Friction is the force of resistance to sliding or rolling motion on a flat surface. Different types of friction defined as ploughing friction and rolling friction. The motion between piston and cylinder liners generates ploughing friction. Friction is not desirable in IC Engines as it directly affects the performance of the engine. Frictional losses are estimated in the tune of about 17-18% in various engines dynamic systems i.e. crank, bearings, piston cylinder valves, pumps, connecting rod etc. Piston-cylinder contributes frictional losses about 35-45% of the total frictional losses. So it is important to understand the mechanism of friction and wear of PRA (Piston Ring Assembly) in IC Engine. Compression rings, oil control ring, piston skirt and piston pin is the main contributing components to PRA friction. Ring width, ring face profile, surface roughness, ring tension, ring gap, ring land width, clearance liner temperature, skirt geometry and skirt bore clearance are the major design factors over and above material property of the ‘pair’ in PRA.
The contribution of various dynamic systems is identified by no. of scientists and estimated system wise in an I.C. Engine are as under.
Distribution of friction losses in I.C. Engine
The figure itself indicates the major contribution is of PRA assembly into total friction losses of the engine. So, it is important to understand the mechanism of friction and wear of PRA friction of an I.C. engine.
The piston and cylinder assembly are the primary link in converting the chemical energy of combustion into usable torque. The performance of the piston, ring and cylinder interface directly affects the power, fuel consumption, emissions and gas blow-off by the engine. The piston assembly accounts for nearly half of the total mechanical friction losses and is very complex to measure.
1.2 Advantages and limitations
Advantages
1. Test rig is having enough space so that it can accommodate all the equipment, which are necessary for trial.
2. Test rig is designed in such a way that all the equipment of test rig should have enough gap in between the equipment mounted on the rig so while performing test necessary adjustment can be carried out in minimum time.
3. Test rig is designed in such a way that required parameter could be noted easily.
4. Equipment are selected, mounted and installed, keeping in mind the required margin for experiment so number of test and in each test, number of set reading can taken so that better analysis can be carried out.
Limitations
1. Vibration takes places during performing test.
2. Test rig is heavy.
1.3 Application
1. It is used to measure frictional force between piston and cylinder liner.
2. To find out suitability of different lubricating oil with respect to geometry of piston ring.
1.4 Summary
The above discussion shows that the friction losses in piston ring assembly (PRA) system found about 45% of total frictional losses. So it is important to study and find out the way of reducing the frictional losses particularly on piston ring arrangement. Therefore it is found the area of interest. In the next chapter the detailed literature have been surveyed to find out the domain where the further research work can be conducted.
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Chapter 2 LITERATURE REVIEW
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2.1 Overview
Over the period of time considerable research and development work has been done in the area of friction between piston ring assemblies. In today’s modern technological world, still this process is widely applicable in reducing friction. The research effort which has been done in this area is reviewed and discussed in detail.
2.2 Literature
Atul.S.Shah, Dr.D.V.Bhatt [P1] has investigated on friction Contribution of Each Element in PRA System under Different Variables on Multi-cylinder Motorized Test Rig. The research study is mainly focused on experimental work and for this the Motored engine friction test method (strip method) is used. The authors have put efforts to find out Frictional power loss variation under individual piston ring under different speed and lubricants.
Fig.2.1 Block diagram of test rig
The fabricated test rig, 800-CC multi cylinder internal combustion engine system with crank mechanism, piston cylinder head, engine lubrication system, and engine cooling system, without gear box is used. Crank shaft is coupled with induction motor to drive the engine. For varying the speed of test rig, the A.C. motor drive /variable frequency drive is used. The multi functional watt meter is used to measure the performance in terms of power consumption.
The main objective of this research paper is to analyses effects of different operating parameters i.e. speed, lubricants and PRA system element. Experimental work is carried out on development of multi-cylinder IC engine test rig under different variable like speed, lubrication.
In experimental work the strip method is used the set of work experimental work carried out on variable like speed, lubricant but without changing piston geometry. From experimental results it is observed that without 1st or 2nd piston ring individually, the power consumption is less than the power consumption under standard assembly condition. The role of piston oil ring is observed critical in PRA system.
H R Mistry, D.V.Bhatt [P2] has investigated on tribological parameters of SI engine. The objective of the research paper is to measure PRA friction of multi cylinder 800 engine systems indirectly by measurement of power consumption by strip method. The set of experimental work were carried out under different operating conditions in set of all 3 pistons and set of 2 pistons (without 2 pistons) in speed ranges from 600 to 2400 rpm. The response variables are power consumption & PRA friction. Also PT100 type temperature sensors ranging from 0 to 50 celcius are used.
The result concluded are frictional power loss contribution by individual piston ring varies under different speed. Nature of curve of power consumption of PRA system with is in line with Stribeck curve. The paper offers that in PRA system the minimum frictional losses occurs when the engine runs with 2 ring only and maximum friction losses occurs, when engine running without both ring.
D.V.Bhatt, K. N. Mistry [P3] has experimentally studied on friction under different variable on piston ‘cylinder Assembly. Frictional losses in any I.C engine observed between 17-19% of total induced horsepower. 35-45% frictional losses observed due to piston ring assembly only from the above-referred total frictional loss. Therefore many researchers have study and tried to find out the way of reducing the frictional losses particularly on piston ring arrangement. The objective of research paper is to analyze the effect of various parameters like speed, lubrication viscosity and ring geometry on piston ring assembly to reduce friction losses.
Experimental methodology is adopted based on certain assumption and simulated the entire system with an extra drive system by electrical motor with a provision of various speed availability. The major limitation of this study is that the system should be rigid and vibration free.
From the analysis it may be concluded that elasto-hydrodynamic condition was a prevailing during the speed between 750 to 1200 rpm, later on part stiff rise in FF observed in almost all the cases that may be due to change in lubrication condition from hydrodynamic to mixed or boundary lubrication.
