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Okay, boost 101 here.
In layman's terms, what exactly does a 'boost' do and why should I want to cut it off?

BTW, I love the spit, she's a delicate, irascible machine, and when she's good, she's 'very' good , but when she's bad, she's 'very' bad.

G'day Jrazz,

I am sure someone more technical could split hairs, but here is a "mug's" (me!) guide.

Boost is the atmospheric pressure + or - the pressure created in the engine by the fuel system.

Zero boost is 1 "atmosphere" or roughly 30 inches of Mercury. (or 14 point "something" pounds per square inch)

As soon as the engine starts, the suction of fuel into the carby creates low pressure. You will see at low pressure a "minus" boost figure..meaning that the current pressure in the system is less than 1 atmosphere.

The more throttle the greater the pressure in the system, as more fuel is flowing through it at a higher rate. . Now the Merlin in the early Spits is capable of generating about 18psi which is more than the engine can safely handle without self destructing due to overpressure and fuel detonation etc. .

Enter the Boost Cut Out. The boost cut out ensures that anything over the specified maximum (6 1/4 lb per sq in with Merlin III and 9 lb per sq in with the Merlin XII) is vented to the atmosphere..thus avoiding melt down.

This is achieved by a sprung piston arrangement. Unfortunatley if this piston fails, the consequence is that potentially ALL the pressure is vented and your Merlin becomes a large counterweight to the tail..and nothing else.

So, enter the ABCO-O or Automatic Boost Cut Out - Override, the red finger tab. If the BCO fails and starves the engine of fuel, this override "sticks its finger in the Dyke" so to speak and gives back full pressure. In an emergency the engine could then be nursed at low throttle back to base. If you gave it high (or full) throttle then you would blow the whole thing up very quickly with the 18psi that had become "re-available", making the whole "avoid engine failure by ABCO-O" thing a bit of a mute point.

Put such a toy in the hands of a pilot, much less a fighter pilot and he will, in short order, find another job for it..in this case as an "emergency overboost".

Problem is, that the pilot, in his "emergency", is "disinclined" to restrict throttle to less than full..so 18lb per sq in emergency also fries the engine.

To avoid this the technical boffins had a flash of genius....drill a couple of little holes in each cylinder head so that the maximum stays at 12 and the rest vents. .. EVERYBODY happy!!!!

I do know of one Squadron Leader who was quite keen on getting more power and didn't mind cooking his engine to do it...when they investigated how he had gone through so many engines they found that he had managed a flash of brilliance himself and found a system to beat the system . He banged a match into each of those little holes..neatly restoring the full 18psi. He was renouned for having the fastest but most often repaired/overhauled/replaced Hurricane in Fighter Command!

Darryl


Edited for general spelling and tidy up.

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Great explantion there Darryl! And a couple of nicely "turned" phrases as well

The stories are rather interesting as well.

Paul

Cheers Paul!

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

ME:
Thanks for the explanation, Darryl.
But let me see if I understand this in practical terms.

YOU:
Boost is the atmospheric pressure + or - the pressure created in the engine by the fuel system.
Zero boost is 1 "atmosphere" or roughly 30 inches of Mercury. (or 14 point "something" pounds per square inch)
As soon as the engine starts, the suction of fuel into the carby creates low pressure.
You will see at low pressure a "minus" boost figure..meaning that the current pressure in the system is less
than 1 atmosphere.

The more throttle the greater the pressure in the system, as more fuel is flowing through it at a higher rate.
Now the Merlin in the early Spits is capable of generating about 18psi which is more than the engine can safely handle without
self destructing due to overpressure and fuel detonation etc.

Enter the Boost Cut Out. The boost cut out ensures that anything over the specified maximum (6 1/4 lb per sq in with
Merlin III and 9 lb per sq in with the Merlin XII) is vented to the atmosphere..thus avoiding melt down.

ME:
So IOW, the Merlin XII (for instance) can not handle more than 9 pds. psi without being in danger of shutting down?

YOU:
This is achieved by a sprung piston arrangement. Unfortunately if this piston fails, the consequence is that potentially
ALL the pressure is vented and your Merlin becomes a large counterweight to the tail..and nothing else.

ME:
How will I know if this piston fails or is about to fail ?

