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Fuel Injector Series: Part Three - Injector Dynamics

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By: Aloicious

Feb. 17, 2012

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Okay, so now that we understand generally how the injectors work, and how they are controlled, let’s look at some dynamics of their function and problems that arise from these dynamics that we will have to address when we start tuning the injector controller.

First, let’s understand what the injector controller is expecting as far as the fuel delivery.

This diagram shows basically what the controller expects the injector to deliver. In this diagram, the X axis is time, so that represents a single pulse width, pretty straight forward. However unlike the oscilloscope diagrams we were looking at earlier, in this diagram, the Y axis represents the fuel delivery. So as you can see, the controller expects the injector to function 100% along with the electrical pulse.

So the controller expects the injector to basically jump from 0% flow, instantly up to 100% flow, but the injector doesn’t function at the speed of light, since it is a mechanical device there are little delays and complexities. If we compare the above image of what the controller expects, to what the injector function actually is, you can see where these discrepancies are.

So what is causing this difference? Well, to make the explanation a little easier, I’ve taken the same diagram and coded it a little bit, so let’s take a look at that and go into some explanation here.

Okay, so let’s break this down a little bit, if we look at the color differences under the curves, the green color under the first portion of the curve would represent the period of time where the controller is expecting more fuel to be flowing, than what actually is. The yellow portion represents the time that the injector is actually flowing what is expected of it, and lastly the light blue portion represents the period where the injector is flowing fuel when it isn’t expected.

Now look at the top portion of the image with the numbered segments. Remember back to the first article in the injector series where we talked about how the injectors work. Okay, so the segment of the pulse labeled as #1 represents the time while the electromagnetic coil is charging up and building up the magnetic field. The segment labeled #2 represents the time that the seal (the pintle, ball, or disc) is being lifted off its seat until it is fully open.  Segment #3 is the time where the injector is fully flowing as expected, and lastly segment #4 is the period where the electromagnetic coil is discharging, and the spring is forcing the seal back onto its seat, and cutting off the fuel flow.

Well, you can see that these differences will cause descrepancies in what is expected, and what is delivered. These differences are really only a fraction of a millisecond, you may be thinking, well if it’s such a small difference, what’s the big deal? And for most of the time, you’d be correct, there isn’t too much difference, however there are two main periods of injector function where these dynamics are very important. These times would be at very short pulse widths, and at very high duty cycles.

At very low pulse widths, the injector actually doesn’t have time to open fully before the controller commands the injector off. In the same type of diagram as above, here is an example of that situation.

You can see that the injector coil starts charging and before it can finish and pull the seal up to full flow, the injector is commanded off. So, not only does the injector not flow the correct amount of fuel, but since we’re talking of building magnetic fields and other forces that don’t act fully predictable, it also translates into unpredictability in the fuel flow at these small pulse widths, which is commonly referred to as a period of non-linearity, where the fuel flow can be erratic and unpredictable.  Now different injector designs, tolerances, and build quality will dictate how low of a pulse width that the injector can function predictably, but every injector will have a low limit to its linearity. There are ways to combat this in the injector controller tuning, which we’ll discuss during the tuning section of this series.

At very high duty cycles there is the other side of the problem, where the injector doesn’t have enough time to actually close before it is commanded back on.

So when the duty cycle gets too high, due to the injector dynamics, you’ll actually reach static flow before 100% commanded duty cycle. Depending on the injector, this is roughly around 93% duty cycle or so, obviously different injector builds may end up tolerating a higher or lower duty cycle before going static. This part of the issue is where the real problems and dangers stem from, and there is not much you can do for it in the tune because once an injector goes static, it cannot flow any more fuel for that given fuel pressure, and high duty cycles happen at high engine loads, when the engine needs fuel. So if your engine requires more fuel than the injector can supply, then you will start running lean and causing engine damage. We’ll talk about how to choose the right injectors to avoid this problem in the next section, but basically you’ll want to shoot for an injector that peaks it’s duty cycle for your setup at around 80% to give you enough headroom to supply the fuel you’ll need at peak engine loads.

Previous section: Injector Control

Next Section: Choosing the Right Injectors

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