Natural gas vehicles

What is a natural gas vehicle?

A natural gas vehicle is simply a normal car, motorbike, bus or truck that uses natural gas as a fuel rather than petrol or diesel. In these vehicles the natural gas fuel is burnt in an internal combustion engine exactly like conventional liquid fuels (petrol and diesel) through mixing the fuel with air and achieving ignition.

Natural gas vehicles have slightly different components to a typical to petrol (gasoline) or diesel vehicle to allow the gas to be stored and used. More details on these differences can be found below.

Are there different types of natural gas engine?

There are three types of natural gas engine in use (see figure 1).

Figure 1: Types of natural gas engine

Figure 1: Types of natural gas engine

  • Dedicated - Dedicated vehicles have engines that can only run on natural gas.
  • Bi-Fuel - Bi-Fuel vehicles operate on natural gas whilst retaining the ability to use another fuel - usually petrol. The engine can operate on either fuel but not on both simultaneously. The compression ratio of the engine must remain at a level suitable for combustion of both fuels. Currently this type of engine is used almost exclusively on smaller vehicles like cars and vans.
  • Dual Fuel - Dual Fuel engines are derived from diesel engines. A small amount of diesel is injected to ensure ignition of the fuel mix, but the majority of the fuel is natural gas mixed with the incoming air. Dual fuel engines are auto ignited by compression of the small amount of diesel fuel and require no spark plugs, and are currently only applied to truck and bus engines.
  • Dual fuel engines therefore substitute diesel fuel for natural gas, the substitution rate depend on the integration of the fuel system and the type of vehicle operation, typical rates vary from 40 to 80%.


What about the fuel, how is that stored, and where can you fill up?

Natural gas (NG) is stored on board the vehicle either as a compressed gas (CNG) or in liquefied (LNG) form. These approaches ensure that sufficient fuel is available to burn to avoid compromising vehicle performance by limited range.

CNG is stored on the vehicle and at filling stations at pressures up to 250 bar (3600 pounds per square inch). CNG cylinders are manufactured to internationally approved standards and are typically made from composite materials with the advantage of high strength, low weight and low maintenance. Vehicle range depends on the size of on board storage ( UK examples of heavy goods vehicles running on gas show range capabilities up to 550 miles).

L NG is formed when natural gas is cooled to a very low temperature (minus 162 degrees Celsius). The liquefaction process also removes oxygen, carbon dioxide, sulphur compounds, and water. Liquefying natural gas reduces the volume of the gas by about a factor of 600. LNG is stored on the vehicle and at filling stations at atmospheric pressure in cryogenic insulated tanks designed to minimise heat ingress. LNG has a higher energy density than CNG which means more fuel can be stored in the same space, but suffers from very slow fuel loss as heat ‘boils off' the stored LNG.

CNG and LNG refuelling stations are available in limited numbers throughout the UK; full details are available from the NGVA .


Can all types of vehicle run on natural gas?

The simple answer is yes, with adaptations to the base engine technology, natural gas can be used to fuel almost any kind of vehicle from motorcycles and three wheelers through to cars, vans, buses, trucks, trains, boats and even aircraft.

However, the availability of vehicles or conversion equipment varies greatly from country to country depending on local conditions. The UK has an underdeveloped market for natural gas vehicles which means the number of vehicles available to purchase is inevitably limited.

The following are some of the currently or soon to be available vehicles;


What about Biogas?

Biogas is produced from the process of anaerobic digestion (AD) of wet organic waste, such as cattle and pig slurries, food wastes, green waste and plants grown for AD. The AD process produces methane, carbon dioxide and a digestate ( solid material remaining after the anaerobic digestion of a biodegradable feedstock ) that can be used as fertiliser. After upgrading to at least 95% methane biogas can be used just like natural gas, either compressed or liquefied and used to fuel natural gas vehicles. From an environmental perspective biogas has a big advantage over natural gas as its green house gas emissions can be extremely low or even negative, see link below.

Further information on Biogas and vehicles can be found via the Biomethane for Transport Forum here.


What are the green house gas emission benefits?

Green house gas reductions from natural gas and biogas are hard to generalise. Quoted green house gas emission reductions from natural gas are typically 10% and benefits from biogas range from 50 to 200% (>100% is possible if anaerobic digestion captures the methane, a green house gas over 20 times as potent as CO2, which would be emitted into the atmosphere when waste is left to decompose in a landfill site), where the calculations are taken on a well-to-wheel1 basis (see the foot note for well-to-wheel explanation).

However, these generalisations mask a range of options in technology and approach in gas vehicles. As detailed above, engines can be either dedicated, bi-fuel or dual-fuel and the fuel can be either CNG or LNG with differing energy requirements and biogas can be created from a broad range of feed stocks. The 2006 NSCA report2 ‘Biogas as a road transport fuel' presents a well-to-wheels analysis for many of these options.

The well-to-wheel analysis for a 1.6l passenger car is shown in figure 2. Here natural gas (configuration - Port Injection Spark Ignition – PISI) is shown to have a small green house gas emission advantage over a diesel engine and a larger advantage over a gasoline engine. However, the green house gas savings from Biogas (CBG) are far more significant with carbon negative operation possible when biogas is produced from wet manure. Natural gas passenger cars, although common in continental Europe, are rare in the UK.

The well-to-wheel analysis for a 38 tonne heavy goods vehicle is shown in figure 3. Here a dedicated natural engine is shown to have a small green house gas emission advantage over a diesel engine, and diesel/LNG dual-fuel engine to add further limited green house gas savings, again like the passenger car, it is through using biogas (CBG) fuel and 50/50 diesel-biogas dual-fuel that the significant green house gas savings are seen.

