TECHNICAL FIELD
The subject matter described herein, in general, relates to lubrication and. in particular, it relates to lubrication of piston rings used in reciprocating machines. BACKGROUND
Reciprocating machines, for example, reciprocating engines, reciprocating compressors, and reciprocating pumps, have one or more pistons reciprocating inside their respective cylinders. Every piston generally has one or more piston rings mounted on the piston for providing a dynamic seal between the piston and an inner surface of the cylinder called cylinder liner. The piston rings not only provide sealing but also distribute and control distribution of lubrication oil inside the cylinder, and help in stabilizing the piston.
In a reciprocating machine, lubrication is provided between the piston ring(s) and the cylinder liner to prevent wear of rubbing surfaces, i.e.. the piston rings and the cylinder liner. Various techniques of lubrication, such as fluid film lubrication, elastohydrodynamic lubrication, and boundary lubrication have been used for providing lubrication in these machines.
In fluid film lubrication, an oil film is formed between the piston rings and the cylinder liner for maintaining lubrication. One type of fluid film lubrication technique is hydrodynamic lubrication. In hydrodynamic lubrication, the pressure required to develop a full oil film is produced dynamically during a relative motion between two rubbing surfaces. The thickness of the oil film formed between the two rubbing surfaces is proportional to the relative speed between the two rubbing surfaces and the rise in temperature and pressure.

During the reciprocating motion of the piston, the speed of the piston with respect to the cylinder liner increases from a minimum at a bottom dead centre (BDC) of the cylinder to a maximum at mid-stroke length of the cylinder and again reduces to a minimum at a top dead centre (I DC) of the cylinder. Thus, at and around the mid-stroke length, a lubricating oil film is formed between the piston ring and the cylinder liner due to hydrodynamic lubrication. Therefore, there is minimum friction-induced wear of the piston ring and the cylinder liner around the mid-stroke length. I lowever. at reversal points, i.e.. the TDC and the BDC. due to low speed of the piston, the hydrodynamic oil film does not get formed, resulting in a direct contact between the piston ring and the cylinder liner. Thus at BDC and TDC. only boundary lubrication takes place in which there is a metal-to-metal contact. Hence, friction-induced wear is maximum at the reversal points. Further, considerable amount of energy is wasted due to high friction at the reversal points, thereby reducing the efficiency of the engine.
SUMMARY
The subject matter described herein is directed to lubrication of piston rings used in reciprocating machines. A reciprocating machine includes a cylinder, a piston reciprocating inside the cylinder and a piston ring mounted on the piston.
The piston ring of the present embodiment has a C-shaped cross section forming an oil groove in an outer surface of the piston ring. The oil groove receives lubricating oil when the piston approaches a top dead center of the cylinder or a bottom dead center of the cylinder, to form a hydrostatic oil film between the piston and a cylinder liner of the cylinder at the top dead center and the bottom dead center. Thus, the oil groove helps in providing hydrostatic lubrication between the piston and the cylinder wall at the top dead center and the bottom dead center in addition to hydrodynamic lubrication.

These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. BRIEF DESCRIPTION OF DRAWINGS
The above and other features, aspects and advantages of the subject matter will be better understood with regard to the following description, appended claims, and accompanying drawings, where:
Fig. la illustrates an exemplary piston ring, according to an embodiment of the present subject matter.
Fig. lb illustrates a magnified view of an end portion of the exemplary piston ring of Fig la. according to an embodiment of the present subject matter.
Fig. 2 illustrates an exemplary piston ring lubrication system employing an exemplary piston ring, according to an embodiment of present subject matter.
Fig 3 illustrates a graph showing variations in operating parameters of the exemplary piston ring of Fig la.
Fig 4 illustrates a graph depicting variation in friction between two rubbing surfaces when a piston ring according to an embodiment of the present subject matter is employed and a comparison thereof when a conventional piston ring is employed in an IC engine. DETAILED DESCRIPTION
The present subject matter relates to a piston ring lubrication system for providing lubrication of piston rings used in a variety of applications, such as in internal combustion (1C)


