facboy

hi all,

revealing myself as a total n00b, why does the 951 (and other cars for that matter) run in closed loop at part-throttle etc openings, but change to open loop when you go full-welly? what is the reason from an engineering (or other non 'because that's how it is' viewpoint)?

cheers,

chris.

PS in case anybody was interested/didn't know already, the shock tower reinforcement plates from the '88 turbo S (and '89 etc) can be retrofitted to earlier cars. they are in PET and are relatively easy to weld in for anybody looking to stiffen up the front-end slightly (at least in theory). new radiator hose needed too, i believe. i have also seen these plates on a '90 S2 cab, which was odd as i thought they were only fitted to turbos.

alkemyst

I thought there was more to the chassis reinforcement than just the towers (front and rear)...

closed loop I believe is a set group of parameters and no venturing outside it so that the engine and everything else can get into the normal operating range before OBDI/II warning signals start flashing...

under closed loop you really can't see what is happening with the powertrain since there are no options....once in open loop things show up as the computer allows them and you can see the problem areas.

Bri Bro

Closed loop is used to keep the catalytic converter at the correct AFR of 14.7/1 so the car can pass US emission standards. Depending on the input sensors (air flow, TPD etc) the DME looks up a map value, then controls it to maintain the 14.7/1 ratio. If the closed loop control can keep the AFR within 1% of the AFR=14.7/1, the CAT can do it job and reduce emissions.

At WOT, the DME just uses the maps and the AFR is changed to around 12.5/1 to keep out of detention. Emission testing is not done at WOT, it is assumed the engine will spend most of it life cruising at lower RPMs. Also, the NB O2 sensor is designed for a AFR of 14.7/1. That is were it changes from a high level signal to a low level signal. THe DME doesn't have a way of determining an AFR of 12.5/1 so the closed loop opens and only fuel maps are used.

MHT

closed loop operation means that the ecu is looking at the O2 sensor output and using that signal to modify the amount of fuel through the injectors, injector pulse width. When in open loop there is no feedback, the ecu operates on maps that are fixed, and based on eng. temp, eng speed,load etc. As Brian said a slightly richer mixture at WOT is safer and if an o2 sensor were to fail and send a false rich signal under high load the eng would not last long. By the way, as far as our cars are concerned the ecu operates in WOT mode from approx. 60% throttle.