Manifold Pressure

in this video we're going to be talking about the manifold pressure gauge not all airplanes have it but a lot of them do especially those with constant speed propellers well what is it how does it work what does it mean to us let's understand the physics of it okay well let's start with something it says on this gauge we know this is measuring manifold pressure whatever that is and it says back here inches of mercury absolute well inches of mercury is something we need to understand and absolute is something else we need to understand all right let's start with the easy one inches of mercury well you may not understand what that is but you probably are familiar with it as a pilot because your altimeter we've learned we've learned to deal with something um the Coleman window the pressure setting on the altimeter the Coleman window shows you these numbers and they're oddly enough in the same range as this one's showing so this one's showing about 29.91 what inches of mercury okay so what does that mean well let's look at the physics of what that means all right let's we're going to do an experiment we're going to be at sea level we're going to have a dish full of Mercury okay this is full of mercury which is a liquid at room temperature a dish full of Mercury we're going to take a long glass tube and just stick in there okay long glass tube stick it in that dish or it can Where the mercury can flow into it all right it'll look just like that now what we're going to do is we're gonna cap the end of it off and it'll still look like that but then we're going to pull a vacuum on it we're gonna we're gonna we're gonna hook up a vacuum pump and suck all the air out of out of this tube every bit of it until there's no air at all in this tube and what's going to happen the Mercury is going to rise it's gonna rise up to a specific level okay doesn't matter about the vacuum pump it's going to rise up to a specific level now as Pilots we walk over to our airplane because it's always going to be nearby and we go to our altimeter We call we get on the radio and call the local awos and we get an altimeter setting of 29 or 91 just like this 29.91 okay so awos gave us a 29.9 or one inches of mercury inches HG okay 29.91 we're at sea level when we do this experiment so we go to our altimeter Road set the colesman window to 29.91 and we look over at our at our little gauge our little manometer here the vacuum pump has pulled all the air out of it it stopped we closed a little valve here and we take a tape measure and we measure this we measure the height between this fluid at the base and this fluid at the top and what we're seeing is 29.981 inches isn't that something so what did we just do what did we just measure well what we measured is what we measured is this this is a way to measure atmospheric pressure so atmospheric pressure exists everywhere it exists in the room when I'm making this video it exists around my hands right now it exists inside and outside this pin but we don't even think about it we just think there's no pressure we say you know the room is not pressurized we walk outside that's certainly not pressurized but of course it is we just don't notice it it's it's our it's the ambient air it's the atmospheric pressure we can't sense it we can only sense big changes in it as people well this is a device that shows us how much pressure exists where we at we're at so this experiment's done at sea level what we're seeing is the pressure in the atmosphere which is pushing in all directions remember all directions pressure goes everywhere these little arrows are indicating the direction of force from this pressure well this all can't all these arrows they all cancel each other out but whenever you have pressure against this fluid well this fluid has some place to go that doesn't just push out of the out of the the dish here because it's all even but it's not even when we pull a vacuum on this when we pull a vacuum on it we've pulled all the air out that air remember was pressurized too so that air the reason it wouldn't go up the two before is because there was pressurized air in it pressurized atmospheric pressure but when we pull the vacuum that air is no longer there there's nothing there's nothing pressurized anymore and so it pushes right out the tube and it goes to 29.91 inches now you might think well that just that just shows you how much vacuum you pulled on it but actually that's not right uh it doesn't matter how much it well it matters that you pull all the air out and this will read exactly what atmospheric pressure is if the pressure changes a few hours later if it goes up and you call the awos and it goes up to 30.03 you'll see this change also this will rise to 30.03 because the atmospheric pressure increased pushed harder on this fluid and pushed the Mercury up that far that's what's going on that's what that number means okay so we're used to that in an altimeter now if we go back to our airplane now that we understand that you may notice that the pressure that the manifold pressure your engine is off manifold pressure also reads the same thing well this is not quite right this is a little low but you go to your manifold pressure gauge and you notice that it reads about 29.91 as well isn't that something well of course it does because it's reading manifold pressure and manifold the manifold the intake manifold of the engine is connected it's it's the same pressure as the atmospheric pressure okay so let's talk about what an intake manifold is a lot of people probably know this if you're a pilot you probably already have a fair understanding of this but let's go into the details all right in an engine we're going to just draw one cylinder we have a cylinder with a piston that runs inside of it okay this piston has multiple Rings it has a wrist pin we have a connecting rod it comes down here to the crankshaft crankshaft is here and and then we have a crank pin that rotates around the crankshaft Center like that basically like that as this rotates around drives the Piston up and down right on top of the cylinder has a