engines and reciprocating compressors. The piston rings are generally used to provide a sliding seal between a piston reciprocating in a cylinder, and an inner surface of the cylinder called as cylinder liner.
According to an embodiment of the present subject matter, the piston ring lubrication system includes a piston ring having an oil groove, i.e.. the piston ring has a C-shaped cross-section forming the oil groove. The oil groove is provided on an outer surface of the piston ring, i.e.. the surface in contact with the cylinder liner, such that when the piston ring is mounted on the piston, the oil groove is facing towards the cylinder liner. The oil groove is provided in order to facilitate hydrostatic lubrication of the piston ring. Hydrostatic lubrication takes place at top dead center (TDC) and bottom dead center (BDC) positions, when the pressure required to form a lubrication oil film can not be produced dynamically due to little or no relative motion between the rubbing surfaces. In such a case, the pressure required to form the lubrication oil film is provided by an external source such as a pump.
The piston ring lubrication system further includes a pump for supplying lubricating oil at a pressure from an oil sump to the piston ring through a solenoid valve. The solenoid valve is provided to control timings of supply of oil to the piston ring. In one embodiment, the lubricating oil is pumped in the oil groove of the piston ring when the piston is near the TDC or BDC. The lubricating oil thus pumped forms a hydrostatic oil film, thereby enabling hydrostatic lubrication between the piston ring and the cylinder liner.
.Although the piston ring and the piston ring lubrication system of the present subject matter have been described in considerable details with respect to an internal combustion engine, however, it will be understood that the piston ring and the piston ring lubrication system can be

embodied in any reciprocating machine with a piston ring provided on a reciprocating medium, such as in reciprocating compressors.
Fig. la illustrates an exemplary piston ring 100 that can be mounted on a piston. In one embodiment, the piston may reciprocate inside a cylinder (not shown in the figure) of a reciprocating machine, for example, an internal combustion (1C) engine. The piston can be provided with one or more piston rings including the piston ring 100 lor providing a dynamic seal between the piston and a cylinder liner (not shown in the figure).
According to an embodiment of the present subject matter, the piston ring 100 is an open ring having a C-shaped cross section. The piston ring 100 has a first end and a second end substantially close to each other thus forming a gap 102 between the two ends. The gap 102 provides springiness to the piston ring 100. which helps maintain conformity between the piston ring 100 and the cylinder liner even during extreme operating conditions, for example, high temperature and pressure conditions created inside the cylinder during a power stroke. The piston expands and contracts with varying temperature inside the cylinder, thus varying a clearance space between the piston ring 100 and the cylinder liner. The piston ring 100 counters such variations with its springiness.
Further, the piston ring 100 has a C-shaped cross section forming an oil groove 104. The oil groove 104 is provided at an outer surface of the piston ring 100. i.e.. the surface in contact with the cylinder liner, such that when the piston ring 100 is mounted on the piston, the oil groove 104 is towards the cylinder liner. In one embodiment, the oil groove 104 runs from the first end of the piston ring to the second end leaving a small solid space at both ends ol'the piston ring 100. as can be seen in the figure. The oil groove 104 receives lubricating oil from an oil circuit (not shown in this figure). The oil groove 104 transfers the lubricating oil to the cylinder

