Asymmetrical compression ratios

Very interesting. I’ve always understood that roller bearings need an oil mist rather than a full pressure feed but whether that also applies to needle rollers I don’t know.
Delage had similar lubrication problems in the mid 20s when they built three V12s to race at Le Mans. They were a total disaster and only completed 1 or 2 laps.
A mate, who rebuilds vintage racing car engines for a living, managed to cure the problem on one a few years back by massively reducing the oil supply. They were roller big ends but I don’t know what type or if the mains were plain bearings.

OK came across some additional info on this mystery. The asymmetric compression ratio’s only applied to the DB 605 and 603 as far as I can ascertain and not to the earlier 601. And as I explained above was as a result of passing oil into the cylinders from the lubrication system. Daimler Benz lubrication systems were perhaps not up to date when compared to other contemporary high powered aero engines. In particular the way the oil was feed into the mains resulted in it being forced against the centrifugal force of the rotating crankshaft. As a consequence more oil (at a higher pressure) than necessary to lubricate the mains was pumped to ensure that the big-ends were properly lubricated. This resulted in excess leakage (more than necessary for lubrication of the piston and small end, and any cooling requirements) from the bearings on to the cylinder walls and into the combustion chamber. Additionally DB used needle roller big-ends that leaked more oil than plain bearings. Due to the rotational forces more oil was thrown to one side of the engine than the other, hence the asymmetry.

Altering the compression ration to give greater detonation tolerance to the engine seemed to be a rather crude attempt to solve a major problem with the engines. Junkers did not have the same issues with the Jumo211 and 213, but they did a major update to the 211 to create the 213 with an up to date lube system with other mods that allowed the 213 far higher rotational speeds, way beyond what the 605 could survive at.

There is evidence to suggest that DB wanted to build the engines upright but were forbidden by the German Air Ministry. Interruption to production and all that rubbish I presume. And if I remember correctly aeration of the oil was another issue the 605 suffered from and is possibly implicated in the loss of some important German aces when the 109G was first introduced. And if I recall one of the current 605’s in Germany suffered a failed connecting rod a couple of years back.. Perhaps related to these issues?

Lastly I think interesting to note that the direct injection fuel system that allowed the use of lower grades of fuel had its advantages negated by a poor lubricating system, and with none of the considerable charge-cooling available by the use of an injection carburettor the DB engines were at a distinct disadvantage when compared to the Merlin and Griffon, not to mention the R2800 and Sabre. 🙂 🙂 🙂

As TempestNut pointed out the reason was the different length of the airduct, since the blower was positioned closer to the right bank.

I think the torque is related directly to the rotation of the engine, which would not be affected by having different compression ratios in each bank as it would still be turning the same way. What amazes me is how with the limited technology of the time, the engineers were able to find out what was creating the problem and the method of fixing it. There was no plugging it into a computer back then!!
Thanks for the info TempestNut, I love learning about anything technical.

torque?

I was always under the impression that it had something to do with trying to counteract the torque of the engine; I can’t see how that would work myself, but there it is anyways.

cheers

gregv

The above link that referrers to the differing lengths of the air intakes, is possibly only part of a very complicated answer. Detonation is the biggest problem to be overcome in any large high powered aero engine and the Germans had different problems to the British and Americans to overcome due to using direct fuel injection as opposed to a carburettor or an injection carburettor as later RR, P&W and Bristol engines amongst others used.

Basically mixture distribution into each cylinder combined with oil scavenging issues with the inverted engine resulted in the compression ration needing to be lower on one side of the engine than the other as oil would be drawn into some cylinders during operation of the engine. Small amounts of oil in the mixture can dramatically lower the engines resistance to detonation by lowering the effective octane rating (or performance rating) of the mixture entering some cylinders, and there is evidence to suggest that the DB engines had some issue with oil scavenging. Unfortunately I don’t have to hand the exact reason for it being one side or the other.

Additionally German engines used a higher compression ration with lower boost to achieve high power, as compared with RR for example, which used a relatively low 6 to 1 ration and high boost pressure to achieve the same result. This I believe was a contributing factor in making the DB engines sensitive.

This is off the top of my head from memory and I would need to do some digging to get the full explanation out.

Off the top of your head perhaps, but thoroughly impressive nonetheless. Many thanks to all who’ve replied. Tempestnut, if you have the time and are interested please do go ahead and look further. Otherwise you’ve already provided the best answer I’ve heard so far.

The above link that referrers to the differing lengths of the air intakes, is possibly only part of a very complicated answer. Detonation is the biggest problem to be overcome in any large high powered aero engine and the Germans had different problems to the British and Americans to overcome due to using direct fuel injection as opposed to a carburettor or an injection carburettor as later RR, P&W and Bristol engines amongst others used.

Basically mixture distribution into each cylinder combined with oil scavenging issues with the inverted engine resulted in the compression ration needing to be lower on one side of the engine than the other as oil would be drawn into some cylinders during operation of the engine. Small amounts of oil in the mixture can dramatically lower the engines resistance to detonation by lowering the effective octane rating (or performance rating) of the mixture entering some cylinders, and there is evidence to suggest that the DB engines had some issue with oil scavenging. Unfortunately I don’t have to hand the exact reason for it being one side or the other.

Additionally German engines used a higher compression ration with lower boost to achieve high power, as compared with RR for example, which used a relatively low 6 to 1 ration and high boost pressure to achieve the same result. This I believe was a contributing factor in making the DB engines sensitive.

This is off the top of my head from memory and I would need to do some digging to get the full explanation out.

Have a look at this, it sounds plausible
http://groups.google.co.uk/groups?hl=en&lr=&th=e6bc498a2bf381be&seekm=6ogah3%241l2%241%40news1.sol.no&frame=off

My Janes Fighting Aircraft of WW2 has these specs. in the engines section, with higher compression ratios for other DB versions, but no explanation.

Can you point to….

Where these specs came from ?

Just curious ?

I have never seen 2 compression ratio on DB’s

I have absolutely no idea.

Logic dictates (well mine does anyway) that it’s a mechanical reason rather than by design but… having come up with half-a-dozen theories and discarded them all I’m no further forward. 🙂