Thermodynamics

532 | ThermodynamicsFIGURE 9–58Aerodynamically designed vehicleshave a smaller drag coefficient andthus better fuel economy than boxlikevehicles with sharp corners.noise and engine wear during highway driving by decreasing the engine rpmwhile maintaining the same vehicle speed.Front wheel drive offers better traction (because of the engine weight ontop of the front wheels), reduced vehicle weight and thus better fuel economy,with an added benefit of increased space in the passenger compartment.Four-wheel drive mechanisms provide better traction and braking thussafer driving on slippery roads and loose gravel by transmitting torque to allfour wheels. However, the added safety comes with increased weight, noise,and cost, and decreased fuel economy. Radial tires usually reduce the fuelconsumption by 5 to 10 percent by reducing the rolling resistance, but theirpressure should be checked regularly since they can look normal and still beunderinflated. Cruise control saves fuel during long trips on open roads bymaintaining steady speed. Tinted windows and light interior and exteriorcolors reduce solar heat gain, and thus the need for air-conditioning.BEFORE DRIVINGCertain things done before driving can make a significant difference on thefuel cost of the vehicle while driving. Below we discuss some measures suchas using the right kind of fuel, minimizing idling, removing extra weight,and keeping the tires properly inflated.Use Fuel with the Minimum Octane NumberRecommended by the Vehicle ManufacturerMany motorists buy higher-priced premium fuel, thinking that it is better forthe engine. Most of today’s cars are designed to operate on regular unleadedfuel. If the owner’s manual does not call for premium fuel, using anythingother than regular gas is simply a waste of money. Octane number is not ameasure of the “power” or “quality” of the fuel, it is simply a measure offuel’s resistance to engine knock caused by premature ignition. Despite theimplications of flashy names like “premium,” “super,” or “power plus,” a fuelwith a higher octane number is not a better fuel; it is simply more expensivebecause 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 recommendednew car octane number if they start knocking.FIGURE 9–59Despite the implications of flashynames, a fuel with a higher octanenumber is not a better fuel; it is simplymore expensive.© Vol. 21/PhotoDiscDo Not Overfill the Gas TankTopping 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 dueto thermal expansion. This wastes fuel, pollutes the environment, and maydamage the car’s paint. Also, fuel tank caps that do not close tightly allowsome gasoline to be lost by evaporation. Buying fuel in cool weather suchas early in the mornings minimizes evaporative losses. Each gallon ofspilled or evaporated fuel emits as much hydrocarbon to the air as 7500miles of driving.