High altitude vs. low altitude

[silence]

What makes an engine high altitude versus one that's low altitude? Or all altitude?

 

[tomo pauk]

Mostly it comes down to what kind of supercharger system the engine has. The capability to make bigger RPM also helps in providing hi-alt capability, as it is the displacement.
Most of the 'high altitude' engines have had a two-stage supercharger (mechanical two stage system*, as in Merlin 60 and 70 series, or in some P&W engines; or the turbo+mechanical system*, as used on P-47's or P-38's engines). Most of them were using some form of charge cooling (inter-cooler or water-alcohol injection, or both), necessary since the air-fuel mixture was of great temperature once highly compressed.

*there were other two-stage engines used, too

 

[silence]

So it sounds like its a case more of engine accessories rather than the engine itself?

 

[tomo pauk]

More or less. The many times criticized V-1710 was all another ball game once it was supported by turbo, or auxiliary supercharger stage. And even those were behaving differently, depending whether the particular installation was supported by ADI (anti-detonant injection - AKA water injection) or not, and whether there was inter-cooler or not. And then we have a difference between inter-coolers themselves. Usage of inter-coolers most of the times have the drawback of greater aircraft drag, however, most notably on late P-38s and Spitfires, but also at Ta-152H.
We should not neglect the engine's displacement here, too. The DB-605 was a fine high altitude engine, once received the big supercharger from the DB-603A; the resulting DB-605AS/ASM/ASC/D engines were every bit as good as the two stage Merlins, without the need for intercoolers (the single stage supercharger was not heating the compressed air as 2-stagers), but needed the better fuel (C3) and ADI in form of MW-50 system to realize most of their potential. Methanol needed for the MW-50 mixture was hard to come by war's end, however, so the Germans were trying with less methanol in the mixture, resulting with MW-30 (ie. only 30% of the methanol).
The next DB-605, the 605L, was featuring the two-stage supercharger (without inter-cooler), but necessitated both C3 fuel and MW system to deliver the power - 1350 PS at 31400 ft was result. Only a handful of engines was built before ww2 ended.

Fuel was also important thing here, but it was a more crucial 'ingredient' for the power at lower altitudes.

 

[silence]

so the intercooler cools the air that has been heated as a result of compression. A 2-stage supercharger compresses the air twice, so it heats it twice, logically making it hotter than a single stage.

The reason to cool the air is to prevent detonation of the fuel before it gets into the cylinder; this could happen if the air it is mixed with is too hot as a result of the compression. SO, to get more air, you have to compress what you have, thereby making it hotter, thereby requiring the "new" volume of air be cooled prior to mixing it with fuel and then injecting it into the cylinder.

An intercooler and MW-50 (or -30) perform the same function, but may also be used together?

Higher octane fuel means more complete detonation, so more usable energy produced for a given volume of fuel.

Is that about right?

 

[tomo pauk]

so the intercooler cools the air that has been heated as a result of compression. A 2-stage supercharger compresses the air twice, so it heats it twice, logically making it hotter than a single stage.

Basically, that's it.

The reason to cool the air is to prevent detonation of the fuel before it gets into the cylinder; this could happen if the air it is mixed with is too hot as a result of the compression. SO, to get more air, you have to compress what you have, thereby making it hotter, thereby requiring the "new" volume of air be cooled prior to mixing it with fuel and then injecting it into the cylinder.

Not just that, the cooler air is more dense, heavier, meaning you can 'pack' more air cylinders. More air needs more fuel, and the result is more power.

An intercooler and MW-50 (or -30) perform the same function, but may also be used together?

Yes. Among the aircraft using both inter-coolers and ADI are (after late 1943.) P-47, F4U, F6F and P-51H.

Higher octane fuel means more complete detonation, so more usable energy produced for a given volume of fuel.

Is that about right?