ZaidiMohd.Ripin, Horizon Gitano-Briggs, Mohd.Zulkifly Abdullah[P4] has experimentally analyzed on Friction of a Small Two-Stroke Utility Engine via Tear-Down Testing. The main objective of the paper is the analysis of Friction measurements as a function of lubricant quantity and engine temperature.
The methodology use for the measurement of the friction at different speed and temperature are Tear-Down Friction measurement and Oil-fuel ratio frictional measurement. It is observed in the measurement, the friction decrease in the speed of 1200-1800 rpm and gradually decreases when the speed is above 1800 rpm. And at the oil rate of 1:5 to 1:2 the smoke is less emitted while increasing in lubricant rate the friction is approximately less. It is also seen that during compression heat loss takes place and which apparently gives up to the friction. It is concluded that with increase in the speed of the piston of the engine and at proper oil/fuel ratio the friction decreases.
D.V.Bhatt, M A Bulsara and K N Mistry [P5] has experimentally studied on the prediction of oil film thickness in piston ring -cylinder assembly in an I.C engine. Oil film thickness has been predicted by author by identification of important variables affecting OFT and assumed certain parameters as constant having insignificant impact on OFT while developing various models. Some of the major parameters affecting the oil film thickness are, piston speed, lubricant viscosity, ring face profile, boundary conditions, surface roughness, effect of ring twist, bore distortion and ring flexibility.
They concluded that the major factors affecting the oil film thickness are piston speed, lubricant viscosity, ring face profile, boundary conditions, surface roughness, effect of ring twist, bore distortion and ring flexibility. Models are developed either on the assumption of Reynold’s theory of hydrodynamic lubrication or Stribeck curves, the same are established for rotary motion while in PRA system the motion is reciprocating, so results may not have the consistency. A need to develop versatile model for reciprocating motion of piston assembly exists as lubrication regimes varies from Boundary to Hydrodynamic. Film thickness may vary from about 2.5 to 8 micron in power stroke with SAE 10W50 oil at 1600 rpm & no load conditions.
Bedajangam S. K, N. P. Jadhav[P6]the objective of this study was to develop piston ring designs to improve engine efficiency, without adversely affecting oil consumption, blow by, wear and cost. The variable parameters are piston velocity, engine speed, oil viscosity, gas pressure, crank angle film thickness and coefficient of friction.
Fig.2.2 Assembly of lubricating pair Piston Ring and Cylinder Wall
Non variable parameter are system constant, bore diameter, ring tension, ring width, compression ratio, reciprocating mass, piston ring area and piston ring profile. The major assumptions for developing models are either hydrodynamic lubrication theory or mixed lubrication theory of Reynolds equation. A significant share of the total power loss in modern internal combustion engine is due to the friction interaction between the upper compression ring and cylinder wall as shown in figure 2.2.
A schematic of a typical lubrication condition between the ring and the liner is shown Fig.2.3
Fig. 2.3 Lubrication Conditions Encountered by Piston rings.
As shown in figure, due to the roughness of the surfaces in contact, it is possible for certain portions of the two surfaces to have asperity contact and for other parts to be sufficiently lubricated such that the ring is supported by load from the oil film. To simplify this situation, the modes of lubrication are typically characterized by the spacing between nominal lines that define smooth surfaces representing the average of the asperities.
Depending on the distance between the nominal lines, h(x), three different modes of lubrication are possible;
a) Pure Hydrodynamic Lubrication
b) Mixed Lubrication
c) Pure Boundary Lubrication
They concluded that the Boundary friction power loss increases at Low speed and Low load, the oil control ring throughout engine cycle increases contribution from top ring boundary friction around TDC of compression. The boundary friction power loss increases at Low speed and high load, the oil control ring throughout engine cycle and Top ring around TDC of compression/expansion increases contribution from top ring boundary friction around TDC of compression. The hydrodynamic friction power loss increases at high speed and low load, Oil control ring throughout engine cycle increases contribution from top ring boundary friction around TDC of compression. The hydrodynamic friction power loss increases as at high speed and low load, Oil control ring throughout engine cycle and Top ring around TDC of compression/expansion increases contribution from top ring boundary friction around TDC of compression.
2.3 Remarks based on literature survey
From the above literature survey it is understood that there are so many researchers who works on the friction in piston cylinder. The few research papers are available on friction in piston ring assembly. The most of the researchers have numerically found the relationship between lubricating oil and piston ring geometry.
Therefore it is important to understand the mechanism of friction and wear of PRA (Piston Ring Assembly) in I.C Engine. Based on this idea and present literature survey the problem definition of present research work has been formed as under.
2.4 Problem Definition
To investigate the measurement of friction between piston and cylinder by using different piston ring geometry and different lubricating oil and compared experimentally.
2.5 Objective
1. To understand the friction behavior in piston ring assembly.
2. To develop the experimental set-up for investigation of friction behavior between piston and cylinder.
3. To investigate the effect of piston ring geometry and lubricating oil on friction behavior experimentally.
4. To compare the experimental results.
2.6 Summary
In above chapter the detailed literature survey of friction behavior in piston ring assembly has been discussed. The chapter also contains the problem definition of present research work and objectives. In the next chapter the construction parts are discussed in detail.
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Chapter 3 FRICTION FUNDAMENTAL AND LUBRICATION
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3.1 Ring Friction Fundamental
The sliding contact between a piston ring and a cylinder liner hosts a variety of different friction mechanisms during one working cycle of the engine. Owing to the variations in load, speed and counter surface effects, the lubrication conditions in a ring/liner contact are strongly transient, which is reflected by variations in the friction and wear behavior.
The ring friction is determined by the ring load, the surface properties and the lubrication conditions as determined by the sliding velocity and the viscosity and availability of the oil. The ring load comprises the ring pre-tension and the gas forces acting on the back-side of the ring. Experiments by researcher’s with two ring and three-ring pistons have shown that the number of rings influences the frictional behavior of the ring pack, but the total tension of the piston rings in the ring pack finally determines the friction losses.
