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Tallyho1961
05-01-2005, 06:30 AM
A CEM question:

I've noticed that the red line on many, but not all, of the manifold pressure gauges on CSP-equipped allied a/c is located about two thirds of the way along the range of possible pressures. How significant is respecting the red line in terms of engine management?

This may seem like a obvious question, but the reason I ask is that I very often need a power setting above the red line, unless I'm just cruising to and from a combat zone and can afford to throttle back (and drop my RPMs) to what I estimate to be a cruise or even economy power setting.

A good example would be the Kingcobra. I don't have time to load up and check, but I think the red line is around 40, while the gauge reads up to 75 or 80. On others, I think red line is at 40, with max pressure at 50 - which seems more reasonable.

It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

This sort of complexity may not even be modelled in the sim, I'm just curious.

Tallyho1961
05-01-2005, 02:23 PM
I hate to do this, but self-bump.

Any aero engine experts have any thoughts? Or am I off in a corner all by lonesome on this one?

hop2002
05-01-2005, 02:58 PM
It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

The supercharger is designed to generate maximum allowable pressure at a certain altitude. Say, for example, 12,000 ft.

That means that it can generate much more than maximum allowable pressure at a lower level, say 2,000ft.

Especially if the plane is travelling very fast, when air pressure builds up in the inlet, and increases manifold pressure.

The way around this is to fit an automatic boost control, which automatically closes the throttle, reducing pressure to the maximum allowed.

Not all planes had ABC, however.

Redudcing the speed of the supercharger so that it generated maximum allowable pressure at sea level would mean very poor performance at altitude.

FoolTrottel
05-01-2005, 03:03 PM
Hi,

Me no aero enigine expert, got only one thing to for you: a link! Complex Engine Management. (http://www.airwarfare.com/Sims/FB/fb_cem.htm)
(It's got the words 'Manifold pressure' in it, in one sentence combined with 'Keep your eye on(..)!)
Good Luck!

Tallyho1961
05-01-2005, 03:28 PM
Thanks Hop 2002 - that makes sense to me. I know what I am enquiring about may be beyond the bounds of what IL2 is capable of replicating. I've discovered that I find the whole subject of aero engines quite fascinating in its own right.

And thanks FoolTrottel - I've got that one printed, and I've read through it a couple of times. It's very good, but doesn't go as deep as what I am looking for which, as I suspect, may very well be beyond the bounds of IL2 CEM.

Ahh.. the paths this sim takes us down are many and winding http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

Taylortony
05-01-2005, 04:27 PM
Originally posted by hop2002:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

The supercharger is designed to generate maximum allowable pressure at a certain altitude. Say, for example, 12,000 ft.

That means that it can generate much more than maximum allowable pressure at a lower level, say 2,000ft.

Especially if the plane is travelling very fast, when air pressure builds up in the inlet, and increases manifold pressure.

The way around this is to fit an automatic boost control, which automatically closes the throttle, reducing pressure to the maximum allowed.

Not all planes had ABC, however.

Redudcing the speed of the supercharger so that it generated maximum allowable pressure at sea level would mean very poor performance at altitude. </div></BLOCKQUOTE>


Errr an Engine will start to lose power over about 12,000 ft as the air starts to get rarified, IE not as much oxygen in the atmosphere per pound of air, therefore you use a turbo or supercharger to boost this by virtually cramming more air in and hence more oxygen, the effect of this is to increase the Critical altitude where the Engine starts to lose power.
As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off€"at 6000 m (18,000 ft) the air is at half the pressure of sea level. Since the charge in the cylinders is being pushed in by this air pressure it means that the engine will normally produce half-power at full throttle at this altitude but as you climb less resistance in the air means your airspeed will increase. A supercharger is only able to supply so much pressure because the compression increases the air temperature and the engine is limited in maximum charge-air temperature before pre-ignition occurs. This is where the temperature of the air entering the engine being compressed it gets hotter and detonates the fuel charge early... bad news it will destroy an engine...a way round this is to fit an intercooler which in effect passes the compressed air through a radiator allowing you to upp the pressure a little.

