Environmental Engineering Reference
In-Depth Information
It does not specify if ton is short, long, or metric nor if the gallons are US or British gal-
lons. These two problems can be eliminated by using tonne-km per liter instead.
●
It does not specify the type of fuel, which is usually diesel.
●
In the case of products with large volume and low mass, like many food products, the
volumetric capacity of the vehicle is reached before its weight limit and ton-mile/gallon
fails to describe fuel efficiency effectively in these cases.
●
TRANSPORTATION EFFICIENCY
Factors that affect fuel economy
In land vehicles, fuel economy is contingent on forces that oppose to their advance. Opposing
forces depend on intrinsic factors of the vehicle, such as rolling resistance, drag, drive train
losses, and extrinsic factors such as road surface, road flatness, and density of the air. Rolling
resistance is a force that opposes motion as wheels turn. It happens as a result of deformation
and recovery at the point the wheel contacts the ground and the energy resulting from defor-
mation lost as heat. The magnitude of the rolling resistance force is a function of the material
the wheel is constructed of, the load supported by the wheel, and the rolling surface. Because
of low deformation of wheels and rolling surface, steel-made train wheels running on steel
rails have the lowest rolling resistance of all wheeled vehicles. At the other end of the spec-
trum, regular rubber tires on a soft surface (e.g., sand) have the highest rolling resistance.
Losses in the drive train include all the friction of moving components in the engine and
transmission as a result of viscosity of the lubricant, ball bearings, and friction surfaces as well
as loses to auxiliary pumps and compressors.
Of the extrinsic factors, road flatness has an important effect on fuel efficiency. Any vehicle
that rolls on roads is subjected to ups and downs that significantly affect the amount of fuel
consumed. Trains, on the other hand, have the advantage of running on relatively level rail-
roads without severe slope changes, unlike road vehicles; therefore, they can maintain rela-
tively constant speeds and consequently be more fuel efficient.
In addition to land vehicles, planes and vessels are affected by drag, which results from the
speed difference between the solid vehicle body and the surrounding air or water. There are
several types of forces that oppose a vehicles moving through a fluid. The most common are
“pressure drag,” which depends on the shape of the vehicle and “skin friction” that is the result
of viscous forces between the vehicle and the fluid. In planes, besides pressure and skin drag,
there is an additional drag force that is the result of the lift and is called “induced drag,” which
is important (Smith, 1992).
For any vehicle, the force produced by drag, F
d
, can be expressed as:
F
d
=
−
0.5
r
v
2
A
C
d
[13.1]
is the fluid viscosity,
v
2
is the relative speed between the vehicle and the
surrounding fluid,
A
is the characteristic area, and
C
d
the drag coefficient. For calculating the
pressure drag, the characteristic area is normally the cross-sectional area of the vehicle; and
for skin drag the area exposed to the flow also known as “wetted area.” It is important that the
drag coefficient used in any calculations is determined for the characteristic area in use.
Because
F
d
is affected by the square of the velocity, vehicles that move fast are penalized
by this force more severely than vehicles that move relatively slowly.
F
d
is proportional to the
density of the surrounding fluid. Because water is about 800 times denser than air then vessels
are severely affected by this factor. However, even when cargo ships move through a
Where,
r
