Frequency Valve Operation Question

[fastbacker]

I was hoping someone could tell me how the frequency valve works on a cis engine.

I kow it's operated by the lambda brain in reponse to the O2 sensor and throttle position. I'm just unclear what it does. Is it some how connected to the fuel distributor?

Thanks-
Craig Backer

[psalt]

Craig,

The catalytic converter needs a 14.7:1 AFR to operate correctly and survive. No engine management system can provide an accurate enough steady mixture, so they are designed to dither the mixture rich and lean around 14.7:1. EFI does this by altering the injector pulsewidth based on the 02 signal. CIS lambda does this by altering the lower chamber fuel pressure. The amount of fuel delivered by the injectors is affected by both the height of the air flow sensor plate and the differential fuel pressure in the fuel distributor. The way the system works is that the basic fuel distributor setting is too lean, and the pulsing FV at the default 65% duty cycle bleeds off lower chamber pressure richening the mixture to the default fuel curve. When the system goes closed loop, if the 02 sensor reads lean, the duty cycle goes up, bleeding off more lower chamber pressure and richening the mixture. If the sensor reads rich, the duty cycle drops, leaning out the mixture. The throttle switch causes the system to goes open loop after 35% throttle to provide the richer mixture for best power. You can get by without an 02 sensor, but the system needs a functioning FV to get the correct basic fuel curve.

[JohnKo]

That's a very clear and concise explanation. As a technical writer/editor by trade, I'm impressed. I initially questioned your premise that you can get by without an O2 sensor -- it's the input that tells the control module what to do with the frequency valve -- but... are you just saying that the default 65% duty cycle is fine as long as the baseline mixture setting is adjusted so that the engine performs well when warm?
John

[psalt]

Many people assume that since the engine runs fine without the 02 connected, you can get by without the other Lambda CIS components. This is incorrect because by design the system needs the FV to get back to the default fuel curve, an idle (closed) FV raises lower chamber pressure resulting in a lean condition. You can attempt to adjust it out with the mixture screw but the curve will be wrong in many places. It is also a fantasy that disconnecting the 02 sensor somehow improves performance, it is out of the picture after 35% throttle.

[JohnKo]

Originally posted by psalt
It is also a fantasy that disconnecting the 02 sensor somehow improves performance, it is out of the picture after 35% throttle.

Hmmm... that's a new one on me. What's the function that makes that happen? Doesn't sound like an idle switch or a WOT switch. If a heavy load (dyno, long steep hill, etc.) demanded a steady, say, 50% throttle, wouldn't the system still be running closed-loop?
John

[psalt]

No, most modern engine management systems go open loop under WOT. The purpose of the closed loop system is to maintain converter operation, not to maximize power or cruise efficiency. A SI engine makes best power around 12.5:1 AFR, not 14.7:1. Most of the fuel below 14:1 does not actually burn, it is a coolant to allow more spark advance. Turbo cars need even richer mixtures at WOT. CIS lambda does not have a continuous throttle position sensor, it uses three different discrete switches and goes open loop at 35% throttle. A 14.7:1 AFR also does not result in the best efficiency, we are giving up perhaps 20% because we have chosen to demonize 3 emissions. But emissions are a political issue, nothing to do with science.

[JohnKo]

I understand most of that, but I still don't grasp where the 35% throttle, open-loop configuration comes from. I understand idle position switch and WOT switch (and open-loop at WOT) -- what's the 3rd one? Isn't the system running closed-loop when it's off-idle (idle switch not triggered) and before the WOT switch is actuated? Or, are you saying the WOT switch is tripped at 35% throttle?

[psalt]

Yes. On the 911 81-83 lambda CIS there are three switches, idle and 35% throttle (WOT) on the throttle shaft and a separate 15% throttle switch for 2 seconds of cold enrichment sustain below 35C. Once you understand the system, it is pretty clear that Porsche got the initial calibration wrong in 1980 and covered the system with bandaids in 1981, hoping it would go away with Motronic. It did, but it can work suprisingly well, despite being essentially an underground sprinkler system

[JohnKo]

Originally posted by psalt
... it can work suprisingly well, despite being essentially an underground sprinkler system

Ha! Very good! My CIS experience is largely based on other German cars, and I don't currently own a CIS 911, so I didn't realize that 35% and WOT references were essentially the same. Thanks for the clarification. Hope the OP got something out of this!

[fastbacker]

Originally posted by JohnKo
Hope the OP got something out of this!

I certainly did. Thanks all.