A superior German fighter

[wuzak]

Well 2 degrees could be the difference between max power and catastrophic detonation. Lets say for example, if a supercharged engine is pushing 10 psi of boost at sea level (ambient pressure of 14.7 psi, ambient temperature of 24°C @ sea level, the temperature of the air after the supercharger will be 71.4 °C. At 22°C air after the supercharger is 64°C. Now if the detonation point was 69°C air after the supercharger, that 2°C just made the difference. All well in good on just the engine/supercharger, right? Now you have the 50%/50% water-methanol to throw into the equation.

I get 71.5° with inlet of 24°C and 69.2 with inlet of 22°C.

Note that ADI/MW50 is a consumable.

 

[OldSkeptic]

I get 71.5° with inlet of 24°C and 69.2 with inlet of 22°C.

Note that ADI/MW50 is a consumable.

Well said. Those that advocate ADI and NO2 forget that it increases the weight of the plane and takes up volume (always very limited in a fighter).
And of course doesn't last long. Plus you have the logistics issues, maintenance, et al.

RR went for simplicity as far as possible, which was part of their genius, as per the old saying "any fool can make things complex, it takes a genius to keep things simple".

Yes there were NO2 boosted Merlins, used in night fighter Mossies (big enough to take the volume) and water injection was tested during the war and used post war, nearly always just for take off (the Methanol part is an anti-freeze). Heck they even tested liquid oxygen (which was far better than NO2).

But the production decisions RR made were a compromise between sheer raw power, logistics, manufacturing, maintenance, etc.

Hooker's great contribution was his equations, which meant they could focus in the particular areas that mattered. So the Merlin 100 series lifted FTH by 4,000ft with the same power, just by its greater pressure efficiency (comparing similar high/lo variations of course), that's a big gain. Compare the Merlin X vs the XX and you see even greater gains because of the higher supercharger efficiency. (The X was the 2 speed prototype without the Hooker supercharger changes).

But the greatest compliment paid by the Germans was by (in their own complicated way) copying the idea, at least by BMW.
C3 injection. They added fuel into .... the supercharger inlet, cooling the intake and allowing higher boost .. still had fuel injection to every cylinder

 

[Tante Ju]

Well said. Those that advocate ADI and NO2 forget that it increases the weight of the plane and takes up volume (always very limited in a fighter). And of course doesn't last long. Plus you have the logistics issues, maintenance, et al.

I wonder why aircraft like the Hellcat, Thunderbolt, Mustang H etc. bothered at all with ADI when they had all those wonderful, state of the art carburators...

But the greatest compliment paid by the Germans was by (in their own complicated way) copying the idea, at least by BMW.
C3 injection. They added fuel into .... the supercharger inlet, cooling the intake and allowing higher boost .. still had fuel injection to every cylinder

"Copying" what exactly..? That an engine needs fuel feed...? The carburetor? Float carburetor was developed by Herren Wilhelm Maybach and Gottlieb Daimler in 1885 (with notable Italian and Hungarian contribution to carburetor inventions)... or perhaps was it the Otto motor principle that they copied from mighty RR? Until the mid-1930s all large German aero piston engines were carburetor fed (see for example DB 600, the forerunner of the DFI DB 601), they were well aware of the difference between carburetors

The simple answer as to why RR developed its engines (again, quite ingeniously considering their limits) the way it did was that it did not have either the time to develop a completely new engine in wartime in the 30-liter class (the "alternative" to further developing the existing Merlin was waiting for the Griffon until 1943/44..), nor the reliable domestic suppliers of DFI systems: they simply did not have access to Bosch AG products - AFAIK (and could be well wrong) the current DFI systems are essentially the same as the ones developed by Bosch right before WW2.

If the only thing you can mass produce is a 27 liter engine with a carburetor, then its going to be a 27 liter engine with a carburetor plus loads of explanations why this is really the superior way (while frantically developing your own 35 liter engine, the thing others have already done 5-10 years before). If you are stuck with a relatively small engine and trying to compete with big engines in output, you will have to rely on ever extreme amounts of supercharging. In that case you will probably inclined to keep the carburetor a bit longer, as even the minor, ultimately insufficient charge cooling bonus it provides can help to stay competitive. As I said, beggars can't be choosers, though they would probably like to argue that becoming a career beggar was a consciously made choice.

