Automobile Efficiency

Few aspects of energy use have so direct an impact on consumers as the efficiency of our vehicles, the vast majority of which are powered by internal-combustion engines burning gasoline or diesel fuels. Thanks to decades of scientific and engineering progress, efficiency has improved dramatically in recent decades, with more advances on the way.

On average, vehicles have gotten 20% heavier since 1980 (cars are up from 3,100 to 3,500 pounds). Yet in that same period, horsepower has increased more than 80% and fuel economy by 100%. In passenger cars, this has resulted in a 37% decline in energy intensity: from 7,919 British thermal units per vehicle mile in 1980 to 4,949 in 2012. Multiple factors contributed to those results.

Engines Modern vehicles approach the goal of increased efficiency in many ways, including increasing the compression ratio (how much the air–gas mixture is compressed in the cylinder before ignition); replacing carburetors with fuel injectors that pump atomized fuel into the cylinders; and doubling (generally from two to four) the number of valves per cylinder, for more efficient transport of intake and exhaust. 

Additional performance increases result from turbochargers (now found in about 21% of all new cars and 14% of trucks) that thoroughly mix air and fuel via a turbine that propels the mix into the cylinder at above-atmospheric pressure.

As a result, we now get sufficient power from ever-smaller engines. In 1980, the average displacement (the volume through which the piston moves) of passenger-car engines was 3.08 liters. By 2013, it was 2.43 liters—a 21% reduction.

Transmissions Gears change the ratio between how fast the engine crankshaft turns and how fast the wheels turn, allowing the engine to stay at an efficient number of revolutions per minute at different vehicle speeds. 

For years, three- or four-speed transmissions were commonplace. The newest vehicles, however, have automatic transmissions with as many as nine or more gears, which allow the engine to operate at lower revolutions per minute while delivering optimal torque at a given vehicle speed. The U.S. Department of Energy test results show that changing from a conventional five-speed automatic transmission to a modern eight-speed, dual-clutch transmission system can reduce total fuel consumption by 11% in compact passenger cars and 8.5% in sport utility vehicles.

Rolling and air friction The motion of a vehicle is impeded by rolling friction between the tires and the road surface. Modern tires incorporate materials and operate at pressures that minimize the energy loss caused when a pneumatic tire deforms and sticks to the road. At the same time, vehicles are subject to air resistance (drag), which has been progressively minimized in more streamlined designs.

Taken together, these innovations—along with federal mandates—have resulted in substantial improvements, a trend that is certain to continue.