OK...It's dumb question time...again

[Mudge]

What is meant by "manifold pressure"? How it achieved/changed?
I seem to hear about it only in regards to inline engines, not radials.
Would someone 'splain it to me, please?

Mudge the educable

[Holedigger]

No such thing as a dumb question, unless you are already airborne and asking
I had to look that one up some time ago myself, as it is somewhat complicated with all the variables and mechanics involved. Here is a link that explains it fairly succinctly.

http://answers.yahoo.com/question/index ... 230AA74xXB

[Mudge]

OK...let me see if I understand this. The manifold pressure is changed by how much/how little throttle you're applying, right? It's not increased/decreased by a separate control knob/lever of some sort. ie. The more throttle, the higher the MP, (and vice versa), right?

Mudge the enlightened

[bipe215]

Remember that in a normally aspirated engine, the only way for air to go through the induction system is a downward traveling piston with an intake valve open. The MP is measured downstream of the throttle plate. Its not really 'pressure', its lack of pressure, or suction.
If you are at cruise and reduce rpm, you see the MP rise because the pistons are moving less air, creating less vacuum, so MP rises toward ambient pressure.
If you increase rpm at cruise, the MP drops; more suction past the throttle plate, lowers the pressure in the induction system.

Steve G

[RyanShort1]

OK...let me see if I understand this. The manifold pressure is changed by how much/how little throttle you're applying, right? It's not increased/decreased by a separate control knob/lever of some sort. ie. The more throttle, the higher the MP, (and vice versa), right?

Mudge the enlightened

From the FAA's Airplane Flying Handbook:

When performing a climb, the power should be advanced to the climb power recommended by the manufacturer. If the airplane is equipped with a controllable-pitch propeller, it will have not only an engine tachometer, but also a manifold pressure gauge. Normally, the flaps and landing gear (if retractable) should be in the retracted position to reduce drag. As the airplane gains altitude during a climb, the manifold pressure gauge (if equipped) will indicate a loss in manifold pressure (power). This is because the same volume of air going into the engine’s induction system gradually decreases in density as altitude increases. When the volume of air in the manifold decreases, it causes a loss of power. This will occur at the rate of approximately 1-inch of manifold pressure for each 1,000-foot gain in altitude. During prolonged climbs, the throttle must be continually advanced, if constant power is to be maintained.

From the Handbook of Aeronautical Knowledge:

On airplanes that are equipped with a constant-speed propeller, power output is controlled by the throttle and indicated by a manifold pressure gauge. The gauge measures the absolute pressure of the fuel/air mixture inside the intake manifold and is more correctly a measure of manifold absolute pressure (MAP). At a constant r.p.m. and altitude, the amount of power produced is directly related to the fuel/air flow being
delivered to the combustion chamber. As you increase the throttle setting, more fuel and air is flowing to the engine; therefore, MAP increases. When the engine is not running, the manifold pressure gauge indicates ambient air pressure (i.e., 29.92 in. Hg). When the engine is started, the manifold pressure indication will decrease to a value less than ambient pressure (i.e., idle at 12 in. Hg). Correspondingly, engine failure or power loss is indicated on the manifold gauge as an increase in manifold pressure to a value corresponding to the ambient air pressure at the altitude where the failure occurred.
The manifold pressure gauge is color-coded to indicate the engine’s operating range. The face of the manifold pressure gauge contains a green arc to show the normal operating range, and a red radial line to indicate the upper limit of manifold pressure.
For any given r.p.m., there is a manifold pressure that should not be exceeded. If manifold pressure is excessive for a given r.p.m., the pressure within the cylinders could be exceeded, thus placing undue stress on the cylinders. If repeated too frequently, this stress could weaken the cylinder components, and eventually cause engine failure.

Manifold Absolute Pressure (MAP)—The absolute pressure of the fuel/air mixture within the intake manifold, usually indicated in inches of mercury.

Hope that helps. BTW, both of those books are a must-read for flight instructors, so if you're looking for sources of good information for yourself as a student pilot, have a look at them. Most of the Jepp, Gleim, or other books are going to be almost the same stuff.

They can be downloaded as searchable PDFs here: http://www.faa.gov/library/manuals/aviation/

Ryan

[RyanShort1]

Remember that in a normally aspirated engine, the only way for air to go through the induction system is a downward traveling piston with an intake valve open. The MP is measured downstream of the throttle plate. Its not really 'pressure', its lack of pressure, or suction.
If you are at cruise and reduce rpm, you see the MP rise because the pistons are moving less air, creating less vacuum, so MP rises toward ambient pressure.
If you increase rpm at cruise, the MP drops; more suction past the throttle plate, lowers the pressure in the induction system.

