Getting CHTs to be a closer match to each other
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As we know, it is not possible to get CHTs to exactly match, but we do like to have them reading closer to each other. I speculate that one of the differences is that the cowl inlet on 1 and 3 gets the prop blast with the prop turning downward, whereas 2 and 4 get the blast from the prop moving upward. Also number 2 is farther back from the cowl inlet.
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Hi John,
Given your conventional style cowl on the Avid, I would think that you may be interested in junking the plenums and going to a bed style cooling baffle setup. This would take some time away from your flying, but when you are finished it would eliminate the effect of having two sides of the cowl vary from one another, since all cooling air enters the same chamber on top of the engine, and reaches a common equilibrium pressure there. How is your metal cutting, bending and riveting skill these days?
Jay
From: main@JabCamit.groups.io <main@JabCamit.groups.io> On Behalf Of John Miller via groups.io
As we know, it is not possible to get CHTs to exactly match, but we do like to have them reading closer to each other. I speculate that one of the differences is that the cowl inlet on 1 and 3 gets the prop blast with the prop turning downward, whereas 2 and 4 get the blast from the prop moving upward. Also number 2 is farther back from the cowl inlet.
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David Amsler
John, That sounds a bit like what is seen on many sonex installations with 3300s with #2 hotest, and #6 coolest.. The spiraling air from the prop directs the flow up and over #2 and ram air aids flow to the rear cylinder. My fix is probably not directly applicable to your situation with new style cylinders and a 4 banger, but concepts may be relevant. I lowered the roof of the opening into the penum a bit so that more air had to pass through the fins of the first cylinder, then further tapered down the ceiling of the penum to help direct more air down between cylinders, the tapering being in rough proportion to the amount of air i was hoping to direct between cylinders so that air speed over the cylinders was not increased. I also drilled some 1/2" (12mm) holes in the ramp leading from bottom of intake to the first cylinder and inserted a steel bar into those holes and tilted the bar down to leave the holes as small scopes to direct air to the underside of the leading cylinders (also did that for #1 cylinder.) On my older jab and CAMit engines, had to round off top corner of fins on #1 cylinder to fit in the cowl, so did the same on #2 so that incoming air did not hit a vertical wall of fins, this may or may not be possible on new heads. End result was that on my 3300s, middle cylindres both now tend to be hottests. David A.
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glen english LIST
I would have thought at stationary idle, there would be negligible ram air pressure due to the prop root, and that the PROPWASH over the lower cowling leading to negative pressure and thus draw through the fins was dominant. -glen
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On 28/02/2021 2:55 am, David Amsler wrote:
John,
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I ran it from idle to 2200 rpm for plenty of ram. John M Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message -------- From: glen english LIST <glenlist@...> Date: 2/27/21 11:46 AM (GMT-08:00) To: main@JabCamit.groups.io Subject: Re: [JabCamit] Getting CHTs to be a closer match to each other air pressure due to the prop root, and that the PROPWASH over the lower cowling leading to negative pressure and thus draw through the fins was dominant. -glen On 28/02/2021 2:55 am, David Amsler wrote: > John, >
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jabcamit@...
John I think you previously reported that you "adjusted" your CHT indicated readings by locally re-directing airflow to/away from the CHT sensors? If that was (and still is) the case then trying to balance your CHT temperatures is impossible, pointless and possibly counter productive.
You need to have CHT readings that are not affected by local airflow onto the sensor. The sensor needs to "see" the "body" of the head temperature - (not half way up a fin or whatever). And sensor location needs to be the same on each head. Only when you've sorted that (perhaps you already have?) will temperature reductions and balance changes you see from baffle changes etc, become meanigful. BobP
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Dave A
I had to reduce the height of the forward portion of the ducts just to allow my cowl to drop low enough to fit. I will try installing a baffle at the front to deflect the incoming air down through the fins. John M
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Marc Bourget
John, that isn't what "works."
Your need is to develop greater pressure above the cylinders and lesser below (than above).
You should avoid items that add drag to the air going into the cowling to increase the "Delta P"
mjb
From: main@JabCamit.groups.io <main@JabCamit.groups.io> on behalf of John Miller via groups.io <skypics234@...>
Sent: Monday, March 1, 2021 6:18 AM To: main@JabCamit.groups.io <main@JabCamit.groups.io> Subject: Re: [JabCamit] Getting CHTs to be a closer match to each other Dave A
I had to reduce the height of the forward portion of the ducts just to allow my cowl to drop low enough to fit. I will try installing a baffle at the front to deflect the incoming air down through the fins. John M
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Mark You are correct, as indicated by the lower and closer CHT on #2 since I added a downward deflector at the inlet of the duct. Now only 20-25 f difference. EGTs on the higher CHT side are about 400 f higher than the cooler side. Does tilting the base of the carb to the lean side work or is that Jabiru fiction? John M Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message -------- From: Marc Bourget <marcbourget@...> Date: 3/1/21 6:36 AM (GMT-08:00) To: main@JabCamit.groups.io Subject: Re: [JabCamit] Getting CHTs to be a closer match to each other
John, that isn't what "works."
