DIY_EFI Digest V4 #680

[Greg Hermann]

>I haven't seen any specification information that states what the maximum ON
>time is for an injector before damage occurs and this is the data that actually
>determines the extreme end of the duty cycle in the terms of maxium on time.
>After all,  if the duty cycle were restricted to one turn of an engine then at
>600RPM I have, with 80% duty cycle,  a possible pulse width of 80ms which
>is far
>higher than the 12.8ms that I am currently restricting my injector too.

I would say that 80 or 85% is a good maximum duty cycle--at ANY engine
speed, for the cooling reason, and also just to leave a bit of head room at
the top end.

2 ms minimum on time and 2 ms minimum off time, again at any engine speed,
is a decent rule of thumb to avoid fuel delivery inconsistencies.
>

>
>As an aside,  to add my own urban legend,  I've run the Honda injectors
>for more
>than an hour at 6600RPM (without fuel) in the open air using a 12.8ms
>pulse rate
>and 73% duty cycle.  At the end of an hour the injectors are certainly hot to
>touch but not so hot as to burn and certainly not as hot as I would assume the
>area above the intake valve is on an engine at 6600RPM.  I have't tried running
>them with fuel for an hour but I would imagine that they would run cooler.
>
>The stumble only occurs when the throttle is snapped open as fast as possible.
>Normally on aircraft this isn't a good idea but on a hovercraft I could
>see some
>yahoo doing this.  When the stumble occurs the O2 sensor goes off the scale in
>the lean direction so I assume that I should probably make the maxium pulse
>width RPM dependant so if the throttle is snapped open at idle, like an
>accelerator pump, I should give the injector a 40ms pulse (80% duty cycle at
>1200RPM) or so every engine revolution.

Are you saying that you have written your program to restrict the injectors
to 12.8 ms on time, regardless of engine speed?? If this is the case, and
you are getting the correct mixture at 6000 rpm, it is virtually a sure
thing that you are going to be lean at the torque peak (4500rpm??) . The
torque peak will always require a longer on time than the HP peak (more
fuel is needed PER revolution at the torque peak than anywhere else). (I
know, not quite true if the maximum bmep and maximum friction loss torque
do not occur at exactly the same engine speed.

If you are programming in a maximum injector on time of 12.8 ms, I would
bet that this is the source of the lean transition. The nature of the load
curve for the hovercraft (fan, I presume?) is HP increasing with the cube
of the rpm--so--the lean problem MAY only be rearing its head on a
transition, only because you do not have enough steady state load available
at the torque peak to create the full fuel/air demand that the engine is
capable of at that engine speed.

Greg