 

[wuzak]

The simple answer as to why RR developed its engines (again, quite ingeniously considering their limits) the way it did was that it did not have either the time to develop a completely new engine in wartime in the 30-liter class (the "alternative" to further developing the existing Merlin was waiting for the Griffon until 1943/44..), nor the reliable domestic suppliers of DFI systems: they simply did not have access to Bosch AG products - AFAIK (and could be well wrong) the current DFI systems are essentially the same as the ones developed by Bosch right before WW2.

What's the fascination with a capacity class? It is the power class that matters - and there the Merlin was a match for, or better than, the DB 601/605.

It was a concious decision on Rolls-Royce's part to use low compression ratios and high boost to achieve the power. It was Daimler-Benz's approach to use larger capacity and lower boost to achieve the power.

The Griffon was started in 1938/39 and was in service by 1942. It was an all new design. Not sure why you say about waiting for the Griffon until 1943/44. As far as I can tell the main reason why the Griffon didn't come on sooner was that production of existing types and engines took precedent.

If the only thing you can mass produce is a 27 liter engine with a carburetor, then its going to be a 27 liter engine with a carburetor plus loads of explanations why this is really the superior way (while frantically developing your own 35 liter engine, the thing others have already done 5-10 years before). If you are stuck with a relatively small engine and trying to compete with big engines in output, you will have to rely on ever extreme amounts of supercharging. In that case you will probably inclined to keep the carburetor a bit longer, as even the minor, ultimately insufficient charge cooling bonus it provides can help to stay competitive. As I said, beggars can't be choosers, though they would probably like to argue that becoming a career beggar was a consciously made choice.

I don't think there was anything "frantic" about the development of the Griffon.

The Griffon wasn't, originally, to replace the Merlin. It was a response to a request from the FAA for a larger capacity, more powerful engine to help with their heavier (than land based) carrier types.

 

[wuzak]

I wonder why aircraft like the Hellcat, Thunderbolt, Mustang H etc. bothered at all with ADI when they had all those wonderful, state of the art carburators...

The Packard Merlin in the P-51H was the only type of Rolls-Royce engine, or derivative, that had ADI during the war. The reason was to screw even more boost out - from +25psi to +30psi.

The radials used them because they needed more control of temperatures, and had less resistance to detonation (hence they generally used lower boost).

Allison V-1710s with ADI tended to not use intercooling.

 

[Aozora]

I wonder why aircraft like the Hellcat, Thunderbolt, Mustang H etc. bothered at all with ADI when they had all those wonderful, state of the art carburators...

Tssk tsssk, such ignorance... ; the "carburetors" concerned were the Bendix or Chandler throttle body injection units, which were state of the art, and had been adopted by the Americans well before Germany adopted DFI.

 

[Tante Ju]

Style like this

To claim that Rolls-Royce didn't go with direct injection because of their supposed "extreme conservatism" is the usual simplification and myth making of the ignorant

Luftwaffephile always like to describe the problems with fuel starvation during the Battle of Britain but don't seem to recognise that relatively small engineering changes eliminated the problem, which was then bypassed completely when the Bendix Stromberg direct injection carburettor was adopted.

The problem with such a pet theory is that it doesn't allow for the fact that ...

That Daimler Benz - via Bosch - and Junkers developed reliable DFI systems was a great achievement, but to use that in an attempt to damm the Merlin (with some faint praise) and other engines which did not adopt DFI is just ill-informed nonsense.

Tssk tsssk, such ignorance...

will only getting you (a) ignored (b) being short lived on this board.

You just ticked (a) BTW.

 

[Shortround6]

Rolls had run a Merlin on a test stand at 1600hp for 15 hours in 1938, and gotten more power out of for it shorter periods. They KNEW the basic engine would stand up to higher than 1000-1200hp with minor tweaks. This allowed them to put the Griffon on the back burner for a while. If the Merlin had proved to need major work to make 1500hp or more in 1938 then they probably would have worked on the Griffon more.

