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Author Topic: Measuring Air/Fuel Ratio for Tuning  (Read 2739 times)

Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« on: November 04, 2016, 07:23:05 pm »
 I have not found my post explaining some of the unique phenomena that occur in a two stroke engine that causes the O2 reading to mislead the majority of dyno operators trying to use them as a tuning tool.

Here are some links to some of my post on O2 sensors.

http://www.suzukiquadracerhq.com/engine-13/fuel-injected-lt500/msg36770/?topicseen#msg36770

http://www.suzukiquadracerhq.com/engine-13/fuel-injected-lt500/msg11685/?topicseen#msg11685


http://www.suzukiquadracerhq.com/lt500-general-discussion/jetting-advice-(ballpark-starting-point)/msg41226/?topicseen#msg41226

I found some O2 stuff that I had written in response to a question from an engineer that was trying to understand why he could not get the type of O2 readings on his two stroke engine development projects that are common on cutting edge 4 stroke engine projects.


Xxxx  I do not use O2 sensors to tune carbs or EFI on a two stroke, but they can be useful to indicate if and when short circuiting, over scavenging and when poor trapping may be occurring.  The sensor will indicate a lean condition when any of these 3 conditions exist.  Remember misfires, and air/fuel mixture that does not go through the combustion process will cause the O2 sensor to read lean.

You can have the main jet that makes maximum torque and the sensor may read 13:1 at lean best torque on one engine and 15:1 on another engine.  So what do these numbers tell us on a two stroke? ...........

My interpretation of my testing on various two stroke engines is:

The O2 sensor is seeing an average mixture in the pipe of 13:1 on one engine and 15:1 average in the pipe of the other engine.  It does not tell us what the O2 reading was in the cylinder before being released into the pipe where it is now mixed with over scavenged and short circuited mixture from the last or many scavenging cycles ago.

There is the possibility that the engine that had a 13:1 sample in the exhaust pipe, had good scavenging and trapping and actually had a 13: 1 mixture being released into the pipe.      Or....it could have had a 12.5:1 mixture that the cylinder released into the pipe and had a little air/fuel mixture that escaped into the pipe and did not make a trip through the combustion process and added 0.5 points to make the sensor see a leaner mixture.

The engine that had the 15:1 sample taken from the exhaust  could be a little lean but is probably not very lean or it would not have survived a dyno pull without killing a piston.  It does tell me that scavenging and trapping is poor because the O2 reading in the pipe is probably at least 1.5 points or more points higher than it could be and have the engine survive.

An extremely lean reading somewhere in the RPM range on a two stroke is usually a clear indication that short circuiting is occurring at that RPM (Providing that misfiring is not occurring at the RPM where the lean spike is located). This lean spike usually occurs just before the engine comes on the pipe.   A lean mixture on either side of the torque peak may be an indication that poor trapping is occurring or the O2 sensor may be too close the the end of the pipe and is getting some oxygen from the outside atmosphere.  The difficult part is determining at what part of the engine cycle fresh mixture is escaping into the exhaust system.  Is the diffuser signal too negative following blow down, are the scavenging steams not aimed optimally, or is the plugging pulse from the pipe arriving at the wrong time or do we need a different tail cone angle?



 I have written a lot more on using O2 sensors in two strokes and why they are not a useful tuning tools.  I will keep looking through some of my notes.



 

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