[cgrey8]

I've been reading around and it seems there's a cheap way to go and an expensive way to go. The expensive way is to buy a low-impedance injector driver that will fire the injectors when the EEC triggers it.

The cheap way is to simply put 8ohm high wattage ceramic resistors in-line with the injectors to act as a current-limiter. Evidently low impedance injectors don't need the amount of current that is produced by 12-14v across their terminals so running them at a lower voltage with in-line current limiting resistors works. However has anybody worked with low impedance injectors to know for sure whether this will work? Here's the thread I got the idea from:
SRT Forums > How to make a resistor box for low impedance (ohm) injectors.

BTW, the plan is not for me to use low impedance injectors on my 331 build. The plan is to use CNG injectors which are low impedance controlled from a Ford EEC.

[cgrey8]

In further reading, the low impedance driver/converters are offering some function called Peak&Hold that the resistor packs evidently don't do. There's evidently a difference between something called Saturated vs Peak&Hold.

I'm still searching as to what exactly the difference is. I'm sure there's other good info on the NET but here's one thread I'm reading for this info:
Saturated vs Peak&Hold Injector Drivers

Here's the most relevant details from that thread:

Some of you may be wondering what the difference is between high and low impedance injectors. High impedance injectors are used with Saturated Drivers. Low impedance injectors are used with Peak and Hold Drivers or with Saturated drivers when used in conjunction with a ballast resistor (resistor box). The following, taken from SAE J1832 Nov 1989 - Gasoline Fuel Injector, defines each method and also lists the advantages and disadvantages to each.

SA

Injector Driver —An electronic circuit that supplies voltage pulses to an electromagnetic fuel injector for a precise increment of time and at a given repetition rate. The accuracy of these pulses and their repetition is normally ±0.001 ms. The peak-hold driver and the saturated driver are most commonly used by the industry for vehicle applications.

PEAK-HOLD DRIVER—A driver that uses two levels of current to operate the injector. The driver circuit applies battery voltage to the injector until a predetermined current level is reached. The current is then reduced and held at a lower level for the duration of the PW. This type of driver is normally used with injectors having low resistance coils (typically around 2 ohm). The accuracy of the driver peak current level (Ip) and the hold current level (Ih) is held to ±0.50%.

Advantages—The high peak current minimizes OT response and the low hold current minimizes CT response. This method of control results in an increased linear range of injector operation.

Disadvantages—Heat is primarily dissipated at the driver. Circuitry is more complex than that of the saturated driver.



SATURATED DRIVER—A power transistor driver that turns fully on for the entire duration of the injector PW. This type of driver is used with injectors having high resistance coils (typically 12 to 16 ohm) or with injectors having low resistance coils in combination with a ballast resistor.

Advantages—Heat is primarily dissipated through the injector or ballast resistor and not at the driver circuit. Circuitry is simplified compared to the peak-hold driver.

Disadvantages—The inherently slower dynamic response of this system decreases the injector’s usable flow range. The Q of an injector used with this type of circuit is more duty cycle sensitive due to heat dissipation considerations. This driver’s inductive suppression, which may be resistance, capacitance or zener, significantly affects the injector’s Qd rates due to variations in the circuit’s current decay rate. This decay results in a change of the injector’s closing time.

Basically with the ability to tweak with the Injector Offset Curve, Injector Slopes, and Breakpoint, you should be able to run a current-limiting resistor setup and avoid the high cost of Peak&Hold drivers. The only sacrifice will be a small amount of the MAX possible flow of the injectors due to "sluggish" opening. I'm wondering if there's some additional circuitry you can add to alleviate this like a strategically located capacitor on each injector. But I think this'll be good enough.

[liljoe07]

I believe it has to do with the responsiveness of the injector in a way. Low impedance injectors generally react quicker than high impedance.

[cgrey8]

Just posting info...

I took some EV1 and EV6 high impedance injectors along with my CNG low impedance injectors to work to have their impedance measured using an LCR meter. The meter was setup to test at 120Hz and was testing both inductance and AC impedance.

Using a standard DC VOM resistance meter, the EV1 measured 14.5 ohms. On the LCR, they were closer to 17 ohms with ~15mH inductance.

The EV6 measured 12.5 ohms in DC mode. However on the LCR, they were closer to 15 ohms impedance, although the inductance was noticeably higher, in the 22mH range meaning these injectors evidently have more windings than older injectors and thus will give a fair amount more kick-back when de-energized.

The CNG injectors are low impedance and are stamped 3 ohm right on them. Using a DC VOM meter, I found them to be in the 3.3-3.4 ohm resistance range. On the LCR, they were around 5 ohms impedance and only 8.4mH inductance meaning these will give significantly less kick-back than a high impedance injector will. So no parallel or zener diodes will be necessary. The EEC's built in protection will be more than adequate to handle any kick-back they could offer.

