Junk head: Difference between revisions
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{{Short description|Piston engine form}} |
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| ⚫ | [[File:Knight sleeve-valve engine, sectioned head, inlet stroke (Autocar Handbook, Ninth edition).jpg|thumb |
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{{About|the mechanical device|the 2021 film|Junk Head}} |
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{{Lead too long|date=May 2022}} |
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| ⚫ | [[File:Knight sleeve-valve engine, sectioned head, inlet stroke (Autocar Handbook, Ninth edition).jpg|thumb|Section through the junk head of a [[Knight Engine|Knight sleeve valve engine]]|alt=Section through the cylinder head of a Knight sleeve valve engine. The valve sleeves are open to the inlet port. Between and above the sleeves is the junk head. |
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| ⚫ | A '''junk head'''{{Efn|Usually "junk head" around Bristol engines or "junkhead" within Rolls-Royce}} 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]]. |
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| ⚫ | It |
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|title=Allied Aircraft Piston Engines of World War II |
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|chapter=Bristol Hercules |
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|last=White |first=Graham |
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|year=1995 |
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|publisher=Airlife |
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|isbn=1-85310-734-4 |
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|ref=White, Allied Aircraft Piston Engines |
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|pages=136-137 |
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}}</ref> |
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| ⚫ | It is 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 temperatures made it difficult for the materials of the day and gasket failure was common. The junk head requires no head gasket and is sealed by [[piston ring]]s inside the [[cylinder bore]], as for the power [[piston]].<ref name="White, Allied Aircraft Piston Engines, p136-137">{{cite book |title=Allied Aircraft Piston Engines of World War II |chapter=Bristol Hercules |last=White |first=Graham |date=1995 |publisher=Airlife |isbn=1-85310-734-4 |ref=White, Allied Aircraft Piston Engines |pages=136–137 }}</ref> |
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| ⚫ | 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 this required the removal of the pistons from the crankshaft end beneath |
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| ⚫ | 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 [[Planned maintenance|routine maintenance]] still required frequent head removal for de-coking, on a monobloc engine this required the removal of the pistons from the crankshaft end beneath the engine. |
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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 to fit inside the limited space. |
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[[File:Centaurus, junk head.jpg|thumb|upright|Sectioned cylinder and head of a [[Bristol Centaurus]]. Note the copper fins to the junk head]] |
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== Origins of the term == |
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The term "junk head" originates from these sealing rings or "junk rings" and the dummy piston head. The term was previously in use with [[steam engine]]s. Whereas a piston ring must slide within the cylinder and so be made of the best quality long-lived materials, where a fixed cylinder cover was sealed by a ring, this ''stationary'' ring could be of the lowliest materials or "junk", often a greased or [[graphite]]d rope or [[oakum]] packing. Although the high cylinder temperature of an internal combustion engine requires high quality materials even here, the term stuck. |
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== Sleeve valve engines == |
== Sleeve valve engines == |
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Sleeve valves were rarely used in road vehicles, although the [[Knight Engine|Knight engine]] did enjoy some limited popularity in luxury cars.<ref name="Autocar Handbook, Knight sleeve valve" >{{cite book |
Sleeve valves were rarely used in road vehicles, although the [[Knight Engine|Knight engine]] did enjoy some limited popularity in luxury cars.<ref name="Autocar Handbook, Knight sleeve valve" >{{cite book |
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|title=Autocar Handbook |
|title=Autocar Handbook |
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|publisher=[[The Autocar]] |
|publisher=[[Autocar (magazine)|The Autocar]] |
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|edition=Ninth |
|edition=Ninth |
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|year=c. 1919 |
|year=c. 1919 |
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|pages=36–38 |
|pages=36–38 |
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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. |
|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 |
[[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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Where radial engines have used sleeve valves (notably those of the [[Bristol Engine Company]]), the junk head was the usual design of head.<ref name="White, Allied Aircraft Piston Engines, p136-137" /><ref name="Bingham, Major Piston Aero Engines" >{{cite book |
