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Author Topic: Measuring Air/Fuel Ratio for Tuning  (Read 1589 times)

Offline Rensch26

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Measuring Air/Fuel Ratio for Tuning
« on: November 03, 2016, 08:03:13 pm »
Hey everyone,

I was just wondering if anyone on the forum had ever tried to measure the air/fuel ratio with a lambda sensor installed in their exhaust? I don't mind tuning the old fashioned way but I think it would be kind of neat to try. I'm aware the oil will decrease sensor life. There are a few popular brands out there: Koso, AEM, Innovate, etc. I've got an older pipe sitting around and it just got me thinking it would be interesting to install a sensor in it and just use that pipe for tuning purposes.

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Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #1 on: November 03, 2016, 09:54:18 pm »

The O2 sensor will give an accurate reading of the oxygen in the exhaust of a two stroke but it will not give an accurate reading of the oxygen that was left over from the combustion process.  Two strokes and four strokes do not have much in common when it comes to exhaust gas byproducts. 

I have responded to this question in detail somewhere on this forum. 

Offline Rensch26

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #2 on: November 04, 2016, 08:19:07 am »
Yeah that was something I was wondering about as well. Since an amount of fresh air and fuel flow straight through the cylinder when both ports are exposed I would think that the AFR would be thrown off to some degree. Due to this, I would imagine the sensor would read a rich mixture in comparison to a four stroke engine.


Offline Q2W

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #4 on: November 04, 2016, 12:26:19 pm »
Yeah that was something I was wondering about as well. Since an amount of fresh air and fuel flow straight through the cylinder when both ports are exposed I would think that the AFR would be thrown off to some degree. Due to this, I would imagine the sensor would read a rich mixture in comparison to a four stroke engine.

i'd imagine it would read a lean mixture since a good amount of unburnt fuel and air is sucked back in the motor through the pipe.

Offline Rensch26

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #5 on: November 04, 2016, 01:11:16 pm »
Yep, you're absolutely correct. My bad. The fresh air going through the cylinder would cause it read as a leaner mixture. Perhaps a ~15:1 ratio measured by the sensor would actually give you close to the ideal 12.5:1 ratio for maximum power in the cylinder? Would you assume the readings would vary some depending on how far from the exhaust port the oxygen sensor was placed in the exhaust pipe? Due to that very reason you listed above with the fuel and air being sucked back into the cylinder?

Offline Q2W

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #6 on: November 04, 2016, 03:21:22 pm »
Maybe run 2 o2 sensors.  One before the expansion chamber and 1 after.  Take the average of the 2.  Totally guessing here.   +k2

Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #7 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.



Offline Rensch26

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #8 on: November 05, 2016, 06:53:02 pm »
Thanks for the information. All of it was interesting to read and informative. I actually work for Keihin so I obviously support carburetors but in a world of increasing emissions regulations it's inevitable that 2-strokes will all move to fuel injection in order to survive. It's very interesting as to what it takes for EFI to work in 2-strokes reliably.

Offline Q2W

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #9 on: November 07, 2016, 08:16:22 am »
Stickied this post so it's easier to find.  Great info as usual Jerry!

Offline Hotbutta

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #10 on: April 22, 2017, 05:57:01 am »
Come on Jerry, many of us with your pipe and setup would like numbers for proper EGT. Please pick a certain probe, a certain gauge and a certain distance. Putting more Precision in HPR.

Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #11 on: April 22, 2017, 10:58:33 pm »
Thanks for the information. All of it was interesting to read and informative. I actually work for Keihin so I obviously support carburetors but in a world of increasing emissions regulations it's inevitable that 2-strokes will all move to fuel injection in order to survive. It's very interesting as to what it takes for EFI to work in 2-strokes reliably.

Throttle body injection, crankcase injection or port injection does not or has not been able to make a two stroke with a tuned pipe meet the emissions standards of most countries.  I was able to make some of the test EFI test engines we were working on in the early 1990s meet the standards at the torque peak where the scavenging and pipe are working together at their best.   The emissions were off the charts when you are 1000 RPM from the torque peak. 

Some of the military drone engines are using EFI but the governments around the world do not care about emissions on the military stuff.

The boating and snowmobile industry has had success with direct injection into the combustion chamber since around 1995. The price of the two stroke boat engines just about doubled in price with the direct injection technology.  The cost of repair of the direct injection systems are very expensive. 

Engines that operate at a fairly constant RPM where the RPM does not change 1000 RPM or more per second, direct injection seems to work but has not been successful in the two stroke racing engines. 

Rumor has it that KTM will be releasing fuel injected two stroke dirt bikes this year.  It will be interesting to see what they have done and will it be possible to retune it after changing the tuned pipe, ports, reeds or air filter?

Offline MotorGeek - Jerry Hall

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Re: Measuring Air/Fuel Ratio for Tuning
« Reply #12 on: April 23, 2017, 12:47:48 am »
Come on Jerry, many of us with your pipe and setup would like numbers for proper EGT. Please pick a certain probe, a certain gauge and a certain distance. Putting more Precision in HPR.

In the old days we used EGT as a tuning crutch on the enduro go karts with limited success.  The RPM changed slow enough that the durable 3/16'" diameter EGT probes would have time to saturate and keep up with the engine RPM.  In order for a guy to know what EGT was critical to his engine package he usually had to sacrifice a few pistons to determine what the critical temperature was for his engine. 

EGT changes with RPM.  It is not uncommon for the EGT to change 100 to 200 degrees from the start of a dyno run to the end of the dyno run.  Durable EGT probes often take many seconds to heat up or cool down to the temperature of the exhaust flowing over them.  Fast responding probes are fragile and often break due to the vibration and high temperature.  Most of the portable EGT gauge/meters I have used, sample 2 time or so per second.  The data acquisition system in the dyno room has the ability to sample 100K per second.   

One jet size will usually change the EGT on most engine 25 to 50 degrees F.  Moving the timing 1 degree can change the EGT around 50 degrees F.  One compression ratio point can change the EGT 50 to 100 degrees F.  Engines with transmissions  will accelerate 2000 to 3000 RPM per second in the lower gears.

As the compression ratio goes up EGT goes down.   Lower compression  makes the EGT go up.  Advancing the timing lowers EGT.

Considering the above information one can easily see that it is difficult to get accurate EGT readings in the field to correlate with what we see in the dyno room.  Most guys will burn a piston trying to get the EGTs we see on the dyno. 

Use the jet that makes the engine ACCELERATE the best and NOT the jet that gives the highest top speed.  If you optimize the gearing and jetting, the highest top speed will occur when the engine is at it's power peak at about 7600 to 7800 when you have one of our engine packages with the HPR 19 pipe and silencer. 

Running down a road with stock gearing, in high gear, will have the engine RPM way past the power peak where the jetting is usually a little rich.

If you have about stock gearing like most guys run, jetting  a little lean will give the engine more over rev (power after the power peak) while hurting power in the mid range and at the peak.  Higher EGT will shift the power curve to a slightly higher RPM thus giving a slightly higher top speed with the possibility of burning a piston. 


 

 

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