Quote:
Originally Posted by blue62
Ike
That all seems like pretty normal numbers to me.
A couple of things to keep in mind:
14.7 fuel air ratio is not what the engine needs under all conditions to perform best.
An example is WOT.. it should go rich. Heavy engine load requires a richer condition.
Foot off the gas your getting 22: exactly you don't need fuel in the air to coast. your getting 22 because you don't need fuel under that condition but the engine still pulls in air.
Hell you don't need spark. Some modern day engines cut off fuel and spark all together under off throttle conditions. An AFR sensor will see all air (or nearly so) under off throttle conditions.
Another thing: 14.7 fuel to air is the ideal ratio for Catalytic Converter performance.
Not engine performance under all conditions.
On a modern day computer controlled engine the primary function of the DME/ECU system is "Catalytic Converter performance, engine performance is secondary.
The DME/ECU under closed loop conditions constantly fluctuates the fuel air mixture above and below the 14.7 fuel air ratio. That is why O2 sensor signals constantly fluctuate in a sine wave. The DME/ECU sort of looks for an average under closed loop conditions.
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Hey blue,
Thanks for the response. That's interesting about the sine wave pattern, which is exactly what I'm seeing. That seems a bit less efficient than a "hold steady" approach at control but maybe it's more effective to continually correct.
That's also interesting about the cutting off of spark while coasting. It makes sense to do so for fuel economy, as long as the alternator keeps spinning to keep up with current load demand.
The reason the lean tendency was surprising is just the excess heat and likelihood of detonation under those conditions. I see your reasoning though that if the motto is "emissions first, engine second" then it makes sense to do so.
With that being said though, the numbers on my drive this morning were much more centered around 14.7, especially while cruising under moderate load. I really wonder if the short term trims just needed 20 miles to adjust the mixture. I suppose 20 miles of lean isn't too bad considering the detonation sensors/ignition retard built in to the dme.
I think it's fascinating to learn more about how these computers actually do their job. Unfortunately, Bosch me 7.2 is kinda like this black box with very little info about the logic and processes involved. I don't think I've seen a single thread on this forum which is technically detailed regarding this (beyond basic principles of open/closed loop, short/long term fuel trims). I can't even find a good one on nefmoto, which is probably the best site on the web for that type of info.
Anyway, for me the enjoyment will have to be postponed. Looks like I'm leaking oil from the passenger side bank. Probably plug tubes, I should have replaced them when I changed the plugs and coils over the winter. Back on to the stands we go!
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