Piston is the part without which the internal combustion engine could not work. Most people who are keen on cars know the principle of operation of the engine. If you happen to be one of those who don?t, carry on reading. Chemical energy contained in the engine fuel is converted into mechanical and thermal energy. As a result of fuel combustion in the combustion chamber, gases with high temperature and high pressure are formed and they affect bottom of the piston. This energy makes the piston move towards the bottom dead center (stroke).
In the four-stroke engines, the above described process is alternated. As a result, the pair of pistons 1-3 is in the bottom dead center, and the pair of pistons 2-4 in the top dead center (this is where takes place ignition and combustion of fuel-air mixture and pushing it towards bottom dead center).
The piston is one of components which are under the greatest heat in the combustion chamber. On its design depends a number of characteristics characterizing the process of combustion.
The piston is attached to connecting rod. Mechanical energy is transferred to it from mixture burnt in the cylinder. Most of the pistons are made of aluminum and silicon alloys but piston materials are subject to high requirements. These include: high material resistance to high working temperatures, high fatigue resistance, adequate thermal conductivity, small factor of thermal expansion, small mass proper, resistance to friction wear, resistance to corrosion.
The connecting rod is the element connecting the piston with the crankshaft. The piston has a bottom, hub mounting, piston pin mounting, ring part and carrying part. The bottom is shaped according to required solutions concerning the combustion chamber – the simplest solution is flat bottom. In spark-ignition engines but also in those with compression ignition with split combustion chambers, slightly convex bottom forms are used, which improves stiffness of the construction.
The piston hub is used to attach the piston pin. The hub is usually located in the piston’s center of gravity. However, because of its tendency to lateral tilt (leading to faster wear of the piston and greater noise caused by the piston) while positioning the hub in the middle of carrying part of the piston, such solution is not used anymore.
Ring part of the piston is located in its upper part. Two-stroke engines have 1 to 4 piston rings and four-stroke engines 2-4 rings (usually three rings). The number of used piston rings depends on compression. The first piston ring is responsible for sealing the cylinder and is called a sealing ring as it seals the combustion chamber and prevents gases from entering into the engine body. Due to its location, it is exposed to corrosion and the highest operating temperatures. Through the compression rings is transferred up to 70% of heat from the piston to the cylinder. Typically, this ring has a rectangular or spherical shape.
Structural and operational factors influence degradation of material used in engine pistons. Depending on the above factors one can distinguish the following types of wear: friction-caused wear, fatigue wear (variable mechanical and thermal stresses), corrosion-related wear, erosive wear due to gaseous or liquid media. Heat damages appear in the form of cracks in the piston bottom. Such cracks can lead, among other things, to disturbances in the combustion of fuel / gas mixture or to reduction of the chamber tightness. In case of engines with injection into the pre-chamber, the most common defect is cracking of the piston bottoms. The pre-chamber is a combustion chamber in compression engines with indirect compression, usually taking up 20-40% of the total combustion chamber volume. The temperature at edges of the piston bottom in the combustion chamber area can reach even 380°C. In case of contact with some liquid, extreme operating conditions can occur, which leads to cracks or permanent deformation of the piston. The above-mentioned damages to the piston bottom can be caused by penetration of water or fuel into the combustion chamber. These liquids can enter the combustion chamber through, for example, the suction system, damaged gaskets or damaged injection nozzle. Another possible cause of the piston damages is its thermal overload. This phenomenon can occur if oil is replaced too rarely (in cars with self-ignition engine, oil should be changed approximately every year, in case of cars with a spark-ignition engine – approximately every 1,5 years). Moreover, this could lead to clogging of cooling jets with engine oil. 40 to 50% of the mechanical losses in internal combustion engines are the losses caused by friction of the rings and the piston against the cylinder’s surface. This is the reason for ever more frequent reduction of sizes of the ring surface (whereas pressures do not change). This leads to reduction in resilience of the piston rings, which in turn, can cause cracking of rings because of difficult operating conditions. Cracking of the piston rings can also be caused by tribological wear, mechanical overloads caused by combustion disturbances, mounting errors or heavy loads during cold start of the engine.