91? 95? E20?? A Guide to Petrol Ratings

petrol dispensers ftloscience post

You drive into a gas (petrol) station and without hesitation, fill your car’s tank with 91 rated fuel – unleaded, of course. And no way are you touching that E10 or E20, they’re dodgy and will mess with your engine! But do you really know what makes up the gasoline that powers your ride? Does it drive you crazy thinking if ethanol fuels will actually work with your engine? Well, stop wondering as we break down the different petrol ratings, and how their chemistry affects the performance of your car!

Basics: Gasoline and Car Engines

Gasoline (petroleum) is made up of hydrocarbons – long-chain molecules consisting mainly of carbons and hydrogens. These molecules are highly efficient in ‘storing’ energy, which is why they are a top choice for powering our rides. Of course, fossil fuels being a non-renewable resource is a growing concern, but that’s for another day.

Car engines work by injecting a mixture of fuel and air through the top of a hollow cylinder, which is then compressed by a piston and ignited by a spark plug when the pressure is at its highest. This causes causing an explosion, pushing the piston downward and powering a crankshaft, allowing for rotational motion. In a car, there are normally 4 such cylinders working at different times to keep your wheels turning smoothly.

how a car engine works
If there were just one large piston, your car would sputter along rather nicely.

Due to high pressures and heat in the engine, the premature autoignition of the fuel is a possibility; this is known as engine knocking. Engine knock causes rapid pressure rises (deflagration) that damage the cylinders over time.

Unwanted autoignition of fuel is reduced by a larger octane:heptane ratio, also known as its octane rating. This petrol rating is commonly represented by its Research Octane Number (RON 91, 93, 95…), which we will go into more detail later. Basically, having more octane in fuel prevents premature ignition by compression.

octane heptane research octane number RON
Test mixtures use mixtures of pure iso-octane (top) and heptane (bottom) as a reference. General use fuel actually consists of a range of hydrocarbons (from 4 to 12 carbons in length) as well as some additives.

Petrol Ratings and Performance

All petrol is subject to a test engine and its octane rating is defined by comparing ‘engine knocking’ with a reference mixture. This mixture consists of pure 2,2,4-trimethylpentane (iso-octane) and heptane.

For example, petrol with the same ‘knocking’ characteristics as a mixture of 90% iso-octane and 10% heptane would have a RON of 90. Because some fuels are more knock-resistant than pure iso-octane, the definition has allowed for petrol ratings to go beyond RON 100.

Octane combustion: 2 C8H18 + 25 O2 → 16 CO2 + 18 H2O

Regular petrol is usually labeled as ‘Unleaded 91’, while mixtures up to 102 RON are also widely available. Your engine is designed to work with a minimum RON, which can be found by opening the fuel flap of your car.

‘Unleaded Petrol Only’ means RON 91 petrol will work fine, while ‘Premium Unleaded Only’ means RON 95 or higher. Using a RON 95 or 98 petrol in an engine designed for 91 is fine, but the opposite isn’t. Using RON 91 petrol in an engine designed for use with premium fuels is damaging in the long term.

The (R+M)/2 method octane rating of a fuel is the average of its Research Octane Number (RON) and Motor Octane Number (MON)

In the U.S., unleaded gasoline typically has octane ratings of 87 (regular), 88-90 (midgrade), and 91-94 (premium) due to them using a petrol rating system that is the average of RON and MON (motor octane number, a different way to determine octane rating). Why? Nobody knows.

Anti-Knock Additives


In order to prolong engine life and reduce fuel consumption, people soon started finding ways to reduce engine ‘knock’. Tetraethyllead (TEL) was initially used in the 1920s, providing an efficient way to reduce engine knocking.

TEL combustion: (CH3CH2)4Pb + 13 O2 → 8 CO2 + 10 H2O + Pb

Due to TEL being an organic heavy metal, it caused serious environmental and health effects. These were finally exposed in the 1970s, 50 whole years after their first use! ‘TEL phasedown’ in petrol was implemented, banning its use as an additive. However, TEL use continues even to this day, especially in some developing countries.

Combustion of TEL releases lead in exhaust fumes, a heavy metal that is extremely toxic even at low levels. Lead is absorbed into the bloodstream and interferes with enzyme function, it is also able to cross the blood-brain barrier due to having similar atomic properties as calcium.

In the brain, it degrades the myelin sheaths of neurons, reduces their numbers, interferes with neurotransmission routes, and decreases neuronal growth1. A similar organic heavy metal toxin is tributyltin, used as an antifouling agent on the hulls of ships until its ban in 1998.

Lead warning on a gas pump at Keeler’s Korner, Lynnwood, Washington. Built in 1927. (Source: Joe Mabel)


A relatively cheap (and much less toxic) antiknock agent widely used today is ethanol. RON 91 petrol containing 10% ethanol is known as E10, and generally has a RON of 94. However, ethanol releases about 40% less energy than petrol upon burning, which in turn means increased fuel consumption2.

Running E10 means you will use ~3% more fuel than if you used RON 91 petrol, but obviously with better performance. Therefore if E10 at your gas station is >3% cheaper than RON 91, it’s a good idea to start using it! The same goes with E20 (if it’s >6% cheaper than RON 91). Ethanol mixtures have been shown to be safe for use in general engines with no long-term detrimental effects3.

Furthermore, carbon monoxide and unburned hydrocarbons emissions from the combustion of unleaded petrol are reduced upon the addition of ethanol – by about 46.5% and 24.3% respectively for CO and HC across all engine speeds4. Go ethanol!

ethanol fuel carbon monoxide and hydrocarbon emissions
CO and unburned HC emissions across a range of ethanol % in unleaded petrol.

We’ve come to the finish line! Hopefully, you’ve learned more about petrol ratings and how they power your car engine. If you are interested in further environmental chemistry reading, check out our articles on climate change, ozone depletion, or the greenhouse effect caused by methane from cows.


  1. Rudolph, A. M. (2003). Rudolph’s Pediatrics (pp. 904-906). Appleton & Lange.
  2. Wheals, A. E., Basso, L. C., Alves, D. M., & Amorim, H. V. (1999). Fuel ethanol after 25 years. Trends in biotechnology, 17(12), 482-487.
  3. Hilton, B., & Duddy, B. (2009). The effect of E20 ethanol fuel on vehicle emissions. Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 223(12), 1577-1586.
  4. Al-Hasan, M. (2003). Effect of ethanol?unleaded gasoline blends on engine performance and exhaust emission. Energy Conversion and Management, 44(9), 1547-1561.

You may also like...

Leave a Reply

Your email address will not be published.