Closed Loop/Open Loop

[tazgn]

I've finally got the car running decent, and now I'm trying to learn this Felpro system better so I can tune it right. But I have a question that I just can't seem to find the answer to (probably because it's sooo simple that everyone else in the world knows the answer). Can someone clear up for me what open loop and closed loop actually is? I understood it a long time ago, but seem to have forgotten since then. I tried adjusting my rpm for closed loop up to 1200 rpm (a few hundered rpm above idle) and the car just doesn't want to idle smoothly. Thanks to this board, I FINALLY understand the V/E table. Now it's just this other general question. Thank You for any help.

Derrick

 

[bobc455]

Well in general terms, closed loop means that whatever you're using to control something (in this case A/F ratio) has some sort of a feedback device to see what the results are, and then it makes adjustments accordingly.

For example, lets say you have water flowing through a pipe, and you need 50 Gallons / minute (GPM). In every system, there is a controller that tells the pump how quickly to run. A simple way to control this would be to put a knob on the wall for a man to control pump speed. But then if the pump is pumping 55 GPM, the controller (knob) doesn't know that it should do anything to fix it. This is "open loop" control, also known as running blind.

Now lets get a more sophisticated controller for our pump. We'll also add a flowmeter somewhere in our pipe. The controller will watch the flowmeter, and if sees that it's pumping 55 GPM, it will slow the pump down so it reaches it's setpoint of 50 GPM. That is "closed loop" control. The flowmeter "closes" the control loop (gives feedback to the controller so it can make an adjustment).

The ECU does the same thing with the oxygen sensor. If it sees too rich of a mixture, the ECU can make adjustments to what's going into the engine so that it balances out to the setpoint A/F ratio (when the "closed loop" is enabled) based on what the oxygen sensor is reading. So if the oxygen sensor sees 14.0:1, and setpoint is 14.7:1, closed loop control will have the injector back off a bit on how much fuel goes into the motor (injector pulse width).

FWIW, I run with my closed loop enabled all the time (after it's up to temp). My idle speed is around 700 RPM, and I have my high & low at like 550 & 500.

I've found that two things (well a lot more than that, but two biggies) cause a poor idle- trying to idle too lean, and trying to idle with too much advance.

Hope that helps!

-Bob Cunningham
bobc@gnttype.org

 

[tazgn]

Thanks Bob.
That cleared up a lot... No, why would anyone want to use open loop???? Only advantage I can think of is to get rid of the O2 correction factors for getting values for the V/E table.

Derrick

 

[bobc455]

Well in general, there are a lot of situations where open loop control is the only way to go. For example, take your car stereo's volume knob- not only would it be difficult to make a feedback device, but some stuff is intended to be louder than other stuff and the only good way to tell if it's the right volume setting is for a human to listen to it and adjust it according to his/her preference.

Now with regards to the fuel injection (the reason we're all here, after all!), lets say at idle the closed-loop is leaning out by 20% or so at the starting line (an extreme example). Then when you snap the throttle wide-open, the closed loop is still leaning you out by 20% for the first instant - boom! Now seriously if your O2 is correcting by 20% then you have other issues (you haven't tuned your VE tables yet!) and you probably wouldn't be racing, so this isn't much of an issue.

But more realistically, the closed loop has a few other implications. For example, if you idle with the closed-loop active, then your idle speed will oscillate just a bit as it searches for the perfect setting. For another example, as you're going down the track, since the closed loop is constantly making very small adjustments you aren't going to run the same each time. These are extreme examples, but you get the idea. On my car, the O2 sensor is active virtually all of the time (once it's warm).

As a side note, if for some reason your O2 sensor does fail one day and you aren't able to run in closed-loop, you'll be very happy if all of your VE tables are near perfect. I've spent a lot of time tuning, and seldom does my O2 sensor make more than about 3%-4% correction (don't kill yourself trying to get everything that close, I'm just particuarly anal. If you can get within about 6% most of the time, then you're in good shape). If I ever do lose my O2 sensor, I know I have a solid VE table to run from. I could even probably deactivate it now, but since I have it I might as well use it.

-Bob Cunningham
bobc@gnttype.org

 

[Craig Smith]

FWIW in my bracket car, the last 3 passes I made down the track (all of which were in Speed/Density mode and in closed loop) were 11.317, 11.317 and 11.317. I wouldn't necessarily link closed loop operation with run-to-run inconsistency, Bob, but I do think that what you said has plenty of merit. Closed loop only means that there is an opportunity for correction to be applied; it doesn't necessarily mean that any correction will be applied, so simply being in closed loop shouldn't mean anything.

Despite the above info, I do believe that a bracket car with EFI will run most consistently not only in open loop, but in Alpha-N mode as well. My car was doing pretty well that day, but the best bracket cars I've seen keep it simple. I've seen guys run within a few thousandths of a second all day long (over a dozen runs!) over varying weather conditions. This is my opinion, but I would say that working to maintain a given a/f ratio over varying weather conditions will cause more of a performance variance than letting the a/f ratio drift with baro, temp, etc.

Any thoughts on this? I could talk about this for days... I'm not sure we will ever REALLY know.

Craig

 

[bobc455]

Well I was just trying to point out that open loop is desirable sometimes, although for the most part closed-loop control is best. I took a guess and looks like I had bad luck in trying to pick one out, maybe I should've left it at the volume knob...

Seriously, if your VE tables are dead-nuts (or pretty close), then you'll probably be very consistent in either mode (as you demonstrated). It comes down to the individual car and racer - for me, I'll always be in closed-loop as long as my O2 sensor is functioning.

-Bob