Air Pollution
Clean Air for Kids

Motor Vehicle Emission Controls: Fuel Types


In recent years concern about exhaust emissions from motor vehicles has been increasing. To combat this, the motor industry has been promoting the diesel car as cleaner than petrol cars, due to their greater fuel economy and reduced maintenance requirements. However, diesel cars have very different emission characteristics, and an increase in diesel cars at the expense of petrol cars could have important implications on urban air quality, smog formation, global warming and other environmental issues. Emissions of lead are falling due to the banning of leaded fuel in the UK and many other countries. Recently there has been much debate about which fuel, diesel or petrol, is the cleanest in terms of exhaust emissions. Unfortunately there is no clear answer due to the lack of measurements of emissions from both types of fuel, although data from track tests and dynamometers have shown certain trends.

Emissions from Petrol Vehicles

Emissions from petrol cars have been dramatically reduced by the introduction of catalytic converters, which oxidise pollutants such as CO to less harmful gases such as CO2. When compared to petrol cars without catalysts, catalyst cars have much lower CO, HC and NOx emissions, at the expense of CO2 emissions, which increase due to the oxidation of carbon monoxide to CO2. As a consequence of this, a catalyst car will also use slightly more fuel and become less efficient. However, despite these improvements, petrol cars with catalysts still produce more CO and HC than diesel cars, although exhaust emissions of NOx and particulates are much lower than diesel cars. In fact particulate emissions from petrol cars are so low that they are not routinely measured.

Emissions from Diesel Vehicles

Diesel fuel contains more energy per litre than petrol and coupled with the fact that diesel engines are more efficient than petrol engines, diesel cars are more efficient to run. Diesel fuel contains no lead and emissions of the regulated pollutants (carbon monoxide, hydrocarbons and nitrogen oxides) are lower than those from petrol cars without a catalyst. However, when compared to petrol cars with a catalyst, diesels have higher emissions of NOx and much higher emissions of particulate matter.

Cold Start Emissions

Emissions from cars are greatest when an engine is cold. On a cold day a petrol car may take up to 10km to warm up and operate at maximum efficiency; a diesel car may only take 5km. Consequently, diesel cars produce less unburned fuel during a cold start, which will result in lower emissions of carbon monoxide and hydrocarbons. Diesel cars could make a significant impact on air quality in urban areas where most cold starts occur, especially when it is considered that a catalyst on a petrol car would take several minutes to reach its operating temperature. Overall, diesel cars emit less hydrocarbons, carbon monoxide and lead pollution than petrol cars, but produce more noxious gases and significantly more particulates.

Emissions for Road Vehicles (per vehicle kilometre)


Carbon monoxide

Hydro - carbons

Oxides of nitrogen

Particulate matter

Carbon dioxide

* Petrol car without a catalyst






Petrol cars with a catalyst






Diesel cars without a catalyst






* Petrol cars without catalysts have been given a relative value of 100 for comparison

Despite much debate over which car, petrol or diesel, is cleaner, weighing up the advantages and disadvantages is not easy. For example, diesel cars have been promoted, as they produce less CO and HC on average when compared to petrol cars, and they have greater fuel economy producing less CO2 per km. However recent health concerns about particulate matter have given diesels a less environmentally-friendly image, as have the higher emissions of nitrogen oxides compared with petrol cars. As a comparison, petrol cars produce virtually no particulate matter, take longer to warm up, produce more carbon dioxide per mile on average, and emissions of the regulated pollutants are higher.

Cleaner Petrol and Diesel

A method of pollution reduction currently being utilised involves the use of cleaner petrol and diesel. It is cheaper to improve conventional fuels than to use many of the alternatives and no investment is needed for new storage tanks and service stations. Ultra low sulphur petrol is now widely available in the UK.

Alternative Fuels

To replace pollutant fuels (petrol and diesel), alternative fuels are currently being developed. Those put forward as alternatives to petrol and conventional diesel include: compressed natural gas (CNG); liquefied petroleum gas (LPG); city diesel; hydrogen; alcohol fuels; and battery operated vehicles.


On a cycle representing congested urban traffic, both LPG and CNG outperform petrol powered vehicles on emissions of carbon monoxide (CO). Indeed, emissions of CO from CNG powered vehicles are of the same order as those emitted by diesel vehicles. However, emissions of total hydrocarbons (THC) from CNG vehicles are relatively high because of methane, the major component of natural gas. Although methane is a small contributor to the formation of low level ozone it is a major factor in global warming. Emissions of NOx and particulates from both LPG and CNG powered vehicles are significantly lower than those from diesel vehicles. Moreover, emissions of NOx from CNG vehicles are half those from equivalent petrol engined vehicles. A recent study using a small delivery van fitted with a three way catalyst and capable of switching between CNG and petrol, showed that on a modified EU emission test cycle, emissions of CO, non-methane hydrocarbons (NMHC) and NOx were 76%, 88% and 83% respectively lower with CNG than with petrol. Using data from other studies CNG also compares favourably with emissions from equivalent sized diesel-engined vehicles.

City Diesel

City diesel is a petroleum based lower emission diesel developed in Sweden but now available in many European Countries including the UK. Exhaust emissions from vehicles fuelled with city diesel compare favourably with exhaust emissions from equivalent vehicles fuelled with conventional diesel. The main benefit of city diesel is that its combustion reduces particulate emissions by 34 - 84% depending on engine type, duty cycle, test basis and type of particulate measured. An additional benefit of city diesel is that it is a low sulphur fuel, which is necessary for the optimum running of oxidation catalytic converters.


To produce a cleaner environment for all to live and work in, the development of alternative, cleaner fuels is essential. To encourage the use of the fuels, competitive prices combined with good marketing techniques are required.