3.1.1 Piston ring kinematics and kinetics
One of the major requirements on the ring pack is related to the ring dynamics; radial and axial ring motion and ring twist. Ring motion and ring twist about the ring centre affect the operation of the ring, the oil film formation and the friction between the ring and the liner, the wear of the ring and cylinder liner, and the blow-by across the ring pack.
The primary motion of the piston rings is equal to the reciprocating piston motion. In an analysis of the piston ring lubrication, it is necessary to determine the velocity of the piston ring as a function of the crank angle. Apart from the reciprocating motion of the piston, the secondary motion of the piston affects the piston ring operation. The clearance between the piston and cylinder liner allows lateral movements and tilt of the piston according to the forces and moments acting on it.
3.1.2 Piston ring forces and moments
The piston ring secondary motions can be divided into piston ring motion in the transverse direction, piston ring rotation, ring lift, and ring twist. These types of motion result from different loads acting on the ring. Loads of these kinds are inertia loads arising from the piston acceleration and deceleration, oil film damping loads, loads owing to the pressure difference across the ring and friction loads from the sliding contact between the ring and cylinder liner.
The gas pressure above, below and behind the ring produces resultant forces on the ring section. The inertia forces acting on the piston rings, as well as those acting on the other reciprocating crank mechanism components, change proportionally to the square of the engine speed. The side loading of the piston against the cylinder wall is a result of the articulated joint of the connecting rod. The elastic distortion of the piston and liner can affect the effective geometry of the ring face and cylinder liner contact, which causes a non-uniform distribution of the contact pressure between the cylinder liner and the piston ring face. The piston pin is often offset from the piston centerline. This arrangement is applied in order to avoid piston-generated noise or to reduce the thermal load on the ring grooves.
3.1. Forces acting on the piston ring
3.1.3 Effect of piston ring surface finishing and coating
As for any tribological surface, the surface finish is of great importance for the lubrication conditions and the frictional behavior. For decades now, piston rings have been coated, both for suppressing wear and for reducing friction. Hard chromium coatings with oil-retaining porosity or channels on piston rings are good examples of a traditional beneficial piston ring coating.
The influence of the surface finish of the piston assembly on the total frictional losses of an engine has been studied by engine tests by researchers, who have shown that the frictional losses increase when run-in pistons, piston rings and cylinder liners are replaced by new components that need to be run-in. Ma and co-workers have shown that the coefficient of friction is strongly reduced during the running-in stage of the engine operation, and Priest and Taylor have shown the same effect in terms of fuel consumption. The benefits of reducing the surface roughness, in particular for reducing the friction and wear under mixed lubrication conditions.
3.1.4 Effect of cylinder liner surface finishing and coating
A plateau-honed cylinder liner surface profile, which consists of a fairly flat base surface with a network of deep scars, has been found appropriate for the lubrication of the piston assembly. The oil-retaining volume of the honed cylinder liner surface is of substantial relevance for the tribological performance of the system. For some decades, surface roughness parameters like the RSK (profile skewness), RVK (reduced trough depth) and RA (arithmetic average) have been used as measures on the oil-retaining capability of the groove pattern that has been produced onto the liner surface by honing.
According to the work by Galligan and co-authors, the coefficient of friction in the beginning of an oscillating test is lower (?? = 0.1 against 0.13) with a highly polished cylinder liner than with a liner with a standard surface finish. However, after a certain sliding distance the coefficient of friction is at the same level irrespective of the difference in initial surface quality.
As described below, in the sections concerning scuffing, the surface quality of the cylinder liner largely determines the scuffing resistance of the cylinder and piston assembly combination, from which point of view too smooth a surface liner may be unfavorable.
From the above investigations on the influence of the surface quality of the cylinder liner on the engine friction it can be concluded that an optimum surface quality can be established, which considers the aspects of both friction reduction and scuffing suppression. Coatings on cylinder liner surfaces may offer further benefits.
3.1.5 Effect of cylinder liner out-of-roundness
Deviations from cylindricity of cylinder liners cause local variations in the contact pressure between the piston rings and the cylinder. The wear of the cylinder liner and the likelihood of bore polishing and piston ring scuffing is likely to be pronounced on areas subjected to higher contact pressure. In the case of severe roundness errors, areas of particularly low contact pressure between ring and liner are subjected to increased risk of combustion gas blow-by, particularly with stiff piston rings and high crankshaft speeds.
3.2 Lubrication
Lubrication is the process, or technique employed to reduce wear of one or both surfaces in close proximity and moving relative to each other, by interposing a substance called lubricant between the surfaces to carry or to help carry the load (pressure generated) between the opposing surfaces. The interposed lubricant film can be a solid, (e.g. graphite) a solid/liquid dispersion, a liquid, a liquid-liquid dispersion (a grease) or, exceptionally, a gas.
A lubricant is a substance introduced to reduce friction between moving surfaces. It may also have the function of transporting foreign particles. The property of reducing friction is known as lubricity (Slipperiness).
A good lubricant possesses the following characteristics:
1. High boiling point
2. Low freezing point
3. High viscosity index
4. Thermal stability
5. Hydraulic Stability
6. Demulsibility
7. Corrosion prevention
8. High resistance to oxidation
3.2.1 Oil Supply
Oil is needed at the piston ring and liner interface to provide hydrodynamic or mixed lubrication in order to reduce friction and prevent seizure. In addition to the lubricating aspect, the oil acts as a heat carrier that transports heat from the piston and the ring-liner interface. The oil is further needed in the ring groove for preventing the ring from sticking to the groove. Oil is supplied to the piston and piston rings from the crankcase, directly or indirectly.
The oil supply method usually depends on the size of the engine and on the required amount of oil. Smaller, high-speed engines use splash lubrication, as the amount of oil supplied this way is usually sufficient. Larger engines that need a higher amount of oil need to have the oil supplied up into the piston. One solution for oil supply to the piston is that the oil is fed from the main bearing or bearings to the crankshaft and further on to the connecting rod and the piston.
3.2.2 Oil Quality
Oil degrades due to age and contamination. The additives in the base oil fall apart and combustion particles, such as soot and wear particles contaminate the oil. In the ring pack area, this occurs especially in the ring and land regions. The degradation is particularly caused by high temperature and blow-by gases.
