Effects of AFR on EGT, Power

[TimZ]

This is a branch from another thread - I wanted to continue the discussion, but didn't want to hijack the thread. Here's what got me thinking...

 

I would say the fueling is actually over-rich past your torque peak. We lost major power running anything richer than 12.5 almost every point past torque peak. From talking to people in general who have dynoed their cars and noted the changes, it seems that you need to actually pull fuel past torque peak.

 

For instance, on JeffP's car we started at something around 11:1 across the board, we started experimenting pulling fuel at various points watching EGT's as well. Some people said they tuned to 1650F, we didn't get anywhere near that point, but were amazed at higher rpms (with porting and a cam) that in some load bins we were actually running as lean at 13.8!!! Most of the stuff was in the high 12's, which was FAR LEANER than anything before torque peak.

 

I encapsulate this in a short synopsis, it took FAR longer on the dyno than we thought before we realized that running the same AFR past torque peak was actually too rich, and we were loosing considerable power.

 

Just make sure their datalogging equipment is calibrated properly, and stick with the same place to (as noted above) monitor relative changes from your adjustments.

 

I guess I should have worded it differently "I Mis spoke LOL LOL" What I was trying to get across was thyat the rich mixtures are just as damaging to the engine long term on the dyno.

The 11:1-5 AFR's are really bad in my opinion. The excess fuel going out the exhaust, on a turbo car has one more chance to be ignited, WHERE YOU ASK? In the exhaust turbine housing! That is BAD! Take a look at the picture I have on my webpage, the turbine housing got VERY hot and in my opinion the engine started going into thermal run-away. In fact, I will say that if you run the engine to 110 degrees C on a pull like I did ONE time, you just may find yourself with a bad valve seat around #4 in that area.

I put that car through FRICKIN hell that day, at least 25 pulls with full power. So it cost me a little, but I was able to figure out along with Tony that the richer AFR's in the 11 range up to peek torque only to serve to make the engine hotter then it would run with a leaner mixrture.

On the N/A car, the AFR's can be much richer then they need to be, and the waste is in the exhaust, no big deal, just bad fuel economy, but you take that same exhaust and run it through a very hot exhaust turbine housing and you start to run another somewhat of a second engine (the exhaust turbine housing) and that IS BAD.So the excessive heat is bad for the engine (cylinder head) and the pistons, depending on how they were honed to fit the cylinders. I have pictures of one build that the engine got very hot, and the pistons expanded about .0005 with the heat that left a nice 1/2" wide scrape mark in every cylinder. Was it enough to make a difference, in reality no, but none the less that is what happened to that build. The car got to 270 on the gauge and stayed there, the oil went to 130 degrees C and stayed there as well. I was out in the middle of the desert and really had not other alternative but to continue on and hope for the best. The next build the pistons were a little looser in the cylinders to provide the space for the pistons to expand and not cause problems running that hot for short periods of time.

Anyway, I'm all over it, I must find the correct solution to keep the engine cool, and I think I have, but testing will tell all. If the temp gets under control to my satisfaction with the modifications to the head and block I have made, then it will be a goal of 800 Hp, and guys (the nay sayers) I will run the rpm's to at least 9000 rpm's in that process.

So think about that, I don't think the honda boys will be able to compete if I can get the car to hook.

anyway

 

On my last set of dyno pulls, this wasn't exactly the behavior that I observed. As I've noted before my setup always seems to have some pretty high EGTs. Running E85 has helped considerably with its inherently lower combustion temps, but I when I tried leaning the mix out a bit, power went up, just as Tony and Jeff observed, but so did my EGTs. Still not as high as they were on gas, but higher none the less.

 

I've also been speculating that the cause for my high EGTs was fuel burning in the exhaust, and this does seem to be corroborated by the fact that the pistons and chambers looked really good the last time I had the engine apart, when temperatures that I was observing on gas (~1850 degF ) should have left some observable evidence had they been occurring in the combustion chamber. (Full disclosure - I did drop a valve seat, but it was the intake valve, and once we got into it, it appears to have been improperly installed, so not due to a thermal meltdown).

