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Driving injectors 
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TO92 - Vaguely active

Joined: Mon Dec 08, 2014 10:15 am
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Location: Christchurch, NZ
Hey everyone

I'm going to be building an FPGA based EMS over the next while and hope to learn and share my endeavours on here. I'm in the stages of planning at the moment and have a few questions regarding the common approach taken to driving injectors.

My thoughts so far are this: Since injectors are inductive their impedance will change with frequency, hence current flow and thus fuel flow are functions of frequency and voltage. My initial thinking is that each injector should driven with a pulse width relative to the period of one camshaft rotation. My concern is that since impedance increases with RPM, 70% duty cycle won't correspond to 70% of the maximum possible fuel flow. Instead, current flow through requires a closed loop system.

My thoughts aren't clear on this. Can anyone help by sharing their knowledge?

Thanks!


Mon Dec 08, 2014 12:17 pm
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Your thinking is quite literally all wrong. Or rather, you're well outside the frequency domain where you'd approximate right. What is critical is modeling the flow of the injector correctly at low opening times, and modeling the times for open and close and therefore flow-period during an on-pulse. During wide open the flow is linear, or very close to it. It's those fringe areas of nearly open or not quite closed that screw things up.

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Mon Dec 08, 2014 8:21 pm
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Joined: Tue Feb 12, 2013 5:43 am
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Agree with Fred. On the top end, most injectors and drivers (it has to be considered as a set - any testing must be with a specific driver configuration and model injector) are linear up to ~ 90-95% duty cycle. Good practice is to limit yourself to 85%, ESPECIALLY with boosted engines. Past that, and you start going non-linear, where an additional 1% DC does not equate to an additional 1% fuel flow.

At the bottom, it's more of a toss-up. It used to be that people would have idle fueling issues below 2ms, but this was also a function of the controller (MS1). Some users on the MS sites have reported imperically-determined opening times of 0.4ms on specific injector-driver combinations; with the sort of PW resolution possible with FreeEMS or comparable hardware, you can get reliable and repeatable use from a pretty small DC. This would have to be carefully determined with your specific equipment, using a flow bench and oscilloscope. Or just stick with a nice safe 1.2ms ...

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Tue Dec 09, 2014 2:22 am
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TO92 - Vaguely active

Joined: Mon Dec 08, 2014 10:15 am
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Location: Christchurch, NZ
Thanks for your contributions guys.

@Fred
If I understand correctly; an injectors duty cycle doesn't have a linear relationship with its flow rate, hence we need to account for the fringes with some "fuel trims" in an attempt to form a linear relationship.

Also, if injector duty cycles aren't based on the duration of one combustion cycle (one camshaft rotation), what are they relative to/triggered by?

@DonTZ125
What you're saying seems related to what I'm getting at. I think the combination of inductance, frequency, and mechanical resistance are what prevent these narrow cycles from being possible.


Wed Dec 10, 2014 10:45 am
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Stop thinking in duty cycle. That's an output of what's going on. Think in terms of pulse width, dead time, and static flow rate, with an appropriate model for the fringes.

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FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
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Thu Dec 11, 2014 11:16 am
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LQFP144 - On Top Of The Game
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Check out articles at Injector Dyanmics website. There is a lot of good info there


Fri Dec 12, 2014 12:47 am
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