When you open the bonnet of a car you’ll look in a see all sorts of wires, plastic covers, tubes, pipes and metal bits all over the place. It can look very confusing! It’s actually a pretty simple device, but over 140 years or so of development it’s gained lots of additional bits and bobs on there to make it all work a little more efficiently and much, much cleaner. But let’s travel back to the beginning and get the basics straight. We can work on the other bits and bobs at a later date.
Essentially, what we’re doing in an engine is taking some chemical energy in the fuel (in this case petrol) and by combusting it (burning it) releasing heat. We then use this heat to move mechanical parts. In order to make this all work we need to start with burning the fuel. In order to create any fire we always need 3 things: fuel, oxygen and heat. The fuel is the petrol that we put into the car at the petrol station. We then need some oxygen to burn this with. We get the oxygen from the air that surrounds us (air is approximately 20% oxygen). By mixing the fuel (petrol) and air together we can burn this mixture by adding some heat. We add the heat from a spark. You may have heard of spark plugs. The spark plugs are the things that create a spark, which gives us our heat. Combine all three and BANG! Well, not quite actually. If you were to get a small dish of petrol and put a match (or spark) to it in a room it wouldn’t go BANG. It would burn, but much like a candle burns – a sooty yellow flame dancing about. But no bang.
Right, so, how do we get the BANG, and more importantly, why do we want the BANG? Why not just burn like a candle? Well, in order to get an engine to make enough power to propel a large object such as a car around we want to release the energy from the fuel very quickly. Burning large pools (or drops) of fuel in air doesn’t burn very fast. What we want is not a candle but an explosion. This is where things get a little more interesting. Sometimes engines are called “Internal Combustion Engines”. This is the technical name for the car engine. It’s so called because the combustion (the burning of the fuel) occurs inside the engine. Quite simple really. So what we do is we mix the fuel and the air together in a sealed chamber. This chamber is called the cylinder, because, well, it’s cylindrical! Imagine a baked bean can – that’s pretty much the shape we’re talking about. However, the bottom end of this can is able to move up and down inside the rest of the can. This is called the piston.
But what actually happens? How does it make the car move? Ok, so let’s start outside the engine with the air. This is sucked into the cylinder. As it’s sucked into the cylinder fuel is squirted into the air as it enters in very, very fine droplets to give a really nice mix of fuel and air together in a misty cloud. The air is sucked in as the piston (bottom of our can) is being pulled downwards through a valve we’ve opened at the top (imagine making a small hole in the top of the can that we can open and close as we wish). The piston has started at the very top of the cylinder and is moving down sucking this air-fuel mixture with it as it goes. When it gets to the bottom of this stroke we close the hole in the top where the air was coming in. The piston then moves upwards again, squashing, or perhaps squeezing the air and fuel mixture. This makes it warmer.
When we squash or compress a gas (such as air) the temperature of the gas will go up (equally, if we mechanically expand a gas then the temperature will drop – see our experiments at the end). So now we have hot air and gas, ready to catch fire. All it needs is a little spark. Back to the spark plug for this job. Add the spark to the hot, mixed air and fuel in a very confined space. Now we get our BANG! Think TNT type BANG. This bang is the fuel and air mix burning very, very fast. Only a tiny drop of fuel, but if we get the quantities right it releases lots of energy for us very, very quickly (for fuel and air we need about 14 parts air to 1 part fuel evenly mixed for perfect combustion).
Now, as this burns the temperature rises again – fire is hot! As the temperature rises the gas wants to expand (if you heat a gas up it will expand, cool it down and it will shrink – see the experiments at the end) and as it’s heating up very fast the gas expands very fast. This is why we hear a bang noise from an explosion or thunder – it’s the noise of the air heating up very, very fast and therefore expanding very, very fast.
So the gas expands and pushes our piston back down towards the bottom of the cylinder very fast. This is where the power in the engine comes from. Finally the piston moves back up from the bottom once the burning is complete and as it goes up we open a different hole in the top of the chamber and let the hot burnt exhaust gases back out, ready to start again.
This is a 4-stage process that just repeats and repeats. The easy way to remember it is:
1. SUCK – suck in all the air and fuel
2. SQUEEZE – squeeze the air and fuel mixture together into a very small space
3. BANG – ignite the fuel and air mixture to make it burn (or explode if you prefer)
4. BLOW – blow all the old burnt air and fuel out the other end
Suck, squeeze, bang, blow, suck, squeeze, bang, blow, suck, squeeze.... (you get the idea!)
Ok, so we’ve got the bang and how that works, where does the mechanics of moving come in? Well, the piston (bottom of the can or cylinder) is connected by a rod (the connecting rod! – soon you get used to this type of naming, Engineers are generally very straightforward people) to a crankshaft.
The crankshaft goes round and round (instead of the piston going up and down). Think of a bicycle – it’s exactly the same as your legs going up and down (like the pistons) connecting to the pedals going round and round (the crank). Once you’ve got this bit going round and round the rest is easy – a few shafts and gears and we can move the round and round from the engine to a round and round at the wheels – the car moves!
These bangs in the cylinders happen very, very quickly and many times per second. In an average car travelling along a motorway you would expect each of these cylinders in an engine to go bang about 25 times every second! When people talk about the number of cylinders their car has (typically 4, but sometimes 2, 3, 6, 8 or even 12) that is the number of these “bean cans” with the pistons at the bottom in the engine of that car. So, for our average car with 4 cylinders, while we’re travelling along the motorway we have 4 of these going bang 25 times each second, so that’s 100 bangs a second!
That is why you can’t hear an individual “bang” as the car drives along, it’s just a constant noise to you or I.
There’s lots of other things we could then go into about how we control the air and fuel in and out of the engine, how we make it emit less dangerous gases when we burn the fuel (or clean them up before they get back into the air), how we make more power from the engines without making them larger and all sorts of other fun, but you now have the basics. If you’d like any further information there are many websites out there that provide more detail, but below are some links to some of the ones that I think are quite good and also some links to a few animations that may help you understand a little better all the above.
http://www.animatedengines.com/otto.html - simple animations of a petrol engine
https://www.youtube.com/watch?v=OXd1PlGur8M – a great video from Ford, building and then running and engine (if you want to skip the build, head to about 4:00 minutes in to watch it running)
Some further information:
http://animagraffs.com/how-a-car-engine-works/ - I’d recommend this site, excellent animations and explanations of engines
1. Compression and expansion of gases: Gases will get hot if you compress them, equally, they’ll get cold if they are allowed to expand. How can we test this? - http://www.scienceinschool.org/2012/issue24/energy
2. Heating and cooling of gases: Gases will expand if you heat them up and contract if you cool them down - http://www.lovemyscience.com/magiccoin.html
Equally, you may have noticed when pumping up your bike tyres that the pump gets hot. This is from the temperature of the air rising as you compress it into the tyre. Equally, if you open the valve and let the air out quickly it will feel a little cold. This is the temperature of the air dropping as it expands again.