Below the critical altitude the supercharger is actually delivering too much boost that must be restricted or risk damaging the engine. This means that at least some of that 150 horsepower (100 kW) driving the supercharger is being wasted. At lower altitudes the supercharger is not operating at its best efficiency when it is handling the more dense air and that causes an additional load on the engine too. some modernish aircraft still do not have any control over this, the Piper Seneca is one and it is a piece of p*ss to overboost it on the ground, you have to watch it on runs as it can overswing as it accelerates, other aircraft have a simple blow off type valve, that at a given pressure dumps air back to the inlet side of the turbo/ supercharger....I havent seen a restricted throttle, but a lot of wobbly prop you dont use the power lever, but the prop to govern power..

for a stretched unedited version of above see
http://encyclopedia.lockergnome.com/s/b/Supercharger

hop2002
05-01-2005, 06:26 PM
Errr an Engine will start to lose power over about 12,000 ft as the air starts to get rarified,


No, it will start to lose power above 0ft, as the air pressure drops.

Unless it's using a supercharger or turbo to boost the manifold pressure.

If that's the case, it will start to lose power above the altitude at which the supercharge can provide maximum allowable pressure.


IE not as much oxygen in the atmosphere per pound of air

No. Not as much oxygen per volume of air, not weight.

Air pressure drops with altitude, which means air density. That means the weight of a given volume of air is dropping, not the oxygen content (ie oxygen content per volume is decreasing, oxygen content per weight is not (for practical purposes, anyway))


therefore you use a turbo or supercharger to boost this by virtually cramming more air in and hence more oxygen,

Yes. But it's reduction in air pressure, not oxygen percentage.


Since the charge in the cylinders is being pushed in by this air pressure it means that the engine will normally produce half-power at full throttle at this altitude

Yes. Bear in mind this varies with air speed, as the forward speed "rams" the air into the intake, creating extra pressure. This is true for supercharged and normally aspirated engines.


but as you climb less resistance in the air means your airspeed will increase.

Not true in most cases if power is dropping.

WW2 fighters typically have maximum speed at critical altitude, the altitude at which the turbo or supercharger can deliver max allowable pressure. Speed usually declines above that altitude.


A supercharger is only able to supply so much pressure because the compression increases the air temperature and the engine is limited in maximum charge-air temperature before pre-ignition occurs.

Not only. Depending on the fuel and strength of the engine, other components might give out first.

100/130 fuel allowed 18 lbs boost in the Merlin before detonation (pre ignition), but not all Merlins could run at 18 lbs (in fact, not until late 42 early 43 did 18 lbs capable Merlins enter production, iirc)


other aircraft have a simple blow off type valve, that at a given pressure dumps air back to the inlet side of the turbo/ supercharger....I havent seen a restricted throttle,

On the Merlin, the fuel was injected prior to this stage, so you really don't want to be dumping the excess air pressure, because you're dumping fuel with it.



A good example would be the Kingcobra. I don't have time to load up and check, but I think the red line is around 40, while the gauge reads up to 75 or 80. On others, I think red line is at 40, with max pressure at 50 - which seems more reasonable.

It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

One thing that might explain the difference with the Kingcobra is fuel grade. I have no idea whether it's true or not, but it might have been intended to run on 100/150 or 115/145, which would allow much more pressure.

If running on 100/130, that would account for a guage that reads som much higher, with a low redline.

I haven't got around to reinstalling IL2 yet, and I can't say I noticed before, but does the game model the increase in manifold pressure you get at high speeds?

Taylortony
05-01-2005, 06:52 PM
Originally posted by hop2002:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Errr an Engine will start to lose power over about 12,000 ft as the air starts to get rarified,


No, it will start to lose power above 0ft, as the air pressure drops.



Unless it's using a supercharger or turbo to boost the manifold pressure.

If that's the case, it will start to lose power above the altitude at which the supercharge can provide maximum allowable pressure.


The main thing the supercharger does is to raise the critical altitude


IE not as much oxygen in the atmosphere per pound of air

No. Not as much oxygen per volume of air, not weight.

Air pressure drops with altitude, which means air density. That means the weight of a given volume of air is dropping, not the oxygen content (ie oxygen content per volume is decreasing, oxygen content per weight is not (for practical purposes, anyway))


therefore you use a turbo or supercharger to boost this by virtually cramming more air in and hence more oxygen,

Yes. But it's reduction in air pressure, not oxygen percentage.


Since the charge in the cylinders is being pushed in by this air pressure it means that the engine will normally produce half-power at full throttle at this altitude

Yes. Bear in mind this varies with air speed, as the forward speed "rams" the air into the intake, creating extra pressure. This is true for supercharged and normally aspirated engines.




but as you climb less resistance in the air means your airspeed will increase.

Not true in most cases if power is dropping.

WW2 fighters typically have maximum speed at critical altitude, the altitude at which the turbo or supercharger can deliver max allowable pressure. Speed usually declines above that altitude.


