If German have access to high ocatane fuel, how does that impact the performance of their fighters? (1 Viewer)

[RichardSuhkoi]

I watched a video on youtube, the author explained that, even though BF-109 is lighter and has bigger engine than P-51, the P-51 ended up being much faster because USA has fuel with much better octane rating of upto 150 while German fuel octance rating is around 80-100.
So my question is, what would happen if we give FW-190, Ta-152 150 octane fuel ? Would they get a huge boost in performance? Or their engine can't handle it?

From an engineering point of view an interesting core aspect of octane and knock is combustion chamber design. Which engines / variants had the best engineered chambers? For example, if you did test cell work and monitored oxygen content to compare at identical mix ratios, then adjusted boost to match combustion chamber pressure prior ignition and mapped up to detonation, what would the data say? An idea chamber avoids pressure reflections causing detonation after ignition, and avoids sharp corners that heat up causing hot surface pre ignition. But here is a not widely known German attribute; surface gap igniters. Spark plugs with protruding electrodes risk the electrode edge reaching ignition temperature. Some German engineers realized they could design spark plugs with semi conducting flat insulators that avoided this issue. These spark plugs permitted sparking at much higher pressures than air gap and also permitted lower voltages out of magneto thus increasing reliability of cables and distribution at altitude. The patents were dissolved and taken up by various USA manufacturers and the concept proved critical to success of gas turbine engines.

 

[Engineman]

Surface discharge sparkplugs for piston engine or automotive use are a niche product. Generally, piston engine combustion benefits much more from moderate amounts of spark plug electrode projection into the combustion chamber. Modern high performance spark plugs are made to survive in very harsh conditions and are mechanically strong. The electrodes are made to not cause pre-ignition or unwanted detonation. They also last very well, 40,000 miles is a common service interval for modern high spec spark plugs. Also, the fine quality and small size of Iridium type electrodes give maximum exposure to the flame kernal, while offering minimum resistance to gas flow.

Eng

 

[RichardSuhkoi]

Surface discharge sparkplugs for piston engine or automotive use are a niche product. Generally, piston engine combustion benefits much more from moderate amounts of spark plug electrode projection into the combustion chamber. Modern high performance spark plugs are made to survive in very harsh conditions and are mechanically strong. The electrodes are made to not cause pre-ignition or unwanted detonation. They also last very well, 40,000 miles is a common service interval for modern high spec spark plugs. Also, the fine quality and small size of Iridium type electrodes give maximum exposure to the flame kernal, while offering minimum resistance to gas flow.

Eng

But, on the WW2 German engines where used, the attribute of interest was the reduced tendency to become a hot surface ignition source. Plume shape can be controlled and plume size increased with spark energy. Electrode wear wasn't a concern when engine TBO was 100 hours. Automotive spark energy is tiny, maybe 20 mJ. From memory, the big Seimens mags on WW2 German engines were at least several times that. It would be interesting to compare spark energy from say DB605 to Merlin. The other constraint is quench at high combustion pressures. Very good boost was available at max operating altitude so combustion chamber pressure at spark event was very high, but outdoor ambient pressure very low. These 1940 engines were flat rated due supercharge out to ~25,000 feet altitude. With air gap igniters say on Merlin, the gap had to be reduced so it would avoid quench at the peak open circuit firing voltage available. With 1940 materials, it was impossible to run extreme voltages without flashover at mag or cable. So the gap selected was not the gap ideal for ignition but rather the biggest gap they could set for the limited reliable peak open circuit voltage. Meanwhile the surface gap would fire at much lower voltages. I think someone told me of aero engines where mag 1 ran different gaps than mag 2, the intent to get optimal ignition at majority operating condition and avoid miss at max boost. I remain in awe of WW2 technology.

 

[tomo pauk]

These 1940 engines were flat rated due supercharge out to ~25,000 feet altitude.

Hello,
Can you please elaborate on this?