Your need is to develop greater pressure above the cylinders and lesser below (than above).
You should avoid items that add drag to the air going into the cowling to increase the "Delta P"
mjb
From: main@JabCamit.groups.io <main@JabCamit.groups.io> on behalf of John Miller via groups.io <skypics234@...>
Sent: Monday, March 1, 2021 6:18 AM To: main@JabCamit.groups.io <main@JabCamit.groups.io> Subject: Re: [JabCamit] Getting CHTs to be a closer match to each other Dave A
I had to reduce the height of the forward portion of the ducts just to allow my cowl to drop low enough to fit. I will try installing a baffle at the front to deflect the incoming air down through the fins. John M
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David Amsler
John, Jabiru style baffles may help even out temperatures by creating turbulence, but that turbulence kills ram air effect and can reduce overall cooling, A deflector that squeezes the air passage down a bit starting where youwant more air to push through the fins, and extending all the way back as a smooth surface avoid the turbulence. David A.
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jkcarpe@frontier.com
Following all the talk of CHT's, is there a temperature that is too low. I see Rotec advertises LCHs to run at 185 F, would that be acceptable, is there any downside to below 200 f head temps?
On Monday, March 1, 2021, 11:04:17 AM PST, David Amsler <energyhawk@...> wrote:
John, Jabiru style baffles may help even out temperatures by creating turbulence, but that turbulence kills ram air effect and can reduce overall cooling, A deflector that squeezes the air passage down a bit starting where youwant more air to push through the fins, and extending all the way back as a smooth surface avoid the turbulence. David A.
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glen english LIST
is the Rotec system pressurized ? or run at STP ?
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also- remember- the jab and most systems are suck out systems, rather than ram air in. best basic solution is to force the air through as much of the fin area as possible, and ensure your cowl is generating good neg pressure. and seal the cowl ! on reassembly, I am going to put Ally batwings above and below the bores and on the head gaps to force more air over the middle for the head fins.
On 3/3/2021 11:47 AM, jkcarpe@frontier.com wrote:
Following all the talk of CHT's, is there a temperature that is too low. I see Rotec advertises LCHs to run at 185 F, would that be acceptable, is there any downside to below 200 f head temps?
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I run Rotec LCH’s and they do not run at 185. My coolant runs 185-200, as does my oil. The CHT’s run 275-300 under the plug, but keep in mind that the head is probably cooler on the interior unlike air cooled heads which are at their coolest on the exterior of the head. Anyway, my CHT’s are running at the low end of acceptable range. No signs of any of the typical Jab top end issues. I run my EGT’s hotter (leaning like a normal engine) than Jab recommended range because I know the guides and seats are running cool.
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Jay
On Mar 2, 2021, at 5:47 PM, jkcarpe@... wrote:
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glen english LIST
Jay is your setup thermo regulated ?
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On 3/3/2021 11:59 AM, Jay Scheevel wrote:
I run Rotec LCH’s and they do not run at 185. My coolant runs 185-200, as does my oil. The CHT’s run 275-300 under the plug, but keep in mind that the head is probably cooler on the interior unlike air cooled heads which are at their coolest on the exterior of the head. Anyway, my CHT’s are running at the low end of acceptable range. No signs of any of the typical Jab top end issues. I run my EGT’s hotter (leaning like a normal engine) than Jab recommended range because I know the guides and seats are running cool.
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Bob
Today I removed the #4 baffle and BEHOLD my number 2 CHT dropped a bit and my number 4 , which always read 50 F lower that the next highest is now within 10 degrees of number 2. Can't believe it, my CHTs are all now running around 300 F plus or minus 20 F in 70 F OAT. Couldn't be more pleased. John M
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No, I have coolant temp gauges in the panel and servo adjustable exit air from the radiators, so I manually adjust to get my coolant where it is acceptable and limit drag. The ratios are the same as the P-51 radiator plenum design, so maybe I get a little thrust out of them 😉
Jay
-----Original Message-----
Jay is your setup thermo regulated ?
