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Manifold Pressure


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I'm a mechanic by trade, but this always gets me confused:

 

What exactly IS manifold pressure?

 

This is where I run into trouble. I know that between the carb and the intake valves there is no pressure unless a supercharger or turbocharger is pushing in the air. Since these Lycomings have no super/turbo charger, where does this pressure come from? The only thing between the carb and intake is a vacuum.

 

Am I missing something here?

 

Your answer may may cause an epiphony.(sp?)

 

Later

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Witch, Witch, Witch.

 

Manifold pressure is the measure of pressure between the intake and the cylinders, the pressure will always be lower than atmopheric in a naturally aspirated machine it is the pressure in the manifold relivate to the outside pressure.

 

Turbo/super chargers force air into the manifold thus causing a pressure higher than the outside pressure resulting in greater hosrepower output.

 

 

clear as mud?

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The pressure inside the engine comes from the outside air, barometric pressure. In a naturally aspirated engine, the manifold pressure will never be higher than the ambient air pressure. With the engine shut down the manifold pressure gauge is measuring the outside air pressure.

 

When the engine is running, the suction from the cylinders decreases the air pressure inside the intake tubes, because the throttle butterfly restricts the amount of air that is allowed into the engine. As you open and close the throttle the size of the restriction changes, allowing more or less air into the engine. If more air is allowed in the manifold pressure will be higher, if less air is allowed in then the manifold pressure will be lower. There is never a vacuum in the intake tube.

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Witch, Witch, Witch.

 

Manifold pressure is the measure of pressure between the intake and the cylinders, the pressure will always be lower than atmopheric in a naturally aspirated machine it is the pressure in the manifold relivate to the outside pressure.

 

clear as mud?

Nope.

 

OK, so if the outside pressure is about 30" Hg, and the manifold pressure is say 20" Hg

 

Aw crap, I get it now. So when the throttle is closed, the manifold pressure will be low because of the partial vacuum and when the throttle is open, the manifold pressure will be higher because of lack of partial vacuum.

 

I guess I kept thinking suction pressure for air conditioners; the pressure of the refrigerant before it gets to the compressor, or low side pressure. Maybe that makes sense.

 

Out of curiosity, what happens to the engine if you go above the 25" Hg in the Robbie? I've heard that it burns the valves but I honestly don't know. Most of the engines I worked on were diesel and turbines. We only had one type of equipment that had a Continental 6-banger. I hate replacing exhaust shrouds.

 

Later

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Nope.

 

OK, so if the outside pressure is about 30" Hg, and the manifold pressure is say 20" Hg

 

Aw crap, I get it now. So when the throttle is closed, the manifold pressure will be low because of the partial vacuum and when the throttle is open, the manifold pressure will be higher because of lack of partial vacuum.

 

ya almost got it right, it's just the opposite of what you posted, a closed throttle creates a slower volume of air movement, and higher when open thus reducing the internal pressure.

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less vacuum = higher manifold pressure? A manifold pressure of 25" is still a vacuum compared to 30" outside. 25" is max power wide open in the right conditions even on the r22. Less power = more vacuum. Has anyone owned an old vehicle with cacuum advance wipers? They are top notch when coasting down a hill but acellerate or climb a hill and you may need to help them with a coat hanget-hand out the window style return. The more the throttle closed the less manifold "pressure". I now wonder if the the sensor is on the inside of the butterfly to analyze this better. Does anyone know the answer to this on the robbie 22 or the lycoming 360? I would think it must be on the atmospheric side of the butterfly.

 

I would think manifold pressure can be measured in a positive value that is less than what we think of as zero but we are actually under pressure ourselves. While 15" of manifold pressure is way less than we live in it is still not a true vacuum. Truly it is pressure. Remember that what we think of as zero is actuallr 30" of pressure.

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Manifold pressure is measured in the intake manifold, after the throttle. Assuming the motor is at a set rpm, the intake vavles will stay open the same amount of time on each cycle. If the pressure in the manifold is low, only a small amount of air/fuel will get into the combustion chamber and will produce a small amount of power. If the manifold pressure is increased, more air/fuel will be forced into the chamber thus creating more power.

 

The throttle restricts the amount of air/fuel entering the intake manifold. Since the motor is pulling out more air than the throttle is allowing in, it creates a vacuum (vacuum meaning anything under atmospheric pressure). The more the throttle is opened, the more air/fuel is allowed to enter resulting in an increase in manifold pressure. If the engine is running, the manifold pressure will always be in a vacuum (under 29.92 inHg).

