Many small power systems, including automobiles, harness the energy produced by burning fossil fuels. These converters use combustion to produce heat from fuels such as gasoline, natural gas, propane, and other materials containing carbon. For this reason, they are called combustion heat engines.
Combustion heat engines tend to be relatively inefficient, because the heat that drives the machine also tends to dissipate easily into the environment. Energy is lost in the friction of moving parts, in overcoming rolling resistance (e.g., from irregularities in the road and flexing of tires), from air resistance, and from power-train resistance. Energy is also consumed by lubrication and cooling systems, which are necessary to make sure the heat engine does not destroy itself.
The remainder of the heat is used by the engine to develop power. Because a great deal of heat is lost during engine operation, the efficiency of a gasoline engine may be as low as 15 percent and seldom is higher than 30 percent.
Only a small fraction of the energy used by a typical new car actually moves the vehicle.