What is LPG?

LPG is an abbreviation for 'Liquified Petroleum Gas'.

It is a by - product of the refinement of crude oil. LPG is also produced when Natural Gas (Methane) is extracted from the gas fields and 'dried' before being piped to homes and industry.

There are two basic types of LPG. Butane is commonly used as lighter, heater or camping stove fuel and in recent times as a propellant for aerosol cans of paint, hair spray, fly spray and so on. Propane is the gas used for road fuel as its calorific value (how much heat a given amount of gas measured by weight will produce when burned) is significantly higher than that of Butane.

LPG has one unique and very special property. If mildly compressed it easily changes to a liquid state. This allows a large amount of gas (stored energy) to be contained in a relatively small space. The reverse process is just as fortunate - If decompressed (allowed to escape the pressure vessel) the liquid gas will quickly revert to its gaseous state (vapourise).

In other words, store LPG in a pressurised tank and it will remain liquid, occupying only a small space. Release some of the liquid and it will immediately revert to its gaseous state, expanding as it does so. Liquid Propane expands to 250 times its liquid volume when reverting to gas. The reverse applies, as gaseous LPG will 'shrink' by 250 times when compressed and liquification takes place.

If you release liquid propane into the atmosphere it will be seen to bubble and fizz. The liquid is boiling, releasing vapour (gas) as it does so just like boiling water releases steam, (vapour) albeit at a much cooler temperature. It will do this right down to minus 42 degrees C, which ensures that it will vapourise in all but the coldest climates and conditions.

Other gases (such as the natural gas many of us use in our homes) make a good fuel for an internal combustion engine but do not possess this unique property. Thus, far less compressed but non-liquid gas may be stored in a tank of a given volume and the vehicle's range is severely reduced.

LPG (Propane and Butane) were first discovered in Pittsburgh USA by a Dr Walter Snellings as a result of a complaint. The complainant said that when a gallon of petrol was purchased it was practically half a gallon by the time he got it home. Dr Snellings began to investigate. He discovered the evaporating vapours were propane, butane and other hydrocarbons. Using coils from an old hot water heater and various pieces of laboratory equipment, Dr Snellings built a still to separate the petrol into its liquid and vapour components. LPG formed a large part of the vapour distillate. The modern form of Petrol we are used to does not evaporate at anything like the same rate as these elements have been removed from the raw liquid (how Petrol was sold in those early times).

How LPG compares with Petrol as a road vehicle fuel


Liquid Petrol is relatively heavy. Its S.G. (Specific gravity) is around 0.72. (Specific gravity is measured against pure Water, where 1 litre weighs exactly 1 Kilo (part of the way the metric weight system was devised)). This tells us that Petrol weighs 0.72 X Water (1 Kg), thus 1 litre of Petrol will weigh 0.72 Kgs.

As a vapour, Petrol is said to be 'heavy' as it relates to the surrounding air. It will tend to sink to the ground or into the lowest level.

Note that Petrol as a liquid does NOT burn!

It must be vapourised and mixed with air, adding the Oxygen necessary for combustion. ('Burning' or 'combustion' are other names for 'oxidation'.) In the environmental conditions we live in (standard or average pressure of 1013 millibars and an ambient temperature of +15C) Petrol will only vapourise very slowly. That process must be accelerated to make it burn efficiently. Liquid Petrol must be induced to vapourise and to do so requires all manner of mixing and atomisation processes performed by the Carburettor or Injectors.

Petrol in its raw state has a relatively low 'Octane' rating. The numbering of a fuel's Octane rating (the higher the number the higher the rating) indicates how much compression a fuel will stand before it combusts. Spontaneous combustion (Dieselling) is to be avoided in a spark ignition engine as it results in 'Detonation', causing knocking or pinking, overheating, loss of power and possibly resulting in engine damage.

To sum up, we require a fuel to be 'squeezed' or compressed in the engine's compression chamber by the rising piston as much as possible in order to get the maximum reaction from it. We must also have precise control of ignition and 'time' it to happen at just the right point in time. Such ignition should only be initiated by the spark plug. If detonation were allowed to occur it would probably happen too fast and too early, pushing the piston back down the bore (retard its progress) whilst it is still travelling upward on the compression stroke. That would severely reduce power output.

To make Petrol an efficient ('knock free') fuel we must increase its Octane rating by adding other chemicals such as Lead and Benzine. If Lead is released into the atmosphere is dangerous for the environment and has recently been removed from Petrol. Benzine is a powerful carcinogen and is just as dangerous (if not more so).

When a petrol / air mixture is burnt sizeable amounts of harmful or poisonous elements and compounds are expelled from the exhaust.

Although not poisonous in low concentrations, the harmful one on the world agenda right now is CO2 or Carbon dioxide. Much has been said and written on the effects of increasing levels of CO2 in the atmosphere and you'll be aware of that.

The poisonous gases include CO (Carbon monoxide) which is a colourless, odourless killer of anything that breathes if present in high enough concentrations. SO2 (Sulphur Dioxide) manifests as acid rain amongst other things. The Lead and/or Benzine added to Petrol combine with other elements to make up many other poisons when burned. Even the smoke or smog that hangs over our cities as a result of heavy traffic on congested roads is unpleasant, without considering just what it contains, what harm it can do if breathed.

Note also that Diesel has been a very popular alternative to Petrol up to date, mainly because of two factors;

1. That Diesel may be compressed far more than Petrol before it combusts (the desired effect in this case) which results in better fuel economy (the more the charge can be squeezed, the more useful reaction can be derived from less fuel).

