Investigation Into Piston Failures Engineering Essay

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Abstract

The previous extensive discussions about the significance of the functions, composition, and failure of the mechanical parts of an engine have been quite controversial. Such investigations and studies gave rise to various questions. Therefore, the work efficiency of the engine block as per the piston's operations is related to the piston's mechanism. The pistons of an engine are the most intricate components in rest of the field components of automotive as well as of other industry. If engine is considered as the heart of a car then the piston is regarded as its most important part. Various researched have been conducted so far proposing new geometries, materials and manufacturing techniques for engine pistons which further accounted for a constant improvement for the past decades and demanded comprehensive analysis of the smallest details. Despite of all these vast researches, there is a mass number of piston failures. There are various origins in damage mechanisms which area basically related to temperature, wear and fatigue. Further the fatigue damages prominently include thermal fatigue and mechanical fatigue, which are either at high temperature or at room temperature.

This study is not just related to the causes, diagnosis and solutions of piston failures but also related to an overwhelming and a specialist exploration of the points of investigation. Various interlinked aspects and methods corresponding to the piston failures or fatigues have to be considered to construct a reliable, overwhelming and coherent investigation about engine piston failures. Thus, the following three basic approaches are employed to analyze the fatigue mechanism.

Total Life Approach,

Crack Initiation Approach, and

Crack Propagation Approach.

Various characteristics such like the piston-materials, the piston-operations, the total life approach or the other related operations classify lubrication operations as the nature of the engine piston failure. The fatigue total-life approach corresponds to the piston life method that analyzes the stresses damaging the piston. In the similar context, the materials of the piston focus on the fatigue matter in which a piston is most likely to damage or crack due to the use of the forged materials. Such cracks or damages relate to the aspects of material-life, usage-life, stress-life, and operation-life. Nevertheless, the cyclic fatigues are commonly elastic where the material-stress life curve is used and the piston fatigue mechanism is basically plastic deformation. The characteristic of the crack initiation related to the dilemma of piston fatigue is usually employed to determine the operation's processes influencing the efficiency of the engine piston.

Table of Contents

Table of Contents 3

1.Introduction to Pistons 4

1.1 Background on Pistons: 4

1.2 Aims and objectives: 6

1.3 Piston function 6

1.4 Piston Parts 8

2.Piston Design 11

2.1 Piston Shape investigation: 11

2.2 Piston crown: 12

2.3 Piston Rings: 12

2.3.1 Compression Ring 12

2.3.2 Wiper Ring 13

2.3.3 Oil Ring 13

2.4 Piston skirt: 15

2.5 Materials used for manufacturing: 15

2.5.1 The significance of silicone: 16

2.5.2 The types of aluminum alloy: 16

2.5.3 Forged versus Cast: 18

References 19

Introduction to Pistons

1.1 Background on Pistons:

Let's study the historical background by reviewing the initial development of the piston ring. During the primitive times of mankind, the most important goal was to draw water more efficiently. Agricola, the Renaissance author published a comprehensive description, in 1556, about the piston pumps used by the miners to draw water. The author pointed out the use of metal discs and leather washers in which the purpose of using the leather was to avoid friction on the pump rods. The rapid growth of the urban population at that time gave rise to the need for pumps to raise water. In 1582, the very first waterwheel-driven pump was set up in London. During the late seventeenth century, the severe issues related to the mining industry in England were observed with the agglomeration of water in to deeper shafts put forward the opportunity to commence the use of steam power for driving reciprocating engines. Ever since, the reciprocating engines have been constantly developing, and there is no predictable end towards their usage and continued augmentation. During the years of their use, one specific problem has stimulated pervasive developmental efforts, which is the leakage of fluid in between the piston and the cylinder bore. This problem is experienced in various types of machineries, combustion engines, water pumps, hydraulic motors, air compressors, hydraulic pumps, and cylinders. Initially, the efforts were applied to excessively narrow down the gap however the efficiency of such a sealing method was found to be very low. It was considered impossible to develop a piston and bore with adequately small tolerances to ascertain low leakage. This problem was resolved by isolating the sealing function with the help of a separate machine element that was called the piston ring. The purpose of the piston ring was to better adjust the contact surface of the cylinder bore or liner. In 1774, the first attempt was made to create such a ring, when rope packing was incorporated in to a steam engine for acquiring a seal that enhanced the engine's thermal efficiency to 1.4 percent. This subsequently followed the development of piston ring out of metal (Dowson, 1998). In 1860, Etienne Lenoir patented the internal combustion (IC) engine functioning at much higher temperatures and pressures in comparison to a steam engine. The use of ferrous material for the piston rings was prompted with the development of this type of engine since it has the mechanical properties of bearing or tolerating high temperatures. Ever since, the piston rings are mostly made up of cast iron or steel. Since 1889, the continuing attempts to improve piston rings have derived the creation of various scientific papers (Hersey, 1944). There is yet significant research interest in developing our comprehension about the tribological phenomena taking place among the piston rings, the cylinder liners and the piston in mechanisms such as that of internal combustion engines (Andersson et al., 2002). Early on, the improved performance was experienced due to the improvement in the form and the reduction in the waviness of the cylinder bore. Afterwards, the honing technique was developed to improve the degree of surface roughness. In the internal combustion engines, the honing of cylinder bore is being done for almost a century where as the augmented plateau-honing technique is being employed for the last half century (Suzuki, 1997). A flat land surface is created between the cross-hatched grooves' pattern through the plateau honing technique. The valleys or grooves basically serve to retain oil. In order to decrease the amount of retained oil for the purpose of

Reducing the oil consumption, the trend of using a land surface between the valleys with higher bearing capacity was followed which further signified an improvement in scuffing resistance. Therefore, the valleys became narrower and shallower where as the land surface turned smoother (Lenthall, 1997). What forces lead towards the improvement of the operation and efficiency of piston rings in Internal Combustion Engines, which are presently employed in the development of about fifty million automobiles per year? The basic driving forces are definitely stricter environmental regulations and concurrent needs for the augmentations such like reduced wear and loss of friction.

