Thermodynamics

Chapter 9 | 531use per vehicle in the United States has increased to 603 gallons in 1999(worth $1206 at $2.00/gal) from 506 gallons in 1990 (Fig. 9–56).Saving fuel is not limited to good driving habits. It also involves purchasingthe right car, using it responsibly, and maintaining it properly. A car doesnot burn any fuel when it is not running, and thus a sure way to save fuel isnot to drive the car at all—but this is not the reason we buy a car. We canreduce driving and thus fuel consumption by considering viable alternativessuch as living close to work and shopping areas, working at home, workinglonger hours in fewer days, joining a car pool or starting one, using publictransportation, combining errands into a single trip and planning ahead,avoiding rush hours and roads with heavy traffic and many traffic lights, andsimply walking or bicycling instead of driving to nearby places, with theadded benefit of good health and physical fitness. Driving only when necessaryis the best way to save fuel, money, and the environment too.Driving efficiently starts before buying a car, just like raising good childrenstarts before getting married. The buying decision made now willaffect the fuel consumption for many years. Under average driving conditions,the owner of a 30-mpg vehicle will spend $400 less each year on fuelthan the owner of a 20-mpg vehicle (assuming a fuel cost of $2.00 per gallonand 12,000 miles of driving per year). If the vehicle is owned for5 years, the 30-mpg vehicle will save $2000 during this period (Fig. 9–57).The fuel consumption of a car depends on many factors such as the type ofthe vehicle, the weight, the transmission type, the size and efficiency of theengine, and the accessories and the options installed. The most fuelefficientcars are aerodynamically designed compact cars with a smallengine, manual transmission, low frontal area (the height times the width ofthe car), and bare essentials.At highway speeds, most fuel is used to overcome aerodynamic drag or airresistance to motion, which is the force needed to move the vehicle throughthe air. This resistance force is proportional to the drag coefficient and thefrontal area. Therefore, for a given frontal area, a sleek-looking aerodynamicallydesigned vehicle with contoured lines that coincide with the streamlinesof air flow has a smaller drag coefficient and thus better fuel economythan a boxlike vehicle with sharp corners (Fig. 9–58). For the same overallshape, a compact car has a smaller frontal area and thus better fuel economycompared to a large car.Moving around the extra weight requires more fuel, and thus it hurts fueleconomy. Therefore, the lighter the vehicle, the more fuel-efficient it is. Alsoas a general rule, the larger the engine is, the greater its rate of fuel consumptionis. So you can expect a car with a 1.8 L engine to be more fuelefficient than one with a 3.0 L engine. For a given engine size, diesel enginesoperate on much higher compression ratios than the gasoline engines, andthus they are inherently more fuel-efficient. Manual transmissions are usuallymore efficient than the automatic ones, but this is not always the case. Acar with automatic transmission generally uses 10 percent more fuel than acar with manual transmission because of the losses associated with thehydraulic connection between the engine and the transmission, and the addedweight. Transmissions with an overdrive gear (found in four-speed automatictransmissions and five-speed manual transmissions) save fuel and reduceFIGURE 9–56The average car in the United States isdriven about 12,000 miles a year, usesabout 600 gallons of gasoline, worth$1200 at $2.00/gal.30 MPG20 MPG$800/yr$1200/yrFIGURE 9–57Under average driving conditions, theowner of a 30-mpg vehicle spends$400 less each year on gasoline thanthe owner of a 20-mpg vehicle(assuming $2.00/gal and12,000 miles/yr).