Observations on WWI Petrol Octane

[Dan_San_Abbott]

Hello Bletchley:
I was wondering about other engines and the Special aviation fuels:
Maybach MbIVa 245 Ps, used inthe Ru.C.VII.
Benz BzIIIav 195 Ps used in the Rol.D.VIb.
Mercedes D.IIIaü 180 Ps used in the Fok.D.VII, Pfalz D.IIIa,D.XII, Alb.D.Va and Rol.D.VIa.
Siemens Schuckert ShIIIa 160 Ps used in the SSW D.III, D.IV, & Pfalz D.VIII.
Happy New Year Bletchley,
Dan-San

[Bletchley]

Hello Dan-San

I think most, if not all, of the late-war overcompressed engines could use either Flugbenzin or Fliegerbenzin, but the BMW IIIa appears to be (possibly) the odd one out here - I think that the Fliegerbenzin probably had a 'highest useful compression ratio' of around 6 to 1, at least the winter version, as in chemical composition it looks very similar to Ricardo and Kewley's 'super' Borneo. The BMW, with a compression ratio of 6.4:1, would therefore have to be throttled back (as it was, to 1400 rpm) at ground level even with this fuel to prevent detonation. With Flugbenzin (or worse, Leichtbenzin) then I think the fuel would probably have been detonating even when throttled back to 1400 rpm and 185 PS at ground level.

I think that the Mercedes D.IIIau was designed to run on Flugbenzin, but would give better low altitude performance when running on Fliegerbenzin. When the British tested it, using British (i.e.Shell) aviation fuel (very similar to Flugbenzin), they found that this engine, although it was designed to run at around 160 PS / 1400 rpm at ground level, was mechanically quite able to run at 1500-1600 rpm for periods of half an hour or longer. At these higher rpm, however, it would start to loose power above 1500 rpm (around 170 hp at 1500 rpm, hardly any better at 1600 rpm). German pilots, on the other hand, appear to have been getting 180 PS from this engine at ground level. I think with Fliegerbenzin the engine could be unthrottled to around 1600 rpm at ground level to deliver 180 PS without loosing any power to detonation, due to the higher aromatic content of this fuel.

The Maybach Mb.IVa had a very peculiar carburettor - fully described in my 'altitude compensating carburettors' thread. It could be run on a rich mixture at ground level (as the British did, when testing it), to give a high powered low altitude engine that could develop around 300 hp (it was a very heavy engine, and could clearly take the mechanical stress). As used mostly by the Germans, however, it would function well on a weak mixture at ground level (245 PS, held constant) as a high altitude engine. I would guess that when used in this way, without a rich mixture to prevent detonation at ground level, it would need to run on Fliegerbenzin to prevent detonation (it had, from memory, a high compression ratio of 6:1).

I don't know much about the Benz BzIIIav or the ShIIIa carburettors, so I am rather reluctant to comment on either.

Although many of the late-war overcompressed engines appear to have been capable of running on either Flugbenzin or Fliegerbenzin, I do not think that it would have been likely that these fuels would have been used alternatively on a day to day basis - one of the characteristics of a high benzol content fuel is that the benzene content gives it a strong solvent effect. If an engine has been run on gasoline, and is switched to a high benzol/gasoline mixture, the benzol will strip off all the carbon deposits left by the previous gasoline fuel and gunk up the engine. So, in switching from gasoline to a high benzol fuel the engine would have to be cleaned thoroughly to remove all this first. Also, I have no idea how much Fliegerbenzin was produced - if in only small quantities at first, it may have been reserved for those engines that really needed it (Mb.IVa, then BMW IIIa), until the demand for Flugbenzin started to outstrip supply.

Bletchley

[Bletchley]

Yavor,

Bullinger, Kriegserfahrungen aus der Flugzeugmeisterei, Bericht IV: Die Betriebstoff-Versorgung.

I don't think this is a book, but a reference to a document in the Bundesarchiv:

Dip.Ing. Bullinger, Kriegserfahrungen in der Motorenabteilung der Flugzeugmeisterei. RH 61/1133

2.Abteilung A
2.2 Gruppe II (Kriegsrustung und Kriegswirtschaft)
2.2.5 Rohstoffe
2.2.5.5 Energie und Brennstoffversorgung

Bletchley

[Dave_Watts]

Hello Bletchley,

Your postings are superb!

Dan-San has been hitting around what I've been asking over on the BMW "F" thread, and Kacey posted on this thread asking if the "F" designation of the Fokker D.VII could possibly be an abbreviation for Flugbenzin or Fliegerbenzin?

I think it is a good candidate.

