Boost (Hurricane 1 and Merlin II/III)
Hurricane 1
It would seem that the majority of Hurricanes that went to France (BEF and AASF) had the Watts two blade prop,(and some with fabric covered wings).
Considering the limitations (take off and climb rpm) they performed well against the 109 with its VP.
Another factor is the diameter of the Watts prop decreased the ground clearance compared with the Rotol 3 blade and this must have led to 'issues' with the poor surface on the French airfields.
Proving another point it did not seem to limit Pilots like Cobber Kain from scoring well against the odds !!!
Considering the limitations (take off and climb rpm) they performed well against the 109 with its VP.
Another factor is the diameter of the Watts prop decreased the ground clearance compared with the Rotol 3 blade and this must have led to 'issues' with the poor surface on the French airfields.
Proving another point it did not seem to limit Pilots like Cobber Kain from scoring well against the odds !!!
So it seems correct to think that a boost gauge will read zero when at rest, whereas manifold pressure, which is an absolute pressure, will be indicated as a positive value (which will vary according to local conditions, altitude, etc)
Boost, Absolute, MAP
-4, 10.7, 21.8
0, 14.7, 29.9
+12, 26.7, 54.3
The boost gauge is merely telling you how much above or below sea level ambient you are. -4 boost = 14.7-4, +10 boost = 14.7+10
Spitfire boost gauge
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Typical MAP gauge for a turbo/super charged engine
Last edited by megan; 21st Oct 2017 at 02:01.
“That’s not a manifold pressure gauge, THIS Is a manifold pressure gauge!”
If a boost pressure display was reading absolute pressure, then at sea level, normal atmospheric pressure, blah blah blah, it would read +14.7, not zero.
Xray4277,
You make a valid point, except that boost pressure gauges are not "gauge pressure" devices. They are reading absolute pressure, but 14.7 psi is subtracted from the reading, to give a nominal "zero boost".
The boost pressure on the ground at sea level will not be zero, unless it is a standard pressure day, just as manifold pressure gauges will very rarely read 29.92" of mercury. The gauge on the 182 I fly usually reads about 25" before start.
You make a valid point, except that boost pressure gauges are not "gauge pressure" devices. They are reading absolute pressure, but 14.7 psi is subtracted from the reading, to give a nominal "zero boost".
The boost pressure on the ground at sea level will not be zero, unless it is a standard pressure day, just as manifold pressure gauges will very rarely read 29.92" of mercury. The gauge on the 182 I fly usually reads about 25" before start.
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I find it useful to remember that a piston engine is an air pump. The engine tries to suck the air out of the inlet manifold and the atmosphere does its best to keep it filled. At higher rpm and/or lower throttle openings, the engine will win (giving a relatively low MAP reading) and at low rpm/larger throttle openings the atmosphere will win (giving a relatively high MAP reading).
A normally aspirated engine can, even at full throttle, only ever have at best, ambient atmospheric pressure in the manifold. A forced induction engine can have more than atmospheric pressure, mainly depending on throttle position, although rpm does have an effect. As already stated, with increasing altitude/reducing air density, eventually the manifold air pressure would reduce to be less than normal sea level pressure. That's where the two stage (or alternatively, two speed) supercharger comes in handy. The later Merlin engines had one supercharger blowing into the second one - they ran in series.
I have a supercharged car. It has a boost gauge (which naturally reads close to "zero" with the engine switched off). When driving long distances on road I normally try to keep the boost gauge at a negative figure by using a very small throttle opening because that results in better economy and stresses the engine less. As soon as I open the throttle further so that the boost gauge reads more than the "zero" position, I'm aware that I'm asking the engine to do more than it could if the supercharger wasn't fitted.
A normally aspirated engine can, even at full throttle, only ever have at best, ambient atmospheric pressure in the manifold. A forced induction engine can have more than atmospheric pressure, mainly depending on throttle position, although rpm does have an effect. As already stated, with increasing altitude/reducing air density, eventually the manifold air pressure would reduce to be less than normal sea level pressure. That's where the two stage (or alternatively, two speed) supercharger comes in handy. The later Merlin engines had one supercharger blowing into the second one - they ran in series.
