Normally Aspirated High-Altitude Engines?

[elbmc1969]

How do you optimize the design of a normally-aspirated engine for high altitude?

(I really should know this ...)

 

[ThomasP]

Large(er) displacement, high cylinder compression, fuel injection (either direct or via the carburetter), relatively long air intake tube (to help decrease surging effects and increase RAM effects), low(er) rpm to a degree (everything else being equal) to allow more efficient cylinder charge flow/scavenging, and individual cylinder or individual exhaust port jet exhaust manifolds (or possibly a 'tuned' set-up) for the jet thrust provided and more efficient scavenging. Coil pack type ignition to prevent arcing if operating altitudes are above ~25,000 ft.

Note that the above only includes things available pre-space age.

It will also depend a bit on what altitude and speed you are hoping to achieve - eg high altitude sail plane or multi-altitude fighter, etc.

 

[elbmc1969]

I read somewhere ("The First Air War"?) that during WWI the British and French built for high altitude performance, while the Germans built for maximum power. Or maybe it was inter-war? No idea if it's true.

Anyhow, no jet exhaust manifolds at the time.

 

[Shortround6]

I think that is backwards ?

The Germans built at least one engine that used high compression for it's time. So high that you could not use full throttle at ground level without blowing up the engine.
The throttle assembly had 3 gates. One for low level, one for medium height and the last for high. As the pilot moved the lever from gate to gate the lever could be pushed further forward to open the throttle more. So you didn't get full throttle until you were at high (for 1917-18) altitude.
the engine was large and heavy if you want to fly at low altitude.

 

[elbmc1969]

Looks like I did have it backwards, at least to a degree. The D.VIIF had the BMW IIIa, with many of the features described above. Wiki says:

"The engine was successful, but the real breakthrough came in 1917, when Friz integrated a basically simple throttle butterfly into the twin-barrel "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane fuel of gasoline blended with benzole, the carburettor adjusted the richness of the fuel-air mixture according to the aircraft's altitude. It enabled the engine, now dubbed BMW IIIa, to develop a constant 200 horsepower (150 kW) up to an altitude of 2000 meters – a decisive advantage over competitors' engines."

There was also the Maybach Mb.IVa:

"It was tested on Wendelstein (mountain)[2] at an altitude of 1800 m and rated there at 245 hp.[3] This would theoretically correspond to rating of about 300 hp at sea level; however, the engine was not designed to withstand such power[4] - it needed to be carefully throttled down at low altitude, so it would not exceed the safe level of 245 hp. It had three carburettor settings, to be changed during the flight depending on the altitude."

So not throttle gates?

And in the references for the Maybach article, possibly the most German book title ever:

Kyrill von Gersdorff; Kurt Grasmann; Karl Prestel; Helmut Schubert (1985). Flugmotoren und Strahltriebwerke : Entwicklungsgeschichte der deutschen Luftfahrtantriebe von den Anfängen bis zu den internationalen Gemeinschaftsentwicklungen (in German) (2. erg. und erw. Aufl. ed.). Koblenz: Bernard & Graefe. p. 26. ISBN 3-7637-5283-8.

 

[Bretoal2]

Looks like I did have it backwards, at least to a degree. The D.VIIF had the BMW IIIa, with many of the features described above. Wiki says:

"The engine was successful, but the real breakthrough came in 1917, when Friz integrated a basically simple throttle butterfly into the twin-barrel "high-altitude carburettor", enabling the engine to develop its full power high above the ground. Burning a special high octane fuel of gasoline blended with benzole, the carburettor adjusted the richness of the fuel-air mixture according to the aircraft's altitude. It enabled the engine, now dubbed BMW IIIa, to develop a constant 200 horsepower (150 kW) up to an altitude of 2000 meters – a decisive advantage over competitors' engines."

