|
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. That 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 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 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 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Junk head」の詳細全文を読む スポンサード リンク
|