Microsoft Word - Cengel and Boles TOC _2-03-05_.doc

534 | Thermodynamics

the vehicle’s handling adversely, and hurt the fuel economy by increasing the

rolling resistance. Overinflated tires cause unpleasant bumpy rides, and cause

the tires to wear unevenly. Tires lose about 1 psi pressure per month due to air

loss caused by the tire hitting holes, bumps, and curbs. Therefore, the tire

pressure should be checked at least once a month. Just one tire underinflated

by 2 psi results in a 1 percent increase in fuel consumption (Fig. 9–61).

Underinflated tires often cause fuel consumption of vehicles to increase by

5 or 6 percent.

It is also important to keep the wheels aligned. Driving a vehicle with the

front wheels out of alignment increases rolling resistance and thus fuel con-

sumption while causing handling problems and uneven tire wear. Therefore,

the wheels should be aligned properly whenever necessary.

WHILE DRIVING

The driving habits can make a significant difference in the amount of fuel

used. Driving sensibly and practicing some fuel-efficient driving techniques

such as those discussed below can improve fuel economy easily by more than

10 percent.

Avoid Quick Starts and Sudden Stops

Despite the attention they may get, the abrupt, aggressive “jackrabbit” starts

waste fuel, wear the tires, jeopardize safety, and are harder on vehicle com-

ponents and connectors. The squealing stops wear the brake pads prema-

turely, and may cause the driver to lose control of the vehicle. Easy starts

and stops save fuel, reduce wear and tear, reduce pollution, and are safer and

more courteous to other drivers.

Drive at Moderate Speeds

Avoiding high speeds on open roads results in safer driving and better fuel

economy. In highway driving, over 50 percent of the power produced by the

engine is used to overcome aerodynamic drag (i.e., to push air out of the

way). Aerodynamic drag and thus fuel consumption increase rapidly at

speeds above 55 mph, as shown in Fig. 9–62. On average, a car uses about

15 percent more fuel at 65 mph and 25 percent more fuel at 70 mph than it

does at 55 mph. (A car uses about 10 percent more fuel at 100 km/h and 20

percent more fuel at 110 km/h than it does at 90 km/h.)

The discussion above should not lead one to conclude that the lower the

speed, the better the fuel economy—because it is not. The number of miles

that can be driven per gallon of fuel drops sharply at speeds below 30 mph

(or 50 km/h), as shown in the chart. Besides, speeds slower than the flow of

traffic can create a traffic hazard. Therefore, a car should be driven at moder-

ate speeds for safety and best fuel economy.

Maintain a Constant Speed

The fuel consumption remains at a minimum during steady driving at a mod-

erate speed. Keep in mind that every time the accelerator is hard pressed,

more fuel is pumped into the engine. The vehicle should be accelerated grad-

ually and smoothly since extra fuel is squirted into the engine during quick

FIGURE 9–61

Underinflated tires often cause fuel
consumption of vehicles to increase by
5 or 6 percent.

© The McGraw-Hill Companies/Jill Braaten,
photographer

15

15

20

25

MPG

30

35

25 35 45

Speed (mph)

55 65 75

FIGURE 9–62

Aerodynamic drag increases and thus
fuel economy decreases rapidly at
speeds above 55 mph.

Source: EPA and U.S. Dept. of Energy.