YOU:
So, enter the ABCO-O or Automatic Boost Cut Out - Override, the red finger tab. If the BCO fails and
starves the engine of fuel, this override "sticks its finger in the Dyke" so to speak and gives back full pressure.
In an emergency the engine could then be nursed at low throttle back to base. If you gave it high (or full) throttle
then you would blow the whole thing up very quickly with the 18psi that had become "re-available", making the
whole "avoid engine failure by ABCO-O" thing a bit of a mute point.

To avoid this the technical boffins had a flash of genius....drill a couple of little holes in each cylinder head so
that the maximum stays at 12 and the rest vents. .. EVERYBODY happy!!!!

ME:
OK, but if they solved the problem this way, why is there a Boost Cut-off in the first place?
And as an adjunct to this question - since there is an ON/OFF BCO switch why or when
would I want to turn it off?

indeed +1

mos

Hmmm... interesting. It's even more complex than I initially thought and these answers raised equally many new questions than old ones were answered... but that more often than not is the case in technical and scientific topics.

The location of BCO-valve. Where is it?

A) If it was upstream of throttle plate, you couldn't achieve cut-out boost at any other throttle setting than full open. This is not the case, thus out-ruled possibility (unless there's a bug in modeling).
B) Between throttle plate and venturi, i.e between air limiting device and fuel administering device. Effects would be what we see in the sim.
C) After carburetor. Effect would be what we see in the sim but this would have the downside of BCO venting fuel-air mixture instead of just air, increasing fuel consumption and creating a potential fire hazard.

So, from my hunch, it would be (B). Is this correct?

If the BCO-valve is located downstream of throttle plate, (B) or (C), is there another overpressure valve upstream as well to prevent overpressure from damaging either the throttle plate or the supercharger itself
, if you have the engine running at 3000rpm and you close the throttle suddenly? Or are the components designed to withstand it, 18psi continuous, and potentially more than 18psi for a short period if the throttle is closed abruptly (airflow inertia, ramming effect) (which is something one should never do on any warbird - to make abrupt adjustments toward either direction)?

Then about Merlin becoming just a counterweight to the tail if BCO fails... It kinda made me wonder whether I made the right guess about BCO being located at (B), between throttle and carburettor venturi... because should the BCO fail, it would vent any pressure above ambient to leak out. You'd still have equal MP to ambient air, right? It's not just a counterweight then. Depending on altitude, it can be very little or quite adequate for level flight. You could be able to limp back home. And if venturi was downstream of BCO, it wouldn't even know whether BCO is open or not, or failer or functional, it just sees the airflow in venturi, and mixture should be OK as well. With BCO dead and IF BCO-O didn't exist, shouldn't it be able to still limp back home with severely reduced engine output? If BCO would fail, would it become bi-directional, thus giving you atmospheric pressure even with throttle plate closed, or would reversed airflow direction shut the valve even if the spring that normally closes it was snapped? Not being able to reduce power output below +0 psi would be much more hazardous than not being able to obtain more than +0 psi, especially considering the part when you have to land it.

To me, the tiny holes on cylinder heads seem a very odd solution for giving a second layer of overboost prevention. If they're just holes:
A) wouldn't during intake stroke it leak air into cylinder, screwing up the mixture? (Holes leak into both directions)
B) wouldn't during compression stroke air leak out of cylinder even when boost pressure was less than +12 psi? (Holes have no specific cut-off. They're always open.)
C) during combustion stroke, wouldn't exhaust leak out, reducing volumetric efficiency and thus engine power output? (Holes have timing, they don't know if it's intake air or exhaust.)

To me, it'd seem like they solved one problem (pilot being able to damage the engine, if the pilot is idiot enough to apply full throttle at low altitude) and replaced it with three problems that the pilot cannot fix even if he flew by the book. Or have I mistaken the location of the holes? Did they really drill them to cylinder head instead of intake manifold? But even in manifold, constantly open holes would constantly leak air in (resulting in lean mixture) or out (resulting in poor fuel economy). I'm just baffled because I fail to see brilliance in this solution. Would it have been impossible to fit another similar valve as BCO downstream of main BCO valve, just having a stronger spring that gives in at +12 instead of 6 or 9? Obviously, it's not absolutely failsafe (like drilling holes is) but like with magnetos, it's still a duplicate system and extra layer of protection. And you'd need a second BCO-O switch in case that other valve fails before the main valve does.

