enginehistory.org

[rinkol]

I recently had a chance to examine the valve gear of a RR Merlin and DB601A. These engines both have 4 valves/cylinder with a single camshaft. The noticeable difference is that the RR valve gear is arranged so that separate cam lobes are used for the intake and exhaust valves whereas the DB valve gear uses the same cam lobe for both. The separate cam lobes would have the advantage of allowing the timing of the intake and exhaust valves to be tweaked independently.

To what extent does this make a practical difference?

BTW, the valve gear in the later DB engines (e.g., DB601E) was supposed to have been improved. Does anyone know of the nature of the changes?

Robert

[szielinski]

Maybe the Germans used different rocker arm lengths and positions to change the relative opening & closing of the exhaust and intake valves.
-just a thought since this post has sat here for a while

[rhavemann]

BY stander comment... Indian motorcycles used a single pattern cam..
however when it came time to go fast. They changed to a dual pattern cam profile... IE one for the intake ...one for the Exhaust..Same for Harley Davdison 1928 JD-2 cam...SO I guess 2cams is the hot number for horsepower
_________________
Bob Havemann, The Wrenchmann

[jwells]

The DB 600-series valve gear was unique to WWII aero-engines. Very ingenious and compact, especially compared to the bulky system of cross-over lever tappets used by R-R.

The system was completely symmetrical, so that what ever happened to the inlet valve also happened to the exhaust, for example timing for the DB 600 engine was 8 48 48 8 giving a very modest 16 deg of overlap. However, with the DB system it must be remembered that one cam lobe used the flank of the lobe to hold the exhaust valve open while the other flank was starting to open the inlet!

The early Allison V 1710A also had a symmetrical valve timing of 15 55 55 15 but this was by choice not necessity of design.

The DB 601 engine had a timing of 21 50 50 21, ie 42 deg of overlap whereas the Merlin II had 31 52 72 13, ie 44 deg overlap, so very similar.

The unusual one of the era was the C-model Allison 1710 with 52 66 76 26, ie 78 deg overlap which is significantly more.

The Junkers V 12s are interesting in that the early 211 engines had R-R type cross-over lever tappets while the later 213's adopted the DB-type lever tappets, although with three valves per cylinder, they had the luxury of one cam lobe per valve and, therefore could use exactly what timing worked best.

Jerry W.

[wallan]

It should be mentioned that the Triumph Dolomite Sprint two-litre 16-valve engine used the same concept of the cam operating the inlet valves, directly, and the exhaust valves by rockers. (it won a design award for Spen King) The only real disadvantage to this is that valve timing is identical, and valve overlap cannot be optimised, and trying to tune the engine for higher power causes some of the fuel/air mixture to escape before the exhaust valves close, increasing fuel consumption. However, as the DB series of engines used direct injection, the only thing that would escape would be compressed air, thus there wouldn't be an increase in fuel consumption.