Thermodynamics

534 | Thermodynamicsthe vehicle’s handling adversely, and hurt the fuel economy by increasing therolling resistance. Overinflated tires cause unpleasant bumpy rides, and causethe tires to wear unevenly. Tires lose about 1 psi pressure per month due to airloss caused by the tire hitting holes, bumps, and curbs. Therefore, the tirepressure should be checked at least once a month. Just one tire underinflatedby 2 psi results in a 1 percent increase in fuel consumption (Fig. 9–61).Underinflated tires often cause fuel consumption of vehicles to increase by5 or 6 percent.It is also important to keep the wheels aligned. Driving a vehicle with thefront wheels out of alignment increases rolling resistance and thus fuel consumptionwhile causing handling problems and uneven tire wear. Therefore,the wheels should be aligned properly whenever necessary.FIGURE 9–61Underinflated tires often cause fuelconsumption of vehicles to increase by5 or 6 percent.© The McGraw-Hill Companies/Jill Braaten,photographerMPG35302520151525 35 45Speed (mph)55 65 75FIGURE 9–62Aerodynamic drag increases and thusfuel economy decreases rapidly atspeeds above 55 mph.Source: EPA and U.S. Dept. of Energy.WHILE DRIVINGThe driving habits can make a significant difference in the amount of fuelused. Driving sensibly and practicing some fuel-efficient driving techniquessuch as those discussed below can improve fuel economy easily by more than10 percent.Avoid Quick Starts and Sudden StopsDespite the attention they may get, the abrupt, aggressive “jackrabbit” startswaste fuel, wear the tires, jeopardize safety, and are harder on vehicle componentsand connectors. The squealing stops wear the brake pads prematurely,and may cause the driver to lose control of the vehicle. Easy startsand stops save fuel, reduce wear and tear, reduce pollution, and are safer andmore courteous to other drivers.Drive at Moderate SpeedsAvoiding high speeds on open roads results in safer driving and better fueleconomy. In highway driving, over 50 percent of the power produced by theengine is used to overcome aerodynamic drag (i.e., to push air out of theway). Aerodynamic drag and thus fuel consumption increase rapidly atspeeds above 55 mph, as shown in Fig. 9–62. On average, a car uses about15 percent more fuel at 65 mph and 25 percent more fuel at 70 mph than itdoes at 55 mph. (A car uses about 10 percent more fuel at 100 km/h and 20percent 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 thespeed, the better the fuel economy—because it is not. The number of milesthat 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 oftraffic can create a traffic hazard. Therefore, a car should be driven at moderatespeeds for safety and best fuel economy.Maintain a Constant SpeedThe fuel consumption remains at a minimum during steady driving at a moderatespeed. Keep in mind that every time the accelerator is hard pressed,more fuel is pumped into the engine. The vehicle should be accelerated graduallyand smoothly since extra fuel is squirted into the engine during quick