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 Desired Carburetor AFR Characteristics At Different % Load 
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Joined: Fri Feb 08, 2013 2:37 pm
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The clue here is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or so.

The tuning goal is to find the leanest mixtures which the engine will tolerate without missing or surging in level road cruise and moderate to intermediate acceleration. As load is increased engines will tolerate leaner A/F. Flat level road load will nearly always need to be richer than ¼ or ½ throttle acceleration. In fact, the closer to WOT, the leaner an engine will run, although approximately 15% richer than 1λ A/F (12.5/1) is necessary for best power and engine safety.

The first 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.

The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion.

Using a WBO2 and a vacuum gauge to monitor this you will see improvement as you adjust the primary main jet to find the best A/F for moderate acceleration in the load range between a level road, perhaps 14-12 in. hg. and the point where the power valve opens, perhaps 8-6 in. hg. The engine's part-throttle acceleration will noticeably improve as the AFR is adjusted to the lean side of stoichiometric.

When the air bleeds are configured correctly the A/F will progressively become leaner (from the rich idle) as the throttle is opened, until reaching the low vacuum load point where the PV opens.

The key thing is, 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.

Image

The second graph (Fig. 5-8. below) is from C.F. Taylor's book, “The Internal Combustion Engine in Theory and Practice”. Taylor is summarizing lab work at MIT with commercial engines and single cylinder research engines. The upper (solid) line represents what can usually be achieved with most engines with mild up to moderately wild valve timing. The lower (dashed) line is what you might achieve with an engine that has very good A/F distribution, such as might ideally be achieved with EFI.

The third and fourth (the pair) of graphs (Fig. 11-3. below) are from Edward F. Obert's, “Internal Combustion Engines”. Obert was a Professor of ME and the Chair of The Department of Mechanical Engineering at the University of Wisconsin.


Attachments:
MIT-Taylor AFR.jpg
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Obert AFR.jpg
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Sat Mar 22, 2014 9:32 pm
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Joined: Sun Jun 23, 2013 1:41 pm
Posts: 375
Location: Phila. Penn.
The load vs. fuel requirement test results are really eye-opening. I think a lot of us equate the phrase 'part-throttle' in various books as 'steady cruising' at just about any speed. The same books tell us that as we open the throttle, the mixture gets richer. In both cases, they fail to really describe the throttle, AFR and load relationships.

Hope it is OK to quote you Tuner. This is from the old Innovate forum where a person found less jet was making less steady AF readings on the WBO2.
tuner wrote:
With such high cruising vacuum and sporty valve timing a mixture in the 13’s may be necessary to keep it from “hunting” or outright missing at light load. When the load increases and the vacuum is reduced the mixture should lean fairly quickly to leaner than stoichiometric. The high vacuum and valve overlap result in a high percentage of exhaust dilution and rich mixtures are more easily ignited.

A graphic representation of the A/F vs. load with fuel on the ordinate, rich at the top, and load % on the abscissa, 100% at the right, forms a bowl or “fish hook” shape. 0% load, idle, rich as necessary (13/1?) for whatever valve timing. As load increases the A/F line slopes down and to the right, leaner as the engine needs or will tolerate (16–17/1 for some engines), until the % of load, hence manifold pressure is high enough (vacuum is low enough), to open the power valve and the A/F hooks back up to the WOT mixture (12.5?). Clear as mud, right? In other words, at a 20 ft.lb. load on level ground with lots of EGR from the valve overlap a fairly rich mixture may be necessary, but at the 200 ft.lb. load you can apply by crowding the throttle for a lane change, a much leaner mixture will run without misfiring and save a lot of fuel.


The graph below you also posted on that forum shows that clearly. Its very similar but maybe more extreme than the one from Taylor.
(archived innovate thread at http://britishqueen.myfreeforum.org/vie ... 97&start=0 see post #17 )



tuner wrote:
This third 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.

The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion.

Using a WBO2 and a vacuum gauge to monitor this, as you lean the jet which controls the A/F in moderate acceleration in the load range between a level road, perhaps 14-12 in. hg., and the point where the power valve opens, perhaps 8-6 in. hg., the engine's part-throttle acceleration will noticeably improve as the AFR is adjusted to the lean side of stoichiometric.

The key thing is, 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.


Attachments:
Larew AFR Graph with notation.jpg
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Sun Mar 23, 2014 9:11 am
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