The air resistance force of a vehicle in motion is proportional to the air density, the front area of the vehicle, its aerodynamic coefficient Cd and the square of its speed V2. Here the speed to be considered should take into account the possibility of a component of wind opposing motion.
The front area of present-day passenger vehicles can vary from 2 to 3 sq.m. and that of a truck from 8 to 10 sq.m. The front area is mainly reduced by reducing the width of the vehicle at the level of the heads of the occupants.
The aerodynamic coefficient, often further refined in a wind tunnel, varies between 0.30 and 0.35. On average, these values were reached many years ago but recent progress has been obtained for example by bonding windshield joints or wrapping the lower part of the engine compartment.
The value of the Cd results from any cause creating air turbulence that consumes energy instead of a laminar system. For example, this is the case for dusty bodywork, roof bars, a radio aerial and open windows. Rear-view mirrors, currently streamlined consume less energy than in the past.
Example of a typical value for a middle-range passenger vehicle:
Cd = 0.30; S = 2 sq.m.; air density = 1.3 kg/cu.m.;
We obtain F = 0.39 V2. (V in m/s, obtained by dividing V in kph by 3.6)
Thus, at 36 kph (i.e. 10 m/s), F = 0.39 x 100 = 39 N and power F x v = 0.39 kW
50 kph F = 0.39 x 192 = 75 N 1.05 kW
90 kph F = 0.39 x 625 = 244 N 6.1 kW
130 kph F = 0.39 x 1304 = 509 N 18.4 kW
Note that these force values are those required “at the wheels”. The corresponding power consumed by the engine must take the output into account. For example, for a conventional engine with an output of around 0.30 at 130 kph, the power consumed is around 61 kW… and the energy for 100 km traveled at this speed is close to 47 kWh which corresponds to a fuel consumption of around 4.5 l /100 km.
Research into improving the aerodynamics of vehicles often comes up against safety, length, space design or ease of baggage stowing capability requirements.
The major progress achieved with the Cd is often offset by the increase in the front area of certain present-day passenger vehicles.
In the field of trucks, the front area is four to five times larger and the Cd difficult to optimize, particular for semi-trailers. The spoiler on cabs, the new cab profiles, the windshield inclination and the use of lateral sliding panels have enabled a substantial gain to be made for these vehicles. But the largest gain in energy is mainly achieved here at a lower mean speed than for passenger vehicles on roads or freeways.