 

The thing I'm a bit hung up on is that if we are measuring a rich mixture (and even 13.8 is still quite a bit rich compared to stoich), then there should not be any oxygen in the exhaust to support the further burning of the excess fuel. So how's the rich mixture burning in the exhaust?

 

I think that I have an answer to my setup at least. I'm using a pretty big turbine housing - it's a full T4 split housing with a 1.15 AR and a full 4" exhaust, plus a 60mm wastegate with essentially its own 2.25" exhaust. Here's a plot of my exhaust Backpressure on my best dyno pull:

 

As you can see, the exhaust backpressure (measured at the turbine inlet) is the same as or a fair amount lower than the intake manifold pressure. To add to this I have a fairly aggressive cam - here's the profile as installed at the overlap period:

 

So, it follows logically that I'm blowing some unburned mixture through during the overlap period, which is how the oxygen is getting in there.

 

Now, if this is the case, then adding extra fuel should shouldn't result in higher temperature, since it will still be a rich mixture when it blows through to the exhaust, and the oxygen getting injected into the exhaust will only be able to burn so much fuel and the extra fuel will still be unburned.

 

Further, this is only plausible if the exhaust backpressure is lower than the manifold pressure - otherwise you would get exhaust reversion rather than blowthrough. Now from what I remember, Jeff's turbine housing and AR is smaller than mine, so I'm having trouble imagining his exhaust manifold pressure being low enough to cause appreciable blowthough.

 

So what do you guys think? What am I missing here - for some reason Jeff and Tony observed a markedly different result, and I believe that this is what they observed. Seems like something else is going on here - Gents?

[cygnusx1]

Tim, you are assuming all the fuel and oxygen you are putting in gets used up in the chamber. Misfires, partial burns, all dump "leftovers" into the exhaust side... along with your valve overlap ingredients (under the right flow conditions).

 

In my limited experience with my car and turbos in general, I could get my turbine to glow with lean mixtures AND with rich mixtures. There was somewhere in between that resulted in a cooler turbine housing. I don't have EGT gauges but I can tell when the turbine is hot by it's color, relative to how hard I had been driving the car. Not very scientific, but valid observations.

 

I also notice that when you put a "free flowing" exhaust system on a Z, you get popping in the exhaust system. Especially during decel.

[TimZ]

Tim, you are assuming all the fuel and oxygen you are putting in gets used up in the chamber. Misfires, partial burns, all dump "leftovers" into the exhaust side... along with your valve overlap ingredients (under the right flow conditions).

 

Yes, I am making some assumptions. However, remember that in the cases stated, the wideband was reading rich, which says that there shouldn't have been unburned oxygen in there - unburned fuel/air mix usually reads lean, due to its oxygen content.

 

I'm trying to concentrate specifically on a situation where richening the mixture alone causes the EGTs to increase, and understanding why this happens. I'm thinking there may be a linkage between flame propagation speed and fuel mix - perhaps the burn rate slows down enough with a rich mix that the fuel is still burning after the exhaust valve opens. The root cause of this would be a combination of timing and fuel, though, and not the same as the fuel being ignited in the exhaust. I know that propagation speed changes at the extremes for E85 - anybody have data on C16?

 

I also notice that when you put a "free flowing" exhaust system on a Z, you get popping in the exhaust system. Especially during decel.

 

This is a different effect, caused by the throttle transition often leading to a lean mix from either a decel fuel cutoff, or just a lean condition in general on decel. This rich/lean transition introduces oxygen into the the exhaust, allowing the previously rich mix to burn.

[cygnusx1]

What does an O2 sensor (UEGO) read in a ZERO oxygen environment. I think the sensors read "affinity for O2", not actually the O2 content. Just a question.