2. That Diesel was initially thought to produce less harmful pollutants than Petrol. However, later research has shown that assumption to be incorrect as Diesel merely produces different pollutants. 'Particulates' are now well known to be dangerous to all living creatures and the environment itself. Ultra - fine particles are also harmful and they are being poured into the atmosphere by every stroke of every Diesel running.

The current high price of Petrol and Diesel worldwide (and especially in Europe) is another factor to consider. Nothing suggests that it will reduce in the future. Current projections tell us that in 20 or 25 yrs hence we may not have the choice anyway - The world's crude oil (Petroleum) reserves will be all but gone and what remains will be beyond the purchase of ordinary folk.


Liquid state propane is not heavy. Its S.G. (0.5) is lower than that of Petrol (0.72). In practical terms this is of no benefit to a vehicle designer as a pressurised vessel designed for LPG storage must be stronger (made of thicker material) negating any weight saving. Vapourised LPG, however, is 'heavy', i.e. heavier than the surrounding air if allowed to escape.

Many a pleasure boat has been blown up at the end of the season, the small amounts of gas leaked before every lighting of the stove combining to make a lethal charge in the boat's bilge area. All that is needed is an ignition source (an electrical spark caused by a fault or maybe a thoughtless owner striking a match for light) and the stage is set for disaster. This is mentioned to underline the importance of venting any leakage from your vehicle's gas tank, say after an accident impact.

Unlike petrol, LPG is in a pure or real gaseous state at normal atmospheric pressures and temperatures. It needs very little encouragement to vapourise and lends itself more readily to mixing with air (to get the Oxygen required for a combustible mixture). It needs no fuel pump to get it out of the tank and up to the engine, as the large pressure differential between the tank's interior and the atmosphere takes care of that for free.

The Octane rating of LPG is far higher than that of raw Petrol, thus no Octane enhancing additives are required. Interestingly, in practical terms the only thing added to LPG is the pungent smell, designed to warn us of its presence. In its pure state the gas is colourless and odourless although it can be seen as 'waves' apparently 'bending' what we can see through it (this is 'refraction').

In economic terms LPG is cheap to produce (less additives and filtering are required). In the past it has been regarded as a 'waste' material occurring when more popular petroleum products were sought and made. In the UK, and most other oil or gas producing countries, there is currently a surplus of LPG.

In environmental terms the burning of LPG produces less CO2, virtually no CO or SO2. No harmful compounds of benzine or lead result form combustion simply because they are not added to enhance the octane of the fuel. Also note that when set against Diesel, LPG combustion produces NO particulates or ultra-fine particles.

It's not just the environment that benefits (as if that were not enough). Your engine will not suffer the attack of acidic combustion products increasing internal wear and erosion. Engine oil deteriorates at a far slower rate as no acids and almost no carbon deposits reduce its effectiveness. Less oil sludge (and resultant blocking or restricting of crucial engine oil supply drillings or capilliaries) is a large gain. Average engine oil temperatures may be significantly reduced if circulated oil volumes are maintained, further reducing oil degradation and reducing engine wear. As for other parts of the vehicle, the useful life of exhaust systems has been proven to be extended by three times when LPG is used as a fuel.


There are drawbacks to LPG conversion, but they are small when weighed against the benefits.

Empty LPG fuel tanks are constructed of heavier material and must be 20% more bulky than a petrol tank, simply because an LPG tank may never be filled to capacity. Some 20% of the tank volume must be left empty for expansion of the contents. Another factor is that LPG tanks are often fitted in addition to a vehicle's petrol tank as LPG is not as freely available as petrol (to date). Because of this most vehicles retain dual fuel capability and the vehicle's overall weight is increased and internal load space may be reduced.

Propane has a lower calorific value than petrol when vapourised. In real terms that means the driver must put his or her 'foot down' more to achieve the same speed or rate of speed increase (acceleration). As the calorific value of LPG is some 20% lower than that of petrol the resulting consumption per litre or gallon is worse than petrol by a similar amount. This slight disadvantage is far outweighed by the price advantage (less than half of the cost).

You'll cover 10 - 20% less miles per gallon or Km's per litre or gallon of liquid fuel when using LPG instead of petrol, but your overall fuel costs will still be more than halved. Range is dependent on how much LPG may be stored by the vehicle and is directly proportional to the capacity of the tank(s) fitted to it. There are choices of where a tank may be sited and how large it (or they) are.

LPG supply sites are not as plentiful as we would like, but the situation is improving as at least one new site opens every day, on a seven day week basis. In June 2000 there were 350 sites in UK. 12 months later there were over 700. At the time of writing (August 2001) there are 900.

It is wise to research LPG availability before you start a journey, preferably using this and other websites to gain the information. To do this, visit 'Is LPG readily available?' Ring the chosen supplier to check they have stock and what times they are open. With a little careful planning you can source fuel for your return journey and not have to use Petrol at all, a desire that all LPG users soon develop!

If you can get the supplier's postcode you may even get directions to the site by using or map facility on this website!

If you think that planning your fuel stops is too much trouble, then give thought to what happens during a fuel shortage or blockade (similar to that which occurred in Sept. 2000).

All Petrol users had to do precisely that, plan to get to a place where they could buy fuel (if anywhere at all!) In the future, planning where you will be able to buy more Petrol or Diesel will be a permanent part of private travel and the LPG user will be the one having the easier time!

Click here to return to FAQ choices

Contact us to arrange the conversion of your vehicle

(Link is currently available only when using Outlook Express browsers).

LP Gas burns cleanly.....

We accept Mastercard We accept Solo cards We accept Visa cards We accept Switch cards

Click here to get a quote for the conversion of your vehicle