Piston is the most important reciprocation part of an IC engine. Years of research has made it almost perfect and breakthrough designs hardly make their make their way into production. So what are the common features that all pistons employ? The article explains all about piston, piston rings and the undesirable phenomenon, blowby, in detail.
Other than transferring the energy of the hot gases to the connecting rod, the piston has other important functions like to induce swirl in the fuel mixture and aid better atomization etc. This is the reason that in engine, we don’t see flat top pistons anymore. Some piston crowns have very intricate designs that are very functional (see the banner picture above, the negative relief is built into the piston crown for valve opening) and even help reduce emission and aid better combustion. All these designs are generally arrived at by conducting intensive CFD (Computational Fluid Dynamics) and CAE (Computer Aided Engineering) studies.
A normal car’s piston is made by the casting process. Pistons of high performance engines also use forged pistons which are capable of withstanding higher compression ratios. This makes them lighter and more robust.
All pistons have grooves cut in them to accommodate piston rings. As the designs evolved, the number of rings that each piston uses, has reduced. In case of most pistons there are three piston rings and each serves a specific function.
Compression ring - The top most ring is the compression ring that helps seal the combustion chamber and prevents leakage of the exhaust and fuel mixture into the crankcase.
Second ring - Just below the compression ring, the second ring seals the combustion ring in addition to limiting the supply of the oil to the cylinder liner that lubricates the piston and top compression ring.
Oil ring – The function of oil ring is to scrape the excessive oil from the liners back into the crankcase, where it gets collected for reuse.
The piston rings manage to seal the combustion chamber quite successively. However, still some amount of combustion gases and fuel mixture finds its way into the combustion chamber. This is primarily due to the piston ring end gap and the passage that the rings create by moving within the grooves.
So as to prevent direct blowby from the piston ring gaps, the gaps are kept about 120 degree apart at the time of installation.
Traditionally the crankcases were ventilated, however with strict emission controls, the blowby is reintroduced into the combustion chamber via PCV (Positive Crankcase Ventilation) valve where it can be used as charge for combustion.
Much has changed over the years but not the symmetrical design of the piston that's still very much in use. However, now the asymmetrical piston design is gaining popularity. It has a higher skirt area on one the side that experiences larger side thrust due to the sudden downward movement and smaller skirt on the area that experiences lower thrust. This helps reduce the contact pressures and effectively reduce wear on the higher thrust area. The lower skirt area of the other side helps reduce friction and increases effective efficiency.