Why Your Car is a Chemistry Lab on Wheels

Chris Woodford
Grist Media

One billion cars on the planet. Count them! Stack them on top of one another and you’d get a pile 170,000 times higher than Mount Everest – tall enough to reach to the moon four times over. Park them bumper to bumper and they’d stretch across the United States more than 1,200 times. A billion (a thousand million) is the kind of number most of us find hard to picture. To put it in some kind of perspective, the world’s population is a smidgen over 7 billion; between us, we guzzle 2 billion cups of coffee a day. There are roughly 6 billion mobile phones in circulation– and anywhere between 1 and 2 billion sheep. If you can get past a mental image of a sheep tucked behind the wheel of a Jaguar, bleating into the mobile phone that’s clamped, cloven hoof-style, to its jaw, can you possibly imagine a billion cars?

The interesting thing for me isn’t that there are so many cars in the world, but why. What is it about cars that’s made them, in the space of just over a century, one of the most successful inventions of all time? Not surprisingly, the answer is all to do with science.

What’s so good about cars?

Cars are chemistry labs on wheels. And though that might not sound so interesting, it accounts absolutely for their ubiquity. Take away the leather seats, the gleaming chrome, the go-faster stripes and all the rest, and you’re left with a handful of tin cans called cylinders, where petrol explodes into power. Cars are built around engines, and engines (or internal combustion engines, to give them their full name) burn petrol with oxygen in the air to release the energy locked inside it. We think of burning as a way of making fire, but it’s essentially a chemical reaction between oxygen and fuel that just happens to produce heat and fire as a by-product. The simple science of cars is so utterly mundane that we scarcely give it a thought: pump the petrol in your tank, turn the key and off you go. Think about it more closely, however, and you’ll see how astonishing it really is.

Suppose a typical modern family car does about 40 miles to the gallon or, in metric terms, 100 km for every 7 litres of fuel. That means if you have a teaspoon of petrol (about 0.004 litres), it contains enough energy to roll your car about 60 m (200 ft), or roughly 15 times the car’s own length. Consider how hard it is to push a car, even once you’ve got it going from a standstill, and I’m sure you’ll agree that’s quite remarkable. The simple fact is that petrol is absolutely chock full of energy: short of uranium (nuclear fuel), it’s just about the most energy-rich material there is. That, more than any other reason – including the freedom, independence and social status that cars give us – accounts for their popularity.

What’s so bad about cars?

Driving a car is the next best thing to being a human cannonball. It can shoot you over the ground at a blistering speed for an amazing distance on a single tank of fuel. If your tank holds 70 litres (about 15 gallons), and your engine can manage 100 km (62 miles) on 7 litres (11⁄2 gallons) of fuel, a single fill-up at the petrol station will power you a rather surprising 1,000 km (600 miles). So, stopping to refuel four times, you could just about drive across the United States from New York City to Los Angeles.

That might sound impressive, but it’s nowhere near as good as it might be (or should be). If you want to get across the States, a car is probably a much better bet than bicycles – at least if you want to expend minimal effort and make the trip as quickly as possible without flying or taking the train.

Chris Woodford

However, suppose you wanted to climb Mount Everest and such a thing were possible on foot, by bike and by car. Instantly, we find ourselves wondering ‘Do I really need to drag all that metal to the top?’ Carrying a bike would be bad enough, but if you drive a car up a hill, you’re lifting not just your own weight (something like 75 kg/165 lb), but the weight of the vehicle as well (which could easily be 1,500 kg/3,300 lb). This is the real drawback of driving a car. Wherever you go, it’s like having a ball and chain shackled to your leg, except that the ball is about 4.5 m (15 ft) long and weighs 20 times as much as you do. If you chug to the top of a mountain, that means about 95 per cent of the energy you need is wasted lifting the sheer bulk of the car. Only 5 per cent does the useful job that you actually care about: moving your own body to the summit. That’s why cars gobble down so much more fuel and air than cyclists. What applies to climbing mountains applies equally well to any other kind of driving: you’re still shifting extra metal and wasting energy, wherever you go. It’s no coincidence that the Ariel Atom, one of the world’s fastest cars, is also one of the lightest. It weighs less than 500 kg (1,100 lb), which is about a quarter to a third the weight of a typical small car.

