Thermal engines use the combustion of a fuel mixed with oxygen from the air, the pressure of burned/combusted gases act on the pistons to supply the desired mechanical energy.
The efficiency of thermal engines (or energy efficiency) varies widely according to the conditions of use. Substantial gains have already been made, in particular with direct injection and thermal-electric hybridization.
Other advances are entering mainstream such as downsizing in association with supercharging.
The two main fuels used for the automobile are gasoline (petrol) and diesel. However, the dwindling fossil resources and the determination to reduce polluting local emissions as well as carbon dioxide (CO2) which contribute to global warming have led to the use of alternative fuels: liquefied natural gas, synthetic fuels and biofuels (agrofuels). Certain motors can operate with a varying proportion of biofuels ranging from zero to 100 %. Liquid fuels are favored for their high energy density.
Emissions are closely linked to the type of fuel being used. In numerous countries the eradication of lead and the significant reduction in the amount of sulfur in fuels represent a considerable advance. On the other hand the treatment of tailpipe gases has significantly reduced the emissions of carbon monoxide, unburned hydrocarbons (HC), particles and nitrogen oxides.
But CO2 emissions associated with the burning of hydrocarbons are still cause for concern.