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(walter@advancedpilot.com@203.171.81.6) by mail.carneal.com with ESMTPA; 16 May 2013 22:38:44 -0000 From: Walter Atkinson Mime-Version: 1.0 (Apple Message framework v1283) Content-Type: multipart/alternative; boundary="Apple-Mail=_A53AA5D6-720D-4B69-9C8C-1E4C2C1CAA09" Subject: Re: [LML] EGT/TIT Calibration - the MYTH X-Original-Date: Fri, 17 May 2013 08:38:49 +1000 In-Reply-To: X-Original-To: "Lancair Mailing List" References: X-Original-Message-Id: X-Mailer: Apple Mail (2.1283) --Apple-Mail=_A53AA5D6-720D-4B69-9C8C-1E4C2C1CAA09 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=iso-8859-1 Fred: The EGT raw value is not important. We only care about the relationship = to peak EGT. The TIT raw number is important. The turbine is rated to = withstand max RPM at the TIT temp limit indefinitely. If the temp is = above the limit at has RPM, blade creep will allow the blades to scrape = against the housing. Fortunately we rarely operate at max altitude = requiring max turbine RPM, so there is a tiny bit of wiggle room\m. HoW = much wiggle room? We don't have any way to know form the cockpit. <> While that's a logical explanation, that's not why the peak temps are = different when coming from ROP or LOP. The reason is the change in = volumetric efficiency of the cylinders due to the difference in CHT, = there for e a minor change in F:A ratio. If one leans slow enough, the = temps and volumetric efficiencies stabilize and the numbers will be the = same. Walter Atkinson (225) 939-7508 On May 16, 2013, at 9:22 PM, Frederick Moreno wrote: Forget calibration. It is a waste of time. All you will EVER get in = the cockpit is relative temperature indications: when the exhaust gases = are (probably) hotter or (probably) cooler. That is ALL the EGT/TIT = number tells you. The absolute value of the numbers means not much. = The numbers are for comparison purposes only. =20 Measuring gas temperature in the exhaust pipe of a piston engine is very = tough stuff if you want to really know the exact answer. Reason? There = are many confusing effects, and the temperature and flow are constantly = changing. See attached picture for a bit more understanding if you want = to wade into some of the details.=20 =20 The temperature displayed by the instrument is the temperature read by = the thermocouple which is buried inside a probe which is sheathed with a = stainless skin and filled with some kind of ceramic insulator. These = combine to yield a very slow response time, much slower than the cycle = time of the engine (40 times per second at 2400 RPM). So the = thermocouple is time-averaging the temperature in the vicinity where the = thermocouple is located. =20 I found that the EGT SYSTEM (thermocouple plus sheath plus surrounding = thermal mass of exhaust pipe and cylinder head) take sabout three = minutes to come to full equilibrium.=20 =20 Don't believe me?=20 =20 Do GAMI lean test. Start rich, Lean down in steps waiting, say, one = minute per step while you record data. Then reverse, going from lean = back richer. Then plot the data. The curve going leaner will not lie = on top of the curve going richer. Why? Because when you took the = reading, it was still changing, slowly, but still changing. It takes a = lot of time and patience to get good data.=20 =20 But drawing the curve has value because it shows which cylinder goes = lean first, second, and so forth. But the absolute numbers are subject = to huge errors as noted below.=20 =20 The instrument temperature reading is related to the gas temperature in = the same way that a stopped clock is accurate twice a day. That is, the = EGT reading is only accurate when the GAS temperature is rising through = the reading, or falling through the reading, which may occur once or = more each cycle.=20 =20 Look at the figure. The exhaust event lasts about 180 degrees out of a = total 720 degrees required to complete the four stroke cycle. So you = get WHOOSH for about a quarter of the time, and then nothing except some = ringing of the exhaust gas column back and forth as the pressure pulse = oscillates up and down the exhaust pipe. There is nothing like "steady = state" (smooth and unchanging) occurring in the exhaust flow. =20 Similarly, there is nothing like steady state about exhaust gas = temperature. When the exhaust valve opens, the temperature of the = escaping gases is highest, but falls as gas expansion occurs in the = cylinder as the gas exits the cylinder. So the temperature starts out = high, and ends up lower at the end of the exhaust pulse. =20 So both flow and temperature are constantly changing, but flow is nearly = zilch about 3/4 of the time.