liner to form a hydrostatic oil film for providing hydrostatic lubrication between the piston ring 100 and the cylinder liner. As is known in the art, hydrostatic lubrication takes place when the pressure required to develop a full oil film is provided by an external pump.
Fig. lb illustrates a magnified view of an end portion of the piston ring 100. The piston ring 100 is provided with a capillary 106. which is provided at or near the first end of the piston ring 100 and bends to end in the oil groove 104. In one embodiment, the capillary 106 has a bend of approximately 90 degrees. The capillary 106 receives lubricating oil from an oil circuit (no shown in this figure) including an oil pump, and provides the lubricating oil to the oil groove 104. The lubricating oil forms a hydrostatic oil film between the piston ring 100 and the cylinder liner to provide hydrostatic lubrication. In one implementation, the hydrostatic lubrication is provided when the piston is at a top dead centre (TDC) of the cylinder, where the speed of the piston is not enough to form a hydrodynamic oil film. Similarly, the hydrostatic lubrication can also be provided at a bottom dead centre (BDC) of the cylinder for similar reasons.
The hydrostatic lubrication reduces wear of the rubbing surfaces, i.e.. the piston ring 100 and the cylinder liner, by reducing the friction there between. The reduction in friction and wear improves the life of the piston ring 100 and the cylinder liner. Moreover, reduced friction decreases energy losses at the TDC or the BDC. which were usually noticed in conventional machines employing piston rings having a rectangular cross section.
Fig. 2 illustrates an exemplary piston ring lubrication system 200. according to an embodiment of present subject matter. In said embodiment, the piston ring lubrication system 200 can be employed in an IC engine and includes the piston ring 100 and an oil circuit 202. The piston ring 100 may be mounted on a piston (not shown in the figure) and the oil circuit 202 max


provide lubricating oil to the piston ring 100 through a channel provided in a piston skirt of the piston.
The oil circuit 202 includes an oil sump 204. an oil filter 206, a pump 208. a pressure relief valve 210. a pressure gauge 212, and a solenoid valve 214. The pump 208 draws lubricating oil from the oil sump 204, and supplies the lubricating oil to the capillary 106 through the solenoid valve 214. In one embodiment, the oil sump 204 can be present in a crankcase of the 1C engine. Further, the pump 208 can be a gear pump or a reciprocating pump or any other type of pump known in the art.
The lubricating oil from the oil sump 204 first passes through the oil filter 206 for removal of any impurities before the oil enters into the pump 208. The lubricating oil from the pump 208 is then transferred to the pressure relief valve 210, which is provided between the pump 208 and the solenoid valve 214 to maintain the pressure at which the lubricating oil is provided to the capillary 106. The pressure relief valve 210 by-passes the lubricating oil to the oil sump 204 if the pressure increases above a certain preset valve, the pressure of the oil is indicated by the pressure gauge 212.
The solenoid valve 214 receives the lubricating oil from the pressure relief valve 210 and releases the lubricating oil into the oil groove 104 through the capillary 106 and the channel provided on the piston skirt. The channel is provided with an opening through which the lubricating oil enters the capillary 106 when the piston reaches a predetermined position. When the piston nears the predetermined position, the solenoid valve 214 opens to allow the lubricating oil to enter the channel. In one embodiment, the solenoid valve 214 opens and closes with respect to the angle of rotation of a crankshaft of the IC engine, measured in degrees, for example, in one implementation, the solenoid valve 214 opens at 20 degrees before the TDC and


closes at 20 degrees after the TDC to release pressurized lubricating oil into the oil groove 104 of the piston ring 100. Further, if the hydrostatic lubrication is provided at BDC also, then the solenoid valve 214 may open to allow the lubricating oil to enter the channel when the piston nears a predetermined position near the BDC.
The piston ring 100 thus helps in converting boundary lubrication at the TDC and BDC to hydrostatic lubrication. This helps in reducing frictional losses at the TDC and BDC to minimum, leading to appreciable reduction in fuel consumption in the IC engines.
In another implementation, the solenoid valve 214 may receive signals, to open and close, from an electronic control unit (not shown in the figure) based on the position of the piston reciprocating inside the cylinder.
In operation, the pump 208 pumps lubricating oil to the solenoid valve 214. The solenoid valve 214 then releases the lubricating oil from the solenoid valve 214 into the oil groove 104 through the channel and the capillary 106. In one embodiment, the solenoid valve 214 transfers the lubricating oil into the oil groove 104 when the piston approaches a predetermined position, for example, when the piston approaches the TDC or the BDC. The lubricating oil from the oil groove 104 is then transferred to the cylinder liner to form the hydrostatic oil film between the piston ring 100 and the cylinder liner.
Fig 3 illustrates a graph 300 showing variations in operating parameters of the exemplary piston ring 100. Due to the motion of the piston, operating conditions as well as associated parameters, such as friction force, thrust load, and sliding speed, vary across the operating range. These parameters depict the lubrication between the rubbing surfaces. Theoretically, the lubrication between two rubbing surfaces is a collective effect of hydrodynamic. hydrostatic and boundary lubrications between them, which can be expressed in terms of oil film thickness.

