At what altitude would you exceed the max TIT, at say 65% of max rated power?
I don’t ever recall reading a TB21 pilot saying explicitly that he has to fly well ROP in order to limit the TIT, but it makes sense.
Ground level to around 5000 feet.
What would be the explanation for that? The compression ratio is lower than the TB20 engine (which is 8.5 to 1) and that runs at about 1530F EGT at 65% at sea level (23" 2400rpm 11.5 USG/hr). The TB21 is turbo normalised so there is no extra MP.
Lower compression ratio leads to higher EGT.
I think one variable is the control system. I believe controlling the engine via wastegate should be more efficient than throttling the whole thing. I know that engine is supposed to have automatic wastegate control, but I have no idea how it works. However, reduction of expansion ratio could kill the benefit (this is not a power recovery turbine, no way to convert that wasted energy into propulsive force; well, aside from jet stacks).
Another variable is the temperature limit of the turbine. This is an issue in cars as well. You might not be able to run the most efficient setting because it would bake the turbine so you have to sacrifice fuel for its protection.
Then you have intercooler, I can’t recall if this engine has one.
PS:
Silvaire wrote:
Lower compression ratio leads to higher EGT.
It should since you extract less energy.
In that case I ask why the TB21’s TIO540 has a lower CR than the TB20’s IO540.
If the lower CR partly cripples the turbo add-on, why have it? A huge advantage of the TB20 over just about all other GA SEPs (apart from some Mooneys with long range tanks) is its great range of some 1300nm to zero fuel.
Years ago, someone (@Placido I believe) tried to organise a group of people on the Socata owner site to fund a turbo STC for a TB20. Tornado Alley, from vague memory, quoted something like $200k for the STC. There was zero interest… Placido then bought a TB21 anyway.
Peter wrote:
In that case I ask why the TB21’s TIO540 has a lower CR than the TB20’s IO540.
It often happens in car engines as well. Octane rating is limiting. You need to lower compression ratio so the engine works at high loads (the charge doesn’t ignite on its own). And unfortunately there is symmetry – compression ratio is the same as expansion ratio (nobody is actually using that Atkinson’s mechanism and nobody AFAIK implemented in an aero engine Miller cycle). And lower expansion ratio means less energy is extracted from the combustion gasses.
Peter wrote:
If the lower CR partly cripples the turbo add-on, why have it?
In the words of Jeremy Clarkson, power!
Peter wrote:
A huge advantage of the TB20 over just about all other GA SEPs (apart from some Mooneys with long range tanks) is its great range of some 1300nm to zero fuel.
Is there really a significant difference compared to e.g. Commanders or SR22s? Not to mention piston PA46s. Or Bonanza with tip tanks. Some Mooneys will zoom past 1,300 like it’s nothing. Lancair IV?
My TR182 has the same engine as the NA R182 with the same high compression ratio cylinders (8.5:1). It was turbo-normalized by Cessna, not Lycoming, the Lycoming turbos have 7.3:1 or 8:1. The waste gate is manually controlled. The POH of both aircraft show the same speed and consumption at the same power settings. The TR182 generally makes its 2000h TBO without any engine or cylinder work but does require some exhaust work.
Not every turbo’ed Lycoming/Continental installation is more fragile than NA. The combustion engine in general is clearly moving towards turbo charging and there is no reason it can’t be made to last. Plenty of 500,000km Mercedes CDI taxis around even though for years people would say those turbo engines wouldn’t last… And the Centurion 2.0s turbo diesel which is less robust than its car counterpart (much lighter) but develops 15hp more than the strongest car version with no time limit for 100% BHP has a whopping 2100h TBR now.
Peter wrote:
A huge advantage of the TB20 over just about all other GA SEPs (apart from some Mooneys with long range tanks) is its great range of some 1300nm to zero fuel.
The Mooney Ovation will give you 1300 NM without long range tanks.
Piper Mirage with long tanks will do 1500nm with reserves when run LOP.