The main purpose for the cutout switch is to 1) allow the engine to be nursed home in case the cutout piston fails. and 2) to allow the pilots (who as combat pilots do) use it for something other than designed (to get extra power in combat). The holes were drilled to ensure that if the pilots did use the switch they wouldn't kill the engine, but they'd still be able to get extra power out of it and be able to still use it for its original purpose. A-BCO was intended as a last-resort thing for a failing engine, but it became a poor-man's WEP (which is actually more correctly ADI or "Anti-Detonate Injectant/Injection") allowing for extra power in case of emergency. When you operate the A-BCO, you'll still hurt the engine and wear it out at a faster rate, but it won't be catastrophic damage with the holes drilled in the throttle body like it would with the original setup.

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Whoa!! Re Whiic's post, I'm more confused than ever.

I'll be happy if Darryl or Whiic or anyone else could just answer the questions I raised with Darryl.

To wit:
1. So IOW, the Merlin XII (for instance) can not handle more than 9 pds. psi without being in danger of shutting down?

2. How will I know if this piston fails or is about to fail ?

3. OK, but if they solved the problem this way, why is there a Boost Cut-off in the first place?
And as an adjunct to this question - since there is an ON/OFF BCO switch why or when
would I want to turn it off?

4. And I have to now ask this further question; why is it idiotic to go full throttle at low altitudes?

I used to fly a Cessna 152 "back in the day" and I flew those quite well VFR and by "the seat of my pants". But all this
techno-speak escapes me since all I had to worry about back then was following a VOR to an airport and hope that the
weather didn't close in on me before I got there. The only attention I ever paid to the engine was whether or not to
apply Pitot or carb heat or not.

Jrazz,

It is easy to overcomplicate and overanalyise things!!! Put simply:

1. When it first appeared (and after a little refininement) the Merlin II/IIIs were using 87 Octane fuel. With this fuel severe detonation (or "pre-ignition if you like) would result if boost pressure rose above 6 1/4 lbs. per sq. in. The Merlin XII was stronger in many respects and could withstand 9 lbs. At above these values the damage caused by detonation would, in short order, be catostrophic, the engine would not shut down as such but literally shake itself to pieces. The actual size, shape, design of the fuel system "physically" allowed a Merlin of either type to generate 18lbs. So "unrestrained", the pilot could have literally destroyed the engine with the throttle. The Boost Cut Out stops this problem by venting any "extra" pressure.

Now when 100 Octane fuel was introduced THAT fuel was more resistant to detonation, so higher pressure (more fuel) could be induced without it pre-initing... more fuel to the cylinder, more power..ie more "Boost".
So now there was "wasted" capacity,.... the fuel could run at 12lbs (or above) but the system would only allow 9lbs in the Merlin XII (and, of course 6 1/4 in the III). BUT to access that power by overriding the BCO with the "red finger tab" risked 18lbs and that was still too much for the strength of the engines. The answer was simple and brilliant. Drill the holes (of a very specific diametre) to allow pressure above 12 to vent. The Merlin III's were strengthened a little as well. BUT the use of this still seriously degraded engine life...just not to the 30 seconds or so it would last at 18lbs.

2. The boost will fall off rapidly and or engine run rough as the pressure falls too low to sustain the cycle. When this happens and there is no other readily apparent cause (engine knocking, vibrations, oil leaks visible, oil pressure drop etc etc...) then failure of the piston could be expected. A piston will usually fail to "opening" not closing, so try the ABCO-O. If power is restored...then that is most likely the problem.


3. Simply because the power level it allows is far beyond that which gives a reasonable engine life. 12 lbs could result in as little as a two hour total engine life!!!

4. It isn't, necessarily. At low altitude the air is thicker, so more pressure (higher boost) can be achieved. At 20,000ft the engine simply will not deliver 12lbs boost, even if the ABCO-O is used. Above "Full Throttle Height" the power (boost) will reduce even at full throttle. So throwing the lever and using full throttle at 20,000ft does not result in any change to your boost achievable and does not damage the engine at all.
But at sea level, she will run at 12lbs if you do that and you'd better be watching the stopwatch and your gauges or it will be tears before teatime.