=20 =20 Now the poor thermocouple is shielded from all this variability by the = stainless steel sheath. The sheath is heated by convection (passage of = gases), and heated by thermal radiation from anything hotter than the = sheath (portions of exhaust pipe, or exhaust valve perhaps?). And the = sheath loses heat to the surroundings via radiation going to any surface = that is cooler than the sheath (some portion of exhaust manifold pipe = probably, certainly the aluminum exhaust port in the head).=20 =20 And the sheath is also losing heat to the outside via conduction to the = cooler exterior. =20 So the sheath is arriving at some time-averaged temperature that is = probably close to the thermocouple temperature, but which is influenced = by a heat balance of gain and loss between gas, conduction, and thermal = radiation with the surroundings. And the gas is active only 1/4 of the = time, and the flow rate and temperature of the gas are changing during = this short interval. =20 Getting the picture? The thermocouple is related to the gas = temperature, but is delivering a signal to the instrument that is = averaging out all these variations in time and thermal balance heat = flows.=20 =20 In short, we do not really know what the gas temperature is. We only = have a number that kinda sorta represents some sort of average of gas = temperature and flow, installation geometry, and ambient environment = under the cowl (cooling air blowing on exhaust pipes and thermocouples, = radiation environment inside and outside of exhaust pipe, conduction = losses and on and on). =20 The number in the cockpit is only valid for comparison with other = numbers obtained during other power and mixture settings and flight = conditions. =20 Some observations can follow from considering the figure and discussion = above, such as: Installation details count a lot. The depth of the thermocouple, = thermocouple construction details, thermocouple mounting details, = location in the exhaust pipe, distance from the cooler aluminum exhaust = port, nearness to a bend in the pipe, effect of the cooling air blast = coming off the cylinders onto the exhaust pipe - all these things and = more will affect the "EGT/TIT reading" up or down independent of the = "real" gas temperature (which is constantly changing each cycle). This = is why you get a different number in the airplane as opposed to on a = dyno stand at ground level.=20 The TIT will read higher than EGT of a single cylinder. Why? The EGT = gets gas blasted about 1/4 of the time, but the TIT theremocouple on a = twin turbo 6 cylinder engine gets blasted 3/4's of the time. Any = surprise it reads hotter? For this reason, the exhaust pipe is also a = bit hotter at the turbo inlet than below an individual cylinder. If you wrap your exhaust pipes, the pipe wall will be hotter and will = affect the radiant exchange and thermal losses raising the apparent gas = temperature - probably quite a bit- as well as raising the actual gas = temperatures in the pipe, at least near the pipe wall (in the thermal = boundary layer). For all these reasons, EGT spread normally means little unless one = reading is WAY off the program in which case it does mean something = (blocked injector, fouled spark plug, etc). But the value of EGT/TIT is = the TREND and THE COMPARISON to other operating conditions. =20 What about TIT limits, typically 1650F or sometimes 1750F? The TIT is = the best data we have, and if the turbo guys say we can expect trouble = sooner or later if these temperatures are exceeded, it is based on = experience and some safety factor using the best data available to the = pilot. Respect these limits. Inaccurate data is better than no data in = such circumstances. =20 But don't worry about absolute values or calibration. They are = meaningless values and exercises. Watch the trend and comparison with = other operating conditions. Those tell the important tales we need to = know and understand. =20 Fred Moreno =20 -- For archives and unsub = http://mail.lancaironline.net:81/lists/lml/List.html --Apple-Mail=_A53AA5D6-720D-4B69-9C8C-1E4C2C1CAA09 Content-Transfer-Encoding: quoted-printable Content-Type: text/html; charset=iso-8859-1 Fred:

