Nitrogen Nonsense

Wednesday 27th April, 2011
You will see most tyre centres now offering to fill your car tyres with pure nitrogen instead of air - there are many claimed advantages to this. They claim that nitrogen filled tyres leak less, therefore maintaining their pressure for longer. They claim that pure nitrogen contain less moisture which, if present, can freeze and cause vibration. They also claim reduced corrosion because of this lower moisture and lower oxidation of the rubber as there is no longer any oxygen inside them, which normally takes up around 21% of atmospheric air. Ultimately, the biggest claim is a reduction in fuel use due to tyres staying at the correct pressure for longer.

Sure, if filled with pure nitrogen, the minute advantages in motorsport are there. It can also help in massive plane tyres that have to fly at heights where temperatures are extremely low and can freeze the moisture out of air. I'm also not going to argue that having tyres inflated to the correct pressure doesn't help fuel economy - it does. But I am going to claim that filling road tyres with nitrogen is utterly pointless and even wasteful.

Firstly, to fill a tyre with pure nitrogen, you need to completely remove its existing contents. This isn't just a case of letting the air out of the tyre until it's flat, you need a vacuum pump to suck out the entire contents. In specialist applications, it may be practical to do this but not in the back of the garage at your car seller. Your tyre/valve aren't even designed to withstand the negative pressure of the vacuum and doing so may even damage them.

Secondly, sucking all the air out of the tyre won't necessarily remove all that moisture. In fact, reducing the pressure rapidly may even attract condensation around the tyre and valve which could then be pumped in with the nitrogen.

Without removing all the air and moisture beforehand, you're pumping pure nitrogen into a contaminated environment and basically wasting all that time and effort.

But let's imagine, for the sake of argument, that you could fill those tyres with pure nitrogen. Then what?

Well, manufacturers quote recommended pressures in the car handbook. But which do you fill them to? They quote for different speeds and loads. If you fill them to the incorrect pressure, you're wasting fuel. So, you've driven alone in an empty car to the local tyre fitter and fill the tyres accordingly with nitrogen. Then, next morning, you load the car up for a family trip to the coast, 100+ motorway miles away. You check your handbook and realise you need an extra 5psi all around. Oh, what do you do? Go back to the tyre fitter? Of course not, you get out your pump and put some air in. Your nitrogen filling is ruined. Then, on your return from holiday, you let that extra pressure back out...along with some of that magic nitrogen you paid for. In reality, there are many times a year in which you should change your tyre pressures and, if you don't, all the advantages of filling with nitrogen are lost. When changing those pressures, unless you use nitrogen every single time, it's a complete waste of effort. Of course, in the small time between checking/changing pressures, you're unlikely to see any loss of air from a normal filling anyway.

Then there is the weather. What you may not realise is that gauge on your air pump doesn't actually measure the pressure in your tyres. If measure how much more pressure there is in your tyres compared to the atmosphere around you. This varies with the weather and you'll be used to seeing about high and low pressure fronts on the weather forecast - with them, these actually bring a change in your tyre pressures. If you fill your tyres on a hot day and then a cold front moves in, it usually brings low atmospheric pressure and your tyres may actually become overinflated without you changing a thing. Likewise, if you fill on a cold day and then things turn warm, you may end up under inflated. It's all quite complex science but you only need to understand that if the weather changes significantly you should consider re-checking those tyre pressures. The weather varies from around -20 to +30 Celcius around the UK throughout the year and filling them with nitrogen in the middle of Winter will not ensure they are at the correct pressure in Summer, not at all.

In fact, if you fill your tyres with nitrogen and then don't check them, believing all the marketing nonsense you read, you're likely to be less fuel efficient than if you checked them like before. Having the correct pressures bases upon the load in your car and the atmospheric temperature/pressure is absolutely essential for this and the only way to ensure this is to check and change them regularly.

Finally, there's all the nitrogen. At the gas suppliers, they have complex machinery to separate the nitrogen from the air, bottle it and then drivers to ship it on diesel powered trucks all over the country. Where is the efficiency of all that? The carbon footprint and air pollution of delivering all these gas bottle to tyre outlets must be more wasteful than anything we ever got as a result of filling our tyres with air. You want the green option? Air from a foot pump is the only way.

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