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Heads and Barrels

 
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mroots



Joined: 20 Nov 2014
Posts: 4

PostPosted: Fri Nov 06, 2015 15:33    Post subject: Heads and Barrels Reply with quote

Hi all.
Continental and Lycoming flat air-cooled engines have the heads screwed to the barrels. I understand that the barrels are of cast steel and the heads are of cast alluminium alloy and that the heads are warmed when fitted to achieve an interference fit when cooled. Presumably, the heads are not screwed on to tight but just to aligned with the barrel.
A friend of mine was working on the Gloster Gladiator at Duxford and had the Bristol Mercury engine stripped for overhaul and the barrel/head arrangement is familiar. That engine was derived from the Jupiter designed during WW1 designed by Roy Fedden (which had barrels machined from solid). Apparently he disliked the open barrel and separate head arrangement and used the 'poultice' head.
Most other air-cooled engines in trucks, cars and bikes seem to have separate barrels and heads, often clamped together with long studs back to crankcase. The Gipsy engines were similar.
My question is why was the screwed on cylinder head accepted as the norm and what is it's history, i.e. was Fedden the first to use it? Thanks, Marc
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kmccutcheon



Joined: 13 Jul 2003
Posts: 191
Location: Huntsville, Alabama USA

PostPosted: Mon Nov 09, 2015 09:25    Post subject: Reply with quote

Quote:
Continental and Lycoming flat air-cooled engines have the heads screwed to the barrels. I understand that the barrels are of cast steel and the heads are of cast alluminium alloy and that the heads are warmed when fitted to achieve an interference fit when cooled. Presumably, the heads are not screwed on to tight but just to aligned with the barrel.


Correct. Even so, there is considerable torque involved with barrel insertion. The process was once done by hand, but is now assisted by automated equipment.

Quote:
A friend of mine was working on the Gloster Gladiator at Duxford and had the Bristol Mercury engine stripped for overhaul and the barrel/head arrangement is familiar. That engine was derived from the Jupiter designed during WW1 by Roy Fedden (which had barrels machined from solid). Apparently he disliked the open barrel and separate head arrangement and used the 'poultice' head.


Although the Mercury followed some Jupiter conventions, it was a completely new engine, designed in 1926 for the 1927 Schneider Trophy Races. It employed a completely new cylinder design similar to the ones you first described above. The Jupiter adopted this same cylinder design in 1929. Fedden stuck with the poultice head despite the results of work accomplished in 1915-1916 at the Royal Aircraft Establishment by A.H. Gibson and S.D. Heron. Aluminum casting technology in 1916 was primitive, and Fedden thought making the Aluminum head/steel barrel scheme work would be risky.

The Gibson/Heron team established the basic principles of air cooled cylinders:
1) the cylinder head should be of aluminum, since less weight of this metal is required to conduct away heat at a given rate than of any other material;
2) the head should be all one piece, since metal-to-metal joints are difficult to maintain in good thermal contact;
3) the head should be designed to offer the shortest possible escape along a path with the largest possible cross section for the heat generated at the hottest point of the head.
Heron's embodiments of these principles consisted of an aluminum head and cylinder with a shrunk-in steel or cast iron cylinder liner.

Quote:
Most other air-cooled engines in trucks, cars and bikes seem to have separate barrels and heads, often clamped together with long studs back to crankcase. The Gipsy engines were similar.


This construction requires head gaskets, which are often problematical.

Quote:
My question is why was the screwed on cylinder head accepted as the norm and what is it's history, i.e. was Fedden the first to use it?


The screwed-on cylinder head was first tried by J.D. Siddeley for his water-cooled Puma. Once it was made to work, it quickly achieved superiority to other construction schemes. An in-depth discussion of this appears in George Genevro's article, please see
http://www.enginehistory.org/air-cooled_cylinders_1.shtml
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mroots



Joined: 20 Nov 2014
Posts: 4

PostPosted: Tue Nov 10, 2015 15:19    Post subject: Reply with quote

Great answer Kimble, thankyou very much.
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