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Thread: Injector Dutycycle explained

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    Injector Dutycycle explained

    Here I will attempt to dispel some widely advertised misconceptions about what Injector Duty Cycle (IDC) is. It is a subject that many people get wrong and even many professionals seem to miss the point as well.

    It is a widely believed notion that IDC is a state of operation which dictates how long the injector is open and that an IDC of 100% would result in a statically open injector. Meaning it would simply remain open all the time until IDC's fall below 100%. This suggests that the supplied voltage is constant. This could be because people are confusing "duty cycle" in terms of how the injectors are being tested with "injector duty cycle" in terms of being a state of engine operation

    With this belief, this would suggest that IDC has control of the injector and dictates when it should open and close which conflicts directly with the very idea of what the IPW is and does when in fact IDC is only a final calculated result of its environment. The first thing to consider is that the IPW is the only thing which defines when an injector is opened and how long it remains open, and the violation occurs when suggesting the injector remains open all the time which would result in an infinite IPW. Which is absurd. An injector that is static will be flowing during all four strokes of the engine cycle. IDC, simply put, is the result of various conditions... not the influence on those conditions.

    The ECU derives it's IPW based on displacement, flow, air/fuel concentration, VE, load, RPM and other compensations. If your injectors are not big enough for the application, and the conditions are right, the IPW will actually be longer than the time it takes the piston to complete one stroke. The less you can decrease that time the better (to a point).

    This is where the concept of duty cycle comes into play. To put it as simply as I can, IDC is how much of the intake stroke the injector is firing and is represented in % of intake stroke. The IPW is how long the injector is actually open and is represented in milliseconds. So if your intake stroke lasts for 20ms and your IPW is 22ms, then your resulting IDC will be 110%

    To explain this, imagine the piston beginning it's intake stroke. The ecu commands the injector to open. When the piston gets half way down the cylinder, the ECU commands the injector to close. The resulting IDC would be 50% or half the intake stroke. If the piston makes it to the bottom of the intake stroke and the ECU commands the injector to close then, the resulting IDC would be 100% or all of the intake stroke.

    In a situation where the injectors are not big enough for the application, the ECU will command the injectors to come on at the beginning of the intake stroke but not command it to close until some time after the piston has begun its compression stroke. So lets say your IDC's are 115%. What this means is that the ECU has defined an IPW that will last 100% of the intake stroke and 15% of the compression stroke for a total of 115%.

    It is widely recommended that you keep your IDC below 90% in worst case scenarios. It is important to know that if your IDC's are good today they may not be tomorrow. This could be because of changing conditions due to environment or fuel. For instance if your IDC's are 95% today at 7000ft above sea level, and you take a trip to sea level, your IDC's will be well in the red. The same thing could occur between summer and winter weather and different grades of fuel.

    Some reasons to track your IDC's are...

    1. Excessive IDC's can result in very inconsistent AFR's or a leaning out condition. Because the ECU is commanding an IPW to be greater than the intake stroke time, the engine may not get all the fuel it needs because the valve will be closing while the piston begins it's compression stroke and the injector is now firing on a closed valve. This could also result in pooling of fuel at the intake valve resulting in unatomized fuel entering the cylinder upon the next intake stroke.

    2. The lower you can get your IDC the better the intake charge has a chance to mix resulting in very consistent AFR's and smooth operation.

    Now with that said, you don't want your IDC or IPW to be to small either. Too large an injector could result in IPW's being so short in time that the injector doesn't have time enough to start flowing fuel resulting in choppy idle and inconsistent fuel trims during lower RPM operations.

    That's all I got for now. If anyone has any other info I would be happy to update this info.

    George
    **END**
    Last edited by HolyCrapItsFast; 02-23-2012 at 10:06 AM.

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