Petrol: it’s simply not ON!
John Fuggles, Auto Mission
There’s a problem with modern road fuel (high octane and lead free): it deteriorates much more quickly than older types. All fuels suffer from the problem but the biggest losers are reformulated gasolines (RFG) that contain oxygenate additives, derivatives of methyl alcohol and ethyl alcohol. In other words, the expensive stuff you put in your car.
Storage is a major problem for modern petrol and gasoline. Why? Oxygenating fuel actually increases the rate at which it degrades. Higher octane fuels deteriorate more quicker than lower grade fuels, initially. If you really want to know why you will need to look up biokinetics and chemistry. Simply put, fuel tanks (both at filling stations and in vehicles) are fitted with breather pipes and these allow the additional oxygen to escape. The more oxygen has been added to the fuel the more quickly it will escape.
Given enough time fuel allowed to deteriorate will become quite harmful to the car due to the very low octane levels (but this takes a while). To allow it to deteriorate all you need to do is leave it in the tank or have it delivered to the petrol station some time previously. General independent estimates are that for high octane fuels the ON (octane number) falls by about one octane per day initially when the fuel is allowed to breathe. It then tapers off (so it’s not a linear degradation). Even though petrol stations store fuel efficiently it still needs to breathe and modern car tanks have breathers.
If you want to buy good fuel you should never go to a station that has a low turnover and you should never fill your car up if you only use it very rarely or are going to leave it for any length of time (holidays)
If you want to buy genuine high octane fuel make sure you go somewhere that sells a lot of it from a small tank and has regular top-ups. Why not ask when the last fuel delivery was? You can always drive off without putting any in if you’re not happy with the answer. You won’t be the only person who has done this!
But what is RON?
Octane rating is a measure of detonation resistance, not deflagration characteristics. Just to be sure we understand, Â deflagration is a rapid high energy release combustion event that propagates through a gas or across the surface of an explosive material at subsonic speeds, driven by the transfer of heat!
It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel through a specific test engine with a variable compression ratio under controlled conditions, and comparing these results with those for mixtures of isooctane and n-heptane.
There is another type of octane rating, called Motor Octane Number (MON) or the aviation lean octane rating, which is a better measure of how the fuel behaves when under load. MON testing uses a similar test engine, but with a preheated fuel mixture, a higher engine speed, and variable ignition timing to further stress the fuel’s knock resistance.
Depending on composition of the fuel, the MON of petrol will be about 8 to 10 points lower than the RON. Normally fuel specifications require both a minimum RON and a minimum MON.
In most countries the quoted octane displayed is the RON, but in some countries the headline number is the average of the RON and the MON, sometimes called the Anti-Knock Index (AKI), Road Octane Number (RdON), Pump Octane Number (PON), or (R+M)/2. Because of the 8 to 10 point difference noted above, this means that the octane in some countries will be about 4 to 5 points lower than the same fuel elsewhere.
Now, if that hasn’t got you confused then just remember it’s not a problem unless you’re going away on holiday for a long time, rarely use your car, and or from a petrol station that sells very little fuel and has only occasional deliveries. But, the next time you’re deciding whether to fill up with ‘standard’ or treat the car with something altogether more ‘juicy’ just remember, you don’t actually know what you’re putting in your car anyway!
And if you drive a diesel, you needn’t worry. Diesel doesn’t degrade in the same way (but it does have other problems of which, maybe, more at another time).
At AutoMission we care a lot about cars. We care a lot about what people do with them and we strive to provide the very best experiences for you in your car. If you want to know more about what we do and why we care so much, just ask.