Superchargers?

Turbo-supercharger ('turbo' from now on here), in ww2 engines, was one of the stages of what was a 2-stage system; sometimes it was noted as 'auxiliary supercharger'. Other stage, often called 'engine stage', was another supercharger, this one driven by the engine itself. The B-29 used two turbos to compress the air, that was then compressed by engine-stage compressor and pushed into intake manifolds and then into cylinders. The another role for the engine-stage supercharger was to provide an uniform air-fuel mixture, the fuel being injected just before the impeller of the e-stage S/C.

In case the engine was outfitted with just one stage of supercharging (one impeller), the turbo version offered far more power at altitude; the cost of weight and size was justified. In case the engine was outfitted with 2-stage system, where both stages were driven by engine itself, that supercharger system was often equal in performance at altitude, once we account for additional size weight of turbo installation. The engine-driven 2-stage system was usually better under 20000 ft than turbo.

The turbo in cars is the only supercharger, much of the need for the separate one was avoided by injection of the fuel either in intake manifold or directly in cylinders/cylinder heads. The turbo in today's cars has barely noticeable turbo lag, due to introduction of variable-geometry turbines, and/or use of sequential turbos (smaller for low power and low lag, bigger one for greater power). The variable geometry turbines likely draw the ideas from Szydlowski-Planiol supercharger, 1st used on Hispano Suiza 12Y engine that powered the Dewoitine D.520. Those two gentlemen founded the company, named 'Turbomeca', in late 1938 - the rest is history, as often said.

Koopernic said:

Intercooler: = a heat exchanger that is placed between the engine and compressor to cool the air down thereby preventing preignition and knocking in the engine. Also reduces the workload in pumping in the air though it is actually throwing away energy.

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Intercooler was a trade-off that pays off. Cooler air is more dense. So more air can be crammed into intake manifold(s) and cylinders, and more power can be provided by the engine. It was even useful with low-boost, single stage engine like Jumo 211J. US and UK militaries were asking for 50% of 'intercooling'. Shortcoming of the intercooler is that can provide a drag penalty, in case it is not smartly faired in the airframe. There is some weight penalty, too. Benefit is that intercooling also helps on all power settings, not just at maximum boost.

The air (or air-fuel mixture) can be also cooled via injecting the water in a convenient spot. Alcohol was added to the water, it's role being anti-freeze. Sometimes both intercooler and water injection were used to improve engine power, sometimes only one of them. Benefit of water injection is that drag penalty is as good as zero (there is some weight penalty), shortcoming is that its duration was limited, mostly between 5 and 15 minutes.

The US turbo chargers in WW II had an overhaul life about equal to the overhaul life of the engine/s they were used on or a bit more.

It was possible for a turbo to fail, sometimes catastrophically, but since nothing else offered the same sort of altitude performance (turbo can help cruise performance for hours on end) the extra maintenance and danger were accepted.

thanks guys i've heard these terms for years without really knowing how they worked. at least now i understand better how they work

Do not exceed flight times for GE turbos varied a bit as time went on but reached 1300 hours late in the war.

In round numbers, what be the altitude differential and performance differential between a single-stage, two-speed supercharger vs a two-stage supercharger?

I read in "Two-Stage Supercharging" by Richard S Buck, that a single-stage, single-speed supercharger provided adequate performance up to 14,000 feet. A two-stage supercharger increased that altitude to 32,000 feet. What altitude performance could be expected from a single-stage, two-speed supercharger?

From what I've read, due to shortages in two-stage superchargers, while the F4F-3 Wildcat had two-stage superchargers, the F4F-3A Wildcat had single-stage, two-speed superchargers. According to Wikipedia, the F4F-3A's "poorer performance made it unpopular with U.S. Navy fighter pilots." How much poorer?

Thanks.

The altitude depends on the amount of boost desired and the year. A single stage supercharger could compress the air around 3 times the ambient intake pressure. A little less in 1939, a little more in 1944. Adding extra speeds to the drive didn't change the altitude (high) all that much. Most engines in the 1930s were called either fully supercharged or moderately supercharged depending on the altitude of the engine. The moderately supercharged engines had less power at high ( over 9,000ft or so) altitude but more power for take-off and low altitude. The two speed drive simple gave you both. There is a limit to how fast you can turn the impeller and most of the fully supercharged engines were getting close. The two speed Merlin X picked up about 1500ft over the single speed Merlin III.

