What is the stoichiometric ratio and lambda

In a nutshell the carburetor or fuel injectors work by mixing air and fuel in a ratio to provide optimum combustion/ energy to turn the wheels.

Back to the basics of fire, you need the following for a fire – heat, oxygen and fuel – take anyone of those away and the fire will die. So if we were to just put fuel into the engine and try to ignite it, nothing would happen. So the carburetors/ injectors let the engine draw in air and mix it with the fuel so that it can burn. So what is the ideal fuel air mixture?

Stoichiometric ratio

The stoichiometic ratio is the ideal mixture of fuel to air measured in mass of each substance and relates to octane/hydrocarbon based fuels, stoichiometrically this is the ideal mix in which there is just enough air present in order to burn off all of the fuel, although it’s never 100%  achieved. The typical stoichiometic mixture ratio a carburetor (or fuel injection) has to achieve when mixing fuel and air is 14.7 of air mass to 1 of fuel mass – 14.7 : 1. So if you have 100 grammes of fuel, you then need 1.47 kilos (1,470 grammes) of air – given that air weighs practically nothing, this is a lot of air needed to reach the ideal combustion point in our engine. If the stoichiometric mixture is lower then this means the engine will be running fuel rich which means fuel is wasted and harmful engine emissions will be higher – you’re taking away oxygen for the fuel to burn. If the stoichiometic ratio is higher then this means that the engine will be running lean and will struggle to give a smoother running performance and will generally struggle to maintain combustion – you’re taking away the fuel for the fire to burn.

Why is Stoichiometry important?

Stoichiometry is important as with it we can not only get the most efficient use of fuel but also the most power from the fuel, that means we go faster for longer! Stoichiometry is why we fiddle with carburetor settings or get the ECU remapped on our motorbikes – it’s also the reason why we have lambda sensors which feedback to the engine to let it know how much air is left or rather how much fuel is unburnt in the exhaust gases (there should be no oxygen). So in fuel injection bikes when the engine cuts out or runs poorly or there is excessive smoke it could mean the first place to check is the lambda sensor as the bike may not be getting the correct information back to the ECU to inform it on how to mix the petrol and air. Also it could mean that your injectors or fuel filters are blocked again effecting the lambda sensor. So we need to know this so that we can then understand how carburetors work and the principles of how fuel injection works.

How does lambda (?) relate to stoichiometry?

Lambda can mean a lot of things but in this instance it is the oxygen sensor in a vehicle used to measure the stoichiometric point. A lambda rating of 1.0 is perfect, less is running rich, more than 1.0 is running lean. Lambda is calculated as follows:

? = Detected AFR / Stoichiometric AFR

Where the detected AFR is the actual Air Fuel Ratio calculated as air mass divided by fuel mass by analysing the exhaust gases and this is then divided by the stoichiometric Air to Fuel Ratio for any given fuel (14.7 for petrol).

So what is the use of knowing lambda/stoichiometry?

In short, better performance for your motorcycle. When tuning carburetors or fuel injection, not only are you trying to get as much fuel and air into the engine as possible but as near to the perfect ratio as possible. Of course as you go faster or slower the flow of air to the engine will change so that’s why you have several jets in a carburetor and all those settings on the ECU, to ensure that no matter what speed the engine is running at and no matter what the air flow is, the AFR should be as near to perfect as possible.

When you change your airbox for pod air filters for instance, you alter the airflow which is why you need to remap the ECU or rejet the carbs as it begins to suck too much air in, altering lambda. If you significantly change altitude and the air is thinner then you need to adjust as there will be a lower mass of air.

Without this principle your engine simply would not run!

Possibly related?...

  1. How to calculate compression ratio