Q. What cars can use natural gas? Return to top

Natural gas can be used within a modified internal combustion engine to provide motive power. The gas makes an ideal fuel for spark-ignition engines due to its high octane rating, low levels of volatile organic compounds and to the fact that it mixes easily with air prior to combustion. This offers lower idling speeds, better performance, easier cold starting and a more complete combustion, all of which help to reduce exhaust emissions. Due to the low number of gas refuelling stations most natural gas cars are bi-fuel conversions. These are able to operate on gas or petrol, the fuel being selected at the flick of a switch.

In most respects current bi-fuel and dedicated (mono-fuel) gas engines have a performance similar to conventional power units. There are, however, several advantages of using natural gas. These include reduced engine noise and an extended engine life due to the fuel’s clean burn characteristics which reduces engine stress. Conventional (petrol) three-way catalytic converters continue to be used by most natural gas bi-fuel cars, whereas dedicated gas vehicles use catalysts that are optimised for methane, further reducing vehicle emissions.

The most significant difference between natural gas and conventional cars is the method of fuel storage. Given that natural gas is gaseous at room temperature and pressure, it is stored on-board either as compressed natural gas (CNG) or as liquefied natural gas (LNG), the latter cooled to -190 degrees Celcius. CNG is the most common option for cars, the gas being stored in pressurised cylinders (at 200 bar), which are located within the boot space. Cars are typically fitted with a single steel cylinder of around 90 litres capacity that can hold 16 kg gas, equivalent to 23 litres of petrol. Being pressurised these are heavier than conventional fuel tanks and increase a car’s total weight by around 60 kg. Although they occupy a similar volume to those designed for petrol, this can be a problem for bi-fuelled vehicles where two tanks have to be accommodated. The result is that the extra cylinder reduces the amount of luggage that can be carried and can preclude the ability to carry a spare wheel.

Natural gas is a tried-and-tested green car fuel. There are currently over 1.2 million vehicles using natural gas worldwide in Argentina, Italy, the former Soviet Union, the USA, Canada and India. In the UK, there are around 500 natural gas vehicles in use. Only a small number of these are bi-fuel natural gas cars or vans, the majority being dedicated (mono-fuel) heavy-duty vehicles (lorries and buses).

Q. What are natural gas cars like to drive? Return to top

For the most part driving a natural gas car is no different to using a conventional vehicle. The controls are largely unchanged; starting, engaging and stopping the engine are all done in the normal way. The main additions are a switch, usually located by the gear lever or on the dashboard, which allows the driver to select gas or petrol operation, and a fuel gauge that shows the remaining amount of both fuels. Compared to petrol operation, drivers of bi-fuel cars may notice a small loss of power at full throttle when in natural gas mode. Under most driving conditions, however, the difference is hardly discernable. Dedicated natural gas cars provide a vehicle performance equivalent to conventional fuels.

Q. How do I refuel a natural gas car? Return to top

Natural gas re-fuelling systems are of two types: fast-fill units use high pressure compressed natural gas to refuel vehicles in a matter of minutes; slow-fill compressor units 'trickle charge' one or two vehicles over 5-6 hours. To refuel, a flexible hose is connected between the dispenser and the car and is locked into place creating a sealed system. For fast-fill systems, the amount of gas required is then pre-selected before being automatically dispensed. Slow-fill units continue to operate until the tank is either full or the filling process is halted by the user.

One of the main barriers to the use of natural gas vehicles is the low number of gas refuelling stations. Although the UK has the advantage of having an extensive national gas grid, at present there are only around 30 CNG filling stations of which only 12 are fast-fill stations accessible by the general public (some of which require the setting up of an account). On a more positive note, slow-fill units are being developed for home refuelling - the only requirements being off-road parking (a garage or drive), a natural gas supply and a suitable location for the compressor - see useful websites and information.

See the Energy Saving Trust website for locations of UK natural gas stations.