Higher octane fuel means that engine will stand up to a greater manifold pressure before detonation occurs. Greater manifold pressure means more power. Engine need to be stressed and tested to withstand greater stress power, otherwise it would've been destroyed. The engine cooling oil systems might be pushed to their limits, too, so in most of the cases the 'overboost' is used, there is 5 min limit for that.
When 100 oct fuel went into use with RAF, Merlin III was capable for 1300 HP with +12 lbs per sq in boost in at 9000 ft, while with 87 oct it was capable for 1030 HP with +6.25 lbs per sq in at 16000 ft. graph
For German engines, with B4 fuel ('lower octane', similar to Allied 87 oct), the manifold pressures were not going above 1,45 ata (no MW-50 and no intercooler used). With C3 (hi-oct fuel, comparable with Allied 100/115 oct), the manifold pressures went to 1.55-1.8 ata (again, without MW-50 and/or intercooler).

 

[Shortround6]

Higher octane fuel means more complete detonation, so more usable energy produced for a given volume of fuel.

Is that about right?

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.

 

[tomo pauk]

Back to the supercharger as the main 'determinant' whether an engine is regarded as high-alt or low alt. There were several single stage supercharger set-ups for engines deliberately 'trimmed' to make greater power at desired altitude. The Soviet Mikulin engines, AM-35 and AM-38, shared most of the parts, the main difference being in supercharger system. The supercharger system of the AM-35/35A was using up more power to drive, than it was the case for AM-38's system. Net result was that AM-35/35A was capable for 1200-1250 HP at 3-6 km, against 1450-1700 HP the AM-38/38F were capable for, between SL and 2 km. The AM-38 series was also offering 300-400 more HP for take off than AM-38 series, the Il-2 aircraft being major users of AM-38s.
Again, please note that both wide-produced Mikulins were employing single speed, single stage superchargers; there was several prototypes with either two-speed, or 2-speed 2-stage, or even turbo charged Mikulins (mostly variations of the AM-39?).
some charts: here, table (on Russian - sorry)

Back to the DB-605 series, the hi-alt DB-605AS was having slightly less power than the 'plain' DB-605A (40 PS less at SL). The really low-alt V-1710-87, used on the A-36, was giving 1325 HP (military power) at only 2500 ft, vs. the 'mid-alt' V-1710-93 (used aboard of P-39Q) with 1125 HP, but at 'reasonable' 15500 ft. Both having single stage, single speed supercharger of 9.5 in diameter, but one was rotating at 7.48 times of crankshaft rotation speed (= using less engine power to drive), other one at 9.60 times of crankshaft speed (= using up more power).
Once above 10000 ft, the low-alt V-1710-87 was a dog, of course.

 

[silence]

Ok, now I think I get it. Thanks for all the explanations, gents.

 

[Reluctant Poster]

so the intercooler cools the air that has been heated as a result of compression. A 2-stage supercharger compresses the air twice, so it heats it twice, logically making it hotter than a single stage.

This statement is incorrect. The act of compression follows the ideal gas law PV=nRT. The rise in temperature is proportional to the compression ratio (there is an additional temperature rise due to waste heat as a result of compressor inefficiency). Whether there is one stage or two is irrelevant, what matters is the total pressure rise. Rolls Royce selected two stages simply because the overall efficiency of the compressor rose, meaning less HP to drive the compressor and less waste heat added to the compressed air
It is interesting to note that Germans were moving toward intercooling towards the end of the war. One of the major advantages of the TA152 over the FW190 was the incorporation of an intercoolerT

 

[tomo pauk]

Good call re. heating of the compressed air.
We might also note that Ta-152 were always to use a two-stage engines, the Ta-152C will not have inter-coolers for it's engine, using only the MW-50 to cool the compressed air.

 

[Shortround6]

Rolls Royce selected two stages simply because the overall efficiency of the compressor rose, meaning less HP to drive the compressor and less waste heat added to the compressed air

R-R selected two stages because the pressure ratio (compression ratio) they wanted was not obtainable from a single stage compressor at that time.