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OK, let’s clarify

 

What you are seeing on an MP gauge (or a barometer), is a mechanical representation of the basis of “inches of mercury”. Take a 36” tube, fill it with mercury, invert it so the open end is in a pool of mercury (which is open to air pressure). The mercury in the tube will fall to a specific level which represents how hard the outside air is pushing on the pool of mercury, and hence how hard that pool is pushing the mercury up the tube. So when we talk about 28” Hg, we really are talking about inches of mercury. The higher the air pressure, the higher the mercury will rise in the tube, and vise-versa. In a barometer or MP gauge, we replace the three-foot tube of liquid metal with a calibrated bronze bellows connected to gears and a needle.

 

Now onto engines. This topic has been done to death on this forum already, so I’ll keep it simple. Just like a vacuum cleaner, if the engine is not running, the pressure will be equal inside and outside the intake manifold, and the MP gauge will give the same reading as a barometer sitting in the cockpit (not the altimeter setting). Just like a vacuum cleaner, when you start the engine, it starts pumping air. The faster the engine goes, the more air it pumps. At 2600 RPM, an O-360 is trying to pump 3250 CFM – at idle, about 1370 CFM.

 

In a normally aspirated engine, the major impediment between the air pump and the outside air is the throttle butterfly, so if the butterfly is closed, it restricts the outside air pressure from being able to equalize the pressure in the manifold, so the pressure in the manifold drops. The MP gauge reads this pressure (say 14”), but it is still pressure – 14” of mercury will still be supported by this air inside the manifold.

 

Open the butterfly, and more outside air is allowed into the manifold – the pressure rises, being allowed to equalize by removing the impediment. Of course there are other things in the way, the air filter, the corrugations of the intake hoses, the carb venturi or throttle body, so even at WOT the pressure cannot fully equalize, and will usually max out about 1” – 2” below the ambient pressure.

 

Looking at the amount of air an engine moves, you can see that RPM has a noticeable effect on MP, as it is harder to move 3250 cubic feet/minute of air through a 1” opening than it is to move 1370 CFM. Put a MP gauge on that and for the same throttle opening, MP will rise as RPM falls (this is why MP rises during mag checks).

 

If you add a supercharger to the mix, the theory stays the same, but now the supercharger compressor can move enough air to increase the pressure in the intake manifold to above ambient.

 

Wow, that didn’t take long!

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I now wonder if the the sensor is on the inside of the butterfly to analyze this better. Does anyone know the answer to this on the robbie 22 or the lycoming 360? I would think it must be on the atmospheric side of the butterfly.

 

 

The pressure source for the manifold pressure gauge comes from that small steel tube that is attached to the #3 cylinder (left side of the helicopter, front cylinder) on the intake valve side.

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Ok, first......"pressure" does not mean anything higher than ambient. Just because you have "Manifold Pressure" doesn't mean that that pressure is "high".

 

Get away from using words like suction and vacuum, both are incorrect for our use. A vacuum is a totally evacuated area and the term "suction" just isn't right. Pressure moves matter from high to low--simple as that. If the engine creates a low pressure on the cylinder's downstroke, the differential with the [higher] ambient pressure will cause an air movement.

 

The manifold pressure gauge reads that pressure from the #3 cylinder intake stack. The more the throttle is opened, the higher the pressure into the manifolds (still below ambient).

 

So if ambient (NOT barometric) is around 30", the max manifold pressure you can probably obtain is around 28". If you're up in the mountains or Denver, the ambient is going to be around 25" or less--so the max manifold presuure you'll obtain is about 23" (well below the red line on the MAP gauge.) This is the whole purpose of the limit manifold pressure chart.

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Out of curiosity, what happens to the engine if you go above the 25" Hg in the Robbie? I've heard that it burns the valves but I honestly don't know. Most of the engines I worked on were diesel and turbines. We only had one type of equipment that had a Continental 6-banger. I hate replacing exhaust shrouds.

 

Later

 

I've never heard that, and I doubt it because both the R22 and R44 engines are very significantly derated.

 

BUT. I would worry about the main rotor system. The engine can make the additional power, the rotor system may not be able to handle it. Robinson had a number of crashes caused by rotor system failures in Australia which they attributed to salt air corrosion and high MPs.

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Sehr interessant.

 

OK, so I'll try to not have the MP so high next time. My IP was pointing at the MP guage and it was above 25. I guess I was climbing too fast and noticed that the altimeter was at 900 feet. Oops, too high.

 

Later.

