Please explain ATA.

So as a noob that I am in aircraft or general engines I have a question that is still coming to my mind. The Bf 109 or Fw 190 with such powerfull engines, one would think that the pressure inside would be much greater. Let's say 1,2 ata - thats only 1,2 atmosphere? I have more pressure in my car wheels. Am I wrong in this? Please someone explain.

ATA is pressure inside intake, not cylinders. Intake pressure in the range of one to two atmosphere is quite normal for late WWII fighters. For comparison one can take a typical sports car, e.g. 1980 Porsche Turbo, which is 1.77 ATA manifold pressure.

DB601 with 6.74:1 compression ratio means without ignition internal cylinder pressure is 6.74x higher than intake. During combustion perhaps cylinder pressure is 100 atmospheres.

DB601 with 6.74:1 compression ratio means without ignition internal cylinder pressure is 6.74x higher than intake

Not quite! Be careful - compression ratio isn't the same thing as pressure ratio. In your example, with theoretical, adiabatic compresion, pressure would rise about 13-ish times. In real life, given he scavenging losses, it would probably drop below 10, but would still not be the same.

@ Pruciak - we don't know how much you know about piston engines in general (car or an airplane, doesn't matter, general principles are the same), but I'd recommend these articles, which explain both normally aspirated and charged aero engines nicely:

https://www.avweb.com/features_old/pelicans-perch-15manifold-pressure-sucks/

https://www.avweb.com/features_old/pelicans-perch-31those-fire-breathing-turbos-part-1/

https://www.avweb.com/features_old/pelicans-perch-32those-fire-breathing-turbos-part-2/

Actually, I would recommend remainig articles of that guy (about mixtures, props etc.) as well

In engines with compression ration 10.5:1 you would get about 14 bars on gauge while testing compression pressure.

Compression pressure is showing overall state of the engine, how good valve slots are, how good piston rings and cylinder liners are. Due to low crank speed while testing, any significant pressure leaks will impact peak compression pressure. IF you get lower then 10 bars in 10.5:1, engine is pretty much dead.

ATA is pressure measured from vacuum. So when ATA gauge is showing 1.2 ATA this mean that in intake manifold pressure is 0.2 atm above 1atm

In plane's engines the main thing is, to what alt engine can maintain this pressure.

So at SL at full military power ATA shows 1.46. This mean that engine boost = 0.46 bar.

But at 6100 m when ATA shows 1.46. Actual engine boost = 1 bar, When you engaged mw50 you will get 1.8 ATA, engine boost will be 1.34 atm

For displacements 30-35 liters ish, this boost is quite big.

I can say that tuned V-1650-7 (engine from P-51) can run MP 120"-150" this will translate to 4-5 ATA.

So as a noob that I am in aircraft or general engines I have a question that is still coming to my mind. The Bf 109 or Fw 190 with such powerfull engines, one would think that the pressure inside would be much greater. Let's say 1,2 ata - thats only 1,2 atmosphere? I have more pressure in my car wheels. Am I wrong in this? Please someone explain.

you have about 3.5 ATA in your tires.

Thx guys, will read the articles, thank you!

Why compression pressure is higher then compression ratio.

Because compression ratio account to decrease volume charge/air in cylinder.

so with comp ratio 6:1 if you had initial pressure 1 atm that would end up with peak pressure of 6atm. But it is higher. Air while compressing is heating up and this alone is increasing pressure too. Now combine this 2 effects and you have compression pressure which is in most cases higher then com ratio.

Extending this subject a little bit.

every piston driven plane will contain two main engine gauges

One is Intake Manifold Pressure(MP/ATA/Boost) and rpm gauge, Combination of MP and RPM is main thing which determine power developed by engine. And allowing pilot to read out engine power.

Bf 109 with automatic prop speed unit is probably one of the easiest to operate, closely followed by Fw190.

In spitfire and P-51 you have separate control levers for MP and RPM, this makes possibility of engine damage by miss managing power settings.

In spitfire 0 boost = 1 ata approx?

In spitfire 0 boost = 1 ata approx?