If your engine needed 9lb of boost (48in) and your supercharger could provide a 2.8:1 pressure ratio then your altitude was a bit under 15,000ft. IF your engine needed 4 1/2lbs boost(39in) and your supercharger provided 2.8:1 pressure ratio then you had an altitude of around 19,500ft.

These are simply figures and do not take into account the heating of the air in the supercharger which means a less dense charge.

As an indication: The Germans made widespread use of single stage two speed superchargers in the BMW801D, Jumo 211 and Jumo 213A and they seemed to be quite satisfactory to 6500m or about 21500ft. Below that altitude they matched the allied fighters with turbos or the two stage Merlin in speed, above that they started falling behind. The exception being the Mustang which had an aerodynamic advantage. They latter improved their supercharger fluid dynamics and added about 1000m, something not well documented but mentioned in Rudiger Kosins "the German Fighter". The DB605 did a little better due to its unique design and latter enlarged supercharger.

When the Allison V1710 and latter DB605L received two stage superchargers, both with infinitely variable drives, they did not use inter coolers but instead relied on water injection to obtain full power. Inter cooling or turbo adds some bulk that cuts out a little bit of the advantage. One variant of the Jumo 211 had a single stage two speed supercharger with inter cooler.

The DB-605 was doing okay because it was big enough ( 34? vs. 27 L for the Merlin and V-1710) and strong and heavy enough, while powering a fighter that was initially designed around ~20 L engine with 600-650 HP and 2 LMGs - no wonder that more than twice the power will transform a decent fighter into a performer, despite the additions of guns, ammo, fuel and protection.
It can be argued that, with intercooler, any engine will provide more power. Again, if one starts with 34 liters, it can be pardoned not to have an intercoller that the 27L engine dearly needs. The 2-stage V-1710s were about as good as single stage DB-605s, but not that good as 2-stage Merlins until it was too late to matter (ie. before ww2 ended; applicable for service machines).

ohogain said:

In round numbers, what be the altitude differential and performance differential between a single-stage, two-speed supercharger vs a two-stage supercharger?

I read in "Two-Stage Supercharging" by Richard S Buck, that a single-stage, single-speed supercharger provided adequate performance up to 14,000 feet. A two-stage supercharger increased that altitude to 32,000 feet. What altitude performance could be expected from a single-stage, two-speed supercharger?

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Stating those altitudes is applying a too wide a brush to paint an accurate picture. Not all engines with two-stage S/C were capable to do at 32000 ft what a decent engine with single stage S/C was doing at 14000 ft. Then we have a question whether an intercooler was installed, or was there an anti-detonant system aboard (water-alcohol injection usually), or maybe both of those?

The single stage superchargers were capable, in quite a number of engines, to provide a good power at altitudes of 18-22000 ft. Merlin XX and 45, BMW 801D, DB-605, Mikulin AM-35A, for example. Their superchargers being driven either via a single speed, two speed, or infinite speed drives. Some of these engines were being outfitted with enlarged superchargers, like the DB-605AS or 605D, or Merlin 46 and 47, were quite useful even at 25000 ft, or above. In case the engines were outfitted with a 'smallish' S/C from the get-go (like V-1710, R-2600 and single stage R-2800), they were topping at 12-16000 ft, depending on the variant and year.
In case the bigger supercharger was used, and was driven via a single-speed drive (like Merlin 46 47), the power at lower levels was suffering, since the S/C was using up too much power, and was heating the compressed air too much. The AM-35A was using sort of a 'swirl throttle' to circumvent those issues.

From what I've read, due to shortages in two-stage superchargers, while the F4F-3 Wildcat had two-stage superchargers, the F4F-3A Wildcat had single-stage, two-speed superchargers. According to Wikipedia, the F4F-3A's "poorer performance made it unpopular with U.S. Navy fighter pilots." How much poorer?
Thanks.

