Open loop at WOT (was Re: Bosch Mono-Jetronic)

[Zublin, Bryan (SD-MS)]

Michael F. Sargent writes:

>So I'd like a little clarification here. I know that OEM's go open loop at 
WOT
>and end up running rich (my Toyota P/U accelerates faster if you go WOT and
>then back off just a touch), but I don't know why. I can't see any danger
>running at stochiometric at WOT (but I can sure see why you don't want to 
go
>lean). I always assumed that manufacturers went open loop to run richer 
(more
>power) at WOT. Anyone know for sure?

Yes, it runs rich at WOT to obtain the best power.  I'm not sure if running 
*exactly* stochiometric at WOT is bad for the engine, but the *process* of 
getting to stochiometric causes the engine to run lean some of the time. 
 Since the O2 sensor has such a abrupt change at stochiometric (not to 
mention the engine's uneven combustion from one cycle to the next), the 
control loop has to adjust the mixture lean-rich-lean-rich repeatedly so 
that the average is close to stochiometric.  I don't know how lean it goes 
during this process, but I wouldn't want to find out.

If you ever look at the O2 sensor voltage when the engine is running closed 
loop, you can see the voltage change from close to 0V to close to 1V a few 
times each second.  This is a slow loop, and has to be to allow for the 
delays from the fuel injector to the O2 sensor.  Now imagine the engine 
speed changing (accelerating) at WOT throttle.  The ecu trying to maintain 
stochiometric conditions using the O2 sensor would be nearly impossible. 
 That's why closed loop mode is used only at stable engine speeds (ie. 
cruise condition).

For a turbocharged car, running rich or even excessively rich under boost 
will reduce the chance of detonation.  I read somewhere that the highly 
turbo charged race cars used this technique to compensate for lower octane 
race fuel.

Bryan Zublin
bzublin at gi.com