Diesel and Petrol Cars - A Comparison

    In recent years concern about exhaust emissions from motor vehicles has been growing. 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. Whatever the fuel, the internal combustion engine emits a complex mix of pollutants including:     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 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.

    Although better in terms of the regulated pollutants than petrol cars without catalysts, an increase in diesel cars at the expense of petrol cars with catalysts may result in air quality standards being more difficult to achieve, e.g. increasing the proportion of diesel cars is likely to offset some of the expected reduction in NOX emissions, following the introduction of catalysts to petrol cars. However in congested traffic, diesels will produce less CO and HC than a petrol car fitted with a catalyst, but NOX emissions will be higher.

Cold start emissions

    Emissions from cars are greatest when an engine is cold. On a cold day a petrol car may take up to ten kilometres to warm up and operate at maximum efficiency, whereas a diesel car may only take five kilometres. Therefore diesel cars will produce less unburnt fuel during a cold start, which will result in less emissions of carbon monoxide and hydrocarbons. This would mean that diesel cars would 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 get to its operating temperature. For example, a journey of one kilometre could lead to emissions of CO being as much as 14 times higher from a catalyst equipped petrol car, compared with a diesel car (QUARG, 1993).

    Therefore in overall terms, on average, diesel cars emit less hydrocarbons, carbon monoxide and lead than petrol cars, but produce more noxious gases and significantly more particulates.

Petrol vehicle emissions

 In recent years exhaust emissions from petrol cars have been dramatically reduced by the introduction of catalytic converters, which oxidize pollutants such as CO into 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, as shown in Table 1, 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.

Table 1: Emissions for road vehicles (per vehicle kilometre)
Vehicles  Carbon monoxide Hydrocarbons Oxides of Nitrogen Particulate matter Carbon dioxide
Petrol car without a catalyst* 100  100 100 ---  100
Petrol cars with a catalyst 42  19 23 ---  100
Diesel cars without a catalyst 2  3 31  100 85

Source: DOT, 1994 (NB*petrol cars without catalysts have been given a relative value of 100 for comparison)

Carbon dioxide emissions

    Whenever fossil fuels are burnt, the carbon held within them is released in the form of carbon dioxide, which contributes to global warming. The diesel car has been promoted as a way to reduce CO2 emissions, as diesel cars, particularly those with direct injection engines are much more efficient than petrol cars. This is shown in Figure 1.

Figure 1 Carbon dioxide emissions from different types of cars

Source: QUARG, Diesel vehicle emissions and urban air quality, 1993

    Figure 1 shows the emissions of CO2 for various vehicles, travelling at various speeds. The results clearly show that emissions of carbon dioxide are typically lowest from diesel cars and that average emissions from a petrol car fitted with a catalyst are considerably higher.

New technologies to reduce emissions

    New technology is being developed to reduce emissions from both petrol and diesel vehicles, which could have substantial impacts upon the emissions from each type of car. For example, pre - heated catalysts are being developed to reduce the cold start emissions from petrol cars and carbon canisters are being fitted to petrol tanks to reduce evaporative emissions of petrol (petroleum vapour rises into the carbon canister, which then returns the petrol to the tank, hence avoiding a loss due to evaporation).
Oxidation catalysts are also being tested for diesel cars and may become compulsory before the end of 1996. Such catalysts would reduce both hydrocarbon and particulate matter from diesels. Therefore, until these technologies come into widespread use, it is difficult to predict what effect they will have on exhaust emissions, thus complicating further the debate over which car is more environmentally acceptable, diesel or petrol.

Conclusion

    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 and produce 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 nitrous 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. Table 2 compares the emissions of pollutants from various types of cars.

Table 2: A comparison of emissions from petrol cars (with and without catalysts) and diesel cars.
Pollutant
 Petrol car without a catalyst Petrol car with a catalyst Diesel car without a catalyst Diesel car with a catalyst
Nitrogen Oxides ****  * ** ***
Hydrocarbons  **** **  *** *
Carbon monoxide **** *** ** *
Particulate matter **  *  **** ***
Sulphur dioxide * * ****  ****
Carbon dioxide ***  **** * **

* = lowest emissions **/*** = intermediate emissions **** = highest emissions

Source: QUARG, Diesel vehicle emissions and urban air quality, 1993

     In theory a petrol car with a catalyst and a carbon canister or a diesel car with an oxidation catalyst will be the cleanest to run. However there is no clear cut winner in the environmental stakes, as both cars have their advantages and disadvantages, which are further complicated by the various new emission technologies which may become standard in the future. Perhaps the best way to reduce future vehicle emissions is to use engines and fuels which are less polluting (see the ARIC briefing note: 26/95), to limit the use of road vehicles and to plan new road schemes to minimise pollution.

References

Quality of Urban Air Review Group, Diesel Vehicle Emissions and Urban Air Quality, Second report, 1993.
National Society for Clean Air and Environmental Protection (NSCA), Choosing and Using a Cleaner Car, January 1995.
Department of Transport, Transport Statistics Great Britain 1993 Edition, HMSO September 1993.
Written by Julian Atherton-Pusil (March 1996)
Last updated: 12 March 1996 by Matt Whitehead

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