 

[Engineman]

But, on the WW2 German engines where used, the attribute of interest was the reduced tendency to become a hot surface ignition source. Plume shape can be controlled and plume size increased with spark energy. Electrode wear wasn't a concern when engine TBO was 100 hours. Automotive spark energy is tiny, maybe 20 mJ. From memory, the big Seimens mags on WW2 German engines were at least several times that. It would be interesting to compare spark energy from say DB605 to Merlin. The other constraint is quench at high combustion pressures. Very good boost was available at max operating altitude so combustion chamber pressure at spark event was very high, but outdoor ambient pressure very low. These 1940 engines were flat rated due supercharge out to ~25,000 feet altitude. With air gap igniters say on Merlin, the gap had to be reduced so it would avoid quench at the peak open circuit firing voltage available. With 1940 materials, it was impossible to run extreme voltages without flashover at mag or cable. So the gap selected was not the gap ideal for ignition but rather the biggest gap they could set for the limited reliable peak open circuit voltage. Meanwhile the surface gap would fire at much lower voltages. I think someone told me of aero engines where mag 1 ran different gaps than mag 2, the intent to get optimal ignition at majority operating condition and avoid miss at max boost. I remain in awe of WW2 technology.

Spark plug technology was advancing very fast in WW2. Some of your points are technical aims and research data that do not reflect the reality of WW2 German production.
The German magneto's on DB 601, 603, 605, 606, 610, JUMO 210, 211, 213, 222, BMW 801 production engines and almost all development engines had Bosch designed Magneto(s).
The Bosch magneto's were generally well designed and most had better designed ignition timing adjustment methods than the Merlin types.
German sparkplugs introduced ceramic insulators in the late 1930's. However, the service plugs during WW2 suffered badly from being limited to simple nickel-steel electrodes. Additionally, the German sparkplugs suffered from internal gas leakage throughout the war, and never really got the technology to match the allied Merlin sparkplug development.
Pressurised Magneto's for high altitude were produced by both Allied and Axis manufacture.

Eng

 

[RichardSuhkoi]

Hello,
Can you please elaborate on this?

"Flat rated " just means that instead of the typical power output of a normally aspirated engine, where when you plot horsepower against air density (altitude), instead of power dropping the line stays flat up until a certain altitude. This is accomplished by using turbo/super chargers to maintain a constant absolute manifold pressure. Let's say you have an engine that arrives at its thermal or mechanical limit at sea level (1 atm) plus 1/2 atm so 1.5 atm, then if your supercharger is able to maintain 1.5 atm at the manifold all the way to 20,000 feet, you say "flat rated to 20,000 feet". When adjustable boost systems came available such as two speed gearbox driving superchargers things got complicated as the curves intersected and where manual gear shift was required the pilot wanted to avoid fighting at the altitude where the lines intersected so to speak. I read that the FW190 the control system was automatic but was only functioning properly in late 1944 I think.

 

[RichardSuhkoi]

Spark plug technology was advancing very fast in WW2. Some of your points are technical aims and research data that do not reflect the reality of WW2 German production.
The German magneto's on DB 601, 603, 605, 606, 610, JUMO 210, 211, 213, 222, BMW 801 production engines and almost all development engines had Bosch designed Magneto(s).
The Bosch magneto's were generally well designed and most had better designed ignition timing adjustment methods than the Merlin types.
German sparkplugs introduced ceramic insulators in the late 1930's. However, the service plugs during WW2 suffered badly from being limited to simple nickel-steel electrodes. Additionally, the German sparkplugs suffered from internal gas leakage throughout the war, and never really got the technology to match the allied Merlin sparkplug development.
Pressurised Magneto's for high altitude were produced by both Allied and Axis manufacture.

