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.