Cylinder head

The cylinder head closes the combustion chamber of an internal combustion engine from the top. The term "top" is to be understood in this case from the viewpoint of the oscillating piston. Even with boxer engines and the engines that are installed hanging ( such as aircraft engines), one therefore speaks of a cylinder head, although this space here is not "up". All of the modern four-stroke engines, it houses the intake and exhaust ports and valve control for gas exchange processes, oil passages to lubricate the valve train and for water-cooled engines, coolant channels, in gasoline engines the spark plugs in gasoline direct injection engines, the injectors or diesel engines, the injector and the glow plugs. Due to the complicated production of cylinder head belongs next to the engine block of the most complex and most expensive parts of a vehicle.

Typology

Combustion engines and their cylinder heads can be classified according to the following characteristics:

  • Two -stroke engines ( without valves ) or four-stroke engines ( with valves ).
  • Otto engines ( spark plug ) and, optionally, injection valves, or diesel engines ( without spark plugs, fuel injectors, instead ).
  • Naturally aspirated ( carbureted ) or injection engines (without carburetor ).
  • Below Horizontal or overhead camshaft.
  • Single or double camshafts.
  • Cross-flow cylinder head or counterflow cylinder head ( reverse flow flushing).

Components

Depending on the combination of the above criteria includes a cylinder head:

  • Intake valves: it is either by air is sucked ( in diesel engines ) or generated by the carburettor or of the injection system the air-fuel mixture. As this suction gas is cold (maximum motor temperature normally aspirated or turbocharged engines with charge air temperature ) the intake valves is not so exposed to high temperatures, such as the exhaust valves.
  • Valves: This exhaust escapes into the exhaust pipe ( exhaust manifold ), colloquially known as exhaust. Exhaust valves are therefore exposed to high temperatures.
  • Camshaft (s ): For engines with single overhead camshaft ( OHC = overhead camshaft) or double overhead camshaft (DOHC = double overhead camshaft) is home to the cylinder head, one or two camshafts. These open and close the valves directly or indirectly, and this, for example, driven by a timing chain from the crankshaft. The rotational speed of a camshaft is always half the size of the crank shaft because it is a four-stroke engine, the need for each work cycle two crankshaft revolutions.
  • Spark plugs: In four-stroke spark-ignition engines, this type in each second crankshaft revolution for initiating a spark from the combustion in two-stroke engines for each crankshaft revolution. Diesel engines are diesels and have no spark plugs, glow plugs, but as a cold start aid.
  • Location of the injection nozzles: For diesel direct - injection engines they inject directly into the combustion chamber and are correspondingly high pressures and temperatures exposed. In pre-chamber or swirl chamber diesel engines inject it into the pre-chamber and are exposed to lower pressures.
  • Gasoline injectors: partly it is properly mounted on the cylinder head (often with single nozzle for each cylinder, "Multi- Point" ), but partly also on ahead of the cylinder head intake manifold (with " single-point " injections ). In conventional petrol injection engines they lie in the cool intake area in the cylinder head or the manifold before and are separated by the inlet valves of the combustion chamber. With the newer gasoline direct injection, however, they sit in the cylinder head, inject directly into the combustion chamber, where they are exposed to higher temperatures and coking.
  • Inlet and outlet ports: In the charge cycle, so the ejection of the gas from the cylinder and the re- filling of the cylinder with fresh gas channels lead from the outside to the valves through which the gases are passed. In the reverse flow flushing are the intake and exhaust ports on the same side of the cylinder head, wherein the cross-flow purge on the opposite sides of the cylinder head.

Design Features

A combustion process in a cylinder exerts on the cylinder head of the same force as in the flask. Therefore, the cylinder head is exposed to very high forces that push him to the outside. For this reason, it is often not on the cylinder, but directly bolted to the crankcase, with long tie rods or bolts which are inserted through corresponding holes long running through the engine block. The clamp load produced by tightening the screw acts on the long bolt elastically resilient. Thus, the intermediate block is clamped reliably with the seals and the engine remains tight.

The expansion bolts are bereckt by the intended tightening and may be used only once. Therefore, it is very important that when mounting a torque wrench is used so that the predetermined by structural design torque and clamp load this highly loaded bolt connection are observed. An exact compliance with the established procedure regarding screw- order torque graduations and any duty of control after the timing period is necessary.

The moving parts are included in all modern engines in the oil circuit. The lubricating oil pressure is usually passed through a cylindrical channel and the head gasket to the cylinder head, in rare cases and in old engine types by a separate screw oil line. To protect against the leakage of pressure-free spray oil on the cylinder head, a simple thin-walled cylinder head cover with rubber or cork seal is sufficient.

At the bottom camshaft, the rocker arms of the valves generally above plungers and bumpers that are parallel to the cylinders and transfer an axial movement, is connected to the cylinder head situated outside the camshaft. Overhead camshafts are driven by either a timing chain or by a toothed belt. Rare are Stirnradkaskaden or on the cylinder longitudinal waves with Kegelradantrieben, so-called ' bevel ' is used.

There are also four-strokes at a very simple cylinder heads: for motors with side -lying valves ( SV - arrangement, "Side Valve "). These cylinder heads are similar in their flat design, two-stroke heads, as the valve assembly are part of the cylinder and not the cylinder head here. But SV engines are long gone state of the art, even if they occasionally (Russia, People's Republic of China) are still being built.

Cooling is achieved by longitudinal or transverse flow of cooling water. In the cross-flow, the cooling medium flows from the outlet to the inlet side, or vice versa; wherein the longitudinal flow substantially from the rear side in the longitudinal direction toward the front of the crankshaft. A cross-flow is advantageous because the individual cylinders are approximately equally well cooled and the pressure loss is low, so the water pump requires less driving power.

Cylinder heads of marine engines are often surrounded with a walk gallery to work comfortably it can, and are equipped with pressure and temperature sensors, which integrate the cylinder heads into the information system that allows the machine from the bridge or from the steering station remotely monitored and controlled will.

Adjacent parts

Back to top of the cylinder head is finished with the valve cover ( cylinder head cover ). It contains the upper actuation elements of the valve drive and prevents the leakage of the lubricating oil as well as the ingress of air into the engine. In between is the valve cover gasket.

Down the crankcase follows. In between is the head gasket.

Typical work on the cylinder head

  • Check the spark plugs: See there.
  • Adjust the valve clearance: either with appropriate adjustment or with ground spacing plates suitable thickness ( "shims " ) (now often lash dispensable )
  • Grind valves to restore a sufficient compression ( rarely needed only by worn engine or defect in a single valve )
  • Replace the cylinder head gasket
  • Replacing the timing belt / timing chain along with tensioner ( tensioner ) - for timing belt: Tension Pulley (s)
  • Compression test: Instead of the spark plug, a pressure measuring device is used. This makes it possible to assess the general engine condition; extent to valves, cylinders, pistons and piston rings are worn and an overhaul is pending. Important primary is not the absolute level of compression, but an approximately equal value on all cylinders. A compression check is useful for when buying a used vehicle with high mileage.
838647
de