Sorry for noobs question, but what is manifold and what is manifold's pressure?

I drive a car with automatic gearbox, so I haven't a clue ;-)

A Manifold is an assembly of ducts which are directed to the intake or from the exhaust ports of the cylinders on an internal combustion engine.
When one refers to manifold pressure, this is usually the pressure which is measured in the inlet manifold assembly; exhaust manifold pressure is rarely measured in common engine applications.
It is irrelevant wether an engine is fitted to a manual or a automatic gearbox.

Manifold pressure has a close relationship to engine power output.

Depending on how open or closed the throttle is, it will restrict air flow into the engine, such that the pressure after it (ie, in the inlet manifold) is reduced. Fully open, the throttle allows maximum flow, maximum manifold pressure, and the most power.

It also read as a unit of measure, such as inches, on some gauges in the airplane to let the pilot know how much pressure or force the engine is having to work against. This pressure fluctuates based on throttle/mixture settings and the pitch of the prop.

The more throttle, the bigger the combustion, and therefore more pressure. The air works as a resistance as the propeller spins through it.
This is not modeled to the full extent in the game, for example using a lower prop setting (steeper pitch causes more resistance) with a high throttle position would result in very high manifold readings that would be dangerous for the engine.

I guess for a car it would be like trying to go uphill in fourth gear, and slamming your foot on the gas. It puts more load on the engine, though i'm not sure manifold pressure is measured the same as it would be in an aircraft engine. Putting the car in first gear, would be like raising the prop pitch setting, so there is less pressure exerted on the engine parts.

Its also possible to see fluctuations in manefold pressure as the speed of the plane changes, again not modeled. Its really something to keep an eye on when flying, but not quite so in this sim.

Every time the piston drops into the “intake” stroke it is literally pulling or sucking air into the cylinder. Your manifold pressure is actually a reading suction not ram air pressure.

The more air and fuel we can pump or pull into the cylinders, the more power the engine can develop (which makes us fly faster). The manifold pressure gauge measures how much air pressure is in the induction system, just before the air / fuel mixture enters into the cylinders and this gives you good idea of how much power you are developing.

So to simplify, because one can not hold all those details in a head while dog fighting, manifold pressure is simply the power transferred from the engine to the prop shaft.

Is this simplification right?

WITHOUT a supercharger.

WITH a super or turbo charger.

These articles are for modern pilots and written by a long-time highly experienced (to say the least) pilot.

quote:

We have learned that manifold "pressure" in a normally aspirated engine with a wide open throttle is nothing more than ambient pressure, reduced slightly by the drag of a filter, and by additional drag from a pipe with bends in it. By closing the throttle, we constrict (or "throttle") that airflow even further, creating suction (lower pressure for the purists) in the pipes downstream of the throttle plate. This suction is created by the action of the pistons literally pumping air, sucking it into the engine.

With a supercharger the faster the engine runs the more pressure gets fed into the manifold so throttle open or
closed does work to regulate the boost. Some planes did have Automatic Boost Control not mentioned below but it
was there -on some, probably many planes- and did keep the engine from being over-boosted.

quote:

In general, these gear-driven supercharger systems will produce much higher manifold pressures (and greater power) than the equivalent normally-aspirated engine. Some will produce up to 60 inches instead of the usual 29 inches at sea level in a normally-aspirated engine, and some of the Reno racers jack the MP up to as much as 120 inches! Most of these installations will produce far more than maximum power at full throttle, so it is usually necessary for the pilot to stop the throttle at some point well before reaching the mechanical stop, to avoid over-boosting the engine.

The manifold pressure shown on aircraft gauges is that of the engine's intake manifold, which is the passageway that carries the engine's fuel from it's carburetor or fuel injector to the combustion chamber to be burned.

The gauge shows if it's pressure is above or below atmospheric pressure.

In IL2 you may need to watch this on aircraft where you have to manually shift the speed of the supercharger as it will let you know when the engine needs it shifted.

On these aircraft, note the manifold pressure at low alt on supercharger low gear. This is the pressure you want to try to stay close to as you gain altitude.

As you do gain altitude and there is less air for the engine to use, the manifold pressure will drop. If you can shift the supercharger up to a higher speed so the manifold pressure increases closer to but not above what it was at low altitude, then you have done it right.

If you shift the supercharger too early, manifold pressure may go above what it was at low altitude and low supercharger speed, and also above what the octane rating of your fuel and engine design can use, and this can damage the engine.

So too low a manifold pressure will simply cause you to lose power, but too high can blow the engine.

TaDa!

quote:

... but too high can blow the engine.