Eng

True, I am using 35 year old memories on some of this stuff. I cut up an old spark plug

Spark plug technology was advancing very fast in WW2. Some of your points are technical aims and research data that do not reflect the reality of WW2 German production.
The German magneto's on DB 601, 603, 605, 606, 610, JUMO 210, 211, 213, 222, BMW 801 production engines and almost all development engines had Bosch designed Magneto(s).
The Bosch magneto's were generally well designed and most had better designed ignition timing adjustment methods than the Merlin types.
German sparkplugs introduced ceramic insulators in the late 1930's. However, the service plugs during WW2 suffered badly from being limited to simple nickel-steel electrodes. Additionally, the German sparkplugs suffered from internal gas leakage throughout the war, and never really got the technology to match the allied Merlin sparkplug development.
Pressurised Magneto's for high altitude were produced by both Allied and Axis manufacture.

Eng

I haven't worked with mags for about 35 years, so not surprised I had forgotten about pressurized mags. I sort of remember now the old Bendix mags having relief valves and supply regulators. Did WW2 ignition tap bleed air from the inlet manifold or was there a pump used?
I remember seeing an ignition cable from a vintage engine that was used inside a sheet metal conduit and it seemed to be glass fibre (asbestos?) wrapped but had a disintegrating coating. What was the dielectric withstanding on those old cables? Have you seen a German surface gap ignition system? On the old cable I looked the conductor was heavier than expected and my guess plated copper. I'd love to look at different ignition systems and measure characteristics to see what they were up to. From memory, I tested piston spark plugs against quench at 150 PSIA nitrogen and tested mags using a ball gap tester at I think the same 150 PSIA N2 and the test gap was not wide, I think 0.02". I was told by a very old ignition guy that the German mags ran more energy. Have you seen measurements or side by side waveforms? For comparable engines using air gap, for example DB605 vs Merlin, was gap similar? I once cut open a vintage "BG" spark plug and it was mica insulated. I have assumed without knowing that the higher performance spark plugs of WW2 were all alumina insulator equipped and sealed with tamped MgO or similar, now I'm curious.
I can say that gas turbine igniter leakage is still a challenge. Thermal shock cracking metal to inter metallic to glass seals.

 

[tomo pauk]

"Flat rated " just means that instead of the typical power output of a normally aspirated engine, where when you plot horsepower against air density (altitude), instead of power dropping the line stays flat up until a certain altitude. This is accomplished by using turbo/super chargers to maintain a constant absolute manifold pressure.

Perhaps we should not mix up the engines that were 'just' supercharged (by a gear-driven S/C) together with the engines that were with a S/C and a turbo-supercharger. The engines you are refering to (= Merlin, DB 601) belonged to the 1st group, while the military-grade engines of the second group were mostly produced in the USA (worth speaking about for 1940: a supercharged R-1820 + turbo for the B-17, a supercharged R-1830 + turbo for the P-43, a supercharged V-1710 + turbo for the P-38).

See here for the graph involving the altitude vs. horsepower for the DB 601A, dated October 1940 - lines are not flat, not just because the S/C was unable to maintain the best boost above 4.5 km.
Engines from the second category were indeed 'flat rated' (a.k.a. 'sea level rated') from S/L to 20000-25000 ft, thanks to having two superchargers working in series = 2-stage supercharging in effect.

Let's say you have an engine that arrives at its thermal or mechanical limit at sea level (1 atm) plus 1/2 atm so 1.5 atm, then if your supercharger is able to maintain 1.5 atm at the manifold all the way to 20,000 feet, you say "flat rated to 20,000 feet".

Agreed all the way.
Please note that the S/C of the DB 601A was perhaps able to maintain the 1.5 ata (close enough vs. 1.5 atm) up to 3 km, dropping down to 1 ata at 6.9 km (~22640 ft). At ~20000 ft (~6 km), boost was about 1.1 ata.

When adjustable boost systems came available such as two speed gearbox driving superchargers things got complicated as the curves intersected and where manual gear shift was required the pilot wanted to avoid fighting at the altitude where the lines intersected so to speak. I read that the FW190 the control system was automatic but was only functioning properly in late 1944 I think.

