Injector Driver Module

[Greg Hermann]

Hi again, guys,

This switched power supply business sounds promising for injector drivers.

Be aware: With 8 sequence fired injectors on an 8 cylinder motor sized for
an 83% duty cycle at WOT you are going to have either 6 or 7 of them open
and drawing current at one time at WOT. That's how it is. If the duty cycle
went above 87.5% at WOT, you would have either 7 or 8 of them open at any
given time at WOT.

It really would also be nice to avoid the parasitic power use that would go
along with dissipating the power rather than controlling it.

It really would be a beautiful thing if the injector power supply were set
up so that the peak current, the length of the peak current pulse, and the
hold current were all three able to be controlled through software and
stabilized by the ecu at the software set values.

Perhaps a common injector power supply on the plus side (voltage adjustable
through software) and then individual drivers on the ground side capable of
either going saturated or letting through only the hold current after a set
(software adjustable) peak pulse time (without having to dissipate a lot of
power)? You tell me=--pwm with filtering, switched power supplies,
whatever.

The common power supply on the plus side would even be a good idea for
driving saturated coil injectors. Especially if it were capable of boosting
system voltage when required (cold starts?) or knocking system voltage down
for those fools among us who want to run a 24v electric system.

I'm not really a fool for wanting 24v. i'm doing an enormous motor project
for a motorhome, and intend to mount a Leece-Neville 195 amp @ 24v.
alternator on it, and use a 4KW Trace inverter to run things in the coach
when going down the road. Trying to run 12v. would verge on the ridiculous
in a case like this.

Want to bet that the stories out of Canada about saturated coil injectors
not working too well in cold weather originate out of what happens when
battery voltage is low from -40 or whatever, and gets lower when cranking?
Like low to the point that the injectors won't open when cranking? Sorry,
GM has sold lots of #$%^ that don't work--remember the Vega motor? early
Corvairs? And I even liked the 348!!

A voltage boost capability in the common injector power supply would likely
cure such syndromes.

It's really impossible to overemphasize how important absolute precision,
repeatability, and tunability is to operating the injectors for good
performance. See Passini's words about Weber's secrets, quoted elsewhere.
This is the place where it is worthwhile and will be rewarding beyond your
dreams to spend both design effort and money on hardware. I know I'm
preaching here, but have a little faith, and be rewarded!

I would even bet that one could run so-called saturated coil injectors with
a reduced hold-open current (with the type of drivers I'm talking about)
and make them run better and longer.

Have fun with the ideas!

                                        Regards, Greg








>Don, thanks for your reply.
>
>  I am going to fire them sequentially, like you indicate below. Yes, I
>also see that the factory heatsinks on the driver transistors are not big,
>and that is what surprises me.
>  The only explanation I have so far is that the injectors used by the OEM
>modules are the saturated type (16 ohms, and the driver is not working on
>the linear region, but saturated, and hence very little power is
>dissipated there). Unfortunately, I need to drive peak-and-hold injectors
>(1.5..2.3 ohms).
>  Over the 360 crank degrees, the injectors will be fired approximately
>like this:
>#1 (0 degrees), #8 (45 deg), #4 (90), #3 (135), #6 (180), #5 (225), #7
>(270), #2 (315).
>
>  Every driver would be on up to 85% of the time, so no matter what the
>timing sequence is, every power driver will conduct the Hold current (1
>amp) for 85% of the time, so the average power dissipated on the driver
>module box is:
>  [ (Battery Voltage)-(Hold Current)*(Coil Resistance) ]*(Hold
>Current)*(Duty Cycle)*(number of injectors) =
>  so assuming worst case (Maximum battery voltage):
>  [ (16)-(1)*(1.5) ]*(1)*(0.85)*(8) = 98.6W!
>
>  How much power is being dissipated in your module?
>
>>>> <Don.F.Broadus at ucm.com> 08/10/98 10:47AM >>>
>I'm working on an  injector driving module also. If you are going to fire
>the injectors sequentially, then one injector would be on at a time in the
>firing order sequence (1-8-4-3-6-5-7-2 GM V-8)  If you are going to batch
><snip>