Wheels: Simulation Parameters

Parameters for Wheels Road Load Simulator

Road Load Parameters
The energy necessary to propel a vehicle is determined by several road load parameters.

C_d Coefficient of drag - a dimensionless constant that describes the aerodynamics of the vehicle.

Frontal area The total area contained within a longitudinal view of the vehicle projected onto a plane in front of or behind the vehicle. In the simulation, it acts with the drag coefficient to determine the force on the vehicle due to air resistance at a given speed.

C_rr Coefficient of rolling resistance - the forward force exerted by the tires as they roll under the weight of the vehicle. For a given tire design, rolling resistance is mainly a function of vehicle weight. Hence it expresses the force opposing vehicle motion per unit vehicle weight. Since force and weight are both units of force, the quotient is dimensionless, making the rolling resistance coefficient a dimensionless constant. Possible values range from 0.005 lb/lb (special high-pressure, experimental tires) and 0.011 (ordinary radial tires) .

Curb mass The mass of the vehicle itself, without driver or payload.

Payload The additional mass of the driver and any other items such as luggage that are assumed to be present during the test.

Rotational inertia factor Estimates the inertia invested in rotating powertrain components, such as engine pistons and crankshaft, transmission components, driveshaft, and wheels. This inertia affects energy and power demand similarly to the mass of the vehicle, but mass only accounts for translational inertia. When inertia is calculated, the rotational inertia factor is applied to the vehicle mass to artificially increase mass enough to account for rotational inertia.

Wheel diameter The outermost diameter of the wheel/tire assembly at the drive wheels. This information is needed to compute the wheel rotation speed, which is one of the outputs of the simulation.

Air density Air density is important to computation of aerodynamic drag. Air density varies with temperature, pressure (elevation), and humidity.

Other parameters

Fuel heating value Thermal energy available in a gallon of fuel. The default value of 115,000 btu/gal is the lower heating value for gasoline.

Average engine efficiency The average thermal efficiency of the internal combustion engine for the overall duty cycle that results from following the driving cycle. Although the peak efficiency of en engine can be as high as 45%, the average efficiency over a duty cycle will usually be much lower.

Average drivetrain efficiency The average efficiency of the mechanical components between the engine output shaft and the wheels (transmission, differential, etc.)

Average regenerative braking efficiency The percentage of braking energy successfully captured, stored, and returned to the wheels by means of a regenerative braking system. This includes not only the mechanical efficiency of the components involved, but also the percentage of braked energy that actually enters the system rather than being friction braked.

Wheels Road Load Simulation • Virtual Car Home Page

C_d	Coefficient of drag - a dimensionless constant that describes the aerodynamics of the vehicle.
Frontal area	The total area contained within a longitudinal view of the vehicle projected onto a plane in front of or behind the vehicle. In the simulation, it acts with the drag coefficient to determine the force on the vehicle due to air resistance at a given speed.
C_rr	Coefficient of rolling resistance - the forward force exerted by the tires as they roll under the weight of the vehicle. For a given tire design, rolling resistance is mainly a function of vehicle weight. Hence it expresses the force opposing vehicle motion per unit vehicle weight. Since force and weight are both units of force, the quotient is dimensionless, making the rolling resistance coefficient a dimensionless constant. Possible values range from 0.005 lb/lb (special high-pressure, experimental tires) and 0.011 (ordinary radial tires) .
Curb mass	The mass of the vehicle itself, without driver or payload.
Payload	The additional mass of the driver and any other items such as luggage that are assumed to be present during the test.
Rotational inertia factor	Estimates the inertia invested in rotating powertrain components, such as engine pistons and crankshaft, transmission components, driveshaft, and wheels. This inertia affects energy and power demand similarly to the mass of the vehicle, but mass only accounts for translational inertia. When inertia is calculated, the rotational inertia factor is applied to the vehicle mass to artificially increase mass enough to account for rotational inertia.
Wheel diameter	The outermost diameter of the wheel/tire assembly at the drive wheels. This information is needed to compute the wheel rotation speed, which is one of the outputs of the simulation.
Air density	Air density is important to computation of aerodynamic drag. Air density varies with temperature, pressure (elevation), and humidity.

Fuel heating value	Thermal energy available in a gallon of fuel. The default value of 115,000 btu/gal is the lower heating value for gasoline.
Average engine efficiency	The average thermal efficiency of the internal combustion engine for the overall duty cycle that results from following the driving cycle. Although the peak efficiency of en engine can be as high as 45%, the average efficiency over a duty cycle will usually be much lower.
Average drivetrain efficiency	The average efficiency of the mechanical components between the engine output shaft and the wheels (transmission, differential, etc.)
Average regenerative braking efficiency	The percentage of braking energy successfully captured, stored, and returned to the wheels by means of a regenerative braking system. This includes not only the mechanical efficiency of the components involved, but also the percentage of braked energy that actually enters the system rather than being friction braked.