load sub-model.  Click on any plot title to see more details.

The simple engine load sub-model creates no specific summary quantities.

#### Sensors:

There are no sensors which read variables from the simple engine load sub-model.

#### Actuators:

The table below lists all actuators which can attach to the engine on which the simple engine load sub-model is applied.  Click on any actuator type for more details.

 ACTUATOR TYPE VARIABLE UNITS Power   This actuator adds or removes shaft power to an element.   A positive value adds shaft power while a negative value removes shaft power.   If more than one is attached to an element, the results will be additive.   ATYPE=POWER in the CTL:ACTUATOR block. W Shaft Torque   This actuator adds or removes torque to an element.   A positive value adds power while a negative value removes power.   If more than one is attached to an element, the results will be additive.   ATYPE=TORQUE in the CTL:ACTUATOR block. N*m

#### Notes:

When either the torque or power actuator is attached to the engine and is used as the driver for the simple engine load sub-model, it does not add power to the engine.  Rather, it is the driving load torque or power.