Junk head
Encyclopedia
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
. This form has also been termed a "poultice
head".
It is 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 ring
s 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 this required the removal of the pistons from the crankshaft end beneath.
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.
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.
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. These used a junk head.
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.
Where radial engines have used sleeve valves (notably those of the Bristol Engine Company), the junk head was the usual design of head. Individual cylinder junk heads, sealed by a single ring, were also used on the inline H-block sleeve valve Napier Sabre
engine.
Cylinder head
In an internal combustion engine, the cylinder head sits above the cylinders on top of the cylinder block. It closes in the top of the cylinder, forming the combustion chamber. This joint is sealed by a head gasket...
, 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
Cylinder block
A cylinder block is an integrated structure comprising the cylinder of a reciprocating engine and often some or all of their associated surrounding structures...
. This form has also been termed a "poultice
Poultice
A poultice, also called cataplasm, is a soft moist mass, often heated and medicated, that is spread on cloth over the skin to treat an aching, inflamed, or painful part of the body. It can be used on wounds such as cuts...
head".
It is obviously essential for any piston engine to seal the joint between block and head. This is usually done by means of a head gasket
Head gasket
A head gasket is a gasket that sits between the engine block and cylinder head in an internal combustion engine. Its purpose is to seal the cylinders to ensure maximum compression and avoid leakage of coolant or engine oil into the cylinders; as such, it is the most critical sealing application in...
, a flat gasket
Gasket
thumb|sright|250px|Some seals and gaskets1. [[o-ring]]2. fiber [[Washer |washer]]3. paper gaskets4. [[cylinder head]] [[head gasket|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 ring
Piston ring
A piston ring is a split ring that fits into a groove on the outer diameter of a piston in a reciprocating engine such as an internal combustion engine or steam engine.The three main functions of piston rings in reciprocating engines are:...
s inside the cylinder bore, as for the power piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
.
An alternative solution was the monobloc engine
Monobloc engine
A monobloc or en bloc engine is an internal-combustion piston engine where some of the major components: cylinder head, cylinder block, or crankcase are formed, usually by casting, as a single integral unit, rather than being assembled later...
, 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.
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.
Origins of the term
The term "junk head" originates from these sealing rings or "junk rings". The term was previously in use with steam engineSteam engine
A steam engine is a heat engine that performs mechanical work using steam as its working fluid.Steam engines are external combustion engines, where the working fluid is separate from the combustion products. Non-combustion heat sources such as solar power, nuclear power or geothermal energy may be...
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
Graphite
The mineral graphite is one of the allotropes of carbon. It was named by Abraham Gottlob Werner in 1789 from the Ancient Greek γράφω , "to draw/write", for its use in pencils, where it is commonly called lead . Unlike diamond , graphite is an electrical conductor, a semimetal...
d rope or oakum
Oakum
Oakum is a preparation of tarred fiber used in shipbuilding, for caulking or packing the joints of timbers in wooden vessels and the deck planking of iron and steel ships, as well as cast iron plumbing applications...
packing. Although the high cylinder temperature of an internal combustion engine requires high quality materials even here, the term stuck.
Sleeve valve engines
The junk head was only widely used with sleeve valveSleeve valve
The sleeve valve is a type of valve mechanism for piston engines, distinct from the usual poppet valve. Sleeve-valve engines saw use in a number of pre-World War II luxury cars and in USA in the Willys-Knight car and light truck...
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
Knight Engine
The Knight Engine was an internal combustion engine, designed by American Charles Yale Knight , that used sleeve valves instead of the more common poppet valve construction.- History :...
did enjoy some limited popularity in luxury cars. These used a junk head.
Radial engines
Radial engine
The radial engine is a reciprocating type internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel...
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.
Where radial engines have used sleeve valves (notably those of the Bristol Engine Company), the junk head was the usual design of head. Individual cylinder junk heads, sealed by a single ring, were also used on the inline H-block sleeve valve Napier Sabre
Napier Sabre
The Napier Sabre was a British H-24-cylinder, liquid cooled, sleeve valve, piston aero engine, designed by Major Frank Halford and built by Napier & Son during WWII...
engine.