BMW 801 'kommandgeraet' was working well already by 1941. It were the other things plaguing it until 1941 - 1st the overall reliability (had a lot to do with material for the exhaust valves), later the supercharging system was insufficient vs. the Allied best.

FWIW, here is the power chart for the Merlin III (and the Merlin 60). Note that neither is 'flat rated'; Merlin III was with 1-speed S/C drive.

 

[Daggerr]

........................

FWIW, here is the power chart for the Merlin III (and the Merlin 60). Note that neither is 'flat rated'; Merlin III was with 1-speed S/C drive.

Note that maintaining a constant boost with increasing altitude will never result in a constant BHP.

Ambient temperature drops with altitude. So manifold temperature also drops with altitude.

Net result is an increasing BHP with altitude, up to full throttle height, due to higher manifold gas density. Above FTH boost drops and consequently BHP drops.

 

[tomo pauk]

Note that maintaining a constant boost with increasing altitude will never result in a constant BHP.

Ambient temperature drops with altitude. So manifold temperature also drops with altitude.

A lot depends on the 'target altitude' and the supercharging used. If that is around 20000-25000 ft - let alone more - and the turbo + engine-stage S/C is used (as it was the norm on the American turboed engines), the manifold temperature gets increased due to the ambient air being pressured two times. If the intercooler is of low capacity, like it was the case with early P-38s, already the carburetor temperature (ie. after only one stage of compressing) is too high and limits the boost and thus the power. See here.
You can also check out the manual for the P-38s, from the D to H.

Once the P-38s received the better intercoolers, their low- and high-altitude power was the same, vs. the earlier models having the better power at lower altitude.

Net result is an increasing BHP with altitude, up to full throttle height, due to higher manifold gas density. Above FTH boost drops and consequently BHP drops.

For the engines without the turbo in the picture and without the variable drive for the S/C - certainly. Note that the lower power at lower altitude had a lot to do with butterfly throttles being inefficient.
For the engines with the turbo addition, power graph is flat-ish until the rated altitude, and after that both the boost and BHP indeed drop with altitude.
Engines with variable drive for the S/C have the gradual drop of the BHP above the altitude where the throttle is opened, and the more severe drop past the rated altitude.
Engines with swirl throttle (that, at the end of the day, were much more efficient than the butterfly throttles), like the Mikulin's engines, or the Jumo 213s, again have a peculiar power graph shape; on the 213s, the power is ever greater the lower we go. FWIW.

 

[bf109xxl]

Engines with swirl throttle (that, at the end of the day, were much more efficient than the butterfly throttles), like the Mikulin's engines

AM-35 only, IIRC.

or the Jumo 213s, again have a peculiar power graph shape; on the 213s, the power is ever greater the lower we go. FWIW.

Or VK-107А.

 

[tomo pauk]

AM-35 only, IIRC.

It started out with the latest M-34 version, and carried through all the way with AM-42.

 

[GrauGeist]

The core of the issue, is that the engine designers (Allied & Axis both) developed the engines based on available fuel sources of the time.

Not what "could have should have".

 

[Daggerr]

.......................

Note that the lower power at lower altitude had a lot to do with butterfly throttles being inefficient.
......................

That seems to be a common misunderstanding but is not correct.

The difference in BHP between sea level and FTH for Merlin, or any other engine running at a constant S/C speed, is for 100 % the result of the change in ambient temperature with altitude.

For example the Merlin III in the power graph that you posted earlier:
At sea level, +12 psi, BHP = 1187 hp at 15 oC (288 K) ambient. At its FTH of 9,000 ft, +12 psi, BHP = 1303 hp at -3 oC (270 K) ambient.
BHP went up by a factor 1.098 while absolute ambient temperature dropped by a factor 1.067 resulting in a manifold temperature drop by a factor 1.067 and consequently a manifold density increase by a factor 1.067
In addition the S/C power consumption per kg air decreases by the same factor as its inlet temperature drops, so that also gives some increase in BHP which could bridge the gap between 1.098 and 1.067.

