Seeking the true definition of fuel injector latency 2

[GordonLee]

There seems to be a lot of talk of fuel injector latency and offset these days. I haven't found anyone that can actually explain what is being measured. Is there some so-called standard that is used when the manufacturers use when programming the ECU.
Is it:
The time that it takes the pintle to completely open after the drive voltage reaches a certain percent of its maximum?
Or the time it takes for fuel injector to start flowing a certain amount of fuel?

I have spent countless hours testing this and the measured results rarely are close to the values that are put in by the manufacturer.
I also wonder why no one seems to be concerned about the injector closing times.

Hopefully someone in the automotive manufacturing world can enlighten me.

Regards,
Gordon

 

[ivymike]

automotive manufacturing
are you talking about gasoline-fueled car engines, or do you count diesel ship engines in this also?

 

[GordonLee]

I was asking about gasoline automotive injectors.

Gordon

 

[dgallup]

This is a value calculated by measuring static and dynamic flow rates or you can also do a linear regression through a series of dynamic flow points such as 3, 4, 5, 6 & 7 ms at 10 ms pulse repetition period (100 Hz). Check out SAE spec J1832 for the standard definition.

If you plot flow on the vertical axis and pulsewidth on the horizontal axis the x-intercept is the offset or dead time in milliseconds. This is a function roughly of both the opening time and the closing time but it's more complicated than that because the flow/lift curve and pintle bounce also contribute. Closing time is very important in getting good linearity to small duration of injection.



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[TStaples]

Dave,

I think you are referring to the linearity calculation, and then the calculation of the theoretical x-intercept (offset). I think he was just asking about the way ECU manufacturers refer to this. In my experience, it's different than how injector manufacturers normally think of it.

GordonLee,

In the world of programmable ECU's, injector latency, sometimes referred to as offset, is the term that is used that correlates to opening time of the injector. (To an injector design engineer, Opening time is different than offset, as Dave points out. In the SAE spec he references, it defines both, and how to calculate them. In my experience, the ECU manufacturers use this term interchangeably. Latency is really "opening time" = the time elapsed from the start of the logic pulse, to the start of fuel delivery from the injector. In a port injector, this is a function of many design parameters and calibration spring load in the injector used. It's also directly affected by the voltage supplied to the injector by the driver and the operating delta pressure across the injector. Peak and Hold driven injectors are less affected by the battery voltage, compared with saturated drive injectors, but they are still affected.

For example, when I calibrated my 3.8L Porsche club racer with a MOTEC controller, I needed the effective opening time at the dynamic calibration point for a range of battery voltages from 6-16V. I got opening times measured at 1V increments, and used that for programming the look-up table in the ECU.

Dave is correct about closing times being very important in having good linearity. Closing time is not as important to the controller, however, as it primarily impacts the effective duration of injection. (i.e. fuel keeps being delivered after the logic pulse ends) I say it's "not as important" because most ECU's now have so much control authority than they can adjust (lengthen or shorten) the pulse width being commanded of the injector by a large amount, and the effective duration of injection is sort of lost in the noise, as long as it's not excessive. Modern ECU drivers for high impedance injectors employ a zener diode to shunt the flyback current, which allows the magnetic field to collapse very quickly after the drive pulse ends. Assuming reasonable spring loads for most production injectors, most port injectors have closing times in the .4-.7ms range. The important part is that it needs to be CONSISTENT, in which case the ECU's capabilities mask it, making it SEEM unimportant. Opening time, or latency, is more important to the ECU from a timing perspective, to get the fuel shot delivered at the appropriate time. Since most port injectors have opening times in the 1-1.5ms range, this is more critical to overall timing.

I hope that helps.

