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Liquefied Petroleum Gas (LPG), Liquefied Natural Gas (LNG) and Compressed Natural Gas (CNG)

LPG, LNG and CNG are well suited to particular types of vehicles and have advantages in each case, but they are all fossil fuels

Liquefied Petroleum Gas (LPG), Liquefied Natural Gas (LNG) and Compressed Natural Gas (CNG) are fossil derived fuels and therefore release, one way or another, sequestered greenhouse gases into the atmosphere.
As vehicle fuels, they are suitable for use in the two dominant internal combustion engine technologies; spark ignition and compression ignition. Although capable of working in either type of engine there are practical factors which limit their applications to one rather than the other. Broadly speaking LPG is compatible with petrol (gasoline) engines and LNG and CNG with heavy diesel vehicles.
Their main advantages are that they produce much less tailpipe pollution and can be significantly cheaper per mile to run, especially in the UK where users can benefit from government grants, reduced excise duties and other charges. Many vehicles using these gases are dual fuel (aka bi-fuel) and there is a reasonable structure of filling stations, particularly for LPG; as a result they are practical and are 'here and now'.

Liquefied Petroleum Gas is a fuel which can power cars, buses and lorries, however due to factors discussed below, and other alternative fuels being available, LPG is best suited to light vehicles such as cars and small vans which normally run on petrol. Our estimate of energy density* is 65% compared to diesel and about 75% compared to petrol (gasoline).
Typically the gas is predominantly propane (C3H8) with some butane (C4H10) derived mainly from oil refineries (also North Sea gas, in the UK). We give the chemical formulae because it is the ratio of carbon to hydrogen which is important; the smaller the ratio of C to H, the better for the environment. It follows that methane (CH4) is a better gas in this respect but only if fully burnt!
The gas is liquefied by moderate compression at normal temperatures and is stored in appropriate tanks and cylinders. The liquefaction is necessary to provide a reduction in volume and produce acceptable energy densities. In general this moderate, well tried, process gives it a portability and makes it a fuel with a myriad of applications but the main application discussed here is as a fuel for motor vehicles.
LPG vehicles need to be purpose built or they can be converted. It seems that conversions are only practically applicable to petrol vehicles, not diesel because diesel engines need significant modification for this particular gas. Normally they are Bi-fuel which means that they can be run on either LPG or petrol at the flick of a switch, even while motoring.
The most notable difference between LPG and petrol or diesel, for cars and vans, is the cost of fuel. As a rough guide, in the UK, the cost per gallon is halved (ref. 2004) compared to petrol, because the government have reduced the duty by a very substantial amount. In addition to this concession, DETR grants are also available to carry out conversions but they only apply to vehicles less than five years old: grants for cars and light vans were about £700 to £800 in 2004. We also noted in January 2005 that LPG vehicles, eligible for grants, (amongst other low exhaust-polluters) should qualify for 100% exemption from the London Congestion charge.
From a local environmental point of view LPG is cleaner than petrol and also diesel, although it is still a fossil fuel and thus its use, as a whole, contributes to global pollution and climate change. At the vehicle exhaust there are less CO, hydrocarbons, nitrous oxides and particulates emitted and it deposits less sulphur in the engine.