There was also the Maybach Mb.IVa:

"It was tested on Wendelstein (mountain)[2] at an altitude of 1800 m and rated there at 245 hp.[3] This would theoretically correspond to rating of about 300 hp at sea level; however, the engine was not designed to withstand such power[4] - it needed to be carefully throttled down at low altitude, so it would not exceed the safe level of 245 hp. It had three carburettor settings, to be changed during the flight depending on the altitude."

So not throttle gates?

And in the references for the Maybach article, possibly the most German book title ever:

Kyrill von Gersdorff; Kurt Grasmann; Karl Prestel; Helmut Schubert (1985). Flugmotoren und Strahltriebwerke : Entwicklungsgeschichte der deutschen Luftfahrtantriebe von den Anfängen bis zu den internationalen Gemeinschaftsentwicklungen (in German) (2. erg. und erw. Aufl. ed.). Koblenz: Bernard & Graefe. p. 26. ISBN 3-7637-5283-8.

The process was also known in France, where Hispano-Suiza produced some models called "surcomprimés" (overcompressed) - and maybe other motorists. The principle was to build an engine with a much higher compression ratio than standard, and to prevent it from operating at full intake near the ground (otherwise = detonation...).

With increasing altitude, atmospheric pressure decreases and pilot could gradually open the throttle a little more step by step, until a point where the full opening combined with the reduction in ambient pressure (at 2,000 m: approximately 800 hPa, i.e. -20% from ground, at 3,000 m: about 700 hPa, i.e. -30%) gives an EFFECTIVE compression ratio sustainable, owing to the low octane gasolines from these times.

It was therefore necessary to design for these engines some special carburettors, that could more or less automatically limit throttle opening at low altitude, and correct the air/fuel ratio according to the density of the air.

Supercharged engines have the same problems : they must be "throttled" below rated altitude.

 

[thunderbird]

You can’t. All naturally aspirated engines produce more power the denser the air is. increasing altitude always lowers the intake manifold pressure, in a way similar to closing the throttle at a given altitude pressure. its like saying, how do i get a N/A engine to produce peak power at a fifty percent throttle position.

Saying that, one could design an engine so that its intake pressure is limited to say, 7 psi, or so. And by some means, mechanical, or electronically, limit the intake manifold pressure to a max 7 psi. That would alow the engine to produce constant power up to the altitude that is equivalent to the maximum inta manifold pressure. The engine would still be as heavy as an engine designed to use full atmospheric pressure at sea level, and the unavailable power would be as useless to a pilot as runway behind him or sky above him.

 

[Simon Thomas]

Lumsden "British Piston Aero-Engines and Their Aircraft", p96

 

[elbmc1969]

Lumsden "British Piston Aero-Engines and Their Aircraft", p96

Looks like the Jupiter VI is what S Shortround6 was thinking of with the throttle gates. The earlier German engines had carburettor settings or other carburettor manipulation.

Thanks to all who contributed.

 

[AMCKen]

You can’t. All naturally aspirated engines produce more power the denser the air is. increasing altitude always lowers the intake manifold pressure, in a way similar to closing the throttle at a given altitude pressure. its like saying, how do i get a N/A engine to produce peak power at a fifty percent throttle position.

Saying that, one could design an engine so that its intake pressure is limited to say, 7 psi, or so. And by some means, mechanical, or electronically, limit the intake manifold pressure to a max 7 psi. That would alow the engine to produce constant power up to the altitude that is equivalent to the maximum inta manifold pressure. The engine would still be as heavy as an engine designed to use full atmospheric pressure at sea level, and the unavailable power would be as useless to a pilot as runway behind him or sky above him.

Unless you build one that has, say, 16:1 compression ratio. You can't use full throttle (or even half) at lower altitudes, but at higher altitude where the air pressure is half or less of sea level, it would act like an 8:1 ratio. ?

 

[elbmc1969]

Could people PLEASE read previous posts before replying? It's really counterproductive to have replies that completely ignore concrete examples that were posted a few messages before.