Andrew Birkett's nobugs.org
(firstname.lastname@example.org, June 2001)
A while ago, I wondered “how does an engine really work”. I wasn’t just thinking of explosion in cylinders pushing pistons up and down. I was thinking of the little details. This file is a tidied up version of my random notes and questions.
If you want more detail, then grab a copy of the book Internal Combustion Engine Vol 2. Personally, I learned most of what I know from a book called Vehicle and Engine Technology. It tells you how engines, carburettors, chassis, tyres etc work.
We want the engine to turn the back wheel. Or, more practically, to turn the gearbox since we want the bike to move both a very slow speeds and very fast speeds. The range of engine speeds available is roughly 1000 to 10,000 rpm which is a factor of ten. We want to ride a motorbike at anything from 1 to 100mph which is a factor of 100. So, we need a gearbox to provide a suitable range of gears. The engine drives the gearbox which drives the wheel. How do we get rotational motion from the engine?
We do this by a piston which provides periodic linear motion, connected by a con-rod to a crankshaft which turns the linear motion into rotation. The piston experiences a downward force on the ignition stage, which increases its monentum. The crankshaft acts as a flywheel which moves the piston through the other three stages (exhaust, intake and compression).
What affects the force on the piston when we ignite the fuel/air mixture?
Petrol contain mostly hydrocarbons. Hydrocarbons are basically anything which contain carbon atoms and hydrogen atoms. They’re useful as a fuel, because a carbon atom can combine with two oxygen atoms to make carbon dioxide, and two hydrogens can comine with an oxygen to make water (H2O). Actually, as well as the carbon dioxide reaction, you get get a single carbon reacting with a single oxygen to make carbon monoxide (CO). This can then go on to react with more oxygen to make carbon dioxide. All these reactions release energy, which is what makes our piston move downwards.
Petrol is a mixture of about 500 different hydrocarbons, which contain between 3 and 12 carbon atoms. These hydrocarbons are liquid (obviously, since petrol is liquid) and they boil between 30 and 220 degree celcius. So, on a hot day the smallest molecules are going to start evaporating off if they’re not in a sealed container.
Hydrocarbons are either saturated (only contain single bonds) or unsaturated (contains at least one double bond). Saturated hydrocarbons tend to burn cleanly, whereas unsaturated hydrocarbons burn smokily and are unstable.
Now, you want to get the biggest possible bang out of your fuel/air mixture, and you can do this by compressing it before igniting it. This pushes all of the fuel and air molecules closer together. A compression ratio of 6:1 gives 25% efficiency, whereas raising it to 12:1 gives 35% efficiency.
However, when you compress the mixture it gets hotter – up to 600 degrees celcius in a running engine.. If you compress it too much, it’ll get hot enough to ignite itself, which is bad! If this happens in a running engine, it’s called knocking or preignition and it can damage your pistons. Of course, if the engine is hot to start with, it’ll heat up the mixture and make preignition more likely. We’ll return to this in a minute.
So, the efficiency of your engine is related to the compression ratio, which is how much you squish the fuel air mixture before igniting it. The fuel/air mixture gets sucked into the cylinder on the intake stroke when the inlet value is open. Then it gets squished during the compression stoke, until the piston reaches its highest point. If there is 400 cubic centimetres above the piston at it’s lowest point, but only 100cc at it’s highest point then you have a compression ratio of 4:1. A real engine would have a compression ratios of something like 12:1.
We’d ideally want to have the compression ratio as high as possible, but we’ve seen that that’ll cause the fuel/air mixture to ignite early (before the spark plug has sparked) and damage our engine. So, we look for some magic ingredient to stop this happening.
One such magic ingredient is Tetra-ethyl lead, which gives leaded fuel it’s name. It reduces the tendency of the mixture to preignite. Unfortunately, it’s bad for the environment and has been phased out in the UK. TEL acts to lubricate the exhaust value, so there’s some concern that switching to unleaded fuel can lead to the exhaust valve being lubricated less.
Another problem with using tetra-ethyl lead (TEL) is that it would eventually gum up the engine and exhaust with lead, so you also have to add scavengers to the petrol. Scavengers are chemicals which react with the lead to make a volatile chemical which can pass out of the exhaust. An example would be lead halide salts.
It’s worth noting that petrol usually contains some Sulphur before it gets refined. Sulphur defeats the anti-knock properties of TEL and so has to be removed. It’s hard to remove it all though, and the sulphur which is left in the fuel reacts with air to form sulphur dioxide. The sulphur dioxide gets into the atmosphere via your exhaust and dissolves in water to form sulphuric acid – causing acid rain.
