McFeng

3. Pressure measurements  

These measurements help to provide a partial solution to the problems of engine cooling and passenger cabin air-conditioning.

Pressures are expressed as a non-dimensional coefficient independent of speed:

Cp = P-Po
1/2 V2

Bernouilli's equation : 1/2 pV2 + P = Constant along an air fillet is valid as a first approximation for the front of the vehicle. It proves that high-pressure areas have low air speed. Conversely, where the air fillets cling to the body, there is a depression. (The drawing will help you to understand this relation between air speeds and pressures at any point).

Pressure measurements make it possible to trace isobars (lines perpendicular to the air fillets visualising the lines along which pressure remains equal at a given value) on the vehicle's form.

The choice of the position of the air inlet for passenger-cabin air-conditioning will lie in the pressure zone at the foot of the windscreen.

AN AIR OF ECONOMY  

The power that the engine must develop in order to overcome different forms of resistance (apart from accelerations and gravity) is represented by the formula:

W wheels  = N. Wm = 1/2QCxSV3 +f. M.V.

where:

• W wheels = power on driving wheels
  

• N = transmission efficiency
  

• Wm = power on engine output shaft
  

• Q = air density
  

• S = frontal area of car
  

• Cx = coefficient of drag
  

• f = frictional coefficient
  

• M = vehicle mass
  

• V = vehicle speed
  

• 1/2 CxSV3 = aerodynamic resistance
  

• f.M.V. = running resistance
  

The two following graphs represent respectively the formulae above, applied to a current standard production car, and the car's consumption in litres per 100 km according to speed. We can note the importance of streamlining for speeds approaching and over 90 km/h /56 mph), and the similarity between resistance and consumption curves, demonstrating the important role played by aerodynamics in fuel saving.

To travel at 120 kph (75 mph)

• the B2 of 1921 with a CxS of 1,437 required 75 bhp

• the Traction of 1934 with a CxS of 1,230 required 56 bhp

• the DS of 1956 with a CxS of 0,817 required 48 bhp

• while the GSA X3 of 1980 required only 31 bhp thanks to its very low CxS of 0,575.

When a car's CxS is improved by 10 %, consumption at 120 km/h (74.6 mph) goes down by 7 %, at 90 km/h (56 mph) by 5 %, in town traffic by 1 %.

The characteristics of motor cars' resistance to motion can be improved by entirely redesigning their bodywork.

The bodies of current 4-door saloons have Cxs coefficients lying between 0.575m2 (6.19 sq.ft) for the best and 0.900 m2 (9.69 sq.ft) for the least good, or CxS of between 0.32 and 0.48.

The improvement of known aerodynamic forms can also be achieved by fitting corrective adjuncts.

The drag of the Citroën GSA X3 has been reduced by the addition of a forward spoiler (which acts as a deflector and reduces the air-flow under the body) and of an aerofoil beneath the rear window (which reduces drag by modifying the rear lift characteristics), and by fitting a better streamlined rear-view mirror. The spoiler improves the CxS by 2.7 %, the aerofoil by 7.5 %. Both together improve the CxS by 10 % and reduce petrol consumption by 7.5 % at 120 km/h (74.6 mph).

And yet, if in the future every manufacturer is to be faced with achieving the best aerodynamic shapes, the results of these studies will always remain subordinate to sociological and legal possibilities (impossibility of producing extremely long, low cars, for instance), just as it will remain combined with the stylist's own trends.

In this respect, let us point out the misconception of which stylists, manufacturers, journalists and public alike are guilty about items such as spoilers and aerofoils, considered by one and all as optional extras for a sports car, whereas in fact, when, properly designed, they are first and foremost energy economisers which could easily and cheaply by provided for in standard production models.

Knowing that the shape of the body weighs heavily in the decision to buy a car, it is the stylists, at Citroën's, who combine the results of body studies with respect of the various restrictions they must comply with:

1. volume restrictions regarding overall dimensions, inside spaciousness, the engine, location of the tank, the spare wheels and the boot.
   

2. accessibility restrictions for the number of doors and visibility restrictions for the windows.
   

3. standards for shock absorbers, head-lamps, rear lights, traffic indicators, number plates etc...
   

4. restrictions connected with production, such as stamping and assembly problems with sheet metal whose nature imposes the shapes being cut up in various elements.
   

The aerodynamicist, for his part, has to check the results found with the first few shapes made up, and to suggest possible improvements to the stylist. The entire effectiveness of their collaboration takes form in their definition of an ambitious aerodynamic performance project and in their aptitude to achieve it in a seductive form.

This mode of collaboration between stylist and technician has been and remains characteristic of the genesis of Citroën models.

Long before the energy crisis was upon us, and by mere logic and a desire for a coherent approach to car-body design, Citroën models had already achieved exemplary CxS values recognised by specialists the world over.

These studies undertaken years ago now ensure for Citroën a far from negligible advance where the influence of "the right shape" on reducing fuel consumption is concerned.

Below - one of the first -styling sketches for the CX

Aerodynamic co-efficient values for various Citroëns in standard road-going configuration

This article originally appeared in Double Chevron #59 © Automobiles Citroën 1980

© 1999 Julian Marsh