100LL and O2 sensors; your thoughts

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DENSO First Time Fit® oxygen sensors are coated with a special formulation of aluminum oxide to trap impurities outside of the ceramic element. In head-to-head testing against the leading competitors, DENSO oxygen sensors were rated two times more resistant to silicone and lead poisoning. At DENSO, we bring together everything we know as an OE supplier to provide aftermarket oxygen sensors that truly are Better By Design™.

And while I hate to bring it up, the race engines guys swear by them. Their biggest tip was that the O2 sensor should not be put into place when tuning a leaded fuel motor until the mixture was close. Putting an O2 sensor into an exhaust that was rich would kill it, so dial it in close, then add O2 and finish the mapping.

This was a common tip, and guys said the O2 sensors lasted years.

I think this is a valid concern vs the shade tree guys just starting a random motor then say 'muh O2 senor died'

 

Well, if avgas lead won't deposit over 250C which is below the 316C minimum operating temperature of O2 sensors, and lead doesn't poison but just smothers the sensors. It can't be the lead in avgas that kills O2 sensors.
So, what is it?

 

"lead doesn't poison but just smothers the sensors"

The lead doesn't 'kill' the sensor. It simply coats it. The sensor is alive and well, but since no exhaust gas can reach the element, it gives bad readings.

Ninja edit: Even O2 sensors in unleaded fuel cars 'wear out' not because the sensor materials are bad, but soot/debris clogs the porous grid, cutting off the exhaust gas from the sensor.

[Moderator Edit]

 

This is a pretty good video



Now just think, if you coat, with lead, the outer element so exhaust gas cannot get to it, the potential will always be 0 as nothing is pulling the ions from the inner element. Nothing is 'broken' its just that the electrical circuit is never completed, so the ECU trips a code.

Most upstream O2 sensors are pretty far from the exhaust port. Usually after the manifold/downpipe connector, at a foot or so away from the exhaust valve, giving the gaseous lead time to solidify and coat the the element.

My idea would be to move the O2 sensor much closer to the actual exhaust ports, where the temps are still quite high and the lead is still gaseous. How do I know this works? Because every 100LL engine relies on this principal. When you have lead fouling on the plugs, you raise the RPM and lean the mixture to literally melt the solid lead (which is doing nothing more than coating the spark plug, acting as an insulator) off the plug. The plug is never damaged by the lead, just needed cleaned off, and you do that with HEAT.

Every O2 sensor can live in a leaded environment, the issue is managing and extending that life to a financially viable level. They will still clog, but if there is an easy way to take advantage of the additives in the 100LL fuel now, it could be as simple as moving the sensor closer to the engine where it would not see solid lead, or the rate of buildup is significantly decreased.

 

Gee, I guess that you have solved a problem that nobody else has managed to do!

 

Sounds like using electrolysis could work....

 

So I know jack about O2 sensors, but I do know what I've been told by three people who manufacture and sell EI's and EFI systems that are used on Lycoming aircraft engines running 100LL, and that is that a wideband O2 sensor will last 100 - 200 hours in normal usage, and given their relative cheapness, you throw it away and replace it as part of the CI each year. Seems like a cheap enough maintenance item to me, given that 15W-50 oil is $10/quart.

 

How many auto conversions are running stock ECUs with O2 sensors in closed loop and 100LL? I think even SDS EFI is open loop, and O2 sensors are only used during tuning (I will admit that its is just a guess from reading the web site).

I have yet to see a modern conversion where the o2 sensor was moved from the stock location. Well, in an lead fuel environment that's not going to work, or if it does, the O2 sensors are no swapped out until failure, instead of as a consumable with limited lifetime.

I'd like to see your auto conversion. Is it running closed loop? With 100LL?

[Moderator Edit]

 

Agreed, I think the sensors are not a huge issue, and that they need to be treated as consumables. But for an auto conversion, the stock location may not be in the best place for longevity.

The conventional wisdom is that the O2 sensors will die, the ECU goes into limp home mode, and your airplane bursts into flames and you die.

If the O2 sensors goes out of spec, you go open loop, waste a lot of gas, and get a CEL light. On a V type engine, you'll have banked O2 sensors, I'm not sure how the ECU will handle that, use the O2 data for all injector trims? Go back to the maps?

But its not like the engine stops and you die.

 

So, here's a bit of actual experience... I installed a Ballanger AFR unit with a Bosch wide band lambda sensor in my IO-360 powered Velocity several years ago. I use an input on my Garmin G3X to monitor it, since I didn't want to cut a new hole in the panel for the display. The O2 sensor is located maybe 4 - 6 inches from the cylinder (i.e., close). I'm sorry I don't have a picture to post. I'll dig one up or take one if someone is interested.

The first sensor lasted maybe 20 hours. The second sensor lasted about the same. I did install a third sensor, since a trusted friend told me that the problem was that I needed to let the heater pre-heat the sensor before starting the engine to make it last. It lasted about the same amount of time as the others (although I have to admit that I didn't always remember to pre-heat...). I've only used the Bosch sensors, and they're running $70+ each... kinda spendy.

It is nice when working to see the actual air/fuel ratio. I used it to really understand and verify my mixture settings. I typically am flying someplace, and my normal X/C power setting is around 2550 RPM, WOT, and I lean to around 9-10 GPH. With the O2 sensor I know that's leaving me around 15-16 lbs of air per pound of fuel, i.e., pretty lean. My engine runs really well very lean since I'm running two Light Speed Plasma III ignitions.

However, since the last sensor failed, I've not chosen to replace it. I feel comfortable knowing where I'm at, and I don't see the need to keep spending $70+ a pop to verify what I already know.

