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The Daimler-Benz DB 605 was an inverted V12. The compression ratio of the 2 banks varied slightly. Why did the designers do this?

Compression ratio: 1:7.3 right bank, 1:7.5 left bank (museomotori.unipa.it)

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The 605 (601E above) used a "wild" valve timing. This valve overlap would start letting in the air/fuel mixture, while the exhaust valve was still open. So the intake would help push out the exhaust. This hurts the low RPM performance, so a special low-speed intake was also added.*

This overlap, the rotation direction, the compressor (supercharger) being closer to the left bank, are some of the reasons that resulted in the right bank having a lower compression ratio than the left. This affected the 603 and 605, but not the 601 because it didn't have such a wild timing overlap.**

But isn't compression ratio constant due to the fixed volume (a reply to a comment):

No, dynamic compression ratio is a thing that is affected by valve timing. An example (not 605 related) is given as:

However: intake valve closure (sealing the cylinder) always takes place after BDC (bottom dead centre), which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds (...)


* enginehistory.org

** ww2aircraft.net (translated from another link)

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  • $\begingroup$ The compression ratio is a dimensional ratio, that has nothing to do with pressures or valve timing... $\endgroup$ – xxavier Oct 13 '18 at 14:39
  • $\begingroup$ Not when a supercharger is involved. The path to the banks is not equal, nor does the timing allow full compression. Compression ratio of jet engines is also not fixed, e.g., the inlet providing extra compression with altitude/speed that varies. $\endgroup$ – ymb1 Oct 13 '18 at 14:56
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    $\begingroup$ regarding the "wild" valve timing, cam overlap (as it is usually known) also reflects the fact that it takes time to fully open (and then close) the valves. For this reason, the cam has to begin lifting the exhaust valve out of its seat early so as to have it all the way open by the time the piston hits bottom dead center. Similarly, the intake valve has to begin opening early, before BDC. the faster the engine is designed to turn, the greater the degree of overlap required to optimize performance- at the cost of smooth running at idle. $\endgroup$ – niels nielsen Oct 13 '18 at 18:45
  • $\begingroup$ This is very interesting, but I was a bit disappointed that no-one felt any need to discuss the question of whether a compression ratio is a function of two volumes or of pressures. A bit of a missed opportunity in my opinion. $\endgroup$ – Daniele Procida Oct 14 '18 at 0:04
  • $\begingroup$ Daniele, this is an excellent point as PV = nRT. Using fuel injection, one has to know how much air is in the cylinder to inject the proper amount of fuel. Using a supercharger, this (although technically incorrect) may have been factored in to "compression ratio" values. The cylinder would exhaust to atmospheric pressure, and potentially be recharged with air at a different pressure. The REAL meaning of compression ratio, for this engine (properly translated), might help. $\endgroup$ – Robert DiGiovanni Oct 15 '18 at 3:18

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