manifold pressure question

F

FOD

Well-Known Member
Had an DE ask me about this scenario (just BSing not a checkride):
He first said, you have this:
2400RPM 24inches MP = gives you 65% power
2400RPM 26inches MP= gives you 75% power
Then he said hypothetically speaking would you belive the above? i said yeah

Then he goes alright you have two planes (ones turbocharged and the other is not):
Both 200 Horse Power
Both at 2700 RPM
Both at sea level
The normally aspirated aircraft is showing 32inches of manifold pressure and the turbocharged one is showing 42inches of manifold pressure, but they both produce the same amount of power. Why the difference in manifold pressure?

At the initial time i didnt have much of an answer, just said that i imagine it was because the air of the turbocharged aircraft is more dense. But after thinking about it a little bit more, im thinking its because they are both at sea level, thus both producing the max amount of power available despite the differences. But still kinda caught up in details that explain why the difference in manifold pressure (im to lazy to go review that stuff).

Anyone have any ideas?
 
[ QUOTE ]
Had an DE ask me about this scenario (just BSing not a checkride):
He first said, you have this:
2400RPM 24inches MP = gives you 65% power
2400RPM 26inches MP= gives you 75% power
Then he said hypothetically speaking would you belive the above? i said yeah

Then he goes alright you have two planes (ones turbocharged and the other is not):
Both 200 Horse Power
Both at 2700 RPM
Both at sea level
The normally aspirated aircraft is showing 32inches of manifold pressure and the turbocharged one is showing 42inches of manifold pressure, but they both produce the same amount of power. Why the difference in manifold pressure?

At the initial time i didnt have much of an answer, just said that i imagine it was because the air of the turbocharged aircraft is more dense. But after thinking about it a little bit more, im thinking its because they are both at sea level, thus both producing the max amount of power available despite the differences. But still kinda caught up in details that explain why the difference in manifold pressure (im to lazy to go review that stuff).

Anyone have any ideas?

[/ QUOTE ]

MP doesn't indicate power. At full power, the throttle plate is opened, so generally speaking, the MP is allowed to increase to ambient. Turbocharged allows boosted air to increase this pressure (up to the wastegate limit). Same sort of concept with radial engines that operate with blowers.
 
[ QUOTE ]
Then he goes alright you have two planes (ones turbocharged and the other is not):
Both 200 Horse Power
Both at 2700 RPM
Both at sea level
The normally aspirated aircraft is showing 32inches of manifold pressure and the turbocharged one is showing 42inches of manifold pressure, but they both produce the same amount of power. Why the difference in manifold pressure?

[/ QUOTE ]

MAP dosen't indicate power, it indicates how hard the engine is sucking in air.

You are corect MAP + RPM = HP, BUT this equation is diffrent for every engine.

Now the diffrence between the two airplanes is the turbocharged airplane's MAP is artificially bosted by the turbo.

The turbo increases the density (and therefore pressure) without increasing volume.

Aircraft engines with turbos are turbonormalized. They don't boost power (and decrease reliability), but they help maintain sea level power at higher altitudes.

Refer to this article from Avweb.

http://www.avweb.com/news/columns/182081-1.html
 
[ QUOTE ]
Aircraft engines with turbos are turbonormalized. They don't boost power (and decrease reliability), but they help maintain sea level power at higher altitudes.


[/ QUOTE ]

This is only partially true. There are two general types of turbocharged aircraft engines. Turbonormalized and ground boosted. Most (if not all) factory engines made by Lycoming and Continental are ground boosted. These engines are optimized for use with a turbocharger. These modifications include higher flow injectors, a lower compression ratio, magnetos specially designed for high altitude operation (usually by being pressurized or by being made larger), and higher strength components to withstand the higher horsepower. Ground boosted engines are capable of producing more than 30" of MAP and it is not uncommon to takeoff with over 40" of MAP and cruise in the mid 30" range.

