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  #1  
Old 05-29-2004, 10:46 AM
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Relationship between diesel (and jet) exhaust temperature and compression

I have been watching the temps on my exhaust manifold and was guessing that the hotter manifolds went to cylinders with higher compression. I did a compression check and found the opposite to be true.

I have not cleaned and tested my injectors yet and so I have no indications how that variable will play into this.

I am guessing that lower compression means later ignition and less efficient burn so more fuel burning on the way out the exhaust.

Is there something to this? Know of any good links or references on this?


Last edited by TwitchKitty; 06-05-2004 at 02:38 AM.
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  #2  
Old 05-29-2004, 12:48 PM
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I don't think you will find a relationship on this..
I think the difference will be in small differences in fuel amount supplied by different injectors....... or ..... in differences in the cleanlyness of the holes in the precombustion chambers... these make a difference in how well the fuel is spread for the combustion process....
I think once ' compression ignition' is reached that the tiny tiny tiny difference in when it happens in the compression stroke will not be measureable... or will be so much smaller compared to the other variables that you will not be able to isolate it...
I don't believe in references... why confuse people with facts ? I just make up answers....
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  #3  
Old 05-29-2004, 08:33 PM
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The higher the compression, the more efficient the energy transfer into work, so the lower the exhaust temp for a give load.

Essentially, you are right -- the fuel is burning slower, and it's being expanded less, so it's hotter coming out.

Note that the opposite is true in turbines -- the more energy you extract (and the higher the compression ratio), the slower the gas stream moves and the higher the heat transfer to the turbine blades.

Fan jet engine turbines run incredibly hot.

Peter
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  #4  
Old 06-01-2004, 07:35 PM
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Peter,

The exhaust gas temperature of a gas turbine will behave in similar manner to a piston engine. When the gas turbine is brand new, and all the internal seals are just about perfect, the engine will have an EGT margin of about 30 degrees C. What this means is that the engine, at maximum takeoff power, will be running 30 degrees less than the maximum allowable temperature. As the engine gets older, and things begin to wear, and the compression ratio drops, the EGT margin will erode and the EGT will climb. The engine can continue in service until the EGT margin gets to 0 degrees C. At this point, the engine will be at the maximum allowable EGT at maximum takeoff power. Any further erosion in the internal clearances for the engine will result in a negative EGT margin. Normally, this is when the engine requires removal for a rebuild.

A typical temperature for the exhaust of a large turbofan is about 600 C. (1112 F.). Their compression ratios typically run up to about 40:1 or so.
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  #5  
Old 06-01-2004, 10:26 PM
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Brian:

Some run a bit hotter -- the engine intake air on the SR-71 was about 500C or more at speed (test stand operation requried running a JT5 at full throtte in front of it to pre-heat the air!). The exhaust stream was incandescant, somewhere in the temperature range of a hot acetylene torch, I believe.

Engine itself was orange on the leading edges, and the exhaust section was cherry red.

No maker's plate on the side, either -- prototype engine sucked it off the rivets and "ate" it -- not very successfully as I heard. That may be the engine at Wright-Pat with the big gouge in the front compressor blades.....

Not your typical AirBus engine, though.

I'd consider 600C incredibly hot for engine parts!

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #6  
Old 06-01-2004, 10:46 PM
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Peter,

The SR-71 engines were specially designed turbojet engines to produce massive amounts thrust without any regard to fuel consumption. That aircraft has many specifications that are downright amazing. I believe that it holds the speed record for cross-country flight with a time of something like 80 minutes or so.
When it is cruising along at 2100 mph it has a turn radius of 150 miles! After a run of 80 minutes at full speed, it's just about out of fuel!

My previous discussions were regarding the more conventional large turbofans such as the GE CF6 or the Pratt and Whitney PW4000 engines.
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  #7  
Old 06-01-2004, 11:09 PM
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Brian:

The SR-71 supposedly had about the same fuel economy as a Boeing 727. Not all that good by modern standards, but that 727 used to be the most common aircraft in use (or was it the 737?). Anyway, 80 min at nearly three times the speed of sound is a LONG way! A coworker of mine some years back worked on the DEW line radar stations as a nurse, and heard the controllers giving clearance for an SR-71 to land in Kansas City as it went overhead in northern Canda....

Probably a trans-polar run over the Soviet Union from Iran, that was the typical flight pattern in those days.

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #8  
Old 06-01-2004, 11:49 PM
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Peter,

I have to laugh at the fuel economy when compared to a B-727. The 727 burns something like 8,500 lb. per hour. At 500 mph, it will take 5 hours to go 2,500 miles and it will burn about 42,500 lb. of fuel to do it. This is a bit of a stretch for the 727. It would have to have a very light passenger load to make this distance.

The SR-71, will also travel about 2,500 miles on fuel. At 2100 mph, it will cover this distance in a little more than 1 hour. However, to go this fast, it will consume nearly 100,000 lb. of fuel! This is what you need when you are running full afterburners to overcome the massive friction created by a speed of 2100 mph.

The Concorde is another good example. The aircraft weight is just about 400,000 lb. on takeoff from London. It lands at JFK at a weight of less than 200,000 lb. It consumed 200,000 lb. of fuel in about 3 1/2 hours. Same deal. Run four big engines at 1400 mph and watch the fuel gauge drop!

