Ethanol V Petrol: there's no clear winner

There’s been a lot of controversy, but ethanol in petrol is here to stay. Holden will have E85-compatible engines here very soon in an attempt to get fuel companies to supply even higher ethanol blends than are currently available. Amid all the anti-ethanol hysteria and fuel-company hype, not very much has ever really been explained about the key differences between the two fuels.


Most cars can run E10 – a 10 per cent blend of ethanol in petrol. To check compatibility with your car, contact the manufacturer. Ethanol doesn’t destroy engines, but it can eat into the materials used in incompatible fuel systems (sealers and linings; that kind of thing) and the byproducts of that corrosion can block fuel injectors, which can be costly.


It’s a basic alcohol, the same stuff as in beer, wine and spirits. It can be made by fermentation of a variety of different energy-dense foods. In Australia, the majority of ethanol is produced from wheat. It can also be made from sugar and many other starchy crops – as well as from industrial, domestic and agricultural waste using chemically engineered microbes.


Petrol contains more energy than ethanol. If you burn one litre of ethanol you get 23.5 million joules of energy. If you burn one litre of petrol you get 33.5 million joules. That means there’s about 30 per cent less energy in a tank full of ethanol, compared with the same volume of petrol. And that means there’s about three per cent less energy in E10 compared with regular petrol. And that means fuel consumption goes up by three percent if you use E10. So if it’s not at least three per cent cheaper than regular petrol, you’re ripping yourself off. It also means if you ran a car on 100 per cent ethanol, fuel consumption would increase by more than 40 per cent. (Although it’s not advisable to do that. Car engines don’t like to start in the cold on E100. Which is why E85 is pretty much the upper limit for ethanol/petrol blends.) You need to burn 1.4 litres of ethanol to get the same energy as one litre of petrol. (Cruising range drops on E10, too – something long-distance drivers need to consider.)


One litre of ethanol produces 1.5kg of CO2 when you burn it. One litre of petrol emits 2.2kg of CO2. But since 1.4 litres of ethanol and 1.0 litres of petrol contain the same amount of energy, maybe that’s a better CO2 yardstick comparison. And 1.4 litres of CO2 emits 2.15kg of CO2 – so, joule-for-joule, there’s not much in it from a greenhouse perspective.


The elephant in the room here is the engine itself – which is invariably optimised for petrol, not ethanol or ethanol blended fuel.

Engines don’t just burn fuel – they compress the air around it first. Compressing the fuel/air mix prior to combustion allows the expansion to occur over a greater range than just burning it out in the open, increasing the efficiency of the operation. More efficiency means more of the 33.5 million joules in every litre of petrol get converted into torque at the crankshaft. Less is lost out the exhaust pipe as waste heat.

So, the more the compression, the more the efficiency. Only there are physical limits to the amount of compression the fuel/air mix will tolerate before it begins to burn autonomously (think: pinging/knocking).

Octane rating is a measure of a fuel’s resistance to combustion (self-ignition or auto-ignition, and also knock/pinging) when compressed. So, higher-octane fuels resist auto-ignition better; they can be compressed more. So they can produce more torque and have greater efficiency. But if you run a high-octane fuel in an engine designed for low octane, there’s no benefit. This is essentially what you’re doing when you run an ethanol blend in a petrol engine.

A common misconception is that high-octane fuels have more energy in them. That’s crap. They allow greater compression, delivering more torque at the crank via increased thermodynamic efficiency.

Octane is a carbon chemical (eight carbon atoms and 18 hydrogens) which petrol approximates. The chemical with an octane rating of 100 (to which the 91, 95 and 98 fuels commonly onsale compare in percentage resistance to pinging terms) is an isomer of octane called 2-2-4 tri-methyl pentane, which is a specific architecture of eight carbons and 18 hydrogens. They all have the same amount of energy on offer in the various arrangements of the atoms, just the resistance to pinging (or knock) changes.

Ethanol offers a greater octane rating than petrol. So putting an ethanol blend in an engine designed for petrol means in theory it could handle more compression (produce more power and operate more efficiently). Only the compression ratio is set at the factory (it’s a function of piston sweep and combustion chamber shape, including piston crown shape). And that’s usually set for petrol.

This means ethanol blends won’t be as thermally efficient in most engines, compared with straight petrol. (Modern engines adapt slightly to higher octane fuels by increasing the spark advance, but the benefit is tiny compared to increasing the compression to suit the octane rating.) The only real way to solve this problem is to vary the compression, and the only real way to do that is via variable-boost turbocharging, where the effective compression ratio can be varied to suit the fuel’s octane rating by changing the pressure of the post-turbo inlet air mix.) E50 can tolerate something like 19:1 compression, with the right additional controls in place. Ordinary petrol likes 10.5:1 or thereabouts – slightly more with direct injection.

Subject to all these caveats the bottom line is that, broadly, to perform the same job as an equivalent volume of petrol, you’d need to burn about 40 per cent more pure ethanol, and it would emit the same (or very similar) amount of CO2. And if the blend of petrol and ethanol varies, hi-tech turbocharging will be required to exploit the fuel to the best effect.