Now with that said, (and to elaborate on what Dan-San was asking);

Did they use Flugbenzin in the Mercedes D.IIIau or at least in the D.IIIauv? This may be asking too much for a definitive answer.

If you think they likely used the Flugbenzin for all late production flugmotors, then are we saying they would have only had the "hot" Flugbenzin at petrol for all front-line fighter aircraft?

The problem is did they have to run two different fuels at the front? One for the new BMW aircraft and another for the "standard" Mercedes D.IIIau aircraft?

My angle on these questions is driving towards an answer to the "F" designation question.

As I pointed out on the BMW "F" thread, Fokker built D.VII's with an "F" designation painted right on the side of the airplane, clear as day to see. Albatros and OAW included the "F" designation on their attached factory builder's placards, but nothing readily visible, and there is no "F" designation on the official Military placard for the same aircraft. Point being this designation appears to be utilized by the builders indicating BMW or equivilant powered aircraft, but not Idflieg. Any ideas?

Thanks for your help, I know you don't have all the answers.

Best,
Dave W.

[Bletchley]

Hello Dave,

The only references I have seen to the actual use of such a very heavily benzolated fuel (35% or more benzol content, Fliegerbenzin) are references specifically to the BMW IIIa, in the German publication by Dechamps & Kutzbach (published in the early 1920s, but with much of the data derived from work done and tests made at Adlershoff in 1918), or in British tests on this engine in 1919, where they used both 20/80 and 50/50 mixtures of benzol and gasoline.

This is not to say that it was not used in other engines - the very steep rise in the compression ratio in all the late-war 'hohenmotoren' point towards the use of a more aromatic fuel than either Leichtbenzin or Flugbenzin, and there is evidence that they performed better with Fliegerbenzin than Flugbenzin.

On the other hand there was a general prejudice against 'heavy' fuels, as it was generally considered, until the research done by Tizard and Pye after the war, that the 'lighter' gasolines were more suitable for aero engines and the 'benzol rich' fuels were inferior because of their lower calorific content (this is the reason that Shell 'Light Borneo' was considered to be inferior to 'Sumatra'), and in the case of low compression engines this was probably correct, although exagerated (only a maximum 2% difference in power).

Also, I have no idea how much of this benzol-rich Fliegerbenzin was produced or delivered to the Front. In my opinion the BMW IIIa would have had some problems with detonation on Flugbenzin, even when it was throttled back to low-altitude running (i.e without use of the second throttle or 'overgas'), and would have required at least the 'winter' Fliegerbenzin or something else very similar. Anecdotal reports have been posted here, by Stephen Lawson, of US pilots after the Armistice taking Fokker D.VII aircraft for a spin and suffering from fatal crashes because of the wrong fuel:

"'Benzine' was the German fuel and so high in octane that you couldn't run Allied gasoline through a Fokker D.VII 180 hp Mercedes D.IIIau without it choking the engine off. This is the fatal mistake U.S. Aero Sqdns made after the war. Many of the post war pilots KIC were due to the fact that Allied aviation gas was used to top off the tanks of the captured German aircraft rather than 'Benzine'"

I asked Stephen if he had any more details on this, and he said he didn't, because I thought then that there may have been a confusion here between the Mercedes and BMW. I have read several accounts, from British memoirs, of post war RAF pilots 'acquiring' a personal D.VII, all of them apparently of the Mercedes type, and none of them reported any problems of this kind (and all survived to write their memoirs). Also, British tests on the D.IIIau using a British fuel (admittedly, more aromatic than the US fuel) indicated that it would run OK up to around 1500 rpm (but would cut-out if the throttle was advanced to far into the altitude range at low altitude).

My best guess is that Fliegerbenzin would have been reserved, initially, for the BMW IIIa engines (possibly, also, the high flying photo recon. version of the Rumpler with the Mb.IVa), but as fuel shortages at the front started to bite and the advantages of this anti-knock fuel were appreciated then its use might have spread to the other high-compression engines such as the Mercedes D.IIIau. One can imagine, in a unit operating both Mercedes and BMW Fokkers, that the Fliegerbenzin might have been set aside for the BMW Fokkers, until someone filled up a Mercedes tank with the fuel (either by a mistake, or because of a shortage of Flugbenzin) and found that it gave the Mercedes better low-altitude performance (up from 160-170 PS to 180 PS). Or a unit operating BMW Fokkers may have been allocated just the Fliegerbenzin, in the knowledge that it would do the Mercedes no harm (like putting high octane petrol in an car designed to run on low octane petrol).

