[Toobuilder]

I have a situation with temperatures that I don't understand and was wondering if any of you have run across the same thing. I have found that the CHT and EGT can be managed very easily at relatively low altitude by running LOP. If the CHT starts to get away from me, I simply lean it a little more and can get it right down to where I want it. Since I have dual electronic ignition, I can lean to 100+ cooler than peak or more with the engine still running smoothly. The problem I am having is the higher in altitude I go, the more the engine splits in EGT and CHT. 1 and 3 respond together, as do 2 and 4, but the temp split between the two sides of the engine gets quite wide with altitude. What this means is that if I'm trying to bring my hottest cylinder under control by leaning, my coldest cylinder is already way too cold and gets worse with every turn of the mixture knob.

Notice the major spread in CHT and EGT between the left and right side of the engine in this picture at high altitude. Trying to lean further to bring #3 under control would cause the already too cold #4 to plummet further.

Now, here at low altitude, also WOT and LOP, things are much more under control. The temperature split between EGT is very narrow, and CHT are nice and cool. This is what I want it to look like at altitude!

This is a 200 HP (angle valve) IO-360, Bendix FI with a standard Vans cowl, and baffles are new and sealing against the cowl. Also, the "tape over the cowl split" trick did not seem to make any difference.

So does anyone have any theories why I get this split with altitude, and more importantly, how do I fix it?

Thanks.

 

[DENMACRES]

split

the fuel flow is very low at that alt. the fuel divider tries to equalize to each cyl. thinking eng. is at very low power. (idle) this screws up your balanced injectors. Dennis IO-360, -6

 

[Toobuilder]

So...

...Is this normal? I've never had an engine instrumented to this level before - is this another case of too much information?

 

[DanH]

I seem to recall Don Rivera saying that at anything less than 7 GPH, flow division was at the divider, not the injector orifice.

 

[Toobuilder]

I seem to recall Don Rivera saying that at anything less than 7 GPH, flow division was at the divider, not the injector orifice.

Not sure I follow you there Dan... Do you mean to say that at such low fuel flows, the nozzle ceases to perform a metering function, and the flow divider becomes more influential?

Perhaps I need to give Don a shout...

 

[ao.frog]

Check..

... this tread: http://www.vansairforce.com/community/showthread.php?t=74484&highlight=CHT

Maybe that could solve your problem too?

 

[Ironflight]

...Is this normal? I've never had an engine instrumented to this level before - is this another case of too much information?

Interesting discussion Michael - I often wish that I would take the time to research the accuracy, repeatability, hysteresis, etc of our transducers. We might very well find out that we are chasing instrumentation in some cases.

For this specific question? I agree with the idea of calling Don at Airflow.

Paul

 

[Toobuilder]

... We might very well find out that we are chasing instrumentation in some cases...

For all the wonders of the digital age, sometimes it makes one long for the good old days where we just leaned to a touch rich of rough and forgot about the engine for the rest of the flight.

 

[Toobuilder]

... this tread: http://www.vansairforce.com/community/showthread.php?t=74484&highlight=CHT

Maybe that could solve your problem too?

Thanks for that, but I tried that trick too. No difference..

 

[David-aviator]

My take on the situation - if the transition from peak to LOP is occurring at or near the same fuel flow with each cylinder, the fuel delivery system is balanced. Individual CHT and EGT values are interesting but of no real consequence.

In Mike Busch's forum on going beyond TBO (his Cessna twin is doing well at 200% TBO) the #1 item is controlling rust and #2 is CHT. CHT should never be allowed to go over 400 and as a norm kept below 360. CHT is controlled with fuel flow and/or air flow.

If your injection system is balanced, the variance in CHT is an unbalanced air cooling issue. The challenge we have is that the conditions of flight are so variable it is nye impossible to balance CHT's all the time. My #4 runs about 20 hotter due to air stolen for oil cooling. Yours is running cooler, that doesn't make much sense assuming the cooler is located at #4. That's just the beginning of the mysteries surrounding CHT. As long as they run cool, I don't sweat it.

Your cylinders are running so cool, 299-338, they will last forever. That is good.