 

I like the theory of slow propagation causing high exhaust temps.

[TimZ]

What does an O2 sensor (UEGO) read in a ZERO oxygen environment. I think the sensors read "affinity for O2", not actually the O2 content. Just a question.

 

Not absolutely sure, but I believe that it would read full rich wit zero oxygen - I am pretty sure that misfires will show a lean condition.

[cygnusx1]

Raw fuel on the sensor will show lean. But that because raw fuel has no interest in oxygen. In other words, raw fuel does not want to react with O2, so the sensor see no demand for O2.

 

A rich condition also dumps CO into the exhaust which in and of itself is combustible. CO will react with any O2 that might be introduced through valve overlap and any other Oxygen that can be robbed from other agents in the exhaust...like nitrogen dioxide.

 

You might be seeing some "catalytic burn" in the exhaust turbine...without actually having the catalyst there.

[RTz]

What does an O2 sensor (UEGO) read in a ZERO oxygen environment. I think the sensors read "affinity for O2", not actually the O2 content.

 

 

A WBO2 has a built-in air pump. It pumps O2 to keep the sensor at stoich. It's this volume of air, accurately metered, that provides the means to calculate AFR.

[ktm]

perhaps the burn rate slows down enough with a rich mix that the fuel is still burning after the exhaust valve opens.

 

Time to fry the noodle. Richer mixtures have a faster flame speed than leaner mixtures.

 

Check out the following two links: http://forums.turbobricks.com/showthread.php?t=170270

 

http://www.honda-tech.com/showthread.php?t=2445210&page=2 (post #37)

 

I did some reading a while back and found these. Anything with a chart is bookmark worthy to this engineer.

[Lazeum]

A WBO2 has a built-in air pump. It pumps O2 to keep the sensor at stoich. It's this volume of air, accurately metered, that provides the means to calculate AFR.

 

Interesting reading there

 

Some extra info regarding WBO2 from Innovate Motorsports

[Tony D]

Man, I have to WORK today and I can't devote time to this thread. Damn damn damn damn! I'll link JeffP to it so he can theorize, he has less work to do than me today (I assume...)

 

Good questions. Especially about his 0.63A/R housing (or 0.82, I forget which).

[TimZ]

Time to fry the noodle. Richer mixtures have a faster flame speed than leaner mixtures.

 

Check out the following two links: http://forums.turbobricks.com/showthread.php?t=170270

 

http://www.honda-tech.com/showthread.php?t=2445210&page=2 (post #37)

 

I did some reading a while back and found these. Anything with a chart is bookmark worthy to this engineer.

 

Interesting - I was going to go find and show that flame speed vs. mixture graph in my next post, then your link was using the same graphic. I'll show it again here just to keep things simple:

 

I remembered that Ethanol slows down appreciably at higher mixes, but couldn't remember what gas did - thanks for the link!

[ktm]

I remembered that Ethanol slows down appreciably at higher mixes, but couldn't remember what gas did - thanks for the link!

 

Tim, by higher mixtures do you mean leaner? The charts are for fuel-air equivalence ratio. In this case, 1 is equal to an AFR of 14.7, 0.9 is an AFR of 15.9 and 1.1 is an AFR of 13.

 

On my last set of dyno pulls, this wasn't exactly the behavior that I observed. As I've noted before my setup always seems to have some pretty high EGTs. Running E85 has helped considerably with its inherently lower combustion temps, but I when I tried leaning the mix out a bit, power went up, just as Tony and Jeff observed, but so did my EGTs. Still not as high as they were on gas, but higher none the less.

 

I seem to run high(ish) EGTs on my setup as well. I routinely hit 1600 F under boost and can watch my EGTs climb on Logworks under load. I've tried to lower them as well without much success.

