The exhaust pipe of a car engine discharges a variety of burned and unburned hydrocarbons, carbon monoxide, nitrogen and sulfur oxides, and traces of various acids, alcohols and phenols. The three main types of automotive emissions are evaporative emissions, refueling losses and exhaust emissions. It is important to note that the car does not always need to be running to emit emissions. Exhaust emissions are the result of fuel combustion in the engine.
The Environmental Protection Agency (EPA) requires all fuels and vehicle types to meet ever-lower, near-zero thresholds for tailpipe emissions of air pollutants and particulate matter. Of primary concern are regulated emissions of hydrocarbons, oxides of nitrogen (NOx), carbon monoxide (CO) and carbon dioxide (CO). Natural gas vehicles (NGV) have an advantage in meeting these stringent standards with less complicated emission controls. Natural gas vehicles continue to provide lifecycle emissions benefits, especially when replacing conventional vehicles.
Natural gas vehicles with ultra-low NOx engines can produce near-zero NOx emissions, meeting the California Air Resources Board's optional near-zero emissions standard of 0.02 NOx. Exhaust emissions are a mixture of different gases and particles emitted by vehicles when the engine is running. The goal of engine optimization is not to minimize pollutant emissions from the combustion system or to maximize pollutant reduction in the ATS, but rather to achieve a target level of system-wide emissions. This may require an increase in the emission of some pollutants from the combustion system if the performance of the ATS is high enough to allow the design objective to be met.
When used as a vehicle fuel, natural gas can offer lifecycle greenhouse gas (GHG) emission benefits compared to conventional fuels, depending on vehicle type, duty cycle, and engine calibration. Diesel vehicles are particularly in the spotlight because of their higher emissions levels compared to gasoline cars. Regulations on marine sulfur dioxide emissions are tightening, however, only a small number of special areas around the world have been designated for the exclusive use of low-content diesel fuel. One external combustion engine that has received wide support as a low-emission energy source is the Rankine cycle steam engine.
Diesel engines are responsible for approximately 85% of all NOx emissions from mobile sources, mainly in the form of NO. H2O, CO2 and N2 are the most desirable by-products of combustion, and automotive engineers strive to create emission control systems that allow a vehicle to produce only these three chemical groups. Studies have shown that exposure to diesel exhaust gases causes lung damage and respiratory problems, and there is concrete evidence that diesel emissions can cause cancer in humans. This section explains the four main pollutant emissions (CO, HC, PM and NOx) from the diesel engine.
It is an important component of emissions from motor vehicles (and stationary internal combustion engines), which can also include crankcase exhaust and evaporation of unused gasoline. Globally, all major users of gasoline and diesel engines in thousands of different applications have to comply with current and future stringent exhaust emissions mandated by governments.