Crucial product features of an automobile, e.g. fuel consumption, maximum speed and acceleration depend on air resistance. This is illustrated on the basis of power demand:
Pbed = PRoll + Pa + PL
with the air resistance power
PL = 0,5 * cw * A * ΡL * v3
Air resistance is described by:
FL = 0,5 * cw * A * ΡL * v2 with
cw Air resistance co-efficient,
A Projected cross-sectional area of vehicle,
v Vehicle velocity,
ΡL Density of air
The vehicle velocity appears as a square in the calculation of the air resistance. The diagram shows the rapid increase of the fuel consumption at a velocity of about 100 km/h. At a speed of 120 km/h, about 80% of the engine power is used to overcome air resistance. The vehicle manufacturer can influence the cw value and the projected area. The cw is a measure of the quality of the aerodynamic form. For a better understanding, the theoretical basics of fluid mechanics are described.