Megasquirt Support Forum (MSEXTRA)

Trying to decipher this DS203 scope. This is a trace of one of the injectors firing.

Jim

Here is a better one. I need to learn which part of the trace is injector dead time, can't say I've done that before. But I wasn't able to find anything RTFM

Jim

You can't see injector dead time because it is an electro-mechanical effect. You have to measure it by flowing fluid through the injectors.

That trace looks close, but not exactly what I'd expect. All the times I scope injectors I see a flat at the top of the flyback. You are seeing a spike. The voltage looks ok, but it decays away not quite as I'd expect. I'm not sure if that's a problem - it certainly doesn't look like a grossly broken driver anyway.

James

Thanks James. So how do you actually measure DT then? Some kind of sensor to show fuel coming out of the injector nozzle? I'm sure I don't have anything like that. I guess I could figure out a way to use a dropping resistor to measure current...

I have 4 injectors on each channel, could that have something to do with the differences in the waveform?

In an aside: I tried switching to Alpha-N and it had no effect. That would seem to rule out the MAP sensor wouldn't it? Doesn't seem to be much left. Seems like I owe you all an apology for being something of an ass, this may not be fixable via the controller.

Maybe I need to call my cam grinder and see if he can offer any insight.

Jim

https://m.youtube.com/watch?v=lJgi8l-2XG4

James

I see, so you are extrapolating the DT from the delivery curve. I watched a few of those other vids too. Interesting the way the blip in the inrush current moves around, but when I rigged an ampclamp my current trace didn't look like that. Maybe the injectors saturate, IDK.
It's going to take me a good bit of time to set up a bench for flow testing. That isn't going to happen this week and probably not next either and I will need to drive the car before it can be done. But IIRC the guys at the 2015 MegaMeet got it to idle reasonably well so I'll go looking for that file, attempt to load that and see what happens. More tomorrow, gotta go.

Jim

You don't need a test bench. I have measured deadtime at least 6 times, and always using the car. 4 and 6 cyl engines, so not so mutch to disassamble as a v8. Somtimes a fuel hose has to temporary be changed for a longer one to get a container under the injector, and maybe the injectors need to be clamped to the fuelrail during testing to prevent leak.
You need to keep the voltage stable during testing. Easy with a 12v supply with adjustable voltage as I use, but it can be done with a charger or another cars alternator. Use light, fan, etc if needed to keep voltage lower.
First time I measured deadtime and flow on a 6 cylinder BMW it took about 2 hours.
Base deadtime at about 13.2V is most important, but if possible I would advice to measure on a lower and a higher voltage at at least one injector, so you have the voltage correction.
But flow all injector at least one time to make sure the flow is even.
This is a easy thing to to, but can be a bit work if the injectors is hard to reach.
Remember good ventilation and precautions against fire. Fuel fumes in the air builds up fast even if you use a narrow container.

Espen

Actualy just remeberd I have a filmclip of the firs time I measured deadtime and capasity. Uploded it to youtube to give you an idea of how it cold be done in a engine bay.

https://youtu.be/SoKEbQtnuCo

Thanks. I did watch a bunch of vids so all that helps. Yours too.
I have an 8 station bench but it needs a little work. Maybe I can get something going a little later this week. Wouldn't hurt to verify my earlier flow numbers, and do the test at different pulse rates and I can see where battery correction could help. If I have to pull all the injectors I might as well test them all at once. I can run a hose and wires from the car.

Below is a pretty good scope trace I think. The waveforms look a lot like what I saw in the vids. Based on the two blips the injector is physically open for 2.5ms I think? (Based on the distance between the blip on the current trace and the blip on the voltage trace and depending on where on the blip you count from) It would seem to indicate 1.0 or 1.1ms DT, but I noticed that the DT measured with the "blip" method may be longer than that calculated from the flow graph. Does this seem consistent? I noticed also that DT is dependent on battery voltage and fuel rail pressure.
According to my datalogs at smooth idle my injector PW is about 3.7, which subtracting the DT setting of 1ms leaves 2.7 which is at least close, but not datalogged from this scope shot. Shorter than I'd like to see for fast idle certainly but usable. I understand we changed to 4 squirts to try to help idle control, isn't the other side of that coin a pulse width half as long, which of course would make DT at least twice as critical.

I have an old tune from 2 years ago that had a stable idle but I don't think it was all that much slower than what I have now. A couple guys at the megameet did that. I can't load it because I get an error, I probably updated the software or firmware and caused that. But what I can do is compare the settings and the maps. I'll try to do that to see if I can get any insight and hopefully an idea or two may pop up.

Jim

Hose an wires from the car should do the job

It's actually not uncommon for engines I have tuned that PW on high idle is lower than on low idle.
Depends on cam, and many engines need richer AFR on lower idle rpm.
Above about 2ms PW(DT included) you should be above the critical unlinear area on most injectors.
So say DT is 0.8ms, it should not be a problem with most injectors idling a warmedup engine above 1.2ms PW in TS/MLV( 2ms scooped).

OK, well from the current trace it looks like my DT should be in the ballpark at least. I have it set at 1.0ms and the trace starts to dip at 1.0-1.1ms. This was with the engine running at around 1400rpm or a bit higher I think. I didn't verify that and I didn't check the voltage. But at least the preliminary indications are good. I'm not off by half a millisecond or something apparently.

I am repairing the test bench which requires me to do a little machining and I had to order a reamer which should be here in a few days. In the meantime I can continue with tuning efforts.

At this point though I'm not cure that this oscillation can be tuned out. It might be the best I can hope for is to move it down. But, once the bench is leak-free I will pull the injectors and do a proper test series.

Jim

How many times do I have to post this? - You CANNOT measure dead time on a scope.

(Nor can you measure deadtime by checking for injector dribble.)

James

I accept what you are saying, simply to conform with the community's tuning methods. Just using what I have on hand to try and get a better understanding.

Can't do much else for the next few days.

Jim

Jim_Blackwood wrote:I accept what you are saying, simply to conform with the community's tuning methods.

It's not "the community's tuning methods" - it is the physics of an injector.

James

It has nothing to with comforming with the comunitys tuning methods. You simply wil not find the injectors physical deadtime with the scoop. No need to invent the (poorer)wheel again this time.
While it may be a fun thing to do before you can measure deadtime, you will not find the physical deadtime that way. And you can confirm the injector drivers are working that way.
There are other methods to figure out approximately deadtime, but I don't think it wil work at all on your engine in the state it's running.
It's easyer and more accurate to pull the injectors and measure the deadtime physical.

Even if measuring DT is too intimidating for now, measuring flow rate is much simpler. Once that number is verified "req_fuel" can be set correctly and one variable will finally be out of question.

A scope trace can give you an idea of how long it took the pintle to travel from the closed position to the open position and how long it took to travel back, but it will not be able to tell you how much fuel flowed through the injector while the pintle was in an intermediate position. Consequently, using a scope to measure dead time will result in an estimate that is too high.

Must have something set wrong, I can't get the injector test to start. I enable Test Mode, turn on the fuel pump, but the injector testing "Start" button stays greyed out, the stop button is bold.

Output interval is 100ms
RPM 1200
Mode: All or One, no difference
channel: Inj2
PW: 4.0
Total number of injections: 1000
PWM parameters: Standard

Coil testing section
100
12000
Off
CoilA
3.0
Am I missing something simple here?

Post a screenshot?

James