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What is Natural gas car?

By admin in , | November 25, 2009 | Comments Off on What is Natural gas car?

Natural gas is a naturally occurring mixture of gaseous hydrocarbons consisting of at least 80% methane (CH4) with lesser amounts of propane, ethane and butane. In the UK, the main uses of natural gas are heating and electricity generation.


What cars can use natural gas?

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 gasoline, 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 (gasoline) 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 gasoline. 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 gasoline, 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.


What are natural gas cars like to drive?


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 gasoline operation, and a fuel gauge that shows the remaining amount of both fuels. Compared to gasoline 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.


How do I refuel a natural gas car?


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.


Are natural gas cars better for the environment?


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 gasoline – 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 gasoline (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 gasoline 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.


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