Junk head: Difference between revisions
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== Radial engines == |
== Radial engines == |
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[[File:Bristol Perseus sleeve valve radial engine.jpg|thumb|right|[[Bristol Perseus]] |
[[File:Bristol Perseus sleeve valve radial engine.jpg|thumb|right|[[Bristol Perseus]] engine |
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|alt=A Bristol Perseus air-cooled radial aircraft engine, with sleeve-valves. The engine has been sectioned for display. One of the upper cylinders has been sectioned, showing inside the cylinder and cutting into the junk head. |
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[[Radial engine]]s, almost universally air-cooled aero engines, almost all use single cylinder blocks, with a separate head for each cylinder. Most use monobloc blocks and heads, referred to as 'barrels'. The single-cylinder arrangement of each head, together with air-cooling, remove some of the restrictions on the use of a junk head. Accordingly, most of the junk head designs or number of heads manufactured have been for air-cooled radial engines. |
[[Radial engine]]s, almost universally air-cooled aero engines, almost all use single cylinder blocks, with a separate head for each cylinder. Most use monobloc blocks and heads, referred to as 'barrels'. The single-cylinder arrangement of each head, together with air-cooling, remove some of the restrictions on the use of a junk head. Accordingly, most of the junk head designs or number of heads manufactured have been for air-cooled radial engines. |
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Revision as of 16:14, 23 December 2009
A junk head is a form of piston engine cylinder head, where the head is formed by a dummy piston mounted inside the top of the cylinder. In most other engine designs, the cylinder head is mounted on top of the cylinder block.
It's obviously essential for any piston engine to seal the joint between block and head. This is usually done by means of a head gasket, a flat gasket on the surface of the block. In early engines, the high pressures and high temperatures made this a difficult trial for the materials of the day and gasket failures were common. The junk head requires no head gasket and is sealed by piston rings inside the cylinder bore, as for the power piston.
An alternative solution was the monobloc engine, where the block and head were formed as one piece. This solved the sealing problem, but complicated manufacture and maintenance. Particularly when routine maintenance still required frequent head removal for de-coking, on a monobloc engine
Water cooling is difficult with a junk head, as the water ports must be connected to the removable head and there is little space available within the cylinder diameter. If the engine uses the common form of overhead poppet valves, these would also be difficult
Sleeve valve engines
The junk head was only widely used with sleeve valve engines. These sealed on the inside of the sleeve, also sealing the sleeves themselves from the combustion pressure. It would also have been difficult to arrange a flat head gasket on the top of the moving sleeves.
Sleeve valves were rarely used in road vehicles, although the Knight engine did enjoy some limited popularity in luxury cars.[1] These used a junk head.
Radial engines
Radial engines, almost universally air-cooled aero engines, almost all use single cylinder blocks, with a separate head for each cylinder. Most use monobloc blocks and heads, referred to as 'barrels'. The single-cylinder arrangement of each head, together with air-cooling, remove some of the restrictions on the use of a junk head. Accordingly, most of the junk head designs or number of heads manufactured have been for air-cooled radial engines.
Where radial engines have used sleeve valves (notably those of the Bristol Engine Company), the junk head was the usual design of head.
References
- ^ Autocar Handbook (Ninth ed. ed.). The Autocar. c. 1919. pp. 36–38.
{{cite book}}:|edition=has extra text (help)
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