Understanding The Relationship Between Energy & Fuels
In addition to beaming re-runs of I Love Lucy into space, human beings can do something amazing, which no other animal can. We can expend more energy than we derive from our food. The rest of the animal kingdom, from the lowliest earthworm in your compost heap to the king of beasts cruising the Serengeti for wildebeest, must devote less energy to catching dinner than they recover from eating it. Otherwise, they will wither and die.
Not us. Not by a long shot. In the developed world, food is a contrivance, a given. We have bigger things on our minds. We can build cars, litigate, use iPods, launch space shuttles, fly business class, film TV sit-coms and rise to the absolute pinnacle of the food chain because of one thing alone: we are no longer shackled to the live-or-die imperative of energy conservation. (Having opposing thumbs has helped, too.)
So transparent has this luxury become that few of us are aware it even exists. Take the simple process of cooking. More energy is spent heating the food than we ingest from it – it is our mastery of fuel that allows the luxury of eating food in states other than ‘raw’.
Energy fuels public debate. Yet few people know how fuel really works, what energy is, how we depend upon it in every aspect of our lives from maintaining the ‘core temperature’ of our bodies to enjoying the freedom of driving, and the convenience of night-time illumination.
This is the crash course on energy and fuel.
WHAT IS FUEL?
Fuels, from firewood to coal to 98RON PULP represent one thing: stored energy. The energy sits dormant in the chemical bonds between atoms in complex molecules. Good fuels store a lot of easily accessible energy in a small volume, so in that respect petrol is better fuel than firewood. Diesel is likewise better than coal, which is why (coal-fired) steam locomotives were superseded.
The most common elements in fuels are carbon, hydrogen and oxygen. Because the taxonomy of science is unimaginative, fuels containing only hydrogen and carbon are called ‘hydrocarbons’. These can be gases like LPG (a.k.a. propane; three carbon atoms and eight hydrogens) or liquids like petrol (petrol approximates ‘octane’; eight carbons and 18 hydrogens).
Fuels containing all three elements are called carbohydrates – sugars and starches, as well as the cellulose in firewood and paper. Alcohols like ethanol are hydrocarbons with only a little oxygen built in, slightly different to sugars and starches.
To exploit a fuel’s energy, the bonds between the atoms need to be shattered. And the only practical way to do that is via burning, a.k.a. combustion. Energy is released because the combustion products contain less stored energy than their parents.
Technically, burning is the same thing as adding oxygen, or ‘oxidising’. (This, in part, explains why carbohydrates and partly oxidised hydrocarbons like ethanol don’t produce as much energy as petrol.)
Oxygen from air, plus petrol vapour, compressed in a combustion chamber, is like a hand grenade. The spark is like pulling the pin. The octane molecules disintegrate; some oxygen combines with the carbon to make carbon-dioxide, and the rest hooks up with the hydrogen to make water (as steam, a colourless gas). These are the two basic combustion products, so you can see that carbon dioxide, hydrocarbons and energy are inextricably linked. Each of the eight carbon atoms in octane goes off and collaborates with two oxygens to form eight molecules of CO2. Every molecule of octane yields eight molecules of CO2. Every kilogram of petrol (1.43 litres) produces a little more than three kilograms of CO2. (The ‘extra’ two-ish kilos is donated from consumed atmospheric oxygen.)
No amount of clever engineering can subvert the fuel-CO2 relationship; it is intrinsic to the energy-extraction process, and perhaps the biggest problem currently facing humanity. And it’s not just petrol. Similar intrinsic CO2 relationships exist for all other carbon-based fuels. If efficiently burnt, every atom of carbon in every fuel becomes part of a new CO2 molecule, which explains why everything from a campfire to a coal-fired power station inevitably yields CO2 to the greenhouse effect.
Meanwhile, back in the combustion chamber, something marvellous is occurring. Because water and CO2 contain far less stored chemical energy than petrol, a great deal of energy has just been liberated, in the form of heat. In practice, this forces the violent mechanical expansion of all the gasses in the chamber (read: controlled explosion). The result is what physicists call ‘work’ – the piston is shoved downwards, hard. This process takes place 50 times each second – per cylinder – at 6000rpm, and from the driver’s seat, things start getting really interesting…
BUT WHAT ABOUT POLLUTION?
