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Old 23-Sep-2012, 10:34 PM
pguenet pguenet is offline
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CO values for mixture

I am finaliy finishing the work on the ST and looking at setting up Tps / TB balance and mixture.

Tps is good
Tb balance as well

I have acquired a nice snap on reader and some Ducati header copper pipes. So proper tools now.
I started wih 1 quarter turn out on the front airbleed and would read 0.48% CO.

I have investigated a bit everywhere and 0.5% CO seems like a perfect value by most people's standards. Yet for the Ducatis, the recommendation seems to be to run at 4%.

Can somebody explain why such a difference before i start riching it up.

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Old 23-Sep-2012, 11:17 PM
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The air/fuel ratio that maximizes power has been shown to be about 12:1 or 12.5:1 and is about 20% richer than stoichiometric (14.7:1). 14.7:1 yields exhaust CO levels of less than 1%. 12:1 give about 6% CO. 0.5% is a target for emission control to address environmental concerns, not for engine power.

Since engine power is air-limited (not fuel-limited), for maximum power, you want to use up all of the air that gets ingested by the engine. So you richen up the mixture until the best tradeoff between over-rich power 'loss' and air utilization gives you maximum power. So up to a certain point a richer mixture burns slower, but produces more power. There’s a fuel/air ratio for optimum flame speed. Too lean, it slows; too rich it slows.

I refer you to:

Heywood, John B., Internal Combustion Engine Fundamentals, McGraw-Hill, 1988, p. 395

According to Heywood:

"Both flame development and burning angles show a minimum for slightly rich mixtures ([phi is approximately] 1.2) and increase significantly as the mixture becomes substantially leaner than stoichiometric. ... Faster burning engines (which have higher turbulence) are less sensitive to changes in mixture composition, pressure, and temperature than are slower burning engines (which have lower turbulence)."

In other words, richer (up to a point) burns faster (smaller burn angle), not slower. The minimum burn time appears to be right around the max power air/fuel ratio (phi about 1.2).

See pages 402 to 403, particularly Figure 9-25 at the bottom of page 403. Keep in mind that the figure is for a laminar flame, and which is not exact for the turbulent flame typical in an engine, but it is a clue and a start and not a bad guess for guessing the effect of changing parameters.

That said and to address your question, we find that generally peak power output comes at 4% CO.
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Old 23-Sep-2012, 11:44 PM
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All good stuff as expected

Just in case, some interesting setup info:


Cheers - Frank
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