cylinder head it'll have an exhaust valve an intake valve draw an intake valve that's open and an exhaust valve that's closed okay so exhaust would go this way to the exhaust pipes intake goes this way all right so this intake valve is open that's where that's where whenever the Piston pulls down it pulls air into the cylinder The Strokes of the engine are intake piston goes down compression stroke it com the intake valve closes and seals it off and it compresses all that air well it's got fuel in it right compresses air and fuel the spark plug fires ignites the air and fuel explodes push the piston piston down and the power stroke and then it rolls right back around exhaust valve opens and it blows the exhaust out on the exhaust stroke and it comes back down to do the intake again that's the that's on a four-stroke engine okay we're only going to be looking at the intake strip that's what we're interested in on the intake stroke you have an open intake valve air and fuel are going to flow into it all right it's going to flow around the valve that's open into the cylinder because the Piston pulled down sucking air in now where'd that air come from well it came from a pipe connected to the intake Port this pipe would be called the intake manifold at the end of that somewhere is another device I'm just going to draw this like a little pipe running through it the carburetor okay and in front of the carburetor somewhere is a filter an air filter all right so air air flows through the filter through whatever piping connects to the carburetor through the carburetor leaves the carburetor with fuel in it let's draw some fuel going in here so fuel gets in here gets gets mixed with the air and then goes through the intake manifold to the head through the intake valve and into the cylinder okay this is happening really fast okay one big thing that we need to need to recognize in the carburetor is there is a valve a butterfly valve called the throttle valve okay that throttle valve here and it and it swings this way like this okay if you were to if you were to look straight up at it if you were to look look down the pipe at it it would do this it would it would close open so if it was open we would draw it in dashed lines like this that rotates this direction okay just like that okay so at idle when you go to start your engine remember we have an airplane sitting there at sea level we just did this experiment we noticed that our altimeter we notice that this red 29.91 inches we notice our altimeter is at 29.91 and we notice that our manifold pressure is also at 29.91 so that manifold pressure is being read from right here somewhere about right here it's in the intake manifold that's probably coming from the carburetor but we got a gauge here manifold pressure okay so it's reading like that pretty high you go to start your engine where's that throttle valve it's closed it's got a little hole in it it's either cracked open slightly and it won't close all the way or it's got a hole or a couple of holes drilled into it and what that does is it allows only a tiny amount of air just enough air to mix with just enough fuel to end to run the engine at idle and no more and only idle at a certain RPM so it's metered so that when that thing is closed say let's say your engine idles at 700 RPM the reason it doesn't idle at 1200 or 1500 or 200 is because there's just enough air going through here and mixed with just enough fuel to have just enough combustion in the cylinders to run it at that speed that valve right there is what regulates the speed of your engine literally the power of your engine which also regulates the speed at least at least a basic engine just idling all right so when you get in your airplane and push the throttle forward or you push a lever or you push the the cable when you throttle it up and the engine goes well what you're doing you're just pushing a cable that's connected to an arm that opens that valve it's closed and it opens it and oh by opening it all you're doing is you're allowing more air to flow through here the carburetor mixes more fuel with that air and you get more power in the cylinders how's this happening well the air doesn't just flow through it gets pulled through by the Piston so when the Piston comes down on the intake stroke you have an open intake valve and the Piston comes down really fast and it sucks pulls it sucks air in it actually pulls pretty hard in idle see we have a closed valve here with tiny holes we're talking real small like the small that this pin would this pin head is probably bigger than the holes very very small so a very small amount of air is getting through here so that piston as you can imagine as it pulls is pulling pretty hard it's sucking really hard and you can hear it if you're around this valve you could open the ports up and listen to this valve it's sucking really hard on it if you could use your hand instead of the valve it would hurt it would pull your hand really hard so you can imagine when we started this before we started this engine we noticed the manifold pressure gauge here open atmosphere through a little tiny hole but still open it was reading the same pressure as atmospheric pressure but when we started it now the Piston is pulling down and sucking it's pulling the pressure of the atmosphere down uh down because of the suction and so the manifold pressure gauge decreases so this is what it looks like maybe when it's running or or more likely when it's sitting and when you start it it pulls it way down here below 10 even pulls it way down because it's got a high vacuum on it and it can't it's pulled the pressure that the atmosphere exerts on this whole system way down okay but then you go to then you go to to run the engine up or you go to take off and you push the throttle in and then the engine revs up and starts making all kinds of Power well what you've done is you've opened the throttle up and now when that piston comes down for a breath instead of sucking real hard it's got an open it's got an open valve and a pretty free air filter so it just it just breathes it just the air