Yes

A jet engine takes air in the front, compresses it to give more explosive force, add fuel to it, ignites it and the resulting explosive force drives the aircraft forward and also turns the compressor.

A supercharged or turbo-supercharged engine is, essentially, jet engine with a reciprocating engine instead of a burner section. The piston engine either directly drives the supercharger or the exhaust gases drive the turbo charger. The turbo/supercharger compresses the intake air.

The intake manifold pressure is an easy method of measuring the explosive potential of the air being used by the cylinders, a critical piece of information.

Normally aspirated (no turbo or supercharger) engines always have an intake manifold pressure that is less than 1 atmosphere even at full throttle.

While 2 atmospheres or 70 inches of manifold pressure does not sound like a tremendous amount at about 28-32 psi, it is a tremendous amount of explosive energy.

And there is also an incredible amount of explosive energy stored in an inflated tire. The 52 psi in your car tire, were it to be charged with vaporized aviation gasoline and ignited, would result in an explosion of tremendous volume and force.

Un-ignited air in a tire failure carries tremendous force.

2 atm is about 60" of mercury not 70"

1013.25hPa = 29.92" of mercury standard atm

But at 6100 m when ATA shows 1.46. Actual engine boost = 1 bar, When you engaged mw50 you will get 1.8 ATA, engine boost will be 1.34 atm

I don't quite get that one. Where in the whole intake system is the pressure sensor for the ATA gauge located and why would pressure on the outlet of supercharger be that different from pressure in the manifold if there is no wastegate inbetween (or is there?)? Yo-Yo?

2 atm is about 60" of mercury not 70"

1013.25hPa = 29.92" of mercury standard atm

I wasn't actually implying that it was but I wasn't clear. I was using examples of maximum boost levels seen in aircraft from each system of measurement.

I don't quite get that one. Where in the whole intake system is the pressure sensor for the ATA gauge located and why would pressure on the outlet of supercharger be that different from pressure in the manifold if there is no wastegate inbetween (or is there?)? Yo-Yo?

Typical concept behind boost is to measure excessive pressure above ambient pressure, Boost gauge in spitfire will not account the ambient pressure drop so even boost gauge in spitfire shows 0 lbs boost at high alt in intake manifold pressure will be higher then the outside pressure.

So at SL where ambient pressure is 1atm ~ 30inHg. Supercharger in DB605 is boosting 0.46atm ~ 13.8inHg, so 1atm + 0.46atm boost = 1.46 ATA = 43.8inHg (take note that spitfire can reach 67inHg = 2.2ATA wich is insane :))

ATA is absolute pressure in side intake manifold.

But at 6100m where ambient pressure is 0.46 Atm ~ 13.8inHg, In order to have 1.46ATA boost have to be much higher(1atm), so 0.46Atm + 1atm boost = 1.46ATA = 43.8inHg

Other way

On the ground SL when engine is off ATA gauge will show about 1.0, so when you apply full power supercharger has to boost only 0.46 ata to hit 1.46ata

but if plane would be parked at airfield at 6100 m elevation, ata guage would show 0.46ATA when engine is off. So at Military power supercharger would have to increase intake pressure from 0.46ATA to 1.46ATA = 1ATA

For me Boost = Intake pressure - Ambient pressure.

Supercharger out let pressure will be almost the same as intake manifold pressure, There are loses in housing between supercharger and the engine intake so Supercharger outlet will have always the highest pressure. I think DB605 has different compression ratio on left and right bank cylinders to account inequality of cylinder feed, think because the location of supercharger outlet.

I wasn't actually implying that it was but I wasn't clear. I was using examples of maximum boost levels seen in aircraft from each system of measurement.

Oh, you are right, sorry then.

No, it's incorrect. Very wide-spread mistake. English system of boost is referred to SURFACE STANDARD PRESSURE. Generally the English gauge measures the same ABSOLUTE pressure but the scale shows it minus standard pressure.

And, by the way, 1 ata is not equal to 760 mm of Hg (101325 Pa) because it is 1 kG/cm2.

