DIY_EFI Digest V4 #679

[John Dammeyer]

Hi Phil,

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.

Now I realize that's a bit overstated but I understood that the 80% rate is to
avoid two things:

1.  Injector overheating.

2. Injector float due to signal removal and re-application at a rate faster than
the voltage can decay in the coil due to coil inductance.  The injector then
positions itself at an indeterminate state because the energy never really
leaves the coil before the next application of voltage.  This is one of the
places where Peak/Hold injectors are far superior to Saturated.

Please _do_ post a copy of the injector data sheets that state the maximum on
time of an injector before damage will occur.

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.

BTW,  I'm also running sequential injection (not bank fire), so in fact at low
RPM I'm firing an injector every 180 crank degrees.  You could balance a coin on
the engine it idles so smoothly.

Cheers,

John



>Date: Tue, 07 Dec 1999 03:12:56 +1100
>From: Phil Lamovie <phil at injec.com>
>Subject: Re: cycles
>
>Hi John et al,
>
>
>...but an engine cycle is 720 degrees and therefore, at 7000RPM,  with
>an 80% duty cycle the max pw is 13.7ms.
>
>Yes John your quite right a 4 stroke IC engine has a working cycle
>that
>encompasses 720 degrees of crank. On the other hand an Injector is
>not any IC engine and has a duty cycle that is of one revolution only.
>
>This does of course give rise to the question... are you only
>injecting every second revolution of the engine ? If so that may
>establish part of the reason for the stumble you are experiencing.
>
>Phil
>
>