I'm loving the discussion - please keep it going - but the question I'd ask is, if you're not going to close the loop, why install an O2 sensor?

Dave

 

If you want to close the loop manually you might want it. Maybe you just want to know and would rather use AFR than EGT for leaning. You might also want it while programming/tuning an open-loop system.

That's funny, because most of the EFI systems available for airplanes right now run open-loop. It works pretty well and still gets good fuel savings. Closed loop would be nice and convenient but it works well without.

In a car, yes. Not in an airplane, especially during critical phases of flight. Last thing I want is limp-home mode triggering at 400ft after takeoff, because my OEM ECU is looking for some parameter the car provides but my airplane does not...

 

tuners can disable almost all the unneeded sensors so you won't trigger limp home. Without knowing exactly what WILL trigger limp home, its kinda hard to say why its bad. Manufacturers do sell 'stand alone' ECUs for engines that run things like pumps, generators etc and its basically a stock unit that's had the 'car stuff' turned off.

And guess what, your $50k certified engine can break, with no warning, at that same 400ft. So to say that you fear the ECU is more superstition than reality. Its just another old wives tale that people refuse to let die.

Thank you for the information. I found the following and you seem to not have fared as well:

http://wbo2.com/lsu/lsu4.htm

Depending on the lead content of the fuel used, the expected service life time is: (preliminary data)

• for 0.6 g Pb/l: 20,000 km
• for 0.4 g Pb/l: 30,000 km
• for 0.15 g Pb/l: 60,000 km
• for 0.00 g Pb/l: 160,000 km (ie. unleaded)

Extrapolating this Bosch data suggest that at 1.5 g/l the life may be as low as 2,000 km or just 20 hours at 100 km/h. AvGas 100LL has 0.56 g/l Pb, suggesting a sensor life of 200 hours. Compare this to a suggested life of 1,600 hours for unleaded.

 

I think the real question is to determine the operational cost difference between open and closed loop mode and see if there is enough fuel savings to justify the extra cost of treating the O2 sensor as a consumable.

I've read that there is 20% or more fuel savings with close loop. At your 10gph cruise just became 9hph (or 8gph if running well)

In a modern conversion engine I don't think open loop is an economical option. You'd be better off with a carb and EI so you could throw any fuel available at it.

But looking at v6 automotive engines, the recent engines are making HP that last gen motors couldn't reach even if highly modified.

Could the power and weight of an LV3 realized in a 2000-2010 era motor?

 

I think a real issue you're going to run into trying to make your Lycoming or Continental motor into a closed loop system is the lack of a knock sensor. To really stretch out the LOP operation, especially at greater than, say, 75% power, you'd better have a way to avoid detonation (like a knock sensor).

To me, I added the Lambda sensor just to make sure I understood where I'm operating. After operating with a Lambda sensor for awhile I feel like I've manually closed the loop (at least enough for myself).

I lean based on power settings (% power calculated by my Garmin G3X), and my fuel flow rate. I don't spend time looking for EGT peaks, because that just increases the dwell time in the highest cylinder pressure portion of the leaning operation. I follow what Mike Busch says... just chop the mixture fast and get to the fuel flow you want. Without a knock sensor, that seems to me to be the safest way to LOP operations...

Dave

 

I agree a knock sensor is critical, but most certified engines (under 200hp) rarely hit higher than 8:1. Where as a modern conversion is at 11:1 on 87 octane.

While knock and detonation is bad, are certified engines are more susceptible to damage due to the construction??

I mean 100LL, low compression, and low RPM. Where does the knock come from? If I check the ECU on my cars, I doubt that it would have recorded any knock triggers/events.

 

compared to what??? when the first O2-sensor EFI (closed-loop) were introduced in 1986 (catalytic converters made compulsory), the fuel consumption of the same engine went up by around 5-10% due to the forced lambda 1 operation, cutting out the lean mode in part load. Even in an aircraft application, you are better off with an open-loop system as the ECU can be programmed to run rich at full power (12.5-13.5 for max power) and lean in economy cruise (up to 17 depending on engine). Going closed-loop, you lose that ability to control the mixture (most systems still go open-loop under full power based on mapped data but no feed-back). Best option is a wide-band sensor with a display and a manual fine-tune feature (or eco, normal, power modes) for the ECU once it is fully mapped. For that, the sensor doesn't need to be in the full exhaust gas stream and you can reduce the exposure to Pb.

 

My preference would be see if you could get the 200 hours of life out of the O2 sensors, heck even 100 hours would be great for fuel savings.

Maps are nice, but fuel trim is where the money is saved. In an AC engine you can lean ROP/LOP, with an ECU you could even add in a method to alter injector timings.. but then you have a workload issue. I'd prefer to devise a way to let the ECU do whats it's so good at, engine management.

Manual fuel trim is an option, but to me, not a preferred one.

 

what criteria for performance evaluation might one use....temperature, head pressure..... Lycoming has temperature limits, but head pressure ....I do not think so. How would you know if what you set is safe....knock sensor? Really out of my experience I usually prefer stock but I am curious.

 

I would rely on the ECU and the knock sensor. With a modern conversion there is no need to push the limits. They make great HP and when the fuel system is set up properly, will have respectable GPH numbers.

200hp is 200hp, so I don't expect to get it to 5gph, but closer to 7 is better than over 10.

My guiding principals are mostly economic, not efficiency. I'll take 10gph if that meant open/mannual loop vs 10 hour O2 sensor lifetime, or worse, not flying at all.

 

Depending on LOP your going to need close to 12 gph to get 200 hp closed loop or not.