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Motoculture: cylinder head and cylinder

A single-cylinder engine, whether two or four-stroke, or diesel, consists of a central cylinder block, a generally finned cylinder head for better cooling and a crankcase, These parts, separated by seals to ensure a perfect seal, are held together by studs which tightening compress the joints, allowing them to ensure a perfect seal.

A spark-ignition engine or a single-cylinder diesel is most often composed of three superimposed elements:

  • the breech,
  • the cylinder block,
  • the casing (from top to bottom).

The most frequent assembly is done by long studs, acting as columns, screwed into the crankcase block, the upper threaded portion receives the nuts that block the three elements on top of each other, between which are interposed joints.
They must imperatively be changed each time they are disassembled.
More and more often, the joints are replaced by a joint paste which polymerizes with heat.

Engines in two and four-stroke tiller

Engines in two and four-stroke tiller

Two-stroke engine

The small two-stroke compact single-cylinder (for example, chainsaws) often have only two elements, the absence of tumblers making it possible to be satisfied with a blind cylinder. In this case, the cylinder-cylinder block, in one piece, is made of cast alloy; he is wearing the cooling fins.
This block is then always jacketed: the shirt, special cast iron, is press-fitted and can hardly be changed, because it requires a very complex tool to heat the block while cooling the shirt with a gas liquefied. In this case, the cylinder-head block is fixed to the housing by four or six screws. On the simplest motors, the plane of joint between the two elements also carries the bearing housings of the crankshaft.
The tightness of the seal must be perfect because it plays an important role in the operation of the two-stroke engine. Normally, a centering pin prevents reassembly of the two elements other than in the original position, which defines the flow of gases in the lights. On smaller two-stroke engines, the carburetor attaches directly to the crankcase. This type of construction reduces to two the number of joints to have a perfect seal.

Four-stroke gasoline engine

The existence of the tumbler practically prevents to be satisfied with two elements, with cylinder drilled blind. Indeed, the rockers are always fixed in the cylinder head. On the lighter engines, cylinder head and cylinder block, as well as crankcase, are made of light alloy.
Some large power engines still retain a cast iron cylinder block. This construction also offers the advantage of allowing easier shirt exchange, since the jacket and block are made of materials having adjacent expansion coefficients; they are therefore assembled by a simple tight fitting.
But there are also single-cylinder engines without a shirt, where the cylinder is directly pierced inside the block. The general principle of construction consists in using the cylinder block as an assembly element, the cylinder head and the casing coming to refer to it by stud fasteners.
On some recent machines, this construction has been simplified by casting the entire cylinder and crankcase (this is the case, for example, Honda engines). A cover fixed by screw allows access to the crankshaft. It is placed at an angle to allow extraction of the piston-rod assembly from below. This construction makes it possible to renovate an engine that has lost its compressions with a minimum of disassembly.

Diesel motor

The very high compression ratio Diesel engine requires a more robust construction. The end-of-combustion temperature often prevents the light alloy from being used for the cylinder block unless a solid cylinder liner is used.
In practice, only diesel single cylinders retain a cast iron block. This also offers the advantage of better vibration absorption. The bolt is fixed to the block always using solid studs. The cylinder head seals made of bi-metal material are generalized because it allows a stronger clamping pressure. On the diesel monocylinders, we often find a boss with very thin fins, which receives the seat of the injector; this element is indeed subjected to extreme temperatures and pressures, and its overheating can deteriorate it quickly.

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