dynamho

I don't know if I'm disrupting the forum by reviving an old thread, but I'm having the "darndest" time finding a convincing answer to the question: Why do car manufacturers these days program cars to go open loop at WOT? Why not stay in closed-loop mode at WOT?

1. I've read a post saying that at WOT, the computer can't keep up with all the monitoring and reacting. This is hard to believe since stuff happening inside an engine is actually pretty slow (kHz) when compared to computer calculation cycles (mHz). Also, good O2 sensors are very fast. So I don't buy this argument unless I'm missing something.

2. I've read a post saying that it's to keep exhaust temps down to make the catalytic converter last a long time. Is this true?

Any other reasons?

zoom44

both are correct. the systems cant monitor and react fast enough at higher rpms and you cant run stoich at higher rpms or you'd burn the cat. it is more of a hardware issue than a computing issue. but the PCM does use the fuel trims it learns during closed loop to trim the open loop fuel requests so it should be fairly accurate to what its asking for. in open loop the front 02 sensor acts mostly as a cat temp sensor to watch for over temp situs.

thank you for bringing up the older thread instead of starting a new one. anyone else that has question scan review this thread and hopefully find the answers they are looking for

Hymee

iTrader: (1)

No need to appologise for reviding an old thread!

Zoom44 is correct. The main problem (I believe) is that by the time the "last" does of fuel is injected / compressed / burned and then heading down to the cat, and then the O2 sensor does it's thing and taken it's measurement, the engine will most likely be at a different load point - after all, in WOT the RPM's would normally be rising pretty fast, especially in the lower gears.

I also believe the MoTeC can do fast lambda, and you can have a target AFR/Lambda table where you can enter "non-stoich" values such as required at WOT. I haven't seen this in operation, and I believe you also need to get the base mappings close to start with, otherwise to many iterations of the feedback loop would be requied, and then the problems as mentioned about with latency become an issue. If you had a computer controlled dyno/retarder, you could spend more time at each load point and get it accurate - either "manually" or automatically via this closed loop style of approach.

Cheers,
Hymee.

Rasputin

Most manufatcurers still use switching HEGOs (0-1 V narrow band) sensors for closed loop fuel control. It is only suitable to target stoich AFRs. Anything rich or lean is impossible to target with it. And, as Zoom said, at high speeds and loads conditions, the fuelling MUST be rich, over-rich even, to prevent the cat temp to go above about 970°C as it would result in a reduction of cat longevity.

Now, the RX8 is fitted with a wide band lambda sensor. Why they haven't used it to run in closed loop in all run conditions is a mystery to me.

Fabrice

dynamho

Thanks for all the great replies. It seems like latency is the issue.

brillo

I'm not sure I'm totally clear on all the functions of our stock wideband.

As I understand it, the(front O2) narrow band serves the typical closed loop feedback function to keep the car near 14.7. The front O2 sensor does not serve as a cat temp sensor.

The wideband rear 02 sensor serves as the cat temp sensor under pretty much all conditions and as an idiot check for the narrow band sensor. What I'm not clear on is whether or not the wideband is working in both open and closed loop or just one of them. I would think it wouldbe helping under the open loop scenario to better dial in the fuel under WOT. Does the wideband also report back under closed loop? I would think it would, just as a check.

Most cars get away without a wideband currently, so Im guessing the open loop monitoring serves an emmissions function (as it helps dial in afr and helps protect the cat).

I'm revisiting some of these issues in the context of some interesting LTFT observations I've been reading about, I'm thinking that most folks would see greater hwy fuel mileage by reseting the ecu before each trip.