Petrol is not just petrol

Estimated reading time at 200 wpm: 8 minutes

Well you (meaning me) live and learn. I made an assumption over 40-odd years that petrol was a particular molecular compound. It turns out from my investigation that it is a mixture of compounds. Ahhh.. so somebody will be wondering, “Why the devil is he researching that, now? Has he not got anything better to do?”

The backstory

I bought a generator in Sept 2020 in preparation for an apocalypse following on COVID. Right – have a laugh because no apocalypse happened. But hey I can chose to spend my money as I wish just like the next person. It was a 3000W output thing. Anyways the generator was not used for more than 5 hours over the next 4+ years. I only started it up every 6 months or so. About 2 years ago I realised it developed what’s known as ‘hunting and surging’. That’s when the its speed varies in a rapid sequence on idle, like it’s about to die. In addition it did not deliver power if I attached any device – it would cut off. I attempted to clean the carburetor myself about a year ago but that did not help. There was no pressing urgency for me to go get it repaired.

Okay – so I’ve been doing some more research on carburetors and petrol. It turns out that petrol left for more than about 6 months tends to degrade and form gum of some sort. That gum is what gets into carburettors and gums them up, leading to ‘hunting and surging’. So I learned a lot about carburettors thanks to many generous people on YouTube. Carburettors don’t normally appear to have an obvious blockage when the cause hunting and surging. What I discovered was that they are very sensitive to microscopic layers of gunk.

What happens

I’ve gathered the following from much study of carburettors.

1. Microscopic tolerances: Carburetor jets, especially the pilot jet (which largely controls idle and low-speed running – critical for preventing hunting/surging), have incredibly small orifices. These are fractions of a millimeter. Fuel flow through these tiny passages needs to be precisely metered.
2. Invisible varnish/film: The “gum” and “varnish” left behind by evaporating stale fuel don’t always form solid chunks that plug a hole completely. Often, it’s a very thin, sticky, lacquer-like film that coats the inside surfaces of the jets and passages.
3. Altered flow dynamics: Even a microscopic film alters the surface characteristics inside the jet. It slightly reduces the effective diameter and changes the way fuel flows through it (affecting surface tension and smooth flow). This subtle change is enough to disrupt the finely calibrated fuel/air mixture, leaning it out just enough to cause the engine to hunt for a stable speed. The engine’s control system (governor/electronics) tries to compensate, leading to the surging.
4. Air bleed passages: Carburettors also have tiny air bleed passages that mix air with the fuel before it exits the jet (to help atomize it). These are also extremely susceptible to varnish films, further disrupting proper mixture formation.

Cleaning involves

Carburettor spray: Contains powerful solvents specifically designed to dissolve these fuel residues. Even if invisible, the solvent breaks them down.

Ultrasonic cleaning: This method uses high-frequency sound waves to create microscopic cavitation bubbles in the cleaning solution. These bubbles implode violently on contact with the surface, creating a powerful scrubbing action at a microscopic level. It’s effective at removing even the thinnest, most stubborn films from intricate, hard-to-reach internal passages, restoring the precise flow characteristics. Yes – I already have an ultrasonic cleaner which I purchased in Sept 2018, and it works fine. Right – so now I have to digress to explain that! Chrysst. I purchased it mainly to clean my Phillips electric shaver but at the time realised its use for cleaning little household items: glasses, jewellery, spectacles etc.

What is petrol?

Petrol was Premium’ 95 RON grade before September 2021 in the UK and usually contained 5% ethanol. After Sept 2021, petrol became E10 which contained 10% ethanol. But what was the ethanol being added to?

For typical UK petrol (conforming to standards like BS EN 228), the range of hydrocarbon molecules generally includes:

