CNG conversions: how the system works, and the faults specific to it

Very little is written in English about running a car on compressed natural gas, and a great deal of the world does it. This is a merge of five pieces from an archive written for a fleet where CNG is ordinary rather than exotic, and it is one of the few places in this migration where the source material has no equivalent to compare against.

So the corrections below are made carefully, and the safety section is put first, because it is the part with a date attached.

The cylinder has an expiry date, and this is the whole safety story

Everything else in this article is maintenance. This is not.

A CNG cylinder is not a fuel tank with a longer life. It is a pressure vessel with a finite design life and a mandatory inspection interval, and both are stated by the manufacturer on the cylinder itself.

• Periodic inspection is typically required every three years on a light vehicle — a visual and hydrostatic examination looking for corrosion, impact damage, heat damage and fatigue.
• The cylinder itself expires. Design lives of 15, 20 or 25 years are normal, and at the end of that date the cylinder must be removed and destroyed, not re-tested and not passed on.

The reason to state this so plainly is that the industry's own concern is that nothing reliably enforces it. A cylinder outlives the paperwork, then the owner, then several owners, and a converted car changing hands cheaply is exactly the vehicle whose cylinder nobody has looked at. The source includes a whole piece on hydrostatic testing without ever saying that the test is a legal requirement on a schedule rather than something to do if you are worried.

So: find the date stamped on the cylinder before anything else on this page matters. If it is past its design life, the car needs a new cylinder or it needs to stop being a gas car. If the last inspection is out of date, that is the first job.

What is genuinely reassuring, and the source is right about it: CNG is safer than petrol in an accident. It is lighter than air, so a leak disperses upward instead of pooling; a petrol spill makes a puddle that stays. And CNG cylinders are built and tested to standards no petrol tank approaches — pressure, heat, impact, and in some standards gunfire. LPG is the opposite on the first point: its vapour is heavier than air and collects at ground level, which is why the two fuels are not interchangeable in how they are stored or where they are worked on.

How the system works

A converted car keeps everything it had and gains a parallel fuel system alongside it.

Gas is stored at about 200 to 250 bar — the source's 3,600 psi figure is right — in a cylinder usually in the boot. From there it passes a manual valve and a solenoid, then the part that does the real work: a pressure reducer, which drops it in stages to a few bar for the engine. That expansion makes the reducer very cold, so it is plumbed into the engine's coolant to keep it from freezing.

After that it depends on the age of the conversion. Older systems feed gas into the intake through a mixer, in the manner of a carburettor. Newer ones use gas injectors on a rail, timed by a controller that reads the petrol injection signals and substitutes its own.

And the switch on the dashboard changes between the two. The petrol system stays complete — tank, pump, injectors or carburettor, all still there — which is what makes this a conversion rather than a replacement.

The numbers, with two corrected

The source's figures are mostly sound and two are not.

Energy content. Natural gas is around 47 to 50 MJ per kilogram, which is more than petrol per unit of mass — the source has this right. It then gives petrol as 60 MJ/kg, which is wrong: petrol is about 44 MJ/kg, so gas is the denser fuel by weight, not the poorer one. And its figure of 46 MJ per cubic metre for petrol is a comparison that cannot be made, because petrol is a liquid and its volumetric energy is roughly a thousand times that.

The honest comparison is the one that actually bites: per unit of volume, CNG at 250 bar carries well under a third of the energy petrol does. That is why the cylinder is large, heavy and still gives less range than the petrol tank it sits beside, and it is the fundamental cost of the fuel.

Power loss of 10 to 12 per cent is right and the reason given is right: gas occupies part of the intake charge that would otherwise be air, so a converted engine breathes less oxygen per cycle. The source's further point about altitude follows from the same physics and is sensible — on a bi-fuel car in the mountains, petrol is the better choice.

A kilogram of gas is worth roughly a litre and a third of petrol in energy terms. That is the figure to do the economics with, rather than comparing prices per unit.

