Boost (Hurricane 1 and Merlin II/III)

[Genghis the Engineer]

I've been wading through some old manuals as part of a private project looking at the Hurricane 1.

I understand MAP and CS props well enough on modern aeroplanes, but I'm struggling with how this maps to "Boost" as used on the Merlin engine. Can anybody explain what Boost actually is and is controlled - is it just an alternative term for MAP, or as I suspect something defined a bit differently?

At the same time, references to the early CS props on the Hurricane 1 / Merlin II combination refer to a "2 speed" prop, which suggests a CS prop with only two RPM settings - as opposed to the infinitely variable lever I'd see on a modern aeroplane. Is that correct, or is it more modern than I think, or even is it a 2-pitch (fine and coarse) prop such as is fitted on one or two modern motorgliders?

And if anybody knows the answers to those, I'm guessing that they might know the answer to my third question. At what point was the 2 blade fixed pitch prop finally withdrawn from the Hurricane fleet?

Cheers chaps.

G

[Fareastdriver]

Rubbish removed

[Lightning Mate]

Sorry, but boost and MAP are not the same.

MAP is the absolute pressure in the manifold, measured from zero datum ie a vacuum.

Boost is the pressure in the manifold above atmospheric.

The early systems on the Hurricane and Spifire had two blade angle settings only. The rpm in either was controlled by the throttle, unlike modern constant speed propellers.

[Wander00]

From someone's book - cannot remember which - early prop was 2-speed - coarse and fine - with a lever in the cockpit - I have a feeling it was DRSB who crashed an aircraft on take-off - only to have lever point at him like an accusing finger as it was still in "coarse" whereas it should have been in "fine" for take-off

[Lightning Mate]

You are correct. He put it through a fence at the end of the airfield if I remember.


a kit to convert their two pitch propeller unit to a constant speed propeller.

[Fareastdriver]

Sorry, but boost and MAP are not the same.

Good enough for government work.

Those days have gone.

[Lightning Mate]

No they have not.

Try flying a WW2 warbird instead of a helicopter...........

[oxenos]

"take zero boost as 14 ins of MAP. "

Wrong.

Zero boost is approx 30 inches of mercury. + 8 boost is therefore 8 X 2 plus 30, i.e. 48 in MAP

[Genghis the Engineer]

But basically zero boost is effectively idle power, and above that is linear-ish with increasing power presumably?

Does "boost" actually serve any real purpose then, since we seem to be talking about fixed pitch propellers - even if you have a choice of three fixed pitches?

G

[3 Point]

Nope, zero boost isn't idle power, far from it. Zero boost is almost enough to tip a Spitfire or similar on it's nose while doing the run up to check mags. Zero boost would be a typical cruise power setting. Idle power would be off the scale, somewhere below -8 boost.

Zero boost is static atmospheric pressure; often referred to as "static boost" ie 29-30" MAP. They are both indications of the pressure in the inlet manifold and therefore are "the same" in a sense however, MAP uses absolute zero pressure as its datum and (usually) In Hg as the units, "boost" uses the prevailing atmospheric pressure as its datum and (usually) psi as its units.

Positive boost pressures are seen on a forced induction engine (Supercharged or turbocharged) and equate to MAP figures above 29"Hg.

Boost certainly serves a purpose as it allows you to set desired engine power at various RPMs (or with two speed VP (ie not CS) props). I have only known one aeroplane with a fixed pitch prop and a boost gauge. This was an engine which was supercharged to allow it to maintain sea level power output as the altitude increased; when you got high enough such that full throttle gave you zero boost that was effectively full throttle height; any further climb and you would start to lose power.

[Fareastdriver]

I am getting old. I mixed up lbs/inch2 with inches of mercury.
LM I did train on a supercharged radial engined aircraft and also a supercharged radial engined helicopter. Same engine, different gauges.

[Genghis the Engineer]

So in a nutshell - Boost, normally expressed in in.Hg, is MAP minus (MAP at a nominal cruise power setting). So idle is something like -8 Boost, and combat power was around +10 boost, expressed in in.Hg.

It was used, as MAP is, for setting power in general preference to using RPM. The Hurricane 1 did not have a constant speed prop - the earliest models had a fixed pitch prop, the later models had a variable-fixed pitch prop with a choice of two or three pitch settings. So a pitch setting was made, then the powerplant was controlled on the throttle, primarily by reference to boost pressure, rather than RPM.

Have I got that right?

G

[Lightning Mate]

Just about.


Same engine. different guages.

