Passenger cars and light trucks powered solely by burning gasoline in an internal combustion engine are the most popular. Gasoline engines use spark plugs to ignite the air-fuel mixture in the engine. There are several reasons for the popularity of gasoline powered vehicles. They are currently affordable (this depends on the price of gas), easy to refuel (gas stations in just about every town), they meet performance expectations (range, acceleration, and speed), and we are most familiar with the technology. One of the major disadvantages of gasoline is that once the fuel is burned, it is gone forever. In addition, gasoline engines emit hydrocarbons (HC), oxides of nitrogen (NOX), carbon monoxide (CO), and carbon dioxide (CO2). HC and NOX contribute to smog formation. Carbon monoxide can enter the bloodstream, reducing the flow of oxygen throughout the body. Carbon dioxide is considered a greenhouse gas which impacts global climate change (global warming).