In the crank mechanism piston performs several functions , including the perception of the gas pressure and the transmission of forces on the rod , sealing the combustion chamber and heat removal from it . The piston is the most characteristic part in an internal combustion engine as is realized with the help of thermodynamic process engine.Conditions in which it operates piston extreme and characterized by high pressure, temperature and inertial loads. at least from steel. The pistons are made of two ways - by injection molding or by stamping called forged pistons .
Scheme piston engine
The piston is integral structural element , which is conventionally divided into head (some sources call it the bottom) and a skirt. The shape and design of the piston is largely determined by the type of engine , the shape of the combustion chamber and the combustion process occurring in it. Piston gasoline engine has a flat or near- flat surface of the head . It may be provided with grooves for the full valve opening . Pistons with direct fuel injection have a more complex shape. In the top of the piston diesel engine combustion chamber holds a certain form , which provides a nice twist and improves mixture formation .
Below the piston head run grooves for mounting piston rings. The piston skirt has a tapered or curved ( barrel ) form . Such a shape of the skirt of the piston compensates for thermal expansion during heating. When reaching the operating temperature of the engine piston takes a cylindrical shape. To reduce the friction losses on the piston side with a layer of antifriction material ( molybdenum disulfide, graphite) . In the piston skirt has holes with the tides ( the boss ) to secure the piston pin .
Piston cooling is carried out by the internal surface of a variety of ways :
oil mist in the cylinder;
oil splashing through the hole in the rod ;
oil splashes special nozzle ;
injection of oil into a special zone in the annular channel rings;
oil flow through the tubular coil in the piston crown .
Piston rings form a tight connection with the wall of the piston cylinder. They are made of cast iron modified . Piston rings are the main source of friction in the internal combustion engine. Friction losses in the rings reach to 25 % of mechanical loss in the engine.
The number and location of the rings depends upon the type and purpose of the engine . The most common scheme - two compression and one oil ring . Compression rings prevent breakthrough of gas from the combustion chamber into the crankcase . The first compression ring works in the most severe conditions. Therefore, a number of pistons of diesel and gasoline engines forced into the groove of the ring set steel insert that improves durability and allows to realize the maximum compression . Compression rings may have a trapezoidal , a barrel , a conical shape , some are performed with a cut ( notch ) .
Scraper ring removes the excess oil from the surface of the cylinder and prevents the ingress of oil into the combustion chamber . The ring has a plurality of drainage holes . Some designs have a spring ring expander .
Compound piston rod by means of a piston pin which has a tubular shape and is made of steel. There are several way to install the piston pin . The most popular so-called floating finger that has the ability to rotate in bosses and the piston rod head during operation. To prevent displacement of the finger he fixed retainer rings . Much less is applied rigidly fixed ends of the piston pin or rigid fastening pin into the piston rod head .
