What is the ideal afr at idle? My Dodge ram stays right on 14.7 most of the time but however it is EFI.
Thats a balance between reducing emmisions, keeping the cat happy, maximizing fuel economy for that particular engine. The cat will cleanup the high NOx. Pre-cat smog control targetted 14.2 at idle, and did it in combination with retarding timing and increasing idle speed to offset that. That's right in the Service manuals and
tech bulletins.
As lean as it will tolerate at interstate speeds.
By doing some research it seems everyone has different afrs in mind on the subject.
Even throwing out all of the opinions based on conjecture and hearsay, there really is no single answer. There are general trends that were well established by testing and then the details vary with specific application.
If you're just going for engine effiency (in terms of making good power for amount of fuel used) and not dealing with emissions requirements, then the general trend of AFR needs can be described as follows:
"The [relationship] is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or" somewhat more. Cruising, especially at lower speeds, requires little throttle and puts relatively little load on the engine. Half throttle or mid-load would be going up hills or part throttle acceleration. "The thing to understand is most engines respond to being leaner...at part-throttle..."
In summary "at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT." The load where richer is needed varies with engine, gearing and vehicle. It may be 60-70% as shown here, or as high as 90%. Load relates to manifold vacuum, and therefore is used to signal the point where enrichment is needed.
F is
Fuel/Air ratio for gasoline. Invert the numbers to convert to Air/Fuel ratio.
0.08 = 12.5 AFR
0.667 = 14.7 AFR
0.06 = 16.6 AFR
Constant power is any steady throttle condition.
Steady 15% might be something like driving 40 mph on a flat.
Steady 25% might be cruising 60 mph on flat or steady 40 mph up a long grade.
Steady 100 % might be something like towing or dragging max load up up a really steep long grade, foot to the floor, without losing or gaining speed.
The
acceleration loop shows that maximum acceleration (full throttle) has about the the same fuel-air needs as constant 100 % power.
"This ... graph is from Walter B. Larew,
Carburetors and Carburetion. At the time he wrote his book on carburetors he was a retired Brigadier General who taught Military Science at Cornell, among his other accomplishments. He published this carb book in 1967. He didn't specify an engine type for this graph but his information is in the context of engines in general. His sources were most likely military aviation research. The math in his book is from NACA TR-49 and similar publications.
This graph is representative of a richer part-throttle that may be necessary to tolerate with an engine that has radical valve timing and perhaps not so good A/F distribution at part-throttle."
quotes and graph from Tuner on Innovate Motorsports Forum and full text now reposted at RFS