Turbonormalized engines are different, they are set up so that even at full throttle they only make about 30" of MAP, the same as a normally aspirated engine. The advantage of doing this is you can turbocharge a normal engine (often by STC), just by bolting on the turbo system. This is often done as a modification to an existing airplane and engine combination by an aftermarket modification shop like Tornado Alley Turbos. This allows the engine to maintain max cruise power to a much higher altitude, usually around 18,000', instead of around 7,000' like a normally aspirated engine. By doing this you get better takeoff performance at high density altitudes and the ability to cruise at a much higher TAS than an unmodified airplane, without really increasing fuel flow, although you will have to use oxygen to get the most benifit from this type of modification.
 
[ QUOTE ]
Then he goes alright you have two planes (ones turbocharged and the other is not):
Both 200 Horse Power
Both at 2700 RPM
Both at sea level
The normally aspirated aircraft is showing 32inches of manifold pressure and the turbocharged one is showing 42inches of manifold pressure, but they both produce the same amount of power. Why the difference in manifold pressure?

[/ QUOTE ]

This can only happen if the engines are of different displacement. An example would be a 250 hp IO-540 vs. a 220 hp TSIO-360. For the smaller engine to make 220 hp, you can guarantee that it carries significantly more MAP than the larger engine does when it is making the same HP.

MAP alone does not allow a power comparison between engines. You can run a big engine at low MAP and get the same power as a very small engine at a very high MAP.

Formula 1 cars did this back in the 1980's during the 'turbo era' and they were able to get over 1,000 hp from engines that were (If memory serves) about 1.5 liters big. They did this by running ultra high rpm and bosting them to the moon.

So you could say that for our purposes, power depends on displacement, MAP, and RPM (along with about 100 other variables). If you are able to increase any of the three you will get more power.
 
People often confuse aircraft turbos with car turbos.

In a auto engine a turbo is designed to boost HP by craming much more air into the cylinder. WIth this extra 20-40% HP you go much faster, but reliability goes down the tubes. I have ridden a Honda motorcycle that was boosted, it was a genuinly frighting experiance, but really cool. /ubbthreads/images/graemlins/cool.gif /ubbthreads/images/graemlins/wink.gif

An airplane engine's first priority is reliabilty, not speed. So enginers genrally do not try to boost power when they turbocharge an engine. There is some additional power, but not much.

Ground boosted engines usually have fixed or pilot operated wastegates. The wastegate is a valve in the exaust system that allows exaust gas to bypass the turbine wheel. This limits boost to a safe level that the intake manifold was designed to accept. A good example is the Senaca II. In this airplane the turbo has a fixed wastegate that limits boost at low altitudes with out limiting it too much at high altitudes (think about the compromise of a fixed pitch prop). On take off MAP should not exceed 45" (IIRC), but it still only produces 200 HP at sea level. At higher altitudes the MAP behaves just like a normal aspirated engine, only the numbers will be bigger. At about the airplanes critical altitude the HP will increase up to 220 HP due to the increased efficency.

Others have pilot adjustable wastgate. All engines with turbos have a safety overboost valve. This keeps you from hurting your engine too badly, but a mechanic must inspect the pressurized part of the intake for blown seals.

The downside of the fixed wastegate is that at gound level the engine is working harder than it should with higher pressures, and higher temps, with a natural downward effect on reliablity. Also it really increases pilot workload. It's really hard to keep your head outside the cockpit while you're fine tuning the MAP. In my opinion all of this is being penny wise and pound foolish.

True turbonormalized engines have a automatic wastegate that keeps MAP down to 30-32". If the engine made 200 HP without the turbo, it will still only make 200 HP. However it will continue to produce 200 HP to a much higher altitude.I personally wouldn't buy a turbocharged airplane without an automatic wastgate, either factory or STC. It is much easier on the engine, and much easier on the pilot.


If you are flying a airplane with a turbo make all your power changes smoothly. Yanking on the levers really messes up a boosted engine (it's not good for normal ones either).

Make sure that you're oil temps are good and warm before you take off. At full power the turbo spins at aprox 100,000 RPM and at temps close to 2000 deg F. /ubbthreads/images/graemlins/shocked.gif It needs good lubrication.

I hope this clarifies my origional post, I had to do some research to make sure I was explaining this right.
 