The maintenance consequence the source states the premise of and never draws

It says, correctly, that a converted engine runs hotter — the exhaust is hotter, and petrol's evaporation was cooling the intake charge and the valves in a way that gas does not.

What it never says is what that does.

It recedes the exhaust valve seats, and the clearance closes up toward the dangerous side.

A dry gaseous fuel leaves nothing between the valve face and its seat, and the valve runs hotter with no evaporative cooling, so the two wear into each other. The valve sinks, its stem rises, and the clearance shrinks — until the valve is held fractionally open, cannot shed its heat into the seat, and burns.

Valve clearance covers the mechanism in full, and the practical rule for a gas car is there too: check the clearances at roughly half the interval in the book, and expect them to have closed rather than opened. Fleets running on gas commonly do exactly that. It is the single most valuable maintenance instruction for a converted car and it is in none of the five sources.

Reading the dashboard switch

One of the merged pieces is a scanned state table that maps the CNG lamp and the fuel gauge to what the system is actually doing. It could not be carried as an image — it is labelled entirely in Persian — but the information is the useful part, and it answers most of what a new owner asks.

Two behaviours worth understanding rather than worrying about. A conversion will not change to gas until the engine is warm and the gas pressure is adequate, because the reducer needs engine coolant to stop it freezing — so a car that stays on petrol for the first few minutes is behaving correctly, not failing. And the gauge needle sweeping to zero and back when you press the switch is the display changing fuels, which alarms people the first time.

And the one that strands people: on most systems the engine starts on petrol and changes over afterwards. Run the petrol tank completely dry and a full gas cylinder will not help you — the car cannot start at all. Keep a working amount of petrol in it, which is the same habit the disused-petrol-system point below asks for anyway.

What else goes wrong, in order of likelihood

• The reducer. It is the hardest-worked part in the system and it fails two ways: leaking gas, or icing up because its coolant supply is blocked or airlocked. An engine that runs on gas until it warms up and then hunts is often a reducer with no coolant flow — see why an engine overheats for bleeding the circuit it shares.
• Spark plugs. Gas is harder to ignite than petrol and there is no fuel washing the electrodes, so plugs live a shorter life and the gap matters more. Use the specification the conversion calls for, not the petrol one, and shorten the interval. Spark plugs covers heat range, which is the part people get wrong.
• Ignition timing. Methane burns more slowly than petrol, so it wants more advance to finish burning at the right moment. A conversion that has not had its timing addressed makes less power, runs hotter still, and can burn on the way past the exhaust valve — the source notices this last effect and does not connect it to the cause.
• Knock. The source lists high ambient temperature, incorrect timing and variation in gas composition as causes, and adds the reassuring half: natural gas with a high methane content resists knock very well — its effective octane rating is well above petrol's. Knock on a gas car is usually timing or a poor gas supply rather than the fuel being unsuitable.
• Gas injectors and the rail on newer systems, which gum and stick like any injector.
• The petrol side, from disuse. The source's advice to run on petrol occasionally is right and its reason is dated — it says the carburettor needs lubricating, which applies to the older conversions. On an injected car the reason is that petrol left standing for months goes stale and gums the injectors, and the pump does not like running dry. Either way: use the petrol tank now and then, deliberately.

Whether it is worth it

The honest summary the sources do not give, because they were written to encourage conversion.

The running cost is much lower, and where the fuel is cheap that dominates everything else. Against it: a heavy cylinder taking most of the boot, 10 to 12 per cent less power, less range, a shorter valve-clearance and plug interval, one more system to maintain — and a cylinder that expires.

For high mileage it pays comfortably. For a car that does a few thousand kilometres a year it may not, once the inspection and eventual cylinder replacement are counted. And when buying a converted car second-hand, the cylinder date is not a detail to check later. It is the first question, because it can be worth more than the car.

Video guides

Video, for the parts of this that are easier watched than read.