The carburettor controls the fuel/air mixture on a motorbike, and you often hear ‘lean’ and ‘rich’ being used to describe the fuel/air mixture. Let’s look at what effect this ratio has on the engine.
Firstly, there’s a theoretically optimal fuel/air mixture. This is called the stoichiometric mass/volume and it tells you how much air (ie. oxygen) you need to completely burn an amount of fuel. If you have less air than this, the mixture is rich. If you have too much air, the mixture is lean. You can look at it in terms of fuel. Too much fuel gives a rich mixture, too little gives a lean mixture.
The stoichiometric mass is related to the carbon/hydrogren ratio in your fuel. This makes sense, since each carbon atom needs two oxygen atoms to make CO2, and each hydrogren needs on average half an oxygen atom. So you can presumably just add up the number of carbon and hydrogen atoms and do a bit of maths to work out how many oxygen atoms you’re going to need.
If you have the ‘perfect’ amount of oxygen for your petrol you can expect to get about 45 mega-joules of energy for every kilogram of petrol you’ve got. However, engines aren’t perfectly efficient. For a start, to get the maximum amount of work out of the explosion, you’d have to let the gases expand until they’ve cooled down to the surrounding air temperature (look up Carnot cycles somewhere). In a real engine, the gases only get to expand as long as the piston is moving down. When the exhaust port opens, and the piston moves up to punt the exhaust gases out, the gases are still hot. That’s why the exhaust pipe gets hot! A normal engine has an efficiency of about 20-40%, so it only gets 20-40% of the theoretical maximum amount of energy out of each explosion. The rest of the energy goes to warm up the engine coolant, the exhaust and the engine’s surroundings.
All these hot exhaust gases go out of the cylinder, passing by the exhaust value. This makes the exhaust value pretty hot – up to 300 degrees celcius. Because of this, the exhaust value takes more of a hammering than the inlet valve, since the gases passing into the cylinder are at air temperature.
Apparently, for petrol you get stoichiometric combustion (that’s complete combustion) when you have a fuel/air ratio of 1:15 (that’s 15 parts of air to one part of fuel). You can get more power out of your engine by running a richer mixture of 13:1, but you’ll be producing some partly burned fuel leading to smoky exhaust and a gunky engine. You get maximum thermal efficiency (most energy for a given amount of fuel?) when you have a lean mixture such as 17:1.
Let’s look at what happens when the spark plug fires when you’re running a lean mixture. There’s less fuel molecules to go around, so the flame moves across the cylinder more slowly. This leaves more heat in the cylinder walls and cylinder head, which can lead to overheating. If the fuel/air mixture is very lean, then the flame can still be present when the inlet valve opens, which causes backfire!
If you’ve got a compression ratio of 12:1, with an engine speed of 1500rpm the flame will move across the cylinder at something like 15 meters per second.
As the engine speed increases, there’s less time for the mixture to burn completely. An engine running at 1000rpm spends 0.06 secs in each cycle, which drops to 0.006 secs when it’s running at 10,000rpm. One way to combat this drop in available burning time is to fire the spark plug a bit earlier when the engine is running fast – this is called the spark advance. If you increase the spark advance too much, it can cause knocking. However, if the engine is running fast then there’s less time for reactions to occur on front of the flame front, which tends to decrease the chance of knocking.
Preignition can be caused by the mixture getting too hot. It can also get caused by hot surfaces, such as the exhaust value or the spark plug or hot deposits on the piston. The spark plug deliberately ignites the mixture once per cycle, but if it doesn’t get rid of it’s heat quickly enough it can remain hot enough to ignite the next charge of fuel when it comes in. The exhaust value gets hot because the hot exhaust gases squeeze past it on their way out. If it gets too hot, it can ignite the fuel mixture too. This leads to “runaway surface ignition” because if a hot surface ignites the fuel mixture, it’ll keep getting hotter. You can end up with melted pistons.
Misfire can be caused by a failure in the electrical system, which means that the spark plug won’t be making a spark. It can also be caused if the spark plug gets fouled by deposits. This happens more if the engine is running a rich mixture.
Engines are made from metal, because it’s one of the few materials which can withstand the forces of the explosions, withstand the high temperatures and which can be made into the right shapes. The cylinder is made of metal. The piston and piston rings are made of metal. What stops this from scraping against each other in a nasty way?
The answer is oil. If you’ve got two metal surfaces rubbing against each other you need at least a thin layer of oil between them. If you run out of oil, or the oil film breaks at some place in your engine, the metal will rub together and get hot very quickly, potentially welding themselves together.
Why is oil good for this purpose? Most obvious, it’s slidy. If you have two metal surfaces sliding against each other, then putting a layer of oil between them will decrease the friction.