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You might want to check out the wwiiaircraftperformance.com, and compare different Widlcats/Martlets with different engines.

tomo pauk said:

The DB-605 was doing okay because it was big enough ( 34? vs. 27 L for the Merlin and V-1710) and strong and heavy enough, while powering a fighter that was initially designed around ~20 L engine with 600-650 HP and 2 LMGs - no wonder that more than twice the power will transform a decent fighter into a performer, despite the additions of guns, ammo, fuel and protection.
It can be argued that, with intercooler, any engine will provide more power. Again, if one starts with 34 liters, it can be pardoned not to have an intercoller that the 27L engine dearly needs. The 2-stage V-1710s were about as good as single stage DB-605s, but not that good as 2-stage Merlins until it was too late to matter (ie. before ww2 ended; applicable for service machines).



Stating those altitudes is applying a too wide a brush to paint an accurate picture. Not all engines with two-stage S/C were capable to do at 32000 ft what a decent engine with single stage S/C was doing at 14000 ft. Then we have a question whether an intercooler was installed, or was there an anti-detonant system aboard (water-alcohol injection usually), or maybe both of those?

The single stage superchargers were capable, in quite a number of engines, to provide a good power at altitudes of 18-22000 ft. Merlin XX and 45, BMW 801D, DB-605, Mikulin AM-35A, for example. Their superchargers being driven either via a single speed, two speed, or infinite speed drives. Some of these engines were being outfitted with enlarged superchargers, like the DB-605AS or 605D, or Merlin 46 and 47, were quite useful even at 25000 ft, or above. In case the engines were outfitted with a 'smallish' S/C from the get-go (like V-1710, R-2600 and single stage R-2800), they were topping at 12-16000 ft, depending on the variant and year.
In case the bigger supercharger was used, and was driven via a single-speed drive (like Merlin 46 47), the power at lower levels was suffering, since the S/C was using up too much power, and was heating the compressed air too much. The AM-35A was using sort of a 'swirl throttle' to circumvent those issues.



You might want to check out the wwiiaircraftperformance.com, and compare different Widlcats/Martlets with different engines.

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Thanks.

Shortround6 said:

The altitude depends on the amount of boost desired and the year. A single stage supercharger could compress the air around 3 times the ambient intake pressure. A little less in 1939, a little more in 1944. Adding extra speeds to the drive didn't change the altitude (high) all that much. Most engines in the 1930s were called either fully supercharged or moderately supercharged depending on the altitude of the engine. The moderately supercharged engines had less power at high ( over 9,000ft or so) altitude but more power for take-off and low altitude. The two speed drive simple gave you both. There is a limit to how fast you can turn the impeller and most of the fully supercharged engines were getting close. The two speed Merlin X picked up about 1500ft over the single speed Merlin III.

If your engine needed 9lb of boost (48in) and your supercharger could provide a 2.8:1 pressure ratio then your altitude was a bit under 15,000ft. IF your engine needed 4 1/2lbs boost(39in) and your supercharger provided 2.8:1 pressure ratio then you had an altitude of around 19,500ft.

These are simply figures and do not take into account the heating of the air in the supercharger which means a less dense charge.

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Sorry for my ignorance, but why would an engine need different boosts? Was it engine specific? Did one engine require 9lb of boost and a different one 4.5lb of boost?

Depends on fuel and engine design, A lot of mid 1930s engines used under 4 1/2lbs of boost (39 in or around 1.3ata) The British began pushing things with 6lbs (42in) of boost in the Merlin on 87 octane ( and British 87 octane may NOT have been the same as other peoples 87 octane. depends on the amount aromatic compounds used in the fuel) and the US was changing to US specification 100 octane ( with 2% or less aromatic compounds so the rich mixture response, (not measured at that time) didn't change much. It did allow boosts of 44in or a bit more more in Allison, Wright and P&W engines.

The Germans and French tended to use larger displacement engines with less rpm and lower boost than the British and Americans. You have to know what the supercharger was actually doing if you want to compare the supercharger design and not claim one was better than another based on the "full Throttle height" of the engine when the two different engines might very well have superchargers providing rather different pressures even though the engines power and height performance were similar.

