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

Engineman · Sep 1, 2024

hobbes154 said:
Having belatedly read The Secret Horsepower Race, apparently German engines were not getting the maximum boost possible out of their existing fuels (especially C3 but even B4) for most of the war due to those issues.

Main benefit of C2 was that it would not have reacted with fuel tank lining or got into the lubricating oil the way C3 initially did due to its different chemical composition. So the DB601N might actually have worked.

Yes, reading and understanding the detail in TSHR by Calum Douglas is the most accurate way to relate the German problems with their engines, and some of their aircraft decisions as well! Most of the German problems were basic resource issues, fuels and metals.
So, the DB 601 N was a successful upgrade at the time, but that upgrade was only ever a short-term improvement for the DB 601 A and used in the late Bf 109 E and some Bf 110. The slightly late development of the DB 601 E then forced the DB 601 N to be used in the Bf 109 F1/F2 and the problems with long-stored "C3" (as it was at that time), the rubber fuel bag-tank contaminating the fuel and the oil dilution problems, combined to cause big problems. But that was just the start of it!

Eng

RichardSuhkoi · Sep 1, 2024

hobbes154 said:
Having belatedly read The Secret Horsepower Race, apparently German engines were not getting the maximum boost possible out of their existing fuels (especially C3 but even B4) for most of the war due to those issues.

Main benefit of C2 was that it would not have reacted with fuel tank lining or got into the lubricating oil the way C3 initially did due to its different chemical composition. So the DB601N might actually have worked.

http://www.wwiiaircraftperformance.org/Analysis_German_Fuels.pdf

This collection was a surprise to me.
Separately, is not oil dilution driven by factors such a cold cylinder wall condensation, excess large droplet size, excess ring gap leakage, low oil temperature, and others? I know alcohol fuel is very prone to cause oil dilution and I associated this with low vapour pressure but don't really know.

hobbes154 · Sep 1, 2024

RichardSuhkoi said:
http://www.wwiiaircraftperformance.org/Analysis_German_Fuels.pdf

This collection was a surprise to me.
Separately, is not oil dilution driven by factors such a cold cylinder wall condensation, excess large droplet size, excess ring gap leakage, low oil temperature, and others? I know alcohol fuel is very prone to cause oil dilution and I associated this with low vapour pressure but don't really know.

They may all be factors but the point Calum Douglas emphasised was that C3 had a high boiling point, so once it was in the oil it was much harder to get out again (incidentally easier on BMW 801 vs DB 601 because the air-cooled engine ran hotter). So I should have said "stayed in" rather than "got into" - fixed.

henshao · Sep 30, 2024

Shortround6 said:
There is a story that one P&W engineer ran a R-2800 at well over 100in on a factory dyno. Over 3.0 ata. He was in the next test cell to some guys working on the R-4360 (28 cylinder engine) and he was trying to beat them. He was using enormous amounts of water/alcohol. It was only for a few seconds. The engine survived (as supposed to have been a "B" series engine and not one of the later "C"s.
This is why they have confidence it rating engines they way they do for service in the real world (engines with several hundreds of hours on them maintained by 18-20 year old mechanics, sleeping in tents, eating lousy food.
What an engine did once or twice on a test stand is interesting, what it could do for a number of hours is a lot more interesting.

The US tested a P-47 in Europe using 44-1 fuel (100/150?) and ran it at 65in without water and 70in with water. Things were OK in level flight but in climbs there were problems with both cylinder head temperatures and carburetor inlet temperatures. Low airspeed did not provide enough cooling airflow for either the engine or for the intercooler/turbocharger.
Not breaking the engine on the test stand is step 1.
Keeping the engine alive using the service radiators/oil coolers, etc is a lot harder.

In mid 1944 the 8th AF tested all their fighters on 100/150 octane fuel, the P-47 was operationally cleared for 70" manifold pressure at this boost level with water injection, peaking at more than 2800hp. Mustang and Lightning also saw increases in operational boost clearance with this detonation resistant mixture

150 Grade Fuel

Although 2800hp is an eye-popping figure the biggest gain IMO is the new 65" military power rating, giving some 2350hp whether the plane had water or not

Luft.4 · Sep 30, 2024

If I may chime in here, I would say that access to high octane fuel would have changed little overall because performance was also dependent on other factors.
One example: the MW using DB 605s are commonly stated to run at compression ratios of 8.3/8.5 which is not correct. They ran at 7.3/7.5, even dropping down to 7.0 by Feb. 1945. Why? Because of factors such as poor metallurgy etc. They wanted the D series wanted to run on C3 and 8.5 compression, but the pistons could not take it, hence the drop back down to 7.5 … and this was decided in spring 1944!
I'd say the prime of the DB 605 was between mid-September 1944 and late January 1945 where they could still run them with MW at 7.5CR on C3 without reduced performance. To push beyond what the Luftwaffe already had, they would need better metallurgy rather than better fuel (which was pretty good btw, they just couldn't take full advantage of it hence why the fuel is not the only issue).

tomo pauk · Sep 30, 2024

Luft.4 said:
If I may chime in here, I would say that access to high octane fuel would have changed little overall because performance was also dependent on other factors.
One example: the MW using DB 605s are commonly stated to run at compression ratios of 8.3/8.5 which is not correct. They ran at 7.3/7.5, even dropping down to 7.0 by Feb. 1945. Why? Because of factors such as poor metallurgy etc. They wanted the D series wanted to run on C3 and 8.5 compression, but the pistons could not take it, hence the drop back down to 7.5 … and this was decided in spring 1944!