In short, there’s a basic inefficiency to petrol-powered cars (all that extra weight) that we simply can’t dodge. And that’s before we go anywhere near considering their real inefficiency. The fundamental problem with cars is that a mere 15 per cent of the energy locked in petrol actually moves you down the road. The rest is wasted in various ways, including heat losses in the cylinders, frictional rubbing in the gears, the sound the engine makes, powering the electrical system and much more besides. If cars were 100 per cent efficient, and all the energy in the petrol were perfectly converted into kinetic energy blasting you down the road, you’d be able to go five to ten times further at least – well over half a kilometre or even more on every teaspoon of fuel.

The more people you pack into your car, the bigger the useful load you’re carrying compared to the useless weight of the vehicle, so the greater the efficiency you’re getting. That’s why things like trucks, buses and trains work out as efficient forms of transportation even though they’re bulky and use heavy diesel engines. However, no matter how efficient you make a car, or any other vehicle powered by internal combustion, it’s still burning petrol and belching out pollution of one kind or another, from soot or smog to the carbon dioxide implicated in global warming. How, then, can we build a better, cleaner, more efficient kind of car? What kind of pointers can we get from science?

Better than petrol?

Wacky inventors have come up with all kinds of ways of powering cars. Before petrol came along there were steam cars, for example, but steam engines are by far the most inefficient types of power – and coal is heavy, filthy and spews out smoke. Diesel engines, which are like industrial-strength petrol engines, are almost as old and work on the same basic principle. Although we tend to think that electric cars are radically modern, they date back to well before Henry Ford’s time in the late 19th century. Ferdinand Porsche, best known as the father of the modern luxury sports car, originally made his name pioneering hybrid electric cars in 1900.

Although it’s easy enough to sketch a sleek new kind of car on paper, it’s much harder to come up with a credible design that moves you as quickly or as far as petrol. You might think 100 years or more of progress would have made electric cars supremely better than petrol ones, but there’s a basic problem: batteries don’t pack in anything like as much energy as liquid fuels such as petrol, kerosene (aeroplane fuel) or alcohol (used to power rockets). Even a lump of wood or a bag of sugar holds more energy than the equivalent weight of rechargeable batteries. Moreover, while you can completely refuel your petrol-powered car in just a minute or two, fully recharging the bank of batteries in a silent, electric runaround will keep it docked and useless for several hours at a time.

Environmentalists love to imagine that a giant, petrol-headed conspiracy has kept electric cars whistling and waiting in the margins of technology, while dirty, expensive gas guzzlers continue to pollute the planet. The truth is more prosaic and less sensational: petrol is – and for the time being will continue to be – a far more effective and far more efficient medium for carrying energy than batteries. Science, not politics, is the simple reason why most of us are still driving petrol-powered cars today.

Our electric future?

We won’t necessarily be driving petrol-powered cars tomorrow. No one can predict with any certainty when oil will run out – by which we mean when it will become so expensive that market forces make the alternatives more attractive. That day will, however, arrive eventually: it’s taken hundreds of millions of years to make our planet’s entire oil supply from rotted plants and sea animals, but little more than a century to use up virtually all the oil at our disposal. Oil is being formed every day; assuming we stopped using it tomorrow, if you came back in millions more years, you’d find plenty of new oil underground to drill to the surface and burn. So, regardless of the fact that batteries aren’t as good at carrying energy in portable form as petrol, that’s the way the future is heading, like it or not.

Bulky batteries might be a drawback of electric cars, but these sleek and silent, sparky vehicles have plenty of things in their favour. In theory, they’re much lighter than petrol- powered cars because you don’t need that monstrous engine, those fiery cylinders with their pistons pumping up and down, and that grinding gearbox. In practice, of course, what you need instead is almost as bad: a load of extremely heavy batteries. Even so, electric cars generally work out lighter, which makes them more efficient.