The issue is the allowable amount of boost in absolute units measured after the supercharger. There are various design parameters that affect the ability of the engine to operate without detonation for a given level of supercharger boost:

- fuel octane and whether water injection was used or other means of cooling the charge (such as the use of a rich air-fuel mixture)
- compression ratio (a lower compression ratio should allow increased supercharger boost)
- the amount of charge heating contributed by the supercharger (depends on supercharger efficiency and the intercooler performance (if used))
- cylinder head design (airflow and temperature)

There were also physical constraints such as the strength of the critical engine components and the ability to maintain acceptable temperatures. Engines such as the HS 12Y and various Gnome Rhone 14K derivatives were designed to operate with fuels available in the early/mid 1930s and lacked the strength to benefit from really high boost levels.

Rolls Royce deserves comment for making particularly effective use of increases in fuel octane to obtain high power levels from the Merlin, a relatively small engine. This did involve progressive improvements to the supercharger and the mechanical components of the engine though, interestingly, it was achieved without changes to the engine RPM or compression ratio. There is a paper by Lovesy, "Development of the Rolls-Royce Merlin from 1939 to 1945," that can be found on the web and is well worth reading.

In order not to clog another thread, I've moved this here.

ThomasP said:

In response to Shortround6 post#25 and the other posts related to the GE, P&W, Wright, and German superchargers,

The Germans had figured out the supercharger problems by about late-1942, if by no other way than the reverse engineering of the Merlin XX and later engines. The US also had figured out the supercharger problems, if by no other way than through the UK providing the Merlin XX and 60 series supercharger designs.

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Germans probably have had nothing new to learn from supercharger installed on Merlin XX and the like. They experimented with 2-stage superchargers on DB 601C/D, luckily nothing came out from these engines.
US have figured in 1930s that 1 stage of supercharging is not good enough past 20000 ft, thus USAAC went with turbos (thus havig two stages of supercharging for engines), while USN co-funded 2-stage superchargers at P&W (and probably at Wright)

However, both the Germans and the US (US in the early part of the war at least) had other fish to fry, so to speak.

The Germans had to plan on maybe not having high quality aviation fuel, and the Eastern Front was using up most of their focus and resources. In addition they used direct fuel injection on all(?) their high performance engines, which had some advantages over carburetor types, but did not allow for the fuel injection into the eye of the supercharger.

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Advantage of not having a carburetor is that there is no obstruction to the airflow due to the carb, plus one can have better consumption figures. No problems with icing, either (thus no need for ice guard, heating passages for airflow, de-icing carbs etc).

The US had institutional inertia to overcome, and not just relative to the turbo installations. The Allison V-1710 did not evolve to the level of the single-stage Merlin until the Rolls Royce Allison Merlin V-1710'G' series. The V-1710'G' could aptly be described as a 'Merlinized' and 'Hookerized' model with a slightly modified Merlin ⌀10.25" impeller, Merlin 6:1 cylinder compression ratio, and Hooker geometry for the airflow (I am not sure where the fuel was injected in the P-38L V-1710'G' series system). P&W did not come into their own until late-1944 with their R-2800 'C' series engines, and Wright not until their R-2600 'C' series? in 1945? (maybe not during the war?)

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(by bold)
There was no 'institutional inertia' vs. non-turbo engines at US Navy. At USAAC/AAF, turboed engines worked well in service before RR 2-stage engines went in service. Let's not think that Bristol and Napier engines employed 2-stage engines, institutional inertia or not.
V-1710G was not a Merlinized V-1710 - no common-shaft impellers, no intercooler, no cooling of S/C housing, no flame trap, no 2-speed S/C drive. Hooker geometry for airflow??
P-38L never used V-1710G. P&W R-2800 2-stage engine powered the XF4U-1 (prototype Corsair) in 1940, mass production of 2-stage R-2800s started by early 1942, mass production of 2-stage R-1830s started by late 1940. The 2-stage R-2600-10 powered XF6F-1 (prototype Hellcat) in 1942.
Bolded part is load of bull, even if it is a joke.

What I am saying by the above, any jokes aside, is why did the Germans and the US not simply put the Merlin supercharger on their engines? The answer is the very large amount of design, retooling, and trouble shooting that would be required. I can not say for sure how long the delay/disruption in production such a switch would have caused in 1942-43, but even today it would take a year, plus or minus a couple of months. For the US at least, while what they had for superchargers may not have been upto comparison with the Merlin, it was 'good enough' in an operational sense for the most part.

