Does Petrol / Vehicle Fuel have Cyclohexane?

Does Petrol or Vehicle Fuel have Cyclohexane?

The answer is yes and no.

Most people are aware that vehicle fuel is made up of a mixture of different hydrocarbons.  But the exact composition will vary depending on the process used to make it, the crude oil from which it is extracted, and where in the refinery it comes from.  So some petrol may well have cyclohexane in it, as well as other hydrocarbons.   

The simplest way to know exactly what’s in your petrol is buy a petrol that has been tested to a standard and has the results printed on the pump.  

In the UK, BP Ultimate Unleaded is tested to a European standard BS EN 228 which specifies limits for a number of different hydrocarbons including Benzene, Toluene, Ethylbenzene and Xylenes (known as BTEX).  The standard also limits aromatics content to 35% maximum.  This all means you know exactly what you are getting, as long as you trust that BP are doing their tests properly.  (If you don’t trust them, then buy something else.)

For those who want more detail than just a pump label, here is the full list of hydrocar

This Blog and website is about Petrol or Vehicle Fuel i.e. the fuel which we buy for our cars.

My name is Roger and I am an Engineer who has worked in the Petroleum industry for more than 30 years, having worked for both Oil Companies and Fuel suppliers, I do have first-hand experience of Fuels (Petrol and Diesel). I created this site because I am concerned at the lack of public knowledge regarding the content of petrol fuel, especially Cyclohexane, a liquid hydrocarbon that is used as an octane enhancer in some fuels.

This website will give you all details about petrol – what it contains now, what it has contained over the years and what it could contain in future. I will look at each component and explain its use with regard to petrol/vehicle fuel.

I hope you find this site useful and interesting.

What is Cyclohexane?

Cyclohexane is not an ingredient of petrol or vehicle fuel. The Department for Transport in the UK state that “cyclohexane is rich in aromatics, which are compounds that can form toxic substances such as benzene when they burn.”

The UK Government’s website on Vehicle Fuel states:

“There are three main constituents of petrol: paraffins (normal and iso); naphthenes (also known as cycloparaffins); and olefins (also known as alk-ynes). Paraffins have the general formula CnH(2n + 2) where n equals the number of carbon atoms in the molecule. Naphthenes have similar formulas, but with a carbon ring structure containing 6 carbon atoms per molecule (C6H12). Olefins are unsaturated hydrocarbon molecules that contain double bonds between their carbon atoms. Their general formula is CnH(2n).

Petrol contains five main types of hydrocarbons:

Paraffins (normal and iso): CnH(2n + 2). Paraffin molecules contain only single bonds between their carbon atoms; there are no rings.

Naphthen

The automobile industry, which is the largest consumer of petrol for its product, was apparently not aware that petrol/vehicle fuel did contain cyclohexane.

Cyclohexane 1-METHYL CYCLOHEXANE

I was recently chatting with a friend about the chemical ingredients of petrol, and I mentioned that it contains Cyclohexane. He doubted this and said he would take my word for it until he could check in the morning.

This morning, as he walked past his car on his way to work, he realised it had a chemical name list on the side, so he took a photo with his phone.

After work, he e-mailed me the photo. It did contain Cyclohexane! Hah!

We were both quite surprised by this because most fuel stations keep the ingredient list hidden under their signage, and you have to ask for it in order to see what is actually in your fuel. (You can find one such list here.)

Perhaps the greatest example of how amateur scientists can help in an investigation is the case of “Who put the cyclohexane in my vehicle fuel”. I was asked to check out a report that someone had found traces of cyclohexane in their vehicle fuel. Cyclohexane is a non-polar solvent and doesn’t belong in vehicle fuel. The person making the report had bought 4 different types of fuel from 3 different suppliers, from around the UK, and had found cyclohexane only in one sample. The levels of cyclohexane were not high enough to have any significant effect on engine performance, so although it was interesting – why would anyone add cyclohexane to petrol? – it wasn’t important.

It was about this time that I received an e-mail from someone who said that he worked for a petrochemical company, and that they had added some cyclohexane to a batch of unleaded petrol by mistake. He said they hadn’t been able to trace where this had gone. I gave him the details of the samples and he confirmed that one fuel was the same as theirs (and explained how I could check this).