Wossa guage please?

[Genghis the Engineer]

Wossa guage please?

It's what the engine innards start to do if you don't keep the needles in the green.

G

[good spark]

wossa a gauge please?

bit like a plum

gs

[oxenos]

Lightening Mait got a dishunary for his birfday

[Lightning Mate]

i'm like honured innit i meen like fings ere ar like good innit

fred is like well drifting like innit

[Lancman]

Boost pressure and manifold air pressure are two different names for exactly the same thing. American engine manufacturers favored MAP and the British favoured boost. Only the scaling on the gauges/gages differed. If you put a boost gauge and a MAP gage next to each other at sea level on an ISA standard day the boost gauge needle would point at 0 boost on the scale and the MAP needle to a shade below 30"Hg.

The early Merlins weren't fitted with constant speed propellors, they were fixed pitch. Later propellors could be selected to either of two fixed pitches any time that the engine was running. RPM varied as a function of engine power and TAS. If a take-off was attempted in coarse pitch the RPM, and thus engine power, would initially be kept low by the very high torque load on the engine but they would increase as the aircraft accelerated. If that Hurricane had had a much longer runway it would have got airborne eventually just as the Schneider Trophy seaplanes did. Incidentally, the Schneider Trophy racers, with their very coarse fixed pitch props, used to start their take-off runs at 90 degrees to their intended take-off direction just to allow for that torque.

[Lightning Mate]

Boost pressure and manifold air pressure are two different names for exactly the same thing.

.................:ugh:

[3 Point]

Hi Genghis,

we are getting close but not quite there yet, re-read my second paragraph.

"Zero boost is static atmospheric pressure; often referred to as "static boost" ie 29-30" MAP. They are both indications of the pressure in the inlet manifold and therefore are "the same" in a sense however, MAP uses absolute zero pressure as its datum and (usually) In Hg as the units, "boost" uses the prevailing atmospheric pressure as its datum and (usually) psi as its units."

The usual units for boost is psi and for MAP is In Hg. Think Fahrenheit and Celsius; two systems for measuring temperature but each with a different datum point and measured in units of different increments. Two different systems of measuring the same thing.

Happy landings

3 Point

[Vitesse]

Could somebody explain what (a) boost cutout is and when it might be used. Thanks.

[stevef]

You can call a boost gauge a manifold pressure gauge/indicator but you can't always call a manifold pressure gauge a boost gauge. For that, the engine needs to be super/turbocharged.
Many normally-aspirated light aircraft have manifold pressure gauges.

[Fareastdriver]

I will try again. The gauges do the same thing. They measure the pressure in the inlet manifold and convey the result to the cockpit in either lbs per square inch or inches of mercury. With everything stationary both instruments will read the ambient pressure.
The checks for both are the same. You wind your altimeter to Zero and if the subscale is lower than standard 1013.2 then the boost or MAP gauge will read either either below Zero or below 29.92 ins. With an inches subscale they will read the same.
The ratio of boost to MAP is 2.036, or 2 for short; so for every pound of boost pushed into the engine a MAP gauge would increase by 2 ins and when the engine goes into suction mode, vice versa.
An old check on the Hercules used to be a climb at Zero boost and see what height the supercharger coudn't maintain boost any more.

[Lancman]

A "boost cutout" was more accurately a boost regulator cutout. The boost regulator was part of the linkage between the throttle lever and the actual throttle valve, and limited the throttle valve opening in order to avoid over-boosting the engine in normal operations. In an emergency the boost cutout lever could be pulled and an extra couple of pounds of boost gained.

It also slowly opened the throttle, without moving the throttle lever, as the aircraft climbed in order maintain the selected manifold air pressure until you reached full throttle height.

Of course the term may have found other meanings since then.

I've slowly come round to the conclusion that the Americans were right; manifold air pressure is a much more useful term than boost pressure.

[Lightning Mate]

With everything stationary both instruments will read the ambient pressure.

Not so my friend.

The MAP gauge will, but the boost gauge will read zero - hardly the same thing.

I use this illustration, drawn by myself, for instructional purposes. It shows what each gauge will read with the engine shut down at sea level:

http://i636.photobucket.com/albums/uu82/Lightning_29/boostMAP.jpg

[Fareastdriver]

I agree, my friend. At 1013.2 at sea level. OR 1013.2 ambient pressure at ANY height.
I can still remember my checks from 1960 and checking that the boost gauge agreed with the QFE was one of them.

Anyway they don't agree, if the boost gauge is at Zero then the MAP should be at 29.9 ins Hg.

