Factors to consider

• Exhaust pollution of regulated noxious emissions

  LPG offers substantial improvements, particularly compared with even the cleanest of diesels where particulates (PM10 and PM25) and nitrogen oxides (NOx) are still a real problem. This should be considered the prime issue since it has been estimated that over 24,000 people die prematurely in the UK each year as a result of this pollution.

  Local and Regional Noxious Emissions

  Most developed countries and a growing number of developing countries with serious urban pollution problems now regulate noxious emissions from cars that undermine local and regional air quality. The most commonly regulated emissions are carbon monoxide (CO), hydrocarbons (HC), nitrous oxides (NOx) and particulate matter (PM). LPG is made up of chemically simple and pure hydrocarbons and so mixes easily with air allowing almost complete combustion and producing less smog-forming emissions. Consequently, vehicle emissions from autogas are generally lower than those from conventional fuels.

  LPG Emissions (data supplied by Shell)

  Compared To ULS Petrol Compared To ULS Diesel

  [ULS = Ultra-Low Sulphur]
  11% to 13% less carbon dioxide	80% to 95% less particulates
  15% to 80% less oxides of nitrogen	99% to 99.8% less ultra fine particles
  20% to 40% less Hc hydrocarbons	90% to 99% less oxides of nitrogen
  30% to 35% less carbon monoxide

  Third generation autogas engine and fuel-injection systems are now able to achieve emissions that meet Californian Ultra-low Emission Vehicle (ULEV) standards, which are among the most stringent in the world. LPG's comparative performance is even better for cold starts, since petrol needs to be enriched when the engine is cold because it has poor vaporisation at low temperatures. There are no evaporative problems with autogas since it is used in gaseous form in sealed systems. Since most city car trips involve very short distances, urban emissions from autogas are ordinarily much lower than from petrol.

  Autogas also generates significantly lower levels of NOx compared to diesel in both light-duty vehicle (LDVs) and heavy-duty vehicles (HDVs). Tailpipe particulate emissions, a major problem with diesel, are negligible for autogas. This makes autogas particularly suitable fuel for urban areas, especially where particulate or black smoke pollution is a major problem High concentrations of particulates contribute to respiratory problems and are carcinogenic. It is estimated that 24,000 people die prematurely in the UK each year as result of vehicle pollution.

  
  
• Greenhouse Gases, mainly carbon dioxide (CO 2 )

  Autogas vehicles emit less of this pollution than petrol cars. LPG environmental performance is shown to be even better when the energy required to produce and supply the fuel is taken into account.

  Greenhouse-gas emissions for any given fuel are almost directly proportional to the amount of fuel consumed. Thus the main factors affecting emissions are the energy and carbon content of the fuel. Several studies have attempted to compare emissions of CO 2 from different fuels and vehicles focusing solely on tail-pipe emissions or on the full life cycle (including emissions incurred during the production and supply of the fuel). In general, these studies show that autogas yields significant reductions in emissions compared with petrol - see table below.

  Study % of Petrol Emissions Comment

  CO 2 Emission Reductions from Autogas Compared with Petrol (%)
  IEA/AFIS (1998)		Emissions are lower from field LP Gas sources since less energy is used in production
  - Field LP Gas	18-29
  - Refinery LP Gas	7-22
  Delucchi (1998)	15-33	Average is 22%. Analysis takes account of emissions from vehicle manufacture and assumes 55% field LP Gas.

  
  
• exhaust pollution of non-regulated gas

  LPG offers substantial improvements over both conventional and other alternative fuels in this area.

  The environmental advantages of autogas over conventional and other alternative fuels are even greater with respect to unregulated emissions, including air toxics such as benzene, acetaldehyde, formaldehyde and butadiene - see table below.

  LPG Diesel CNG Methanol Ethanol Benzene Benzene (evaporative) Acetaldehyde Formaldehyde Butadiene

  Non-Regulated Emissions for Passenger Cars. (% of Conventional Petrol) Source: Argonne National Laboratory (2000)
  7.3	90.2	3.5	17.7	17.7
  0.0	0.0	0.0	7.1	7.1
  75.0	110.4	95.8	41.7	2645.8
  93.6	103.2	496.8	720.0	416.0
  12.9	108.1	4.8	16.1	24.2

  Evaporative and fugitive emissions of hydrocarbons from motor vehicles and refuelling facilities are known to make a substantial contribution to total HC emissions. This is a particular problem with petrol, due to its volatility. Autogas vehicles have completely sealed fuel systems and pumps have virtually zero evaporative emissions (fugitive emissions are normally limited to the small release of gas when the fuelling coupling is attached and removed.

Other Environmental Benefits

In the event of a fuel spill, autogas quickly dissipates into the air. Thus, in contrast to petrol and diesel, there is never a risk of ground or water contamination.

Lower noise levels from autogas compared with diesel-powered vehicles provide another significant environmental advantage. This is especially important for night-time deliveries or garbage collection in cities.

Conclusion

The 100,000 LPG vehicles currently driving on Britain's roads represent a saving of around 80,000 tonnes of CO 2 per annum compared with petrol and substantial local air quality benefits compared with diesel.

We live in an increasing environmentally conscious world. Vehicle emissions are recognized as one of the main contributors to global warming and air pollution. At the same time consumer behaviour is extremely difficult to change. Public transport in the UK is not delivering the service that it should and in any case could not deliver a total solution. People will need to drive cars in the foreseeable future.

It is therefore of the utmost importance that efforts are made to make cars as environmentally friendly as possible. Both petrol and diesel cars are becoming much cleaner but there is a limit to how much these emissions can be reduced. Much hope must rest with future technology in the form of electric hybrids and hydrogen fuel cell cars. However, these raise environment issues of their own in terms of the disposal of very large numbers of high capacity batteries and the industrial production of hydrogen. In addition, the technology must be considerably developed and massive infrastructure changes in all the industries connected to motor driving will be required. Inevitably vested interests will slow the process down. In a recent speech President Bush envisioned a world where we all drove hydrogen-powered cars - in 2040.