I have a supercharged car. It has a boost gauge (which naturally reads close to "zero" with the engine switched off). When driving long distances on road I normally try to keep the boost gauge at a negative figure by using a very small throttle opening because that results in better economy and stresses the engine less. As soon as I open the throttle further so that the boost gauge reads more than the "zero" position, I'm aware that I'm asking the engine to do more than it could if the supercharger wasn't fitted.
Sorry Shy Torque, both of these are wrong.
Suck, Squeeze, Bang, Blow, is the Ottoman Principle so the inlet pressure MUST be below atmospheric in an unsupercharged engine unless you use ram air irrespective of the rpm or throttle opening.
An example is the old vacuum windscreen wipers used on car in the fifties. At high rpm and closed throttle they went like the clappers but at any rpm with the throttle wide open they slowed to a crawl, but they still worked. They didn't stop mid swipe and go backwards.
The more you supercharge the engine the more air and fuel you can force in; the more you can squeeze and the bigger the bang, so much so that it the exhaust gases aren't cooled so when it comes out of the exhaust the stubs are glowing. There's so much of it that you can use it to drive a turbocharger and so push even more fuel/air in and then you have to use an intercooler to keep the turbine in one piece.
at low rpm/larger throttle openings the atmosphere will win
A normally aspirated engine can, even at full throttle, only ever have at best, ambient atmospheric pressure in the manifold
An example is the old vacuum windscreen wipers used on car in the fifties. At high rpm and closed throttle they went like the clappers but at any rpm with the throttle wide open they slowed to a crawl, but they still worked. They didn't stop mid swipe and go backwards.
The more you supercharge the engine the more air and fuel you can force in; the more you can squeeze and the bigger the bang, so much so that it the exhaust gases aren't cooled so when it comes out of the exhaust the stubs are glowing. There's so much of it that you can use it to drive a turbocharger and so push even more fuel/air in and then you have to use an intercooler to keep the turbine in one piece.
Intercooler is to cool charge air before it gets to turbo - engine power is determined by the MASS of fuel/air mixture going to the cylinders, not volume. Since any inlet will choke beyond a certain point, i.e. no matter what you won't get more than a certain volume of air down the intake, you want cooler air going in (cooler air is denser, therefore more mass per unit volume).
Most pressure displays will have bar (g) or bar (a) (or psi, mm, inches, whatever system of units you are using) to differentiate between gauge pressure and absolute pressure, as defined in my earlier post. Actually, if you look at the two displays in Megan's post, the manifold pressure display is labelled with inches of mercury ABSOLUTE, which is why it is reading 29 inches or thereabouts at rest at sea level. The Spitfire boost gauge shows zero under the same conditions because it shows (engineering wise) GAUGE pressure, which knocks off the sea level pressure of around 14.7 psi (equivalent to 29 inched of mercury).
Come on guys, it's not hard to state what an intercooler does - the very name is a big hint.
An intercooler is a device to reject heat, usually to the atmosphere or another fluid, from the inlet air after compression and hence heating by the turbo or supercharger prior to the inlet of the engine.
It has absolutely nothing to do with turbines and preservation thereof. The engine runs on the Otto cycle.
An intercooler is a device to reject heat, usually to the atmosphere or another fluid, from the inlet air after compression and hence heating by the turbo or supercharger prior to the inlet of the engine.
It has absolutely nothing to do with turbines and preservation thereof. The engine runs on the Otto cycle.
Sorry, still labouring my point I know, but boost indicator does NOT show absolute pressure. Not in the strict engineering sense.
A boost pressure gauge, in spite of its name, differs fundamentally from the fuel pressure gauges, oil pressure gauges, and other pressure-measuring instruments used in an aircraft. Like an altimeter, it records absolute pressure, and not merely pressure above atmospheric. In fact, the American name of "manifold pressure gauge" is more correct, the gauge giving no indication of the extent to which the supercharger has boosted atmospheric pressure. The orthodox British system of boost gauge calibration ignores this fundamental fact, pressures being given in pounds per square inch above or below an absolute pressure of 14.7 pounds per square inch. The latter pressure, which is the mean sea level atmospheric pressure, is arbitrarily known as "zero boost."