Increasing altitude will cause both instruments to reduce. That is the whole reason for a supercharger; to maintain inlet manifold pressure at that altitude.

[Lightning Mate]

Increasing altitude will cause both instruments to reduce

Not so.

A supercharger will hold a set boost up to full throttle altitude, after which it will start to reduce.

It is possible to hold MAP in a climb, but impractible. With a constant speed propeller without a turbocharger it can be done by using reduced power (rpm & MAP) in the initial climb. As altitude increases the throttle may be progressively opened, which will hold the MAP until the throttle is fully open.

Perhaps this discussion would be better by PM. :)

[Peter-RB]

Genghis,

I used to have a full rebuilt Merlin in my reception office when I owned a factory with assorted offices, the Merlin I had was a two stage two speed Supercharger type, just below the main air tube going into the intake manifold (looking from the rear) was a quadrant of levers that would have been connected by rods and bell cranks back to the cockpit controls, but it seemed that the second stage S/Charger would be activated by the same pushrod that activated the first stage (or speed), the same quadrant of levers were all connected as well to two cranked arms to either side of the mopping bucket sized Carburettor. So it always seemed to me that just one lever would operate the functions of stepping up and giving extra motion lotion, and more HP.
All these levers and bell cranks also were connected to a water injection system as well as the a small fist sized fitting at the front of the engine that I imagine was also connected with the propeller speed.

Who ever perfected that system possibly went mad.

I had many people come to see the Merlin, or possibly my well endowed Secretary but no one ever did satisfy my question of how it was all controlled, most were old Flyers who came to touch and remember things in past years.with a few philistines who wanted to know if it would fit into a car!!.

My regards

Peter R-B
Lancashire

[Lightning Mate]

They don't build them like that anymore.

Wish I had one in my office. :sad:

[Lancman]

They're both reading ambient pressure L M, it's only the numbers on the dial that are different.

[Lightning Mate]

My boost gauge would show zero at any ambient pressure because it's a relative or differential pressure gauge.

OK gents, I'll give up here and continue to get my students through professional examinations. :)

[Vitesse]

Lancman - Thanks for the Boost Cutout explanation.

[good spark]

lightning mate, your pic is correct
29 92 and 0 are the same, depending on which side of the pond your from, after 0 the manifolds more pressurized than ambient, but only if the props in inc rpm.

gs


ps this one will go for ages, someone will allways argue.

[barit1]

I have been pondering the implications of both instruments.

Climbing to critical altitude, whilst maintaining constant MAP (i.e. absolute pressure), the back pressure on the exhaust system is decreasing, as is crankcase pressure and thus the force on the bottom of the pistons.

At the same time, ambient temperature is dropping, for greater air density at the same MAP.

I read these two factors as a gradual increase in power during the climb.

On the other hand, a Boost "gage" (Yankee spelling) would tend to even out the first of these factors. I've never flown a Boost engine - maybe someone can expand on this. :E

[Brian Abraham]

From a RAAF (1942) ground school training publication,

The air pressure in the induction pipe is known as boost or boost pressure, and is measured in lbs per square inch above (+) or below (-) that of the standard sea level atmosphere.

Of course, we colonials may not understand the complexities of machinery, but to me that says a boost gauge is exactly the same as a MP gauge. The boost gauge will indicate ambient pressure with the engine shut down.

Re boost cut out. The Lancastrian (transport Lancaster) engine lists a maximum of +14 boost, but with the boost cut out +18 was available. +18 was only to be used on take off if necessary, and if so used, boost was to be reduced to +14 as soon as the +14 take off safety speed (140 knots) was reached.

[happybiker]

LM
"Wish I had one in my office."

Which one? The rebuilt Merlin or the well endowed secretary!

[Genghis the Engineer]

In a previous job I had 18 Merlins (a mix of 35s and 500s) on my inventory. Starting up 4 of those was really quite satisfying.

http://www.sae.org/aeromag/techupdate_5-00/images/12b.jpg

Something wonderfully appropriate about using 4 Merlins to test Typhoon...

G

[Peter-RB]

http://i1152.photobucket.com/albums/p491/PeterDRB/RollsRoyceMerlin137-1.jpg

[Rory57]

Peter R-B

I don't know if this will answer your questions but have a look at

R-R Merlin Automatic Boost Control (http://www.enginehistory.org/Piston/Rolls-Royce/R-RmerlinABC/R-RmerlinABC.shtml)

for a discussion of how Merlin Boost Control works.

