My love to hate engine.

My love to hate engine.

Postby j79guy on Sat Feb 21, 2009 10:25 am

Whew, where to start on this one? Core engine is a great flight unit, but a damn poor industrial engine. The core engine, the F-404 was designed "Sigma-1.2" whereas engines such as the TF39 (LM2500) and J79 (LM1500) where designed to be "Sigma-3". What this means is, on test, the engine has to meet a certain minimum performance standard, for a certain period of time. Let's say, 100% thrust, has to be maintained for 100 hours. In the old engines, this could be comfortably maintained for 300 hours, without incident. The newer engines, like the F404 would comfortably perform this test for 120 hours.
Ok? So obviously some ruggedization was lost, in the quest for lighter weight. Secondly, the newer engines were "simplified". i.e. fewer compressor and turbine stages. This was acheived via higher compression ratio per stage, (Compressor) and more work done per stage. (Turbine.) The new style aero compressor blades are like artwork. Beautifull wide-chord blades, made out of titanium. Compression ratio per stage on the order of 1.4:1, whereas the older engines with their narrow chord steel blades were about 1.12:1 compression ratio per stage. The F404 was designed to replace the J79, with the same developed thrust, 2/3 the weight and 30% fewer parts. Yes, GE acheived that goal and it is a legendary flight engine. Unfortunately the industrial units are legendary too, only in a significantly different way.

There is no denying that the LM1600, when fresh is efficient. However give a few run hours under the belt, and that efficieny falls off rather fast. When fresh overhauled, on a crash shutdown, the engine will lock up for well over four hours, due to tight turbine tip shroud clearance. After a few shutdowns, this lock up time gets less and less, to where eventually, there is no lock up at all. Now with only two stages of turbine, guess how the turbine efficiency is effected? Those impressive wide chord compressor blades? put a couple bug guts on them, and yes, the compressor efficiency falls right off too. As a matter of fact, everyone thinks that the newer engines have a more "efficient" compressor. This is not so. The old J79 compressors, were rated at 86% efficiency. Just about right where the new ones are. Difference being, the older compressors are more resistant to fouling than the new ones by virtue of having more stages with which to develop pressure. That high compression ratio per stage may look impressive on paper, but simply does not have the resilience in a ground power application. A wee bit of fouling on the Lp. stages has a much more profound effect on compression ratio, than on an older engine.

The F404/LM1600 was designed to be light weight. Makes sense, to be a good flight engine, they should weigh as little as possible. However this is what precisely works against the industrial units. The energy of moving 100 Lb of air per second through the engine and releasing 150,000,000 Btu/Hr in the combustor simply shakes the engine, due to energy radiation. It is a well known fact the the physical act of buring a fuel/oxygen mixture makes a "noise" in the 300-500 Hz range. In the F404/LM1600 this noise is intense enough to shake the whole engine, and excite the natural frequency of any component within and without the engine that falls in this frequency range. Yes, this includes a significant percentage of the engine, and those effected components literally self destruct.

Any industrial engine, to be taken seriously, should be able to run 30,000 fired hours, at base load conditions, between overhauls. Sadly, few new industrial units will do this. The older ones would. In order to preserve any scrap of credibility the LM1600 operators have to replace their HP Compressor Stator cases on an 8000 fired hour basis! They are give a "price break" on rebuilt exchange cases, but what does this really acheive? It keeps the engines running, but most importantly, makes the reliability figures falsely look good, at the expense of the operators, who are paying for this false rating out of their own pockets.
In 1996, I was invited to attend an LM1600 user's conference. (One of the last conferences before becoming an independant.) I stood up and called GE every dirty name in the book, regarding this engine. Nothing personal, I was simply goading GE to do something substantial towards really fixing the problems. The problems are well known, and so are the fixes, just that at that time, and still to this day, it is more convenient to put a Band-Aid on the wounds, rather than fix the problems. (Heck, if the operators will pay for the Band-Aids, why not?)

Anybody have good experiences with this engine? I could go on and on, but my main points have been made.

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