What is oil made from? What makes it slidy? Long molecules? Why doesn’t water work?
It has to be stable at temperature – can’t decompose quickly.
What do the different ratings mean?
Piston rings to scrape the oil, block the explosion from getting down past the piston.
What does oil encounter during its lifetime (cold start, hot operating — changes in viscosity).
If you use different metals when building an engine, you’d better be sure that they expand at roughly the same rate. If you make the piston from metal which expands faster (as the temperature increases) than the metal which the cylinder is made of, you’ll have problems.
If you’ve got a rotating shaft, you need to support the two ends of it somehow. The two ends get supported in bearings (since they bear the load) in the engine case. A bearing is just a specially chosen material which reduces the friction between the parts – such as brass. You still need oil though.
The carburettor is the bit of the motorbike which mixes the fuel with air. The fuel comes the tank, through a fuel tap which lets you turn the fuel on or off, then into the carburettor. Normally, we leave it up to gravity to get the fuel from the tank into the carburettor – we don’t need to pump it. This wouldn’t work in an aeroplane which has to fly upside down, but bikes spend most of their life right-way-up so it works fine. The air is sucked in from the outside world, and passes through an air cleaner before reaching the carburettor. There’s a few different types of air cleaner, but their purpose is the same. They remove grit, dust and other undesirable gunk from the air. You don’t want little bits of grit flying into your engine where they’d cause havoc.
The carburettor has its own little reserve of fuel called the float chamber. This work in the same way as a toilet cistern. When the fuel level drops, a value is opened which lets more fuel in. When the fuel level is high enough, the valve closes. You can get an idea of how much fuel is stored in the float chamber by turning the fuel tap off while the bike is still running. It’ll keep running for a minute or so.
The main purpose of the carburettor is to ‘spray’ the fuel into the air stream which passes through the carburettor. This creates a very fine mist, so that each bit of fuel is surrounded by lots of air which it can react with once it reaches the cylinder. It’s no good having big blobs of fuel, because only the fuel on the surface will be exposed to oxygen.
Let’s look at how the carburettor sprays the fuel into the air. There’s a fairly wide passage runs through the carburettor which is where the air passes through. We make a little hole in the side of this passage, and connect it to the float chamber. The idea is that the passing air will suck the fuel out of the hole, and it’ll then mix with the air.
We can make this work much better if we make the air passage narrow before it reaches the hole, and then widen afterwards. This creates a pressure difference (the Venturi effect) which acts to suck the fuel out into the air stream.
Clearly, the size of the hole is going to have a big effect on the resulting fuel/air mixture, so we use a carburettor jet. A jet is like a screw with a hole running down the middle of it. You screw the jet into the carburettor body, and the fuel goes up through the hole in the middle. You can buy jets with different sizes of hole if you want to change your fuel/air mixture.
There’s a problem with this description though. There’s not yet any way for the rider of the bike to control the fuel/air mixture. That’s going to be a problem, because the rider isn’t going to be happy with an engine which only runs at one speed. We can introduce speed control by arranging for a metal needle to sit above the jet. We can slow the engine down by moving the needle down, so that it partially blocks the hole in the jet. This throttles the fuel supply, and so it’s known as the throttle control. The right-hand grip on a motorbike simply controls the height of the needle. When you’re at full throttle, the needle is totally out of the jet. When you’re going very slowly, the needle will be blocking the jet more.
So, that’s the a pretty simple carburettor. The people who first built these things pretty quickly realized that they weren’t perfect. For a start, they didn’t work very well if the engine was running slowly. If the engine is running slowly, the piston is sucking air in at a relatively slow rate. If the air is moving past our jet very slowly, it doesn’t suck out much fuel so we end up with an overly lean mixture. We can fix this problem by putting a second, larger, hole in the carburettor called the pilot jet. This is only operational when the engine is running slowly. It’s larger size ensures that enough fuel is mixed in, even at slow speeds. At higher speeds, it’s not used.
So, what can go wrong with carburettors? Since they’ve got lots of very small fuel passages, they can get blocked quite easily. That’s why you have a fuel filter (usually in the fuel tap). That’s also why it’s a bad idea to run the bike with very little fuel. Chances are, you’ve got little bits of sediment in your fuel tank which have sunk to the bottom. If they get into the carburettor, everything is going to stop. You’ll also get problems if the fuel leaves behind any residue inside the carburettor. This gums up the insides of the carb, and you need pretty noxious cleaning fluids to get rid of it.
