Fuel/air ratio

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 fuel to one part of air). 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.