Reminds me of meeting a British mate in a French garage where he was checking his tyre pressures, "Did not realise the Mondeo had such high tyre pressures", I said. "Why?" said my mate, "what is the problem?" Pointed out that French pressure gauge was in Bar, and he had read it as psi, so had put 3.2 Bar -45psi ish - in each tyre. hate to think what it would have been like to drive like that
Ah well, Megan (and India Four Two) we're just going to have to agree to disagree on this one then!
If I attached a pressure measurement device to any system which was at atmospheric pressure, and the device read zero, any engineer who I asked would say it was measuring gauge pressure, not absolute. If you are subtracting 14.7 psi from the boost gauge reading so that the device shows zero, to me and any other engineer who I know, that's not absolute pressure. The subtraction makes it 'gauge pressure' by definition.
If I attached a pressure measurement device to any system which was at atmospheric pressure, and the device read zero, any engineer who I asked would say it was measuring gauge pressure, not absolute. If you are subtracting 14.7 psi from the boost gauge reading so that the device shows zero, to me and any other engineer who I know, that's not absolute pressure. The subtraction makes it 'gauge pressure' by definition.
Reminds me of meeting a British mate in a French garage where he was checking his tyre pressures, "Did not realise the Mondeo had such high tyre pressures", I said. "Why?" said my mate, "what is the problem?" Pointed out that French pressure gauge was in Bar, and he had read it as psi, so had put 3.2 Bar -45psi ish - in each tyre. hate to think what it would have been like to drive like that
Xray, I see the boost gauge as little different to an altimeter, which too is reading absolute. The zero point though on an altimeter can be set to any arbitrary point, normally QNH or QFE.
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FED, I agree that in practice with a normally aspirated engine running on a simple intake system, the MAP wouldn’t reach fully ambient. But depending on the restrictions to airflow in the inlet system, it can get close. Modern inlet systems include resonator boxes which act almost like “free” forced induction. Modern motorcycle and car manufacturers such as Honda have done a very good job in this respect.
An intercooler has nothing to do with protection of the turbo! It’s there to cool the inlet air temperature to the engine. This both increases charge density and reduces the chances of mixture detonation.
Having the exhaust glow red hot isn’t necessarily a problem now the days of piston engined fighters or bombers have long gone!
An intercooler has nothing to do with protection of the turbo! It’s there to cool the inlet air temperature to the engine. This both increases charge density and reduces the chances of mixture detonation.
Having the exhaust glow red hot isn’t necessarily a problem now the days of piston engined fighters or bombers have long gone!
I have followed this thread out of curiosity and, at times, with a wry smile about how something so simple can be made so ambiguous and potentially confusing! I have converted several pilots who are used to flying piston engine aircraft with a conventional MAP gauge calibrated in ins Hg to one that has a boost gauge in psi. All I say is that the gauge works in the same way but the units are different (2" Hg being approximately 1 psi) and '0 psi' equates to ISA sea level static pressure (approximately 30"). And they always understand and never have any problems! Am I missing something?
Rgds
L
Rgds
L
It is - quite literally - all relative. There is, from one point of view, a certain logic in having a boost gauge with what might be considered an arbitrary zero point. Most of us live at or relatively near sea level, so using ISA as zero is a commonsense approach. And speaking colloquially, 'boost' implies getting something extra, so a positive boost pressure, meaning a manifold pressure greater than ISA, is quite easy to understand. Where it falls apart though is when we talk of 'negative boost'. Again, speaking colloquially, most people would probably interpret this as a 'negative pressure', and of course in absolute terms there is no such thing. Negative values indicated by a boost pressure gauge just mean that the manifold pressure is below ISA, as the supercharger or turbocharger fights a losing battle against diminishing ambient pressure at altitude. There will still be some positive pressure in the manifold come what may. Pressure, in absolute terms, can never be less than zero, in the same way that temperature, on the absolute scale (Kelvin) can never be below zero. Zero Kelvin is when all molecular motion ceases and you can't get below that value. Zero (absolute) pressure is a total vacuum and you can't get lower than that either.
The great thing the rules of physics is that they apply whether you believe them or not!
The great thing the rules of physics is that they apply whether you believe them or not!