When you burn petrol you should get only CO2 and water. In practice, you don’t. There are unburnt hydrocarbons, carbon-monoxide, oxides of nitrogen (NOx), etc. We’ve all heard of them, and they all occur because air, the combustion chamber, and petrol itself, are inherently compromised.
Air is not oxygen. Air is overwhelmingly nitrogen. Only 20-odd per cent of air is oxygen. So there’s four times as much nitrogen as oxygen sucked into the combustion chamber, and a tiny fraction of it gets swept up in all that controlled chemical mayhem. That explains NOx. (However, the overwhelming bulk of nitrogen emerges unchanged from the tailpipe, albeit substantially warmer. About three-quarters of tailpipe emissions are merely inert, unchanged nitrogen.)
Carbon-monoxide, a deadly poison and suicide favourite, is what happens when there’s not enough oxygen to go around with all those carbons; it’s deformed carbon dioxide.
You have to understand that petrol beats modern metallurgy. Potentially, it produces too much heat for your engine. Perhaps you’ve heard of engines that run too ‘lean’ (ie, with too little fuel) and melt down. The reality is that engineers always tip more fuel into an engine than can be efficiently burnt by the air with which it spends its last few moments. This extra tipple of fuel is used merely to cool the chamber. If you ever get petrol on your skin, you’ll note it feels cold. That’s because it evaporates rapidly, and that sucks heat out of the environment. The fancy name for this extracted heat is the ‘latent heat of vaporisation’. Forget what you’ve heard about ‘lean burn technology’. Dictating an excess of fuel stops the combustion chamber reaching temperatures that otherwise would melt its iron, aluminium and steel underpinnings.
Petrol also still contains impurities and additives that are passed out the tailpipe, though these have been lately dramatically reduced. Benzene, tetra-ethyl lead and sulphur were major culprits in the past, but better fuel quality has largely removed them from the pollution landscape.
FUEL … OR BOMB?
Filling the tank with unleaded has become such a mundane act that few of us spare a thought for the mind-boggling amount of energy being transferred.
A modern military torpedo, for example, is powered by kerosene, a common hydrocarbon blend, and oxygen. The energy required for it to hunt down a ship and deliver the warhead – the hydrodynamic equivalent of a cheetah running down a gazelle – is far greater than the amount of energy in the high-explosive tip. And that basically explains why a simple malfunction in the fuel system of a torpedo was sufficient to sink the gigantic Russian submarine The Kursk in August 2000. Around 700 litres of kerosene came into contact with the liquid that supplied the oxygen for combustion (a concentrated form of hydrogen peroxide). The energy released blew the front end off a 23,000-tonne machine twice as long as a Boeing 747, built to withstand hundreds of feet of water pressure. The explosion measured 1.5 on the Richter Scale.
Next time you pump 50 litres of petrol into your car, consider this: A Holden Commodore travelling at 110km/h possesses around 800kJ (kilojoules) of kinetic energy. If that energy is explosively transformed into the car’s structure, say by running head-on into an unyielding, massive object, the impact will be unimaginable. The vehicle and its contents will be rendered unrecognisable. Eight hundred kilojoules is a vast amount of energy.
Twenty-five millilitres of petrol is a ridiculously small volume. A child could hold it in two cupped hands (although the World Health Organisation recommends refraining from skin contact). Those cupped hands would be holding and amount of petrol equivalent to 800kJ of potential combustion energy, the same as the kinetic energy of the Commodore at the legal freeway limit above. (It’s definitely a good reason not to smoke while refuelling, and to earth jerry cans before introducing the refuelling nozzle.) A full tank of 50 litres contains the same amount of energy as 2000 Holden Commodores all travelling at 110km/h.
Petrol is such an amazingly good fuel that the energy locked inside it beggars belief.