Would you be so kind to post some sources that MW50 using DB 605s were dropping the CR down to 7:1 by Feb 1945? Ditto for the 605D having the CR of 7.5:1? Thanks in advance.

Luft.4 said:
I'd say the prime of the DB 605 was between mid-September 1944 and late January 1945 where they could still run them with MW at 7.5CR on C3 without reduced performance. To push beyond what the Luftwaffe already had, they would need better metallurgy rather than better fuel (which was pretty good btw, they just couldn't take full advantage of it hence why the fuel is not the only issue).

By Spetember of 1944, there were the 605s with big S/C in service by good numbers, the basic engine was debugged, so indeed they were in their prime; power at the 'bomber altitudes' (7 km and above for the West) was comparable with the 2-stage Merlin.
What was not there was the 2-stage supercharged DB 605 version already by late 1943/early 1944, to provide an even greater increase of power above 7 km. The 2-stage superchargers required no fancy materials, and DB wasted a lot of time designing complicated 2-stage superchargers that were nowhere. At the end (late 1944), both Jumo and DB used the compact template from the Merlin 61 with both stages being run by the same shaft, but the actual 2-stage supercharged engines were used only in token numbers before end of the war (and probably no 605L saw any combat).

Gemhorse · Sep 30, 2024

Awesome discussion -
(Thanks Wurger & Der Adler ist Gelandant ~ It is great to be back, am feeling very rusty though ~ Cheers )

Luft.4 · Sep 30, 2024

tomo pauk said:
Would you be so kind to post some sources that MW50 using DB 605s were dropping the CR down to 7:1 by Feb 1945? Ditto for the 605D having the CR of 7.5:1? Thanks in advance.

ChefTLR KTB in BAMA. Otherwise see the work Das Jagdflugzeug Messerschmitt Bf 109 by Michael Baumgartl.

tomo pauk · Sep 30, 2024

Luft.4 said:
ChefTLR KTB in BAMA. Otherwise see the work Das Jagdflugzeug Messerschmitt Bf 109 by Michael Baumgartl.

Thank you again.
Is the ChefTLR KTB available on-line at BAMA?

RichardSuhkoi · Sep 30, 2024

henshao said:
In mid 1944 the 8th AF tested all their fighters on 100/150 octane fuel, the P-47 was operationally cleared for 70" manifold pressure at this boost level with water injection, peaking at more than 2800hp. Mustang and Lightning also saw increases in operational boost clearance with this detonation resistant mixture

150 Grade Fuel

Although 2800hp is an eye-popping figure the biggest gain IMO is the new 65" military power rating, giving some 2350hp whether the plane had water or not

The memories are getting all fuzzy now but I recall talking to a well known engine builder who told me he measured 6000 HP for some seconds on his 4430 running alcohol using his home made EFI system. He explained how running rich kept things from melting, but excess fuel diluted the oil.

Luft.4 · Sep 30, 2024

tomo pauk said:
Thank you again.
Is the ChefTLR KTB available on-line at BAMA?

Yes. Other docs may be found in the book and give the earlier date of feb. Those are from the DB archives.

Engineman · Oct 1, 2024

Luft.4 said:
If I may chime in here, I would say that access to high octane fuel would have changed little overall because performance was also dependent on other factors.
One example: the MW using DB 605s are commonly stated to run at compression ratios of 8.3/8.5 which is not correct. They ran at 7.3/7.5, even dropping down to 7.0 by Feb. 1945. Why? Because of factors such as poor metallurgy etc. They wanted the D series wanted to run on C3 and 8.5 compression, but the pistons could not take it, hence the drop back down to 7.5 … and this was decided in spring 1944!
I'd say the prime of the DB 605 was between mid-September 1944 and late January 1945 where they could still run them with MW at 7.5CR on C3 without reduced performance. To push beyond what the Luftwaffe already had, they would need better metallurgy rather than better fuel (which was pretty good btw, they just couldn't take full advantage of it hence why the fuel is not the only issue).

Yes. Original "DB 605 D" information does show that the aspiration was to have the higher CR 8.5/8.3 and many documents, some original some speculative, quote the higher ratio.
It should be noted that the "DB 605 D" started as a projected high altitude DB 605 in 1941 and that had at least two variations, then as a later improved variations through to 1942/3 that were only possibly prototypes. Later in 1943, an engine much like the eventual 605 D was conceived and prototyped. This may have had aspirations for 8.5/8.3 CR, but the paperwork is surprisingly shy about mentioning the CR. In fact, almost all documentation for the 1943-45 DB 605 D versions, that were in production, fails to specify the CR, contrary to normal DB technical documentation. Yes, there are "Leistungsblatt" and other post war data sheets that quote 8.5/8.3 but the actual technical documentation does not and I believe the production DB 605 D in 1944 was 7.5/7.3.
Unfortunately, I do not have sight of the "ChefTLR KTB" or the book Das Jagdflugzeug Messerschmitt Bf 109 by Michael Baumgartl yet, so I do not know what references there are there.

As an aside, Rolls Royce were intending to reduce the CR of some Merlin versions for higher power in about 1943, but the success with fuels and supercharger developments made that
complication unnecessary.

Eng

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

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