The type is a misprint, it was an igso 480 (340HP). I don’t think it was ever used on any Commanders. By the time fuel injection was coming online, Ted was already going for the top of the line igso 540 for the 680F, FL,FLP.
Even though the twin commander line looked very similar over it’s 30 year production run, there really was continuous evolution. On the powerplant side they always had the highest performance, most powerful engine as soon as it came out. At the time when they were adopted, I believe the GO-435, GO-480, GSO-480, and IGSO-540, were the most powerful engines available. We are very fortunate that they decided to also produce the economy model.
The Turbocommanders, of course, also mounted the most powerful Garret available. I guess the 840/900 would be the economy analog for the 500.
I have always like the geared Lycomings from that era. Today, they are considered to be expensive to maintain engines that are dificult to properly maintain at any price. In the 60s and well into the 70s, I think they were well supported, well understood, reliable, and reasonably expensive to maintain. Obviously more expensive per hp, and much more intolerant of pilot and mechanic mistakes than the training engines… O-320/360. The TEO 540 has been out for almost a decade, and it is still having a lot of problems. By the time the IGS0-540 was a decade old, it was so well proven that it was obsolete. It is interesting to compare the two.
Because of incomplete specs on the internet (you can’t tell what accessories are included) it is hard to compare weights. From what I can see they are about the same at about 550 lbs (about 100 lbs heavier than the economy engine).
At full rip on takeoff:
IGSO 540 47”x3400 rpm 380hp and 60 gph bsfc .95!!
TEO 540 49”x2575 rpm 375hp and 43 gph bsfc .59
They both have 7.3:1 compression, so map should be representative of imep. The turbo’d engine will have to push a slightly higher map to compensate for back pressure, but the supercharged engine is running so rich that it will put a 5-10% damper on the amount of power produced in the cylinder. Both engines are essentially boosted to the limit of what can be produced by a 540 cid engine running 100LL.
The igso uses massively rich mixture to cool the engine and suppress detonation. The teo eases some of the heat burden with a variable waste gate, and hopes for a well cooled installation while cutting the detonation margins by relying on electronics to prevent or even correct detonation.
To me it appears that all the extra power produced by spinning the igso 32% faster was soaked up by the reduction gears and friction of higher rpm (15 hp?), the supercharger (75 hp?), and the extremely rich mixture (30 Hp?). There is the significant benefit of a quieter more efficient slow turning prop.
Climbing out at 270hp :
The igso 35x2750. Fuel flow should be somewhere between auto lean 26 gph (.58 bsfc), and auto-rich 32 gph (.72 bsfc). Still cooling with fuel, but not dumping so much fuel in that there is a power loss.
The teo 35x2400 24 gph (.53 sfc)
And at economy cruise of 215 hp:
Igso 29” and 2400 rpm with the props flicking over at 1540 rpm.
The POH shows 19 gph, but the Lycoming manual has auto lean at 16.5 to auto rich at 21 gph
Equals TEO540 57% pwr 2200 rpm 16.5 gph (.46 bsfc)
At lower rpm and lower power setting where the igso does not rely on fuel cooling, the fuel efficiency is very close. Essentially down to a what the gears and the supercharger soak up vs the backpressure of the turbo. I would guess that any energy wasted by gearing and higher rpm is made up for by prop efficiency. Electronic fuel injection and ignition timing help only a few percent at econ power settings. Another way to say it is that the igso is tuned to operate efficiently at 50-60% power and uses rich mixture to compensate at higher power.
What really jumps out at you is the massive amount of fuel used for evaporative cooling at high power in the older engine. Even on short flights though, you spend less than 15% of the flight at high power. So it really has only a small penalty on trip fuel burn. The new engine trades fancy electronics, and the higher expense of intercooled turbo for a pretty small efficiency gain. While a variable waste gate keeps the turbo from working too hard and overheating the induction air, the electric F/I and ignition shouldn’t do anything to help the engine run cool. It really just does a better job of accurately measuring how hot the engine is running. I wonder if the Tecnam has high cooling drag, or if the computer actually ends up running the engine quite a bit richer in the real word. I don’t see anything in the Lycoming charts that TEO is operating significantly lean of peak, or would indicate that the computer, at any power setting, would pull chts down by further leaning the mixture
The primary reason for the electronics is, of course, to try to knucklehead proof the engine while still putting out the high power. While Lycoming is awesome for publishing their manuals online, understanding service difficulties is more like reading tea leaves. Every indication I see is that the engine still has problems. They may be nothing more than a result of not having yet fully developed and disseminated the technical knowledge needed to maintain and operate the plane.
While it would be great to have a smooth, efficient, 375hp turbo’d fadec in a Shrike, I also wonder if Lycoming and Tecnam talked about developing the igso with elec fuel injection and ignition. I think instead of the fadec dream that has failed over and over on recip aero engines, it would make sense to use the vast array of sensors on the engine to function like the “dope slap” system that was used to train me on the operation of big radials many years ago. If the complex and dynamic set of limits that define safe and conservative operation of a high power recip could be communicated with tones/lights/vibration/tattletailing (the way modern turbine aircraft squawk if we get toward the edges of a safe flight profile) it could have the desired effect of improving the reliability and economy of a knucklehead susceptible engine.
While going from supercharger to turbo is an incremental improver of engine performance, it does bring a higher cost. Going from geared to direct drive though is a real compromise for a high performance engine. I think eliminating the reduction gears worked to get the direct drive high power IO 540s nearly as knucklehead proof as the training engines. But it actually works against performance, reliability, and total cost at the very top of the range.
Now that Tecnam is going to try using the Continental GTSIO, I wonder if there will be any effort to knucklehead proof that engine.