I know that, but 1kg/cm2 is close to 101325 Pa, something like 98000 Pa.

But my point was not to calculate units but to show that actual boost will increase with increase of alt in supercharged planes.

That at high alt pressure difference between intake manifold and outside pressure will be greater then at SL at same power setting

Can you tell me what i said wrong about spit's boost gauge ? I said that spit's boost gauge will not change reference pressure. So this gauge will shot correct number only at SL when you set +4lbs than you know that pressure in intake manifold is roughly 4lbs higher then outside pressure. But at 20000 ft this scale will not provide obvious boost level. Its preaty much same measure as ATA or MP but with different scale. Set 0 as 1atm

So as a noob that I am in aircraft or general engines I have a question that is still coming to my mind. The Bf 109 or Fw 190 with such powerfull engines, one would think that the pressure inside would be much greater. Let's say 1,2 ata - thats only 1,2 atmosphere? I have more pressure in my car wheels. Am I wrong in this? Please someone explain.

The gauge that says ATA measures intake manifold pressure. It's the pressure of the air-fuel mixture entering the engine, after the supercharger, which means it can be higher than atmospheric because the air is compressed by the supercharger ("blower") to increase engine power.

ATA means Atmosphere, Absolute. 1.0 ATA means the air pressure in the intake is exactly atmospheric, which is roughly 14.7 PSI if you're at ground level. However, the "absolute" part means the gauge measures pressure relative to an internal standard, not the outside atmosphere. So, if you're at 20,000 feet altitude, the outside air pressure is only about *half* an atmosphere, and the engine's supercharger has to pressurize the air to develop 1.0 ATA manifold pressure. If you're on the ground, close to sea level, the supercharger doesn't have to develop any pressure rise to produce 1.0 ATA at the intake manifold.

The Bf-109 engine runs at a max of 1.8 ATA, which is roughly 12 PSI boost pressure ("boost" means above outside-atmosphere intake pressure) if you're at sea level.

The Fw-190 run about 1.9 ATA maximum, or just over 13 PSI at sea level.

As altitude increases, the boost pressure (intake air pressure above outside atmosphere) must increase to maintain the same total, or absolute, intake manifold pressure because the outside atmospheric pressure drops with altitude. The supercharger absorbs more engine power to develop this higher pressure ratio, which is why engine power drops off as altitude increases, even though the manifold absolute pressure (ATA) stays about the same.

That's why they went through all the trouble to develop expensive & finicky exhaust-driven turbochargers, to get around the problem of absorbing more and more engine power to drive the supercharger as altitude increases.

AD

In spitfire 0 boost = 1 ata approx?

That's right--it's about 14.7 PSI, or ~30 inches of mercury, but only if you're at sea level. If you're at 20,000 feet (roughly 6,000 meters), the air pressure outside is only about half of what it is at sea level, so the gauge reading zero would actually be about 7.5 to 8 PSI boost, even though the gauge says zero boost.

If that sounds weird, the Spitfire "boost" gauge actually shows manifold Absolute pressure, not just boost. So, it's only accurate at sea level. At higher altitudes, the manifold pressure regulator increases boost to maintain a constant manifold pressure, and the actual boost pressure produced by the supercharger is higher than what it reads on the gauge.

18 PSI boost on the gauge is actually 18 PSI boost (above outside atmospheric pressure, ~14.7 PSI) if you're at sea level.

At 20,000 feet, the outside atmospheric pressure is only about 7 PSI, so if the gauge shows 18 PSI, the boost pressure is actually 25-26 PSI.

All of the other 3 warbirds have manifold pressure gauges that show Manifold Absolute Pressure, so they're accurate at any altitude. But, the Spitfire's "boost" gauge is only accurate at sea level, unless you consider it to say "Manifold Absolute Pressure Referenced to Sea-Level Atmospheric", and not "Boost" :)

AD

I think all warbirds used absolute MP gauges, only British + their colonies called them boost gauges but not 100% sure.

Japan used cm of mercury same as Russia iirc.

What about, France,Spain,Italy planes what units were used there?