The graph 300 illustrates the variation in operating parameters when hydrostatic lubrication is provided at TDC alone. It will be understood that hydrostatic lubrication may also be provided at BDC or may be provided at BDC alone, and the operating parameters will accordingly vary.
In the graph 300, positions of the piston at the TDC. BDC, and mid-stroke (MS) are taken as reference positions and are represented along a vertical axis, while the aforementioned operating parameters are represented on a horizontal axis. The graph is plotted for a power stroke, i.e.. when the piston moves from the TDC towards the BDC after a combustion cycle. Curve 302 represents variation in sliding speed (S) of the piston ring 100 against the cylinder liner. At the TDC. the sliding speed is zero. The sliding speed increases from the TDC to reach maximum speed at the MS and decreases again from the MS to reach zero at the BDC.
Curve 304 depicts variations in thrust load (TL). The maximum TL is observed at the TDC due to the thrust applied by the expanding burnt gases on the piston after combustion. The TL gradually decreases as the piston moves from the TDC to the BDC. as shown by the curve 304. and reaches a minimum at the BDC. as the expansion of burnt gases is almost complete at the BDC.
As the relative sliding speed between two rubbing surfaces determines hydrodynamic lubrication between the two. hence, a variation in the hydrodynamic lubrication (HDL) oil film thickness, represented by curve 306. follows the curve 302 representing the sliding speed. Minimum thickness of the HDL oil film is observed at the TDC as the sliding speed is not sufficient to build the pressure required for the formation of the HDL oil film. As the sliding speed increases, the HDL oil film thickness also increases and reaches maximum at the MS and then again decreases to a minimum at the BDC with the sliding speed approaching zero.

The piston ring 100 is pumped with lubricating oil when the piston reaches the TDC with pumping continuing for some time after the TDC. Due to the continuous pumping of the lubricating oil in the oil groove 104 of the piston ring 100. a hydrostatic lubrication (HSL) oil film is formed between the rubbing surfaces. As shown by curve 308. the HSL oil film thickness is maximum at the TDC and remains the same until the MS and then decreases thereon to a minimum towards the BDC. The HSL, oil film thickness decreases as the pressure inside the oil film ceases. As the thickness of both HDL and HSL oil films is minimum at the BDC. hence boundary lubrication (BL) takes place at the BDC. In one embodiment, the HSL oil film can also be provided at the BDC by using the same mechanism used for providing HSL oil film at the TDC.
further, curve 310 represents variations in oil film thickness (OFT) between the rubbing surfaces. The OFT represents overall thickness of the lubrication oil film formed due to both hydrodynamic and hydrostatic lubrication. From the TDC to the MS, the HSL dominates, and therefore the curve 310 follows the curve 308. After the MS, the hydrodynamic lubrication exists, and hence the curve 310 follows the curve 306.
1 he lubrication improves due to employment of hydrostatic lubrication in the present lubrication system. Due to improvement in lubrication between the rubbing surfaces, the wear and tear and the friction force between the rubbing surfaces are reduced. As shown by curve 312. the friction force in the TDC to MS region reduces considerably as compared to conventional lubrication systems employing conventional piston rings. In addition, there is a substantial reduction in the friction force in the MS to BDC region as well.
Further, curve 314 shows that the wear is also reduced in the region between the TDC and the MS and is almost negligible in the region between the MS and the BDC. Even though