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Far less of a trouble would've been making 2-stage supercharged DB 605 and/or Jumo 211 than making brand-new DB 603 and/or Jumo 213s.
US produced perhaps 10 times the turboed engines than RR produced 2-stage S/Ced engines.

For the Germans, leaving out the fuel quality issue leaves engines that are pretty much equal to the US and UK engines in any operational sense, at least until it no longer really mattered.

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Germans, to their detriment, lagged by at least 2-years with the engine as capable as 2-stage Merlin, Griffon or R-2800, let alone turbo R-2800 - the very reason why LW was unable to compete above 20000 ft against Allied best fighters.
2-stage engines can work with 87 oct fuel.

tomo pauk said:

2-stage engines can work with 87 oct fuel.

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They can but it is an awful lot harder. A Merlin 61 at 23,500ft to get 12lbs of boost (54in) was compressing the outside air about 4.6 times. (on book claims 5.1 times)

A DB605 to run at 1.42 Ata (42.5 inches) needs to compress the air 3.58 times at the same altitude. It's supercharger would require less power to run and heat the intake air less.


Now if you want to fly around at 30,000ft where the air is 8.88in Hg instead of the 11.85in Hg at 23,500ft you are really going to have to compress the air and you need a really good way to cool it off before trying to use in the engine if you are using 87 octane fuel. It can be done but you are going to need bigger or better intercoolers (more weight/drag) or use water injection sooner (or more of it) for more installed weight.

The Allied planes with their, oh so terrible, carburetors were running very rich and using the extra fuel as both a charge coolant and an internal coolant for the engine. I don't think the German injection systems were set up to provide the range of mixtures the allied planes were.

Shortround6 said:

They can but it is an awful lot harder. A Merlin 61 at 23,500ft to get 12lbs of boost (54in) was compressing the outside air about 4.6 times. (on book claims 5.1 times)

A DB605 to run at 1.42 Ata (42.5 inches) needs to compress the air 3.58 times at the same altitude. It's supercharger would require less power to run and heat the intake air less.
Now if you want to fly around at 30,000ft where the air is 8.88in Hg instead of the 11.85in Hg at 23,500ft you are really going to have to compress the air and you need a really good way to cool it off before trying to use in the engine if you are using 87 octane fuel. It can be done but you are going to need bigger or better intercoolers (more weight/drag) or use water injection sooner (or more of it) for more installed weight.

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Both Jumo 213E and DB 603LA used 87 oct fuel. 213E was not allowed to use water-injection when in 3rd gear beacuse it was found out that S/C drive is not strong enough. Jumo 213F run on 87 oct, non intercooled, but required water injection.
The DB 605L, with it's high CR (8.6:1 vs. 6.5:1 for Jumo 213 engines) and no intercooler required both C3 fuel and water injection already for 1.4 ata.

The Allied planes with their, oh so terrible, carburetors were running very rich and using the extra fuel as both a charge coolant and an internal coolant for the engine. I don't think the German injection systems were set up to provide the range of mixtures the allied planes were.

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Allied carbs ranged from excellent to terrible.

Hey tomo pauk, re your post#73,

"P-38L never used V-1710G." Sorry, my mistake. I should have used the F-82 in my example.

The Hooker geometry airflow is due to the fact that Hooker was a theoretical mathematician. Using mathematic analysis he figured that pretty much all of the superchargers in service at the time were 'choking' the airflow, causing unnecessary heat build-up and using additional HP. The result of this change allowed an increase in efficiency. If I am remembering correctly, just the redesigned geometry of the inlets and expansion chamber allowed about a 10% increase in efficiency over any other in service. The geometry was actually Hooker's first contribution to the supercharger heat problem.

The R-1830 and early- to mid-war R-2800 2-stage superchargers still used the typical less efficient airflow geometry of the time (all the pre-war/early-war US superchargers used basically the same geometry with about a 60-65% efficiency) and did not use the fuel to help cool the supercharged air in the supercharger. The Hookerized superchargers in the Merlin XX achieved about 90% efficiency, with a consequent reduction in HP absorbed by the supercharger.

