Closed loop O2 sensor considerations.

[Sam Powell]

So it looks like I could try raising the correction value at temps above 200 maybe. I should take the lap top with me and see what the MAT is on those hard, hot start situations. I should say, it starts fine, it just runs badly until the AF ratio stabilizes. I will play with it. Thanks for the idea.

Sam

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[supton]

What O2 sensor are you using? I did some quick reading (here), as I knew some sensors were heated, some are not. It appears heaters are used so that the sensor reports properly, more quickly. If that is true, then I wonder if your readings right after starting might not be as correct as you might think they are.

It appears that O2 sensor temperature is determined off the resistance of the heater, which would require measuring the voltage and current to the heater, which doesn't appear trivial to me to do on a real time basis.

[Sam Powell]

My concern is not the read out of the AF meter. It is the poor, shaky idle. As soon as the AF ratio goes from 17:1 down to a better 14.5:1 or so the idle is good. If I turn my IAT bias dial up immediately to make the ratio 14.5, the idle gets good. So, it would seem the meter is reading correctly. 14.5 idles well, 17:1 does not.

The major problem with turning up the bias knob is that it is really easy to forget it, and leave it rich. When I have done that the ratio goes richer by about a point. Then it idles at 13.5 instead of 14.5. It idles nicely there, so it does not get my attention. Since the pot rotates a full 320 degrees, it looks nearly the same full, up as full down. Thus it is easy to forget.

This situation with lean hot start ups is much better with the closer location of the sensor. Before moving it, the engine would hardly idle at all until the AF ratio read richer.

The O2 sensor is a wide band, with a heater. Until it gets up to temp, which takes only a mater of seconds, the gauge reads a bogus 14.8, regardless of the fueling.

Thanks for thinking about this. It is nice to get the feedback. What is your experience with EFI?

Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[supton]

My experience with EFI pretty much starts and stops with "I put the key into the ignition, and it starts." I recall going through an S10 carb once, and I've had to mess with Briggs and Stratton motors on occasion, so I have more experience with carbs.

17:1 is pretty lean. When you run that lean, do you have more timing advance? Everything I've read indicates that lean mixtures burn more slowly, thus requiring more timing advance. You see this with vacuum advance on the distributor. When you say you are adjusting only fuel ratio, are you leaving timing advance alone?

[Sam Powell]

17:1 is too lean for the /6 combustion chamber design. It does not work well. That ratio happens only by accident during hot restarts. Read this thread to get the back ground on it, and the band aid that was developed for dealing with it. It works. It is not the ideal solution, but gets you going when you stop at the bank, or to get gas.

http://slantsix.org/forum/viewtopic.php?t=45071&postdays=0&postorder=asc&start=15

http://slantsix.org/forum/viewtopic.php?t=45923

I tried to find the thread that has Gunpilots diagram for the circuit that fools the IAT temp feedback, but could not find it. Anybody know where it is?


The timing thing is actually the reverse of what you said. Leaner mixtures burn hotter, and are more prone to pre-ignition. You must thus retard the timing some with the leaner mix. The magic bullet is lean enough not to preignite under ideal timing placement, which is a matter of getting the spark to happen so the flame gets across the cylinder at the ideal time to impart maximum force downward on the piston, and NOT impede its progress upward.

Guys who pilot small aircraft watch the exhaust gas temp, and lean out the mix at cruise so the exhaust gas is 50 degrees either side of max temp. More lean = higher exhaust gas temps.

It is my understanding it was Lindbergh who first worked on this. That's how old the idea is. It was not readily accepted in the field at the time he flew in China prior to WWII, but he managed to get the guys to listen to him. The realized after a bit he was flying further, and staying out longer than any of the other pilots.

Sam

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[supton]

ah, I think I see--MS controller "thinks" the intake air temp is very high, so it leans it out--too much. So you fool the computer into think it's cooler, by shunting the IAT with an external pot. Hmm, do you have an electric fan on the radiator? It's not unknown for late model cars to run the electric fan for a few minutes after shutdown. I wonder if blowing air over the engine (and radiator) might help get rid of heat buildup. Say 5 or 10 minutes; might want a real temp controller (IOW, if engine temp is greater than 180F, run fan for 5 minutes after shutdown).

Also: on your shunt, have you thought about installing a relay, so that the relay disconnects the shunt? I'm thinking, again, relay on for temp above a certain level; relay off after a fixed time.

But I'm kinda surprised that you're having these issues. Well, maybe not: most MS installs are probably on V motors. And probably a few are after raw power, not mpg and maybe not "nice running". Beats me. Still, I'm kinda surprised there isn't more code options for the issues you're seeing.

[Sam Powell]

The Accel GEN VII ECU I tried to run for awhile had a temp sensor for the manifold. There was a tune tab for manifold temp, and you could change the mixture relative to manifold temp. This allowed me to fix the hot start problem completely. There is no such thing on MSII.

Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.

[supton]

As for the timing: I'm not sure I'm following. [I really need to go read up again, been a few years. Plus, all my info would have been from a Chilton's or from Hot Rod.] I recall lean running engines ran hot because they tended to exhaust still-combusting mixtures out the exhaust. [Which can burn exhaust valves.] One advances timing to "beat" the slower burning characteristics of the lean mixture; but one then can have detonation, because, if it's too advanced, the resultant high pressures in the cylinder (because the piston is still rising) can auto-ignite the mixture elsewhere in the cylinder.

Interesting on the pilots adjusting for max exhaust temp. My first thought (influenced by the above, and likely to be as wrong as the above, if I'm wrong) is that they are leaning out as much as possible. I'm pretty sure it's hard to get detonation in aircraft, as avgas is very high octane, and the engines tend to run low(er) compression, as durability is of paramount concern. I could be wrong though--I'm scared of heights, and even a lowly Cessna can get high enough off the ground for me.

I'll go dig through my library. I recall a Chilton's for late 70's Impala's around here somewhere...

[Sam Powell]

Best economy is 50 degrees off of peak EGT, either side. ON the lean side does not feel as smooth as on the rich side. Pilots tell me if they have passengers they chose the rich side to make the passengers feel better about things. If not, then they chose the lean side. Running at peak EGT can burn valves. None of this has any consideration for the EPA or environment.

Sam

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A REAL professional is someone who knows when to hit it with a bigger hammer.