Taking a look under the hood

26 July 2016

Interview with

Philip Garsed, The University of Cambridge

What actually goes on under the bonnet of a vehicle powered by a combustion Ford Mustang Engineengine burning petrol or diesel? Cambridge University's Philip Garsed got moving with Naked Scientist Connie Orbach...

Connie - Like many people I've got a car and I actually used to drive a lot for my old job, but despite my absolute reliance on gas guzzlers, I've never actually known how they work. To change that embarrassing fact, I asked Cambridge University's Philip Garsed to take me through the basics...

Philip - It's quite simple in many respects. All a petrol engine really is is a series of cylinders, and inside the cylinders are things called pistons, which are basically big bits of metal which kind of seal nicely to the cylinder and can move up and down, and these pistons move up and down, because of the fact that the engine is moving. The engine works by having the piston move up and down in a series of strokes.

So in the first stage you need to get the fuel and the air into the engine, so the piston moves down and it sucks in a mixture of air and fuel into the cylinder, because it's sort of moving down and pulling in this mixture.

At that point a valve closes, the piston continues its movement and starts to move back up again, but because now this mixture is stuck inside the cylinder, it gets compressed really hard and you end up with all this very flammable air and fuel in a very, very small space.

At the point where everything is most compressed we ignite a little spark and that causes this fuel and air mixture to burn very, very quickly, and as it burns it gets hot and that forces down on this piston. That's where we get this mechanical energy out of the fuel - so that's stroke number three.

The final thing that happens is we open a second valve and as the piston continues its journey and goes back up again, it forces all that hot air and gas out of the cylinder and then that goes to the exhaust pipe.

So there's four strokes : bringing in the fuel and air, compressing the fuel and air, igniting it and burning it, and then exhausting it.

Connie - OK. We have this clever four stroke piston but only one of the strokes is producing your power to drive the car forward. So even if it's working at superspeed, that sounds like a pretty clunky journey to me...

Philip - Yes, if you just had one cylinder it would be very, very bumpy. You'd get a periodic big shove of mechanical energy and not much the rest of the time, which is why in real life, in a car you have several cylinders. It depends on the size of the car; you can have 4, 8, 16. You arrange it so that they are doing all of these parts of the cycle at different times and they all go slightly out of sequence with each other, and that gives us much better, much smoother power output.

Connie - Philip mentioned that the movement of the pistons is provided by the movement of the car but, unless I'm very, very confused, most cars start from a stationary position...

Philip - The engine can only keep moving by already being moving, which is why we need things like starter motors. When you're starting your car you turn the ignition and you hear it sort of whirring up and that's an electric motor that's causing the pistons to move up and down in the right way, so that we can at exactly the right time bring in the fuel, burn it, and get the thing going. In the olden days you had to do that by hand and it was a pretty hard job turning the engine over to get it to start.  

Connie - So that's why when my battery's flat I can't start my engine. It's because this initial electrical impulse isn't there to get the thing moving?

Philip - Yes. If you've got no battery, the motor can't turn and your engine is just sitting there. But, if your battery is flat, one thing you can do is push the car along until it's going fast enough, and sometimes, if you get your timing exactly right, you can get it to start that way.

Connie - Wow, I always through a push start was just some kind of...magic?  But, my naeivity aside, petrol isn't the only fuel available - what's different about diesel?

Philip - In some ways it's quite similar. A lot of diesel engines have this four stroke idea but what you do with the fuel is a little bit different.  Because diesel engines tend to use a heavier fuel it means there's a bit more energy in the fuel itself (from a certain amount of it) but it's a bit harder to get that energy out.

And so, instead of bringing in fuel and air when you're sucking things into the engine, you just bring in air. Once you have that air stuck inside the cylinder, you then compress it really, really hard (far more than in a petrol engine) and actually, if you compress air it tends to get very, very hot. And so, when you've got to the point where the air is at its most compressed, then you inject the fuel straight into the cylinder and it almost immediately vapourises and then, because it's so hot there it burns, and it burns nicely and consistently and that's the point at which it forces the piston back down again.

So the difference is in how we put the fuel into the cylinder after that it's kind of quite similar.

Connie - So what is it about diesel and petrol (the fuels themselves) that means you need these two different systems?

Philip - Well petrol is basically a lighter kind of fuel. It evaporates more easily whereas diesel is much heavier but stores more energy. So petrol engines are good because they have fewer pollutants, they tend to be quieter. Diesel engines you can get much more energy out of them, so they're good for really big, heavy duty engines.

Connie - So diesels, though we have them in cars, you're more likely to see them in, what, lorries and trains and things like this?

Philip - Yes. It would be very unusual to see a petrol train or lorry; mostly they use very, very big diesel engines

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