trying to figure out the "ata" gauge

[bDAWg]

hi all
could someone provide a quick and simple explanation of what the ata gauge in the Bf-109 is actually indicating?...is it manifold pressure or boost or are the 2 the same thing?

I've got a pretty good handle on flying and fighting and not blowing any gaskets.etc by controlling the rpm's and by keeping an eye on the oil and water temp...I now would like to always be able to put the engine into it's power band as efficiently as possible and no doubt that's the reason for having the ata gauge....

but I'm lacking the knowledge of what manifold pressure/boost is and how it affects the engines performance...is this something to do with turbo chargers?

[MadBlaster]

Maybe this. The intake manifold is a fixed volume/space. When you increase the throttle in the cockpit, you are mechanically increasing the fuel/air mixture relative to that volume/space of the intake manifold (engine vacume sucks in the air/fuel mix). There must have been a pressure sensor at the intake manifold that reads the fuel/air pressure and feeds the ata gauge its reading.

More air/fuel makes engine cycle faster and must be offset by increased load to keep from over-rpms. Too much fuel/air when the engine is under heavy load means there isn't enough vacume (due to low rpms) to handle the increased fuel/air and the engine lugs.

[bDAWg]

got it ...thanks for the easy to understand explanation...now I'll have an idea of what the figures on the guage represent.

and is this also referred to as boost in other AC?

[Kurfurst]

Originally Posted by bDAWg

hi all
could someone provide a quick and simple explanation of what the ata gauge in the Bf-109 is actually indicating?...is it manifold pressure or boost or are the 2 the same thing?

Its the boost/manifold pressure (same thing) gauge, expressed in atmospheres. 1 ata (= 1 kg / sq.centimeter) is equivalant to 0 lbs/sq.inch, ie. normal ambient air pressure (German boost gauges measure absolute pressure, British ones relative under/overpressure compared to normal pressure.

IIRC 1.7 ata boost (max. boost of later war DB engines) was equivalant to +7 lbs sq. inch, but our DB 601A engine has max. limit of 1.35ata (1.45ata for 1-min WEP), and a corresponding RPM of 2400 (3000/min allowed for 30 secs).

[Viper2000]

*edit*Curses! Kurfurst beat me to it! I'll leave the comic in anyway...*/edit*



Boost and manifold pressure are the same thing; a measure of the pressure in the intake manifold (ie the ducting between the supercharger and the cylinders).

The British convention was to measure this pressure in psi gauge and call it boost; zero boost means that the pressure in the intake manifold is nominally atmospheric (14.7 psi absolute), +12 means atmospheric pressure + 12 psi (i.e. 24.7 psi absolute) and so on. So this system works rather like the pressure gauge on a bicycle pump (except that the scale goes all the way down to -14.7 psi, which would be a perfect vacuum).

Almost everybody else decided to measure manifold pressure in absolute units, such that zero would be a perfect vacuum.

The Americans use inches of mercury; the Germans use ata.

The ata is a rather annoying little unit, because it's not quite what you'd expect.

A standard atmosphere is 101325 Pa
A bar is 100000 Pa
An ata is 98066.5 Pa (ie 1 kgf/cm^2).

So when your engine is switched off on a standard day at sea level you'd expect to see a manifold pressure of:

101325 Pa = 0 psi boost = 29.92"Hg = 1.0332 ata.

So you can see how lightly boosted the German engines were: 1.42 ata is only 20.2 psi absolute or about +5.5 psi boost.

Why do we care?
Too much pressure in the intake manifold means too much pressure in the cylinders, which will cause problems for the engine.

In particular, detonation. High pressure and long combustion chamber residence times increase the probability of detonation, so you can only get away with so much manifold pressure at any given rpm; you can get away with higher boost at higher rpm, but of course there's an rpm limit too...

Also, with a constant speed prop the rpm we get is the rpm we asked the CSU to give us, and this therefore doesn't tell us anything about how much power the engine is producing.

Since the power production is basically a function of the engine's air consumption, it will vary roughly in proportion to engine rpm * air density in the intake manifold.

Since we can't measure the density of the air in the intake manifold directly, pressure is a pretty good substitute because we know that

P*V = roh*R*T

All this stuff about engines needing vacuum to run just isn't true. The engine wants as much air as possible. It only creates a depression in the intake manifold when we throttle it. It can't get as much air as it wants to, and so it sucks down the pressure in the intake manifold.

But if you open the throttle of a naturally aspirated engine then the pressure in the intake manifold will rise to (almost) ambient pressure.

If the engine is supercharged, the throttle is almost always upstream of the supercharger.

An early Merlin has a supercharger pressure ratio of about 2.7 at 3000 rpm. So if the engine ran unthrottled at sea level then the manifold pressure would be about 2.7*14.7 = 39.7 psi, which is +25 psi boost. This would cause rapid engine failure due to detonation, and even if you put 150 grade fuel into it, an early Merlin just wouldn't be physically strong enough to stand up to the cylinder pressures produced for more than about 15 minutes or so (though for that glorious 15 minutes it would put out over 2000 bhp).

[bDAWg]

so the ata gauge is actually used for a few reasons.....for keeping the intake manifold pressure to the values specified in the pilot's notes for peak performance during the different work loads ...and also just as importantly it's used to prevent engine failure due to detonation....or what I've always referred to as "knocking"

great info guys...you know I googled about this earlier today and was unable to find anything about this subject...this will probably help out quite a few of us who are new to this somewhat more demanding CEM.

[Babi]

many thanks for the explanation.

One question: how do we control the manifold pressure/boost? only with the throttle? meaning that if we see the pressure is over the limit for a specific rpm, we have to throttle back? nothing more?

Also, if with increasing altitude the pressure drops, this means we will be able to open the throttle more?

(btw is detonation modelled in the sim? how do i recognize it?)

[Babi]

Originally Posted by bDAWg

.this will probably help out quite a few of us who are new to this somewhat more demanding CEM.

Yep, and a sticky thread would be a good idea too.

[Babi]

did a little testing on detonation with the spit mkIIa.

sea level, throttle to max, boost cut out (the instrument was reading +9, but that's the max), pitch to a minimum (1700 rpm). the plane flew on for 20 minutes. no damage. I thought this was the worst possible scenario for detonation (max pressure, min rpm).

did the same thing with max rpm (3100 rpm) this time intermittent black smoke appeared, (that would go away if i put boost level back to +6), but again no damage. after another 15 minutesi grew tired and closed the radiator, which led to "perforated water radiator" in a few seconds.

what am i doing wrong? i thought the engine were much easier to damage.

[MadBlaster]

Likely the spit's constant speed unit won't allow you to do as much damage with the governor in place, verses 109 where you can directly control the load (in manual prop mode) and screw it up really fast. In other words, they probably designed the CSU as much as possible to mitigate the extreme situations on the engine (lug and over-speed).