High altitude vs. low altitude

[Shortround6]

Not true the Rolls Royce R was running 21 lb/in2 from a single stage supercharger in 1931.

Actually it is perfectly true. For altitude performance you have to compare the ambient air (outside) to the manifold pressure.

Sea level air pressure is 14.7 lb/in2 so a supercharger with a pressure of 2.43 will handle it, perhaps a bit more for flying at 1-2,000ft.

Now at 16,000ft the air pressure is about 7.97 lb/in2 so to get 6lb boostin the intake manifold (20.7 total) you need a compressor that will deliver a 2.59-2.6 pressure ratio.

The Merlin 61 gave 12lb of boost at 23,500ft (26.7lb /in2 total pressure) but the air at 23,500ft is about 5.82 lb/in sq. so you need a pressure ratio of about 4.59:1 . Rolls claimed 5.1:1, they may have used a different pressure chart?

NO single stage compressor could reach much over 4:1 pressure ratio with any sort of acceptable efficiency and most were around 3:1.

 

[GregP]

In 1931 Rolls Royce was in the middle of the Schnieder Cup series and was running the "R" engine. It does NOT surprise me that they could get 21 psi of boost. They got an R to over 2,000 HP at that time and ran the S.6B to 440 mph with the floats attached.

But they were NOT flying at 21 psi operationally in military service ... it was for racing only and very short-lived at that.

 

[wuzak]

In 1931 Rolls Royce was in the middle of the Schnieder Cup series and was running the "R" engine. It does NOT surprise me that they could get 21 psi of boost. They got an R to over 2,000 HP at that time and ran the S.6B to 440 mph with the floats attached.

But they were NOT flying at 21 psi operationally in military service ... it was for racing only and very short-lived at that.

The S6B's record was 407mph on average. It was the Macchi MC72 that managed 440mph.

Merlins and Griffons of the late war period could run +21psi boost, or more, using PN150 fuels.

 

[GregP]

I meant they weren't running 21 psi in active military service during the Schneider Cup years, and they weren't.

I know the boost they were making during WWII, but you know that.

 

[wuzak]

I meant they weren't running 21 psi in active military service during the Schneider Cup years, and they weren't.

I know the boost they were making during WWII, but you know that.

Teach me to not look at the context!

In terms of boost levels run during the Schneider Trophy years I would think it would be not much boost, if any at all. I think late Kestrels, ie after the Schneider Tropy, got up to +6psi. Early Kestrels didn't have superchargers at all.

 

[rinkol]

Also, for racing applications, they would have been prepared to accept a loss in supercharger efficiency as long as they could obtain increased power even if this required special fuels that would not have been acceptable for operational use.

 

[Shortround6]

The Schneider Cup engines were most defiantly using special racing fuels, Usually developed by Rod Banks. Normal AV-gas of the time would have destroyed the engines in short order (seconds) at 4-6lbs of boost.

 

[GregP]

The cutting edge of piston engines was a tough place to be when things went wrong. It was probably tough to find out what failed when most of what is left looks like scrap metal, and that assumes you get it back in whatever pieces it was in.

The "R" led to the Merlin and that was a good thing.

I am still amazed the Germans came up with teh DB 600 / 601 series, the British the Merlin, and the Americans the Allison and ALL came up with aircraft close in performance. The Soviets came up with an improved Hispano-Suiza that never made much power and ALSO came up with planes that could operate in Siberia to very good performance levels.

The Japanese got 350 mph speeds from what was basically an 1,130 HP radial.

So ... it seems everyone was fairly inventive.

Amazing.

 

[Reluctant Poster]

Attached is an interesting paper on the Merlin that includes a discussion on supercharging. Figure 11 shows that although the single stage Merlin 50 could achieve a ratio of 5 to 1, the efficiency fell to 60% as compared to close to 70% for a Merlin 61.

 

[Venturi]

All av-gas had about 18700 BTUs per gallon. The higher octane fuel resisted detonation better allowing for more pressure in supercharging and/or higher compression in the cylinder.

To use Tomo's illustration the Merlin used 2.25 liter cylinders so at best without a supercharger you could get 2.25 liters of fuel/air mixture to burn every two engine revolutions (and only a few racing car/motorcycles get 100% filling of the cylinder/s).

A supercharged engine using 6lbs of boost (normal air pressure being 14.7lb) can get 40% more fuel/air into the cylinder. An engine using 12lbs of boost can get 30% more fuel/air into the cylinder than the 6lb boost engine can. The problem comes in with th efact that trying to use 12lbs boost will heat the fuel/air in the supercharger ( very few, if any, engines had fuel detonate inside the supercharger due to heat) is when the hot mixture is compressed in the cylinder by the rising piston and the temperature rises ( and you have the hot cylinder parts, like exhaust valve/s) the mixture can start burning before the spark plugs fire OR the entire cylinder full of mixture flashes (ignites) at once, Normally it takes about 40 degrees of crankshaft rotation to burn the mixture depending on engine. Spark plugs fire at around 20 degrees before top dead center and burning is pretty much done at around 20 degrees after top dead center. The Flame front advances across the cylinder (piston top). Now the advancing flame front also exerts a pressure wave that further compresses the fuel/air mixture and even if the remaining mixture (last 1/2 say) flashes (detonates) it exerts very high stresses on the engine and can melt holes in the piston top. Having the mixture flash before the spark plugs fire really stress the engine because it can make the engine (or that cylinder) try to run backwards even if it doesn't blow things apart ( one cylinder trying to run backwards can result in bent broken con rods, broken pistons,etc)

High octane is ALL about increasing the auto ignition temperature of the fuel. It allows the use of higher compression ratios in the cylinder which allow more power to obtained from burning the same amount of fuel.

Excellent post.