Friction under (careful) control
Whether we like it or not, the phenomenon of friction accompanies all moving mechanical elements. The situation is no different with engines, namely with the contact of pistons and rings with the inner side of the cylinders, i.e. with their smooth surface. It is in these places that the greatest losses from harmful friction occur, so the developers of modern drives are trying to minimize them as much as possible through the use of innovative technologies.
Not only temperature
To fully understand what conditions prevail in the engine, it is enough to enter the values in the cycle of a spark engine, reaching 2.800 K (about 2.527 degrees C), and diesel (2.300 K - about 2.027 degrees C). High temperatures affect the thermal expansion of the so-called cylinder-piston group, consisting of pistons, piston rings and cylinders. The latter also deform due to friction. Therefore, it is necessary to effectively remove heat to the cooling system, as well as to ensure sufficient strength of the so-called oil film between the pistons operating in individual cylinders.
The most important thing is tightness.
This section best reflects the essence of the functioning of the piston group mentioned above. Suffice it to say that the piston and piston rings move along the surface of the cylinder at a speed of up to 15 m/s! No wonder then that so much attention is paid to ensuring the tightness of the working space of the cylinders. Why is it so important? Each leak in the entire system leads directly to a decrease in the mechanical efficiency of the engine. An increase in the gap between pistons and cylinders also affects the deterioration of lubrication conditions, including the most important issue, i.e. on the corresponding layer of oil film. To minimize adverse friction (along with overheating of individual elements), elements of increased strength are used. One of the innovative methods currently being used is to reduce the weight of the pistons themselves, working in the cylinders of modern power units.
NanoSlide - steel and aluminum
How, then, can the goal mentioned above be achieved in practice? Mercedes uses, for example, NanoSlide technology, which uses steel pistons instead of the commonly used so-called reinforced aluminum. Steel pistons, being lighter (they are lower than aluminum by more than 13 mm), allow, among other things, reducing the mass of crankshaft counterweights and help to increase the durability of crankshaft bearings and the piston pin bearing itself. This solution is now increasingly used in both spark ignition and compression ignition engines. What are the practical benefits of NanoSlide technology? Let's start from the beginning: the solution proposed by Mercedes involves the combination of steel pistons with aluminum housings (cylinders). Remember that during normal engine operation, the operating temperature of the piston is much higher than the surface of the cylinder. At the same time, the coefficient of linear expansion of aluminum alloys is almost twice that of cast iron alloys (most of the currently used cylinders and cylinder liners are made from the latter). The use of a steel piston-aluminum housing connection can significantly reduce the mounting clearance of the piston in the cylinder. The NanoSlide technology also includes, as the name suggests, the so-called sputtering. nanocrystalline coating on the bearing surface of the cylinder, which significantly reduces the roughness of its surface. However, as for the pistons themselves, they are made of forged and high-strength steel. Due to the fact that they are lower than their aluminum counterparts, they are also characterized by a lower curb weight. Steel pistons provide better tightness of the working space of the cylinder, which directly increases the efficiency of the engine by increasing the operating temperature in its combustion chamber. This, in turn, translates into a better quality of the ignition itself and a more efficient combustion of the fuel-air mixture.