In real life, but not in-game.

quote:

Originally posted by WholeHawg:
Every time the piston drops into the “intake” stroke it is literally pulling or sucking air into the cylinder. Your manifold pressure is actually a reading suction not ram air pressure

Not in supercharged engines. A supercharger delivers pressurized air to the cylinders.

quote:

Originally posted by DROBNJAK1960:
So to simplify, because one can not hold all those details in a head while dog fighting, manifold pressure is simply the power transferred from the engine to the prop shaft.

Is this simplification right?

Its not really the power transferred to the prop, but more of a way for pilots gauge the engine load. The measurement read on the gauge is another way to describe output, but its only part of the picture when concerning power output. Your RPM setting has a lot to do with the output of the engine as well.

In game its not that important, but in real life, for planes with sensitive boost systems and superchargers its a very important gauge.

quote:

quote:

Originally posted by na85:
Every time the piston drops into the “intake” stroke it is literally pulling or sucking air into the cylinder. Your manifold pressure is actually a reading suction not ram air pressure

Not in supercharged engines. A supercharger delivers pressurized air to the cylinders.

This is true but the measurement is still the difference in pressure achieved from the rotation of the engine and is a separate measurement from vaccum created by the air that the intake is actually mixing a managing prior to combustion.

I take the reading to mean engine load as well as power output.


Low RPMs + high throttle = what HG reading? ...higher or lower than where it should be.

quote:

Originally posted by BillSwagger:

This is true but the measurement is still the difference in pressure achieved from the rotation of the engine and is a separate measurement from vaccum created by the air that the intake is actually mixing a managing prior to combustion.

uhhh... that was a confusing post. In supercharged engines, there is no suction when the cylinder is on the intake stroke. There is no condition on that statement, it's a fact. If you agree then we're good.

quote:

I take the reading to mean engine load as well as power output.

I agree. I mostly see it as engine load, rather than power output; power = torque x RPM so looking at manifold pressure alone doesn't give a good indication of horsepower.

quote:

Low RPMs + high throttle = what HG reading? ...higher or lower than where it should be.

Pretty sure that for a given throttle setting, if you decrease RPM the manifold pressure goes up. This is why in a real aircraft you can't run, say, 20% pitch at full throttle.

While that *may* be true on a turbo charged engine, it is certainly *not* true on a supercharged engine. Gear or belt driven supercharges are designed to give a specific pressure ratio. Opening and closing the throttle on a aircraft in game only changes what the initial pressure is. The pressure ratio of the supercharger then boosts it to the observed manifold pressure.

Pressure ratio for supers is determined by supercharger size, design, and how fast it spins relative to the engine (In auto terms, pully sizes).

On turbos, you can in theory over boost the engine, though it would have been almost impossible on an aero engine (assuming everything else is working), since you can't slow an aircraft engine down nearly fast enough to seriously overboost. It would likely start turbo surging first, and after that, you won't get any boost at all

quote:

Originally posted by BillSwagger:
It also read as a unit of measure, such as inches, on some gauges in the airplane to let the pilot know how much pressure or force the engine is having to work against. This pressure fluctuates based on throttle/mixture settings and the pitch of the prop.

The more throttle, the bigger the combustion, and therefore more pressure. The air works as a resistance as the propeller spins through it.
This is not modeled to the full extent in the game, for example using a lower prop setting (steeper pitch causes more resistance) with a high throttle position would result in very high manifold readings that would be dangerous for the engine.

I guess for a car it would be like trying to go uphill in fourth gear, and slamming your foot on the gas. It puts more load on the engine, though i'm not sure manifold pressure is measured the same as it would be in an aircraft engine. Putting the car in first gear, would be like raising the prop pitch setting, so there is less pressure exerted on the engine parts.

Its also possible to see fluctuations in manefold pressure as the speed of the plane changes, again not modeled. Its really something to keep an eye on when flying, but not quite so in this sim.

http://xkcd.com/386/

Sigh.

Manifold pressure is the pressure in the intake manifold. End of story.

WWII piston engines don't like running at low rpm & high manifold pressure because combustion takes approximately a constant number of degrees of crank angle rotation. Low rpm therefore implies slow combustion. Slow combustion provides time for what might best be described as "nasty chemistry" to take place, leading to the onset of detonation. It is misbehaviour of the combustion process which leads to engine damage.

As a reasonable first order approximation:

power is proportional to manifold pressure * rpm

In fact, you don't do quite so well because frictional losses increase with rpm. This is the fundamental motivation for running at low rpm.

Ram pressure is not especially significant at WWII fighter speeds. At Mach 0.5, sea level on a standard day, the ram pressure ratio is only about 1.18. So you'd see an extra 5.4 "Hg or so.Mach 0.5 on a standard day at sea level is a little over 170 m/s, which is about 380 mph. Most WWII fighters could not significantly exceed this speed at sea level.

