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
Testing
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
(a)
(c) (d)
(b)
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.
EuroNCAP
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
includes:
• Allgemeiner Deutscher Automobil-Club e V
(ADAC), motoring organisation – Germany
• Bundesministerium fur Verker, Bau- und
Wohnugswesen, government department –
Germany
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.