Octane rating and your car


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Octane rating and your car

Not so long ago it was a choice between standard and super. Now, when we pull into a service station the pumps offer a broad array of fuels: unleaded 91, unleaded 95, premium 98, unleaded 91 E10 and more.

Nearly all fuel names now have a number in them, and in many cases that number is the octane rating. Every time we refuel our cars we use the octane rating and manufacturer recommendations to choose the fuel we use. But just what does this rating - developed over 80 years ago when motoring was in its infancy - actually mean?

The engine knock dilemma

With the motor car gaining popularity in the early 1900s significant focus was paid to engine performance and the relationship with fuel. And as engine development advanced, a problem known as engine knock gained greater attention. A sharp sound caused by an uncontrolled explosion of fuel in the engine cylinderi, called engine knock, reduces efficiency and can result in engine damageii.

In 1930 the Co-operative Fuel Research Committee released the Research Octane Number (RON) based on work done three years earlier by Graham Edgar. The RON for a fuel indicated its anti-knock ability. Edgar used two reference fuels that were given RONs of 0 and 100 respectively to create a rating scale, and using a special designed testing engineiii assigned a RON to the fuels of the dayiv.

The Waukesha CFR Fuel Research Engine Fuel testing machine launched in 1929.

First published in 1932 and made an ASTM standard in 1951, the higher the RON score a fuel had, the better its anti-knock ability. At the time of releasing the rating, fuels generally had an octane rating of between 40 and 60: a long way from the rating of modern fuels.

In the 1920s - despite new methods of fuel production being introduced that resulted in incremental improvements to octane ratings - engine knocking was still a problem. Despite concerns for public health from lead emissions in exhaust, alkyl lead was added to petrol to solve the problem.

The relationship between octane rating and engine compression correlated to engine efficiency and power. The leaded fuel with its 'octane-boosted' anti-knock capacity meant that the development of higher power and more efficient engines was possible.

Leaded fuel continued to be sold in Australia up until 2002, with car manufacturers required to sell unleaded-capable vehicles from 1986v. Moving to unleaded petrol required modifications to the make-up of the fuel as well as engine design changes: unleaded petrol is now available at octane ratings from 91 and up.

Ethanol blends

While the RON values for fuels like unleaded 91, 95 and 98 are self-apparent, with new ethanol petrol blends on the market the octane rating and what it means for your vehicle is potentially less clear.

The price of ethanol blends is generally lower than petrol. However, Ethanol contains less energy than petroleum, so fuel consumption per kilometre will be higher. (Source: http://www.environment.gov.au/atmosphere/fuelquality/publications/pubs/ethanol-flyer-2012.pdf)

Ethanol, produced from renewable sources including wheat, sugar and sorghum, is available in ethanol-petrol blends in Australia. Ethanol contains less energy than petroleum, so all other things being equal fuel consumption per kilometre will be highervi. The price of ethanol blends, however, is generally lower than petrolvi.

Unleaded 91 with Ethanol - also referred to as E10 - has a minimum 91 octane ratingvii. In 2003 the government restricted ethanol blend sales to this level based on testing that indicated that more than 20% ethanol could cause engine problems in older vehiclesviii.

Ethanol 85 (E85) is a blend of between 70 and 85% ethanol with petrol. The Fuel Quality Standards Act 2000 - which defines the standards for fuels in Australia - mandates Ethanol 85 with a target Research Octane Number of 100 (although testing methods are yet to be ratified)ix. The higher octane rating does not mean that you can use it in any vehicle: E85 is for cars that have been built or modified to use it, including recently released Holden Commodores and V8 racing supercars.

So despite the high octane ratings for E85 - and that most standard vehicles should be able to run on E10 - drivers are recommended to check with vehicle manufacturers before using ethanol blended fuels. Drivers should also check if they are required to use 98 or 95 octane 'premium' fuels in their vehicle instead of 91 octane fuels.

For more information on makes and models that can use ethanol see the Federal Chamber of Automotive Industries website. For information on fuels suitable for pre-1986 cars visit the Australian Institute of Petroleum website.


i Blackburn, R., 2010, The dummy's guide to fuels, Drive.com.au, http://news.drive.com.au/drive/motor-news/the-dummys-guide-to-fuels-20100915-15b9l.html

ii Encyclopedia Britannica, 2013, Knocking, http://www.britannica.com/EBchecked/topic/320471/knocking

iii Waukesha Engine Division, Dresser Industries Inc., 1980, The Waukesha CFR Fuel Research Engine, http://files.asme.org/ASMEORG/Communities/History/Landmarks/5519.pdf

iv Totten, G.E., Westbrook, S.R., Shah, R.J., ASTM, 2003, Fuels and lubricants handbook: technology, properties, performance, and testing, p62. http://books.google.com.au/books?id=J_AkNu-Y1wQC&printsec=frontcover#v=onepage&q&f=false

v Australian Government, Department of Sustainability, Environment, Water, Population and Communities, National phase out of leaded petrol, http://www.environment.gov.au/atmosphere/airquality/publications/qa.html

vi Ethanol Answers, Myth Buster (FAQs), http://www.ethanolanswers.com.au/ethanol/myths

vii Shell Australia, About Shell Unleaded 91 with Ethanol, http://www.shell.com.au/products-services/on-the-road/fuels/shell-unleaded-with-ethanol/about.html

viii Department of Sustainability, Environment, Water, Population and Communities, Australian Government, Fuel Quality in Australia - Ethanol Labelling Standard, http://www.environment.gov.au/atmosphere/fuelquality/standards/ethanol.html

ix Department of Sustainability, Environment, Water, Population and Communities, Australian Government, Fuel Quality in Australia - Ethanol E85 fuel quality and fuel quality information standards, http://www.environment.gov.au/atmosphere/fuelquality/standards/ethanol-e85.html#standard