[ QUOTE ]
This can only happen if the engines are of different displacement.

[/ QUOTE ]

Make that : "This can usually only happen....."
 
Alright i went back to the DE and told him a mixture of what was said on here and elaborated on a little more....he said i was still wrong.

He basically says that the hot exhaust air that comes from the engine goes thru the turbo charger then is cooled thru the intercooler, which increases the pressure.
 
[ QUOTE ]
He basically says that the hot exhaust air that comes from the engine goes thru the turbo charger then is cooled thru the intercooler, which increases the pressure.

[/ QUOTE ]

I'm confused by what he's saying there. Is he saying that the exhaust gasses are cooled by the intercooler after going through the turbo impeller? If that's what he's saying, he's absolutely wrong. Also, keep in mind that not all turbocharged aircraft have an intercooler. The turbo I flew for my MEI certainly didn't.

Maybe I'm just missing something. I need my caffiene fix. /ubbthreads/images/graemlins/smile.gif
 
no...i may have said that wrong...your right the exhaust gasses spin the turbine...however most turbocharging systems incorporate an intercoller to cool the air to that has been pressurized or charged.
 
Just sounds like typical DE BS to me. They always like to try to catch you by acting like they know more about something then you do, when in truth they don't have the slightest clue.
 
Well like i said, this wasnt a checkride or anything...he's just teaching a class that im taking and he gives us a little question to think about everynight....i kinda like the idea...

Also i havent really run into any problems with DE's, its the FSDO guys that are a different story
 
[ QUOTE ]
He basically says that the hot exhaust air that comes from the engine goes thru the turbo charger then is cooled thru the intercooler, which increases the pressure.

[/ QUOTE ]

Either he is completely wrong, or you misunderstood him. Don't worry it happens. /ubbthreads/images/graemlins/cool.gif

A turbocharger is like a water wheel conected to a water pump.

Exaust gases turn the turbine wheel. This side gets extreamly hot. Up to 2000 F

The turbine wheel is conected to the compressor by a common shaft. This shaft can turn as fast as 100,000 RPM

The compressor side takes fresh outside air and boosts the pressure to the carburator.

Since this air has been pressurized it has heated up. THe intercooler cools it back down for optimum efficency

The two side never mix.




The reason that the turbocharged engine dosen't produce any more power is that it was specificaly designed not to. Useing boost to get more power has a negative effect on reliability.

Don't be confused by the diffrent numbers on the MAP guage. If you look at the power chart you will see that every power setting with these two hypothetical engines will be about 10" diffrent, yet will produce the same HP.
 
Yeah your right...i was in a hurry and just wrote it down wrong...if you read the rest of the post you will see that
 
[ QUOTE ]
The reason that the turbocharged engine dosen't produce any more power is that it was specificaly designed not to.

Don't be confused by the diffrent numbers on the MAP guage. If you look at the power chart you will see that every power setting with these two hypothetical engines will be about 10" diffrent, yet will produce the same HP.

[/ QUOTE ]

This, I think, is the best SHORT answer to the original question. Two different engines, two different sets of power settings, etc.

I was flying a turbo C-206 quite a bit until recently (NOT turbonormalized - it will boost up above 30" on the ground). To get takeoff power, you need to throttle up to about 36", whereas a normally aspirated 206 will give about the same horsepower at ambient pressure (about 30"). There were some guys who thought that 36" of power included an "extra" 6 inches of "unnecessary" power, and they used 30" at takeoff with the turbo engine. They didn't realize that 30" on the turbo engine doesn't make the same power as 30" on the aspirated version. If they had looked up the power charts, they would have seen that they were shorting themselves of about 60 horsepower at takeoff. When you're taking off at gross weight on a hot day, that's not good...and your takeoff performance parameters become a question mark.
 
Purpose built turbocharged engines usually have a lower compression ratio than the equivalent sized normally aspirated engine. They also have more exhaust back pressure due to the turbo system and all its associated plumbing. This can be part of the cause for the higher MAP.
 
You must log in or register to reply here.
Back
Top