Now note the LCR only uses a minor amount of voltage to test the inductance and impedance with. There's not nearly enough voltage or current to actually energize any of these injectors. In a real circuit, the actual impedance experienced to be seen by the EEC could be lower. The inductance in the injector coils will present as a high resistance when voltage is 1st applied to the injector. However when the coil is energizing as well as doing real mechanical work, that changes the equation of things. The movement of metal (the injector's pintle) within the field of a coil will affect current flow is a positive direction. Not only is the inducing voltage having to energize the coil, but it is also having to move the pintle AND as a result of that movement is overcoming spring pressure pushing the pintle closed...all of which requires current to do. Likewise when voltage is cut off and the pintle is dropping, that movement of the pintle is going to act something like a generator throwing current the coil by itself wouldn't have thrown back out into the circuit. To the circuit, all this looks like higher inductance. But since I didn't have the equipment to do a full-voltage analysis of these injectors, I wasn't able to do a TRUE analysis of them. But Googling around, I was able to come up with someone who did do some full voltage tests of some injectors. If you are interested in what he found, check out this article:
Test Injector Inductance

Towards the bottom of that article, you can see oscilloscope graphs of what voltage and current do over time. I found it interesting that the "mechanical work" is done first. Then once the injector has been opened, the coil continues to energize.

It's amazing how much info there is to know or that you can know on what should be a simple aspect of fuel delivery.

As it relates to the project though, the only concern with in-line resistors on these low impedance CNG injectors will be if there's enough current making it to them to open them at a consist rate. If they are being opened at inconsistent rates, then that could be a problem at idle and thus would require I move to a more technically correct approach of using a true Peak-n-Hold driver which would get me consistent open events as is an absolute necessity with traditional high flow gasoline injectors. Assuming the CNG injector rails and the regulators are equally sized, I estimate these injectors to be equivalent to gasoline injectors in the 20-30lb range. Thus the injector PWs will be about the same as you might expect with a 24lb injector. Idle PWs will not be super-short so some sloppiness is tolerable.

[LeeH]

The difference is the control circuit, think of saturation as a light switch, it's either on or off. On a peak and hold, the sol. is hit with higher initial current to "latch" it, and then the current is tapered off as it takes less current to the coil to hold it open, this not only provides better control of the injector, but is easier on the control circuit.

[cgrey8]

The circuit you are describing is a Peak-n-Hold which is what low impedance injectors are designed for. However when what you have is not Peak-n-Hold, saturation is all you can do. And if the injector opens a little less snappy, hopefully the injector size is small enough that the variability in open time is ignorable. Since low impedance injectors historically are only found on high flow injectors, running a low flow (less than 30lb/hr) injector as a high impedance injector should be fine. But we'll see.

[LeeH]

The circuit you are describing is a Peak-n-Hold which is what low impedance injectors are designed for. However when what you have is not Peak-n-Hold, saturation is all you can do.

Correct, I was just pointing out that installing a resistor will do nothing for the control circuit, saturation is the method used in this EEC, and a resistor does nothing for the driver design.
Present day technology allows us to have larger high impedance injectors, so running low imp. injectors in a saturation circuit is not something I would encourage. Fuel delivery is all about proper and consistent control of the injectors, the peak and hold circuit is just much better and efficient at controlling the injectors, especially very large ones.

[cgrey8]

...Present day technology allows us to have larger high impedance injectors, so running low imp. injectors in a saturation circuit is not something I would encourage. Fuel delivery is all about proper and consistent control of the injectors, the peak and hold circuit is just much better and efficient at controlling the injectors, especially very large ones.

Agreed, running low impedance injectors this way certainly isn't ideal by any means. And as you said, today it's mostly not necessary since very accurate high impedance high flow injectors are now available. However in my case of a CNG conversion system, it's unavoidable since my CNG kit came with low impedance injectors without the option of high impedance injectors. Most CNG kits come with their own computer to handle fuel delivery. However for this project, I'd like to get the Ford EEC to control them.

And from what I can tell, these are not exactly high flow injectors so open control of the injectors shouldn't be as critical as it absolutely is on big high flowing injectors. Thus a lot of the reasons why you would use a low impedance injector doesn't really make sense here unless it boils down to cost. Perhaps low impedance injector coils are cheaper to manufacture in China so they opt for the added cost and complexity of a low impedance peak-n-hold circuitry over making the coils high impedance? I don't know. The company also makes 1 ohm injectors too, but the website doesn't really indicate what makes those different/better than the 3 ohm versions I have.