The EGT raw value is not important.  We only care about = the relationship to peak EGT.  The TIT raw number is important. The = turbine is rated to withstand max RPM at the TIT temp limit = indefinitely. If the temp is above the limit at has RPM, blade creep = will allow the blades to scrape against the housing.  Fortunately = we rarely operate at max altitude requiring max turbine RPM, so there is = a tiny bit of wiggle room\m.  HoW much wiggle room?  We don't = have any way to know form the cockpit.

<<Start rich, = Lean down in steps waiting, say, one minute per step while you record = data.  Then reverse, going from lean back richer.  Then plot = the data.  The curve going leaner will not lie on top of the = curve going richer.  Why? Because when you took the reading, = it was still changing, slowly, but still changing.  It takes a lot = of time and patience to get good data. = >>

While that's a = logical explanation, that's not why the peak  temps are different = when coming from ROP or LOP.  The reason is the change in = volumetric efficiency of the cylinders due to the difference in CHT, = there for e a minor change in F:A ratio. If one leans slow enough, the = temps and volumetric efficiencies stabilize and the numbers will be the = same.

Walter Atkinson
(225) = 939-7508


On May 16, 2013, at 9:22 PM, = Frederick Moreno wrote:

Forget = calibration.  It is a waste of time.  All you will EVER get in = the cockpit is relative temperature indications: when the exhaust gases = are (probably) hotter or (probably) cooler.  That is ALL the = EGT/TIT number tells you.  The absolute value of the numbers means = not much.  The numbers are for comparison purposes = only.
 
Measuring gas temperature in the = exhaust pipe of a piston engine is very tough stuff if you want to = really know the exact answer.  Reason?  There are many = confusing effects, and the temperature and flow are constantly = changing.  See attached picture for a bit more understanding if you = want to wade into some of the = details. 
 
The temperature displayed by = the instrument is the temperature read by the thermocouple which is = buried inside a probe which is sheathed with a stainless skin and filled = with some kind of ceramic insulator.  These combine to yield a very = slow response time, much slower than the cycle time of the engine (40 = times per second at 2400 RPM).  So the thermocouple is = time-averaging the temperature in the vicinity where the thermocouple is = located.
 
I found that the EGT SYSTEM = (thermocouple plus sheath plus surrounding thermal mass of exhaust pipe = and cylinder head) take sabout three minutes to come to full = equilibrium. 
 
Don't believe = me? 
 
Do GAMI lean test.  Start = rich, Lean down in steps waiting, say, one minute per step while you = record data.  Then reverse, going from lean back richer.  Then = plot the data.  The curve going leaner will not lie on top of the = curve going richer.  Why? Because when you took the reading, = it was still changing, slowly, but still changing.  It takes a lot = of time and patience to get good = data. 
 
But drawing the curve has value = because it shows which cylinder goes lean first, second, and = so forth.  But the absolute numbers are subject to huge errors as = noted below. 
 
The instrument temperature = reading is related = to the gas temperature in the same way that a stopped clock is accurate = twice a day.  That is, the EGT reading is only accurate when the = GAS temperature is rising through the reading, or falling through the = reading, which may occur once or more each = cycle. 
 
Look at the figure.  The = exhaust event lasts about 180 degrees out of a total 720 degrees = required to complete the four stroke cycle.  So you get WHOOSH for = about a quarter of the time, and then nothing except some ringing of the = exhaust gas column back and forth as the pressure pulse oscillates up = and down the exhaust pipe.  There is nothing like "steady state" = (smooth and unchanging) occurring in the exhaust = flow.
 
Similarly, there is nothing like steady = state about exhaust gas temperature.  When the exhaust valve opens, = the temperature of the escaping gases is highest, but falls as gas = expansion occurs in the cylinder as the gas exits the cylinder. So = the temperature starts out high, and ends up lower at the end of the = exhaust pulse.
 