Where radial engines have used sleeve valves (notably those of the [[Bristol Engine Company]]), the junk head was the usual design of head.<ref name="White, Allied Aircraft Piston Engines, p136-137" /><ref name="Bingham, Major Piston Aero Engines" >{{cite book |
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|isbn=1-84037-012-2 |
|isbn=1-84037-012-2 |
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|ref=Bingham, Major Piston Aero Engines |
|ref=Bingham, Major Piston Aero Engines |
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|pages= |
|pages=52–53 |
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}}</ref> Individual cylinder junk heads, sealed by a single ring, were also used on the inline H-block sleeve valve [[Napier Sabre]] engine.<ref>{{cite journal |
}}</ref> Individual cylinder junk heads, sealed by a single ring, were also used on the inline H-block sleeve valve [[Napier Sabre]] engine.<ref>{{cite journal |
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|title=The Napier Sabre Aero Motor |
|title=The Napier Sabre Aero Motor |
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|date=24 March 1944 |
|date=24 March 1944 |
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}}</ref> |
}}</ref> |
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== Cooling == |
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[[File:Fleet Air Arm Museum. RNAS Yeovilton. Somerset (8437697371).jpg|thumb|View above the head of a [[Bristol Centaurus]]]]Cooling is difficult with a junk head, with either air or water. 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 to fit inside the limited space. Air cooling is also difficult, as the shape of the head is deeply re-entrant and it is difficult to arrange airflow to the deep well around the spark plug or injector. Bristol developed a two-part head to address this problem, with copper conducting fins.<ref name="Ricardo, High-Speed Internal Combustion Engine, junk head">{{cite book |last=Ricardo |first=Harry R. Sir |title=The High-Speed Internal Combustion Engine |publisher=Blackie |date=1953 |edition=4th |location=Glasgow |pages=310–311 |chapter=Piston Aero-engines |ref=Ricardo, High-Speed Internal Combustion Engine, 4th ed. |authorlink=Harry Ricardo }}</ref> |
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== Notes == |
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{{Notelist}} |
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== References == |
== References == |
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{{Reflist|colwidth=35em|refs= }} |
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{{reflist}} |
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[[Category:Engine technology]] |
[[Category:Engine technology]] |
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[[Category:Internal combustion engine]] |
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[[Category:Sleeve valve engines]] |
[[Category:Sleeve valve engines]] |
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{{mech-engineering-stub}} |
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Latest revision as of 22:46, 29 November 2024
 | This article's lead section may be too long. (May 2022) |
.jpg)
A junk head[a] 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 is 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 temperatures made it difficult for the materials of the day and gasket failure was common. The junk head requires no head gasket and is sealed by piston rings inside the cylinder bore, as for the power piston.[1]
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 this required the removal of the pistons from the crankshaft end beneath the engine.
Origins of the term
[edit]The term "junk head" originates from these sealing rings or "junk rings" and the dummy piston head. The term was previously in use with steam engines. Whereas a piston ring must slide within the cylinder and so be made of the best quality long-lived materials, where a fixed cylinder cover was sealed by a ring, this stationary ring could be of the lowliest materials or "junk", often a greased or graphited rope or oakum packing. Although the high cylinder temperature of an internal combustion engine requires high quality materials even here, the term stuck.
Sleeve valve engines
[edit]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.[2] These used a junk head.
Radial engines
[edit]
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.[1][3] Individual cylinder junk heads, sealed by a single ring, were also used on the inline H-block sleeve valve Napier Sabre engine.[4]
Cooling
[edit].jpg)
Cooling is difficult with a junk head, with either air or water. 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 to fit inside the limited space. Air cooling is also difficult, as the shape of the head is deeply re-entrant and it is difficult to arrange airflow to the deep well around the spark plug or injector. Bristol developed a two-part head to address this problem, with copper conducting fins.[5]
Notes
[edit]- ^ Usually "junk head" around Bristol engines or "junkhead" within Rolls-Royce
References
[edit]- ^ a b White, Graham (1995). "Bristol Hercules". Allied Aircraft Piston Engines of World War II. Airlife. pp. 136–137. ISBN 1-85310-734-4.
- ^ Autocar Handbook (Ninth ed.). The Autocar. c. 1919. pp. 36–38.
- ^ Bingham, Victor (1998). "Bristol Hercules". Major Piston Aero Engines of World War II. Airlife. pp. 52–53. ISBN 1-84037-012-2.
- ^ "The Napier Sabre Aero Motor". The Aeroplane. 24 March 1944.
- ^ Ricardo, Harry R. Sir (1953). "Piston Aero-engines". The High-Speed Internal Combustion Engine (4th ed.). Glasgow: Blackie. pp. 310–311.