4 Inductors - 4 Flyback Diodes OR 4 inductors - 1 Flyback Diode

I am trying to design a fuel injector driver using Arduino. I am however having trouble deciding how to arrange the flyback diode for the energy collapse at the injector turn-off (typically a 60 V voltage spike). A fuel injector is essentially a 9 mH inductor with 15 ohm resistor in series drawing about 1 Amp of current.
The maximum switching frequency in this case will be 100 Hz (driving MOSFET with an Arduino).
Tried to search the internet but did not get my answers.
Here are my problems: (also adding attachments)

4 injectors - 1 bigger MOSFET OR 4 injectors - 4 MOSFETs

I guess this one is up to my choice

Problem No 2. :
Flyback diode arrangement.

As far as I know, the circuit on the right is a Zener diode allowing the injector to discharge quickly, but to me, it seems a little dangerous, as not all power supplies can sink current. (2nd picture)

Problem No 3. :
Use 1 bigger diode OR 4 separate diodes?

This is the most important one for me. Not sure if using 4 diodes provides even distribution of the current, I feel like it would be a mess. (1st picture)

4 MOSFETs is safest - if you parallel the actuators they can have mutually circulating currents
if the inductances aren't closely matched.

Anyway injectors are meant to individually synchronized to each cylinder's intake cycle aren't they?

yup, they need to fire individually, matched to cylinders compression stroke before ignition on petrol, or at ignition on diesel (high timing precision needed to avoid detonation), on 4-stroke inline 4, usually pistons 1-4 and 2-3 move together but the valves are at different strokes, one is at compression while the other is expelling exhaust, one explodes while other intakes air, even if you fired in pairs, you would get fuel in exhaust on 1 pair at all times wasting it, and ruining the catalytic converter in the process when it explodes from exhaust heat, let alone firing 4 at once, quadrupling the gas mileage and damaging the engine... Getting the correct stroke is highly dependent on engine design, since the crank turns twice to complete the cycle, research your engine...

So, 4 mosfets with flyback each, also don't be cheap and use mosfets of double power rating at least.

why zener btw? to protect injector from overvoltage as well???

ultrafast avalanche rectifier diode antiparalel to injector is the way to go about this since they don't leak, (2nd picture left), on turnoff the diode shorts it and disipates the field into heat, zeners are usually too frail for this and more expensive.

The inductance becomes the source of oposite polarity, diode represents almost a short circuit to it, but not to the supply, so it can't interfere with anything else. Each their own, thats why its not messy.

Or you can buy diode integrated mosfets so the reverse current bypasses the fet (that would cause the supply to "feel" the transient).

or use a snubber circuit that uses capacitance to pass transients thru a resistor much like the antiparalel diode.

or maybe even all of it since it needs to be reliable
btw, be carefull of radio interference on control electronics, a car can be very noisy, but its a whole other conversation

how do you plan to time them, or calculate the pulse width for correct fuel-air mix?
you need sensors, engine position, speed, and mass air flow or manifold vacuum, lambda for fine tuning, and a pid program running the calculation, then you need a correct map

Why not use a part designed to drive the injectors, there are many sources. The Ti LM1949 "is designed to control an external power NPN Darlington transistor that drives the high current injector solenoid. The current required to open a solenoid is several times greater than the current necessary to merely hold it open; therefore, the LM1949, by directly sensing the actual solenoid current, initially saturates the driver until the “peak” injector current is four times that of
the idle or “holding” current. This dramatically reduces system power and keeps components cooler. Another option is the CY335 by Bosch Semiconductors. They have examples on there websites to save you the design effort. These devices are designed to survive to about 175C in the engine compartment, the Arduino is definitely not. Packaging will be of a concern to you as well.

the Arduino is definitely not. Packaging will be of a concern to you as well.

One could theoretically stuck it in the cabin, and have the driver close, but thanks for the post, I tend to forget there is a chip for almost every single purpose already made :slight_smile:

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