BL takes place in the proximity of the BDC. however, a minimum TF calls for reduced wear. The reduction in wear increases the life of the piston ring as well as the cylinder liner. With the employment of the piston ring 100. the engine can save considerable energy lost in overcoming frictionai force.
Fig 4 illustrates a graph 400 comparing variations in friction force between the rubbing surfaces when the piston ring 100 is employed and when a conventional piston ring is employed in an IC engine. The vertical axis of the graph 400 represents the friction force, whereas the horizontal axis represents the crank angle. The graph 400 is drawn for a four-stroke IC engine, wherein curves 402 and 404 depict variance in the friction force when the piston ring 100 and the conventional piston ring are employed, respectively. The conventional piston ring employed has a rectangular cross-section with dimensions similar to that of the piston ring 100. As shown, the friction force is reduced considerably during the entire working cycle of the engine.
The aforementioned versions of the subject matter and equivalent thereof have many advantages, including those, which are described below. Due to the employment of the piston ring 100, the lubrication between the piston and the cylinder liner is improved considerably. The new design will help in converting boundary lubrication at the TDC to hydrostatic lubrication. This would help in reducing frictionai losses at the TDC to minimum, leading to appreciable reduction in fuel consumption in IC engines. Further, the reduction of friction as well as wear and tear and increases the life of the piston ring 100 as well as the cylinder liner. Reduction in friction also increases the efficiency of the engine, as less energy is wasted in overcoming frictionai forces.
Although embodiments for a piston ring lubrication system employing a piston ring have been described in language specific to structural features and/or methods, it is to be understood

that the invention is not necessarily limited to the specific features or methods described. Rather. the specific features and methods are disclosed as exemplary implementations for the piston ring.

I/We claim:
1. A reciprocating machine comprising:
a cylinder having a cylinder liner; a piston reciprocating inside the cylinder: and a piston ring (100) mounted on the piston; characterized in that.
the piston ring (100) has a C-shaped cross section forming an oil groove (104) along an outer surface of the piston ring (100).
2. The reciprocating machine as claimed in claim 1, wherein the piston ring (100) includes a capillary (106) to provide the lubricating oil to the oil groove (104).
3. The reciprocating machine as claimed in claim 1 further comprising an oil circuit (202) to provide the lubricating oil to the oil groove (104).
4. The reciprocating machine as claimed in claim 3, wherein the oil circuit (202) comprises a solenoid valve (214) configured to release lubricating oil in to the piston ring (100).
5. The reciprocating machine as claimed in claim 3. wherein the piston includes a channel provided on a piston skirt to direct lubricating oil from the oil circuit (202) to the piston ring (100).
6. The reciprocating machine as claimed in claim 3. wherein the oil circuit (202) is configured to provide lubricating oil to the oil groove (104) when the piston approaches at least one of a top dead centre of the cylinder and a bottom dead centre of the cylinder.
7. A method comprising:
pumping lubricating oil to a solenoid valve (214):
releasing the lubricating oil from the solenoid valve (214) into an oil groove (104) in a piston ring (100), when a piston approaches a predetermined position: and
transferring the lubricating oil from the oil groove (104) to a cylinder liner to form an oil film between the piston ring (100) and the cylinder liner.
8. The method as claimed in claim 7, wherein the lubricating oil is released into the oil groove (104) near a top dead centre of a cylinder.
9. The method as claimed in claim 7. wherein the lubricating oil is released into the oil groove (104) near a bottom dead centre of a cylinder.
10. A piston ring (100) comprising:
an outer surface having a C-shaped cross section forming an oil groove (104): and
a capillary (106) provided proximate to a first end of the piston ring (100). wherein the
capillary (106) opens into the oil groove (104) to provide lubricating oil to the oil groove
(104).
1 1. The piston ring (100) as claimed in claim 10. wherein the piston ring (100) is implemented in
a reciprocating machine.