  There are drawbacks to converting vehicles to LPG:
  • The cost of conversion of a petrol car seems to work out at about £1,100 to £1,500 and it may take a few days to do (a few weeks lead time). According to Andrew Frankel in an article in the Sunday Times (13 Jan. 2002) the estimated cost was greater, from about £1,500 to £2,300. Whichever, the resultant capital outlay dictates that you must do a lot of miles to recoup the cost.
  • There are relatively few filling stations around the UK, but no doubt more will become available as there are more users (in January 2003 we read that there are well over a thousand stations).
  • We have also heard of problems caused by non-standard filling nozzles.
  • The mpg will be less than with conventional fuel (our guess is it will be reduced to about 3/4) but even so fuel costs should be lower, at about 2/3 [ie 1/2 X 4/3 = 2/3].
  • The range of a tank full of LPG is likely to be less than a tank of petrol, however, with bi-fuel vehicles the total range will be the sum of the two so there is no need to be concerned about finding LPG stations.
  • The extra volume taken up by the gas tank will reduce the available load space.
  • There may be a slight performance deficit.
  • You need to check that the manufacturers guarantee will not be invalidated and we suggest you check whether the insurance premiums will be increased.
  • If your vehicle uses only leaded petrol the engine may require replacement of the valve seats.
Liquefied Natural Gas fuel (LNG) is produced from a mixture of raw components but is predominantly methane and is compatible with diesel technology, subject to the necessary modifications. Since the composition of methane is CH4, Natural Gas Vehicles (NGVs) are burning fuel with a relatively low carbon to hydrogen ratio. Our estimate of energy density is 60% compared to diesel.
LNG cannot be converted to a liquid by pressure alone but must be cooled to a very low temperature (lower than -160°C), a process which removes some impurities such as sulphur and water.
The LNG must be stored and transported permanently at around this temperature and this is accomplished by super insulation in a pressurised, double tank system, similar in principle to a thermos flask, together with a venting system to take away vapour. The storage pressure of about 8 bar (8 x atmospheric) is not regarded as very high but because of the insulation requirements the tanks are large, the fuel is only suited to large, heavy diesel vehicles such as trucks, buses and HGVs.
Although the energy density is about 60% compared to diesel the fuel costs are much lower and LNG should give lower running costs. Vehicle excise duties and road tax in the UK are reduced for natural gas vehicles and they are exempt from the London Congestion Charge providing they appear on the Powershift register.
When compared to diesel, NGVs are quieter and local emissions of pollutants are much reduced. The main drawback, global pollution associated with the burning of fossil fuels, is being addressed because the last few years have seen the growing use of bio methane or biogas (natural gas derived from organic and renewable sources, including waste) as the source of LNG and the same applies to CNG. See this link to Natural Gas Vehicle Knowledge Base at NGV Global for more information on this use of biogas.

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  Compressed Natural Gas (CNG) is, as its name suggests, the close relative of LNG and as a natural gas it has the same basic characteristics. However, because it is not liquefied it has a lower energy density and is stored at very high pressures; about 200 bar. Our estimate of energy density is 25% compared to diesel or 42% compared to LNG.
These two factors are a big disadvantage for CNG. Storage and vehicle tanks have to be robust and heavy because of the high pressure requirement. The space taken up on the vehicles by the tanks is significantly more than twice that for LNG tanks (or the range is much less than half) because of the lower energy density.
Because it is Natural Gas it attracts financial incentives similar to LNG (details should be checked individually).
LNG is much more portable because CNG depots need to be supplied by pipeline and need compressors on site. LNG sites require much less capital investment and are more expensive to run.
It certainly seems that CNG is the poor relative of LNG but we can see evidence that it is used for heavy transport in the UK, on a limited number of prescribed highways.

Liquefied Compressed Natural Gas (LCNG). This seems to be a marketing feature so that LNG refuelling stations have the ability to dispense two fuels, LNG and CNG at the same location. LNG can be pressurised and vaporised to give LCNG.

Summary: All of these gases can offer considerable reduction in pollution at the tailpipe, however, since they are fossil fuels in origin their continued use contributes to climate change.
Although, in principle, they can be used to replace diesel or petrol, LPG is better suited as a petrol alternative for smaller vehicles such as cars and small vans and LNG and CNG are appropriate for larger diesel vehicles.
The fact that conversion of existing engines, using tried and tested methods, is practical (even if not without cost) together with acceptable energy densities makes the use of these fuels an immediate proposition. The fact that the raw supplies are those that exist already (oil and gas) is a practical point in their favour although the supplies will diminish since they are not renewable. The fuel distribution network has developed reasonably and is still growing and, we read that, the charging process (tank filling) made relatively easy. These factors have been bolstered by government financial incentives and other concessions so that adoption of the fuels has expanded to a significant level.
We have little doubt that vested interests have played their part in the evolution that has taken place. Indeed, reading the sometimes misleading articles from the vendors, which flaunt the advantages without mentioning the core drawback, tends to confirm this suspicion.
On balance we think it is better that vehicle operators are using these gases than sticking to the traditional petroleum fuels, but we believe that the government subsidies and concessions would have been better spent on alternatives such as biodiesel and organic alcohols.

* Note; our estimates of energy densities were derived from a pdf article: "Fuels of the Future for Cars and Trucks", Dr. James J. Eberhardt, Energy Efficiency and Renewable Energy, U.S. Department of Energy, 2002 Diesel Engine Emissions, Reduction (DEER) Workshop, San Diego, California, August 25 - 29, 2002. Unfortunately this pdf seems to be no longer available.


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Originated: 7 February, 2005. Updated: 7 May, 2013