The following is excerpted from a letter by Jerry Wells in "Torque Meter" Volume 2, Number 2 (Spring 2003) - magazine of the Aircraft Engine Historical Society. As can be adjudged from the article, little is known about German WWII aero engines, but this article is the "best guess" from experts in the field. A factual, definitive answer would seem still to await us ie original German documentation which as likely as not no longer exists being lost in the annuls of time and the massive destruction of WWII.
On a further point of interest regarding CRs, J.J. notes, quite correctly, the 8.3/9.5 figures that pertain to the left and right cylinder banks of the Daimler-Benz 603 when running on 100 octane fuel.
One of the great curiosities in aero-engine literature is that authors often meticulously list the differential CRs of the DB 603/605 V-12s, but never get round to explaining why the engine manufacturer deemed it necessary to do this! By any standards, it is an odd thing to do - I cannot think of any other piston engine that was supplied with the CR for some cylinders deliberately set lower than others.
Hunting through the literature, three theories materialize - perhaps an indication of how little we know about the German WWII aero-engines. The first suggests the differential CRs were due to articulated con-rods, the reasoning being that in a V-engine the pistons moved by the articulated rods would always travel slightly further in completing each stroke compared to their master-rod counterparts. All else being equal, this would produce a slightly higher CR on the link-side of the engine. However, this theory falls down very quickly on two counts:
1) It is a relatively simple matter to adjust the shape of a combustion chamber or piston top to compensate for the slightly longer stroke.
2) The DB 600 series V-12s were never fitted with anything but fork and blade con-rod pairs!
Theory two centers on the fact that the supercharger on the DB 600 V-12s as located on the left hand side of the engine and, thus, from such position would naturally provide mere “huff” to the cylinder bank nearest to it. In order not to (relatively) over-boost the left hand bank, the CR would have to be lowered slightly on that side.
In their book, Die deutsche Luftfahrt - Vol 2, (if you haven't got a copy of this in your library, you're missing a treat) ven Gersdorff & Grasmann write, regarding the DB 603, “Die Verdichtung der beiden Zylinderreihen war unterschiedlich, die linke hatte 7,3 din rechte 7,5, um die durch die unterschiedliche Lange der Ladeluftleitung – bedingt durch den einseitig angeordneten Lader- geringen Ladedruckunterschiede auszugleichen.” An exact translation of this reads, “The compression ratio of the two cylinder banks was different, the left had 7.3:1, the right had 7.5:1, in order to balance the small boost-pressure differences due to the differing lengths of the supercharger air delivery pipes resultant from the off-set arrangement of the supercharger.”
One hesitates to question the veracity of two such respected German authors (especially when they are writing about German Flugmoteren (aircraft engines)) but their explanation appears to be flawed. For one thing, even though the the DB supercharger was offset, the delivery pipe to the inlet manifolds actually has a gentler, less angular pathway down to the midpoint between the two cylinder banks than is the case with a cross mounted Allison/Merlin supercharger installation. If it discharges at or very near to the midpoint between the banks then only equal pressure will be fed to each side. Having reached the midpoint of the rear most cylinders, the DB induction pipe bifurcates to feed the two banks. However, the two branches do not end blindly at the forward-most cylinder but instead rejoin at this point so that the entire manifold is ring-shaped, making the likelihood of any one-sided pressure differences impossible.
The third possibility is that the lubrication system was at the heart of the matter. One of the disadvantages of an engine design which features inverted or downward pointing cylinders is that the cylinder and piston interiors form containers into which the oil flows and accumulates due to gravity. While this augers well for piston cooling and cylinder lubrication, the problem of oil moving past the pistons and into the combustion chambers in excess quantities crops up. Mixing oil with petrol (gasoline) rapidly causes a lowering of the octane rating of the fuel thus increasing the likelihood of detonation.
Tucked away in one of the very early editions of the R-RHT magazine, Archive is a reprint of, “Comments on a Visit to Germany - July 24th to August 12th, 1945.” Two paragraphs are of particular interest: “A good example of the (excessive) Air Ministry control lies in the inverted DB engine. The DB people said that both from a technical and production point of view they would have preferred to make an upright engine but they are compelled to make it inverted by the (German) Air Ministry.”
