Engine performance and testing

Development engineers prepare to test an engine in

a computer-linked test cell to establish the optimum

settings for best performance, economy and emission

levels. With the increasing emphasis on

performance with economy, computers are used to

obtain the best possible compromise. They are also

used to monitor and control prolonged engine testing

to establish reliability characteristics. If current

engines and transmissions are to be used for a new

model, a programme of refining and adapting for

the new installation has to be initiated. However, if

a completely new engine, transmission or driveline

configuration is to be adopted, development work

must be well in hand by this time.

Aerodynamics and wind tunnel


Aerodynamics is an experimental science whose aim

is the study of the relative motions of a solid body

and the surrounding air. Its application to the design

of a car body constitutes one of the chief lines of the

search for energy economy in motor vehicles.

In order to move over flat ground, a car must

overcome two forces:

1 Resistance to tyre tread motion, which varies

with the coefficient of tyre friction over the

ground and with the vehicle’s mass.

2 Aerodynamic resistance, which depends on the

shape of the car, on its frontal area, on the density

of the air and on the square of the speed.

One of the objects of aerodynamic research is to

reduce the latter: in other words to design a shape

that will, for identical performance, require lower

energy production. An aerodynamic or streamlined

body allows faster running for the same consumption

of energy, or lower consumption for the same

speed. Research for the ideal shape is done on

reduced-scale models of the vehicle. The models

are placed in a wind tunnel, an experimental installation

producing wind of a certain quality and

fitted with the means for measuring the various

forces due to the action of the wind on the model

or the vehicle. Moreover, at a given cruising speed,

the more streamlined vehicle has more power left

available for acceleration: this is a safety factor.

The design of a motor car body must, however,

remain compatible with imperatives of production,

of overall measurements and of inside spaciousness.

It is also a matter of style, for the coachwork must

be attractive to the public. This makes it impossible

to apply the laws of aerodynamics literally. The evolution

of the motor car nevertheless tends towards a

gradual reduction in aerodynamic resistance.

Aerodynamic drag

The force which opposes the forward movement

of an automobile is aerodynamic drag, in which

air rubs against the exterior vehicle surfaces and

forms disturbances about the body, thereby

retarding forward movement. Aerodynamic drag

increases with speed; thus if the speed of a vehicle

is doubled, the corresponding engine power

must be increased by eight times. Engineers

Figure 1.17Interior styling model

(Ford Motor Company Ltd )

24Repair of Vehicle Bodies

body shape, underbody contours and projecting

parts. The fewer disturbances which occur as air

moves past the vehicle, the lower its drag. Threads

on the vehicle exterior as well as smoke streams

indicate the air flow, and enable test engineers to

see where disturbance exists and where air flows

are interrupted or redirected, and therefore where

reshaping of the body is necessary in order to produce

better aerodynamics (Figures 1.19 and 1.20).

Prototype production

The new model now enters the prototype phase.

The mock-ups give way to the first genuine road

going vehicle, produced with the aid of accurate

drawings and without complex tooling and machinery.

The prototype must accurately reproduce the

exact shape, construction and assembly conditions

of the final production body it represents if it is to

be of any value in illustrating possible manufacturing

problems and accurate test data. The process

begins with the issue of drawing office instructions

to the experimental prototype workshop. Details of

skin panels and other large pressings are provided

in the form of tracings or as photographic reproductions

of the master body drafts. As the various

detailed parts are made, by either simple press tools

or traditional hand methods, they are spot welded

into minor assemblies or subassemblies; these later

become part of a major assembly to form the

completed vehicle body.

Prototype testing

Whilst still in the prototype stage, the new car has

to face a number of arduous tests. For these tests

a mobile laboratory is connected to the vehicle by a

cable, which transmits signals from various sensors

on the vehicle back to the onboard computer for

collation and analysis. The prototype will also be

placed on a computer-linked simulated rig to monitor,

through controlled vibrations, the stresses

and strains experienced by the driveline, suspension

and body.

Crash testing (Figure 1.21) is undertaken to

establish that the vehicle will suffer the minimum

of damage or distortion in the event of an impact

and that the occupants are safely installed within

the strong passenger compartment or safety cell.

The basic crash test is a frontal crash at 30 mile/h

(48 km/h) into a fixed barrier set perpendicularly

Figure 1.18Theoretical drag curves for four types

of vehicle, all reduced for comparison purposes to a

front section of 2 m2. Since air resistance increases

in proportion to the square of the speed, a truck with

Cd 1.0 requires 35 bhp at 100 km/h, whereas a coupé

with Cd 0.2 requires only 7 bhp

express the magnitude of aerodynamic drag using

the drag coefficient Cd. The coefficient expresses

the aerodynamic efficiency of the vehicle: the

smaller the value of the coefficient, the smaller

the aerodynamic drag.

