Diesel engines are compression ignition engines; they do not have spark plugs. When thinking of diesel, a medium or heavy-duty truck might first come to mind. Diesel powered specialty vehicles (e.g., garbage trucks, school buses, and fire engines) and semi trucks pulling trailers are very common. Diesel engines have also become popular in light trucks. Recently in the United States there has been a resurgence of diesel powered cars. In Europe, diesel powered cars are fairly common, making up about one-half of new cars. Diesel fuel has more energy per gallon as compared to gasoline, making it more efficient for every gallon of fuel burned. Diesels emit NOX, sulfur dioxide (SO2), and particulate matter (PM), in addition to greenhouse gas pollutants. Sulfur dioxide contributes to acid rain. Particulate matter can be microscopic particles like dust, soot, or smoke that are able to penetrate into the lungs and cause a variety of health problems. Ultra-low sulfur diesel and newer engine designs with particulate traps have greatly decreased the amount of PM emissions.