[TimZ]

Tim, by higher mixtures do you mean leaner? The charts are for fuel-air equivalence ratio. In this case, 1 is equal to an AFR of 14.7, 0.9 is an AFR of 15.9 and 1.1 is an AFR of 13.

 

No, I meant rich - the ethanol/methanol curves exhibit a pronounced peak in flame speed, at 1.1 equivalence (.91 lambda), and then decrease noticeably as the mix gets richer. The gasoline curve is much flatter.

[jeffp]

Yes the 11:2-3 AFR did make my turbo exhaust turbine very hot. As for the engine, it seemed to run about the same water temps.

I was reasonibly sure the excess fuel was igniting in the turbine housing. I concluded this when I started tuning for 12:2-3 AFR's the turgine was not as hot as before. The power was up and the engine sounded better, smoother through the RPM range.

 

A quick test is to look at the exhaust turbine itself. If the wheel is a white/crusty look then the engine was lean. If the wheel was a sooty black, then it was rich. The brown crust, like rust is a good burn.

Flow will follow the path of least resistance, so if your exhaust back pressure is 1 Lb less then the intake, guess where it goes. So that being said, it sufices to say this is the ideal condition for the engine, exhaust back pressure less then the intake pressure.

Another thing you can check, I was walking behind the car once on the dyno at JWT. The car was running around 11:1 and when the exhaust hit my face and my eyes, I imediately started sneezing like crazy and my eyes were burning a good amount, but the one thing that let me know exactly what it was is when the exhaust hit my face, it started to burn. That was nothing less then fuel dense enough to make me notice it on my face, and it really smelled like gas as well. The car was hotter then hell and had to cool down that time.

I think it is enough fuel getting ignited in the scroll of the housing to create the additional heat, and the turbine housing was glowing. I have also seen a lean condition, and that appears to happen, or have more effect in the engine, and not so much in the turbine housing. Lean just overheats the pistons, and you can see that by looking at the color/texture of the pistons in the quiench area of the part. You wont see near the heat in the exhaust turbine housing with a lean condition, you destroy the engine, and the water temp will be the indicator of that.

The O2 readings, well raw fuel will not trigger an O2 sensor, what the sensor reads is the PPM O2 of the sample taken, the sensor then makes a calcuated measurement of the O2 and compares that to "free air" or normal breathing air, unless you live in OC then you get to become a human air filter, to make the calculation. Convert that into what we know as AFR's or in reality it is a PPM measurement.

anyway

[ktm]

Tim, I always wanted to come back to this discussion. I've been reading quite a lot about reversion, overlap, LSA, etc. and the impacts on detonation and engine performance. I've also read extensively about turbo back-pressure between the turbine and head. I think I have an answer to your question:

 

The thing I'm a bit hung up on is that if we are measuring a rich mixture (and even 13.8 is still quite a bit rich compared to stoich), then there should not be any oxygen in the exhaust to support the further burning of the excess fuel. So how's the rich mixture burning in the exhaust?

 

Edit: I see you postulated this already and I am basically agreeing.

 

If you look at your EBP and MAP plot, you are running a ratio of less than 1 (EBP to MAP) up to about an RPM of 7000ish. This means your MAP is higher than your EBP. You could easily be getting oxygen in your exhaust during the overlap. The more oxygen there is for oxidation, the longer you can sustain combustion before the oxygen is consumed.

 

By the way, what are you using to measure your EBP? This issue (EBP) has really piqued my curiosity and is causing me to re-evaluate my turbo turbine selection.

Edited February 26, 2010 by ktm

[Dtsnlvrs]

From the SDS website

Tuning Via EGT vs, Wide Band/ Narrow Band Meters

There seems to be a lot of mystery and misinformation about using exhaust gas temperatures to tune engines. Claims by many EGT gauge manufacturers about it being the best way to tune an engine must be qualified. The BEST way to tune an engine is on an engine dyno- PERIOD. What EGT is good for is a reference for where the engine made maximum torque at wide open throttle. Once removed from the dyno, a similar air/fuel ratio can be established at a later date by dialing in the mixture to achieve the target EGT. It is really the AFR that is important, not the EGT. Most engines will make maximum power at an AFR of between 12.0 and 13.5 to 1 however, the EGT may vary from 1250F to 1800F and is dependent on many factors.

