swan

'ello all,

Does the ECU actually use the input for anything besides measuring the efficiency of the catalyst?

stevegarner

Swan,
Yeah, the ECU will use the O2 sensor to alter the air/fuel ratio while it's running in closed loop control. In open-loop conditions (i.e. high load) it'll just get the AFR directly from the map and won't bother using the O2 sensor to correct it.

There's some good info on this on the ECUTEK site:

http://www.ecutek.co.uk/frame_tuning.html

Cheers,
Steve.

swan

Thanks Steve. To confirm my understanding, on idle/low revs it's in closed loop mode, value should be betwen 0 and 0.9. At some point (any idea what?) this switches to open loop and the ECU reads the map. The O2 sensor readings should then (according to the Ecutek site), not drop below 0.7.

A few questions:
When does the stock ECU/Prodrive ECU switch between open and closed loop?
At idle what should the O2 readings be, 0?

Pete Croney

Swan

The 01-03 cars use a wideband lambda senor for fuelling and this is in the exhaust manifold. They also have another lambda sensor after the downpipe and this is used to determine cat efficiency.

The classic shape cars have 1 lambda sensor and this is used only to determine fuelling on closed loop. On the classic, the lambda sensor is before the cats and can be in the exhaust manifold (early cars) or in the downpipe (97-00).

swan

Pete, for the 01-03 cars, is the lambda sensor then the only one used for fuelling? What's the O2 sensor in the down pipe used for if not as above? Is the last lambda after the down pipe only used for cat efficiency or does the ECU use a mix of signals from the two lambda's and the o2 sensor?

*Sonic*

If the O2 (Lambda) Sensor is only used in low revs/idle, and reads between 0-.9v how come the AFR sensor does the disco lights thing when on low revs, and also how come the AFR only reads properly on WOT, when I asume this is when the Lambda is no longer used, and the MAF is

Or is it that at that point it switches to the MAP, the Lambda should be reading roughly 0.9v

Steve

swan

Sonic, may be a case of the blind leading the blind but:

At lows revs the O2 signals vary and so too would the AFR ratings, esp. if there's air fuel correction taking place. I'd say it stabilises as soon as the circuit opens (obviously WOT qualifies here) as the map is then read and no fluctations/corrections take place. That's also when you want a high reading to ensure it's not running lean. My understanding is a minimum of 0.7, 0.8 ideal, 0.9 rich.

dnb

The MY96 UK cars also have lambda sensors in the downpipe, rather than in the exhaust headers...

As I understand things:

The output signal of a lambda sensor is essentially a step response to the mixture of the exhaust gasses. What I mean is that it outputs a low voltage for lean and a high voltage for rich. There is no in between - so the signal will oscillate up and down (at a frequency of approx 1 Hz) as the ECU adjusts the closed loop fuelling to achieve an AFR of 14.7:1

When in open loop mode (eg under boost) the lambda sensor just sees rich exhaust gasses, so outputs a high voltage.

Some AFR gauges "average" the readings from the lambda sensor so that it doesn't appear to dance about under closed loop, and some don't...

swan

But the voltage reading in open loop mode varies too which means that it is still generating "accurate" figures as opposed to a "high" figure.

dnb

It's not precicely a step response, so there is a litte bit of "slope" to the characteristic... So yes, you will see some varying readings of rich and the same for lean.

I very much doubt it is linear, so I wouldn't put much faith in using it to say how rich or lean the car is running...

swan

So in open loop the oxygen sensor isn't necessarily giving an accurate reading?

dnb

That's my understanding, yes. (Unless of course it's a wide band sensor and you are still in its range of operation.)

You'll be able to see that the car is running rich or lean, but not get an accurate AFR reading from it.