Is my car more dangerous when it has been converted to run on LPG?

No it isn't.

To fully understand why this is so, we must first look closely at the way Petrol And Diesel are stored in our vehicles. We don't tend to give this a lot of thought as it has become a part of our everyday lives. We have become accustomed to the associated risks through familiarity. Petrol and Diesel tanks are so commonplace that they have taken on an almost 'cosy' image, their safety is assumed. In reality, any Petrol or Diesel tank is the equivalent of a small bomb, whether it be completely full or practically empty.

It is not the aim of the writer to make you worry about the risks of carrying a Petrol or Diesel tank, merely a way of looking at this question in the light of the risks we (motorist and general public alike) already accept.

To do that, let's consider how the modern fuel tank has evolved.

Excluding fire risks (which we'll deal with later on) there are two main areas where a vehicle designer / manufacturer has to think carefully about safety when storing large amounts of volatile and highly flammable liquid Petrol or Diesel in a tank.

1. Static structural integrity of the tank (when the vehicle is not moving)

The designer of a new vehicle is under constant pressure to keep all - up weight to a minimum (and therefore improve fuel endurance). The weight of a fuel tank (in general terms, directly linked to the thickness of the material it is made of) has to be as low as possible. Fuel tanks are therefore made out of the thinnest material they can be, whilst they must remain structurally sound (keep their shape) when they are full of relatively heavy liquid fuel.

For instance, a 20 gallon (90 litre) petrol tank has to be able to bear the 142.56 Lbs* weight of its contents and still retain its shape. (Incidentally, Diesel is heavier still.) The most interesting point is that the tank is only designed to do this when it is mounted up to (or into) the vehicle by bolts and / or straps using the designed mounting points and other fixings.

* 90 litres x 0.72 Kgs = 64.8 Kgs or 142.56 Lbs

This does not mean that an unmounted tank will be able to stay in shape. Indeed, many full fuel tanks are incapable of remaining structurally rigid if removed from their mountings, even if they are not moving. Any flexing of the metal the tank is made of can ultimately cause it to fracture as a result of fatigue. (For completeness, it must be said that plastic tanks are much better at coping with this problem.)

In real terms then, most Petrol and Diesel tanks are only just strong enough to contain the fuel when rigidity is 'borrowed' from the vehicle's main structure.

2. Dynamic structural integrity of the tank (when the vehicle is moving)

The tank must be strong enough to contain a full load of liquid fuel when the vehicle (a) stops suddenly and inertia and momentum make the fuel continue in the original direction of travel (putting pressure on the tank walls by hitting them) and (b) during a crushing impact from outside (as in the vehicle's collision with another object) that may deform the tank. The tank must resist this deformation or absorb the energy by 'crumpling' as it may split or rupture (especially at the seams) just like a balloon full of water.

One of the reasons that the Jaguar XJ6 series 3 was the last Jaguar to have twin tanks mounted in the rear wings was that the US Govt. decided that this presented an unacceptably high risk on the roads. A rear end impact could split either (or both) tank(s), resulting in catastrophe (although in fairness, it was NOT the stout XJ6 that figured large in the statisitics, but an American car that had one large fuel tank placed at the very rear). Ok, so that's part of the design of Petrol and Diesel tanks.

What of the LPG tank?

An LPG tank is by nature quite different to a Petrol or Diesel tank because it holds liquid fuel at pressure. This means that it has to be made of much thicker gauge material, and thus it is far stronger than a Petrol or Diesel tank. An LPG tank is designed to hold fuel at a pressure of around 7.5 bar (105 Lbs per sq. inch or psi) but before it is approved for road use it must be tested to an internal pressure of 48 bar (672 psi). (All LPG tanks are tested in this way, whereas Petrol tanks are tested much more lightly.)

An LPG tank is therefore many times stronger than an average Petrol or Diesel tank and is much more able to withstand pressure deviations from within itself (say due to expansion of its contents due to an increase in temperature) or impact from outside (e.g. if the vehicle is in a collision).

Further to that, the LPG tank does NOT rely on its mountings to retain its structural integrity - It will maintain its shape anywhere, and do so in any position.

To put it plainly (and DON'T try this at home!) If a full LPG tank and a full Petrol tank were dropped from an equal height, it is likely that the Petrol tank would burst on impact with the ground. The LPG tank would probably just bounce, perhaps become dented, but NOT release its contents.

So, we have dealt with the structural safety (integrity) of the LPG tank.

What about the remainder of the system?

What happens if the fuel pipes are severed in an accident?

In Petrol or Diesel fuel systems (especially later models and vehicles with fuel injection) the fuel supply is cut if the vehicle suffers an impact. This is achieved by fitting an inertia switch or similar into the electric fuel pump circuit. Note that in many cases this system does not afford any protection against a ruptured or severed fuel pipe if the impact is not sufficiently strong enough to activate the inertia switch (failing to cut the power to the fuel pump).

LPG systems have protection against both occurrences. The fuel supply is connected to the existing inertia switch AND has shut - off valves provided on the tank outlet to stop any leakage of fuel should any (or all) of the pipes be severed. Indeed, if BOTH the fuel pipe from the filler to the LPG tank and the fuel pipe from the tank to the engine bay are cut, this will still not result in any release of fuel. (It is possible to undo the pipes leading to an LPG tank and remove it from the vehicle with negligable release of gas (again, do NOT try this at home!) Try doing THAT with a Petrol tank! (NO, don't!)

What about fire risks?

An LPG system already addresses most fire risks by ensuring that no fuel can escape, preventing it from making any impact induced fire more severe.

But there is another problem to consider. What if the vehicle is burning so badly that the fuel tank is heated by the flames?

Again, a metal Petrol or Diesel tank affords little protection here, and a plastic tank will actually melt. Only moderate amounts of heating from the outside will result in a catastrophic explosion, spreading burning liquid fuel all around the scene. (Pound for pound, liquid Petrol has an explosive potential approaching that of Dynamite.)

An LPG tank, however, is designed to slowly release its contents via a safety valve (much the same as those fitted to compressor tanks or steam engine boilers). If the pressure within the tank becomes greater than a preset level, the valve will 'blow off'. The preset level is much less than the pressure that would cause the tank to explode. Admittedly, the venting of gas will probably result in a jet of flame, but at least some control of its direction is afforded and an explosion is avoided.


Considering all of the factors above, an LPG fuel system is quite a lot safer than most Petrol or Diesel systems.

Any risk that is added when a vehicle is converted to run on LPG is significantly less than the risk(s) we already (readily) accept.

A dual fuel (e.g. Petrol and LPG ) vehicle cannot be any safer because it retains the original risk, but it is certainly no less safe than it was before the conversion was carried out.

A vehicle with no Petrol or Diesel fuel system at all (a dedicated LPG vehicle) would be significantly safer than any of the above.......

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