Two gaseous fuels that can be used in spark-ignition engines are Liquefied Petroleum Gas (LPG) and natural gas. Natural gas is a naturally occurring mixture of hydrocarbons consisting of at least 80% methane, with lesser amount of propane ethane and butane. LPG is also a mixture of hydrocarbons, but its main constituent is propane, with lesser amounts of ethane and butane.
Both LPG and natural gas can be used within a modified spark-ignition (petrol) engine. These gases make ideal fuels for combustion engines due to their high octane rating, low levels of volatile organic compounds and the fact that they can readily mix with air prior to combustion. These characteristics result in a more complete combustion, which helps to reduce exhaust emissions. The gas' clean burning characteristics also reduce engine stress, therefore extending engine life.
Due to limited availability of these fuels, most gas adapted vehicles are bi-fuel conversions, able to operate on either gas or petrol; the fuel being selected at the flick of a switch. Modern conversions use electronically controlled gas injection systems which allow optimised performance from either fuel type.
The most significant difference between gas and conventional cars is the method of fuel storage. While both LPG and natural gas are gaseous at room temperature and pressure, LPG can be easily liquefied under pressure which makes it the more popular of the two 'road gases'. The steel tanks most commonly used to contain the fuels under pressure the fuel can add up to 60 kg to a vehicle's weight, and in the case of bi-fuel cars, can reduce overall fuel consumption as two fuel tanks are required, .
Driving an LPG and natural gas car is similar to using a conventional car. The main additional control is a switch, usually located by the gear lever or on the dashboard, which enables the driver to select LPG or petrol operation, and a duel-mode fuel gauge. Regarding performance, drivers of bi-fuel cars may notice a small loss of power at full throttle when in gas fuel mode. Both LPG and natural gas are tried and tested green car fuels – in the UK, over 100,000 cars and light-duty vans currently use LPG.
To refuel an LPG vehicle, a flexible hose is locked into place to create a sealed pressurised system. The amount of LPG required is usually pre-selected before the gas is automatically dispensed, which takes around the same time as a conventional refill. At its peak, LPG was available from around 10% of all forecourts. While these have reduced in number slightly, as of 2014 there are around 1,400 forecourts selling LPG across the UK.
For locations of LPG refuelling points in the UK, view the LPG refuelling map.
Natural gas vehicles can be refuelled in two ways – 'fast fill' uses a high pressure system to refuel in minutes and 'slow fill' takes several hours (usually overnight). Both are essentially carried out by connecting a flexible hose between the dispenser and the vehicle, which is locked into place to create a sealed system. The main barrier to the use of natural gas is the low number of publicly accessible gas refuelling stations in the UK which currently number less than 14. One alternative is to use a home-based slow fill system such as Gasfill.
In principle, carbon emissions from LPG and natural gas are reduced due to their low carbon content and high octane level. Methane is the main constituent of natural gas, which is also an important greenhouse gas and so must be taken into account when assessing the impact on global warming. Taking both carbon dioxide and methane into account, as well as the fact that using LPG increases fuel consumption by around 30%, bi-fuel gas cars show an improvement of around 10-15% as compared to those using petrol, and are therefore comparable with diesel greenhouse gas emissions.
Regulated emissions are reduced for LPG and natural cars compared to conventional fuels. Even compared to petrol, which itself is low in NOx, the best quality bi-fuel gas engines produce fewer NOx emissions and virtually eliminate emissions of particulates. For natural gas vehicles, the unburned hydrocarbons (such as methane) also 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, bi-fuel vehicles are the most common LPG car type available in the UK.
However, the greenhouse gas emission benefits of using gas vehicles is changing over time, as the fuel economy of conventional petrol improves. Before to long, the benefits of the 'road gases' will be confined to regulated emissions and noise reduction.
Some ownership costs for LPG and natural gas cars are higher than for their petrol equivalents. This is principally due to the higher purchase price or to the cost of conversion of an existing car. Typically, conversion costs are in the range £1,500-£2,500 for natural gas and £1,200-£2,000 for LPG. Additional capital costs are incurred if a home based natural gas refuelling compressor unit is installed – the cost of units start at around £2,000 for a slow-fill system.
Gas fuelled vehicles fall into the 'alternative fuel' category for 'road tax' (Vehicle Excise Duty); although owners will only save £10 per year compared to a petrol car. Depreciation rates for both LPG and natural gas are now slightly higher than those of petrol cars, a reflection of the decreasing popularity of these fuels, the increasing advances in conventional engines and the relative lack of refuelling locations.
Whereas in the 1990s, the switch to 'road gases' was driven by fuel cost savings, gas prices are on the increase thanks to a steady (and continuing) reduction in the fuel tax breaks, and also to the improving fuel economy of conventional cars. That said, LPG still offers a reduction in fuel costs of around 25%. Note, however, that when comparing the price of LPG per litre with petrol, LPG's lower energy density means that 30% more fuel is required per mile.
The availability of LPG and natural gas vehicles has never really been an issue as the vast majority are essentially conversions of conventional petrol cars – either converted by the manufacturer or retrofitted after being sold as a new car.
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 recommended by the UKLPG Association should be used.