Again, this is testing the basic structure of the engine, getting the engine to MAKE 1500-1800hp at altitude is another story.

Fuel injection would NOT have gotten as much power out of the big radials as water-injection did. As already mentioned, they were temperature limited. When running at full power they consumed large amounts of fuel that had nothing to do with burning it for power, they were using it not only to cool the intake charge (like Rolls) but to cool the engine from the inside. When fitted with water injection the carburetor had de-enrichment circuit and they used 30-40 gallons LESS fuel per hour while making 250-300hp MORE horsepower. Fuel flows in the charts are given on a per hour basis even when the engine was limited to 5 min. at that power setting.

Fuel injection can do some very good things but lets not claim it could do things that it actually could NOT do.

I am in agreement with Wuzak about the constant harping on the displacement of the Merlin. Power per liter matters to engine designers. AIR FRAME designers could care a flying ***** about it. The Single stage Merlins and the Allison and the DB 601 and the Jumo 211 in 1939/40/41 were ALL fairly close to each other in BOTH power and WEIGHT which is what interests the air frame designer. HOW they got that power at that weight is up to the engine designer. Small displacement and high rpm or large displacement and low rpm. NOBODY's boost was all that high in 1937-39 when the engines were designed and brought into service.

 

[DonL]

@ wuzak

Yes, the air fuel mixture would be high for the draw through system (like that used in the Merlin).

But, it won't be as high as the air in a system where no fuel is added (ie direct injection, like DB 6).

It also highlights the different philosophies of the two engine manufacturers - Daimler-Benz opted for capacity, high compression and low boost (relatively). Rolls-Royce followed the path of the R engine - high boost, low compression ratio.

For the high boost route the fuel into the eye of the supercharger was invaluable in lowering induction temperatures.

What's the fascination with a capacity class? It is the power class that matters - and there the Merlin was a match for, or better than, the DB 601/605.

It was a concious decision on Rolls-Royce's part to use low compression ratios and high boost to achieve the power. It was Daimler-Benz's approach to use larger capacity and lower boost to achieve the power.

The Packard Merlin in the P-51H was the only type of Rolls-Royce engine, or derivative, that had ADI during the war. The reason was to screw even more boost out - from +25psi to +30psi.

I have some questions:

1. Could it be, that the different design philosophies had to do with the quality of fuel and the knock resistance of the expected fuel?
From all I could read about german engine development, the engineers were well aware, that they were stuck basically on B4 fuel, also that the possibility to use and produce high grade B4 fuel wasn't given until 1936, as the IG Faben get the patent of lead tetraethy from Standard oil.
The germans had never the chance of the whole WWII to produce high grad fuel with/out of isooctane, because they don't have isooctane and don't know the development.
To my understanding this was one of the main reason they developed from the beginning to more capacity engines, because for higher boost (ata), you are in need for high grade fuel.

2. Also there was (Jumo 211, Jumo 213; compare to DB 601, DB 605, DB 603) a different design philosophy between the german engine factories Junkers and DB.
DB built it's engine from dry weight very light. The DB 601/DB 605 is at the same weight class of the Allison 1710 and RR Merlin with 33/35,9 Liter compare to 28 Liter engines, the DB 603 44 Liter at the same class as the RR Griffon and the Jumo 213 35/36 Liter engines.
At first sight to me, DB bought this "light weight" with relative low RPM's. After the development of the Jumo 213 out of the Jumo 211 with high pressure water cooling, Junkers was able drive the Jumo 213 with 500 rpm more then the Jumo 211. DB was not able to manage a much higher RPM at the DB 605 (100 RPM higher then the DB 601), although the DB 605 was converted also to a high pressure water cooling and sleeve bearings instead of roller bearings.
The DB 605 was next to the DB 603 compare to RR (Merlin, Griffon), Allison (V1710; Junkers (Jumo 213), the inline engine, that was driven with the lowest RPM.