The Merlin S/C delivers a certain pressure ratio at a certain speed.
To maintain a certain boost (S/C discharge pressure) the S/C inlet pressure has to have a certain value so as to satisfy the pressure ratio.
The throttle valve lowers the ambient pressure to the required S/C inlet pressure by partly closing.
As altitude increases and ambient pressure drops the throttle valve will be less closed until its is fully open at FTH.
Neither the S/C nor the piston engine have any knowledge of that and they don't care.
Below the FTH the engine does not say: "OMG, the throttle valve is wasting energy so my BHP drops".
Below the FTH it really says: "OMG, the ambient temperature rises so my BHP drops".

That is not to say that a swirl throttle on a Merlin would not result in extra BHP due to less power consumed by the S/C. A swirl throttle changes the angle at which the air enters the S/C thereby changing the delivered pressure ratio and consequently the S/C power consumption. A swirl throttle (nowadays called variable inlet guide vanes) is a cheap alternative to a variable speed drive.

I will not further elaborate on this because that would be off topic as this topic is supposed to be about high octane fuels.

Calum Douglas has in the past years talked several times about German avgas composition and PN on his twitter account.
Bottom line: The Germans knew very well how to make good avgas.
Their best avgas C2 was at rich mixtures even better than the allied 100/150 but they were not able to make a lot of it.
Main difference between allied and German avgas was aromatics content.

 

[Shortround6]

Below the FTH the engine does not say: "OMG, the throttle valve is wasting energy so my BHP drops".
Below the FTH it really says: "OMG, the ambient temperature rises so my BHP drops".

There are at least 3 things going on, maybe more.
If the engine is running at the same rpm the supercharger (in a single speed supercharger) is running at the same speed (25,768 rpm for a Merlin III).
You are going to get pretty much the same temperature rise through the supercharger. Inlet air temp vs outlet temperature (manifold air temperature)
What you do get is lower intake air temperature at higher altitudes.
Sea level.... 59 F = 15 C
9000 ft.....26.9 F = -2.8 C
19,000ft..1.94 F = -16.7 C

For given octane rated fuel that helps limit the amount of boost you can use.
Merlin III will give a lot more than 12lbs boost at sea level, it could make 16lbs at 5500ft and could probably make around 23lbs at sea level.
The Merlin III will compress the air at 16,000ft ( I am going to round things off a bit) 2.6 times the ambient air pressure. It will do the same thing at sea level.

What the "throttle" has to do is restrict the pressure of the incoming air to the impeller to about 16.4in Hg (not accounting for the temperature). Supercharger will then 'boost' the pressure back up to 42.6 in Hg for the rated 6.25lbs of pressure for take-off.

This assumes that everything in the intake tract is operating the same and obviously with some sort or restriction/s (throttle plates?) blocking off 1/2 the airflow that is not the case.
The supercharger itself is not operating very effectively, flow is a combination of pressure and mass, We are 'tricking' the supercharger by blocking the intake to restrict both pressure and mass (actual air in lbs/min) and spinning the supercharger much faster than it needs to flow the volume (mass) of air we need to get the power (fuel/air burned) we want. The supercharger is operating way off it's optimum and that really screws things up.
Modern supercharger map.

Supercharger design had gotten a lot better but basic function/principles will apply. Supercharger is designed for best performance at a certain RPM and air flow. In the chart above we can get 60lbs of air flow at 78000 rpm and be in the 80% zone. But if our supercharger is fixed at 78000rpm and we want to flow only 30lbs/min we are approaching the surge line and the efficiency has dropped from 80% to 68% (?). Merlin III supercharger was about 70% at best.
Now for the 'mysterious' loss of the power. The Merlin supercharger drive shaft is taking the same amount of power to turn (or close) but the supercharger isn't using the power effectively. But less is going to actually compressing the air. I highlight a sentence earlier.