Steve G

You CAN have a higher manifold pressure (ie. not really a suction) if your aircraft is equipped with a Turbo or Supercharger. Hence bipe's mention of a NORMALLY aspirated engine. However, a lot of the more famous warbirds were equipped with superchargers or turbochargers at one point.
There may be a suction effect, but the gauge is measuring the absolute pressure inside - which may be reduced by the draw from the engine.

Ryan

[Taigh Ramey]

Hello Mudge,

The manifold pressure instrument simply measures air pressure in inches of mercury. On a standard day at sea level it should measure 29.92, or just under 30 inches of pressure. This is the pressure pushing down on a column of mercury in a glass tube.

On an aircraft the manifold pressure instrument is connected through a simple tube, typically 1/4 inch, right to the intake manifold between the carburator and the engine. It measures air pressure, or vacuum, inside the intake manifold hence manifold pressure.

At rest the instrument will indicate ambient air pressure which should be the same as the altimeter setting for the airfield.

When the engine is running manifold pressure will vary with throttle position. When you start the engine the manfold pressure drops as the engine creates less than ambient pressure inside the intake manifold as the pistons draw in air (and fuel). When you add more power with the throttle the manifold pressure increases.

With a turbocharged or supercharged engine there is a compressor which helps to pump more air into the engine especially at altitude where the air is thinner. This will giver the engine more available power at higher altitudes.

Many engines in warbirds had both turbos and superchargers.

I hope this helps a little,
Taigh

[51fixer]

Of course the Brits muck these things up. Their indicators are is Pounds of Boost which I believe is the same as PSI or Pounds per Square Inch.
As I will be entering the British Golden Age of aircraft when the MK XVIII Spit shows up here next week what is the conversion rate of MP to lbs of boost?
Rich

[Mudge]

So to answer the second part of my question (inline vs. radial) MP is not something that is exclusive to inline engines. Right?

Mudge the quick

[RyanShort1]

So to answer the second part of my question (inline vs. radial) MP is not something that is exclusive to inline engines. Right?

Mudge the quick

Correct.

Ryan

[bipe215]

Of course the Brits muck these things up. Their indicators are is Pounds of Boost which I believe is the same as PSI or Pounds per Square Inch.
As I will be entering the British Golden Age of aircraft when the MK XVIII Spit shows up here next week what is the conversion rate of MP to lbs of boost?
Rich

John Deakin's explains in pretty well in his Hurricane pirep:

Manifold pressure is automatically limited to 12 lbs. sq. in. in this engine, but for emergency use, that feature can be cut out with the "Automatic Boost Cutout." Yes, I said "pounds per square inch," and not "inches of mercury" so common in the USA. The manifold pressure gauge is marked so that "0" is ambient pressure. At less than ambient pressure, "Boost" is negative (about -6 at idle), and at higher power settings, "Boost" is positive. There are 2.036 inches of mercury to each 1 pound per square inch, so a takeoff "Boost" of 12.0 is equivalent to 54.35 inches of manifold pressure at sea level (12 x 2.036 + 29.92). All this is of little consequence, because we just use the appropriate "Boost" numbers.

[Mudge]

OK, there's a MP/Boost gauge on the panel. Why? If I'm at idle or full throttle or cruise, why do I care what the MP is. Does it give me an indication of how well/poorly the engine is performing?
Sorry to prolong this as I'm sure it's getting boring to some, but I like to know how/why things work.

Mudge the bulldog

[retroaviation]

Okay, so now it's ME that's showing my ignorance here, but........

If the boost gauge shows "0" at ambient pressure, how does it show a difference (with the engine not running) at different altitudes. For example, if it shows "0" at sea level, will it show "-2" or so, when here at Midland, which is just shy of 3,000 ft. above sea level? Our manifold pressure gauges only show roughly 27" (give or take, depending on barometric conditions) here.

Am I making any sense at all? Heck, now I've gone and confused myself.

Gary

[RyanShort1]

The rule of thumb is that you lose 1" of mercury per 1,000' of altitude, so if sea level pressure is 29.92. and you're close to 3,000' above sea level, 26.9 to 27.0 would be approximately accurate.
It's mainly due to the outside air pressure. When the engine's not running, it's going to show approximately the same air pressure as the surrounding air.

Ryan

Last edited by RyanShort1 on Fri Nov 14, 2008 2:26 pm, edited 1 time in total.

[51fixer]

OK, there's a MP/Boost gauge on the panel. Why? If I'm at idle or full throttle or cruise, why do I care what the MP is. Does it give me an indication of how well/poorly the engine is performing?
Sorry to prolong this as I'm sure it's getting boring to some, but I like to know how/why things work.

Mudge the bulldog

In warbirds and other aircraft with controllable propellers the power settings you run the engine at depends on setting a rpm with the prop and a power setting with the throttle. Say full power in a P-51 is 60" MP and 3000 RPM. Everything forward. Cruise might be 24" and 2450 RPM. That is why the gauge is important.
Rich