-Tony Staples

http://www.tscombustion.com

 

[mattsooty]

Gordon,

I think that Tony has it covered but, in a nutshell: -

The 'Effective Injection Pulsewidth' is the time that the injector is held open, during which fuel flows at the static flow rate at that condition [F=(dp over injector tip, rated flowrate of injector)]

As the injector driver goes 'on' there is a finite time for the flow to ramp from zero to full, likewise when the driver goes 'off' there is a finite time for the flow to ramp down to next to nothing. This total time is the latency, confusingly the only initial ramp is also refered to as 'the latency' at this is the time between on & full flow with the the ramp down being described as 'lag'.

The main two reasons are for the maximum injector are: -
Maximum on time to ensure full load fuel requirements without overheating drivers etc
Ensure that predicted aircharge per cylinder is dealt the correct fuel at the correct time

MS

 

[GordonLee]

I just purchsed the SAE J1830 spec and reviewed it and see that the term latency is not in it.
It seems that a lot of people are using the terms latency, lag, and dead time interchangeabily, and they may not mean the same thing to everyone. Now there is a quite of talk of offset that some are thinking it is the same and the others.

If a person requests a latency measurement, it appears that it may be a good idea is to ask them what they really require for their ECU's input. Although many may not know what they actually me need.

I look at the method used in measuring the offset in SAE J1830 and can't help but wonder if there was a better or easier to do it. I guess it doesn't matter as long as everyone does it the same.

Really appreciate all the information to date.

Gordon

 

[patprimmer]

I would tend to use the terms opening latency or lag or offset, closing latency or lag or offset and total latency or lag or offset. That way all confusion should be eliminated. Other terms that spring to mind as possible descriptions are total dwell seat to seat and dwell at full flow.

Regards
Pat
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[TStaples]

Pat,

Your description of the terms that you would use is a great example of why we need standards. Depending on who is working on the technology, what part of the world they are in or from, and so on, there could be many different terms for the same basic concept. That's one of the primary functions of the Society of Automotive Engineers, of which I am a proud member. They have standardization groups that cover every kind of technology used on automobiles. One of the key functions of these groups is to develop standards that define how things are to done, or measured, or utilized, or applied, and so forth. On of the keys to this is that terminology is standardized. That way, automotive engineers, working for different companies, in different countries, can refer to the same things using the same terminology. It eliminates confusion and reduces the chance for errors. So, if you have an experienced Automotive Engineer like dgallup (whom I worked with for almost 18 years!) who deals with the OEM's, he is going to use the standardized terminology for, in this case, fuel injection. However, and the main reason I posted, my experience in automotive fuel systems has shown that many aftermarket manufacturers don't utilize the standards and don't always use correct terminology. In the case of aftermarket ECU manufacturers, they act like they have never looked at an SAE specification for an injector and create all sorts of differing terms for things that are already well defined. I've run into this before in the case of latency, vs. the correct term, "Injector Opening Time". I've also seen offset misused, which has a very specific mathematical definition per SAE. It is sometimes misapplied when the aftermarket manufacturer really means "Injector Opening Time"

BTW - Total dwell seat to seat is defined by SAE as "Effective Duration of Injection". That is equal to (in simple terms) (the duration of the logic pulse applied)-(opening time)+(closing time) typically referred to in ms. So, for a port injector with a 1.25ms opening time, and a 0.50ms closing time, the effective duration of injection for a 3.00 ms injection pulse is 2.25ms. Dwell at full flow isn't really much different. Good port fuel injectors are designed so that the transit times from fully closed to fully open for the armature/ball, or armature/pintle, or just armature in the case of a plate style injector are very fast relative to the logic pulses. These are typically on the order of 50 microseconds or less; the best designs are a lot less. The valves are also designed so that the curtain area above the metering section has very little pressure drop relative to the metering section. That means that if the armature moves 50 microns from closed to open (typical of many PFI injectors,) the injector is up to within a few percent of full flow after about 20-30 microns of motion. That helps ensure linearity of the injector at lower pulse widths. If an injector design exhibits really slow transit times for opening or closing, it will result in more non-linear behavior at short pulse widths, and more piece-to-piece variation amongst a set of injectors.

-Tony Staples

http://www.tscombustion.com