Piezo injector operation. Current vs voltage

[Technician]

Hi guys
Based on what I've read so far in this thread is this guy incorrect?

https://m.youtube.com/watch?v=fPB5_9o11tI

Pico themselves have a ranage of videos, one such one is here;

https://www.youtube.com/watch?v=4MnP5koZH9k

Frank Massey also have done a video here;

https://www.youtube.com/watch?v=DFLc7hEKnUk

[Darren Fraser]

Thanks Technician
Frank Masseys video was very informative and backs up what Steve said earlier about the reverse current is actually the piezo stack reverse bias into the injector circuit once power is switched off by the ECM.

Barnaby Donahue on the other hand from the Pico Technology video implies that there are two separate events from the ECM to the injector, one to open the injector, and an additional event to close the injector.

Listening to Barnaby Donahues presentation I would be inclined to agree with the author of this thread that there is a separate event from the ECM that reverses the current, but listening to Frank Masseys presentation I would be inclined to agree with him and Steve that the reverse current is the result of the piezo discharging back into the injector circuit once the power supply to the injector is switched off.

I've done A LOT of reading this weekend on piezoelectric injectors and I have a much better understanding now thanks to the information shared. As it stands now, Steve and Frank's understanding that we aught to look at the piezo stack as a condenser reversing the current makes the most sense to me..

Thank you all for your contributions to this thread, I am so grateful for the better understanding I have now.

On a lighter note, whilst reading up yesterday on piezoelectric injectors this song came to my mind and I simply cannot resist sharing it....

https://m.youtube.com/watch?v=DDTvLldOgZs

[Iver]

Hi, I am aware that when firing a piezo diesel injector that you have to fire current through the piezo crystal stack in one direction to open the injector and reverse the direction of current to close the injector. You also need high voltages to achieve this. But what I am confused about is how the voltage doesn't change polarity but the current does? How is the current flowing in the opposite direction but the voltage is staying high on the same wire like in picture.

Rym

I would say that your illustration is wrong. See below

[Technician]

Hi thanks for your reply. In the picture the blue trace is voltage and the red trace is current. Current flows in one direction and then the other all the time voltage is applied in the same direction?

RYM,

Some people don't see it this way, but voltage is just present, the voltage levels change but voltage is always there, it has no direction.

[Iver]

I don't think we can say

voltage is just present, the voltage levels change but voltage is always there, it has no direction.

Voltage does have a direction, We only have to look at the + & - on voltage scales in Pico. As it crosses the Zero line it has changed direction.


Back to these injectors we only need to look at the voltages used to fire these injectors - ~130v.
This starts as a 12v system. Someone has already said that the Piezo stack behaves like a Capacitor.

The ECU uses Capacitors to activate these injectors, they are charged to that ~130v. When the Injector is switched off some very precise switching occurs in the ECU so that the Collapsing Stack energy is returned to those capacitors. Not necessarily the same one !!

We can see this clearly on the CURRENT trace, it changes direction from open to close. The Voltage just dissipates away to 0v very quickly.

It is the wizardry of the switching & capacitors within the ECU that makes that magic happen.

[Technician]

I don't think we can say

voltage is just present, the voltage levels change but voltage is always there, it has no direction.

Voltage does have a direction, We only have to look at the + & - on voltage scales in Pico. As it crosses the Zero line it has changed direction.


It is the wizardry of the switching & capacitors within the ECU that makes that magic happen.

We'll have to respectfully disagree on that point then. What you are seeing on the voltmeter is not direction of voltage but an increase or decrease of voltage.

There is also no wizardry or magic in electronics, it's pure science.

[liviu2004]

So when my fluke shows me 12V or -12V, it does not show me voltage direction (a bit improper word).

[Iver]

So when my fluke shows me 12V or -12V, it does not show me voltage direction (a bit improper word).

Not an improper word at all.

I remember being taught about this on a Bosch Course. Not so sure it is clear in the course notes (which are not with me right now) However I do remember the Trainer explaining the function of these injectors.

Will look it up in a day or two.

But you are correct we can agree that voltage does not change direction as such but a Circuit such as a H Bridge will Change the Direction at the Consumer / Injector.

We don't need to agree to disagree.

When your Fluke Display goes from + to - it is not waffling nonsense. It is accurate.

[Technician]

I don't remember anyone saying it is waffling nonsense. I've listened to many people over the years talking about electricity and the understanding of the subject, and many training providers know that electricity is difficult to explain and understand, which is why the language seems to change in training schools/colleges over the years. I'd be interested in knowing what Bosch actually said about voltage in your training notes, which I can then compare to what I was told back in the 1980's. I'm pretty sure the goal posts will of moved somewhat!

During my aTa technician assessments over the years the Master Tech asked us a question about a voltmeter reading, he asked us, what does 'OL' on the display mean!

He looked at me probably because I was the oldest there at the time, so I advised him that Fluke advised it meant 'overload', well he jumped on that immediately implying that it did not mean that and actually meant 'Out of limit'.

I looked at his age then and realized that time has moved on and words change to help clear up the understanding. On the second day of my training I took the Fluke voltmeter manual to the training centre with me, showing him the exact wording as written by Fluke some thirty odd years ago.

Now the Master Tech had nothing to say. Generations change and believe what they are told at the time.

[Fat Freddy]

Some people don't see it this way, but voltage is just present, the voltage levels change but voltage is always there, it has no direction.

Agree. It's a relative thing. It's a measure of potential difference. It's why bleed down resistors exist.
https://youtu.be/9YmFHAFYwmY?t=64
In this video you can see the PD being equalized relative to gnd he gains half a million volts. To the lines he lost half a million volts. All relative. Was it -'ve or +'ve?