Today's Message Index:
----------------------
1. 03:00 AM - Re: Temps (Lynn Matteson)
2. 06:09 AM - Re: Temps (BobsV35B@aol.com)
3. 02:40 PM - Re: Temps (Mark Hubelbank)
4. 04:47 PM - Re: Temps (BobsV35B@aol.com)
Message 1
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Bob-
Just to clarify, when you said: " If you want to use more than
seventy-five per cent power, you need to run at least 150, preferably
250 degrees F above peak EGT. At those higher power settings, we need
the extra fuel to slow the burning rate so that we can avoid
detonation," I'm sure you meant to say **150, preferable 250 degrees
richer (or cooler) than peak EGT** didn't you?
Mark, Bob, and others-
Where are you getting the figures for the percentages of power?
Everybody talks about these percentages, but I don't see the flows,
the manifold pressure numbers, or whatever the criteria is for
establishing these percentages written as it pertains to the Jabiru
line of engines.
Lynn Matteson
Kitfox IV Speedster, taildragger
Jabiru 2200, #2062
Sensenich 62"x46" Wood (summer)
Electroair direct-fire ignition system
Rotec TBI-40 injection
Status: flying...1054 hrs (since 3-27-2006)
Message 2
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Good Morning Lynn,
Yes! Glad you caught that. I was thinking about needing more fuel, but I
sure did not get it written that way!
The reason we want to be that rich is so that the rate of burn (Actually it
is a bit more complicated because the burn is just as fast but the fuel
takes longer to get burning that fast).is slower. That moves the point of
peak cylinder pressure further along on the stroke and reduces the peak
cylinder pressure. The actual temperature of the EGT is immaterial. It is just
that EGT is a good, low cost, and convenient, indicator of what is happening
to the combustion process.
They key is to control that peak cylinder pressure.
The history of using EGT as an indicator is all about how Al Hundere
brought us to using his ALCOR unit (Al's Corporation.)
He was trying to figure out some way to add a torque meter to our small
engines when he noted the variances of the EGT on his dynamometer. He noted
that while the EGT gauges of the day were not extremely accurate, they did
indicate the peak quite accurately. By advising us to run richer or leaner
than peak EGT he gave us the same information as was used in those big round
engines that were equipped with torquemeters. That is why ALCOR never used
precise temperatures. The actual EGT temperature means nothing. It is it's
relation to peak EGT that tells us where the compression process is going.
What we really want to know is where the mixture is in relation to best
power mixture!
Does that help a bit or have I muddied the water again? <G>
As to where the numbers come from for the Jabiru, in my case it is a WAG
based on the principle that all internal combustion engines run the same and
are affected the same way, but our aircraft engines are all designed to run
rich to cool properly at full power and to cool properly at cruise power
using best power mixtures.
That is why Continental says to run rich above sixty-five per cent power
and Lycoming uses seventy-five percent. If you try to nail either's engineers
down to something more precise, they will tell you it all depends and my
WAG is that the JABIRU engineers went through the same process as did the
others.
Happy Skies,
Old Bob
In a message dated 10/23/2010 5:01:08 A.M. Central Daylight Time,
lynnmatt@jps.net writes:
--> JabiruEngine-List message posted by: Lynn Matteson <lynnmatt@jps.net>
Bob-
Just to clarify, when you said: " If you want to use more than
seventy-five per cent power, you need to run at least 150, preferably
250 degrees F above peak EGT. At those higher power settings, we need
the extra fuel to slow the burning rate so that we can avoid
detonation," I'm sure you meant to say **150, preferable 250 degrees
richer (or cooler) than peak EGT** didn't you?
Mark, Bob, and others-
Where are you getting the figures for the percentages of power?
Everybody talks about these percentages, but I don't see the flows,
the manifold pressure numbers, or whatever the criteria is for
establishing these percentages written as it pertains to the Jabiru
line of engines.
Lynn Matteson
Kitfox IV Speedster, taildragger
Jabiru 2200, #2062
Sensenich 62"x46" Wood (summer)
Electroair direct-fire ignition system
Rotec TBI-40 injection
Status: flying...1054 hrs (since 3-27-2006)
Message 3
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Lynn,
I don't have a manifold pressure indicator yet so I am going by
fuel burn. At best a spark ignition engine generates 15 HP per gal per
hour so at 6 GPH one at best is generating 90 HP which is 75% of 120 HP
(for a 3300). I am treating this as worst case and figure anytime it
won't nicely lean to 6 GPH or under, it has to run at the rich settings.
For a Jabiru, I then follow their recommendations and set the mixture
for 1250 F max. Even that is a moving target as it changes as one gains
altitude.