A lubricant consists of base oil and additives. The additives vary depending on the operational environment for which the oil is designed. The most common additives are boundary friction reducers, viscosity index modifiers, and anti-wear additives such as ZDDP. Molybdenum dialkylthiocarbamate (MoDTC) is a base oil additive that reduces the boundary friction in a surface contact. In starvation conditions the friction with the MoDTC-modified oil may decrease to become equal to that of non-friction modified oils.
3.2.3 Contamination in the oil
The reason for engine oil changes is that the oil is degraded in terms of viscosity and oxidation, and that solid or liquid contaminations become mixed or dissolved into the oil. The presence of contaminations in engine oil is generally undesired, as solid contamination particles potentially cause abrasive wear and liquid contaminations may cause corrosive attacks, tribochemical wear and viscosity changes. Undesired polishing of cylinder liners in diesel engines during operation, commonly called bore polishing may occur if corrosive species and small abrasive particles are present in the lubricating oil.
The contaminants in the engine oil build up over time. This disadvantage is normally maintained by replacing the contaminated oil with a new batch. The oil change intervals can be based on running hours, particularly in the case of smaller engines, or on the condition of the oil.
Solid contamination particles: The detrimental effect of solid particles in the engine oil is particularly obvious with softer materials, like piston skirts and journal bearings, and in highly loaded contacts like the cam-follower contact. Crankcase oil that is contaminated by abrasive particles leads to wear of the piston skirt below the piston rings, which is indicated by a matt appearance of the surface below the rings, particularly on the thrust sides of the piston skirt.
Piston rings do suffer from abrasive wear, if particles of sufficient size are present in relevant concentrations in the engine oil. Bore polishing is another undesired effect of small abrasive particles in the lubricating oil while larger particles can cause scratches in the bore. The origin of the contamination particles may be soot from combustion, ingested dust in the shape of silica dust and similar minerals, and wear particles consisting of ferrous, copper, lead, chromium, aluminum, tin and nickel alloys. The main categories of solid particles in crankcase oils are the carbon, or combustion, particles and the metallic, or wear, particles.
3.2.4 Lubrication regimes
The ring-pack area experiences different kinds of lubrication regimes owing to local speed, load and surface roughness variations, and to variations in the oil supply. The desired lubrication conditions are the fully flooded ones, as the wear of the surface is negligible in this regime. This is, however, nearly impossible to achieve with today’s engine power demands. High combustion pressures and piston speeds exceed the optimal ones in terms of lubrication. Therefore, mostly mixed and boundary lubrication occurs in the ring-pack area. Oil transport to the ring in mixed lubrication is more or less insufficient. For example, a high combustion pressure may disrupt the oil film. The different lubrication regimes are briefly described below:
1. Fully flooded ring lubrication: The oil film covers the whole ring surface area. The load is carried solely by the oil film. These conditions typically occur when the piston ring sliding velocity is high and a low pressure acts on the back-side of the ring, as are the conditions during piston mid-stroke.
2. Partially flooded ring lubrication: Only a part of the ring is lubricated with oil. In this area the load is carried by the oil film, while the rest of the load is carried by the surface asperities and gas forces. This occurs in the vicinity of the dead centers, where the speed is lower and/or the pressure on the backside of the ring is high.
3. Starved ring lubrication: Oil availability on the liner is at its minimum. The oil is forced away from the ring surface area owing to insufficient oil supply and/or a strong gas pressure gradient over the ring. Increasing speed increases the oil film thickness to certain level, after which the speed is too high for the oil film to withstand.
3.2.4.1 Viscosity effects
The effect of the oil viscosity on the frictional behavior of piston rings has been investigated by researchers. The oil viscosity affects friction values under conditions of pure hydrodynamic lubrication when the rings are fully flooded. Higher friction values occur at higher viscosity. Suggestions are made that a slight increase in friction, which is observed at mid-stroke of the piston motion, could be partially caused by high-speed shear. The rings experience a very high contact pressure at mid-stroke, which could lead to oil starvation and thus friction increase.
3.2.4.2 Surface roughness and surface pattern effects
Surface roughness and textures have a considerable effect on the ring-pack friction. Sui and Ariga have investigated the influence of surface patterns on oil film thickness and ring friction. The oil film thickness of the top ring is hardly at all influenced by a change in the surface pattern and therefore the change in the surface friction is negligible. The second ring and the oil control rings, on the other hand, show differences in friction depending on the direction of the surface texture. A longitudinal pattern gives the highest friction and a transverse groove pattern gives the lowest friction. The influence of the surface pattern, or texture, on the friction occurs under conditions of mixed and boundary lubrication, where contact between sliding surfaces occurs. Sui and Ariga conclude that in situations where the surface roughness cannot be reduced to an optimal level, roughness orientation optimization should be considered.
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Chapter 4 CONSTRUCTION DETAIL
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4.1 Overview
In this chapter the construction detail for fabrication of set-up has been given and properties and function of different parts has been described as follows.
4.2 Different parts required
The different parts required in this project are as follows:-
1. Piston
2. Piston rings
3. Cylinder liner
4. Crankshaft and connecting rod
5. A.C motor
6. Bearings
7. Belt and pulley arrangement
8. Springs
9. Acrylic plate
10. Nuts and bolts
4.2.1 Piston
The piston is the cylinder-shaped component that moves up and down in the cylinder bore. Though pistons are of cylindrical shape, however they aren’t usually perfectly round, and they don’t usually have perfectly straight sides.
Fig. 4.1 Piston
Piston consists of grooves on which piston rings are mounted. Piston movement allows the engine to perform useful work. The piston must withstand a lot of physical force as well as tremendous heat during an engine’s operation.
The piston of an internal combustion engine is acted upon by the pressure of the expanding combustion gases in the combustion chamber space at the top of the cylinder. This force then acts downwards through the connecting rod and onto the crankshaft. The connecting rod is attached to the piston by a swiveling gudgeon pin. The pin itself is of hardened steel and is fixed in the piston, but free to move in the connecting rod. All pins must be prevented from moving sideways and the ends of the pin digging into the cylinder wall, usually by circlips.
