What a can of worms.
Is the fact that the system is closed or open determaned by the ECM or the O2 sensors or both? Where I am headed with all this is why a race tuner is used if all you did was replace pipes and air intake. I kinda get why it's necessary if you do cams or another major change but I'm stuck in the distributor days and the computer systems are screwing with my brain [or what is left of it]
Both closed and open loop systems start with a basic "map" that allows the ECM to calculate fuel injector pulse width and ignition timing based on input from various sensors. For instance, the "map" has lookup tables that correlate things like throttle position, rpm, manifold pressure (vacuum), engine temp, intake air temp, etc. The ECM takes all the inputs and calculates how long to keep the injector open to supply the correct amount of fuel to meet the originally programmed target AFR. There are even various versions, with some using rpm versus throttle position to estimate engine load, while others use manifold pressure sensors for instance. With the open loop systems, this is the end of the line. The ECM varies timing and amount of fuel based on the programming and the available sensor inputs, but it doesn't have any way to self adjust if a sensor drifts slightly from totally accurate values, or the injectors or fuel pump lose a little flow, etc. Over time, even though the ECM is still making the same decisions based on the map and the sensor inputs, the actual AFR can change significantly enough to affect emissions and performance. Enter the closed loop system, which adds downstream O
2 sensors to check the actual results and provide feedback to the ECM which can then fine tune the pulse width calculations.
Once you get involved with closed loop systems, there are many variations. The simplest ones use narrow band sensors that are basically just plus and minus switches. The sensor compares the oxygen level in the exhaust to the level in the atmosphere and chemically creates a small voltage based on the difference. That voltage is read by the ECM, values below a certain point signal the ECM to add fuel, and values above that point signal the ECM to reduce fuel. What this winds up looking like if you plot it on a graph of AFR versus time is a rapidly oscillating wave that is centered at 14.7 AFR. The term narrow band comes from the fact that this type of sensor only works effectively in a small window of AFR values centered at 14.7:1. A wide band sensor, on the other hand, works across a much wider range of AFR's.
EFI systems that are just used to meet emission regulations, like the one on current model Harleys, only go into closed loop operation under conditions where the EPA regulates emissions. For instance, a rich mixture is required for cold starts so the system stays in open loop mode until the engine reaches a certain temperature. Also, the regulations do not apply at high throttle openings and high rpm, since those operating modes also require a richer mixture. Once again, in those circumstances the system goes to open loop operation. The reason this is important is that while you are in open loop mode, there is no fine tuning of the mixture. Whatever amount of fuel the map tells the ECM to inject is what you get, no matter if you are running stock or you're running open exhaust and no air filter. Obviously the real AFR is going to be way off if you've made major changes to air flow. This is why I caution folks who heard from this or that expert that they don't need to retune when changing pipes and air cleaners to be very careful. Fortunately H-D tends to run their stock maps slightly rich in the higher throttle opening and rpm ranges, so most likely you can remain safe with small changes in air flow, but odds are the resulting AFR will not be close to optimum.
More sophisticated systems, and those intended for performance applications rather than emissions, use wide band sensors and controllers/ECM's which can directly read a range of AFR's, not just the 14.7 of the narrow band system. With these, it is possible to keep the system in closed loop mode even under those conditions where richer mixtures are required such as hard acceleration and high rpm. This allows you to keep your feedback system working to maintain the exact AFR your tuner intended. Is this really needed for the average rider? No it isn't. Remember, we survived quite well for the first 100 years with just carburetors and no fancy electronics or sensors at all. But if you are looking to maximize your control over the combustion process, a wide band closed loop system is definitely preferable to the cheaper emission regulations based systems.
Hope this helps.
Jerry
Author
Topic: FUEL INJECTION QUESTION (Read 1433 times)