532 | Thermodynamics
noise and engine wear during highway driving by decreasing the engine rpm
while maintaining the same vehicle speed.
Front wheel driveoffers better traction (because of the engine weight on
top of the front wheels), reduced vehicle weight and thus better fuel econ-
omy, with an added benefit of increased space in the passenger compart-
ment. Four-wheel drive mechanisms provide better traction and braking thus
safer driving on slippery roads and loose gravel by transmitting torque to all
four wheels. However, the added safety comes with increased weight, noise,
and cost, and decreased fuel economy. Radial tiresusually reduce the fuel
consumption by 5 to 10 percent by reducing the rolling resistance, but their
pressure should be checked regularly since they can look normal and still be
underinflated. Cruise controlsaves fuel during long trips on open roads by
maintaining steady speed. Tinted windowsand light interior and exterior
colors reduce solar heat gain, and thus the need for air-conditioning.BEFORE DRIVINGCertain things done before driving can make a significant difference on the
fuel cost of the vehicle while driving. Below we discuss some measures such
as using the right kind of fuel, minimizing idling, removing extra weight,
and keeping the tires properly inflated.Use Fuel with the Minimum Octane Number
Recommended by the Vehicle Manufacturer
Many motorists buy higher-priced premium fuel, thinking that it is better for
the engine. Most of today’s cars are designed to operate on regular unleaded
fuel. If the owner’s manual does not call for premium fuel, using anything
other than regular gas is simply a waste of money. Octane number is not a
measure of the “power” or “quality” of the fuel, it is simply a measure of
fuel’s resistance to engine knock caused by premature ignition. Despite the
implications of flashy names like “premium,” “super,” or “power plus,” a fuel
with a higher octane number is not a better fuel; it is simply more expensive
because of the extra processing involved to raise the octane number
(Fig. 9–59). Older cars may need to go up one grade level from the recom-
mended new car octane number if they start knocking.Do Not Overfill the Gas Tank
Topping off the gas tank may cause the fuel to backflow during pumping.
In hot weather, an overfilled tank may also cause the fuel to overflow due
to thermal expansion. This wastes fuel, pollutes the environment, and may
damage the car’s paint. Also, fuel tank caps that do not close tightly allow
some gasoline to be lost by evaporation. Buying fuel in cool weather such
as early in the mornings minimizes evaporative losses. Each gallon of
spilled or evaporated fuel emits as much hydrocarbon to the air as 7500
miles of driving.FIGURE 9–58
Aerodynamically designed vehicles
have a smaller drag coefficient and
thus better fuel economy than boxlike
vehicles with sharp corners.
FIGURE 9–59
Despite the implications of flashy
names, a fuel with a higher octane
number is not a better fuel; it is simply
more expensive.
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