You are going to get pretty much the same temperature rise through the supercharger.

I sort of lied. That only applies if you are running at full throttle. If you throttle the engine down (restrict the intake) and make it run in area of the map it doesn't like the extra power doesn't disappear, it is converted to heat and it heats up the intake charge a lot more. If you are 'wasting' 10hp in the supercharger due to lower efficiency you are getting 10hp (7460 watts) of energy heating up your intake charge and that is going to play hell with the intake charge temperature, which is going to play hell with both the intake charge density/mass
and the detonation limits in the engine.

Look at the Merlin VIII engine or Merlin X engine in low gear to see the effect. Same supercharger turning slower is allowed to use higher boost and/or heats the intake air less.

Some people just lumped everything going on at low altitude into 'throttle loss' rather than go through a long explanation.

 

[Daggerr]

You are going to get pretty much the same temperature rise through the supercharger.

I sort of lied.

Yes, you lied.
Hooker et al make the same mistake in their famous booklet.

The temperature rise through the S/C is definitely not always the same, but varies with inlet temperature.

The Merlin S/C delivers a pressure ratio depending on its speed. Associated with that pressure ratio is a temperature ratio, not a fixed temperature rise.
A pressure ratio of say 2.6 would result in a temperature ratio of 1.45 at an efficiency of say 70 %.
If the S/C inlet temp were say 260 K then the S/C outlet temp would be 377 K, a rise of 117 K.
If the S/C inlet temp were say 250 K then the S/C outlet temp would be 362 K, a rise of 112 K.
So temp rise decreases with inlet temp, and a 10 K drop in inlet temp results in a 15 K drop in S/C outlet temp.
Less temp rise means less S/C power consumption per kg air, as its power is simply mass flow times temp rise times specific heat.

Most of what you say is similar to what I said except you used more words and numbers. However some statements I don't agree with.
The throttle valve does not make the S/C run less efficient when the objective is max power for a given boost.
If the throttle valve would be used to achieve a big reduction in BHP it would become a different story.

When operating at sea level or at FTH both at the same boost of say +12 psi and the same RPM of say 3000 the inlet pressure is in both cases equal to MAP divided by the S/C pressure ratio of say 2.6, so suction pressure is about 10 psia both at sea level and at FTH. The only difference the S/C notices between sea level and FTH is the change in it's inlet temperature due to the change in ambient temperature. The S/C does not know (and does not care) what the ambient pressure upstream the throttle valve is. The throttle valve simply maintains that 10 psia inlet pressure by partly closing and creating the necessary pressure drop from ambient pressure to 10 psia. That pressure drop does not affect the S/C power and the Merlin engine BHP.
All this does not result in enormous operating point moves in the supercharger map that would result in enormous changes in S/C efficiency. There will in this case be little, if any, difference.

A detailed calculation of Merlin BHP would not involve the throttle valve as there is no such thing as a "throttle loss" that needs to be taking into account in such calculation.
The BHP rise between sea level and FTH at a certain boost is only caused by the drop in ambient air temperature, nothing else.
If ambient temperature would be constant from sea level up to space then the Merlin BHP curve would be perfectly horizontal between sea level and FTH instead of rising with altitude.

Personal note: I am a chemical engineer with decades of experience as a process design specialist for the petroleum and petrochemical industry. Almost every plant design I worked on had at least one centrifugal compressor in it. I am very familiar with the working of compressors, and all other equipment in such plants, and of course also with thermodynamics and chemistry. I only mention this so that you and others don't think that I am just an amateur who does not really understand what he is talking about.

 

[Engineman]

Hi Guys,

I must suggest that anyone particularly interested with WW2 Supercharging buy's Calum Douglas' superb co-authored tome, Turbo/Supercharger Compressors and Turbines for Aircraft Propulsion in WWII (TSCT). Not cheap, but unsurpassed.

Eng