instead of just instead of suction and a little bit of air flowing the air just breathes in real fast and it doesn't develop a lot of vacuum therefore the manifold pressure does not suck down instead when you open the throttle up the manifold pressure goes up so your gauge here your gauge here as you as you increase your throttle the gauge will open up it will go up and so you're so now you're taking off and we're making a lot of power and we've got real high manifold pressure and you start climbing you're climbing a thousand feet a minute and let's assume this is a fixed pitch propeller you're climbing a thousand feet a minute and and uh and you start noticing every thousand feet you look at your manifold pressure gauge and something's happening about every thousand feet you the needle goes down and an inch and one inch of Mercury and another thousand feet and it goes down another thousand it goes down well that's because that's because approximately you lose uh you might remember this from the colesman window for every one inch of mercury of pressure change is about a thousand feet so as you climb a thousand feet your manifold pressure goes down and that's if you keep it at wide open throttle that's if you keep the throttle shoved all the way forward and you're making full power so so that means that there's less pressure in the atmosphere to go into your engine less pressure means there's less there's actually less air that can flow through your carburetor which will pick up less fuel with at least when it's leaned correctly and that means there will be less power so the manifold pressure gauge is really an indication of how much power your engine is making it's a direct indication of how much pressure is in the intake manifold but what you're what this is telling you is what kind of power the engine is producing so when you climb up ten thousand feet you're going to be at wide open throttle you're going to be right here something like that and that tells you that's not you're not making very much power because there's not much power to be had there's not enough air up there to mix with the fuel that you have to make pressure to make a a lot of power so so that's really what it's all about so when you're in a constant speed now in a fixed pitch propeller you're used to just running based on RPMs and if you have so you just might you just use the throttle to control engine speed but in a constant speed prop you use the propeller lever to set engine speed it sets the speed of the propeller which drives the speed of the engine and then you set your but you set your power your engine power with manifold pressure and that's what and that's how how that works so when you set let's say uh you know somebody might say well I was how fast were you flying what power setting and they say 25 squared 25 squared or anyway 25 squared what does that mean s that means I set my manifold pressure at 25 inches of mercury and I set my engine speed at 2500 rpm so 25 inches of Mercury 25 00 RPM so this is with the prop you set this with a prop lever and you set this with a throttle now in my example of climbing up real high you climb for 10 000 feet you can't and most engines uh well you wouldn't be able to at 10 000 feet you wouldn't be able to run at 25 squared because you don't have that much pressure in the atmosphere so you're going to be at a lot lower power setting and uh and what that squared what that means when people say that is if they do 23 squared see it's 25 23 inches of mercury on the manifold pressure adjusted by the throttle and 2300 RPM and there's other reasons for that but uh but that's what that stuff means okay manifold pressure inches of mercury inches of mercury or is telling you the pressure If you measured with a Mercury device like this the pressure that that would exert a specific rise in inches of mercury it's coming from your Mana intake manifold it's being driven by pulling the pressure that's available in the atmosphere which is an absolute pressure not a gauge pressure so when we talk about when we talk about pressure we see a pressure gauge like this this is we're only if we say this is 20 something's 20 psi or a zero like this gauge reads zero that doesn't mean there's no pressure in the atmosphere where that gauge is that just means there's no pressure at this port relative to the air around it okay but in the air around it we already know there's pressure so we don't deal with that that's that's called gauge pressure now this is also a gauge but uh it's called gauge pressure so this is called we measure this in absolute pressure that's why it even says on your gauge absolute letting you know that this is not just talking about some relative pressure but absolute pressure so the throttle on the engine the throttle lever controls the throttle valve which determines how much resistance is in the intake system the more resistance here the more suction from the Piston occurs and the more it pulls your manifold pressure down it actually pulls the available the available air pressure from the atmosphere down giving you lower power and it it when you're idling it's really low uh when you're running the engine really fast and you have the throttle back like when you're descending at a high engine speed you get the lowest amount of amount of manifold pressure because it's a really high vacuum that means it's pulling it's it's pulling really hard but the throttle plate is cutting it off so when you push the throttle forward open it up you're getting more of the atmospheric pressure in there to push the air into the into the combustion chamber and produce power that's what it means when we look at a manifold pressure gauge so again real quick on a constant speed propeller you set your you set the speed of the engine with uh with a prop lever that's your speed you set the power and you measure power by manifold pressure and you do that with a throttle that's all there is to it

Channel: Layton Soaring

Views: 3,520

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Id: 9_rwwfnzZB0

Length: 23min 13sec (1393 seconds)

Published: Wed Aug 23 2023