Fuel Injector Transmission Line Model

[acamer14]

Hello to all:

With all of this talk recently about buying electric vehicles, I thought I would see if I could improve upon a 27 year old design. I have a 1995 S10 2.2L I4. I wanted to play around with the idea of making the computer stop injecting fuel when I completely let off of the throttle. Modern day vehicles do this. Obviously there are some issues making sure the engine does not cut out when in neutral or the clutch is press in. Also, the signal from the O2 sensor will have to be intercepted.

I want to put a relay before the injector on the 12V line. The engine computer has the 12V at the injector then the computer pulls the line low, completing the circuit making the injector fire. I am sure it does this with a internal MOSFET and supporting circuitry.

Here is a rough circuit diagram:
+12V ------ injector ---------- Computer

As Modified:
+12V ------- Relay -------Injector ---------Computer

I was talking about this to a friend and he said, "The resistance will burn up computer." High resistance would cause lower current and would not "burn up" anything. I believe what he was referring to is impedance. The injector is fired as a pulsed circuit making the wiring an AC circuit transmission line. Impedance matching on transmission lines is very important to avoid voltage spikes, both positive and negative, and reflections. However, I remember reading a rule of thumb. If the length of the transmission line is electrically short, <1/10 of a wavelength, it is not considered a transmission line and impedance matching is not important.

Assuming the redline of the engine is 6000 RPM the injectors fire with the cam shaft at 3000 RPM. That is 50 Hz. The wavelength is around 6,000,000 Meters. The wiring is significantly shorter than one-tenth wavelength. By the rule of thumb transmission line characteristics do not matter. However, I feel like reflections still happen. At short distances the reflections still happen but decay fast enough to still transmit the signal.

I would like to gather your thoughts on this. Do I need to take into account transmission line characteristic? Should I put a zener diode at the computer to source and sink positive and negative reflections? How would you handle this if it were your vehicle? Anything else I should know that I have not mentioned?

Thank you ahead of time for your participation and thanks for reading my long post.

 

[BrianPetersen]

Are you sure it doesn't already have "deceleration fuel cut"? It's not hard to implement, even for a 1990s-era ECU. It probably already has that, to forestall catalyst overheating due to misfire during shut-throttle overrun conditions.

 

[bradrs]

Why would the resistance burn up the computer? What resistance?

One thing you'd need to worry about potentially, is engine diagnostics. This is not unsolvable, and probably also not worthwhile. It's been done plenty of times before with commercially available products. If you take away the 12V supply, the diagnostics will not see that on the injector driver line with the injector off. That will be a real concern. I don't know off the top of my head what that particular ECU would have in it for injector diagnostics. But unplugging an injector and running the car should tell you.

You should be able to toss any concerns about impedance matching the lines out. The injector is on, or its off. If you try to interrupt the power to the injector while its injecting, you can have a lean injection, and possibly an unwanted voltage spike then. That spike isn't from transmission lines, but rather from the inductor in the injector when you suddenly chop the current off going through it. More of the commercial products focused on intercepting the driver side of the injector signal, so that they can sense when its off, and then not turn on. That way you don't shut off the injector in the middle of an injector cycle.

 

[LionelHutz]

Go to tuning forums and see if you can reprogram the ECM to enable DFCO, if it's not already enabled. It should be part of the the ECM programming.