On 10/23/2010 5:58 AM, Lynn Matteson wrote:
>
> Bob-
> Just to clarify, when you said: " If you want to use more than
> seventy-five per cent power, you need to run at least 150, preferably
> 250 degrees F above peak EGT. At those higher power settings, we need
> the extra fuel to slow the burning rate so that we can avoid
> detonation," I'm sure you meant to say **150, preferable 250 degrees
> richer (or cooler) than peak EGT** didn't you?
>
> Mark, Bob, and others-
>
> Where are you getting the figures for the percentages of power?
> Everybody talks about these percentages, but I don't see the flows,
> the manifold pressure numbers, or whatever the criteria is for
> establishing these percentages written as it pertains to the Jabiru
> line of engines.
>
>
> Lynn Matteson
> Kitfox IV Speedster, taildragger
> Jabiru 2200, #2062
> Sensenich 62"x46" Wood (summer)
> Electroair direct-fire ignition system
> Rotec TBI-40 injection
> Status: flying...1054 hrs (since 3-27-2006)
>
>
--
Mark Hubelbank
NorthEast Monitoring
2 Clock Tower Place
Suite 555
Maynard, MA, 01754 - USA
mhubel@nemon.com
978-443-3955
Message 4
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Good Evening Mark,
I think your 15 HP per gal of fuel is a good number for an engine with a
compression ratio of at least 8.5 to one if it also has very good fuel
distribution to each cylinder. If there is any unbalance in the amount of fuel
per cylinder, that ideal power per gallon of fuel cannot be reached.
Lower compression ratios will mean substantially less horsepower for each
gallon of fuel used.
One simple way of checking for balanced fuel mixtures with a fixed pitch
propellor is to lean for a drop in RPM. Starting at full rich and leaning the
engine should result in a slight increase in RPM. once the peak RPM is
reached, further leaning will cause a drop in RPM. For an engine the size and
RPM range of the Jabiru, a drop of at least one hundred RPM before any
roughness is noted signifies good distribution. With excellent distribution,
the rpm drop could be a couple of hundred RPM. If you can discern no
increase in RPM before roughness ensues, The distribution stinks and you will
get
way less than 15 HP per gallon of fuel.
The Jabiru that I fly has a Bing carburetor and we have no way of manually
adjusting the mixture. Any changes have to be done by changing out the fuel
rods and jets.
There is no easy way to analyze the Bing equipped Jabiru, but with careful
analysis of a good six cylinder engine monitor, some conclusions can be
reached.
It is NOT a simple process.
It would not be at all unusual for an engine equipped with a Bing to
develop as little as ten HP per gallon of fuel.
I envy Lynn's adjustable mixture capability! <G>
As Always, It All Depends.
Happy Skies,
Old Bob
In a message dated 10/23/2010 4:40:43 P.M. Central Daylight Time,
mhubel@nemon.com writes:
--> JabiruEngine-List message posted by: Mark Hubelbank <mhubel@nemon.com>
Lynn,
I don't have a manifold pressure indicator yet so I am going by
fuel burn. At best a spark ignition engine generates 15 HP per gal per
hour so at 6 GPH one at best is generating 90 HP which is 75% of 120 HP
(for a 3300). I am treating this as worst case and figure anytime it
won't nicely lean to 6 GPH or under, it has to run at the rich settings.
For a Jabiru, I then follow their recommendations and set the mixture
for 1250 F max. Even that is a moving target as it changes as one gains
altitude.
On 10/23/2010 5:58 AM, Lynn Matteson wrote:
> --> JabiruEngine-List message posted by: Lynn Matteson <lynnmatt@jps.net>
>
> Bob-
> Just to clarify, when you said: " If you want to use more than
> seventy-five per cent power, you need to run at least 150, preferably
> 250 degrees F above peak EGT. At those higher power settings, we need
> the extra fuel to slow the burning rate so that we can avoid
> detonation," I'm sure you meant to say **150, preferable 250 degrees
> richer (or cooler) than peak EGT** didn't you?
>
> Mark, Bob, and others-
>
> Where are you getting the figures for the percentages of power?
> Everybody talks about these percentages, but I don't see the flows,
> the manifold pressure numbers, or whatever the criteria is for
> establishing these percentages written as it pertains to the Jabiru
> line of engines.
>
>
> Lynn Matteson
> Kitfox IV Speedster, taildragger
> Jabiru 2200, #2062
> Sensenich 62"x46" Wood (summer)
> Electroair direct-fire ignition system
> Rotec TBI-40 injection
> Status: flying...1054 hrs (since 3-27-2006)
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