Radials: Four valve layout?

[SwampTiger]

There were several families of radials with four valve heads, Cosmos/Bristol and Armstrong Siddeley. What were the differences? The Bristol valves appear to have the intake valves inclined to open into(V) one another as do the exhausts. This would limit either the valve lift, timing or both. How was the Armstrong Siddeley's head layout?

 

[rinkol]

Inclined valves can be larger and provide some cross-flow. This configuration is common in racing cars. For aircraft use, the frontal area tends to be greater and more complex valve gear may be needed.

With correct operation, the valves shouldn't interfere with each other. If necessary, piston heads can have recesses for the valves.

Most of the Armstrong Siddeley engines seem to have had two valve cylinder heads e.g., http://www.enginehistory.org/Piston/A-S/A-S1/DSCN0904.jpg

 

[MiTasol]

Agree with Rinkol
Typical AS cylinder from Cheetah

 

[SwampTiger]

Inclined valves work when intake inclines towards exhaust. AIUI, in the Bristol engines, the intakes inclined/moved towards one another, not parallel. Same but less limiting for the exhausts.

For the A-S engines, I got the info off Wiki. My bad.

Still, why didn't Bristol, or a licensee, change the layout/pathway for the valves?

 

[pbehn]

Knowledge of scavenging and ideas of "squish" was changing all the time in that era, the Merlin was designed with a ramp head originally.

 

[MiTasol]

Inclined valves work when intake inclines towards exhaust. AIUI, in the Bristol engines, the intakes inclined/moved towards one another, not parallel. Same but less limiting for the exhausts.

For the A-S engines, I got the info off Wiki. My bad.

Still, why didn't Bristol, or a licensee, change the layout/pathway for the valves?

Bristol designed a very complex valve train that compensated for temperature changes (all cylinders and crank cases grow fractionally in length as the temperature rises) and had very low internal friction compared to many other designers. I suspect that, by the standards of the day, the air flow was considered excellent.

Licences can include a prohibition on making changes because if the modified unit has a spectacular failure that usually reflects badly on the original designer, not on the modifier.

 

[SwampTiger]

Thanks. That explains somewhat. It would explain the basic single row radial licenses. Something like the Alfa Romeo 135 would not be bound by such legalities. Same with the Japanese derivatives. The original license would cover the licensed copies, but not engines derived from the core engineering. That would come under patent licenses. If you change the valvetrain geometry, you are not legally bound by the patent or the license. See Gnome-Rhone's two valve head design on the basic Cosmos Jupiter engine core.

 

[msxyz]

Alfa had a license-collaboration agreement; their engines were not mere copies but included several modifications of the base design that were also passed back to Bristol

 

[Simon Thomas]

According to Lumsden, the early Leopards had 4 valve heads. They look to operate in a similar fashion to the Bristol actuating system.

 

[MiTasol]

According to Lumsden, the early Leopards had 4 valve heads. They look to operate in a similar fashion to the Bristol actuating system.

Thanks Simon
Any pictures?

This is the Pegasus valve gear - looks a nightmare on paper but much better in the flesh.

 

[Simon Thomas]

 

[MiTasol]

That is one ugly engine. A little thought in running the ignition leads in bundles and clamping them so they do not thrash themselves to death in the slipstream would have helped the looks and the maintenance.

 

[cammerjeff]

Interesting, that would make the early 4 valve Leopards one of the few Multi Row Radial engines ever made with 4 Valve Heads. A very complicated design on a 2 row radial engine.

Learned something new toady. Thanks

 

[SwampTiger]

So how did the later twin row Alfa 135 and Nakajima four valve geometry compare? Did they follow the Bristol pattern of intakes inclined to open towards one another rather than parallel to each other?

From a modern design perspective, this appears a poor design. Does anyone have access to discussion of reasoning? Compared to early two valve designs, it would be superior. However at the low compression and timing of earlier engines, it may not matter. The layout should enhance flow rate and turbulence. But why not improve the design over time?

Was the issue the conservatism at Bristol, and by other manufacturers, to limit costs, and thus changes to the design? They were able to keep up with competitors through the mid to late Thirties without major design changes. Bristol never made a two row poppet valve engine.

 

[MiTasol]

Shinpachi may know of the Nakajima layout and surely someone in Europe can answer for the Alfa engines.

Costs were probably a significant factor because in those days the numbers of engines you could amortize the expense over was far less than in war time and you were competing against many more other manufacturers than exist now.

On the other hand Bristol were able to cut their cost per engine considerably by licencing to multiple other manufacturers.

 

[Bretoal2]

Thanks Simon
Any pictures?

This is the Pegasus valve gear - looks a nightmare on paper but much better in the flesh.

Jupiter valve gear was identical for latest versions with forged alloy cylinder head.

Earlier versions, with four parallel valves and "poultice" head, were slighlty different with forked actuators.

 

[msxyz]

So how did the later twin row Alfa 135 and Nakajima four valve geometry compare? Did they follow the Bristol pattern of intakes inclined to open towards one another rather than parallel to each other?

From a modern design perspective, this appears a poor design. Does anyone have access to discussion of reasoning? Compared to early two valve designs, it would be superior. However at the low compression and timing of earlier engines, it may not matter. The layout should enhance flow rate and turbulence. But why not improve the design over time?

Was the issue the conservatism at Bristol, and by other manufacturers, to limit costs, and thus changes to the design? They were able to keep up with competitors through the mid to late Thirties without major design changes. Bristol never made a two row poppet valve engine.

This layout was designed in the 20's and it was very advanced for the time. Bristol engines were the first to solve the problem of valve burning thanks to their design: two smaller valves are easier to cool than one large, plus the exhaust valves are placed at the front thus they're better cooled by the incoming rush of air. The penta roof chamber was also a novelty (though it was used in some race cars at the time) and provided better combustion.

The advent of better materials for the exhaust valve (nichrome alloys), the advent of the hemispherical combustion chamber (which allows both large valves and an optimal geometry for combustion) surely made this design obsolete in a radial but Bristol at the time was investing into sleeve valves research so I guess they didn't have also resources in keeping up with the latest trends of the time. Besides, The basic Jupiter/Pegasus design evolved from 450-500Hp to 1000HP which was about the same power offered, using a similar displacement, of the Wright R-1820