Yes i know but that was not in the context i was putting it


A supercharger is only able to supply so much pressure because the compression increases the air temperature and the engine is limited in maximum charge-air temperature before pre-ignition occurs.

Not only. Depending on the fuel and strength of the engine, other components might give out first.

100/130 fuel allowed 18 lbs boost in the Merlin before detonation (pre ignition), but not all Merlins could run at 18 lbs (in fact, not until late 42 early 43 did 18 lbs capable Merlins enter production, iirc)


yes but adding extra fuel is the norm to cool the engine, all engines run rich




other aircraft have a simple blow off type valve, that at a given pressure dumps air back to the inlet side of the turbo/ supercharger....I havent seen a restricted throttle,

On the Merlin, the fuel was injected prior to this stage, so you really don't want to be dumping the excess air pressure, because you're dumping fuel with it.




A good example would be the Kingcobra. I don't have time to load up and check, but I think the red line is around 40, while the gauge reads up to 75 or 80. On others, I think red line is at 40, with max pressure at 50 - which seems more reasonable.

It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

One thing that might explain the difference with the Kingcobra is fuel grade. I have no idea whether it's true or not, but it might have been intended to run on 100/150 or 115/145, which would allow much more pressure.

If running on 100/130, that would account for a guage that reads som much higher, with a low redline.

I haven't got around to reinstalling IL2 yet, and I can't say I noticed before, but does the game model the increase in manifold pressure you get at high speeds? </div></BLOCKQUOTE>

will give you that one, we tend to measure fuel and the like in weight as it is more accurate

I know the Merlin premixes it, i was talking generically.... the one I work on now and then BTW has a Griffon fitted to it... I was going through the fuel system the other week on that in the books discussing the idling problems on it, which hopefully will be sorted soon

Yes true and a Jet Engine in theory should have a turbine for each stage of the compressor but this dont happen.......... It is normally reckoned that above about 12 grand you will notice a significant drop off of power not below

I was trying to put this in laymans terms without getting to technical for here, I could start wittering on about BMEP FMEP IMEP and PMEP etc after all there the best way to measure output but it starts to get a bit deep for most in here......

Tallyho1961
05-01-2005, 07:06 PM
Thanks guys - I find this really interesting. a bit deep, yes, so I'll put on my bathing suit http://forums.ubi.com/groupee_common/emoticons/icon_smile.gif

SkyChimp
05-01-2005, 08:59 PM
Originally posted by hop2002:

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">A good example would be the Kingcobra. I don't have time to load up and check, but I think the red line is around 40, while the gauge reads up to 75 or 80. On others, I think red line is at 40, with max pressure at 50 - which seems more reasonable.

It just seems odd that an engine would be designed to generate so much of its pressure above a known 'danger' level, especially on a combat a/c which would be expected to undergo a lot of stress.

One thing that might explain the difference with the Kingcobra is fuel grade. I have no idea whether it's true or not, but it might have been intended to run on 100/150 or 115/145, which would allow much more pressure.

If running on 100/130, that would account for a guage that reads som much higher, with a low redline.

I haven't got around to reinstalling IL2 yet, and I can't say I noticed before, but does the game model the increase in manifold pressure you get at high speeds? </div></BLOCKQUOTE>

The red mark on the P-63's MAP gauge indicates the point at which time-limited boosts are achieved.

The red mark is at 51" hg. The P-63C-5's Allison V-1710-117 achieved military power at 52" hg. At that point, time limitations should have been observed. At 51" hg and below, power could be used continously (limited by temp).

The V-1710-117 could achieve the following (on 100/130 grade fuel):
1,800 hp @ 24,000 feet @ 76" hg (wet)

Water injection began automatically at 58" hg.

Tallyho1961
05-02-2005, 09:40 AM
Thanks SkyChimp! - that really nails it down. I take it you're a pilot?

Dave.

SkyChimp
05-02-2005, 06:02 PM
Originally posted by Tallyho1961:
Thanks SkyChimp! - that really nails it down. I take it you're a pilot?

Dave.

No, just an amateur WWII aviation enthusiast.

SkyChimp
05-02-2005, 06:03 PM
Originally posted by Tallyho1961:
Thanks SkyChimp! - that really nails it down. I take it you're a pilot?

Dave.

double post

SkyChimp
05-02-2005, 06:12 PM
Stupid forums http://forums.ubi.com/groupee_common/emoticons/icon_mad.gif

hop2002
05-02-2005, 06:41 PM
Thanks Skychimp. I know very little about the P-63, and I don'[t think I've even flown it properly in IL2.