Injector Duty Cycle.

[John Dammeyer]

>
>Date: Tue, 7 Dec 1999 07:52:41 -0700
>From: bearbvd at cmn.net (Greg Hermann)
>Subject: Re: DIY_EFI Digest V4 #680
>
>>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
[snip]
>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.

I think that's an unreasonable assumption.  I've found that at idle using the
stock Honda injectors (24lb/hr) that idle on a warm engine uses about 2ms pulse
width for Stoich.    I found when I tried to use 4ms as my minimum time that the
engine barely ran.  It may be that the injector at that pulse width is not
opening completely but adding a too large value into the total injector time
screws up all the other calculations.


>>
>
>>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.
[snip]
>Are you saying that you have written your program to restrict the injectors
>to 12.8 ms on time, regardless of engine speed??

yes.

> 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.

In the Honda's case the peak torque is up at 5000RPM which makes it so nice,
with a 2.4:1 reduction drive, for aircraft.  The Hovercraft Fan is pitched to
provide maximum thrust at 2100RPM so this works out nicely.  And yes,  the
mixture at 5000RPM is not 12.8ms.  It's less.  In fact watching the EGT it's
fascinating to adjust the pulsewidth at that RPM by a tiny amount and watch the
EGT go up as the mixture is leaned out and the EGT go down as the mixture is
richened.  Aiming for a particular EGT and O2 value has demonstrated that
adjusting the fuel on either side of optimum causes an increase in either EGT or
fuel usage without an increase in engine speed.  This is a WOT.

>
>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.

At 4000RPM and a partially open throttle, a sudden open to WOT creates a nice
smooth (and quite fast) acceleration to 5600 with the Hovercraft fan so I'm not
having acceleration enrichment problems with partially opened throttle.  Just
from idle to 2000RPM with a barely open throttle to WOT.

The 12.8ms is to prevent a 'dramatic' cancellation of injection if a timer
interrupt is missed because the injector is told to turn on (via compare with a
register) after the compare point has already passed.  It's the way I wrote the
software.  Perhaps I could have done it differently but I had one compare
register to share among 4 injectors.  This works fine if Injector 1 has to turn
on before Injector 3 but if the mixture requirement changes so that on the
intake stroke for Injector 3 a lot more fuel is required then it's possible that
Injector 3 may actually have to turn on before Injector 1 because Injector 1
value was calculated for a different mixture requirement.  The timer is already
programmed to turn on Injector 1.

The solution has been to immediately turn on Injector 1 if the turn on point for
Injector 3 is earlier than for Injector 1.  But this type of transition doesn't
appear to occur under 3000RPM since I've set my maximum pulse width to 12.8ms.
What I will try is to use the RPM as an index into maximum pulse widths so that
I can set a larger pulse width for acceleration at lower RPM.

Perhaps I should phrase a question in this way.  For folks working with 1.6L
engines that are fuel injected,  what size injector do you use and how long an
injection pulse width do you use if the engine is idling at 1000RPM and the
Throttle is suddenly moved to WOT?  ie: What Acceleration Enrichment do you use?

Regrds,

John