I couldn't fathom this to the ground, but there must be different design philosophies.

 

[wuzak]

1. Could it be, that the different design philosophies had to do with the quality of fuel and the knock resistance of the expected fuel?
From all I could read about german engine development, the engineers were well aware, that they were stuck basically on B4 fuel, also that the possibility to use and produce high grade B4 fuel wasn't given until 1936, as the IG Faben get the patent of lead tetraethy from Standard oil.
The germans had never the chance of the whole WWII to produce high grad fuel with/out of isooctane, because they don't have isooctane and don't know the development.
To my understanding this was one of the main reason they developed from the beginning to more capacity engines, because for higher boost (ata), you are in need for high grade fuel.

I think that is a distinct possibility.

Rolls-Royce did follow that path when they went racing with the R, requiring special fuels to be developed.

When the Merlin was being developed the fuel they had on offer wasn't that great. But there was better fuel being developed in the US. So that may have allowed them to believe that even better fuels would be available in the future.

By the same token, the requirements from the engine manufacturers also drove fuel development.

And when better fuels were available Rolls-Royce were able to take full advantage.

Tetra-ethyl lead was used to give British and American fuels the better knock resistance. Not sure if Germany also had that, but alcohol is also an octane booster.

2. Also there was (Jumo 211, Jumo 213; compare to DB 601, DB 605, DB 603) a different design philosophy between the german engine factories Junkers and DB.
DB built it's engine from dry weight very light. The DB 601/DB 605 is at the same weight class of the Allison 1710 and RR Merlin with 33/35,9 Liter compare to 28 Liter engines, the DB 603 44 Liter at the same class as the RR Griffon and the Jumo 213 35/36 Liter engines.
At first sight to me, DB bought this "light weight" with relative low RPM's. After the development of the Jumo 213 out of the Jumo 211 with high pressure water cooling, Junkers was able drive the Jumo 213 with 500 rpm more then the Jumo 211. DB was not able to manage a much higher RPM at the DB 605 (100 RPM higher then the DB 601), although the DB 605 was converted also to a high pressure water cooling and sleeve bearings instead of roller bearings.
The DB 605 was next to the DB 603 compare to RR (Merlin, Griffon), Allison (V1710; Junkers (Jumo 213), the inline engine, that was driven with the lowest RPM.

I couldn't fathom this to the ground, but there must be different design philosophies.

You're right about the Junkers having a different philosophy to Daimler-Benz. Something I never really considered, and always seems to be glossed over, the discussion usually returning to DB 6 vs Merlin/Griffon.

Did Junkers have 2 or 3 speed superchargers, rather than the variable speed superchargers of the DB series?

 

[DonL]

Did Junkers have 2 or 3 speed superchargers, rather than the variable speed superchargers of the DB series?

Yes Junkers worked with 2 speed superchargers and not variable speed superchargers like DB, till the Jumo 213E, which had the first two stage 3 speed supercharger of any german engine in production.

 

[Shortround6]

In 1935-36 everybody had 87 octane fuel, they knew 100 octane was coming (Howard Hughes set the world speed record in 1935 using 100 octane) but they did not WHEN 100 octane was coming as a PRACTICAL fuel ( Howard's fuel cost 4.00 a gallon, many times normal aviation fuel). The basic decisions as to engine size, rpm and such were made at this time.
This is one reason for the British Rolls Royce Vulture, While they may KNOW the Merlin could STAND UP to 1600-1800hp they could not get that power from a service engine using service fuel.
A DB 601 was about 22% bigger than the Merlin in displacement but the early ones ran at about 20% slower so the actual air through the engine was within a few % of each other at similar boost levels. and the early engine weighed very close to each other and put out similar amounts of power.
Once production lines are established it becomes a question of how much power can be squeezed out of the existing design and here is where the British had an advantage in fuel. The British went through at least FOUR different fuel specifications/capabilities.
WHile they knew 100 octane fuel was coming NOBODY had any idea IF or WHEN such things as 100/130 or 100/150 would ever show up. It was almost science fiction in 1937-38.

