Vehicle Power Requirements
How much power does it take to go 60 mph? 80 mph? The weight of the
car and its speed all are factors that affect the amount of power required.
The force balance of a vehicle is shown in
Figure 4. Force Balance
The forces acting on the car are caused by internal, tire, and air
resistance. The resultant of these forces, the total drag force, FD, can be estimated by the following
cR = coefficient
of rolling resistance
The coefficients of rolling resistance and drag are determined from experiment.
A typical value for the coefficient of rolling resistance is 0.015.
The drag coefficient for cars varies, a value of 0.3 is commonly used.
m = mass of vehicle
A = frontal surface
g = 9.8
density of are, 1.2 kg/m3 @STP
The power output requirement can be determined from the drag force given above
and the vehicle velocity.
P = FDV
Given the mass of a vehicle and its frontal surface area, a plot can
be drawn showing the power requirements for a range of speeds. The
Power Requirement Applet plots this relationship.
required to accelerate to a given speed is also of interest. More power is required for more acceleration.
Applet compares the power required to accelerate from 0 to 60 mph
for a range of times.
The basic definition for the acceleration force (neglecting drag !) is given by:
F = ma
Assuming that the force required to accelerate a vehicle from 0 to 60 mph
can be determined from the above equation, then the power necessary to accelerate
to a given velocity is:
P = maV
m = mass of the vehicle
a = acceleration =
DV = 60 -
0 = 60 mph = 26.82 m/s
V = final velocity, 60 mph