SUCK, SQUASH, BANG AND BLOW

Few of us care what goes on under the bonnet of the car – unless, of course, the engine doesn’t start when it’s supposed to. But to have any bluffing credibility at all, you need to have a basic understanding of how it all works. These days, a car’s inner workings are usually hidden behind a baffling-looking apparatus of plastic covers, boxes, wires, cables and pipes joining one mystery object to another.

However, if yours is a diesel- or petrol-powered car, the way automotive power is provided is pretty straightforward, even if describing the processes involved isn’t. Basically, the fuel, which is highly combustible, is squirted into a confined space where it’s set alight. This makes it explode, and the energy created drives the engine, which in turn drives the car.

Petrol is more willing to go bang than diesel, as any recidivist arsonist will attest after dousing their flammable project with the stuff, setting light to it and being blown into the next postcode. This is known as external combustion and should not be attempted by anyone with half a brain. In the context of current petrol cars, what happens instead is internal combustion. Some petrol is squirted into a cylindrical space in the engine called a bore (as in ‘bore hole’), where it’s mixed with air – also sucked into the space with the help of valves that open and close at strategic moments – and the resultant vaporous cocktail is ignited by an electrical spark from something called a spark plug.

This produces a lot of energy. In fact, howstuffworks.com reckons that enough is made ‘to propel a potato 500 feet’, which is a deeply pointless thing to do, and anyway, how do they know it’s 500 feet rather than 450 or 600?

Unlike the average man, a crankshaft can multitask.

Instead, the petrol/air vapour shares the bore with a ‘piston’, which sits very snugly and moves up and down inside it, helped by being attached by a pivot to a connecting rod.

(Don’t worry, nearly there.) The other end of the connecting rod is attached by a second pivot, amusingly known as a ‘big end’, to a piece of heavy engineering called a crankshaft – which in profile looks like a Cubist drawing of the up and down lines on an oscilloscope – so when it goes round and round, the connecting rod goes up and down, forcing the piston to join in.

As the piston heads for the bottom of its travel, fuel and air are sucked into the space above it, which is then sealed by the valves, so that as the piston goes back up again, the fuel/air mixture is squashed, which means that when the spark plug sets light to it, a lot of pressure has built up and even more energy is released, forcing the piston down again, filling the bore with all those nasty gases produced by the explosion.

When the piston next heads to the top of the bore, a valve opens and this pushes the spent gas out and into the exhaust system, before starting the process all over again.

Cars have had anything from one to 16-plus cylinders, but most of them make do with four, and a single crank (to which all the pistons are attached) but at different points in the combustion cycle, so that they rhythmically bob up and down, taking turns to suck in the fuel and air, blow it up then chuck out the filth. Another way to think of it might be ‘Suck/Squash/Bang/Blow!’, but that sounds like the title of a certain type of adult movie and probably shouldn’t be repeated in polite company.

Moving on, unlike the average man, a crankshaft can multitask, and one end of it is connected to a car’s transmission, which in turn uses drive or propeller shafts to make the wheels go round and the car move.

Why do you need a transmission? Well, if you’ve ever ridden a bicycle you’ll know that changing the gears impacts on how easy it is to ride, that lower gears help get it up hills without exhausting you, and higher ones allow it to go faster on the flat or downhill. This is what a car’s transmission does, sometimes in a way that involves you deciding when it changes gear, or in some cases doing this for you, but this is digressing from the limitless bluffing potential of engines.

Diesel engines do much the same thing with the same bits as petrol ones but without spark plugs. Diesel, or heavy oil, is less prone to exploding than petrol and so the way diesel engines persuade the fuel to combust is to keep squashing it. If you’ve ever put your thumb over the end of a bicycle pump and pumped, you will know that compressing the air you’ve trapped will make it get hot. So a diesel power unit is officially known as a ‘compression ignition’ engine because it keeps squishing the fuel and air until they give up and set light to themselves.

When this happens, diesel generally burns more efficiently than petrol does and the combustion process is more complete, which is why diesel is normally more economical. However, it spews forth thoroughly unpleasant soot particles and muck, requiring even more filtering and cleaning up.

There are other forms of automotive power that could be described in minute and fascinating detail, including petrol/electric and diesel/electric hybrids, which have both petrol or diesel and electric motors and can switch between the two or use one or the other. (Just remember the word ‘hybrid’ and posit that they’re a halfway house on the way to full electricity.) Then there are rotary petrol engines that only have three moving parts. Designed by a German called Felix Wankel (don’t even think about it), they’re smooth but often have an insatiable thirst, can be a bit dirty, and have been known to suffer from an early burnout as a result of frazzling their mechanical extremities.

And that’s all there is to it. Simple, really.