Piezo injector operation. Current vs voltage

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RYM6746
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Piezo injector operation. Current vs voltage

Post by RYM6746 » Tue Feb 27, 2018 10:07 pm

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.
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victor2k
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Re: Piezo injector operation. Current vs voltage

Post by victor2k » Wed Feb 28, 2018 7:00 am

Hello,
At the opening of injector you must supply the crystal stack but this isn't a conductor so you must to short to the gnd to put him in to the initial state(that is the reason why you can't be allowed to disconnect the piezo injectors when the engine run).Also you can see the crystal stack as a capacitor:first you charge the "capacitor" - the volume of crystals will increase,then you discharge the same "capacitor"-the volume of crystals will decrease.
Regards
Last edited by victor2k on Wed Feb 28, 2018 9:00 am, edited 1 time in total.

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Re: Piezo injector operation. Current vs voltage

Post by RYM6746 » Wed Feb 28, 2018 8:31 am

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?

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Re: Piezo injector operation. Current vs voltage

Post by victor2k » Wed Feb 28, 2018 8:52 am

Hello,
What reference was used to make your capture?The engine ground?Or was used ij+ and ij- as a points of voltage measurement(In this case I think you will see a reverse of the supply voltage)?Is a Siemens VAG piezo injector in your example?
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Re: Piezo injector operation. Current vs voltage

Post by Steve Smith » Wed Feb 28, 2018 11:57 am

Hello and thank you for the posts.

This is an interesting topic and one that came up at the Roadshow this week

The Guided Test here has a great explanation of the Piezo Stack activity during each injection event
https://www.picoauto.com/library/automo ... o-current/
Scroll down to the Technical Information section

To get the full picture we need to capture the voltage at both sides of the injector during the injection event.

Thank you to Volrem, I have just downloaded one of his Volvo XC60 captures from the Waveform library where he has captured this event beautifully.

Victor hit the nail on the head here when he mentioned the Piezo stack is not a conductor as we know it.
It behaves more like capacitor where there will be a momentary current draw during the activation stage (charge) but once this is complete, even though there is a voltage pressure differential across the injector, no further current will flow!

I hope the image below will help.
Piezo injector operation.jpg
Piezo Injector Operation
You can see above how peak current (during activation) is achieved almost instantaneously where Channel A records a voltage of 33 V approx. Channel B 0 V approx. and Channel C 7.4 A approx.

Given we have a pressure differential across the injector (Captured at Ch A & Ch B) current will flow into the Pizeo Stack (charge) resulting in expansion and the commencement of injection.

The voltage continues to increase on Channel A where it plateaus at 114 V approx.
Even though Channel B remains at 0 V (so we have a huge pressure differential) no further current flows as the Piezo stack is not a typical conductor.

Maintaining the voltages in this state (Ch A & Ch B) ensures the Piezo stack remains in the expanded state for the duration of injection

You will then notice how the voltage begins to fall (Ch A) as the current begins to reverse through the Piezo stack.

Had this been a typical conductor, the voltage at CH A would have been turned off by the PCM where we would have captured an instantaneous fall to zero volts (maybe with an induced voltage spike) accompanied with an instant drop to zero current flow. (Think of an ignition primary circuit)

However, this is not a typical conductor and what we have here is the discharging of the Piezo stack (reverse current flow) during the contraction event. The result of which is a progressive fall in the voltage to zero volts (Ch A) and a reverse current flow as the Piezo stack discharges into the injector circuit.

This is counter intuitive as we are taught that for current to flow there must be a pressure differential. This is true of a typical conductor, not of a Piezo Stack

Below is the Toyota Piezo injector method (Thank you to mdlamber on our Waveform Library) where the PCM will send a 5 V injector activation and duration signal (IJT) to the Injector Driver assembly. Whilst we only have one side of the injector voltage here, we can confirm the activation and duration periods based on the IJT signal.
PIEZO IJT METHOD.jpg
TOYOTA IJT
These are just two examples of Piezo injector control. Don’t assume one side of the injector will always be at zero volts with the other being high at 100 V +

Various manufacturers use a number of elaborate “control techniques” during injection activation and duration. Both sides of the injector are controlled in such a fashion to optimise injector operation whist limiting the effects of switching high frequency voltage and current

I hope this helps, take care…….Steve

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Re: Piezo injector operation. Current vs voltage

Post by kona » Wed Feb 28, 2018 5:58 pm

Hello,

But why in the first example current starts changed  before than changing voltage
These two events must begin at the same time or how ?
Explain please.
Thank you

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Re: Piezo injector operation. Current vs voltage

Post by victor2k » Wed Feb 28, 2018 6:25 pm

Hello,
Here is a capture with voltage from ij+,ij- related to the gnd,injector current and the comand pulse from ECU to EDU.
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Re: Piezo injector operation. Current vs voltage

Post by RYM6746 » Wed Feb 28, 2018 6:49 pm

Thanks for the replies, that has answered all of my questions. I can now look at piezo injector current and voltage waveforms with confidence

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Re: Piezo injector operation. Current vs voltage

Post by RYM6746 » Wed Feb 28, 2018 8:27 pm

Important note: Once the stack is in the injector open position, it does not need to have a continuous supply voltage and will remain in the open state until the close command. Hence the positive and negative current spikes in the waveform. For this reason, never disconnect the injector's multi-plug with the engine running. If the injector is in the open state it could hydraulic-lock the engine with the diesel fuel. The engine will cut out if the control module detects any faults with the injectors or driver circuit.
Surely this statement is partially incorrect in the guided test section under peizo injector technical information. when it says it does not need to have a continuous voltage and will remain in the open state?

It would appear in order to keep it in the open state a continuous voltage needs to be held even though there is no more current flow?

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Re: Piezo injector operation. Current vs voltage

Post by liviu2004 » Wed Feb 28, 2018 8:32 pm

kona wrote:Hello,

But why in the first example current starts changed  before than changing voltage
These two events must begin at the same time or how ?
Explain please.
Thank you
You can see the circuit ecu-injector as an oscilator circuit, electrons go from ecu to inj and reverse, but voltage to ground does not change polarity. Hard to explain.

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