Gas sealing is achieved by the use of piston rings. These are a number of narrow iron rings, fitted loosely into grooves in the piston, just below the crown. The rings are split at a point in the rim, allowing them to press against the cylinder with a light spring pressure. Two types of ring are used: the upper rings have solid faces and provide gas sealing; lower rings have narrow edges and a U-shaped profile, to act as oil scrapers. Pistons are cast from aluminum alloys. For better strength and fatigue life, some racing pistons may be forged instead. Early pistons were of cast iron, but there were obvious benefits for engine balancing if a lighter alloy could be used. To produce pistons that could survive engine combustion temperatures, it was necessary to develop new alloys such as Y alloy and Hiduminium, specifically for use as pistons.
Piston types and geometry
There are many piston types developed according to the operating requirements of different engine types. The piston types are commonly categorized by their cooling arrangement, by their primary field of application or by their structure. Four classifications of piston types, according to their structure, are presented as follows:
1. Uncooled or oil spray-cooled, cast or forged monometal light-alloy pistons for high speed automotive and small utility vehicle engines.
2. Uncooled or oil spray-cooled cast light-alloy pistons with ring-groove insert for high-speed heavy-duty diesel engines.
3. Single-piece or composite pistons with a cooling gallery for high-speed heavy-duty and medium-speed diesel engines.
4. Pistons for two-stroke, low-speed diesel engines.
The most essential areas of the piston are the piston top, the ring belt including the top land, the pin support, and the skirt. The geometry of these areas can vary significantly in compliance with the field of application.
4.2.1.1 Piston skirt materials
Typical piston materials are light alloys, cast iron, nodular cast iron, and alloyed steels. The pistons for high-speed engines are primarily made of aluminium silicon alloy.
To achieve low mass forces, high resistance against deformation and fatigue failure, and good sliding properties, the piston materials should fulfill specific requirements, such as:
1. low density,
2. high strength under temperature variations,
3. high heat conductivity,
4. good wear characteristics, and
5. favourable heat expansion.
The surface coatings used for pistons can be divided into the following application fields:
1. Coatings to improve sliding characteristics.
2. Coatings to increase wear resistance.
3. Coatings to improve thermal properties.
4. Coatings to increase the knock resistance.
4.2.2 Piston-ring
In the early steam engines no piston rings were used. The temperatures and the steam pressures were not as high as the corresponding parameters in today’s internal combustion engines, and the need for considering thermal expansions and clearances was smaller. Increasing power demands required higher temperatures, which caused stronger heat expansion of the piston material.
Fig. 4.2 Piston-ring
This made it necessary to use a sealant between the piston and the cylinder liner to allow a decrease in the clearance in cold conditions, i.e. when the clearances were at their maximum. The first piston rings used in an engine had the sole task of sealing off the combustion chamber, thus preventing the combustion gases from trailing down into the crankcase.
Piston rings for current internal combustion engines have to meet all the requirements of dynamic seal for linear motion that operates under demanding thermal and chemical conditions. In short, the following requirements for piston rings can be identified:
1. Low friction, for supporting a high power efficiency rate.
2. Low wear of the ring, for ensuring a long operational lifetime.
3. Low wear of the cylinder liner, for retaining the desired surface texture of the liner.
4. Emission suppression, by limiting the flow of engine oil to the combustion chamber.
5. Good sealing capability and low blow-by for supporting the power efficiency rate.
6. Good resistance against mechano-thermal fatigue, chemical attacks and hot erosion.
7. Reliable operation and cost effectiveness for a significantly long time.
4.2.2.1 Main functions of piston rings
The functions of a piston ring are to seal off the combustion pressure, to distribute and control the oil, to transfer heat, and to stabilize the piston. The piston is designed for thermal expansion, with a desired gap between the piston surface and liner wall. The rings and the ring grooves form a labyrinth seal, which relatively well isolates the combustion chamber from the crankcase. The position and design of the ring pack is shown in Fig. 4.3.
Fig. 4.3 Piston and piston rings
The ring face conforms to the liner wall and moves in the groove, sealing off the route down to the crankcase. The sealing ability of the ring depends on a number of factors, like ring and liner conformability, pre-tension of the ring and gas force distribution on the ring faces. Some of the combustion chamber heat energy is transferred through the piston to the piston boundaries, i.e. the piston skirt and rings, from which heat transfers to the liner wall.
Furthermore, the piston rings prevent excess lubrication oil from moving into the combustion chamber by scraping the oil from the liner wall during the down stroke. The piston rings support the piston and thus reduce the slapping motion of the piston, especially during cold starts where the clearance is greater than in running conditions. The rings are generally open at one location, at the ring gap, hence easily assembled onto the piston, see Fig. 4.3.
4.2.2.2 Piston ring materials and coatings
A piston ring material is chosen to meet the demands set by the running conditions. Furthermore, the material should be resistant against damage even in emergency conditions. Elasticity and corrosion resistance of the ring material is required. The ring coating, if applied, needs to work well together with both the ring and the liner materials, as well as with the lubricant. As one task of the rings is to conduct heat to the liner wall, good thermal conductivity is required. Grey cast iron is used as the main material for piston rings. From a tribological point of view, the grey cast iron is beneficial, as a dry lubrication effect of the graphite phase of the material can occur under conditions of oil starvation. Furthermore, the graphite phase can act as an oil reservoir that supplies oil at dry starts or similar conditions of oil starvation.
Coatings for rings are widely used. One example of such a coating is chromium, which is used in abrasive and corrosive conditions where running conditions are severe. Hard chrome plating is particularly relevant for the compression ring.
