Energy Density & How Some Fuels Offer More Bang Per Litre

Filling one’s tank is so utterly mundane an experience that few spare a passing thought for the staggering volume of energy routinely transferred in the process. Occasionally some fool makes a Molotov of himself at the bowser with an unwitting static electricity discharge into the air-fuel vapour mix around the filler neck. Captured on CCTV, it’s the merest blip on the popular radar that petrol is anything other than a benign, unremarkable liquid.

The truth, however, is rather different.

Petrol and its more viscous sibling, diesel, are almost perfect energy storage mediums. They cram so much energy into such a small volume, and they weigh and cost so little, that other fuels – especially alternatives – have real trouble measuring up.

Say you fill up on unleaded. The nozzle might click off automatically at 58 litres. You’re ready for another 600km of driving. For a mere $77 you have just tipped 43kg of stored energy into your tank. It doesn’t sound like a big deal until you actually measure the energy, which in this case is a mammoth two billion joules.

A joule is the fundamental unit of energy. For measuring money, it’s dollars. For length, it’s metres. If you want to quantify energy, you need a yardstick calibrated in joules.

Putting joules into context, if you lift a one-kilo bag of tomatoes from the floor to the kitchen bench, it acquires about 10 joules. When a 100kg front-row forward sprints up a three-metre staircase to the second floor of his beachside condo, he acquires 3000-odd joules.

Two billion joules – two gigajoules – is mind-bendingly immense. It’s enough to lift a two-tonne car more than 100km into the air (and by then it’s in space). It’s enough to punt 32kg of bacon to escape velocity – so hard and fast that earth’s gravity could never reclaim it (a sizzling 11.2 kilometres per second). Convert it to food, and you’d meet the energy requirements of 250 people for a day (fat westerners, too, not third-world types).

With oil on the wane and alternatives apparently waxing all over the shop, are you speculating about the day when you’ll switch to hydrogen? Ethanol? Battery-powered cars like the GM Volt? These may all happen in time, but let’s look at how they stack up on fundamentals when compared with petrol today.



Energy: 2 billion joules

Volume: 58 litres

Mass: 43kg

Biggest problem: It’s running out while demand is skyrocketing, plus greenhouse contribution



Energy: 2 billion joules

Volume: 52 litres

Mass: 45kg

Biggest problem: see petrol, plus the carcinogenic particulate emissions. And additional up-front engine cost



Energy: 2 billion joules

Volume: 80 litres

Mass: 40kg

Biggest problem: See petrol, plus cost of retrofitting alternative fuel system


Uncompressed Natural Gas, a.k.a. Methane

Energy: 2 billion joules

Volume: 56,500 litres

Mass: 37kg

Biggest problem: Where, exactly, do you put 56 cubic metres of gas? (Needs a box seven metres long, four high and two wide.) Even if you compress it to some extent cruising range is severely limited.


Liquid Hydrogen

Energy: 2 billion joules

Volume: 199 litres

Mass: 15kg

Biggest problem: Boils at minus 253 degrees C, can freeze air (turn it solid) is currently produced from methane (dirty), uses potentially dirty energy to liquefy, and even the most efficient storage vessels lose 1.7 per cent of the liquid every day via evaporation. No delivery infrastructure, and exceedingly hard to handle.

Conclusion: More than a decade away


Compressed Hydrogen Gas at 5000psi

Energy: 2 billion joules

Volume: 590 litres

Mass: 15kg

Biggest problem: Dirty production, plus dirty energy used to compress the gas, fuel tank needs to be 10 times bigger than petrol tank for the same onboard energy storage.

Conclusion: More than a decade away



Energy: 2 billion joules

Volume: 85 litres

Mass: 67kg

Biggest problem: Not really green, although touted as such, competes with food production, and intrinsically increases fuel consumption by 30 per cent

Conclusion: Only a second-rate stop-gap measure


Lead-acid Battery

Energy: 2 billion joules

Volume: 8500 litres

Mass: 15,800kg

Biggest problem: Where would you put it; how would you carry it? They die if fully discharged. Anorexic energy storage capacity per unit volume and mass means severely restricted cruising range. Needs to be recharged with potentially dirty energy.

Conclusion: No good even in a golf cart.


Lithium-ion Battery

Energy: 2 billion joules

Volume: 2060 litres

Mass: 3500kg

Biggest problem: Only looks good when compared with lead-acid batteries. Can’t compete with hydrocarbons on bang for buck, per kilogram, or for compactness. Needs to be recharged with potentially dirty energy.

Conclusion: Only a super-short-haul solution.


Solar (Photo-voltaic) Cell

Energy: 2 billion joules

Biggest problem: Cars don’t run on PV cells except as a stunt. Performance drops at night, to zero. For 2 billion joules, you need an array of 300 square metres of reasonably efficient solar cells and seven hours of sunlight – so you can store it in your car’s 3.5-tonne lithium-ion battery (see above).

Conclusion: Solar cars are pie in the sky.



Energy: 2 billion joules

Volume: 157 litres

Mass: 133kg

Biggest problem: cars don’t burn wood, but it’s an interesting comparison

Conclusion: Still the number one fuel for billions in the third world, and not at all bad for heating water



Energy: 2 billion joules

Volume: 76 litres

Mass: 95kg

Biggest problem: Engines don’t burn coal any more but it’s where most of Australia’s electricity and greenhouse emissions stem from

Conclusion: Hampers the transition to clean energy in countries like Australia with vast reserves, and needs more than oxymoronic marketing (think: so-called ‘clean coal’) to overcome poor enviro-fundamentals


Uranium-238 (nuclear fission, 2 billion joules)

Mass: four grams

Volume: tiny

Biggest problem: Turns into a bomb spontaneously if you store 52kg or more in the one place, highly radioactive, hard to dispose of responsibly when spent, lightweight advantage of fuel more than offset by considerable mass (and servicing requirements) of small-scale nuclear reactor required under the bonnet…

Conclusion: Cleaner than coal, but with long-term waste issues