 

[Toobuilder]

Well, that's kind of the mystery David... At low altitude I can get the LOP values well within a 10 degree spread (often besting my "low altitude" picture in the first post). The cylinders fall right in line as well with a fairly even spread. As I go high, the EGT splits (as shown) between the 1-3 and 2-4 (left side, right side). The thinner air makes cooling more difficult, but it does seem that EGT is driving the CHT to some degree. Trying to keep my "hottest" EGT cooler than 50 drives the outliers way cold, and the CHT's follow. And yes, the oil cooler is on #4, which I agree does not make sense. I almost started thinking the thermocouples were bad, but they are dead on at ambient, and also at low altitude. I'm not ruling it out, but a bad T/C is not high on my list of probabilities

It's kind of a chicken vs. egg thing. I'm not sure what drives what, but once it splits, it is very hard to get CHT and EGT to play nice again.

 

[DanH]

Not sure I follow you there Dan... Do you mean to say that at such low fuel flows, the nozzle ceases to perform a metering function, and the flow divider becomes more influential?

Yes.

I looked it up in my email and found my mind had slipped only a little

Don said that with 0.028" nozzles the divider would control fuel split below 7.5 gph (not 7 gph). Obviously this is with an AFP system. Don't know about the others.

 

[rich]

Michael,
You might have a couple of spark plugs/wires or coils misfireing at high altitudes. That can cause higher EGT readings.Try running on one EI and then the other and look for any engine roughness.
I had a spark plug that would run fine until I reached 7000 ft. then start to misfire.
Good luck,

 

[Toobuilder]

Rich,

I did a mag check several times in cruise and got the expected EGT rise when operating on the single plug (either one). No roughness, but a definate rise in EGT indicates that both ignitions are working well.

 

[Toobuilder]

Yes.

I looked it up in my email and found my mind had slipped only a little

Don said that with 0.028" nozzles the divider would control fuel split below 7.5 gph (not 7 gph). Obviously this is with an AFP system. Don't know about the others.

So perhaps the trick is to reduce the nozzle orifice enough to increase pressure and overcome the influence of the divider (more pressure at the nozzles than the divider), yet still be big enough to supply the max power requirements at sea level?

...There may not be enough "reserve" pressure in the current system to provide fuel through smaller nozzles at high power. Maybe Don will jump in here.

So the question of the day: For those that run LOP, do your engines behave like this at altitude? I have a stock system, so it stands to reason that many others should be seeing this.

 

[DanH]

So perhaps the trick is to reduce the nozzle orifice enough to increase pressure and overcome the influence of the divider (more pressure at the nozzles than the divider), yet still be big enough to supply the max power requirements at sea level?

That is my understanding, although perhaps not so simple in practice.

I am curious. Although perhaps not directly relevant to a temperature split, what do your CHT values look like when running a best power mixture, say 100-150 ROP? Must you run LOP to get acceptable temperatures?

 

[Toobuilder]

That's a good question Dan... I can't recall ever running it that rich for any length of time in cruise. Climb mixture is generally fat, but the speed is so low that CHT is meaningless. ...I'm just trying to get the climb over with so I can go LOP and cool off. I can tell you that LOP is not required to keep temps in check, but I don't know how they compare to ROP vs. LOP. I guess I'll have to go out and try flying at best power and see what it does.

 

[DanH]

That's a good question Dan... I guess I'll have to go out and try flying at best power and see what it does.

Ok, I ask because of the CHT's in the first photos (at 12,500 feet): 380, 357, 395, and 322, and your comment "Trying to lean further to bring #3 under control would cause the already too cold #4 to plummet further.".

First, best power ROP is a more difficult cooling case, as is a higher altitude (lower density). If you're leaning to get #3 down to 395, it's not going to be good at best power.

Second, compared to your reference cylinder (the cool #4) 1 and 3 are always warm....about 40 degrees in the low altitude case and 60-70 in the high altitude case.

There's a temptation to search for THE problem, but most of the time it's a little of this and a little of that. I'd suggest going back for a look at the baffles before worrying too much about a fuel flow inbalance at low volume that you may not be able to solve.

 

[DENMACRES]

fuel flow

I went to .024's to solve the divider issue, but there was a significant drop in full power on my modified IO-360 (ported, flowed and upped comp.) I have went back to larger inserts. Pulling the throttle back at alt. until the MP just starts to fall will help even the EGTs. (old racer trick) Dennis McCright 936-443-3562 RV-6,900hrs. LOP, 172kts, 6.9gph at 10,000 Have fun out there.