Piston ring surfaces are, in addition to chromium plating, thermally (plasma) sprayed with molybdenum, metal composites, metal-ceramic composites or ceramic composites, as a uniform coating or an inlay coating material .Thin, hard coatings produced by PVD or CVD include coating compositions like titanium nitride (TiN), chromium nitride (CrN); however coatings of this type are currently used exclusively for small series production for competition engines. Surface coatings/treatments for the entire piston ring surface are based on phosphorus, nitrides, ferro-oxides, copper and tin.
4.2.2.3 Types of Piston-rings
Piston rings form a ring pack, which usually consists of 2’5 rings, including at least one compression ring. The number of rings in the ring pack depends on the engine type, but usually comprises 2’4 compression rings and 0’3 oil control rings. The oil control rings used in diesel engines are two-piece assemblies and spark ignited engine oil control rings may be three-piece assemblies as well.
Compression ring
The compression ring acts as a gas seal between the piston and the liner wall, preventing the combustion gases from trailing down to the crankcase. The rings have a certain pretension, i.e. they have a larger free diameter than the cylinder liner, which assists the ring in conforming to the liner. Plain compression rings, with a rectangular cross-section, satisfactorily meet the sealing demands of ordinary running conditions.
Fig. 4.4 Compression ring cross-section
The ring may have a tapered face profile in order to shorten the running-in period, it enables the compression gas pressure to act on the face-side as well and thus relieve the pressure against the liner wall, which reduces the wear rate during running-in Scraper rings, which are usually used as the second compression rings, can simultaneously be used as oil-scraper rings
Oil control ring
In addition to the task of the compression rings to seal off the combustion chamber from the crankcase, there needs to be some mechanism to distribute the oil evenly onto the liner, for this purpose oil control rings are used. The number of oil control rings in a ring pack is one or two. The oil control ring is perforated by slots in the peripheral direction, which provides a way for the excess oil to leave the ring pack area. Oil control rings are not always necessary, contrary to the compression rings. Two-stroke spark-ignited engines, for example, have the lubrication oil mixed in the fuel, and therefore need no oil control rings. The appearance of the oil control ring differs from that of the compression ring; see the Figs. 4.5c and 4.5d.
Fig. 4.5 Compression and oil control rings
4.2.2.4 Piston ring groove
In a large number of piston designs, the piston ring belt consists of three ring grooves. The piston rings are situated in the grooves between the ring-groove flanges. Since the ring groove and the flanges are part of the piston sealing system, affecting the blow-by of the combustion gases and the oil consumption, the surfaces of the flanges have to be of very high quality. Damaged sealing surfaces lead to increased blow-by and reduced effective combustion pressure. At the same time the increased flow of the hot blow-by gases interferes with the oil film between the sliding surfaces and may cause hot gas damage to the piston rings.
To increase the wear resistance of the ring grooves in the pistons of heavy fuel oil engines, the grooves are typically either induction hardened or chromium plated. The wear of the ring groove flanks can affect the effective geometry of the ring face against the cylinder liner. In order to improve ring-groove wear resistance in steel composite piston crowns, a hard chromium layer can be applied.
To protect the first ring groove, sometimes also the second, in high-performance diesel engines against wear, so-called ring carriers made of high-alloyed cast iron are cast-in. Ring carriers are preferably made of Niresist, an austenitic cast iron with a thermal expansion coefficient almost equal to that of aluminum.
4.2.2.5 Sealing ability of piston rings
Piston rings have to meet various functional demands. The piston ring cylinder liner contact is a dynamic environment. In addition to the problems related to the static sealing capability, the varying shape of the cylinder liner in both the longitudinal and peripheral directions makes the sealing even more difficult in dynamic conditions. This requires good conformability of the rings.
Conformability means the ability of the piston rings to conform to a deformed cylinder liner. The deformation is caused by thermal and mechanical loading, cylinder head bolt tightening and abrasion. Conformability can be improved by increasing the tangential load or by decreasing the momentum of inertia.
4.2.3 Cylinder Liner
Since the piston and the piston rings are moving in the cylinder, the cylinder liner constitutes an important tribological element as a sliding surface against the piston and piston rings. The cylinders can be made of cast iron containing phosphorus, manganese, chromium, molybdenum, vanadium and titanium as alloying elements, or steel or aluminum.
The liner surface can be coated with a hard chromium layer to improve the wear resistance of the cylinder liners. Grey cast iron, when used for cylinder liners, is tribologically beneficial, as the graphite phase of the material gives a dry lubrication effect and furthermore acts as an oil reservoir that supplies oil at dry starts or similar conditions of oil starvation. To improve the wear resistance of an aluminum cylinder, a ceramic particulate phase can be cast-in into the aluminum liner during the manufacturing of the block.
Fig. 4.6 Cylinder Liner
A cylinder wall in an engine is under high temperature and high pressure, with the piston and piston rings sliding at high speed. A cylinder liner is a cylindrical part to be fitted into an engine block to form a cylinder. It is one of the most important functional parts to make up the interior of an engine. The cylinder liner, serving as the inner wall of a cylinder, forms a sliding surface for the piston rings while retaining the lubricant within. The most important function of cylinder liners is the excellent characteristic as sliding surface and these four necessary points.
1. High anti-galling properties
2. Less wear on the cylinder liner itself
3. Less wear on the partner piston ring
4. Less consumption of lubricant
The cylinder liner receives combustion heat through the piston and piston rings and transmits the heat to the coolant. The cylinder liner prevents the compressed gas and combustion gas from escaping outside. It is necessary that a cylinder liner which is hard to transform by high pressure and high temperature in the cylinder.
4.2.4 Crankshaft and Connecting Rod
In a reciprocating piston engine, the connecting rod connects the piston to the crank or crankshaft. Together with the crank, they form a simple mechanism that converts reciprocating motion into rotating motion. Connecting rods are best known through their use in internal combustion piston engines, such as automotive engines.
The shaft is subjected to various forces but generally needs to be analysed in two positions. Firstly, failure may occur at the position of maximum bending; this may be at the centre of the crank or at either end. In such a condition the failure is due to bending and the pressure in the cylinder is maximal. Second, the crank may fail due to twisting, so the conrod needs to be checked for shear at the position of maximal twisting. The pressure at this position is the maximal pressure, but only a fraction of maximal pressure.