P&W did not revamp their supercharger geometry for the R-2800 until the 'C' series entered service. I do not know if they ever incorporated the use of fuel to cool the charge during compression in the supercharger?

Fuel injection also allows slightly higher cylinder compression ratios (everything else being equal) without detonation.

There is always institutional inertia (not to be confused with institutional incompetence or stupidity) and I was not referring to any resistance to not using a turbo. Usually it manifests in the form of "if it aint broke don't fix it" or "this is the way we have always done it". Until Hooker figured out the math, no one else understood the problem, hence there was no reason to change. Once the math was figured out, there was an institutional inertia effect in the sense that it takes time and effort to change what the institution is doing - in technical knowledge dissemination, in manufacturing methods, in tooling up for new production, in developing a logistics train, in training of maintenance personal, etc. This always causes delays/disruptions in switching over to new designs when you have limited manufacturing resources.

Also, I did not realize you had switched to this thread, so I have already posted again on the "was the me 262 delay only hitler fault?" thread.

ThomasP said:

...
The Hooker geometry airflow is due to the fact that Hooker was a theoretical mathematician. Using mathematic analysis he figured that pretty much all of the superchargers in service at the time were 'choking' the airflow, causing unnecessary heat build-up and using additional HP. The result of this change allowed an increase in efficiency. If I am remembering correctly, just the redesigned geometry of the inlets and expansion chamber allowed about a 10% increase in efficiency over any other in service. The geometry was actually Hooker's first contribution to the supercharger heat problem.

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Agreed pretty much.

The R-1830 and early- to mid-war R-2800 2-stage superchargers still used the typical less efficient airflow geometry of the time (all the pre-war/early-war US superchargers used basically the same geometry with about a 60-65% efficiency) and did not use the fuel to help cool the supercharged air in the supercharger. The Hookerized superchargers in the Merlin XX achieved about 90%, with a consequent reduction in HP absorbed by the supercharger.

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Seems like the Merlin's superchargers went to 75% efficiency at pressure ratio between 2.5:1 to 3:1 - link, Fig.11.
P&W engines used fuel to help cool the compressed air, as all non-injected engines. Granted, some used it better than others.

P&W did not revamp their supercharger geometry for the R-2800 until the 'C' series entered service. I do not know if they ever incorporated the use of fuel to cool the charge during compression in the supercharger?

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R-2800 was a wholy revamped engine, with most parts being new, and managed extra 100 rpm vs. B series. At inlet of S/C, the fixed guide vanes were installed. All of that, cupled with improved S/Cs, certainly improved altitude power.

There is always institutional inertia (not to be confused with institutional incompetence or stupidity) and I was not referring to any resistance to not using a turbo. Usually it manifests in the form of "if it aint broke don't fix it" or "this is the way we have always done it". Until Hooker figured out the math, no one else understood the problem, hence there was no reason to change. Once the math was figured out, there was an institutional inertia effect in the sense that it takes time and effort to change what the institutional is doing - in technical knowledge dissemination, in manufacturing methods, in tooling up for new production, in developing a logistics train, in training of maintenance personal, etc. This always causes delays/disruptions in switching over to new designs when you have limited manufacturing resources.

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Interestingly enough, there was next to no resistance for the Packard-made Merlins by the USAAC/AAF. The brass was of opinion that too much of AAC fighter force hangs on succes or failure of V-1710 - thus P-44/P-47B (with R-2180/2800) and V-1650 for, initially, P-40s.
By the time Hooker improved the Merlin (spring/summer of 1940), V-1710 was not installed in a service-worthy fighter, so indeed further development would've meant USAAC of 1940/41 having 320 mph (and that is not certain) P-36s as best fighters.
AAC was certainly not the one to languish on the past, even with depression of 1930s and subsequent lack of money - they were funding 2- and 4-engined bombers, 2-engined fighters, 1-engined 2000 HP fighters, turbo engines in bombers and fighters alike, were the 1st to part with biplanes, they switched to V12 engines despite predominance of radials in US production.
Like anyone, they made their fair share of mistakes - no crystal ball and that jazz.

Also, I did not realize you had switched to this thread, so I have already posted again on the "was the me 262 delay only hitler fault?" thread.