So both flow and temperature = are constantly changing, but flow is nearly zilch about 3/4 of = the time. 
 
Now the poor thermocouple is = shielded from all this variability by the stainless steel sheath.  = The sheath is heated by convection (passage of gases), and heated by = thermal radiation from anything hotter than the sheath (portions of = exhaust pipe, or exhaust valve perhaps?).  And the sheath loses = heat to the surroundings via radiation going to any surface that is = cooler than the sheath (some portion of exhaust manifold pipe probably, = certainly the aluminum exhaust port in the = head). 
 
And the sheath is also losing = heat to the outside via conduction to the cooler = exterior.
 
So the sheath is arriving at some = time-averaged temperature that is probably close to the thermocouple = temperature, but which is influenced by a heat balance of gain and loss = between gas, conduction, and thermal radiation with the = surroundings.  And the gas is active only 1/4 of the time, and the = flow rate and temperature of the gas are changing during this short = interval.
 
Getting the picture?  The = thermocouple is related to the gas temperature, but is delivering a = signal to the instrument that is averaging out all these variations in = time and thermal balance heat flows. 
 
In = short, we do not really know what the gas temperature is.  We only = have a number that kinda sorta represents some sort of average of gas = temperature and flow, installation geometry, and ambient environment = under the cowl (cooling air blowing on exhaust pipes and thermocouples, = radiation environment inside and outside of exhaust pipe, conduction = losses and on and on).
 
The number in the = cockpit is only valid for comparison with other numbers obtained = during other power and mixture settings and flight = conditions.
 
Some observations can follow = from considering the figure and discussion above, such = as:
  1. Installation details count a lot.  The depth of = the thermocouple, thermocouple construction details, thermocouple = mounting details, location in the exhaust pipe, distance from the cooler = aluminum exhaust port, nearness to a bend in the pipe, effect of the = cooling air blast coming off the cylinders onto the exhaust pipe - all = these things and more will affect the "EGT/TIT reading" up or down = independent of the "real" gas temperature (which is constantly changing = each cycle).  This is why you get a different number in the = airplane as opposed to on a dyno stand at ground = level. 
  2. The TIT will read higher than EGT of a single = cylinder.  Why? The EGT gets gas blasted about 1/4 of the time, but = the TIT theremocouple on a twin turbo 6 cylinder engine gets = blasted 3/4's of the time.  Any surprise it reads hotter?  For = this reason, the exhaust pipe is also a bit hotter at the turbo inlet = than below an individual cylinder.
  3. If you wrap your exhaust = pipes, the pipe wall will be hotter and will affect the radiant exchange = and thermal losses raising the apparent gas temperature - probably quite = a bit- as well as raising the actual gas temperatures in the pipe, at = least near the pipe wall (in the thermal boundary = layer).
For all these reasons, EGT spread normally = means little unless one reading is WAY off the program in which = case it does mean something (blocked injector, fouled spark plug, = etc).  But the value of EGT/TIT is the TREND and THE COMPARISON to = other operating conditions.
 
What about TIT = limits, typically 1650F or sometimes 1750F?  The TIT is the best = data we have, and if the turbo guys say we can expect trouble sooner or = later if these temperatures are exceeded, it is based on experience and = some safety factor using the best data available to the pilot.  = Respect these limits.  Inaccurate data is better than no data in = such circumstances.
 
But don't worry about = absolute values or calibration.  They are meaningless values and = exercises.  Watch the trend and comparison with other operating = conditions.  Those tell the important tales we need to know and = understand.
 
Fred = Moreno
 
<What-is-the-gas-temperature.jpg>--
For archives and = unsub http://mail.= lancaironline.net:81/lists/lml/List.html

= --Apple-Mail=_A53AA5D6-720D-4B69-9C8C-1E4C2C1CAA09--