Yes, the Cessna 152 is to engine management what Lady Ga Ga is to good taste. The ONLY time it becomes necessary is in carby icing conditions or aerobatics. (but I suppose even Ga Ga displays taste at some time....)

Cheers

Darryl


Oh, Chris,yes, the holes were drilled in the cylinder head top-joint...of the BCO that is. D

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

"Shutting down"... it's a bad word to describe it. Merlin XII "can" and "cannot" handle over 9 psi. It can handle it for short period of time but it costs some durability. It will wear. It cannot handle it continuously.

It will not suddenly just "shut down". Engine failure is gradual and there's no safeguard that would shut the engine down to preserve it. A safeguard like that would be life-threatening. The Boost Cut-Out (BCO) merely open when 9 psi is reached. It opens to the extent needed, to give you 9 psi of manifold pressure. If there's much excess pressure, the valve will open more and release more. If the pressure would be only slightly above 9 psi, the valve is merely cracked. When boost drop below 9 psi, the valve shuts completely and leaks no air in any direction.

What was discussed on BCO valve failing was a rare failure scenario. It's not part of normal operation of the valve... at all. But Spitfire is still equipped to handle that rare failure with BCO override (BCO-O).


I don't even know if it's modeled in Accusim. It certainly isn't mentioned in maintenance hangar. That part of discussion was about real Spitfires... like was my post as well.

I wouldn't mind if some Core-Accusim update included boost cut-out valve failures. But considering there's some peripherals which (even if completely failed to red condition) don't affect anything, I'd like them fixed first. Supercharger and generator seem to be some of them. I've tried breaking and then fixing supercharger while flying at various altitudes but I don't see any improvement in my boost when I repair it. And a red condition generator, I get perfectly fine charge voltage and current without fixing one. I don't remember whether they broke with Accusim 1.1 update of whether they were always that static.

Accusim is not a real Spitfire but it's getting there. And development on Mark V means that systems on Mark I and II will probably improve as there's lots of shared modules.


Engine wear down faster with high power. Early production variants of Merlin were nondurable engines and wore out quickly.

A2A will soon(?) release P-40 Warhawk. That one has Allison V-1710 engine. It was capable of withstanding a lot more running hours, including operation at high boosts. It does have a supercharger inferior to Spitfire so it's a low-mid altitude fighter where as Spitfire is mid-high altitude.


That explains a lot. Unlike a car engine, or a Cessna for that matter, Spitfire has a supercharger. Most cars don't. Cessna doesn't either.

When you apply full throttle on a freely aspirating Lycoming O-235, the manifold pressure is +0 psi (relative to standard air pressure) on sea level. When you do the same with Spitfire with BCO turned off, you get +12 psi. That's almost two times the pressure, all thanks to the supercharger Spitfire has but your Cessna was missing. And if almost twice the pressure is not enough, Spitfire has 27 liters of displacement, Cessna has less than 4. That's 7 times more displacement and a bit less than twice the pressure, meaning more than ten times the oxygen and fuel forced to the cylinders per engine revolution. O-235 rotates at 2800rpm, Merlin at 3000rpm, which are roughly same. Doing the math Spitfire uses about dozen times as much fuel and produces ten times as much horsepower and waste heat. Ok, assuming a lot about them having the same efficiency...

Now, lets check the horsepower ratings instead of guesstimating them from pressures, rpm and displacement: Lycoming O-235 produces around 115 horses depending on variant. Merlin XII produces 1,150 horsepower. Ten times more. It's obviously not quite the same engine as what's installed on Cessna. O-235 is an air-cooled engine. Being air cooled, it can a lot hotter than a watercooled engine. Basically it can be run as hot as the engine metal can handle without becoming brittle. Aluminum alloys start having issues at upward from 250 deg C of cylinder head temperature but staying at low 200's may be perfectly safe. Cessna is more than capable of keeping itself well below critical CHT with full throttle. Merlin has to keep itself below 120 deg C, and despite having ten times the amount of heat to dissipate, it has a tiny box under it's right wing. That's the cooler.

