Tony Cellana wrote:
>Makes a little more sense now.
>
>Next question: what will the lower voltage (due to higher impedance) do to
>the injector in terms of cycles per sec, reaction time, etc?
>
>
Actually the voltage doesn't change at all (as someone incorrectly
mentioned). The way the injectors are wired is with one lead (doesn't
matter which one) hooked up to FULL-TIME (regulated) battery voltage.
This never changes the entire time the ignition is on. The only thing
that happens is when the PCM/ECU gets ready to fire the injector, it
grounds the other lead thus completing a circuit such that current can
flow. Since voltage is staying constant, then a change in resistance
will ONLY affect the current (ohms law V=IR or I=V/R, etc). Now change
this resistance by switching to a different injector and you have ONLY
affected current. Most injector drivers can easily withstand either
little less or more current.
This is not where the problem is. The problem is impedance and
reactance. Impedence can only really be described as a multiple-term
value, a vector-based quantity, usually given in polar or ?? (can't
remember the other) coordinates. What is being offered is really the
resistance of the injector - as measured by a voltmeter set to measure
resistance or "ohms" and leaving out any other (much needed)
information. Words like impedance, inductance, and reactance are used
to describe the behavior of inductors/coils and have multiple-terms to
describe them like ohms and henrys or millihenrys. An injector is
actually more of an inductor than a resistor but has qualities of both.
This number doesn't really tell you much at all about the electrical
characteristics of that injector. You can only find the real
characteristics if you run some current (at some potential voltage)
through the device. Then you will discover the monster that kills
ECU/PCM's. This monster is flyback current. Look at it this way:
Suppose you have some low-imp 65lb/hr injectors - eight of them.
Depending on how you hook them up (individual, series, or parrallel) you
will have lots of current going through them in order to operate them.
Say these injectors are 2.0 ohms each. At 12V potential, when these
injectors are opened, alot of current will be driving them (I=V/R)
compared to say 12 ohm injectors (6 amps instead of 1 amp each). So why
aren't they all one class or resistance? Because it takes lots more
current to slam open big injectors quickly and then shut quickly such
that large amounts of fuel can be delivered. It's this rapid slamming
of large currents that causes the problem. Imagine if you were pumping
6 amps into an injector and then opened the circuit (ECU/PCM lifts
ground disconnecting the ground return path). There is a law of physics
that says "objects in motion stay in motion unless acted upon by other
forces". All this current rushing through the injector holding it open
is shut off rather violently. This current tends to "pile up" and then
it has to go somewhere so it goes the only place it has to go (remember
the ground path is gone now) back up to the injector driver chip(s).
Anywho, this is the flyback current and can be deadly to most injector
drivers if not managed in some way.
The way the megasquirt handles this is via control over the drive
circuit (as well as add-on components to minimize/eliminate this flyback
current in extreme cases). This control is user-programmable. For
example, MS drives high-imp injectors basically wide open as relatively
small currents are involved. When using low-imp injectors (especially
ones in the 65+ lb/hr range) you can "tune" the control of their open
time, open percent, close time, and close percent. You might first
choose to send a 75 percent PWM signal for about 1ms in order to fully
open the injector and then back down to say 25 percent PWM for the
remaining open time for holding it open. If that generates too much
heat in the drivers, you can change to say 65 percent PWM to open and
maybe 20 percent PWM for hold (also can change the duration of the
initial "peak" or open time). These characteristics also affect things
like idle, acceleration enrichment, max pw, min pw, WOT current
limiting, etc.
>Or is it a matter of the additional voltage required to drive them causes
>additional heat in the PCM circuitry, and therefore a higher
>percentage/shorter period to failure in the PCM?
>
Again, see above and remember that voltage is not even in the picture
here and can be ignored because it is held constant. BTW, here is the
definition of impedance:
*impedance:* The total passive opposition offered to the flow of
electric current. /Note 1:/ Impedance is determined by the particular
combination of resistance, inductive reactance, and capacitive reactance
in a given circuit. (188) /Note 2:/ Impedance is a function of
frequency, except in the case of purely resistive networks
Shane
Tony
-----Original Message-----
From: Bernd D. Ratsch <bernd@texas.net>
To: dakota-truck@dakota-truck.net <dakota-truck@dakota-truck.net>
Date: Wednesday, August 20, 2003 11:35 PM
Subject: RE: DML: RE: RE: injectors
"The ECM has power transistors (also called injector drivers) in it to
control the switching of the injectors ON and OFF. These injector
drivers can only handle a certain amount of power or they will fail.
This power is related to the injector impedance. A high impedance
injector will result in lower power passing through the injector
drivers. As injectors get larger, their impedance drops and the power
increases. Make sure the ECM can handle the impedance of the injectors
you install or you could fail the ECM. "
The injectors on our trucks are "High Impedance" - range = 12-16ohm.
("Low Impedance" are from .9ohm-4ohm).
When you change the resistance, you're also changing voltage. (Ohm's
law)
- Bernd
-----Original Message-----
From: owner-dakota-truck@bent.twistedbits.net
[mailto:owner-dakota-truck@bent.twistedbits.net] On Behalf Of Tony
Cellana
Sent: Wednesday, August 20, 2003 7:47 PM
To: dakota-truck@dakota-truck.net
Subject: Re: DML: RE: RE: injectors
And the difference in impedence affects them how?
TonyC
-----Original Message-----
From: Bernd D. Ratsch <bernd@texas.net>
To: dakota-truck@dakota-truck.net <dakota-truck@dakota-truck.net>
Date: Wednesday, August 20, 2003 8:44 PM
Subject: RE: DML: RE: RE: injectors
>Not mistaken:
>
>Impedance
>Factory Injectors: 12 +/-1.2ohms (Factory Spec - '97-'04)
>FMS "High Flow": 14.3-14.5ohms
>Design-III: 15.2-15.3
>
>While you can run different impedance injectors, going to far from
>factory will cause issues. We tested the Design-III's (Orange) on my
>'97, a '98 5.2L, and on a '99 R/T. All of them showed the same problem
>- worked with Bruce Holt on that as well as he didn't know (at that
>time) if they would work properly or not. (This was over 1 year ago.)
>
>- Bernd
>
>-----Original Message-----
>From: owner-dakota-truck@bent.twistedbits.net
>[mailto:owner-dakota-truck@bent.twistedbits.net] On Behalf Of Bob
>Mankin
>Sent: Wednesday, August 20, 2003 10:15 AM
>To: dakota-truck@dakota-truck.net
>Subject: RE: DML: RE: RE: injectors
>
>
>
>I think you're mistaken. If anything I would expect the Design III to
>perform better. Better atomization and resulting mileage. Impedance is
>close enough. If you want to argue that it's not exact match then your
>suggestion of Accel or FMS won't fly since they are not an exact
>impedance match either.
>
>Bob
>
>
>
>
>
This archive was generated by hypermail 2b29 : Fri Feb 06 2004 - 11:46:48 EST