KACEY might be able to add more on the different octane needs of the various engines, and the extent to which the German fuels would have met them, as he has done all the research and calculations on this in the previous post referenced at the start of this one.

Bletchley

[Dave_Watts]

Hi all,

Bletchley,

Thanks for the reply. For clarity, am I correct to understand;

1.The BMW only runs on the Flugbenzin.

2. The D.IIIau runs acceptably on Flugbenzin.

If this is the case, and you're stating if you put non-Flugbenzin/standard av-gas in a BMW it will conk-out and crash, then an "F" designation would be very important.

Now, speaking logistically, when these BMW D.VIIF's were delivered to a Jasta they would have required the Flugbenzin to follow them, correct?

These BMW D.VIIF's were in short supply originally, (as attested by Goring), and no Jasta was fully equipped with exclusively D.VIIF's, there was always a mixture of Mercedes and BMW powered D.VII's. As the majority of D.VII's were Mercedes powered, (77% from my speculative number crunching), the generalized use of Flugbenzin would have not made sense, unless the Jastas were broken into "F"-units and non-"F"-units, but it appears there is nothing to support this theory. By the way the D.IIIauv powered D.VII's shouldn't be taken into account as there were so few and almost none saw service.

We know that JG1, Jastas 4,6, 10, and 11 recieved the first deliveries of D.VII's, (Dan-San, Greg Van, and others would be more specific), mostly D.IIIa and D.IIIau Mercedes powered. While going through Mercedes delivery records there was an unusual delivery of 10 D.IIIau motors directly to Jagdgeschwader Richthofen on April 22nd. I can only think this was in support of the new D.VII's they had recieved. My point being more Mercedes motors not BMW's.

Was this Flugbenzin plentiful? If so, I could see its use for all D.VII's, like you said, running a car on Premium instead of Regular, no big deal, just costs more.

Thanks again,
Dave W.

[Bletchley]

"1.The BMW only runs on the Flugbenzin. 2. The D.IIIau runs acceptably on Flugbenzin."

Assuming that you mean 'Fliegerbenzin', then my guess is "Yes", although I must admit that I have come across no definitive statement or evidence to support this. I think that the BMW would probably have started to 'knock' when it was pushed up to 1400 rpm at ground level or low altitude, if it was being run on Flugbenzin instead, and this would have damaged the engine. On the Fliegerbenzin I think it would have run OK. I think the D.IIIau could have run on either fuel, but with improved low-altitude performance on the Fliegerbenzin.

"Now, speaking logistically, (Dan-San, Greg Van, and others would be more specific), when these BMW D.VIIF's were delivered to a Jasta they would have required the Flugbenzin to follow them, correct?"

Yes. Fliegerbenzin.

"Was this Flugbenzin plentiful?"

If you mean Fliegerbenzin, I would guess "No". I doubt that this would ever have been acceptable as a wholesale replacement for standard Flugbenzin, as it was a "heavy" fuel - either 60% benzol and 40% Light Benzin (summer), or 40% benzol and 60% Heavy Benzin (winter), and it would have produced less power than Flugbenzin in the standard low compression engine. But it had the advantage of containing either less or no Light Benzin (the main element of Flugbenzin), and as there sems to have been a severe shortage of standard Flugbenzin at the Front in the final months of the war, then the Fliegerbenzin may have been produced and used in greater quantities at this time to fill the gap, being used in other high compression engines as well.

Bletchley

[Dan_San_Abbott]

Hi Bletchley:
I do believe that this has been one of the most enlightening threads posted on the Forum. I have learned more about aviation fuel in this set of postings, ever ! I did not realize how the aviation fuel so determined the limits of engine technology in WW1. Thank you very much for the education. I shall continue to follow your efforts to enlightened us on engine technology. This has been most interesting.
Blue skies,
Dan-San

[Bletchley]

Thank you Dan-San - I appreciate your kind remarks, coming as they do from someone who has such a breadth and depth of knowledge in this area. I think a lot of the credit should go to KACEY, however, for his initial post on WWI petrol octane, and on the importance of matching engine development to the existing aviation fuels - I have just been trying to add a "why" to his "what", by digging around in the historical sources, so this thread should therefore be considered as a (rather belated) extension to his excellent and very detailed thread from last year

Bletchley

[Dan_San_Abbott]

Hello Kacey:
It seems I have failed to give you credit for your contribution to the subject of German aircraft fuels. I apologize for my failure to give you recognition of your contribution to this masterful thread.
It is those who ask what and why and search for the answers to those questions that opens the doors of knowledge and expands our understanding of history. Thank you very much Kacey.
Blue skies Kacey,
Dan-San