The ECU absolutely does change the amount of fuel - that is its primary output. Some ECUs have absolutely no control over any sort of air delivery device (aside from for cold-starting). Yes, new electronic throttle cars can vary airflow independently of throttle input, but the main purpose of the ETM is to shut the throttle to allow for effective traction and stability control. Historically, the driver controlled airflow by opening the throttle and the computer responded with an appropriate amount of additional fuel. The AMM and MAP sensors are there to measure the air flow and pressure (respectively) resulting from throttle inputs, and allow the ECU to provide a corresponding amount of fuel. As a side-note, some ECUs monitor only throttle position - neither air flow, nor pressure. Adding more airflow as a lambda correction would raise the idle speed - it's fuel which is trimmed.
More to the point, you absolutely do not always want to be at the stoichiometric mixture. That is fine at idle and on cruise, but engines need to run a richer mix (typically circa 12:1 AFR) on-load. With old narrowband lambda based systems (i.e. a stoichiometric switch), the sensor becomes useless when you put your foot down. The ECU literally ignores it above a certain load point. A wideband can tell exactly where you are, anywhere in the likely AFR range, and allows the ECU to accurately control fuelling everywhere in the load range - hence how later turbocharged Volvos are so much better protected against shoddy remapping. As far as I'm aware, anything ME7-onwards in the Volvo range has a wideband, but I could be wrong on that point. Motronic-equipped cars certainly have narrowbands.
In theory, the "multiplication ratio" between petrol injector pulsewidth and LPG injector pulsewidth ought to be a constant (and some cheap LPG ECUs have a single multiplier value, rather than a map)....but no injector has a flat line for fuel delivery characteristics, so a map of load vs RPM is required for best delivery. One LPG installer told me he sets the system at idle, "then it must be right under all conditions". I thought to myself "no wonder LPG has such a bad name for destroying engines..."!
As a side-note, one LPG installer suggested that it's best to ignore the different stoichiometric ratio for LPG, as the important factor is what the engine is set up to burn. I raised the subject, as I was proposing to use a wideband with an emulated output to report the typical 0-1v curve for the petrol ECU (as if at 14.7:1), but to do it at 15.8:1...hence avoiding mixture fault codes from the petrol ECU. I don't think I agree with what he was saying, but it was an interesting thought.
cheers
James
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