It is true that a two stage compressor will almost always heat the air less than a single stage due to better efficiency of the lower pressure individual compressors. Using two stages for a pressure ratio of 3:1 or so wasn't thought to be worthwhile for what gains there were when balanced against the cost, weight, and perplexity of a two stage system. When you go to a 5.1:1 ratio like the Merlin 60 used you had to use two compressors and did wind up heating the air "twice" but the actual temperature is, as you have stated, pretty much dependent on the actual pressure ratio.

 

[swampyankee]

Moving up to the present day, there have been a few engines built with three stages of turbo-supercharging, for UAVs for extreme altitude flight. Since LOX is pretty cheap, I think that using liquid oxygen as a combined coolant/oxidizer could make for an interesting, albeit short-term, method of getting power at extreme altitudes.

 

[davebender]

Germany had a serious shortage of copper and even aluminum was in short supply by 1945. I suspect that has a lot to do with Germany being in no hurry to use intercoolers.

 

[tomo pauk]

Germany was one the 1st countries to use intercoolers, in the Jumo-211J. Even though it was a singe stage engine, it gave better power at all altitudes than otherwise similar Jumo.211F. The power increase was best felt under 4 km.
The main drive for intercoolers is when supercharger system can really well compress the air - like the 2-stage superchargers. The bigger the pressure ratio, the bigger temperature rise, the greter need for cooling of the compressed air. Alternatively, the water-alcohol injection can be used, and that has it's advantages and drawbacks. Ideally, people were trying to use both methods, in German case with Jumo-213E in the Ta-152H. The Jumo-213F (similar to 213E, but no intercooler), was to have less power, and also smaller drag. The MW-50 sytem uses a consumable, and that was not either in abundant supply from late 1944.

 

[davebender]

Was that made of aluminum or copper?

 

[Reluctant Poster]

R-R selected two stages because the pressure ratio (compression ratio) they wanted was not obtainable from a single stage compressor at that time.

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

 

[gumbyk]

Don't forget the ignition system.
Ignition aircraft magnetos tend break down at low air pressures, and you get misfiring. To counter this, the ignition system had to be pressurised for high altitude flight.
I'm not sure if this was done on all engines, or just engines designed for high altitude work.

 

[parsifal]

does compression ratio make much difference? And as a supplementary, what about stroke length? Finally size of the valves or numbers of valves per cylinder

Nowadays, with electronically controlled timing, its completely different, but back in the 60's and 70's and even the 80's, if you wanted a lively engine, you went for short stroke with big valves. The short stroke gave you a high revving enginethat responded to accelaration issues quickly, but top speed could be affected unless you have a better quality gearbox. if you wanted the vehicle for high speed, or towing, you went for a long stroke, lower revving engine. not as quick but torquey. I just dont have any experience with vehicles of any kind that need to operate at High altitude, but i would think the issue is getting the air into the pot when the air is thin in the first place. if your fuel air mix is lacking in air, I would have thought you would want to advabnce the ignition a bit to give the fuel more time to burn, and you would want a long piston stroke to try and draw as much air in as possible. These of course dont deal with the supercharger which would change things considerably.

As far as fuels are concerned, if long stroke is the way to go for altitude, you would want a slow burning higher octane rated fuel. You dont want the fuel to go bang at the top of the stroke, you want it to burn evenly and continuously on the power stroke to derive maximum power and not generate excessive wasted heat and stress for the engine

most automotive enthusiasts site recommend a roughly 10 degree advance on the ignition timing when operating above 5k as well as a reduction in the fuel jet size

 

[GregP]

For a boosted engine, compression ratio always goes lower. Most racing V-12's at Reno are running 5.0 - 5.5 : 1 CR because of the boost they are running. You can only compress so far before the mixture ignites, so the CR must drop to run higher boost.

For reference, the intake plugs on an Allison fire at 28° BTDC and the exhaust plugs fire at 34° BTDC. These are specs for almost any Allison running today and cover almost every dash number. Merlin plugs fire similarly.