DB605A compression ratios

DB CR’s

There is a direct reference to this matter in a report that was written by Rolls-Royce engineers following a visit to Germany shortly after the end of the War.
The DB engineers they spoke to made two points, 1/ that they were required to build their V-12 engines inverted even though it was against their better judgement and
2/ the use of the differential CR’s in the DB 600 engines was to combat the effects of uneven oil consumption in the two banks of cylinders.
The different CR’s had NOTHING to do with off-set superchargers or articulated con-rods (which these engines never had).
Read the posts by TempestNut – he knows what he is talking about!!

DB605A Compression ratios

Ok I managed to have a quick browse around my references and can add a bit more. The only engines to feature the asymmetric compression ratios were the DB605 and 603 (and all of their double derivatives) As I mentioned above the most likely reason for this arrangement was oil getting into the combustion chamber and causing detonation. Now the one feature of the DB engines that contributed to this and differentiated them from say the Jumo200 series was the lubrication system and in particular the caged roller bearings used on the big-ends. These bearings and the gallery type lubrication system resulted in large amounts of oil being thrown about by the rotation of the crankshaft and more of it ending up on the LB rather than the RB.

A lot has been written about German engineering, but design wise the DB 600 series where not particularly noteworthy despite a number of neat features, and they where not nearly as robust as the Merlin and Griffon or the American Radials, and were way past being state of the art.

I am sure that the answer to this question is to do with oil scavenging and the fact that one side of the engine got more oil into the combustion chamber than the other, and this reduced the detonation limit. To mitigate this issue the engines had a lower compression ratio on one side. This was an issue that the inverted V engines built by Daimler Benz had. I do not have my detailed references to hand but I doubt that the offset supercharger could cause this.

The scavenging of oil on inverted engines has always bothered me; can’t seem to get my head round how you stop the crankcase filling with it.

Not quite sure what you mean by ‘oil in the combustion chamber reduced the detonation limit’. Surely lubrication oil is not volatile enough to cause detonation (or even burn properly) at the compression pressures involved, or have I missed the point?

…the German approach to getting power out of their engines that they ran higher compression ratio’s with lower boost, whereas the British ran lower compression and higher boost…

That’s an interesting observation, two different ways of achieving the same result. My only thought here is that compression ratio is fixed and boost is not so the British approach may have been more flexible.

On the whole the British enjoyed a better supply of superior (mainly American) fuel; German 87 and 96 octane versus ‘British’ 100 and 130 octane fuel.

I agree with you on the master/slave rods, however the compression ratio , whilst being set as you say is a component of the mean indicated pressure in the combustion chamber. That is altered by increased pressure in the manifold. That is why “blown” engines (be it turbo or super) have a lower compression ration than a normally aspirated one.

We’ll have to disagree on that one.

Compression pressure is variable. It changes with all sorts of factors as you mention

Compression ratio is fixed and is defined as I’ve stated. The reason ‘blown’ engines have lower compression ratios is that compression pressures are limited by detonation and the lower compression ratio is used to offset detonation.

It raises an interesting question; the designers of the DB605 must have designed in the different compression ratios, that is it couldn’t have been an accident.

As has been suggested the reason may be something to do with the position of the supercharger.

Different pressures may have existed at the inlets to the different banks effectively leading to a loss of performance in one bank. Raising the compression ratio of that bank may have been the easiest way to restore that lost performance or at least mitigate it.

That would explain it perfectly…if the DB605 had master and slave rods but I’m 99% certain that it doesn’t.

The difference also cannot be explained by pressures in the induction system since compression ratio is only a factor of the swept-volume and the clearance-volume of an engine.

Since the bore and stroke of the two banks are identical the clearance-volume of one bank of cylinders must be different.

The question is why?

I agree with you on the master/slave rods, however the compression ratio , whilst being set as you say is a component of the mean indicated pressure in the combustion chamber. That is altered by increased pressure in the manifold. That is why “blown” engines (be it turbo or super) have a lower compression ration than a normally aspirated one.

A more credible explanation is that I think the Benz engines used master and slave conrods rather than the blade and fork type used by almost all other manufacturers of inline engines.

That would explain it perfectly…if the DB605 had master and slave rods but I’m 99% certain that it doesn’t.

The difference also cannot be explained by pressures in the induction system since compression ratio is only a factor of the swept-volume and the clearance-volume of an engine.

Since the bore and stroke of the two banks are identical the clearance-volume of one bank of cylinders must be different.

The question is why?

I am sure that the answer to this question is to do with oil scavenging and the fact that one side of the engine got more oil into the combustion chamber than the other, and this reduced the detonation limit. To mitigate this issue the engines had a lower compression ratio on one side. This was an issue that the inverted V engines built by Daimler Benz had. I do not have my detailed references to hand but I doubt that the offset supercharger could cause this.

The detonation limit was a problem that vexed all the large Piston Engine Manufacturers and its obvious when looking at the German approach to getting power out of their engines that they ran higher compression ratio’s with lower boost, whereas the British ran lower compression and higher boost and I guess this left the German engines more susceptible to oil in the combustion chamber reducing the detonation limit. This is a complicated issue and I’m sure there were multiple issues that contributed to the asymmetric compression ratios

A very plausable answer , but wouldn’t it have been easier to have a shorter block on the side that the rods don’t quite reach the top (as it must have been apparent from the outset that it was a shorter throw); if I remember correctly the Rolls Royce “R” series changed to master and slave, whilst it is possible that banks were different comp ratio, I’ve not heard it mentioned.

Benz Compression ratios

I too have heard the explanation that the difference in compression ratio was due to the “assymetric” supercharger and/or its plumbing, but I can’t say that it makes much sense. The fact the plumbing to one bank’s intake plenum is longer than that to the other should make no difference whatsover to the pressure.

A more credible explanation is that I think the Benz engines used master and slave conrods rather than the blade and fork type used by almost all other manufacturers of inline engines. With blade and fork conrods the centre of both banks’ conrods is the centre of the crank journal. Therefore the strokes of both banks are the same and the position of the pistons at their lowest and highest points are the same for both banks. With master and slave rods the master rod is centred on the journal, but the slave rod pivots around a point some distance away. Therefore bottom dead centre and top dead centre for the pistons attached to master rods are different to those attached to slave rods. If top dead centre is in a different position for each bank then the compression ratios must be different (assuming the banks have identical cylinder heads).

For this reason all radials (where use of master and slave rods is more or less compulsory unless you want a very long, slightly spiral engine) have at least two different compression ratios – one for the cylinder using the master rod and another for all the others in the bank that use articulated rods. Going off at a tangent slightly, on the Bristol Mercury (and no doubt other Bristols) the slave rods were not spaced in an even circle around the master one, but were at three different radii from it – meaning that the 9 cylinders actually have 3 or 4 different compression ratios, presumably in search of improved smoothness compared with having 1 cylinder having one compression ratio and the other 8 all sharing another.

Niall

There was a discussion of this topic over at the old 12 O’Clock High forum (over a yer ago, at least). If I remember correctly, there were many suggestions but the most plausible was the asymmetric supercharger, mounted on the port side. The induction manifold to the starboard cylinder bank was longer resulting in slightly lower charge pressure which was compensated by a slightly higer compression ratio.

Christer