enginehistory.org

[smoore]

The C/R of early configurations of this engine was the same for all cylinders, which was, I believe, 6.9:1. Development of the engine included inter alia a higher compression ratio, and quite rightly too! However, the C/Rs of the L/H and R/H banks of the later configurations were different, a feature which I find amazing - possibly unique. For instance, the data for the DB605 I have are
Right bank - 7.3 : 1;
Left bank - 7.5 : 1.
One theory I've read accounting for this is that it's to reduce the risk of detonation from the effect of oil contamination of the right bank's cylinders/combustion chambers. However, I find it hard to believe German engine designers were prepared to accept uneven firing thrusts for such a reason - cylinder mouth baffles to reduce oil fling would surely be a better option.
My (purely speculative!) view is the different C/Rs were introduced to equalise the 12 firing thrusts to offset loss of volumetric efficiency from uneven gas distribution to the cylinders through the rather convoluted induction system and offset supercharger fitted to these engines. So - does anyone have any information on the unequal C/Rs?

[jwells]

There is some documentary evidence in existence which confirms that the reason for the differential compression ratios in the DB 600 series inverted V-12 engines was due to oil control problems.

It needs to be remembered that the development of the German inverted V-12 motors took place very quickly. The specifications for high-powered aero-engines were determined by a "think-tank" which was assembled in Berlin at the behest of the RVM in 1928. It was at this point that the requirement for the engines to be of the inverted kind was laid down.

German aero-engine manufacturers had almost no experience of inverted engines, particularly V-type ones.

The development of the Bf 109 illustrates the tight time-line that occurred re the German V-12 engines, eg the 1935 Bf 109V1 had a Kestrel engine installed, the 109V2 had a JUMO 210. The first DB600 engine in a Bf 109 didn't happen until 1937!!

The oil control problem would not have manifested itself for some time due to the rush to get the DB engines into production. Probably, the earlier 601 engines would have got low-compression pistons fitted into the left-hand banks after one or two overhauls.

The literature pertaining to the earlier engines would have been written and published long before the oil control problem was recognized hence no mention of different CR's in the manuals etc.

The interesting indirect evidence for the CR difference comes from a study of the Junkers 211. It was a similar engine so why did it not suffer from the same problem?? Answer - the lubrication system used was very different to that in the DB. End-feed type instead of convential gallery design. End-feed gives much better control over oil flows through the crankshaft and big-end bearings.

The off-set supercharger had nothing to do with this.

[pgeany]

I believe that the needle roller type big end bearings that were used in the DB engines contributed to excess oil being present in the crankcase. This oil then being flung towards the right bank. It could be said that the lubricating system was a weakness and accounts of big end failures are not uncommon with the DB 600 series, especially the 605 in the Me 109. In fact I think their has been one in modern times in one of the Hispano Buchons that have been re-powered with 605's.