Fig. 4.7 crankshaft and connecting rod
Crankshaft is typically connected to a flywheel to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsional vibrations. As a connecting rod is rigid, it may transmit either a push or a pull and so the rod may rotate the crank through both halves of a revolution, i.e. piston pushing and piston pulling. The connecting rods are most usually made of steel for production engines, but can be made of aluminum or titanium for high performance engines, or of cast iron for applications such as motor scooters.
4.2.5 AC Motor
An AC motor is an electric motor driven by an alternating current (AC). It commonly consist of two basic parts, an outside stationary stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the outside shaft that is given a torque by the rotating field.
Fig. 4.8 AC motor
There are two main types of AC motors, depending on the type of rotor used. The first type is the induction motor or asynchronous motor; this type relies on a small difference in speed between the rotating magnetic field and the rotor to induce rotor current. The second type is the synchronous motor, which does not rely on induction and as a result, can rotate exactly at the supply frequency or a sub-multiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet. Other types of motors include eddy current motors, and also AC/DC mechanically commutated machines in which speed is dependent on voltage and winding connection.
Slip: If the rotor of a squirrel cage motor runs at the true synchronous speed, the flux in the rotor at any given place on the rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed. Because the rotating field (or equivalent pulsating field) effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly. Even with no load, internal mechanical losses prevent the slip from being zero.
Specification of motor used are as followed:
1. Single phase, 440V induction motor
2. Rating: 0.5 HP
3. Speed: 2800 rpm
4.3 Summary
The chapter contains the details of fabricated set-up and the properties of the different component used and there function. The working principle and the experimental methodology are included in the next chapter.
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Chapter 5 WORKING PRINCIPLE AND METHODOLOGY
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5.1 Overview
In this chapter the principle of working has been discussed and it also includes the methodology adopted for calculating the stiffness of the spring and procedure for piston ring chamfering.
5.2 Working Principle
The basic principle adopted in the construction of the two stroke test rig is the simple mechanism of reciprocating of piston into the cylinder liner. When the motor is switched on the power is transmitted to the crankshaft by belt and pulley arrangement between motor and crankshaft.
Fig. 5.1 PRA assembly
Due to this power, the crankshaft starts rotating and motion is transmitted to piston through connecting rod. This causes reciprocation of piston into cylinder liner. Due to this movement, the friction is produced between piston and liner. This frictional force will cause deflection of the plate on which liner is mounted. Since the plate is on bearing, movement is easier. The deflection is measured and for accuracy, deflection is to be measured by different instrument like LVDT, Dial gauge and on linear scale and comparison is to be made for different results.
This procedure is to be followed for piston rings having different chamfered angles i.e. 10, 15, and 20 degree. We will use different lubricating oil 2T oil and 20W40. And we will measure deflection and calculate the friction force for each piston ring for different lubricating oil. Hence we will be able to state geometry and oil which will offer minimum frictional force.
5.2 Spring Stiffness Calculation
Stiffness is the rigidity of an object ‘ the extent to which resist deformation in response to an applied force.
The stiffness (k) of a body is a measure of the resistance offered by an elastic body to deformation. For an elastic body with a single degree of freedom (for example stretching or compression of a spring)
The stiffness is defined as:
k = F/ ??
Where,
F is the force applied on the body.
?? is the displacement produce by the force along the same degree of freedom.
Procedure for calculating spring stiffness:
1. Attach one end of the spring to the rigid support.
2. Measure the length of the spring at initial condition (free end).
3. Attach a load at free end of the spring.
4. Again measure the displacement of the spring after applying the load.
5. Calculate the stiffness of spring.
Consider a spring of stiffness (k) whose one end is attached to the rigid support and a load(w) is applied to its free end as shown is fig(6.1)
Fig 5.2 Single Spring
Now the stiffness of the spring can be calculated as:
k=mg/??
Where,
m = mass taken as 0.3kg
g = acceleration due to gravity, 9.81m/s2
?? = displacement of the spring (in mm)
k = spring stiffness
Calculation:
k= mg/??
= 0.3*9.81
94.3
=0.031208 N/mm
Consider four springs in parallel having stiffness of k1, k2, k3, k4
Fig. 5.3 Parallel Springs
Now the four springs are in parallel to each other so the equivalent stiffness can be calculated as:
keq= k1+k2+k3+k4
keq = 0.031208+0.031208+0.031208+0.031208
keq = 0.12512 N/mm
5.3 Piston Ring Chamfering
Fig. 5.4 fixture
The piston ring chamfering is the most delicate process. The chamfering reduces the surface area of the rings which comes in contact with cylinder liner.
Procedure
1. Firstly clamp the fixture in the lathe spindle.
2. Mount three piston rings on the fixture.
3. Fasten the nut tightly.
4. Arrange the tool on the tool post.
5. Start the spindle and bring tool between edges of two piston rings.
6. Thus the sides of two different piston rings get chamfered.
5.4 Experimental Methodology
The experimental methodology includes the method for measuring and reducing friction. Our methodology includes four set of piston ring, which we are going to chamfered at different angles of 10, 15, 20 and remaining one is used as normal ring.
The different lubricants which we are going to use are of two type’s 2T oil and 20W40 and now, on the fabricated test rig the performance of experiment is to be done.
Firstly we will arrange normal piston ring on piston and start the test ring. Then its performance will be observed on different lubricating condition like dry condition, in 2T oil lubricating condition, 20W40 lubricating condition. After that we will calculate value of friction force on basis of deflection of plate and result will be represented in tabular form. Similarly all other sets of piston rings which are chamfered at different angle will be mounted on piston and their performance is observed and result will be calculated.
After that the ranking are to be made for different ring geometry on the different oils, which offers minimum friction will be calculated on the basis of comparison of three ring geometry with the normal piston rings.