There is a good discussion of how and why this manifold pressure business is so confusing in the piece by Jerry Wells in the Torque Meter magazine / fall 05

[Lightning Mate]

Office Decoration circa 1994

Wheressa secretary!!!!!!!

(carefully not responding to ill-informed garbage)

[Peter-RB]

I will ask her if she wants to appear, she was a very good secretary and became a fountain of knowledge on the Merlin and the Griffon 58 (incl Props) that adorned my reception office, so much that I only needed to speak to vistors if they wanted to see me,..

She was a well bult Lancashire Lass with a very good sense of humour!!:ok:

Peter R-B
Lancashire

[barit1]

One more time - Back to the subject at hand:

I know that MAP is measured on an aneroid device - in other words a barometer that measures absolute pressure. It happens to be calibrated just like a US barometer, in inches of mercury. Which means it will (when shut down) vary from day to day, just like a mercury barometer as weather patterns change.

It will similarly read low at a high-altitude airfield, e.g about 24" at 5000', vs nominally 29.92" at sea level.

But the BOOST gage - am I correct in perceiving it as a "PSIG" instrument, as opposed to an absolute pressure device? If so, with the engine shut down, it will read zero boost REGARDLESS OF ALTITUDE or weather patterns.

Am I correct here, or is the boost gauge also an aneroid device? As I re-read Lightning Mate, I believe he and I agree that boost is a "PSIG" parameter.

[Lightning Mate]

She was a well bult Lancashire Lass with a very good sense of humour!!http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gif

Called a prat a prat then presumably. :ok:

Get her on this thread and sort this :mad: out then.

[Lightning Mate]

Is this her?

http://i636.photobucket.com/albums/uu82/Lightning_29/big_tits.jpg

If it is, I'm off with both in burner....................

Ear protection is vital........................

[sycamore]

barit1,you are correct,;don`t you have one on your `hot-rod Chevy..?

[Fareastdriver]

I googled 'boost gauge' and went into Wiki. It says what I have always known, especially one line.
A boost gauge will measure pressure in either psi or bar; many also measure manifold vacuum pressure in inches of mercury (in. Hg) or mm of mercury (mm Hg).

[Brian Abraham]

But the BOOST gage - am I correct in perceiving it as a "PSIG" instrument, as opposed to an absolute pressure device?No, all gauges measuring induction tube pressure are measuring absolute pressure.

An extract from 1943 | 2839 | Flight Archive (http://www.flightglobal.com/pdfarchive/view/1943/1943%20-%202839.html)

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."

The fact that the boost/MP gauge reads ambient pressure prior to engine start is used by some manufactures as a power check on the engine run up before take off. An example from one particular aircraft.

Note MP prior to engine start.
Run Up - Adjust throttle to field barometric pressure as read on MP gauge before starting engine
For propeller type XXX check for 2150 +-50 RPM
For propeller type YYY check for 2250 +-50 RPM

Note
If RPM is too low for given MP, engine is not developing sufficient power and should be checked before flight.

[Fareastdriver]

Hurrah! at last somebody has put it in print what I have being trying to convince Lightning Mate.


Lightning Mate My boost gauge would show zero at any ambient pressure because it's a relative or differential pressure gauge.

OK gents, I'll give up here and continue to get my students through professional examinations.


I suggest you rewrite your lessons.

[Centaurus]

Maybe this will help Ghengis. I had lots of hours as flying instructor on Lincolns.
For take off we would open the power on all engines to zero boost (30 inches MP approx) on the brakes. If every thing looked OK on engine instruments we would release the brakes and open the throttles leading with the port outer a bit more than the others to help directional control until the rudders became effective. Then we selected plus 12 boost and check the RPM was 3000. +12 boost was about 54" MP.

If the runway was short (in those days we did not have take off charts and we hoped we would reach lift off speed by the end of the runway), we would go through a "gate" on the throttle quadrant to obtain maximum power which in the RR Merlin 102 was +18 boost or 66"MP. . At that power setting the noise of the engines in the cockpit was ear shattering and painful. It was just as bad in the Mustang which I flew.

We rarely used +18 boost as it greatly reduced the life of the engine. We wore cloth flying helmets which gave no noise protection and to minimize the possibility of damage to the hearing the technique used by some copilots was to hold one hand pressed hard over the ear nearest to the nearest engine (for the copilot that was the starboard inner known as No 3).