If you want to play around with Mustang IIIs then at Mach 0.55 you'll see at most a pressure ratio of 1.23, which will give you an extra 6.9 "Hg.

IRL you won't realise these ideal ram pressure ratios. Quite a lot of WWII engine performance data was based upon the assumption of 80% ram pressure recovery. I'll leave you do apply the correction factor.

You might also want to test your assumption that this stuff isn't modelled. However, you'll need to fly above your engine's Full Throttle Height. If you don't know what that is then I refer you to Google.

The "Aircraft Guide.pdf" file on DVD 2 tells for each plane that you can what altitudes to switch gears.
The notes tell a number of details for each IL2:1946 plane and show cockpit layout. It is a useful guide
that many players seem to have missed.

quote:

looking at manifold pressure alone doesn't give a good indication of horsepower.

You are correct. Looking at Manifold pressure does not tell you much at all.

Mean Effective Pressure is what is needed to determine horsepower. This is the average pressure inside the cylinder during the power stroke and represents the pressure exerted on the piston head.

It is the BMEP that is important to shaft power and IMEP for indicated power. They teach you this in both college and a Powerplant course for your A&P.

All the best,

Crumpp

quote:

http://xkcd.com/386/

Sigh.

Manifold pressure is the pressure in the intake manifold. End of story.

WWII piston engines don't like running at low rpm & high manifold pressure because combustion takes approximately a constant number of degrees of crank angle rotation. Low rpm therefore implies slow combustion. Slow combustion provides time for what might best be described as "nasty chemistry" to take place, leading to the onset of detonation. It is misbehaviour of the combustion process which leads to engine damage.

I love that cartoon...

It must be remembered that, all things being equal, a lower RPM results in higher cylinder pressures. Thus, high manifold pressure + low RPM puts the greatest stress upon the engine. It doesn't matter in the game, but the speed of the flame front in the combustion chamber can be affected by the mixture. Also, for giggles, the timing of the spark can make a difference in the stresses (pressures) within the cylinder.

In any case, the John Deakin article Gunz linked to is an excellent place to start.

I guess what I'm not understanding then is if there is a spike or increase in the pressure measured and indicated as "MANIFOLD PRESSURE" at lower RPMs because there is too much throttle,

then why is there a higher reading? The pressure indicated is based on the pressure created by the intake stroke, so there should be less pressure at lower rpms. How does more throttle effect the intake pressure?

Salute

I am betting that in SOW BoB, Oleg will give us a complex engine management system which will govern all of the factors which affected these aircaft, instead of the way it is done in IL-2, which is generally unrealistic.

Mixture, engine RPM, prop pitch, boost, all of these interact in a fashion which affects power, performance, fuel economy, heat generation, and engine life.

Using very high boost with a lean mixture setting and at low rpm can destroy your engine in a heartbeat.

Overboosting your engine, by selecting a higher supercharger speed at lower altitudes, or running too long at high boost would do the same thing.

Engine and propellor management is something which a pilot of this era would need to attend to very carefully, it took an enormous amount of his time and effort.

When you add to that, the requirement to trim the aircraft to the various power settings and alignments of the aircraft, you have a pretty full plate.

Now try doing that all while someone is trying to kill you.

The problem with flying a computer instead of a plane, is that you are not getting seat of the pants feedback to tell you when you are not doing the right thing. So it is going to take a lot of gauge-gazing to fly these planes properly.

Unless Oleg has added a proper force feedback system to BoB, which gives you flutter, buffet and other aerodynamic effects.

quote:

Originally posted by BillSwagger:
I guess what I'm not understanding then is if there is a spike or increase in the pressure measured and indicated as "MANIFOLD PRESSURE" at lower RPMs because there is too much throttle,

then why is there a higher reading? The pressure indicated is based on the pressure created by the intake stroke, so there should be less pressure at lower rpms. How does more throttle effect the intake pressure?

There isn't. In fact, with the gear driven supers that the game deals with, you will see a LOWER pressure the lower the RPMs for a given throttle setting. The P-38 and the P-47 behave slightly differently, but not by enough to matter for this discussion.

As for the second part: We aren't talking naturally aspirated engines here. In fact, for NA engines, nobody gives a s*** about manifold pressure really. Plus, if we were, we would be talking about manifold vacuum rather than pressure.

Erco, don't forget that in most (all?) the planes we are talking about, they have multiple plugs for each cylinder, which means that you are talking paired flame fronts. Of course, if we get into that, we need to start getting into cylinder head design, localized hot spots, camshaft profiles, quench, quench pads, piston dome/dish, DCR/SCR, head flow rates, etc