This is a bit difficult to understand and so I will go through Edis figures again with more sources and examples.
Edis wrote:To use all the oxygen in 1 kg of air we need 68 grams of gasoline thus the total exhaust mass will be 1068 grams. Now, if we burn the fuel at lambda 0.5 rather than lambda 1, we need to add 136 grams of gasoline instead of 68 grams, giving a total 1136 gram exhaust. If we go lean to lambda 1.5 instead we will still need the 68 grams gasoline to maintain the power level, then we need 1499 grams of air adding to a total exhaust mass of 1567 gram.
First he refers to the concept of the
air to fuel ratio. Wikipedia says the stoichiometric AFR for gasoline is 14.7. The stoichiometric AFR in engine technology is also called lambda. Lambda=1 means that we use exactly the amount of air to burn all gasoline completely without having any oxygen left.
If we devide 1000g air by the AFR of 14.7 we get 68 g of gasoline for a stoichiometric combustion reaction, which means lambda is 1. The stoichiometric reaction takes the oxygen in 1000 g of air - which is approximately 190 g - to burn 68 g of gasoline completely with no gasoline left and no oxygen left.
Next Edis looks at a rich mixture and a lean mixture. He suggests to use a lambda of 0.5 and 1.5 for this. If we apply the lambda values to the AFR we get the following table.
lambda=0.5 AFR= 7.35 (rich)
lambda=1.0 AFR=14.70 (stoichiometric)
lambda=1.5 AFR=22.05 (lean)
To avoid confusion we will keep the fuel mass constant at 68g. If we do that we get:
68 g fuel x 7.35(lambda=0.5)= 500 g air -> 568 g total rich reaction mass
68 g fuel x 14.70(lambda=1.0)=1000 g air -> 1068 g total stoich reaction mass
68 g fuel x 22.05(lambda=1.5)=1500 g air -> 1568 g total lean reaction mass
We can look at it the other way round and compute the fuel needed for 1 kg of air.
1000 g air : 7.35(lambda=0.5)= 136g fuel -> 1136 g total rich reaction mass
1000 g air : 14.70(lambda=1.0)= 68 g fuel -> 1068 g total stoich reaction mass
1000 g air : 22.05(lambda=1.5)= 45 g fuel -> 1045 g total lean reaction mass
When we look at both tables it gets clear why there is confusion sometimes. If we fix the air in our computations we get a decreasing reaction mass when we go from rich to lean combustion. If we fix the fuel we get a massively rising combustion mass when we go from rich to lean combustion.
Xpensive formulated the claim that the exhaust gas is going through an engine at a fixed ratio to the burned fuel. I said that this can't be true because the mixture can be rich or lean. Edis showed us with his figures that indeed a fixed amount of fuel (68g) will produce a highly variable exhaust gas flow depending of the richness or leanness of the mix. So with regard to xpensive's claim it was clearly shown that it is not true. I think we can all agree on that.
Further I tried to explain why this result looks so unexpected. When you look at table I you see that the air demand is highly variable for the same amount of fuel to burn. So obviously in the real world to achieve those different combustion states with the same amount of fuel you not only have to change the mixture, you also have to change the throttle position to get that amount of air into the engine.
Further I have tried to explain that we can compare the amount of fuel burned with the power of the engine. That is not exactly true because the efficiency will change with the different lambdas but we can use it to make things simpler. If we start with the stoichiometric reaction we find that by going lean on the engine we will need a much wider throttle position because we need 50% more air. If we go to a rich mixture we can significantly go off throttle to achieve the same power because we need 50% less air. As I have already explained the power will not be constant with the same amount of fuel but it explains why we get this unexpected throttle setting when we change the mixture. To keep the power with a rich mixture we need less throttle and to keep it with leaner mixture we need more throttle.
I hope this makes things a bit clearer. I have used Edi's lambda values here because I did not want to confuse things more. I want to add the caveat that lambda values like 1.5 are unrealistic in current F1 engines. I'm not aware where the real values are but I think that 0.7-0.95 should be more realistic. Nevertheless if you do the same computations in that range you would see similar effects for going leaner or richer from a middle value of 0.825.