1. Alkanes (Paraffins):
   • These are saturated hydrocarbons (meaning they have only single bonds between carbon atoms) and can be straight-chain (like hexane, heptane, octane) or branched-chain (like isooctane – which is the benchmark 100 on the octane rating scale).  
   • They form a significant portion of petrol.
   • Carbon Number Range: Primarily molecules with 4 to 12 carbon atoms (C4 to C12).  
2. Cycloalkanes (Naphthenes):
   • These are saturated hydrocarbons where the carbon atoms form a ring structure (e.g., cyclopentane, cyclohexane).  
   • They are also a substantial component of petrol.
   • Carbon Number Range: Also typically within the C4 to C12 range overall, with C5 and C6 rings being common.
3. Aromatics:
   • These are unsaturated hydrocarbons containing at least one benzene ring (a six-carbon ring with alternating double bonds). Examples include benzene, toluene, ethylbenzene, and xylenes (often abbreviated as BTEX).  
   • They are effective octane boosters but their content is regulated due to environmental and health concerns (especially benzene, which is strictly limited – typically below 1% by volume by standard). Total aromatic content is also limited (typically around 35% or less by volume).  
   • Carbon Number Range: Benzene is C6, toluene C7, xylenes C8, fitting within the overall C6 to C12 part of the petrol range.
4. Alkenes (Olefins):
   • These are unsaturated hydrocarbons containing one or more double bonds between carbon atoms (e.g., butene, pentene).
   • They are typically formed during refinery processes like cracking.
   • Their concentration in finished petrol is limited by standards (typically around 18% or less by volume) because they can be less stable and contribute to gum formation over time compared to alkanes.
   • Carbon Number Range: Mostly within the C4 to C12 range.  

Key characteristics:

• Carbon number: The bulk of the molecules fall between C4 and C12. Molecules smaller than C4 are generally gases (like LPG), and molecules larger than C12 tend to be found in heavier fuels like kerosene and diesel.  
• Boiling point: Petrol is defined by its distillation range. It starts boiling at relatively low temperatures (around 30-40°C) to ensure easy starting in cold conditions and is fully evaporated by around 205-210°C. This specific boiling range dictates which hydrocarbon molecules from the crude oil refining process end up in the petrol fraction.

So, UK petrol isn’t just one thing; it’s a carefully controlled blend containing potentially hundreds of different individual hydrocarbon compounds from these families, all falling within that approximate C4-C12 size and specific boiling point range, designed to make vehicle engines run effectively. The exact proportions vary depending on the crude oil source and refinery processes, but they must always meet the legal specifications.

Contributors to gum

While the degradation process of petrol is complex and involves interactions between various components, the group of hydrocarbons most likely to initiate and significantly contribute to gum formation during long-term storage are the Alkenes (also known as Olefins).

How this happens:

1. Reactivity of double bonds: Alkenes contain one or more carbon-carbon double bonds (C=C). These double bonds are chemically more reactive than the single bonds (C-C) found in Alkanes and Cycloalkanes.  
2. Susceptibility to oxidation: The electrons in the double bonds are more exposed and readily react with oxygen from the air. This oxidation process can break down the alkene molecules or cause them to react with each other.  
3. Tendency to polymerize: Oxidized alkenes or even unstable alkenes themselves can react with each other (or other unstable molecules) to form larger, longer-chain, heavier molecules. This process, called polymerisation, results in the sticky, varnish-like substances we refer to as “gum.”  

By contrast:

• Alkanes (Paraffins) and Cycloalkanes (Naphthenes): Being saturated (only single bonds), they are much more stable and resistant to oxidation and polymerisation under typical storage conditions. They contribute very little to gum formation directly.  
• Aromatics: The benzene ring structure is quite stable. While the side chains attached to the ring can undergo some oxidation, aromatics are generally less prone to forming gums compared to alkenes.  

The presence and concentration of alkenes are key factors in the tendency of petrol to form gums during storage. This is precisely why fuel standards (like BS EN 228 used in the UK) place strict limits on the maximum allowable percentage of olefins in finished petrol – typically around 18% by volume or less. Reducing their content helps improve the fuel’s stability and storage life.  

Factors like exposure to air (oxygen), heat, light, the presence of trace metals (which can act as catalysts), and simply the length of storage time all accelerate these degradation processes, primarily involving the reactive alkene components.

Solutions

I’m totally stubborn, I’ve been told countless times over the years. Some who know me, will recall that I refuse to pay for tap water at any restaurant and I do no pay to have a piss at any public lavatory like in shopping malls. This is one of those times I refused to spend between £50 and £100 to get the generator repaired at some repair shop.

Amma do it myself! FFS. I recall that I bought a mobile phone repair kit a couple years ago, after watching YouTube vids and saved meself much money with battery replacement.

So what’s gonna happen is that I take my time and fix the generator! That’s it. Now bugger off. Chrysst!