MoTeC Global Forum

Actually this is a karting question but I can't seem to get the answer anywhere.

The lambda for max. power using petrol is about 0.86. (Karts don't have to worry about pollution regulations...yet)

The question is: Does the max. power lambda change significantly with temperature and pressure?

If it does, are there any formulas which would allow me to calculate the max. power lambda for temperature T1, pressure P1 if I know the lambda for temperature T2, pressure P2?

I was told that the lambda = 0.86 comes from the fact that not all the full is burnt in the combustion chamber (in a 2-stroke at least) and it is not possible 100% combustion because, under normal engine conditions, the shape of the molecules in the petrol don't let the oxygen molecules get at the carbon/hydrogen atoms. Does anyone know if this is true?

Does anyone have an explanation of where the 0.86 comes from (especially if the above is not true)?

GrumpyMiddleAgedGuy wrote:...
I was told that the lambda = 0.86 comes from the fact that not all the full is burnt in the combustion chamber (in a 2-stroke at least) and it is not possible 100% combustion because, under normal engine conditions, the shape of the molecules in the petrol don't let the oxygen molecules get at the carbon/hydrogen atoms. Does anyone know if this is true?

Does anyone have an explanation of where the 0.86 comes from (especially if the above is not true)?

The lambda could be 0.86, or 0.95, or somewhere in between.
There is a small chance it could even be 1.00 .
It all depends on the engine.

Why does the same engine generally need more fuel:Air when the engine is cold?

The only way to know exactly what is best is to measure the torque versus F:A (N Runs - for instance 0.80 to 1.00 at every 0.01 ==21 runs).
Then there is also understanding the spark advance. (N runs, for instance 20 to 40 degrees at every degree = 21 runs).
If the advance and F:A are related then you need to do the tests over 2 variables (N^2 runs == 21x21 runs...)
And there are more subtle effects like the effect of fuel's cooling on the air charge.
If that gets done at every degree C... then it is a 3D space which can be overwhelming.

If you want more power, you probably want to be at a much higher lambda than 0.86.
(I would think somewhere in the 0.89-0.93 range, but I have not idea...)

How the F:A effects cooling and engine temp will need to be worked out.
Also oil generally lowers the octane...

The answer is that the 0.86 was is something that your friend read, and is now like a holy text; and may have more to do with a big American V8, than with a small 2 stroke kart engine.
Unless he has data with the same engine as yours, I would distrust the advise.

If he is a competitor, then I would totally distrust the advise.

GrumpyMiddleAgedGuy wrote:Actually this is a karting question but I can't seem to get the answer anywhere.

The lambda for max. power using petrol is about 0.86. (Karts don't have to worry about pollution regulations...yet)

The question is: Does the max. power lambda change significantly with temperature and pressure?

If it does, are there any formulas which would allow me to calculate the max. power lambda for temperature T1, pressure P1 if I know the lambda for temperature T2, pressure P2?

I was told that the lambda = 0.86 comes from the fact that not all the full is burnt in the combustion chamber (in a 2-stroke at least) and it is not possible 100% combustion because, under normal engine conditions, the shape of the molecules in the petrol don't let the oxygen molecules get at the carbon/hydrogen atoms. Does anyone know if this is true?

Does anyone have an explanation of where the 0.86 comes from (especially if the above is not true)?

I think it gets down to testing. Which is tough as it takes time, $ (US or AU), and can be hard on the vehicle (a lot of vehicles don't lend themselves to chassis dyno testing due to cooling issues etc).

If setup (cam timing etc) is way off, someone might find a max power value different than yours. Their wideband could be off. They may be detonation limited and using fuel as band-aid to cool charge.

After a while I've built up a library of tunes that gives me a general trend and starting point (older motor designs seem to want to run a little richer than newer types) but testing is the only way you'll know.


Test, test, test.