Dynamics of fuel economy
- From: pluto <pluto@xxxxxxxxxxxx>
- Date: Sun, 12 Feb 2006 08:16:59 +0800
Sunday February 12, 2006
Dynamics of fuel economy
The Garage by DADDY FIXIT
A lot of consideration is given to engine, electronics, tyre and suspension
design as a means of achieving fuel efficiency and better speed, but few of
us even stop to consider vehicle aerodynamics as a means of achieving both
fuel economy and speed.
Aerodynamics is the study of liquid flow, and in the case of motor
vehicles, these studies revolve around how the flow of air (a gaseous
liquid) interacts with the forward movement of the car. Air resistance is
the major force that is exerted on the vehicle.
The aerodynamics that allow an aircraft to take off also apply to motor
vehicles. The difference is that in the case of the motor vehicle, the
aerofoil cross-section is mounted upside down, so that a down force,
instead of lift is produced.
Vehicle and aircraft designers use what is termed as the Bernoulli effect
to create down force and lift. The Bernoulli effect means that the faster
the liquid flows past a surface, the lower the pressure on that surface. In
the case of an aircraft wing, air flows past the upper surfaces at a higher
speed as compared to the lower surfaces. As such, the pressure on the upper
surface of the wing is lower than the pressure at the bottom, thus creating
the ?lifting? effect.
As the aircraft speed increases, so does the pressure difference between
the upper and lower surface of the wings, and eventually the difference in
pressure is sufficient to produce a force that is able to lift a
multi-tonne aircraft off the tarmac.
The reverse is true for a motor vehicle. Racing car designers use the
inverted wing principle to create sufficient down force that helps increase
grip, thus allowing racing vehicles to take corners at a much higher speed,
resulting in higher average track speeds.
Vehicle body design
Irrespective of the vehicle?s aesthetic design, the aerodynamic package of
the vehicle is more important than ever before. Wind resistance increases
fuel consumption, and this is a direct cost associated with vehicles that
have high resistance to forward motion. Indirect costs relating to wind
resistance include increased tyre wear, higher engine maintenance,
premature engine wear and possibly higher accident rates.
In addition, poor aerodynamics introduce high wind noise levels that
contribute to driver fatigue.
Vehicle designers use wind tunnels to develop and design car bodies with
shapes that are pleasing to look at and have as little wind resistance as
possible. Although the ideal vehicle design is one of an inverse aerofoil,
in practice this is not possible since this would greatly inconvenience
drivers and passengers entering and exiting the vehicle. There would be a
restraint on passenger cabin space as well.
Good body design is a compromise of the three ? aesthetics, easy access and
loads of cabin space. Once its shape has been decided, there is little room
for change. This means that the aerodynamic study must be done during the
early design stage before the car shape is fixed. Modern day aerodynamic
simulation tests call for a lot of computation power, and most major
testing facilities have a computer dedicated to aerodynamic calculations.
The drag coefficient
Most of us have, at one time or another, heard of the term ?drag
coefficient? or the simple abbreviation ?Cd?. The drag coefficient is not
an absolute number but a relative number that gives us an indication of how
much wind resistance a vehicle is subject to in a forward motion.
Almost every object has a measured Cd number. A man running in an upright
position had a Cd of between 1.0-1.3 while a poorly designed vehicle has a
Cd of approximately 0.8. A well-designed family saloon would have a Cd of
around 0.4-0.6 while a sports car comes in around 0.2-0.3.
On the other hand, the aerodynamically efficient airplane wing has a low Cd
of 0.05 at cruising speeds.
It must be emphasised that the amount of Cd is not proportionate to the
amount of resistance to forward motion. Thus two vehicle of different body
designs but with equivalent Cd?s cannot be expected to have similar
aerodynamic forces when travelling at identical speeds in similar road and
weather conditions.
The amount of resistance a vehicle is subject to is a combination of the
Cd, the frontal area and the speed at which the vehicle is travelling. Thus
a vehicle travelling at 60 km/h can expect only a quarter of the resistant
aerodynamic force compared to a vehicle travelling at 120 km/h, which
explains why driving at high speed is an expensive past time.
Modern vehicle designs are able to take advantage of vast amounts of
computing power in order to optimise their aerodynamic design. As a result,
we are beginning to see massive fuel savings in modern vehicles; some
modern vehicles are at least 30-50% more fuel-efficient compared to
vehicles of the similar engine sizes sold some 10 years ago. Coupled with
better engines, tyre and suspension design, it looks like the fuel
efficient vehicle is set to improve further.
Daddy Fixit is a mechanical engineer with a PhD in automotive engineering.
He worked at a multi-national car and truck engine designer firm where his
area of specialty was software design for engine control. He has co-written
a book on diesel engines. You can write to him at starmag@xxxxxxxxxxxxxxx
http://thestar.com.my
=========================end, and/or end quote================
-pluto
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