Compare the Concorde to a B-747 which will take seven hours to go from London to JFK. It consumes about 30,000 lb. per hour and burns just about the same amount of fuel as the Concorde to go the same distance. However, the B-747 weight is more than double (840,000 lb.) that of the Concorde.

The government would be thrilled if they could get the SR-71 to sip fuel like a B-727
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  #9  
Old 06-02-2004, 12:05 AM
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Brian:

Actually, the SR-71 has been out of service for nearly 10 years now, unless someone has refitted one. They were all built between 1965 and 1969, IFRC, and are OLD by aeronauticals standards, titanium bodies aside. Fatigue is a problem in stainless steel, too.

Range was quite a bit more than 2500 miles, though -- they did routine overflights of the Soviet Union from Iran and Turkey, some transpolar, and many from the US.

As to fuel consumption, they are VERY efficient at 70,000 ft plus and twice the speed of sound, much more so that a Concode, which used straight turbojets. The SR-71 has a number of "funny tricks" about the engine nacells, and runs pretty much as a ram jet at high speed on afterburners. Also used a combination of trailing edge exhaust and upper surface suction to keep the stall speed under 250 mph, and actually produced about 1.5:1 bypass ratio via the exhaust slipstream at low speeds, too. It ate fuel like crazy at low speeds, but the engine was so efficient at high altitudes it has the same fuel comsption over distance of a similar weight turbo fan aircraft. It holds an amazing amount of fuel, too -- has to, it's used for cooling during supersonic flight....

If you've not seen one, go to Wright-Pat in Dayton , Ohio. Quite an impressive museum. The one they have is the Y-R-71, the smaller experimental interceptor version that no one ever figured out a way to arm. They also have a B-52 that made a HARD landing (it's visibly bent) and the only B-36 in extistance. A true monster of a plane!

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #10  
Old 06-03-2004, 09:13 PM
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Peter,

I can imagine the range of the SR-71 being much greater than 2500 miles, however, only if they keep the speed down and not use the afterburners. There is no possible way that it can go more than 2500 miles (give or take) if the thing is cruising along at Mach 3 or so.

I would really like to see one of them. A truly unique airplane.
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  #11  
Old 06-03-2004, 11:21 PM
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Brian:

At 80,000 ft, the air is pretty thin, so drag is minimal. The SR-71 will, as far as I know, fly up to the point there isn't enough air to allow adequate flight control, it's not relaly limited by engine breathing. That high, with that little drag, it doesn't burn much fuel once it's up to speed, at least compared with take off and climb.

It did burn considerably less fuel than the Concorde, but then it's only about 1/4 the size (or less). Concode burned at least half the fuel load getting up to altitude and speed. That's why there was never an engine out emergency -- would have resulted in a ditching, as there wasn't enough fuel on board to return to origination or destination at less than 60,000 ft. Engine maintenance was overwhelming (and I'm sure one of the reasons Concorde never made a dime for anyone). Just like the Great Eastern -- even nearly bankrupted the shipbreakers!

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #12  
Old 06-03-2004, 11:30 PM
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Peter,

I'm not positive about it, but, even at 80,000 feet, to go 2100 mph requires an enormous amount of fuel. The air may be much thinner, but, you still have to push it aside. Just look at the space shuttle after it reenters the atmosphere at something like 140,000 feet. The thing has its wings and underbelly glowing nearly white hot from the friction. The Concorde grows 14 inches in length from the skin heating up at Mach 2 and 60,000 feet. Just imagine the friction at Mach 3!

Do we know, for sure, what it's range is at Mach 3? I cannot believe that it is much more than 2500 miles or so. If this is true, it is less efficient than the Concorde. But, Mach 3 is way more than the Concorde's speed of Mach 2. Speed costs fuel. There is no way around it.
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  #13  
Old 06-04-2004, 05:40 AM
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as little a 5 years a go nasa was using sr 71's for high altatude reserch
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  #14  
Old 06-05-2004, 12:30 AM
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Brian:

It would be fun to look all that up! I'm sure the range was much more than 2500 miles, although less than the U2.

Remember, the SR-71 ran as an almost pure ramjet (with the turbine engine serving as preheater) at high speeds, so efficiency is considerably higher than for a straight turbojet. The engine nacelle used the shock wave from the "nosecone" to reduce the speed of the air to subsonic, so it could the re-accelerated up to supersonic speeds.

The air is dense enough to heat the center frame of the windsheild dull red, and the leading edges of the wings weren't much cooler, but fuel usage was fairly small.

The Concorde was a gas hog, the TU-144 used considerably less fuel, at the expense of a considerably more complex engine system (variable pitch turbofan).

Someday when I have nothing else to do I will see if I can dig up some data.

Peter
__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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  #15  
Old 06-05-2004, 10:06 AM
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From "The Big Book Of Airplanes" (courtesy of my three year old) the pilots would warm their astronaut style tubes of food by holding them against the windows, the engine would expand by 8-10 inches from heating, and the fuel tanks didn't seal until expanding, so they spewed fuel during takeoff and the early flight stages...

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