Piston, Piston Rings and Piston Pins

Main Functions

- transmission of gas force- kinematics guidance- sealing (together with the piston rings) of the crankcase against combustion gas, and the combustion chamber against oil- support for sealing rings (piston rings)- limit and design of the combustion chamber

The piston is subject to stress through:

- gas and mass forces- heat flow of the combustion chamber- frictional forces at the shaft and in the ring grooves- movements perpendicular to the running direction ("tilt“)

The good design and construction must guarantee:

- sufficient strength with as little mass as possible- permissible temperatures through design (heat flow), material and cooling- correct running clearance for all load levels- quiet operation with limited tilt

Setup and Dimensions

- Compression height CH influences the overall height of the engine. It is determinedby the height and overall number of the piston rings.- Piston pin length PL is dimensioned such that there is adequate space for I-ing-type retainers.

Mechanical Stress of the Piston

- caused by gas and mass forces that affect directly atop the piston, their reaction forces in the piston pin boss and skirt, and frictional forces

- The piston pin support is often designed as a moulded hole with pockets on the side to anticipate the tendency of the piston pin to become oval under stress; this design will allow a 20 % higher load carrying capacity.

- During the course of a working cycle, the piston frequently changes its contact side.- The greatest lateral forces occur during the expansion cycle shortly after TDC and lead to an intense contact change from the 'minor thrust face' to the 'major thrust face'.

- Mechanical noises and increased wear of piston, rings, and bearing surface are the consequences of this severe "slap".

- By offsetting the piston pin axis, an additional couple can be obtained at the pistonproviding an improved lateral force behaviour. An offset (0.5 to 2% of D) to the "majorthrust side" causes the piston to contact the major thrust side prior to TDC, and therefore the piston slap noise can be reduced

Thermal Stress of the Piston

- The piston is a component that is exposed to the high amounts of thermal stress.- the piston surface power is often used as a measurement of thermal stress.- The piston surface power output is defined as the effective power output per cylinder, PeZ max relative to the piston surface AK.

Pme - brake mean effective pressure pmeCm - mean piston velocity

np - Rated speedi – combustion cycle per revolutionAk – Piston area

- The piston surface power output is therefore proportional to Cm and Pme, which are the major influence on the average heat flux density.

- The temperatures in the piston are determined by the operating method and combustion process, the type of engine cooling, the engine size, the engine load, and the piston design itself can have influencing effects.

- The temperatures are higher for diesel engines than for comparable gasoline engines.

-For light-alloy pistons, the temperature level is lower than that for grey cast iron pistons.

-The highest temperatures occur in the centre of the piston head.

- To compensate for this thermal deformation, the piston is processed slightlycrowned or conical

Piston Materials

- Some of these requirements (2, 4, 5, 6) are well met by aluminum alloys containing silicon

- Their disadvantage is a high coefficient of linear expansion

Piston Designs

Single-Metal Pistons (Full-Skirt Pistons)

- The simplest pistons are made of one piece and have no fittings made out of a second material.

- Cooling is accomplished through the piston rings, piston skirt, cylinder liner,and splash-oil from the crankcase.

- mainly used for simple small gasoline and diesel engines, and medium-sized low power diesel engines.

Compensating Pistons

- Compensating pistons are made of light-alloy, and have cast-in inserts to influence the thermal expansion behaviour.

- Since the steel inserts and the light alloy are tightly connected, they influence each other during thermal expansion.

Cooled Pistons

- The higher the thermal stress (heat flux density, piston surface power output) and the larger the piston diameter (heat conduction path), the higher the temperatures and the larger the temperature variations become.

- A simple method consists of cooling the piston head with an oil spray of a fixed nozzle or one located in the connecting-rod eye

- forced cooling, during which the oil is supplied to the cooling channels via channels in the connecting rod, piston pin, or through telescopic tubes

Medium and Large Engines:

- The cooling coil piston offers the possibility of force cooling in medium-sized engines. Cooling oil is supplied into the steel coil and cast into the light-alloy piston via the connecting rod and the piston pin.

- For engines that are subject to maximum thermal stress, assembled pistons are used, which consist of two parts that bolted together

- For large engines, assembled pistons that include water cooling are used