Theoretical Vs. Practical Injector Opening Times

[X64v]

Z-ya's sticky identifies my Bosch green top injector (same part numbers) as having a 1ms opening time, so that's what I've used since I got them. I've also run them as Peak and Hold's with pwm settings also per Z-ya's sticky.

 

Today I decided to switch from two squirts simultaneous to two squirts alternating in preparation for running my larger injectors (doubles the instantaneous on time of each bank, giving better idle quality). When I did, I found the engine ran lean through its entire range. I did some researching and found (on msextra.com and some dodge neon forum) that this was because the injector opening time was incorrect, so as it switched the number of times it pulsed, the amount of fuel changed. I also found that the opening time is set correctly when you can change the number of squirts per cycle (or switch between simultaneous and alternating) without causing a change in AFRs (which is most noticeable at idle).

 

I began experimenting with my opening time setting, and found that at .7ms my afr's stayed steady at idle (when they're affected the most) when switching back and forth between the two squirt modes, which seems to mean that's the correct setting. I suspect the true number is closer to .72-.75ms, but MSI only allows .1ms changes.

 

Anyways, why would there be this discrepancy between Z-ya's tested opening time value and the one that works best when experimenting? To be clear, I'm not doubting Z-ya's methods or accuracy, just wondering why the measured value wasn't what worked practically.

[eec564]

Z-ya did his testing with a nice 'scope and amp probe, so his testing was done on the coil inside the injector. Once the injector is fully open and the valve quits moving, the coil begins to saturate. Fully open in this case is where the valve has moved fully inside the housing. Max flow, or even 75% flow is bound to occur before that. Z-ya's settings are good for properly configuring PWM on the Megasquirt so the injector fully opens and does not get burned out.

 

Megasquirt assumes that no fuel flows during the injector opening time but injectors are not perfect on-off devices so setting the injector opening time to the amount of time between the injector receiving a signal and appreciable flow occurring is ideal.

 

The settings for opening times for fuel delivery and coil saturation times seem to be different here, which is very possible and makes sense. When changing your opening times, were you also changing your PWM times? Z-ya's PWM settings look about as ideal as one can get.

 

For your injectors, 3/4 ms could be just about right for fuel to start flowing. Do you have a wide band? At idle (running in open-loop mode) if you change the number of pulses how much does your afr change? That would be a good way to check for opening time settings.

[X64v]

I left the PWM settings alone. I use Z-ya's recommended settings for my injectors (1.4ms time threshold, 30% limit), and they seem to work very well.

 

Yes, I have an innovate LC-1. That's how I came up with the .7ms opening time. At idle with the 1ms opening time, it'd go from 12.3:1 afr at two squirts to ~18:1 at one squirt. At idle with the .7ms opening time, the difference between one and two squirts is only about .1-.2.

 

Switching between one squirt and two is how I came across this in the first place. I switched from two squirts simultaneous to two squirts alternating, which is only one squirt per bank per cycle, and then my afr's went to hell.

 

So yes, that makes sense. The time between voltage start and coil saturation may be 1ms, but the time between voltage start and fuel flow is about .72ms. I'm guessing this number would also be at least slightly dependent on fuel pressure, but I'm not positive.

[eec564]

It's easy to see how Z-ya's "To open" times look a lot like Megasquirt's "Injector opening" times, but the distinction is there. I never really thought about it before, but I remember reading in Megamanual the definition of opening time being something along the lines of "How long it takes for an appreciable amount of fuel to begin flowing. Megasquirt assumes no fuel flows during opening time." Z-ya's times are quite definitely to full open, looking back at his graphs again.

 

I'd imagine fuel pressure would make a difference. Depending on the design of the injector I suppose fuel pressure could either help hold the injector shut or help push it open a bit. I'd suppose fuel pressure could effect closing time more dramatically, since that's trying to plug a hole with pressurized fuel flowing through it. I've never taken apart an injector, so I'm not sure of their exact internal layout.

[X64v]

I'd imagine fuel pressure would make a difference. Depending on the design of the injector I suppose fuel pressure could either help hold the injector shut or help push it open a bit. I'd suppose fuel pressure could effect closing time more dramatically, since that's trying to plug a hole with pressurized fuel flowing through it. I've never taken apart an injector, so I'm not sure of their exact internal layout.

 

That's what I was thinking, that depending on the injector design, the fuel pressure would either help or hinder pintle movement.

 

And ah, I missed the section of the megamanual saying that that was their definition of opening time, which would certainly explain the differences in that number vs. z-ya's actual opening time. I do know the formula megasquirt uses to calculate pulsewidth makes all of this make sense though:

 

Pulsewidth = (Req. Fuel)(VE)(MAP)(Enrichments)+(Accel)+(Inj. Opening Time)

[eec564]

After much searching, I found it just under the table for combinations of squirts vs number of cylinders table about 1/5 to 1/6 down the page.

 

http://www.megamanual.com/v22manual/mtune.htm

 

Injector Opening Time (ms) is the amount of time required for the injector to go from a fully closed state to a fully opened state when a 13.2 volt signal is applied. Since fuel injectors are electro-mechanical devices with mass, they have latency between the time a signal is applied and the time they are in steady-state spraying mode. Typically, this value is very close to 1.0 milliseconds.

The current MegaSquirt® controller code assumes that NO fuel is injected during the opening (and closing) phases. However, it is very likely that a small amount actually is injected. Thus making this value larger will enrich the mix and will have a much greater effect at low pulse widths. MegaSquirt® also uses this value as an additive constant in pulse width calculation, thus making this the lower limit for pulse width.

 

 

I swear, Megasquirt has the LONGEST manual I've ever read that's COMPLETELY FULL of USEFUL information.