The captain had no choice since his left hand was on the control wheel and his right hand had all four throttle levers. As soon as possible after lift off we would pull back the throttles to +12 then at 500 feet during flap retraction we would reduce further to +9 boost and 2650 RPM. At +18 boost the glycol coolant temperatures would heat up very quickly which was another reason to reduce power from +18 as soon as possible after lift off.
Normal cruise in the Lincolns we flew (Mk 31 or Long Nose version) was +6 boost and 2400rpm.

In the circuit we used +4 boost and 2400 RPM then increase to 2650 RPM late downwind. Turning base we set zero boost - which was descent power - and all being well maintained zero boost until late final when, depending on speed, we would gradually reduce power towards minus 4 boost ensuring all throttles were fully closed during the round out. It was then you could hear the characteristic "popping and crackling" noise of the Merlin at idle power. If on three engines downwind with one prop feathered (happened a lot in the tropics) then power was set at +7 boost and 2650 RPM which held the speed nicely.

PS. The noise of the four Merlins in the Lincoln caused a problem one night at Townsville. There was a zoo close to base leg for runway 02 and the zoo had a tall monkey cage. It was the bloody cage that was tall not the monkeys :D
We were doing circuits with new pilots on a still dark night and I understand the monkeys had gone to sleep high on the perches of their cage. The copilot increased the RPM just prior to turning base and the harmonics caused by the increase in RPM and atmospheric conditions must have caused unusual sound waves to affect several monkeys who apparently involuntarily released their hand grips on the perch and fell to their deaths on the concrete floor.
The zoo owner protested to our CO who had little sympathy although he later suggested the zoo owner install a flashing strobe on the top of the cage. That was done eventually and we were instructed to make night circuits wider in order to avoid flying directly over the zoo monkey cage.

[3 Point]

Good explanation Centaurus!

I suppose that Lightning Mate's explanation of the boost as a relative pressure gauge showing plus or minus relative to current ambient pressure might useful and might be the way it functions in some applications. I have flown several engines with boost pressure gauges (including a Centaurus) and I have never seen one which worked this way. I have only ever seen boost gauges which indicate the pressure in the inlet manifold relative to standard pressure. The boost with the engine stopped will vary according to the atmospheric pressure of the day and one sets static boost (ie the same as the reading with the engine stopped) during the run up to check the reference RMP as described by Brian Abraham.

Don't think the Lightning had boost gauges and I wonder who sets the questions for LM's professional exams! Perhaps he could be specific, which aircraft has a boost gauge which shows zero regardless of the ambient conditions?

[barit1]

OK, based on Brian Abraham's elucidation of the workings of the boost gauge, I hereby withdraw the last paragraph of my post #34. :O

[Brian Abraham]

No need for embarrassment Barit. It is a commentary though, that in this age of the jet we have forgotten the basics of what the pilot of old knew ie care, feeding and workings of the recip. From my ground school commercial pilot notes (when the big recip was on it's last legs airline wise, but still plentiful in the military - C121, C97, S2, A26, A1, T28, and more.

http://i101.photobucket.com/albums/m56/babraham227/z082.jpg
http://i101.photobucket.com/albums/m56/babraham227/z083.jpg

Perhaps I should add that the above course notes were written by C. S. Hames, who joined the RAF as an engine apprentice in 1938, on completion of that course remustered as aircrew, trained in the USA, flew operations over Europe during WWII, was part of the Japan occupation force and subsequently graduated from CFS as an instructor.

[Fareastdriver]

I can see in your example above where the confusion as to a boost gauge being an absolute pressure device or a pressure differential indicator comes from.

The last description of the diaphram boost guage makes no mention of the evacuated part of the assembly. Therefore by definition if one side of the diaphram is connected to the inlet manifold and the other side to an open case then when the engine is at rest the gauge will read Zero; at any altitude.

[Lancman]

Please tell me that the young lady at the top of this page has been Photoshopped?

[sycamore]

I think she`s just showing what +18 lbs/ 66" Hg looks like..? Gives you a bit of a swerve on Take-off ....

[Lightning Mate]

Please tell me that the young lady at the top of this page has been Photoshopped?

Prolly!

I would not refer to her as "a young lady"!

Young maybe................

I always said that anything more than a mouthful is a waste.:E


Gives you a bit of a swerve on Take-off ....

Landing more like -when you fall off the bed...................!!!!!

[Peter-RB]

LM

I agree about the mouthfull.......but somtimes its nice to juggle!!;):ouch:

PeterR-B
Lancashire

[Fareastdriver]

This thread has changed to nostalgia rather than aviation and history.