On 3/3/2021 11:59 AM, Jay Scheevel wrote: > I run Rotec LCH’s and they do not run at 185. My coolant runs 185-200, > as does my oil. The CHT’s run 275-300 under the plug, but keep in mind > that the head is probably cooler on the interior unlike air cooled > heads which are at their coolest on the exterior of the head. Anyway, > my CHT’s are running at the low end of acceptable range. No signs of > any of the typical Jab top end issues. I run my EGT’s hotter (leaning > like a normal > engine) than Jab recommended range because I know the guides and seats > are running cool. >
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Someone earlier mentioned referencing Lycoming information when considering numbers for a Jabiru. Lycoming lists 150ºF as the minimum CHT number. I was curious about this as my air-cooled engine (not a Jabiru) runs extremely cool CHTs in the winter time. But after nine years and 250 hours it seems to like the cool temps as it uses no oil and all compression tests and flight parameters remain well within normal ranges. Oil analysis came back with no concerns ...
FWIW ...
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mikeGoog
FWIW & for comparison,
80 h.p. Rotax: 912 heads liquid cooled [barrels finned] on my
Rans S6-116 usually run at indicated 185 F, oil ditto. EGT 700 to 780
C.
It’s in fine fettle only about 850 hours young and has run
like that for the last dozen years.
mikehallam, England.
From: jkcarpe@...
Sent: Wednesday, March 3, 2021 12:47 AM
To: main@JabCamit.groups.io
Subject: Re: [JabCamit] Getting CHTs to be a closer match to each
other Following all the talk of CHT's, is there a
temperature that is too low. I see Rotec advertises LCHs to run at 185 F,
would that be acceptable, is there any downside to below 200 f head temps?
On Monday, March 1, 2021, 11:04:17 AM PST, David Amsler
<energyhawk@...> wrote:
John,
Jabiru style baffles may help even out
temperatures by creating turbulence, but that turbulence kills ram air effect
and can reduce overall cooling, A deflector that squeezes the air passage
down a bit starting where youwant more air to push through the fins, and
extending all the way back as a smooth surface avoid the turbulence.
David A.
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glen english LIST
Jay, I am a bit surprised the CHTs under the plugs run that warm with the water cooling. Probably means the area around the plugs is solid ally . still, that is cool and that is likely the hottest part of the head.
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Unpressurized system ?- IE water circulation but free flow at atmo ? I do not see any downside to low CHTs as long as the dimensions of everything are at a point where the sealing is good on everything. I see good reason to keep bores cool to get heat out of the pistons. The Gen4 should be at significant advantage in that department.
On 3/03/2021 11:59 am, Jay Scheevel wrote:
I run Rotec LCH’s and they do not run at 185. My coolant runs 185-200, as does my oil. The CHT’s run 275-300 under the plug, but keep in mind that the head is probably cooler on the interior unlike air cooled heads which are at their coolest on the exterior of the head. Anyway, my CHT’s are running at the low end of acceptable range. No signs of any of the typical Jab top end issues. I run my EGT’s hotter (leaning like a normal engine) than Jab recommended range because I know the guides and seats are running cool.
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Glen,
Keep in mind that the spark plug body conducts a significant portion of the heat from the interior of the combustion chamber, so under the plug CHT is partially determined by the heat of the sparking end. I mostly just use the values of CHT to compare one cylinder to another. Also, there is no air circulating around my head, since I have narrowed the plenums so as to focus all of the air across the fins of the cylinder barrels. See the photo below. This also increases the temperature measured by the under plug thermocouples. My coolant temperature is taken at the last point before the coolant returns to the radiator.
Jay
-----Original Message-----
Jay, I am a bit surprised the CHTs under the plugs run that warm with the water cooling. Probably means the area around the plugs is solid ally . still, that is cool and that is likely the hottest part of the head.
Unpressurized system ?- IE water circulation but free flow at atmo ?
I do not see any downside to low CHTs as long as the dimensions of everything are at a point where the sealing is good on everything.
I see good reason to keep bores cool to get heat out of the pistons. The Gen4 should be at significant advantage in that department.
On 3/03/2021 11:59 am, Jay Scheevel wrote: > I run Rotec LCH’s and they do not run at 185. My coolant runs 185-200, > as does my oil. The CHT’s run 275-300 under the plug, but keep in mind > that the head is probably cooler on the interior unlike air cooled > heads which are at their coolest on the exterior of the head. Anyway, > my CHT’s are running at the low end of acceptable range. No signs of > any of the typical Jab top end issues. I run my EGT’s hotter (leaning > like a normal engine) than Jab recommended range because I know the > guides and seats are running cool.
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