Most of the 4.6 million cars, trucks, buses and SUVs registered in Massachusetts are powered by internal combustion engines that run on petroleum-based fossil fuels:

  • Gasoline, which generally contains an additive such as ethanol, evaporates easily and is highly flammable. In a gasoline engine, a spark ignites the fuel, but the combustion process is inefficient. About one-quarter of the fuel actually powers the vehicle forward; the rest is consumed by friction and heat, or emitted as exhaust. Learn about reformulated gasoline (RFG).
  • Diesel fuel is heavier, oilier, less evaporative and less flammable than gasoline. In a diesel engine, fuel is ignited through compression of air in the cylinders. Because diesel fuel has a higher energy density than gasoline, vehicles that run on diesel tend to deliver far better fuel economy than those powered by gasoline.

Both gasoline and diesel fuel contain mixtures of hydrocarbons, which are compounds that contain both hydrogen and carbon atoms. In a "perfect" engine, oxygen from the air would convert all of the fuel's hydrogen to water, and carbon to carbon dioxide.

No combustion process is perfect, however, so both gasoline and diesel vehicles are equipped with emission control systems that reduce (but do not eliminate) pollutants that can be harmful to public health and the environment .