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I want to understand why mixture differs between the two conditions.
A big part of it with ramp runs is the delay between the ECU reading all the data, processing it, and applying it when by the time that happens the engine condition has changed slightly - or example, the manifold pressure, fuel pressure (if referenced) will change and there're delays there.
With steady state all the values are stable and unchanging.
Thank you for your answer Gord.
Over time, on many different ECU's, I have found that the mixture tends to be richer in a ramp run, as compared to a previously fine tuned map in steady state, particularly under part throttle conditions.
A recent test with a M150, which has 8! - times faster processing capabilitiy compared to its predecessors, still exhibits the same behaviour.
Considering that the ongoing combustion processes are relatively slow, when compared to modern day ECU's, one would expect that the engine management system, including moving parts such as injectors, would easily handle all that in realtime?
Could it be other aspects of the "equation"?
My guess, and I emphasis that, is it happens mostly with vehicles which use a MAF based setup?
The delay I had in mind was between the MAF reading an increase in air mass flow and that increased air density reaching the cylinder via the plenum between them which needs to be filled first. The ECU reads it as more air = more fuel, when the air isn't quite there yet?
I could be quite wrong, though.
My thoughts would be 2 main contributors.
1) Wall wetting/fuel film. In steady state the fuel film/puddle/wall wetting will be in equilibrium, so effectively the same volume of fuel that you inject into the port will make it into the combustion chamber. The mass of fuel clinging to the port walls and the rate at which it evaporates varies with many factors - MAP, RPM, port temp, air temp, etc, so when you change any of those factors you end up with with a different volume of fuel making it into the chamber than you have injected.
2) Injection scheduling. With port injection the fuel is injected before the intake cycle - so it is possible for the airflow into the combustion chamber to change after the fuel has already been injected based on a different airflow.
Thank you both for answering.
@Gord: Observations from SD.
Have a look at the Inlet Mass Flow and Exhaust Mass Flow channels in the M150 and compare them between steady state and during a ramp run at the same efficiency point.
On WOT it's mainly the fuel film building up. As more is injected as bigger the puddle gets. You can see that well when the turbo spools. On top of it the lambda readings have some delay.
On part throttle I found MAP is just not a very accurate reference for volumetric efficiency. Tps vs rpm on main table with 4d boost compensation just works more accurate for me.