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To answer more directly:

What exactly IS manifold pressure?
It is an indication of the power output of the engine. Robinsons have MP limits because the engines are derated - this is to extend engine life and to reduce harmful stress on the rotor system at cruise speed (B206s have Tq limits for the same reason)
I know that between the carb and the intake valves there is no pressure unless a supercharger or turbocharger is pushing in the air.
As explained earlier, there is indeed pressure. Remember the MP gauge reads absolute, not relative pressure.
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In a normally aspirated engine, the major impediment between the air pump and the outside air is the throttle butterfly, so if the butterfly is closed, it restricts the outside air pressure from being able to equalize the pressure in the manifold, so the pressure in the manifold drops. The MP gauge reads this pressure (say 14”), but it is still pressure – 14” of mercury will still be supported by this air inside the manifold.

 

Open the butterfly, and more outside air is allowed into the manifold – the pressure rises, being allowed to equalize by removing the impediment. Of course there are other things in the way, the air filter, the corrugations of the intake hoses, the carb venturi or throttle body, so even at WOT the pressure cannot fully equalize, and will usually max out about 1” – 2” below the ambient pressure.

 

I stand corrected, damn I hate when I get things like that bassackwards :wacko:

 

that's Fling ;)

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That's ok, I'm still thinking suction pressure.

 

BTW, I noticed that neither bird has an ELT. Are Robbies supposed to have ELT's?

 

Later.

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Guest pokey
At 2600 RPM, an O-360 is trying to pump 3250 CFM – at idle, about 1370 CFM.

 

Fling? after glancing over that article i posted the link to, i noticed thay said a 550 will pump around 400CFM, & a 360 is smaller, so i decided to get out my kal-ka-later & see what happens:

 

360 cubic inches x 2600 RPM = 936,000 cubic inches per minute (divided by) 1728 cubic inches/cubic foot = 541.6 CFM

 

but remember this is a 4-stroke engine, so divide that by 2 = 270.8 CFM.

 

This of korse ! assumes 100% volumetric efficiency.

 

 

BTW? you guys overhaul any landing gear dampers w/ those tools i sent ya? (aint that "pressure balanced" charging tool the greatest?)

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That's ok, I'm still thinking suction pressure.

 

BTW, I noticed that neither bird has an ELT. Are Robbies supposed to have ELT's?

 

Later.

 

 

Nope, ELTs are not required in helicopters. They are not excluded from the ELT requirement, they're just not included. It reads something like "Each airplane must have an ELT....blah, blah, blah." It says "airplane" and NOT "aircraft" like in other regs.

 

But, if you have an ELT installed in your a/c it MUST be working and have a get new battery every 24 calander months regardless if it is required or not. However, it can be removed for up to 30 days(?) to send it in for repair.

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...It reads something like "Each airplane must have an ELT....blah, blah, blah." It says "airplane" and NOT "aircraft" like in other regs....

91.207 Emergency locator transmitters.

 

(a) Except as provided in paragraphs (e) and (f) of this section, no person may operate a U.S.-registered civil airplane unless--

(1) There is attached to the airplane an approved automatic type emergency locator transmitter that is in operable condition for the following operations, except that after June 21, 1995, an emergency locator transmitter that meets the requirements of TSO-C91 may not be used for new installations:

(i) Those operations governed by the supplemental air carrier and commercial operator rules of parts 121 and 125;

(ii) Charter flights governed by the domestic and flag air carrier rules of part 121 of this chapter; and

(iii) Operations governed by part 135 of this chapter; or

(2) For operations other than those specified in paragraph (a)(1) of this section, there must be attached to the airplane an approved personal type or an approved automatic type emergency locator transmitter that is in operable condition, except that after June 21, 1995, an emergency locator transmitter that meets the requirements of TSO-C91 may not be used for new installations.

 

(b ) Each emergency locator transmitter required by paragraph (a) of this section must be attached to the airplane in such a manner that the probability of damage to the transmitter in the event of crash impact is minimized. Fixed and deployable automatic type transmitters must be attached to the airplane as far aft as practicable.

 

(c ) Batteries used in the emergency locator transmitters required by paragraphs (a) and (b ) of this section must be replaced (or recharged, if the batteries are rechargeable)--

(1) When the transmitter has been in use for more than 1 cumulative hour; or

(2) When 50 percent of their useful life (or, for rechargeable batteries, 50 percent of their useful life of charge) has expired, as established by the transmitter manufacturer under its approval.

