Liquefied Petroleum Gas (LPG) is produced from the separation of gases in gas fields, and as a by-product of refining crude oils. It is mostly propane, and also contains butane. The UK produces almost six million tonnes of LPG a year, not all of which is needed for the domestic market, so some two million tonnes a year are exported.
LPG liquifies under pressure so is well suited to being stored in cylinders and as bottled gas. It is commonly used in the UK as a domestic heating and cooking fuel; and a small percentage is used as a fuel for cars, generally marketed as Autogas.
When burned in a spark ignition internal combustion engine, its carbon dioxide emissions are significantly lower than those of petrol and slightly lower than for diesel.
Using LPG in Cars
Autogas is not available at enough outlets in the UK to make it practical to have a car that can only run on LPG, The proportion of fuel stations selling LPG has more than doubled since 1999 but is still only around one-tenth. A few manufacturers make bi-fuel cars for the UK market, but the vast majority of LPG vehicles on our roads are petrol cars that have been converted to also run on LPG.
Converting older petrol cars to LPG is a very sound environmental strategy, because petrol engines manufactured in the days before emissions and fuel economy were given such high priority, tend to be more polluting than modern engines. LPG and petrol have slightly different burning characteristics but almost all engines can be set up so that they switch from one fuel to the other without problem. The largest part of converting a car to LPG is the installation of the fuel lines and the pressurised gas tank needed to store the fuel in liquid form.
Diesel engines are less well suited for conversion to LPG bi-fuel operation. LPG produces lower particulate emissions, less nitric oxides and slightly less carbon dioxide than diesel, but its overall environmental advantages over diesel are smaller than over petrol.
Practical Advantages of Autogas
From the motorist’s point of view, the two advantages of using Autogas are that it is less damaging to the environment, and the financial incentives that make it a cheaper fuel option than petrol; in recent years the cost per litre has been around half that of petrol, because fuel tax on LPG is a mere fraction of that on petrol. A car will cover fewer miles per gallon of LPG, but even taking this into account, greenhouse gas emissions and cost are still lower. However, both these advantages are being eroded.
As conventional petrol and diesel engines become cleaner, the environmental benefits of LPG are less marked; and to reflect this, the fuel tax differential is being reduced year by year.
Other Natural Gas Fuels
LPG is not the only natural gas based fuel used in vehicles, but it is the only one that is marketed to any significant extent in the UK. Compressed Natural Gas (CNC) is mostly methane and produces relatively high hydrocarbon emissions, though low carbon dioxide. It must be stored in highly pressurised tanks, and is therefore most suitable for use in HGVs, buses and other large vehicles.
CNC can be converted into Liquefied Natural Gas (LNG) by cooling it to below -160 degrees Celsius. LNG must be stored in insulated pressurised tanks to prevent it from turning back into a gas; because of the insulation, these tanks tend to be large, so again LNG is generally considered more suitable for larger vehicles. Relatively few vehicles in the UK use CNC or LNG. In other countries several manufacturers offer mono-fuel natural gas vehicles, with potential to use biogases such as bio-methane instead of crude-oil gas.
LPG Versus Other Alternative Vehicle Fuels
LPG is less polluting than petrol or diesel, but it is nonetheless a fossil fuel. Like biofuels, it can be used in existing vehicles; unlike biofuels, it cannot claim to be carbon neutral. But LPG is naturally-occurring and plentiful, so it does not raise issues over land use as biofuels do. It is more accessible than new technologies such as hybrid vehicles and electric vehicles because older vehicles can be converted to LPG; so although the environmental benefits per vehicle may not be as great, these benefits will accumulate if more motorists can switch to LPG than can invest in brand new models.
Another factor to consider is the environmental cost of manufacturing new cars and disposing of old ones; it can be argued that prolonging the life of existing vehicles has environmental advantages. So whilst LPG can only offer a partial solution the problems caused by vehicle pollution, it has brought measurable environmental benefits in the past and still represents an attractive option.