 

[DonL]

@ SR6

I don't disagree with your summary, but fuel was always a serious issue to Germany, and as I said, they did get the patent of lead tetraethy from Standard oil not until 1936.
So to my opinion perhaps german engine designer were a little more conservative/careful and choose capacity against boost.

What do you expect from a "what if" german 28 Liter engine, that would be stuck to B4 or later perhaps C3 with Junkers philosophy and 3200 rpm?
I don't think this "what if" engine would be able to get past 1300 PS (take of power) under the given circumstances.

I also think that MW 50 and GM1 were only developed, because of a compensation of their "low" grade B4 fuel the entire war.
The criticism of Oldskeptip is to me absurd, because he is assuming equality of arms, which was never given at the fuel issue.

 

[altsym]

Where/how did you calculate those temperatures, using what assumptions.

T2/T1 = (p2/p1) y-1/y

Where:
T1= ambient air temperature
T2= temperature after the compressor
p1= ambient atmospheric pressure (absolute)
p2= pressure after the compressor (absolute)
Y = Ratio of specific heat capacities = Cp/Cv= 1.4 for air
Cp= Specific heat at constant pressure
Cv= Specific heat at constant volume

 

[Aozora]

In 1935-36 everybody had 87 octane fuel, they knew 100 octane was coming (Howard Hughes set the world speed record in 1935 using 100 octane) but they did not WHEN 100 octane was coming as a PRACTICAL fuel ( Howard's fuel cost 4.00 a gallon, many times normal aviation fuel). The basic decisions as to engine size, rpm and such were made at this time.
This is one reason for the British Rolls Royce Vulture, While they may KNOW the Merlin could STAND UP to 1600-1800hp they could not get that power from a service engine using service fuel.
A DB 601 was about 22% bigger than the Merlin in displacement but the early ones ran at about 20% slower so the actual air through the engine was within a few % of each other at similar boost levels. and the early engine weighed very close to each other and put out similar amounts of power.
Once production lines are established it becomes a question of how much power can be squeezed out of the existing design and here is where the British had an advantage in fuel. The British went through at least FOUR different fuel specifications/capabilities.
WHile they knew 100 octane fuel was coming NOBODY had any idea IF or WHEN such things as 100/130 or 100/150 would ever show up. It was almost science fiction in 1937-38.

Not quite; in 1937-1938 100 Octane fuel was being trialled on at least three RAF squadrons:

In September 1938 100 Octane was approved for use in Hurricanes and Spitfires:

Rolls-Royce were exhibiting 100 octane fueled Merlin Xs and quoting figures for Merlin IIs in December 1938:

while, at about the same time, the first deliveries of non-trail quantities to Fighter Command stations was underway:

 

[Shortround6]

In the engines designed in 1934-37 it was more a question of of capacity (displacement) vs rpm. Max boost was still pretty low in most engine.

The Kestrel used in the Bf 109 and Ju 87 prototypes used a max of 3.5lbs boost (1.24 ata?). Granted it was an in service engine and not a prototype.

R-R had Built the the 35 liter Condor from 1918 to 1926 or so ( and a diesel version in 1932) plus the Buzzard of 36.7 liters which they developed into the "R" racing engines. The Buzzard only weighed 1140lbs.

I don't know why R-R dropped to 27 liters for the Merlin but they certainly had experience and the capability to build larger engines. But Rolls had only been making aircraft engines for less than 20 years, the idea that the Merlin would still be in first line service 10-15 years in the future would probably have shocked the designers of the time. The Merlin being R-Rs 6th commercial (non racing) engine in under 20 years.

I think you are right in that a 27-28 liter engine with 87 octane fuel would not have made it past 1300 PS no matter who made it.

MW-50 does help with the lower octane fuel ( and helps with the cooling too). GM-1 is sort of split. it provides extra power pretty much ONLY at high altitudes and is sort of a replacement for two stage superchargers. Of course two stage superchargers rather depend on high performance fuel as even with inter coolers the intake charge can be hundreds of degrees F due to compressing the air 5-6 times at altitude so even if the total boost is held to only 6-12lbs that level of boost at 25,000-30,000ft requires a different fuel than the same level of boost at 15-20,000ft.
If the Germans had more access to high performance fuel earlier and could be assured of continued large scale access they might have take a different path.