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Nobody will mind

The R2800 (even the C series) used auto rich and auto lean selections, the rich for high power and climb to provide additional cooling as the richer mixture burned at a lower temperature and additional mass flow carried heat out of the engine. The use of water injection for temporary higher boost values allowed more power due to a change to a better power mixture, the cooling effect being provided instead of by the water meth mixture. USN investigated fuel injection but concluded that the mixture distribution was more efficient with the carb setup!

The biggest issue with shaft driven super chargers, even those with 2 additional stages, was they drew off a lot of power. In high blower at altitude the R2800 used 400 hp just to run the blower! So the 2000 hp SL engine in neutral blower became a 1600 HP engine in high blower at optimum altitude.

Hey tomo pauk,

My understanding is that adding the fuel after the air is already compressed (i.e. after the supercharger) is significantly less effective at reducing the charge temperature before it enters the cylinders.

I believe the 75% shown in the chart ( link, Fig.11) is only taking into account the actual mechanical aspect of the supercharger, and does not include the cooling effect of adding the fuel to the mix before/during the compression. The use of the fuel as coolant in the supercharger reduced the charge temperature at entry into the cylinder by 25ºC, which is equal to about 17% increase in efficiency at SL. This added the other 15% to give the 90% efficiency.

About 15 years ago I built a spread sheet using the standard physics for such things and incorporated Hookers changes into the mix. The results (charge temperature, IHP, S/C HP loss, BHP) for an engine with Merlin XX single-stage characteristics were within 1.5% of historical test records at all altitudes from SL to 40,000 ft when using an overall S/C efficiency factor of 90%. In order to get historical test results for power curves of the Merlin III, V-1710, R-1820, R-1830, R-2600, and R-2800 early- to mid-war engines I had to use between 55% and 65% supercharger overall efficiency factors. The 55% was for the R-1830 2-stage supercharger at high altitude, the 65% was for the single-stage engines. The Merlin III required a 65% supercharger overall efficiency factor, and the Merlin 61 ended up with a 86% overall supercharger efficiency factor at high altitude. In order to reduce the overall efficiency factor to 55% to 65% I had to leave out the 25ºC charge cooling effect of the fuel introduction at the supercharger.

ThomasP said:

Hey tomo pauk,

My understanding is that adding the fuel after the air is already compressed (i.e. after the supercharger) is significantly less effective at reducing the charge temperature before it enters the cylinders.

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Depends who you're asking. On 2-stage V-1710s, relocating the carb from the entry of 1st stage to the entry of 2nd stage earned ~2500 ft worth of rated altitude for military power (1125 HP).

I believe the 75% shown in the chart ( link, Fig.11) is only taking into account the actual mechanical aspect of the supercharger, and does not include the cooling effect of adding the fuel to the mix before/during the compression. The use of the fuel as coolant in the supercharger reduced the charge temperature at entry into the cylinder by 25ºC, which is equal to about 17% increase in efficiency at SL. This added the other 15% to give the 90% efficiency.

About 15 years ago I built a spread sheet using the standard physics for such things and incorporated Hookers changes into the mix. The results (charge temperature, IHP, S/C HP loss, BHP) for an engine with Merlin XX single-stage characteristics were within 1.5% of historical test records at all altitudes from SL to 40,000 ft when using an overall S/C efficiency factor of 90%. In order to get historical test results for power curves of the Merlin III, V-1710, R-1820, R-1830, R-2600, and R-2800 early- to mid-war engines I had to use between 55% and 65% supercharger overall efficiency factors. The 55% was for the R-1830 2-stage supercharger at high altitude, the 65% was for the single-stage engines. The Merlin III required a 65% supercharger overall efficiency factor, and the Merlin 61 ended up with a 86% overall supercharger efficiency factor at high altitude. In order to reduce the overall efficiency factor to 55% to 65% I had to leave out the 25ºC charge cooling effect of the fuel introduction at the supercharger.

Click to expand...

Granted, the 2-stage S/C of the R-1830 was probably the worst of mass-used 2-stage superchargers - about as good as 1-stage superchargers on Merlin XX or DB 601E.
Goes without saying that I'd love to see the tests or math predictions for S/C efficiency. Have you calculated in the losses due the carb being present?