More fuel injector questions

The higher the voltage at the injector, the faster the magnetic field will collapse and the faster it'll close.

75V means that they're using transistors rated for 100V or so, which means spendy. It also means that the magnetic field is collapsing way faster than it was generated. So they're at least tuning the turn-off time.

So can I just use a regular diode like I would with a relay but with a resistor in series with the diode if I want to have the injector close slower? Eric

A resistor in series with the snubber diode makes the injector coil's magnetic field collapse -faster- by dissipating the stored energy that drives that current. Whatever current was flowing through the coil when you switched off the power continues to flow through the diode, whose purpose is to prevent that current from generating a high voltage surge, like an ignition coil or a water hammer in plumbing. The resistor absorbs the energy from the current faster than the diode drop alone. I've seen a relay stay closed for 15 seconds with only a diode drop impeding the current.

-jsw

A resistor won't do it - it will just make the injector draw more power. The relay won't act fast enough. I'd use a diode backwards across the coil - a 1n5404 or 5406 will be plenty. Might get away with a 1n4004 (400 volt 1 amp)

You could - but you really want it to close fast. A resistor could limit the current so you can use a 1n4004 instead of a higher current device.

If you will be installing multiple injectors & drivers, you can also use fast diodes returned to a common "supervoltage" rail, which has a capacitor & Zener down to either ground, or the DC+ rail. The supervoltage rail may float at +75V (say), being charged by the "flyback" pulses from the injectors. This idea comes from an old Epson application report, directed at dot-matrix printers (not so different, if lower power.) If you do this, you will quickly see just how much energy is wasted in heating that Zener. There are ways to reclaim that energy, & return it to the main DC rail, effectively using it to help fire the next injector in sequence. (Sorry I can't discuss the details: they were the property of my employer.)

Thanks Clare. I know that in an engine the injector needs to close fast but in my application, applying tiny amounts of cutting lubricant to a spinning tool, I can get by with a slower closing speed. What I do want is for the injector to last as long as possible. So as long as the injector isn't closing any faster than it would in a car I'm happy. I wonder if the higher viscosity of the lube will be cushion enough? Eric

Well, I don't know how you usually connect a diode with a relay.

The _higher_ the voltage across the coil, the _faster_ the current will decay.

A buck converter, from the 75V rail down to battery, with the feedback coming from the supervoltage rail (because you're trying to regulate that voltage and not the battery voltage).

Not completely easy-peasy, but it should be possible with a handfull of off-the-shelf parts.

If you're talking about a circuit like this, then the HIGHER you make the resistor value, the FASTER the current in the coil will decay, and the FASTER the injector will close.

If you have a website or some other easy-to-access reference that says differently, that was written by someone who actually knows their physics (i.e., not just some columnist for a car magazine), then I'd like to see it.

+12V o ----------------o-----. | | | | .-. | resistor | | | | | | '-' C| injector | C| coil | C| | | - | diode ^ | | | | | o-----' | | | ||-+ ||

I would obtain all four injectors from a junkyard engine and test the worst looking for the minimum voltage and current to open it and the maximum that burns it out, or at least see if it withstands 12V to 14V continuously. Then I would measure the steady state flow rate versus the fluid pressure, or height of the unpressurised reservoir if it's enough.

Once you confirm that they are acceptable and know their limits you can refine your design.

-jsw

There is nothing that will be dammaged by slamming the injector closed, and a slow close can allow the nozzle to drip rather than sprat at the end of the injection - you want the lube on the work, not running down the lube application device.

Unless the reservuir is several hundred feet up, the pressure won't be adequate. He will need a pump, and likely some sort of regulator. a simple pop-off would be adequate and simple.

True. IIRC GM fuel injectors run at about 45 PSI. Any lower, and the spray pattern will be poor. Another IIRC, disc type injectors have a wider spray pattern. Using a pintle type will provide a narrower pattern. If you can get injectors from a GM throttle body, they operate on about 12 to 14 PSI.

And flow a LOT more.