Fig. 5.5 Experimental Setup
5.5 Bill of Materials
Table no. 5.1 Bill of materials
Sr. No. Part name Quantity Cost (INR)
1. Two stroke engine(60cc) 1 1100
2. Electric motor (0.5 HP) 1 850
3. Piston ring 8 320
4. Ball bearing 5 150
5. Deflection spring 4 60
6. Pulley 2 500
7. Angle plate and channel 8 1500
8. Bolts and nuts 10 50
9. Fabrication cost 1000
10. Lubricant oils 2 100
11. Belt 1 100
Total 5730
5.6 Summary
The chapter contains the experimental procedure to perform the experiments which have to be conducted and the working principle. The results obtained by performing experiment will be noted and compared.
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Chapter-6 RESULTS AND DISCUSSION
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6.1 Overview
This chapter contains the results obtained by experiment. Further discussions on the obtained results are made to find the effect of all parameters on the friction force.
6.2 Experimental results
Our experimented results are based on certain parameters like stiffness of spring different lubricant oil and piston ring geometry at constant speed. The stiffness of spring, appears to be the most important parameter for experimental point of view. The friction force measurement is mainly dependent on the spring stiffness. The lubricant plays an important role in our experimental set up. These are intended to reduce the friction and to get smooth movement of piston in cylinder liner.
Variation of friction force with respect to variation in different parameters such as piston ring geometry and lubricating oils is observed and noted.
6.2.1 Experimental results for Dry condition
Variation in friction force obtained for different conditions is tabulated in table 6.1 when working in dry condition. The effect of each parameter on the friction force is discussed depending upon the obtained results. In chapter 5, the calculation for spring stiffness is mentioned. By following that calculation method, friction force is calculated and it is tabulated in table below.
Table 6.1 Experimental results for Dry condition
Types of piston ring Deflection(mm) Average
Deflection(mm) Friction force F=k. ??
(N)
Normal
9
11
13
11
1.37632
10??
4
6
8
6
0.75072
15??
5
7
9
7
0.87584
20??
14
17
20
17
2.12704
From the table 6.1, it is observed that the friction force between piston ring and cylinder liner is increasing as the degree of chamfer on the piston ring is increasing for working in dry condition or without lubrication. It is found that the minimum friction force is observed for 10 degree chamfered piston ring and maximum in 20 degree chamfered piston ring.
Fig. 6.1 Chamfer angle Vs Friction force (Dry condition)
Fig 6.1 shows the graph plotted between friction force and chamfered angle in dry condition. The graph indicates that minimum friction is offered by 10?? both side chamfered piston ring and maximum friction is offered by 20?? both side chamfered piston ring.
6.2.2 Experimental results for 2T Oil
In lubricated condition the variation in friction force obtained for different conditions is tabulated in table 7.2. The effect of each parameter on the friction force is discussed depending upon the obtained results. In chapter 5, the calculation for spring stiffness is mentioned. By following that calculation method, friction force is calculated and it is tabulated in table below.
Table 6.2 Experimental results for 2T Oil
Types of piston ring Deflection(mm) Average
Deflection(mm) Friction force F=k. ??
(N)
Normal
13
9
17
13
1.61656
10??
4
5
4
4
0.50048
15??
7
4
10
7
0.87584
20??
19
18
20
19
2.37728
From the table 6.2, it is observed that the friction force between piston ring and cylinder liner is increasing as the degree of chamfer on the piston ring is increasing when working with 2T oil as lubrication. It is found that the minimum friction force is observed for 10 degree chamfered piston ring and maximum in 20 degree chamfered piston ring.
Fig. 6.2 Chamfer angle Vs Friction force (2T Oil)
Fig 6.2 shows the graph plotted between friction force and chamfered angle in dry condition. The graph indicates that minimum friction is offered by 10?? both side chamfered piston ring and maximum friction is offered by 20?? both side chamfered piston ring.
6.2.3 Experimental results for 20W40 Oil
In lubricated condition the variation in friction force obtained for different conditions is tabulated in table 6.3. The effect of each parameter on the friction force is discussed depending upon the obtained results. In chapter 5, the calculation for spring stiffness is mentioned. By following that calculation method, friction force is calculated and it is tabulated in table below.
Table 6.3 Experimental results for 20W40 Oil
Types of piston ring Deflection(mm) Average
Deflection(mm) Friction force F=k. ??
(N)
Normal
19
15
11
15
1.8768
10??
2
4
6
4
0.50048
15??
5
9
13
9
1.12608
20??
20
19
20
20
2.5024
From the table 7.3, it is observed that the friction force between piston ring and cylinder liner is increasing as the degree of chamfer on the piston ring is increasing when working with 20W40 oil as lubrication. It is found that the minimum friction force is observed for 10 degree chamfered piston ring and maximum in 20 degree chamfered piston ring.
Fig. 6.3 Chamfer angle Vs Friction force (20W40)
Fig 6.3 shows the graph plotted between friction force and chamfered angle in dry condition . The graph indicates that minimum friction is offered by 10?? both side chamfered piston ring and maximum friction is offered by 20?? both side chamfered piston ring.
7.3 Ranking of piston rings for different lubricants
Table7.4 Ranking of piston rings for different lubricants
Lubricants 1 2 3
Dry 10?? Both side 15?? Both side 20?? Both side
2T 10?? Both side 15?? Both side 20?? Both side
20W40 10?? Both side 15?? Both side 20?? Both side
Fig. 6.4 Chamfer angle Vs Friction force (Dry condition, 2T, 20W40 Oil)
Fig 6.4 shows the friction behavior with respect to different chamfered angle and different lubricating condition.
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CONCLUSION
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Experimental work was carried out with different combination of test parameters and individual ring geometry.
From table 7.4 following results can be concluded:
In dry condition, it is observed that 10?? both side chamfered piston ring offers minimum friction force. And hence it seems to be suitable for it.
In 2T oil lubrication, it is observed that 10?? both side chamfered piston ring offers minimum friction force. And hence it seems to be suitable for it.
In 20W40 oil lubrication, it is observed that 15?? both side chamfered piston ring offers minimum friction force. And hence it seems to be suitable for it.