Hello,
Hello!
i have been studying the mat enrichment table that my software was sent with from Marc,and also seen the other tables that are around the forum and i have a few questions....
According to the way megatune figures out the required fuel and the IATs it appears that fueling is reversly proportional to the inlet temperature. n=PV/RT (ideal gas law). According to that we should increase the fueling if the inlet temperatures drop since cold air has more density in oxygen and therefore needs more fuel to have a good burn.On the other hand when IATs are high,the fuel should be lower...
Correct. The basic function of MAT correction is to adjust the fuelling for air density such that the combustion lambda remains constant, at whatever value you have tuned it to in the main VE MAP. (Note here that if the VE map is correct, no control input should be required from the lambda control to achieve the value in the target lambda map)
Here is were it doesnt add up for me. IF the IATs are high (hot day, traffic, bad IC etc) VEMS is actually ordering the map through this table to make the mixture leaner. How is that going to help with the EGTs then???
Its not. Thats a slightly different question - I'll get to it in a minute (keep reading)
Also, if the MAT table was all 100% everywhere on all banks, wouldnt the VE table and lamda table be sufficient enough to keep the mixture controlled all the time?
With closed loop wideband control then yes, the lambda would remain under control, but only due to a control input. The ideal is always to mimimise the required control, hence an open loop correction to the VE table for MAT is nice to have. For those without wideband closed loop, MAT compensation is essential if running rich at high temps and lean at low temps is to be avoided
Which table is the MAT enrichment predominantly affect? Is it the VE table ? is the lamda table?
Both (if its implemented correctly) - again, keep reading....
Also why is it that on higher TPS values i see that the mixture gets even leaner?(or at least below 100%) and on higher inlet temperatures??
Can someone with more knowledge explain to me that? I did understant the gas ideal law explanation,but in reality i see that this could lean to leaner mixture than what there should be. Obviously im missing something,and the map is right..but i have to understand it!
Ah, that is where it gets a bit trickier.
Actually, you have some additional functionality here with a 2 axis MAP to adjust. You got the idea when you mentioned EGT's earlier in your post. In reality there are 2 things to consider - EGT's and Knock. In many ways the two are inter-related, since when you get knock you will need to retard the ignition, leading to higher EGT's requiring additional fuelling.
So, lets start with knock, or at least ignition retard with MAT. If you have this mapped then you will see an increase in EGT's. If this were to cause you to exceed your EGT limit then you want to add some fuel.
Additonally, if you are entering a region where knock is an issue, then richening the mixture will help suppress the knock, so even if ignition retard with MAT is not implemented you may have an advantage by richening the mixture anyway.
At this point, we need to go back to the closed loop issue. If you are running closed loop wideband control, then increasing the VE or MAT enrich table isn't going to do anything - the controller will simply take the additional feed forward fuel back out again. Therefore not only do you need to add MAT enrichment to the VE part as currently implemented, but also to the lambda target part in a matching amount - after all, it is engine lambda that actually determines the combustion temperture - injection pulsewidth is simply how this is achieved. As usual, forgive me Rob if I'm not up to speed, but I'm assuming there isn't a second MAT enrich table that adjusts target lambda?
For those running open loop, they can use the current table to do what I think you are thinking, which is to compensate air density reductions (i.e. go leaner) at lower loads, but then de-compensate again (i.e. go richer) as load increases and knock and/or EGT's become an issue.
Of course those running closed loop can also chose to go open loop under higher load conditions - but if they tend to rely on the closd loop controller to keep everything in check they could be in with a nasty surprise if they simply switch things off....
On the other hand, if you have an engine that doesn't knock under any conditions, then there is no need to retard the spark, and therefore EGT's won't increase significantly, and then therefore no need to increase the fuelling for the higher loads. This is however an unlikely scenario.
Anyway, I hope that starts to explain it - basically your question is well founded, and you have grasped a basic issue of what happens when engines get hot and how to compensate for it. The important bit is that each engine is different, so the basic values in the table give you a starting point based on the change with air density - its up to you to decide if your engine needs to have a modified high load line to de-compensate for its particular problems, and by how much, but this will be in conjuction with values in other maps, and the whole lt will be based on engine parameter such as knock sensitivity and EGT limits. This will be done to a large extent by trial and error, and that my friend is called calibration (or mapping if we are being 'tuner')
thanks
vasilis
You're welcome