“With the inverted engine, they said it was very difficult to obtain consistent oil consumption and due to the rotation of the crankshaft one cylinder bank got more oil (spray) than the other. This oil got past the pistons into the combustion chambers and reduced the anti-knock value of the charge. For this reason the engine was built with a lower cylinder compression ratio on this (?) bank than on the other.”
In the light of this evidence, the third theory seems to be the most plausible. The situation was probably exacerbated by the use of roller bearings in the con-rod big ends. Assuming theory three is true, lowering the CR of one bank of cylinders does seem to be an extraordinary, almost desperate attempt to solve the problem. It also makes an absolute mockery of all the much vaunted, supposed advantages of fitting these engines with precision made, high pressure fuel injection systems. If six of the twelve cylinders per engine were suffering from chronic, unpreventable oil contamination then accurate fuel metering would count for very little. As it was, the DB601 had enough combustion problems due to poor fuel burning resultant from the location of both spark plugs per cylinder on the same (exhaust) side. This undesirable feature alone was responsible for a power loss of seven to eight percent.
From all of this, two other questions beg consideration.
1) Why did predecessors to the DB603/605, i.e. the DB600/601 not also have differential cylinder bank CRs?
2) Did the oil consumption problem affect the well known, close contemporary to the DB 600-series V-12s, vis a vis the Junkers Jumo 211?
In answer to Question 1, it is most likely that the left bank oil burning problem was with the 601 model from the beginning of its production and that the first opportunity to do something about it came with the introduction of the 605/603 types. Even so, as mentioned above, DB's answer to the difficulty was crude to say the least.
With regard to Question 2, it becomes obvious from just a brief comparison of the Jumo 211 and the DB 600 series that apart from being inverted, having a cannon tunnel and featuring an off-set supercharger, the design features of the two makes of engine were very different.
Nowhere is this more apparent than in the respective lubrication systems. The DB had a conventional gallery-type arrangement where, from a central feed-pipe branch lines admitted oil to each main bearing block, thence the oil was forced through holes and grooves into each bearing shell and then into the crankshaft interior via more holes and grooves in the crankshaft main bearing journals. From there, 3/8” pipes took the oil to the hollow crankpin for distribution to the big-end bearings, both roller and plain.
The Junkers' engineers on the other hand, perhaps anticipated possible oil control problems due to the compulsory inverted installation requirement, designed the Jumo 211 with an end-feed system where the oil was fed into the front end of the crankshaft and from this single entry point it was moved along the length of the crankshaft to be fed by centrifugal force to the main and big end bearings. Compared to the gallery-type, the end-feed system is vastly superior because, with the former, as bearing wear takes place there is an increased leakage of oil from the mains journals resulting in less supply to the big-ends. As oil has to be forced into the crankshaft against centrifugal pressure, it gives a reduction in lubrication to the big-ends with increasing crankshaft rotational speed. The limit of life of bearings is the time when the main journal clearances have increased to the extent that the big-ends get insufficient oil, even though the main journal bearings can run on quite satisfactorily with increased clearance.
The advantages of end-feed. lubrication include:
1) The entry of oil into the crankshaft interior not being opposed by the centrifugal force generated by the spinning crankshaft.
2) Making possible a much better proportioning of oil quantities between the main and crankpin bearings,
3) Permitting a reduction in the total oil flow because with the gallery system, excess oil must be given to the mains in order to ensure there is enough for the big-ends.
4) Allowing the deletion of holes and grooves in the main bearings thereby increasing their load carrying capacity.
5) Use of much lower oil pump pressures. In most engines, oil pressure is usually regarded as a vital indication of the health of the functioning engine - any fluctuation signals impending disaster. End feed engines can operate quite satisfactorily on just a few psi if necessary.
An illustration of the tight control of oil flow through the main and crankpin bearings is provided by the Rolls-Royce experience of changing the Merlin over to end-feed late in the WW II period. They found that on the test engines the pistons were in danger of seizing in the bores due to significantly reduced amounts of oil spray coming from the crankpins. They had to modify the design to deliberately make it more “leaky”.
Thus, it is entirely possible that the adoption of the end-feed type of lubrication system spared the Jumo 211 engines of the apparently insoluble oil control problems that plagued the DB 600 series for the whole of its production life.