Q. Are natural gas cars better for the environment?Return to top

In principle, carbon emissions from natural gas cars are reduced due to the fuel's low carbon content and high octane number (which enables a high compression ratio to be used). However, in addition to carbon dioxide, methane (the main constituent of natural gas) is also an important greenhouse gas. Therefore the life cycle emissions of methane must also be accounted for when estimating the impact of natural gas cars on global warming. Taking carbon dioxide and methane emissions into account, natural gas bi-fuel cars (and car-derived vans) show an improvement in greenhouse gas emissions (per mile) of around 10-15% as compared to those using petrol - this means that greenhouse gas emissions for natural gas cars are approximately the same as for light-duty diesel vehicles.

With the exception of methane, regulated emissions (per mile) are reduced for natural gas cars - a bi-fuel natural gas car will reduce NOx compared to petrol (which is itself much lower than diesel). Sulphur oxides and particulates are also virtually eliminated. Furthermore, the unburned hydrocarbons (such as methane) contribute less to tropospheric-ozone formation than do the volatile organic compounds present in petrol exhaust emissions. Larger emission reductions are provided by mono-fuelled (dedicated gas) engines.

However, it is important to note that the relative environmental impact of natural cars is changing. This is due to the development of dedicated gas engines that use optimised catalysts (which reduce methane emissions) and the parallel improvement in the fuel economy of conventional cars. Within a few years, the benefits of natural gas may be reduced or confined to the regulated emissions and noise reductions discussed above. This situation has resulted in a reassessment of the fuel tax advantages that have applied to natural gas over the past five years and underlines the importance that the government attributes to carbon over other emissions. It is possible that, as conventional cars improve, some natural gas tax breaks may be reduced or removed from 2007 (see below).

Q. How much do natural gas cars cost to own?Return to top

Some ownership costs for natural gas cars are higher than for their petrol equivalents. This is principally due to the higher purchase price (if bought new from a manufacturer) or to the cost of conversion of an existing car. Typically, for car and car-derived vans, the additional capital or conversion costs are in the range £1500-£2500. Additional capital costs are incurred if a refuelling compressor unit is installed - the cost of units start at around £2000 for a slow-fill system.

Other changes in ownership costs include vehicle excise duty ('road tax') - natural gas car owners will save up to £50 per year compared to a petrol car. There remains some uncertainty about depreciation rates and resale values for NGVs. Due to the low availability of the fuel, dedicated gas cars probably have a higher than average depreciation rate, simply because buyers of used NGVs are few and far between. However, for CNG bi-fuels models, the latest indications are that depreciation rates are now comparable with conventional cars.

Q. How much do natural gas cars cost to run? Return to top

Although in the past, the switch to NGVs has been dictated by fuel cost savings, this is no longer the case due to the significant recent increases in the cost of natural gas at the 'pump' - despite the fuel's very low fuel duty rate. As a result, in many cases, fuel costs for light-duty NGVs are now broadly comparable with their petrol counterparts. However, fuel cost savings can still be achieved if a home refuelling unit is used - this would result in a gas price of 60p/kg (or less) leading to a per mile fuel cost around half that for a petrol car.

For drivers in and around London, the other major variable cost to consider is the congestion charge. All gas cars in PowerShift Band 4 that appear on the PowerShift Register are exempt from the charge (although owners of NGVs need to register with Transport for London and pay an annual £10 fee). With a £8 payable daily charge, this could provide a potential annual saving of up to £2000.

Note that, as conventional cars are themselves becoming cleaner, the UK government may reduce the fuel tax advantage for gas. However, the government has agreed to pre-announce future (road fuel) natural gas duties three years in advance on a rolling basis.

Q. Where can I buy an natural gas car? Return to top

The availability of natural gas cars has never really been an issue as the majority are essentially conversions of conventional petrol cars - these are either converted at the factory by the manufacturer or are retrofitted after being sold as a new car.

Details of car manufacturers and approved converters (OEMs) who supply natural gas cars are available on the Energy Saving Trust's PowerShift Register. These include models from most of the major manufacturers including: the Vauxhall Zafira and Combo van, the Ford Focus, and three models in the Volvo range: the S60, S80 and V70.

For after-sale conversions, a large number of companies will convert your petrol car to natural gas. Although there are too many to include as part of this Guide, to ensure high quality conversions, it is suggested that only companies that are listed on the PowerShift Register, or recommended by the Natural Gas Vehicle Association (NGVA), should be used.