The market for gas vehicles has improved significantly, with products available in all segments. Most manufacturers now offer dedicated gas rather than dual fuel vehicles, reflecting the improvements in engine technology and availability of refuelling infrastructure.
All new gas-powered vehicles meet or exceed Euro VI/6 emissions standards and therefore comply with Clean Air Zone and Ultra Low Emission Zone requirements.
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Natural gas is a mature technology, reflected in the wide range of vehicles available from mainstream manufacturers. There are two main fuels – CNG and LNG – and two types of gas vehicles – dedicated gas and dual fuel.
CNG and LNG
Compressed natural gas (CNG) is natural gas, primarily methane, which is compressed and stored on the vehicle in high-pressure tanks at around 200 to 250 bar. Liquefied natural gas (LNG) is simply gas that has been compressed at greater pressures until it can be stored as a liquid. On the vehicle LNG has a higher energy density than CNG which means more fuel can be stored in the same space, extending range and reducing refuelling frequency.
Dedicated gas vehicles use a spark ignition engine that runs only on natural gas and biomethane. Fuel is stored on the vehicle in compressed or liquefied form. They are best suited to back-to-base operations with refuelling at the depot, or in an area with good refuelling infrastructure coverage. Manufacturers offer a choice of tank capacity to provide the appropriate range.
Advantages: while less efficient than diesel, the engine is optimised to run on natural gas and therefore emissions are minimised and the combustion process is relatively simple to control.
Maintenance: spark plugs, gas tanks, fuel pipes and filters require regular inspections and servicing. There are no requirements for more frequent checks, however, maintenance costs are typically slightly higher than for an equivalent diesel vehicle.
Dual fuel vehicles combust diesel and natural gas simultaneously in a compression ignition engine.
Older (Euro V) diesel HGVs could be retrofitted with a dual fuel system, but many of these were found to have an issue with emissions of unburnt methane (methane slip). Methane is a potent greenhouse gas, and significant emissions could outweigh reductions in CO2 emissions from using natural gas in place of diesel. Hoiwever, modern dual fuel systems supplied by OEMs on Euro VI vehicles do not have this issue.
Emissions and efficiency are improved compared to a diesel vehicle with dual fuel vehicles best suited to long haul operations, where the displacement of diesel by natural gas can be maximised.
Advantages: a very high diesel displacement rate (90% or more) is achievable on long haul operations. A ‘limp home’ mode can be activated if gas refuelling is not available.
Maintenance: Dual fuel vehicles have similar components to diesel vehicles plus a gas fuel system so there are additional components to inspect and service. There are no requirements for more frequent checks, and maintenance costs should be the same or lower than an equivalent diesel vehicle.
Range and Payload
Early gas vehicles had a shorter range on a single tank of fuel than a diesel equivalent. However, the latest vehicles now offer similar range capabilities; up to 1,600km on a single tank for LNG models.
The extra weight of gas tanks will not reduce the payload of two-axle rigid vehicles of 18 tonnes GVW and three-axle rigid vehicles of 26 tonnes GVW. The Road Vehicles Authorised Weight and Construction and Use Regulations have recently been amended, permitting alternatively fuelled vehicles an additional weight allowance of up to one tonne to compensate for additional fuel tanks or batteries. This additional allowance does not apply to 44 tonne GVW vehicles.