EGTs in climb- the carb'd edition

[DXB]

Based on the related thread in "Modern Mooneys," I've been considering this topic but didn't want to muck up commentary on someone's big bore Continental with balanced injectors by blabbing on about my trusty but primitive carb'd Lycoming O-360.  

Specifically I've been musing about my EGTs and leaning technique in climbs to high DA,, or takeoffs from high DA. Though I've never actually done the latter, I might make it to Leadville one day.   Because leaning to "target EGT" at WOT seems precarious to me, I decided to figure out as accurately as possible what that target value is.  I looked at my JPI data from 50 flights, all of which were from near sea level and done in the colder half of the year- starting in Sept 2015 when the monitor got put in, until March this year.  

There was minimal variation except that two clear patterns emerged - in 70% of takeoffs, EGT on #4 was the hottest, with #3 100 degrees cooler.  In the remaining 30%, #3 was the hottest, in which case #4 ran 100 degrees cooler.  Peak FF was always in the 17-18gph range. Means and std. dev. for peak EGT on takeoff  are shown below for  the 2 patterns:

Cylinder:            Pattern 1 (#4 hottest):                  Pattern  2 (#3 hottest):

1                        1260 +/- 11                                   1280 +/- 9

2                        1296 +/- 14                                   1253 +/- 13

3                        1321 +/- 20                                   1431 +/- 23 

4                        1427 +/- 18                                   1326 +/- 13

CHTs also run hotter in 3 vs 4 depending on which has the higher EGT for a given takeoff, making me think the two patterns reflect real shifts in mixture distribution, not measurement artifacts.  There is no rhyme or reason to which pattern appears- it can shift on consecutive takeoffs under identical conditions on the same day.   FWIW, I suspect neither #3 nor #4 is actually the leanest - #2 EGT usually but not always peaks first on leaning in cruise, and its CHT usually runs second hottest in climb (despite lower EGT), irrespective of whether 3 or 4 runs hottest (both CHT and EGT).  Anyway, my only takeaway so far is that I can safely lean my hottest EGT (#3 or #4) up into the low 1400's to optimize power and improve fuel efficiency on a long climb, as long as the CHTs tolerate it.  It's interesting to note that with a single probe EGT, this might have been a bad idea given the two distinct patterns of mixture distribution.  Reasonable? Any other points to consider?

Also, regarding high DA takeoff rolls (I have no experience), accurately tweaking the mixture to get low 1400s EGT seems precarious as heck .  I have enough trouble staying on the center line, watching the ASI, checking the essential gauges, thinking about abort point, etc. in unison. It might be feasible if I used a particular high altitude field on a regular basis so could learn to set it roughly by instinct.  The more complex technique to lean for high altitude takeoff has previously been covered well here.

[carusoam]

Interesting observation....

Add this to what you are seeing...

When 3 & 4 flip for being hottest, so does the order of 1 & 2.

There is a lot of symmetry, if the fuel tends to go to one cylinder, it is unable to tend towards the other...

Since this is EGT data,  is there a reason for how fuel is preferentially distributed to the back two vs the front two cylinders?

 

an additional thought...

Any idea how far ROP are you at while leaning for 'target'?  When set-up properly, this is usually something like a range from 200 - 300 °F ROP.  While climbing the EGT descends toward 300° ROP.  Lean, to bring it up to 200°f ROP. Climb, lean, climb....

knowing what peak is at SL using your equipment is the basis of executing the 'target egt' method when you get to Leadville.  Using an actual number is a step above what some planes have on their ship's single EGT gauge.

  Realistically, your hottest cylinders are so close to each other.  When they flip-flop, there doesn't seem to be much missed if you were to ignore it.

Saying it again in a slightly different way...

1) you are leaning a few hundred degrees ROP, using the target EGT method.

2) it won't matter much what cylinder you are watching to see exactly when peak occurs. You are enrichening pretty far from peak.

3) this is a different type of situation from when you are trying to run LOP or at peak.  Where you want to know that all cylinders have gone to or through peak.

4) going too far ROP is not beneficial either for a couple of reasons.  Lower power and plug fouling... (Just sayin)

Best regards,

-a-

 

[DXB]

49 minutes ago, carusoam said:

Any idea how far ROP are you at while leaning for 'target'?  When set-up properly, this is usually something like a range from 200 - 300 °F ROP.  While climbing the EGT descends toward 300° ROP.  Lean, to bring it up to 200°f ROP. Climb, lean, climb....

Shouldn't how far ROP you are as a fixed EGT value decrease markedly as you generate less power during a climb?  If I climb to 10,000 DA, I suspect continuously leaning to maintain 1400 EGT will leave me quite a bit less than 200 ROP.  

By contrast, trying to figure out where my EGT peaks at WOT at SL might destroy my engine.  There was a time once when someone  re-leaned the mixture while waiting several minutes for takeoff clearance and then forgot to go full rich when told "clear for takeoff, no delay."   The immediate rough sound of the engine and a quick glance at the JPI led him to simultaneously slam the mixture knob in, emit an expletive, and break out in a cold sweat. The JPI record for a 10 sec interval  showed #4 hitting 1580, so it's probably something >150.  

BTW this person found that the bright yellow EGT bars and red flashing numbers on the JPI900 definitely add a margin of safety.  Mount as close to your line of sight as you can.

[Cruiser]

A couple of thoughts.........

The engine physics are the same for carbed or injected engines in regard to EGT and CHT. Granted the deliver method can pose different challenges.

In general EGT will not vary significantly with OAT. It is purely a function of F/A ratios. Of course cold air is denser than hot air so the F/A ratio will change but T/O EGT should not vary too much since the ratio is always out of balance strongly to the fuel side. i.e. excess fuel vs available air. With that said, look closely at the fuel flow difference between the two patterns. I am betting on a difference although slight that is causing the EGT change between 3 and 4

CHT on the other hand is dependent on both internal  and external cooling Internal due to F/A ratio and external due to OAT and flow over/around the cylinders. The colder the OAT the more effective the cooling.

As for high DA take-offs, pre set the mixture when doing the run up mag check.That should get you close, then you can adjust when you get a chance. If your DA is over 5,000 feet you are not going to hurt anything. Less then that DA and you really don't need to lean on T/O anyway. 

[carusoam]

Dev,

you have now hit on the blue box technique...  Or white box technique when you fly a G1000.

The blue box is printed on the face of the calibrated EGT gauge. It begins about 200°F ROP and extends to about 300°F ROP.  As you climb into less dense air, the mixture gets richer, and the EGT falls out the bottom of the blue box.  This is the sign to lean until the needle is back at the top of the blue box again.

As you climb from SL to 12,500', the EGT will be be a part of your scan. Continuously looking to make adjustments as you go higher.  The width of the box lets you go for a while before needing to make a change.  The price of fuel can keep you attentive to wanting to make a change.

Scanning CHTs is a driving force to keeping EGTs low.  Maintaining 120MIAS in the climb can work pretty well in combination with the blue box technique.

Knowing the peak EGT at SL takes some effort.  Best done with a knowledgable copilot on board.  At least with a JPI900 and fast acting TCs, you can quickly operate the mixture control smoothly through peak while collecting data.  Make sure your data collection rate is set to the smallest interval available. Like two seconds. Review the data later on a computer screen.  You can then refine the technique using FF to get in the range of where peak is going to be and slowly lean through peak knowing there is going to be a two second lag in the data collection.

Keep in my C had neither the calibrated EGT gauge, or FF, or a JPI.  I typically use 120KIAS in the O following the same guidance...

As you know, I am only a PP.  I can easily make mistakes...  Let me know if you see any.

Best regards,

-a-