The new expiration date for replacing (or recharging) the battery must be legibly marked on the outside of the transmitter and entered in the aircraft maintenance record. Paragraph (c )(2) of this section does not apply to batteries (such as water-activated batteries) that are essentially unaffected during probable storage intervals.

 

(d) Each emergency locator transmitter required by paragraph (a) of this section must be inspected within 12 calendar months after the last inspection for--

(1) Proper installation;

(2) Battery corrosion;

(3) Operation of the controls and crash sensor; and

(4) The presence of a sufficient signal radiated from its antenna.

 

(e) Notwithstanding paragraph (a) of this section, a person may--

(1) Ferry a newly acquired airplane from the place where possession of it was taken to a place where the emergency locator transmitter is to be installed; and

(2) Ferry an airplane with an inoperative emergency locator transmitter from a place where repairs or replacements cannot be made to a place where they can be made.

No person other than required crewmembers may be carried aboard an airplane being ferried under paragraph (e) of this section.

 

(f) Paragraph (a) of this section does not apply to--

(1) Before January 1, 2004, turbojet-powered aircraft;

(2) Aircraft while engaged in scheduled flights by scheduled air carriers;

(3) Aircraft while engaged in training operations conducted entirely within a 50-nautical mile radius of the airport from which such local flight operations began;

(4) Aircraft while engaged in flight operations incident to design and testing;

(5) New aircraft while engaged in flight operations incident to their manufacture, preparation, and delivery;

(6) Aircraft while engaged in flight operations incident to the aerial application of chemicals and other substances for agricultural purposes;

(7) Aircraft certificated by the Administrator for research and development purposes;

(8) Aircraft while used for showing compliance with regulations, crew training, exhibition, air racing, or market surveys;

(9) Aircraft equipped to carry not more than one person; and

(10) An aircraft during any period for which the transmitter has been temporarily removed for inspection, repair, modification, or replacement, subject to the following:

(i) No person may operate the aircraft unless the aircraft records contain an entry which includes the date of initial removal, the make, model, serial number, and reason for removing the transmitter, and a placard located in view of the pilot to show "ELT not installed."

(ii) No person may operate the aircraft more than 90 days after the ELT is initially removed from the aircraft; and

(11) On and after January 1, 2004, aircraft with a maximum payload capacity of more than 18,000 pounds when used in air transportation.

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Fling? after glancing over that article i posted the link to, i noticed thay said a 550 will pump around 400CFM, & a 360 is smaller, so i decided to get out my kal-ka-later & see what happens:

 

360 cubic inches x 2600 RPM = 936,000 cubic inches per minute (divided by) 1728 cubic inches/cubic foot = 541.6 CFM

 

but remember this is a 4-stroke engine, so divide that by 2 = 270.8 CFM.

 

This of korse ! assumes 100% volumetric efficiency.

BTW? you guys overhaul any landing gear dampers w/ those tools i sent ya? (aint that "pressure balanced" charging tool the greatest?)

Whups - I was doing the math in my head and used the 144 square inches per square foot conversion.

 

AFIAK, that toolset is as yet unused.

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  • 15 years later...
On 7/8/2006 at 9:54 PM, Witch said:

I'm a mechanic by trade, but this always gets me confused:

 

What exactly IS manifold pressure?

 

This is where I run into trouble. I know that between the carb and the intake valves there is no pressure unless a supercharger or turbocharger is pushing in the air. Since these Lycomings have no super/turbo charger, where does this pressure come from? The only thing between the carb and intake is a vacuum.

 

Am I missing something here?

 

Your answer may may cause an epiphony.(sp?)

 

Later

We live in average pressure of 14.7 psi of atmosphere (measured at sea level).    Another way to measure that same pressure is with a barometer using the mercury scale. 14.7 psi and 29.92 inches of mercury is the same pressure. At any rate, that's what we're breathing.  Turbo charged piston engines create additional pressure artificially with a blower or turbocharger or whatever we want to call it and force it into the intake manifold to increase engine performance. But with non turbo engines, the engine can only suck in air as there is pressure in the atmosphere on the other side of the butterfly valve. I also struggled greatly with this really simple concept during my helicopter ground school. It may have been the language barrier between me and my instructor. I just wish I hadn't wasted so much time trying to wrap my head around it. It's really simple, if properly explained.

As for "derating", that was a rough one too. My longstanding question was always "why are we derating this engine". "Let the thing make the power it's capable of."   But when we get into the charts and see how engine breathing is adversely affected by less atmospheric pressure at higher altitudes, then it makes more sense. Apparently I was thinking in 2 dimensions.

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