Sensor Questions?

[Craig Pugsley]

> > its hard to build a non  microprocessor based fi system without
> > a maf ;   think about it.
> I disagree.  It would be quite simple.  The airflow into the ports per 
> revolution is almost directly related to the manifold pressure for most
> engines with a relatively flat volumetric efficiency characteristic.
> 
> A simple linear mapping between the MAP signal and the injection period 
> with injection timed to occur once per revolution works well.  

This is how (some??) microtech systems work. The steady state injection on
time is only dependent upon the MAP value, not RPM apart from it
triggering the timer (But when on boost there is some RPM compensation)

This assumes that the VE is linear across the RPM scale. (But when on
turbo/supercharger boost there is some RPM compensation). I presume that
this means if the 'slope' is set at RPM of max VE, it will be richer
than nessecary when the VE is lower. As long as this over-richness isn't
hugely too much it should be OK

> Injection every ignition pulse tends to magnify the injector dead time 
> non linearity.  Low load conditions such as idle tend to be erratic since
> the injector pulse widths are close to injector cutoff.

Hence staged injectors eg in 13B turbos, due to wide fuelling requirements

FWIW, the microtech is an analog unit with screwdriver adjustments. I'd
suggest it's a reverse engineered L-jetronic (or similar) using readily
available commercial parts, and a MAP instead of a trapdoor air flow
sensor. It is the cheapest adjustable EFI computer available, at $800aus
($~600 us), but isn't as flexible as the programmable stuff (Haltech,
Motec, Wolf etc).

Cheers,
Craig.


Glossary :-)
MAF = Mass Air Flow sensor (hot wire)
MAP = Manifold Absolute Pressure sensor
RPM = Revolutions per minute
VE  = Volumetric efficiency (how much air the engine can take in as a % of
      the engine capacity
13B = Mazda wankel engine
FWIW= For what it's worth