Piston Rings
Piston Rings
Piston Rings
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NS MUDASSAR HUSSAIN DE-32(MECHANICAL ENGINEERING)
Classif ications: There are two basic classifications of piston rings. The Compression Ring: The compression ring seals the force of the exploding mixture into the combustion chamber. The Oil Control Ring: The oil control ring keeps the engine s lubricating oil from getting into the combustion chamber.
Most pistons have two compression rings and one oil control ring. Most passenger car engines have oil rings with two rails and an expander spacer as shown here. Compression Ring: The purpose of the compression ring is to hold the pressure from the power stroke in the combustion chamber. There are many different cross sectional shapes of piston rings available. The various shapes of rings all serve to preload the ring so that its lower edge presses against the cylinder wall. This serves the following functions: The pressure from the power stroke will force the upper edge of the ring into contact with the cylinder wall, forming a good seal. As the piston moves downward, the lower edge of the ring scrapes, from the cylinder walls, any oil that manages to work past the oil control rings. On the compression and the exhaust strokes, the ring will glide over the oil, increasing its life.
As shown in figure above a second compression ring is also used. The primary reason for using it is to hold back any blow-by that may have occurred at the top ring. A significant amount of the total blow-by at the top ring will be from the ring gap. For this reason, the top and the second compression rings are assembled to the piston with their gaps 60 degrees offset with the first ring gaps. Oil Control Rings: The oil control rings serve to control the lubrication of the cylinder walls. They do this by scraping the excess oil from the cylinder walls on the down stroke. The oil then is forced through slots in the piston ring and the piston ring groove. The oil then drains back into the crankcase. The rings are made in many different configurations that can be one-piece units or multipiece assemblies. Regardless of the configuration, all oil control rings work basically in the same way. Piston Ring Expanders: Expander devices are used in some applications. These devices fit behind the piston ring and force it to fit tighter to the cylinder wall. They are particularly useful in engines where a high degree of cylinder wall wear exists. Conf igurations: P iston rings are arranged on the pistons in three basic configurations. They are: The three-ring piston has two compression rings from the top, followed by one oil control ring. This is the most common piston ring configuration. The four-ring piston has three compression rings from the top, followed by one oil control ring. This configuration is common in diesel engines because they are more prone to blow-by. This is due to the much higher pressures generated during the power stroke. The four-ring piston has two compression rings from the top, followed by two oil control rings. The bottom oil control ring may be located above or below the piston pin. This is not a very common configuration in current engine design.
B enef its: Following are the benefits of piston rings. They maintain a gas-tight seal between the piston and the cylinder wall to prevent blow-by of the gases. P rovide a path for conducting heat from the piston for the cylinder walls. Control the quantity of oil reaching the piston crown and rings, allowing sufficient to ensure lubrication but limiting an excessive quantity which would cause an increase in oil consumption and carbonization.
PISTO N SKIRTS
The main task of the piston is to convert thermal energy into mechanical work. Furthermore, the piston rings seal the combustion chamber from the crankcase and transfers heat to the coolant. The piston skirt acts as a load-carrying surface, which keeps the piston properly aligned within the cylinder bore.
P iston skirt is fitted in both two stroke and four stroke engines. The diameter of the skirt is usually kept slightly larger than that of the piston. This is done to prevent damage to the liner surface due to the piston movement. Soft bronze rings are also fitted in the piston skirts. These bronze rings help during the runningin of the engine, when the engine is new, and can be replaced if necessary. In two stroke engines having loop or cross scavenging arrangements the skirts are slightly larger as these helps in blanking off the scavenge and the exhaust ports in the liner. In four stroke or trunk piston engines the skirt has arrangement for gudgeon pin, which transmits power from the pi ston to the gudgeon pin or top end bearing. As there are no cross head guides in four stroke engines, these skirts help in transferring the side thrust produced from the connecting rod to the liner walls.
P iston skirt taper. The piston skirt is cam ground. This allows a cast piston lo fit the cylinder with only 0.0005 to 0.0025 of clearance. The smaller the clearance, the less chance of piston slap. Expansion takes place perpendicular to the piston skirt. Piston Skirt Conf igurations: Figure A. shows slipper-skirt and trunk-skirt pistons. A full-skirt piston used on longer stroke engines is known as a trunk position. Most of todays automotive pistons are slipper-skirt pistons. A slipper-skirt is designed to clear the crankshaft counterweights on engines with short strokes. The surfaces of the piston skirts that are 90 degrees to the piston pin are called thrust surfaces.
BENEFITS
P iston skirts have different functions for different engines. In large cross head two stroke engines with uniflow scavenging these skirts are short in length and are fitted to act as a guide and to stabilize the position of the piston inside the liner. The piston skirt is the extension of the side profile of piston which controls the piston movement in the bore preventing it from wobbling around and controlling the angular forces present on the piston walls from the angular rotation of the crankshaft.
REFERENCES
[1]. Andersson, P eter, Tamminen, Jaana & Sandstrom, Carl-Erik. P iston ring tribology. A literature survey. Espoo 2002. VTT Tiedotteita Research Notes 2178. 105 p. [2]. Departmet of the US Army. (1985). Principles of Automotive Vehicles, TM 9-8000. Washington, DC. [3]. Gilles, T. (2010). Automotive engines: Diagnosis, Repair and Rebuilding. (6th ed., p. 469). NY: Delmar International Inc. [4]. Gilles, T. (2011). Automotive service: Inspection, maintenance, repair. (4th ed., pp. 271273). NY: Delmar International Inc. [5]. Introduction to piston design for forced induction engines. URL: http://horsepowercalculators.net/category/supercharger-power-parts [Archived in Superchargers P ower P arts] [6]. Kaushik, M. (2010). Piston skirt, Piston Rod and Trunk Piston. Retrieved from http://www.marineinsight.com/tech/main-engine/piston-skirt-piston-rod-and-trunk-piston/ [7]. Leeming, D. J. and Hartley R. (1989). Heavy Vehicle Technology. (2nd ed., p. 43). London, England: Stanley Thornes(Publishers) Ltd. [8]. P ulkrabek, W. W. (2007). Engineering Fundamentals of the Internal Combustion Engine. (2nd ed., p. 23). Delhi: Dorling Kindersley (India) Pvt. Ltd.