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The AFR Target Map

Understanding AFR

Discussion and questions related to the course Understand AFR


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So during the course we covered this pretty well regarding Naturally aspirated engines, and we touched on Turbo and Rotary engines. One thing we didn't cover very much was how we approach this with different fuel types. (I revisited all this today just to double check). While the Fuel table itself will be representative of the BSFC and VE of the engine, and when we actually tune the engine we CAN actually identify what AFR the engine wants to see to help us complete our AFR Table, but what if we are going to road tune? if we were we cant actually identify what the engine wants without the torque optimization tool, so we cannot backwards engineer the table so establishing that AFR Target MAP which is relatively accurate becomes challenging.

So, How would we approach this to be more accurate with our AFR Target Table, what sort of guide lines should we be following given the different fuel types, or what trends have you seen based on your experience. Here's my example based on what I have learned. (rough draft)

https://docs.google.com/spreadsheets/d/1skMzXNHn5F2Thxfpa4Y0GVLuNWxlcIHzY61grQpu9Zs/edit?usp=sharing

I realize now since making this post that I simply was overlooking a few things. Obviously, we cannot be absolutely accurate with the AFR Target Map before seeing what the engine wants for the given fuel type. But as far as fuel is concerned I also realized where i was going wrong in terms of which way we should be going with our targets in relation to fuel type.

With a change in fuel, the efficiency table would be relative to the amount of additional fuel required to reach stoich. But the AFR Target Table will be relative to the performance of the fuel. This means that if the fuel performs better (e.g. Pump 98 to E85) so (less chance of DET, cooler cylinder temps) then the AFR targets could be leaner. So for my example above for E85 would be a little too rich. Therefore the trends we could apply to the AFR Target map to be more accurate would be based upon the fuels performance.

If I am wrong please feel free to let me know, Cheers.

D

As you've observed since your first post, no we can't know ahead of time what specific AFR an engine will need to run. We will have a starting point which is provided within the course that is a safe place to begin tuning from, and then we can optimise the AFR and see how the engine responds to leaner or richer AFR. So ultimately the AFR doesn't need to be 100% locked down before we start tuning - We can always come back and modify this later. As far as road tuning goes, we need to be prepared to make some compromises if this is the technique that we're going to use for our tuning and it's much more difficult to experiment with AFR on the road and see the direct affect of a change.

Typically E85 responds quite well at similar lambda targets to what we would run on pump gas, however due to the cooling properties of E85 and it's higher octane rating, you can get away with running a little leaner.

Thanks Andre, This is what I was after. how much is 'a little leaner'? whats the difference in performance between the different fuels?. Lets make our baseline Pump 95.

Do you have any examples of a general AFR Map of each type of fuel on the same engine. NA and then FI. then we can see the trends ourselves without having to test it out. For those of us who have not had the opportunity to see the performance differences regarding target AFR's yet, this could be really good, and would help us out alot when modeling Realistic AFR Target Maps for different fuels, you know.. just to identify trends. Because if the Performance of the fuel changes the mixtures MBT Points, then we are going to be very inaccurate trying to guess what trend calculations are going to be as we move across the cells.