Automotive Electronics - How to cut fuel injectors

[Popee]

Hey everyone Im new here! I was wondering if you could help me solve a problem. I am developing a Traction Control System for a Formula Student RaceCar www.formulastudent.com

As an Electronic Engineer I head a 2 man team that must design a digital dash and traction control system utilizing a CAN network on the car.

The CAN network and digital dash are almost complete, I am however having one problem with regards to Traction Control.

For the traction controller I am using a PIC18F1320 to read wheel speeds from the CAN network, calculate slip and adjust the firing sequence of the injectors accordingly.

My problem lies with actually cutting the injector pulses so the injectors do not deliver any fuel.

When I was developing the system I designed it with the assumption that the injector is permanently grounded and the Engine ECU supplies the injector with 12v when an injector is required to deliver fuel. The injector cut would be achieved by this simple system.


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However, after reading further literature on websites and texts books; it is evident that in most cases the injector is permanently connected to 12v and the Engine ECU grounds the injector when it wishes to deliver fuel. This is where I am stuck; I am unable to come up with any satisfactory way to interrupt the signal. This is one idea I have:

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The TCS has an input pin that measures the voltage across the engine ECU, when the Engine ECU wishes to fire the injector the voltage will drop as it is now connected to ground. Once I have detected that I could switch off a power transistor in the Traction Control unit keeping the injector turned off; however how would I know when to return the injector back to its normal state? Do I just wait two revolutions of the engine or do you have a sensor that detects when the Engine ECU intents to turn off the injector by returning to a high impedance state?
Im not sure if this will work or if I am over complicating things.

I don’t really wish to try any of the above methods on our engine as it is a brand new engine costing $6000 and if I get it wrong and manage to mess up the fuelling I could destroy the engine and the team has no money for a replacement engine, which means our 25+ strong team would not get to race at Silverstone this year after a year of dedication.

I apologise for the Rant and I hope someone understands what I am trying to acheive as im finding hard to explain it.

Any help would be much apprechiated.

regards,
Lee

 

[divideby0]

I would still be looking at your first scheme.
Put a power FET in series with the +12V there.
The gate normally turned on by a resistor.
Everything should now run normally.

When needed, your tracking control then turns off the FET and injector by putting a 0 (grounding) on the FET gate (check mA sinking capacity of the PIC tho).
The MCU still tries to turn the injector ON but there is no +12V for it.
This scheme is nice because it is all referenced to "earth" and the MCU doesn't know this addition is in there.

Be interesting to see what happens if the injector is ON at the same time as your unit turns it off!
Probably could be made to work as a nice smooth speed limiter (such as we need and should be mandatory for learner drivers).

You could try the FET between the injector and the MCU but this is not so nicely referenced to "earth" ie would the gate "bypass" go to the top of the MCU or to "earth" with the MCU still mixed in there. Also you would not have the nicety of one end of the FET being at + (for ease of bolting to the heat sink).

Can you try this on some other clapped out motor first?

 

[Popee]

Divideby0 are you still refering to the first scheme?

Unfortunatly I am unable to use that scheme as the injector is wired permantly to 12v. I have seen other traction control systems set up as diagram two. Im unsure how to detect and disrupt the pulse.

If possible could you draw a circuit diagram?

regards,
Lee S

 

[VVV]

I believe also that a MOSFET on the +12V side is the way to go.
What does "permanently connected" mean? After all, you already make so may modifications to this car. You should be able to effect one more, even if it takes some inventivity.

 

[Popee]

Thanks for the information guys. I think I have this one sorted now:

**broken link removed**

 

[divideby0]

Yes I thought that you would be able to get into the +12V feed to the injectors.
This would be a much nicer place to fiddle as the unit is then electrically tied to the +12V and to ground as far as bypassing electrical noise.
Also you could add a simple bypass switch or jumper during testing.

When you add the pullup resistors and the grounds and +5V to the PIC for your proposed unit it looks more prone to unexpected results.
Won't you have to do this for each injector?

You probably know that trying to make anything work reliably in an engine bay is veeeeery tricky.
Just so many huge spikes, electrical noise, heat, oil, vibration.
Needs shielded leads, cap bypasses everywhere and a Kryptonite box.
Neighbor has a Peugeot which has an access card with driver's preferred seating etc and it also presets the engine power output.