 

It should be mentioned that the target EGT is valid only on the same engine configuration as was used on the dyno. If you change the ignition timing, cams, pistons, headers etc., the optimum EGT may also change. Raising the compression ratio with no other changes will drop the EGT at the same AFR. Retarding the ignition timing will generally raise the EGT at the same AFR. One engine might make best power at 1350 degrees while a very similar engine might be happier at 1500. You can't guess at this or you are simply wasting your money on the instrumentation. Wankel engines have higher EGTs than comparable piston engines due to their lower thermal efficiencies. 1800F is not uncommon here.

 

Some gauge manufacturers say you should tune to achieve maximum or peak EGT for maximum performance. This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.

 

You can sometimes feel a lean of peak condition as the mixture is hard to ignite and power will be down a bit as well. Once the AFR gets close to 17 to 1 at WOT, generally the engine will start to lean misfire. Most tuners always recommend to begin jetting or programming from a known very rich initial setting and carefully leaning until torque falls off slightly, then going back richer to the point of max torque. Note the EGT at this setting. Be aware that altitude, barometric pressure and ambient air temperature may affect this optimal temperature to some degree.

 

Are EGT gauges better than AFR meters? Conventional narrow band oxygen sensors and digital LED meters are not the best devices to measure AFR in the richer ranges but they certainly warn of a too lean condition immediately and obviously, without translation by the driver and they are affordable. Meters combined with wide band sensors are supposed to be highly accurate and everyone has jumped on the bandwagon with these lately. Unfortunately the naive and impressionable often don't question the accuracy of these devices. We have seen some dyno plots indicating best power was achieved at AFRs of 9.7 to 1 on gasoline. This is PHYSICALLY AND CHEMICALLY IMPOSSIBLE and shows that either the sensor was bad (leaded fuel used possibly) or the meter was not calibrated properly. Again, the wide band sensors have the same limitations as the narrow band- leaded race gas quickly fouls them. We have heard and read many stories now indicating that certain brands of wideband meters differ as much as 2 points AFR in readings between each other. In other words, the accuracy of some of these devices is highly questionable. Extensive testing with laboratory quality instrumentation on aircraft engines universally indicates that best power is NEVER made at AFRs richer than 12 to 1. Airflow and fuel flow rates are independently measured and each cylinder is instrumented with EGT probes.

 

We recently dynoed a shop road racing Celica on a DynoJet equipped with a wide band meter. The meter was saying that the engine was going super lean (17 to 1) at high rpm so we kept upping the fuel there. The engine lost more and more power as we added fuel. The dyno operator was convinced that the meter was right but logic told us with no serious dip in power on the curve and the fact that the engine was still alive that the meter was not correct. We started leaning the engine down more and the engine started gaining power. Finally, when confronted with this information, the operator checked the water trap for the wide band sensor. Once this was emptied, the AFRs looked reasonable again. We didn't need the wide band to tell us this, only the torque curve from the dyno.

 

We have heard of several other instances with people using wide bands getting erroneous readings and tuning their SDS based on these readings. Then they phone us saying that the system is crap. Look at the dyno curve, when the engine makes its best power at a given rpm, that's where it likes the AFR irregardless of what other instrumentation is telling you. Remember, a bad sensor whether O2 or EGT equals bad information. When the engine sounds crisp and makes great power, you're there.

 

I would suggest that mixture meters and EGT gauges are complimentary. EGT gauges have the advantage of working long term with leaded fuel which will clog oxygen sensors. EGT gauges are widely used to set mixture on engines used for steady state high power applications where operation has been carefully documented such as in aircraft. The choice would depend on the application. Both are better if you can afford them.