[Peter-RB]

FED

Nostalgia or history, its good to have a long one... memory that is!
(nearly spelt that wrong) ;)

Peter R-B

[Xray4277]

I realise that I am posting a reply to an old thread, but I came across it while searching for the answer to a question about the relationship between boost, throttle and pitch - Len Deighton, in the afterword to his magnificent novel 'Bomber' mentions having had it explained to him by (if I remember correctly) the pilot of an He111, but he does not explain in in his afterword!

I am not a pilot (I did some flying towards a PPL back in the 1980s but had to give it up for lack of money!), but I have always loved aviation and flying. I am by profession an engineering consultant, and would like to mention that when we talk of pressure it is either referred to as 'gauge' or 'absolute'. Most pressure indicators found in industry show 'gauge' pressure, i.e. they will read zero at normal sea level atmospheric pressure. 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.)

Oh, and having read this thread in its entirety, I do now understand the relationship. I can re-read my Lancaster and Spitfire pilots' notes with new insight!

[POBJOY]

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 !!!

[megan]

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)The boost gauge is an absolute reading also, it will read zero at sea level ambient pressure, where as a MAP gauge will read just under 30 inches, eg relationship is as follows,

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

http://spitfirespares.co.uk/Website%20Products%20252%20(Boost%20and%20Altimeter%20Instru ments)/Boost/20lb%20boost%202%20Front%20large.jpg

Typical MAP gauge for a turbo/super charged engine

https://www.aircraftspruce.com/catalog/graphics/1/10-06454.jpg

[India Four Two]

“That’s not a manifold pressure gauge, THIS Is a manifold pressure gauge!”

[megan]

THIS Is a manifold pressure gaugeBut it doesn't go to 150 inches!!!! (P-51 racer "Dago Red" with Merlin)

[Xray4277]

The boost gauge is an absolute reading also, it will read zero at sea level ambient pressure, where as a MAP gauge will read just under 30 inches, eg relationship is as follows,

Hate to labour the point, but in the strict engineering sense a device which reads zero at normal atmospheric pressure is not reading absolute pressure - the terms absolute pressure and gauge pressure are not the same in the engineering world. A bit like weight and mass which are used interchangeably in common parlance, but in the scientific and engineering worlds they are different.

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.

[India Four Two]

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.

[ShyTorque]

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.

[Fareastdriver]

Sorry Shy Torque, both of these are wrong.

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

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.

[Xray4277]

Sorry Shy Torque, both of these are wrong.



...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).

[Xray4277]

Xray4277,

You make a valid point, except that boost pressure gauges are not "gauge pressure" devices. They are reading absolute pressure,

Sorry, still labouring my point I know, but boost indicator does NOT show absolute pressure. Not in the strict engineering sense.

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).

[mustafagander]

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.

[Xray4277]

Yes - slight typo in my post - the intercooler cools air after the turbo! Cooler (denser) air is then available for the fuel/air mix which goes to the cylinders.

[megan]

Sorry, still labouring my point I know, but boost indicator does NOT show absolute pressure. Not in the strict engineering sense.
Maybe not in your strict engineering sense, but it did back in the day read absolute pressure, in as much you just added 14.7 to the figure displayed. From https://www.flightglobal.com/pdfarchive/view/1943/1943%20-%202839.html authored by JOSEPH LOWREY, B.Sc.(Eng.)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."

[Wander00]

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

[Xray4277]

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.

[Xray4277]

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

Uncomfortable but not disastrously so! I took delivery of a new VW some years ago and drove it around for the best part of a week thinking that the ride was a bit hard - checked the tyre pressures and they were still at 'storage' values of 60psi (to stop the sidewalls sagging during long-term storage). So much for the dealer's pre-delivery inspection...:hmm:

[megan]

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.

[ShyTorque]

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!

[LOMCEVAK]

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

[Xray4277]

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! :ok:
http://www.pprune.org/moz-extension://23fe6c90-333a-453e-9c84-c939e380a718/informenter-marker.pnghttp://www.pprune.org/moz-extension://23fe6c90-333a-453e-9c84-c939e380a718/informenter-marker.png

[Lancman]

Nobody talked about "negative boost", anything less than zero was "minus".

[Xray4277]

Nobody talked about "negative boost", anything less than zero was "minus".

Maybe, but that's just semantics. Negative or minus, a layperson would likely interpret that differently from an engineer or scientist.
http://www.pprune.org/moz-extension://23fe6c90-333a-453e-9c84-c939e380a718/informenter-marker.png