What is the function of urea solution used in vehicles?

[MacSadie]

The primary function of urea solution used in vehicles is to reduce harmful nitrogen oxides in the exhaust into nitrogen and water. Urea solution is required only for vehicles equipped with diesel engines, and is not needed for gasoline-powered vehicles. Most buses, trucks, semi-trailers, and heavy-duty vehicles are equipped with diesel engines, and these vehicles require the addition of urea solution. The working principle of diesel engines differs from that of gasoline engines. In gasoline engines, after the compression stroke, the air-fuel mixture is ignited by spark plugs. Diesel engines, however, do not require spark plugs. After the compression stroke, the pressure and temperature of the air-fuel mixture are sufficiently high to enable spontaneous combustion.