as above realy! Ive read many threads on it but need it explaining in plain terms What happens exactly?
Thanks

Gary.

 

I presume you're refering to fueling...

The car has a MAF sensor - it measures the incoming air

There's also a lambda sensor in the exhaust - it measures things in the exhaust gasses to determine what the air/fuel mixture in the engine has been.

In the open loop case, the car ECU looks at the incoming air flow from the MAF and the RPM of the engine and decides how much fuel to throw into the engine. It doesn't take account of the lambda sensor - it all comes from the ECU making a decision on the engine inputs.

In closed loop, it's a little different. The ECU still looks at the MAF and adds fuel, however it also looks at the lambda sensor to see what the air fuel ratio was. If it indicates lean, then the next cycle round, the ECU adds a bit more fuel than it was originally going to do. If the lambda says rich, then it doesn't put as much fuel in.

Closed loop control is used to maintain an AFR of 14.7:1 for emissions testing (this is the point where the emissions are lowest)

HTH

 

Or another way is when you're cruising on light throttle the ECU tries to run as close to 14.7:1 Air To Fuel ratio to get some reasonable economy using the lambda sensor, this is in closed loop mode.

If however to put your foot down to extract some performance it switches to open loop mode where fueling is based on tables in the ECU.

In both cases the MAF is used. The way I remember which is which, is that open loop mode is at Wide Open Throttle (WOT)

 

Rite then, so basicaly, on WOT the lamda sensor is left on the shelf and all the AFR goes over to the MAF to control in conjunction with the ECU (obviously)
Then, on light load or idle the lamda sensor gives a hand simply to try and maintain some decent ecomomy and to keep emmisions legal while at idle
Hope thats rite.
This would explain why i was once told that cats are virtualy useless on WOT.
Thanks guys.