So, drive it 3000rpm +12psi and it will overheat at any altitude. It's just ridiculous amounts of heat produced. And even if you used it multiple times for short durations preventing it from having time to overheat, the high rpm takes it's toll on camshafts, bearings, etc. and +12 psi takes it's toll on piston rings, cylinder walls, creating scores, etc. This, even without overheat. While +12psi is obviously more straining than +0psi, 3000rpm on a 27-liter engine is definitely more straining than 2800rpm on a small engine that would fin in a car. Merlin doesn't. Bigger engine => bigger components => even with same rpm, speeds are much greater thus is the wear.

If you want to fly Spitfire "like a Cessna", you would never exceed +0 psi, even at low altitude. Running it like that, and using a more sensible rpm suitable for lower boost (like 2000...2400rpm), that big Merlin will last a long loooong time before overhauls (almost like a Cessna). And that's how people fly old warbirds these days. Replacement engines cost a fortune. There's no war, and there's no War Ministry funding the engine replacement or overhaul.

PS. Remember when I said Merlin doesn't fit into a car? I LIED.
http://www.youtube.com/watch?feature=pl ... 5aZg#t=44s

PPS. Forgot to hit "Submit". Killratio managed to answer some of the questions before me.

Well, thanks for the replies, everyone. Although my eyes glazed over occasionally when I couldn't quite wrap my mind
over the technical stuff; all in all I 'sort' of learned a bit about the Merlin.

Okay and now, FWIW, I finally made a successful flight in the spit; to the point of making an almost 'kiss' landing.
If I recall correctly, my cruise settings were around 200 True airspeed, the prop was around 2200 rpm, and I had the Boost Cut-off OFF,
I landed at about 150 True airspeed with no flaps -- since it seems to me the flaps make the Spit harder to land instead of easier--
and I simply kept the spit more or less in a three point attitude and let it settle down by itself until it kissed the runway.
I know I didn't do everything by the book, but the remarkable part is that when I took it to the shop, I had ZERO damage
and like the saying goes, you can't argue with success.

Jrazz,

Just remember that the flaps are actually more "airbrakes" and treat them accordingly. Lower than as you are arching around onto final, as a last step in the slow down. I rarely have them out much before the final straighten to go "over the fence". At a guess they would not be down for more than 20 seconds total (from out, land, check directional control, to raise.)

Final thoughts on BCO.
Non technically, think of the engine and boost cut out as a balloon with a hole in both ends, that you are holding shut one end while someone blows it up the other. They keep blowing and you open and close the hole enough to stop the balloon bursting.. but enough that it can stay inflated. If you let go and leave the hole there..they can't blow it up. If you keep the hole closed altogether, sooner or later the balloon will burst...YOU are the BCO. When I come up behind you with a lump of wood and knock you out..YOU just had a BCO failure. Luckily your friend has some duct tape and jams it over the hole..HE is the ABCO-O but NOW the "blower" must be careful not blow too much or the balloon will burst.... and Robert is your Mother's Brother. (and Lady Ga Ga is still a ...oh,... never mind.....)


Darryl

_________________

...Some say he never blinks, and that he roams around the woods at night foraging for Merlin parts...

Non-standard landing speeds work with medium sized and bigger modern, paved airports. Spitfire was designed to operate from much smaller field. And I mean "field" literally: a place where grass grows. If you try some of the authentic fields, like RAF Digby
screenshot
airfield information and download (freeware)
...you'd get a better idea of what it was like taking off and landing a Spitfire.

Proper landing speed is 65-70 mph. Not 150 mph. And yes, flaps do make it slightly instable, especially closer to stalling (landing speed is on the stall threshold). It'll warn you before it stalls completely, by shaking quite visibly/audibly.

[Proper landing speed is 65-70 mph. Not 150 mph. And yes, flaps do make it slightly unstable, especially closer to
stalling (landing speed is on the stall threshold). It'll warn you before it stalls completely, by shaking quite visibly/audibly.]

Yes, I did know that, but I'm not quite good enough with simulation landings to attempt one close to stalling speed.

The thing is that -as far as I'm concerned anyway - landings in real life were easier. The reason was that I was able to see things in 3-D
and therefore had a good idea when to flare out. Landing in a sim is like landing in real life with one eye closed.