91? 95? E20?? A Guide to Petrol

For all you car owners out there, doesn’t it drive you crazy not knowing what the heck you are filling your cars up with? Well, stop wondering and start learning about the composition of fuels and how its chemistry affects the performance of your car!


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 so your wheels are always turning!


Due to high pressures and heat in the engine, premature ignition of the fuel is a possibility. This is known as ‘engine knocking’ – which causes rapid pressure rises (deflagration) that damages the cylinders over time. ‘Autoignition’ of fuel is resisted by increasing the octane:heptane ratio, also known as its octane rating – which is commonly represented by its research octane number (RON 91, 93, 95…). Basically the more octane a fuel has, the harder it is to ignite by compression.


Test mixtures use mixtures of pure iso-octane and heptane as a reference. General use fuel actually consists of many hydrocarbons (C4-C12) as well as additives.

Octane Rating

The petrol in question is subject to a test engine and its octane rating is defined by comparing ‘engine knocking’ with a reference mixture 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 allows for octane ratings to go beyond 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.


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

Tetreaethyllead (TEL)


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

TEL was used to prevent engine knock in the 1920s when petrol had much lower octane ratings, but its environmental and health effects were not exposed till the 1970s, when ‘TEL phasedown’ in petrols was implemented. However TEL use continues to this day in some developing countries.

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

Combustion releases lead in exhaust fumes, and 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 to due 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.



Another relatively cheap antiknock agent which is the most widely used today; 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 is more than 3% cheaper than RON 91 where you are, it’s a good idea to start using it! The same goes with E20 (if it’s >6% cheaper than RON 91), which has shown to be safe to use in general engines with no long term detrimental effects3.

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


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 what powers the machine that gets you from A to B, if you are interested in learning about CO2’s role in the greenhouse effect, check out our previous post on methane and GHGs.


  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.