Like a dormant volcano, the immense energy in unleaded petrol waits to bubble to the surface.

From barrels of crude oil through local refineries that do just that, they ensure your hydrogen and carbon (hydrocarbons, y’see) blend can make a big bang.

It matters nought whether you fill a Ferrari or a Ford, because hidden inside every litre of regular and premium unleaded is a whopping 34.2 million joules of energy waiting to emerge. That’s right, that’s fixed, whatever the petrol pump picked and price paid.

Regular vs Premium vs Super Premium vs E10 – what does it all mean?

But while the energy density of unleaded is as still as today’s Vesuvius, the pace of internal combustion engine (ICE) technology has exploded exponentially, and this is where a choice of regular unleaded, premium unleaded, super premium unleaded, and 10 per cent Ethanol (E10) come into play – it’s all about what an engine can squeeze out of the energy per litre.

And it’s less like a volcanic eruption and exactly a controlled combustion inside an engine, where injectors spray fuel vapour into a cylinder atop each piston along with air, and a spark ignites the mixture to begin combustion. Bought a four-cylinder? Four pistons. Simple.

What petrol bowsers offer are four versions of unleaded with research octane numbers (RON) of E10 (an ‘up to’ 10 per cent ethanol blend with the next number), 91, 95 and 98. This is just a sliding scale for a fuel’s tolerance for being compressed before it ignites, where the piston within each cylinder draws to its top-dead-centre position and the combustion chamber does its eruption (sorry, combustion).

The higher the RON, the more tolerant a fuel is for high compression. Higher compression allows the piston to reach up a bit higher before ignition, compressing the mixture a bit harder and expanding each power stroke to deliver more torque to the crank with the same amount of fuel. Think of high compression as a rowing coach telling a rower to just extend arms a little further, then pull back a little harder.

In Europe, 95RON is now the basic standard, so anything from a Volkswagen Polo to a Porsche 911 has been optimised to the compression standards of this brew. Such engines will ‘knock’ themselves to death should an owner fill with 91RON, so be aware.

Can you run a 91RON engine on 95 or 98RON?

But should an engine designed for 91RON be filled with 95RON or 98RON? While low-RON fuel shouldn’t be used in engines designed for high-RON, it can go the other way. If such an engine has been run on high octane fuel, then switches to low octane, it will still audibly ‘knock’ for a moment. But then, knock sensors monitor in a loop how well the fuel is burning during combustion, and can ‘advance’ or ‘retard’ the ignition timing to suit the fuel.

Best think of this, then, as that rowing coach encouraging rather than shouting. And, depending on the electronic mapping of a particular engine designed for the regular stuff, premium unleaded can provide a slight increase in power and a bit of extra efficiency. But it’s certainly no high-compression-ratio shouting match.

For most people, this is where dollar signs come in.

91 vs 95 vs 98 RON unleaded pricing

Over a rolling 12 months in New South Wales, 91RON has averaged 136.5 cents per litre (cpl), 95RON has clocked 149.5cpl and 98RON has asked 156.2cpl. Fill up a 73-litre fuel tank in a V6-powered Holden Commodore and that’s $100 per tank for 91RON but $109 for 95RON and $114 for 98RON – a 9.0 per cent hike for (depending on each specific engine) an output boost, according to most reports, of 3.0 to 5.0 per cent depending on the mapping.

Ah, but what about the environment and claims that higher octane fuel is ‘cleaner’ for your engine? Well, there’s some truth there. Claims by certain branded servos that their ‘special’ brew of premium contains cleaning agents that wash your engine is partly true but mostly marketing hyperbole. More importantly by the Australian government’s own admission our fuel standards are simply behind that of other nations, and the fuel dominates the agents.

Burning a litre of any unleaded will emit 2.3kg of carbon dioxide (or CO2). But 91RON is mandated to include no more than 150 parts per million of sulphur, which burns in the combustion chamber and is emitted in particles that contributes severely to urban smog and respiratory illness. Meanwhile 95RON/98RON are mandated to 50ppm.

But even then it isn’t enough. European fuel is mandated to 10ppm, and future engines from the likes of Volkswagen will be engineered with particulate filters (exhaust treatment traditionally for diesels) that are incompatible with our fuel. The government has forecast 10ppm regulation for between 2022 and 2027, but it has not yet announced a date and means Australia may miss out on emissions-friendly engines.

Where does E10 fit in?

And what of that outlier, ethanol? There are mandates in NSW and Queensland that require E10 to be sold state-wide, and the ‘up to’ 90:10 ratio between 91RON and ethanol is cheaper at a year average of 133.6cpl. Ethanol even polls 108RON, occasionally lifting the 91RON it mixes with to 94RON if it is branded as such.

And this type of alcohol is made predominantly on the NSW south coast from waste starch and plant extracts that have already captured carbon from the atmosphere. In this way biofuel-supporters call it carbon-neutral as burning it just returns it to the world, and local production also helps with Australia’s energy security; however, the process is cost- and energy-intensive, leading that Manildra plant to use CO2-burning coal to power it.

The core reason it is cheaper, despite being such a hefty production process, is because the government fuel excise on unleaded/diesel is 40.3cpl versus 5.3cpl for ethanol. Yet even then E10 is only 3cpl less than 91RON.

Add the fact that burning a litre of ethanol creates 30 per cent less energy than unleaded, and an E10 blend will therefore increase fuel consumption by about 3.0 per cent. An E10 blend emits slightly less CO2 per kilometre (2.2kg versus unleaded 2.3kg) and it contains less particulates as it burns more cleanly, but burning coal to create a less energy intensive fuel that is then taxed less than others, means ethanol has its detractors.

Brilliant ideas live or die based on popularity, too, and with Australia’s interest in premium cars (most of which use premium unleaded) rising sharply over the past decade, E10 peaked with an 18 per cent share of other fuels in 2010, falling to 10 per cent nationwide today.

Extracted from