They might have pushed too hard for fuel economy given their fuel situation which hurts peak power production. The Allison used higher compression than the Merlin up until the post war engines, it gave better cruise performance (power per unit of fuel burned) but limited the amount of boost that could be used at any given time in comparison to the Merlin. The Post war engines used in teh P-82 dropped the compression to the same as the Merlin and used more boost than war time engines.

raising the compression ratio raises the peak pressure in the cylinders (requiring heavier construction) faster than raising the boost to get the SAME power.

 

[Shortround6]

Not quite; in 1937-1938 100 Octane fuel was being trialled on at least three RAF squadrons:


In September 1938 100 Octane was approved for use in Hurricanes and Spitfires:


Rolls-Royce were exhibiting 100 octane fueled Merlin Xs and quoting figures for Merlin IIs in December 1938:


while, at about the same time, the first deliveries of non-trail quantities to Fighter Command stations was underway:

There was a big difference between 1935-36 and 1938. And in 1938 the British didn't understand ( nobody really did) what the actual differences were between British 100 octane fuel and American 100 octane fuel and how it would affect engine performance. as for 100/130 fuel?? the performance number scale that the 130 part comes from didn't exist until late 1040 or early 1941. You can't have OVER 100 octane fuel. The octane rating scale is based on comparing the test fuel to known reference fuels which were blends of iso-ocatane and heptane. 87 octane fuel has to ACT like a blend of of 87% iso-octane and 13% heptane even if it contained little, if any, of those two compounds. Once you hit 100% iso-octane you had no way to measure performance above that level until a new rating scale was established.
They knew they could get better performance that 100% iso-octane but they didn't know how to measure it or how to specify it. Or test batches of gas BEFORE putting it the engine or aircraft. The idea that you could have a fuel that would allow 30-50% MORE performance than 100% iso-octane in 1938 was close to science fiction. People could and did predict it was coming but nobody could say when, or how.
British 1940 "100 octane" was NOT 100/130 fuel. It was usually 100/115 to 100/120 depending on batch and/or refinery once the test procedures were instituted. Of course once you CAN TEST the result it is a lot easier to vary small batches to come up with the results you want.

 

[tomo pauk]

Seem the discussion is now in less bickering and more in data, informations and analysis - thanks to the contributors.

 

[Balljoint]

It may be useful to this discussion to review the several mechanisms to allow fuel to burn rather than detonate in an engine. The bad actor is actually the O2 molecule. When subjected to temperature and or pressure, the O2 forms highly reactive free radicals which come in two progressive reactivities. The more reactive, given the more extreme conditions that cause its formation, causes cascading reactions, i.e. detonation. Thus limiting fuel charge pressure and temperature is the prime cure for detonation.

Tetraethyl lead is just a carrier for the lead in that it breaks down under the high temperature/pressure freeing the lead. The free lead is highly reactive and soaks up the O2 free radicals to prevent detonation.

Yet another distinct approach to high octane rating is to form bigger, cyclic molecules that are more resistant to rapid combustion. This is accomplished through cracking and reforming petroleum feed stocks to end up with ultrahigh octane fuel. Reforming was probably the key to production of large quantities of high octane fuels. Just a thought, but since Germany was starting from coal to make fuel –and munitions- , reformation should have been within their technical capabilities though, as with other needs, developing the capacity may have been problematical.

 

[tomo pauk]

One forum member mentioned earlier that a license-built Hispano 12Y engine (in the UK) would be the good candidate for the Westland Whirlwind. Interestingly enough, the Germans have had under control, from early 1939 on, a factory that was making 12Ycrs engines under license - Avia of Czechoslovakia. Since the 12Y was far lighter than the DB-601 or Jumo 211, and only some 50 kilos heavier than the Jumo 210, why not install these engines on the Fw-187? The power jumps from 680 PS at 3700m to 860 PS at 4000 m; the take off power from 690 to 835 PS.