Figure 1.18 illustrates the improvements in aerodynamic

drag coefficient achieved by alterations to

the shape of vehicles. Over the years, the value of

Cd has been reduced roughly as follows:

1910 0.95 1960 0.40

1920 0.82 1970 0.36

1930 0.56 1980 0.30

1940 0.45 1990 0.22

1950 0.42 1993 0.20

During the wind tunnel test all four wheels of the

car rest on floating scales connected to a floor

balance, which has a concrete foundation below

the main floor area. The vehicle is then subjected

to an air stream of up to 112 mile/h; the sensitive

balances register the effect of the headwind on the

vehicle as it is either pressed down or lifted up

from the floor, pushed to the left or right, or rotated

about its longitudinal axis. The manner in which

the forces affect the vehicle body and the location

at which the forces are exerted depends upon the

The history, development and construction of the car body 25

Figure 1.19Wind tunnel testing of a prototype: front view (Ford Motor Company Ltd )

Figure 1.20Wind tunnel testing of a prototype: side view (Ford Motor Company Ltd )

to the car’s longitudinal axis. The collision is

termed 100 per cent overlap, as the complete front

of the car strikes the barrier and there is no offset

(Figure 1.22). The main requirement is that the

steering wheel must not be moved back by more

than 120 mm (5 in), but there is no requirement to

measure the force to which the occupants will be

subject in collision. The manufacturers use anthropometric

dummies suitably instrumented with

decelerometers and strain gauges which collect

26Repair of Vehicle Bodies


(c) (d)


Figure 1.21Basic frontal crash and side impact (angled side swipe) tests (Vauxhall Motors Ltd )

Figure 1.22Standard frontal impact test

relevant data on the effect of the collision on the

dummies. A passenger car side impact test aimed

at reducing chest and pelvic injuries will be legal

in the USA from 1993. This stricter standard

requires that a new vehicle must pass a full-scale

crash test designed to simulate a collision at an

intersection in which a car travelling at 15 mile/h

is hit in the side by another car travelling at

30 mile/h. This test is called an angled side-swipe:

the displacement is 27 degrees forward from the

perpendicular of the test vehicle’s main axis. The

test is conducted by propelling a movable

deformable barrier at 33.5 mile/h into the side of a

test car occupied by dummies in the front and

rear seats. The dummies are wired with instruments

to predict the risk potential of human injury.

Volvo do a very unusual promotional crash test

which involves propelling a car from the top of a

tall building (Figure 1.23).

The history, development and construction of the car body 27

Extensive durability tests are undertaken on

a variety of road surfaces in all conditions

(Figure 1.24). Vehicles are also run through water

tests (Figure 1.25) and subjected to extreme climatic

temperature changes to confirm their durability.

lowest possible tooling cost and to a high standard

of quality and reliability.

As competition between the major car manufacturers

increases, so does the need for lighter and

more effective body structures. Until recently the

choice of section, size and metal gauges was based

upon previous experience. However, methods have

now been evolved which allow engineers to solve

problems with complicated geometry on a graphical

display computer which can be constructed to resemble

a body shape (Figure 1.26). The stiffness and

stress can then be computed from its geometry, and

calculations made of the load bearing of the structures

using finite-element methods (Figure 1.27).

With the final specifications approved, the new

car is ready for production. At this stage an initial

batch of cars is built (a pilot run) to ensure that the

plant facilities and the workforce are ready for the

start of full production. When the production line

begins to turn out the brand new model, every

stage of production is carefully scrutinized to

ensure quality in all the vehicles to be built.


The governments in most countries have some form

of regulations covering vehicle safety. These regulations

are aimed at giving both the occupants of the

vehicle protection in the case of an accident, and

ensuring that pedestrians and cyclists are not subject

to unnecessary injury if they come into contact

with a car. The regulations are in most cases very

minimal. In the UK the Department for Transport

(DfT) works with a number of bodies on vehicle

safety, much of the DfT work is sub-contracted

to Transport Research Laboratory (TRL) Ltd –

formerly a wholly government funded institution. In

America there is the United States Department of

Transportation (DOT). There is also the EEVC

(European Enhanced Vehicle-safety Committee).

The most pro-active of vehicle safety organisations

is EuroNCAP. The full title is European New

Car Assessment Programme. This programme is

jointly funded and supported by it members which


- Allgemeiner Deutscher Automobil-Club e V

(ADAC), motoring organisation – Germany

- Bundesministerium fur Verker, Bau- und

Wohnugswesen, government department –


Figure 1.23Volvo crash test

(Volvo Concessionaires Ltd )

Figure 1.24Road testing a prototype

(Ford Motor Company Ltd )

The final stages are now being reached; mechanical

specifications, trim levels, engine options,

body styles and the feature lists are confirmed.