Finally to ensure maximum environmental benefit, care should be taken to ensure the gas system fitted to a natural gas vehicle is professionally integrated. Provision should be made to catalyse any methane which has passed through the engine unburned, due to methane's potency as a greenhouse gas.


Figure 2: Green house gas emissions for a 2002 generic 1.6l pasenger car

Figure 2: Green house gas emissions
for a 2002 generic 1.6l pasenger car

Figure 3: Green house gas emissions for a 2002 generic 38 tonne HGV

Figure 3: Green house gas emissions
for a 2002 generic 38 tonne HGV

Any other benefits?

Natural gas has long been established as a clean fuel in relation to diesel, and biogas exhibits the same clean burning properties. This clean reputation is based on the air quality impact of using natural gas as a fuel. Natural gas engines provide very significant Particulate Matter (PM) reductions (>90%) when compared to diesel engines, and dedicated natural gas engines can also provide Nitrogen Oxides (NOx) reductions. Additionally natural gas engine emit less engine noise than a typical diesel engine, which make natural gas vehicles usually quieter in operation.

The European Natural Gas Vehicle Association (ENGVA) offer an emission reduction calculator to estimate reductions in regulated emissions (PM, NOx, etc.) for different vehicle types, this tool is available here.

Complimentary environmental and financial benefits are also available when biogas is generated from waste (Biodegradable Municipal Waste) through a reduction in waste to land fill3 (a new legal requirement in the UK, see footnote) and subsequent uncontrolled methane emissions.


Is there UK policy support for natural gas vehicles?

Both the UK and the Commission of the European Community (CEC) have current commitments to reduce CO2 emissions under the terms of the Kyoto Protocol.

Alternative fuels offer a chance to reduce transport green house gas emissions and with this in mind the CEC published a green paper outlining its strategy for the substitution of oil based fuels (Towards a European Strategy for the Security of Energy Supply: 2000), this strategy has stated an objective of replacing 20% of the oil based fuels in European road transport sector with alternative fuels by the year 2020. Within this objective natural gas will ultimately make up 10% of the total, see figure 4.

In the UK, natural gas vehicles receive support through fiscal incentives. The Energy Savings Trust Power-Shift and Clean-Up grants have now ceased and support focused on taxation advantages. These advantages are as follows:

  • Road fuel duty - fuel duty on natural gas is £0.10p/kg and the government has agreed to maintain the duty differential with diesel fuel on a rolling 3 year basis. The February 2007 gas vehicle report showed this duty differential to diesel resulting in approximately a 50% reduction in fuel cost between diesel and CNG.
  • Vehicle Excise Duty (VED) – duty is linked to carbon dioxide emissions and fuel type. Light duty natural gas vehicles pay less VED than the equivalent petrol or diesel vehicle, see alternative fuel column in figure 5, and heavy duty natural gas vehicles can apply for a reduced pollution certificate to obtain a discounted VED. To find out more about VED see the Vehicle Certification Agency VED database here.
  • Infrastructure Grants - Grants from the Energy Saving Trust (EST) are available to support the capital cost of refuelling infrastructure. For further information is available from the EST here.
  • London Congestion Charge - Users of natural gas vehicles, which are on the Power-Shift Register or Clean-Up Register, can apply for 100% relief from the London Congestion charge. Details are available here.


Figure 4: EU strategy for the substitution of oil based fuels by 2020

Figure 4: EU strategy for the
substitution of oil based fuels by 2020

Figure 5: UK graduated vehicle excise duty (VED)

Figure 5: UK graduated
vehicle excise duty (VED)

What is the current and potential market for natural gas vehicles?

According to the European Natural Gas Vehicle Association (ENGVA) in 2007 there were some five million natural gas vehicles in use worldwide, of which 1.4 million are in Argentina and about one million in Brazil . Italy 's fleet of 380,000 vehicles is by far the biggest in Europe, followed by Germany with 38,000 and France with 8,000. In Spain there are more than 500 public sector natural gas vehicles operating in Madrid alone, including buses and refuse collection vehicles.

By contrast, UK experience with natural gas as a vehicle fuel has been less positive and the current number of natural gas vehicles in the UK is below 6004. In the UK heavy goods vehicle applications have been most successful, and some significant fleets have been developed; by contrast in most other EU countries buses and cars have seen the most widespread adoption of natural gas.


Footnote 1:

Well-to-Wheel (WTW) CO 2 equivalent figures are based on the results of two calculations, a Well to Tank (WTT) evaluation which accounts for the energy expended and the associated GHG emitted in the steps required to deliver the finished fuel into the on-board tank of a vehicle, and a Tank to Wheels (TTW) evaluation which accounts for the energy expended and the associated GHG emitted by the vehicle/fuel combinations. ‘Well-to-Wheels Analysis of Future Automotive Fuels and Powertrains in the European Context', CONCAWE, EUCAR and EU JRC, March 2007.

[Back to the green house gas emission benefits]

Footnote 2:

[Back to the green house gas emission benefits]

Footnote 3:

The EU requirement to divert bio-degradable waste from landfill has been introduced in the UK as the Landfill Tax Credit and Landfill Allowance Trading Scheme (LATS). The latter permits local authorities to trade allowances rather than pay up to £150 per tonne of waste sent to landfill. Each local authority is required to reduce disposal of bio-degradable waste to landfill by progressively increasing amounts up to 65% by 2020

[Back to the other benefits]

Footnote 4:

Natural Gas Vehicle Association 2007