Can anyone explain why the Cont TSIO-360 engine (say EB model as fitted to many Seneca IIs) is rated at 215hp at 12,000ft, and 200hp at S.L?
In the POH there is a reference to an 'orifice' in the turbocharging / induction system being 'set' at 12,000ft. This is an engine with a fixed waste gate from what I can tell, so what is this orifice? I'm guessing that with the throttles fully forward at 12,000ft you'd get 40" MP, descend and the MP would increase (if throttles remain in position).
Seems you have already figured out how this works. In case, you haven't....
Fixed Orifice Turbocharger Control The simplest form of turbocharger control is to have a fixed orifice exhaust bypass. A proportion of the exhaust gases always drive the turbo, from the ground upto the maximum operating altitude. This orifice is preset by maintainence guys. This preset orifice setting results in a critical altitude of 12,000ft density altitude (full throttle, 2600 RPM & MAP 40" Hg). Any higher than that, the aircraft will not be able to maintain this MAP.
Critical Altitude The height above which maximum manifold pressure can no longer be maintained.
One of the main limitations of turbocharged engines is manifold pressure, which is limited to let's say 40 inHg. For example, the aircraft is stationairy at sea level, ISA conditions. The air is entering turbocharger with temperature 15°C and it has a pressure of 40 inHg, so we calculate density of the air:
T = 15°C = 273.15 + 15 = 288.15 K p = 40" = 1.3544 bar = 135440 Pa R = 287 J/(kg*K)
The TCDS for TSIO-360-EB shows that it has a critical altitude altitude of 12.000 ft (maximum altitude at which turbocharger can provide maximum manifold pressure), which means that the air is colder than at sea level and the density of the air is a bit higher:
Since piston engine power is (very, very simplified) more or less a function of the density of the intake air (assuming constant air to fuel ratio, etc.), since if we have constant air to fuel ratio, increasing of density of the air will also increase the mass flow of the fuel, more fuel will burn and the power output will be higher.
So we can calculate difference between densities of the air at MSL and 12.000ft:
1.7865 / 1.6378 = 1.09 = 109%
So if the engine is producing 200hp at sea level, it will produce 200 x 1,09 = 218 hp at 12.000 ft, which is close to 215 hp which is stated in TCDS. Do note that this is over simplified just to show basic principles of increasing power with increasing altitude (up to critical altitude) in turbocharged engines.
So basically, the maximum manifold pressure (40") and maximum RPM (2575) remains the same and the power output is higher at critical altitude than at MSL.
Originally Posted by barit1
Also, at altitude, the back-pressure on the exhaust is less, thus better exhaust scavenging.
I posted this on the tech website also, I may be misreading something in FlyingStones's answer but he was assuming because the air is cooler at 12,000ft it is denser, but doesn't take into account increase in induction air temp due to max impeller speed. Therefor I'll have to go with the simple answer - backpressure. Reduction of this improves cylinder fill with fresh charge of air/fuel, 40" pushing against 18" rather the 30". More air/fuel more power.
The action of compressing the air rapidly increases its temperature, and reduces some of the increase in density which results from the increased pressure, this loss of density may be partially recovered either by passing the air through a Inter-Cooler or by spraying the fuel into the eye of the impeller so that vaporization will reduce its temperature. Will need the manual for the engine or the turbocharger for more details.
Lumps, you are right though, that FlyingStone didn't take into account the temperature action of the impeller.
Thanks BB, so some versions produce same rated power at SL or at critical altitude with same RPM and MP. As far as I can tell only one version has a waste gate.
Back into the dark for me.
In the PDF posted by BB, some versions produce the same rated power at SL and at critical altitude with same RPM and MP. In fact they are also fitted with wastegate.
I will soon be flying the Seneca III which has a TSIO 360KB. The Seneca III has a limitation that Take off Power and 2800RPM can only be used for take-off and for 5 mins max, this power setting will produce 220BHP. But it is not available above critical altitude(14,500') unless you conduct a take-off above that altitude. For all other operations, maximum continuous power at 2600RPM will produce 200BHP, which that PDF doesn't mention.
The Contenental Engine Operators Manual is the reference you need, not the POH.
You are correct, but I need the POH(Seneca III) to read some other stuff.
The link I gave is to the engine Type Certificate Data Sheet. It is one of the prime documents in the maintenance publication hierarchy. The Pilot's Operating Handbook would be at the other end of the scale.
The Continental publications should be used, the POH often has limited info. I was just corresponding with Piper a couple of weeks ago about the POH for another aircraft model. In that case the first amendment in over 20 years had just been issued, I was a little concerned that an AD from 25 years ago had still not been taken into consideration.
I think the figures being bandied about here are just 'rated' power, it has little to do with the actual power that could be developed.
The Continental Operators' Manual will have a variety of graphs and charts which may answer the question better.
The POH is just a handbook, - literally.
I am sure I have seen links to these various books available for free download. They will not be current but will do the job for you.
I've never found free links for the Engine Operators Manual. I bought mine from Essco. If I had some time I'd scan a couple of the graphs. The Engine companies actually know what they are doing. Some (specifically Continental) don't seem to understand quality control or supplier quality assurance. But that's a seperate issue from the engineering.
Its late and I'm a bit pressured for time, so I'm not guaranteeing that I've haven't made a mistake.
But my reading of the performance chart in the Engine Operators Manual is that 40 in Hg manifold pressure at SL yields 100% rated power (200 Hp). But 40 inches at 12,000ft yields 215 Hp. So, for a given manifold pressure the engine delivers more power at altitude. So, its got to be about inlet temperature and / or water vapour content (humidity). Humidity has a significant detrimental effect on power.
The performance graph also suggests that the engine can only maintain 40 inches to 12,000ft and after that it drops off, crossing 200 Hp at 14,000 ft where it makes about 37.5 inches.
From experience, this is a bit academic, because you're likely to tun into TIT and CHT issues at those altitudes and high power levels. As the air gets thinner, it also becomes a less effective cooling fluid.
The engine operators manuals are relatively cheap and worth having. On long flights it can be interested to pull them out and see where the engine / airframe performance plots. I'd recommend buying the books.
I'm with some smart engine people next week. I'll ask. But I don't like the back pressure argument. I think if it was reduced backpressure the effect would not drop off as quickly past 12,000 ft. Also - have you seen the amount of exhaust pipe after the turbo? In the Seneca it about a foot of straight, relatively large diameter pipe.
Also, assuming the manifold pressure gauge reads gauge pressure and not absolute, then its essentially telling you the pressure differential between the inlet manifold and atmosphere, ie its incorporating a measure of back pressure.
I think if the engine develops more power at 12,000ft with a 40 inch differential between inlet manifold & atmosphere than it does at ground level, then something else is at play.