Recycling energy

One of the things that makes petrol-powered cars so inefficient is all the stop-start driving you have to do in cities. It takes energy to do anything at all. If you’ve ever pushed a broken-down car, you’ll know how back-breaking it can be simply to overcome its inertia (the basic laziness of a lump of mass) and get it rolling. If your car weighs a tonne and a half (1,500 kg/3,300 lb) and it’s grumbling through the city at 65 km/h (40 mph), it has quite a bit of kinetic energy. Do the math and you’ll find it’s about 240 kJ, which is nearly enough to climb the Empire State Building.

That might not sound so very much, but here’s the snag. Every time you stamp on your brakes to dodge children chasing footballs or cats unacquainted with basic road safety, those 240 kJ disappear into thin air. As the brake pads meet the brake discs and bring you shuddering to a stop, all that motion energy vanishes in a squeal of tyres and a puff of smoke. In sports cars and Formula 1 racers, the brakes can sizzle up to temperatures of 750°C (1,400°F) – hot enough to set them on fire if they were made of wood. When you step on the accelerator after braking to a standstill, the engine has to pick up speed all over again by turning more petrol into power. So the horribly wasteful cycle repeats itself, over and over again.

Electric cars have a big advantage here in that they’re powered by motors. In its simplest form an electric motor is a fat core of copper wires that whirls around inside a hollowed-out magnet. Feed electricity into the wires and they generate a temporary magnetic field that repels the magnet’s own magnetism. The copper core spins around and we can use it to power anything from a vacuum cleaner to a bullet train. The great thing about electric motors is that you can run the whole process in reverse. If you twist the shaft of an electric motor with your fingers, you get electricity zapping out of the copper wires in the opposite direction: the motor, in other words, becomes an electricity generator. In theory, you could make electricity by taking any electric appliance (a vacuum cleaner, say) and operating it by hand so that the motor rotates, which would pump electricity out of the other end. So if you unplug your vacuum cleaner and give it the kiss of life, the wacky theory is that useful electricity should come zapping out of its power cable; of course, in practice, it won’t work in a vacuum cleaner – but it does work in an electric car.

Electric cars use their motors to great effect. When you’re driving along, the batteries pump power through the wires into the motors, making the wheels spin around. When you hit the brakes you cut the current, but the car’s momentum keeps the wheels turning. Because the motors are still spinning as well, they start to generate electricity, which feeds back into the batteries and slows the car down. Instead of wasting energy when you brake, an electric car saves at least some of its energy by recharging its batteries. It’s called regenerative braking and it improves the efficiency of a typical electric car by 10 per cent (electric trains, by comparison, manage about a 15 per cent improvement, which is the equivalent of running one in every seven trains for free).

What would a perfect car look like?

Suppose you set out to design a car that’s as efficient as you can possibly make it. What would you end up with? You’d want something with as few moving parts as possible, so that it wasted little of the energy you fed in. Ideally, it would be as light as possible too, so you wasted less energy shifting metal, plastic and glass. It would need to run off a widely available fuel that carried piles of energy per kilogram – probably something carbon based and organic. If you didn’t mind low speeds (say a modest 6 km/h or 4 mph) and were prepared to consider fuels like fat or vegetable oil, you’d find that the ideal car looked much like your body. It would be low maintenance, energy efficient and easy to park. It wouldn’t rust or lose value, and – most of the time – would age rather beautifully.

This is an excerpt from Atoms Under the Floorboards: The Surprising Science Hidden in Your Home by Chris Woodford, to be published by Bloomsbury India on March 12, 2015 (336 pages, Rs 399).

Chris Woodford has been a professional science and technology writer for 25 years. After graduating from Cambridge University with a degree in natural sciences, he has gone on to write, co-write and edit a number of science education books, including the best-selling Cool Stuff series. He runs www.explainthatstuff.com, dedicated to explaining the science behind familiar, everyday things.