If it’s a cold day then starting the bike can be difficult. If the walls of the carburettor and cylinder inlets are cold, then the fuel will condense on the walls. The same thing happens if you breath out on a cold window – the water vapour in your breath condenses on the cold surface. If your fuel is all condensing before it reaches the cylinder, the mixture will be too lean to burn. You can remedy this by providing some means of making the mixture much richer when you’re starting the bike. This is what the choke lever controls. You can also see why it’s bad to run the bike with the choke lever on, since you’ll end up with an overly rich mixture. Also, if you put the choke on when you’re starting the bike on a warm day then you’ll get an overly rich mixture too, which will stop the bike starting. Taken to extremes, the fuel will start condensing in the cylinder and stop your spark plugs from doing their job.
A final carburettor problem is called carb icing. This is usually only a problem when it’s quite cold outside and there is high humidity (lots of water vapour in the air). When any liquid evaporates and turns into vapour, it takes in heat from it’s surroundings. That’s why you feel colder if you get wet – the evaporating water is sucking all your heat away. The same thing happens in the carburettor when the volatile fuel turns into fuel vapour. This makes the surfaces of the carburettor colder – by as much as 20 degrees celcius. This isn’t normally a problem, but if the carburettor is already cold then the cooling effect of the vapourizing fuel can make the carburretor body drop below freezing. If this happens on a humid day, then as we suck in air we will also be getting lots of water vapour. The water vapour can condense on the cold interior surfaces of the carb, and ultimately freeze. The ice can block the jets and stop the carb from working.
Bike builders tend not to put parts onto a bike unless they have a purpose. So what is the purpose of the exhaust pipe. Why is is it shaped like that, and what’s inside it?
You could theoretically run an engine without an exhaust pipe. It would be incredibly noisy – the exhaust valve is opened pretty quickly after there’s just been a big explosion inside the cylinder. There’s probably still some burning gases, and the whole mixture is going to still be extremely hot and would burn your legs. So, you need some way of getting these hot gases away.
The exhaust pipe serves three purposes. It takes the noxious gases which come out of the engine and moves them a bit further from the rider. We’ve already seen how carbon dioxide, carbon monoxide and water are formed during the burning of hydrocarbons. Sulphur dioxide is formed when the sulphur in the fuel is burned. Additionally, we find oxides of nitrogen in the exhaust gases. Normally nitrogen is pretty unreactive (it makes up 80% of the air we breathe) but the extremely high temperatures generated by the spark plug can cause it to react with oxygen.
Secondly, the exhaust pipe cools down the gases. It does this by allowing them to expand. The exhaust pipe is quite narrow when it leaves the engine, and then it gets wider. Allowing a gas to expand makes it cooler. Remember how compressing the fuel/air mixture with the piston made it hotter? This is the opposite effect. Once the gases have passed through the expansion chamber they are much cooler. That’s why your exhaust pipe dries out fastest near the engine when it’s been raining.
Finally, the exhaust pipe reduces the noise of the engine. Every time the exhaust valve opens, there’s a pressure wave (shockwave) travels along the exhaust pipe. If the exhaust pipe was just a straight tube, this wave would escape to the outside world and you’d have a noisy bike. To stop this, the exhaust pipe also acts as a silencer. It contains various plates with holes in them (baffles), which force the exhaust gases to take a convoluted route to the outside world. This tends to breaks up and cancel out much of the pressure waves, which results in a quieter bike. If you get a hole in your exhaust before the silencer then the exhaust gases will escape there and everything will get much noiser.
The power of a bike is measured in horsepower (HP). One horsepower is the energy required to move 33,000 lbs by one foot in a minute, which is the strength of some perfect horse. It measure the rate of energy use – which is the same thing which ‘watts’ measure, eg. for lightbulbs. A human being can produce something like a quarter of a horse power. Bike engines produce anything from 10 to 200 horsepower.
In practise, you see bikes rated in brake horsepower (BHP). Brake horsepower is named after the machine which is used to measure horsepower, which is called the prony brake. The bhp rating is the maximum attainable hp for the engine. It doesn’t account for losses of energy through the gearbox, chain, wheel bearings. Even if you have a 50bhp engine, you wont be getting your full 50hp of rotation power at your back wheel if you’ve got a rusty chain and corroded wheel bearings.
An engine clearly delivers a different amount of power at different speeds. When it’s going slowly, it provides less energy each second than if it was going quickly. If you look at the power graph for your bike, you’ll see what power it produces at different engine speeds (rpm).
Torque measures ability of accelerate. Again, your engine will produce different amounts of torque at different speeds. Typically, it has low torque at low revs, maximum torque at something like 60% of it’s maximum speed, and then the torque drops off as it reaches maximum speed. The more torque you have, the faster you can accelerate. That’s why bikes feel most responsive at something like 60% of peak revs – you’re at the point of maximum torque.
Why are bikes less responsive when the engine is cold?