 

[wondersparrow]

That reminds me, I was wondering... How long do you figure an O2 sensor would last with leaded fuel. After I get my engine back together, I want to do some much needed testing and tuning on my track car. Problem is, I like to cut my fuel 50/50 with VP 109. Its got a pretty high compression ratio and to keep the timing up there, I need more than pump gas can offer. Would an O2 sensor last 10 mins? an hour? days? It sounds like an egt sensor should be on my shopping list as well....

[Tony D]

What the gist of my post was getting at was NOT the EGT. Jeff seems obsessed by EGT, as do others, but what my point was is that there is no 'magic number' for AFR. People tuning to 11:1 after torque peak likely are running rich and loosing power.

 

The last post from the SDS link is pretty good in that they are saying the same thing I am: Power Output and AFR are what the primary items that dictate tuning of the engine. If the EGT is too high for materials at peak power output, either improve materials, decrease power output, or figure out why it's too high.

 

We were not (at least I wasn't) correlating power to EGT, Jeff didnt like how hot the car was going, and he was trying to tune to someplace in the 1250 range as I recall... But ultimately the result of the days' experimentation was that setting AFR's at some static magic number was only good for a BASELINE TUNE. After that, pulling AFR gave us more power.

 

That it lowered the EGT's IMO is coincidental, but can partially be explained by some of the graphs and charts already posted, all are well rich of Stoich, but running towards a leaner mix resulted in more power after torque peak so like the SDS guys said, 'that's all that matters'.

 

Getting hung on instrumentation readouts may be good for an indepth analysis and ultimately finding out why something happened. But never loose site of the functional result of more power.

 

The thing to take away from my post was that AFR changing after torque peak (pulling fuel) made more power than adding fuel. And that coincided with what the SDS symptoms in their '17:1' example. Regardless of the AFR, once they added fuel and it lost more power, they pulled fuel, and made more power. It's if-then and-or logic when on the dyno. If you get bogged down with what the instruments are saying when the are doing exactly opposite what the power pulls are telling you, then skip the instruments---they're wrong.

 

Most people set or tune to get an AFR like it's a magic goal. "I'm 11:1 and heres' my pull results." The purpose of the dyno is not to tune your car to identical AFR in all the load cels, it's to get the most power under the curve---and that will require different AFRS in each load bin. And I was simply trying to say 'leaner in the upper bins past torque peak' should be something you want to try if you are at 11:1 thinking this is giving you maximum power. Likely, it is not!

[zxtman]

All this talk about EGTs means nothing without knowing where in the exhaust tract the temperature is taken.

 

There could be up to, and even more than, a 200 degree difference when the probe is located 2" after the exhaust port on the head compared to at the collector right before the turbo flange (even more of a difference depending on the length of the header). Or even more of a difference if the probe is in the down pipe (after the turbo).

[Tony D]

EGT is universally taken at exhaust flange or port.

<CENSOR BOT DICTATED EDIT> T.I.T. and TOT are a different reading, meant to check turbine efficiency.

 

Now some misguided people will refer to <CENSOR BOT DICTATED EDIT>T.I.T. (Turbine Inlet Temperature) as EGT, and technically it could be called that, but tuning to a temperature at turbine inlet is total folly indeed!

 

EGT is at/in/near the exhaust port as possible by the probes being used. Anthing other is another reading.

 

I know of one person who went from 17:1 to 10:1 puking smoke out of his car and complained he didn't see a change on his EGT Gauge (twin gauge of good reputation and fairly accurate when the thermocouple was checked!)

 

Helps if it was someplace useful, and with an AFR gauge, this engineer didn't have the common sense to realize (like the guys in the SDS article) that the gauges were obviously wrong, and to quit using them and follow the instrument that is tracking the physical symptoms that the engine is displaying!

Edited February 26, 2010 by Tony D