1.2 Aims and objectives:

This paper investigates the type of piston ring wear and its progression in relation to various operational parameters. It will enable us better comprehend the friction behavior and the wearing of piston rings. The ultimate objective of this study is to allow increasing the lifetime of the piston rings, with some margin of safety, with the help of the rolling contact fatigue life of the bearings.

This study is carried out to support the mechanical engineering process enduring a highly qualified classification of the problems of the engine block and its parts. Thus, this study must be able to enable a mechanical engineer to develop a general apprehension about preventing future engine piston failures. This research will incorporate to two aspects so as to deal with the practical issues. This first aspect will cover the explanation of the entire concept of the assembly of piston in relation to its surrounding parts. The second aspect will include the investigation of the entire concept surrounding engine piston failures such like fracture, fatigue, sealing, friction and other concerns.

1.3 Piston function

A massive number of processes in the world need some input such as mechanical work, rotation to function and torque. The production of mechanical work by machines is dependent on the mechanism of how they convert energy. The chemical energy that is conserved in the fuel is released in the form of heat as result of a thermodynamic process in an internal combustion engine in order to generate the mechanical work, rotation and torque. The second common way to transform energy in to mechanical work is to use an electric motor. The third usual way is to transform hydrostatic energy in to mechanical work with the help of a hydrostatic transmission of the radial piston type, as in the Hägglunds hydraulic motor where the rotation and the torque are generated by the hydrostatic pressure and flow. In 1957, the design of the radial piston hydraulic motor was first introduced accompanied by a patent for an internal combustion engine with radial piston which uses diesel fuel (Bergström and Omnell, 1996). Hagglunds put forward this patent and thereby, the operating mechanism employed was the conversion of energy from chemical to that of a hydrostatic transmission.

Piston is an essential component of an engine irrespective of the type of the fuel being burnt in it, since it transforms the chemical energy in to a mechanical work from the explosion of fuel mixture. Inside the cylinders, the pistons move up and down with the help of the combustion of fuel. The fuel is first injected in to the cylinder and combusted afterwards which enables the piston to move in the downward direction. The fuel is combusted at a very fast interval in every cylinder. The piston executes four stages which are (1) intake, (2) compression, (3) ignition and (4) exhaust, in order of their occurrence. In the first stage, a mixture of fuel and air or simply air enters in to the cylinder. Then the air passes in to the valve that opens and closes with the help of the camshaft. In the second stage, the compression takes place when the piston moves up that compresses the air or the mixture of fuel and air. In the third stage, ignition is performed in two ways. Ignition in a diesel engine is achieved through the injection of fuel in to the heated and compressed air present in the cylinder where as the ignition in a gasoline engine is done with the help of a spark plug. In the fourth stage, the exhaust occurs in which the burnt gases are forced out by the piston moving in the upward direction. In 1959, the first prototype of the hydraulic motor was tested in laboratory, which involved new concepts of piston assembly, which transmitted the tangential force generating the torque (see Figure 2).

Figure 1: The first prototype of hydraulic motor by Hägglunds in 1959.

A long and narrow gap is usually employed in order to seal off the high hydrostatic pressure between piston and cylinder bore (Ivantysyn and Ivantysynova, 2003). The perfect design for hydrostatic pump and motor incorporates either a long or a short piston guide. The benefits of a long piston guide as opposed to a shorter one are reduced leakage and the support for the side load with reduced frictional loss. Moreover, higher volumetric and mechanical efficiency is achieved through a long piston guide in comparison to the volumetric and mechanical efficiency achieved through a short piston guide.

1.4 Piston Parts

The piston serves as the moving end of the combustion chamber where as the cylinder head is the stationary end of the combustion chamber. Usually, the pistons are made up of a cast aluminum alloy due to its improved thermal conductivity and light weight. The potential of a material to conduct and transfer heat is defined as its thermal conductivity.

A piston includes the following parts.

Head: It is the top surface of the piston that is near to the cylinder head and is subjected to tremendous forces and heat whiles the normal operation of the engine.

Pin bore: It is a hollow capacity with in the side of the piston, receiving the piston pin, which is at an angle of 90 degrees to the piston travel.

Piston pin: It is a hollow shaft joining the connecting rod's small end with the piston.

Piston Skirt: It is the part of the piston nearest to the crankshaft aiding the piston to align as it goes in to the cylinder bore. In order to lower the piston mass and to give clearance for the rotating crankshaft counterweights, some skirts have profiles cut into them.

Ring groove: It is a concaved area positioned around the perimeter of the piston which is employed for the purpose of retaining a piston ring.

Ring lands: These are the two surfaces of the ring groove, parallel to each other, serving as the sealing surface for the piston ring.

Piston ring: It is an expandable split ring providing a seal between the cylinder wall and the piston. The rings of the piston are usually created from cast iron since the cast iron maintains the integrity of its original shape even under the influence of dynamic forces such as heat, load, etc. The combustion chamber is sealed by the piston rings as they transfer heat to the cylinder wall from the piston, and direct the oil back to the crankcase. The size and configuration of the piston ring vary according to the engine design and cylinder material. In general, the piston rings in small engines consist of the compression ring, the wiper ring, and the oil ring which are explained in the further sections in this paper.

Figure 2 - Piston Rings

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Figure 3 - Piston Ring Gap

Piston Design

2.1 Piston Shape investigation:

There are three possible forms of the gap geometry for seal off pressure that the Pistons could have, all of which play a unique role towards the leakage. These three forms of gap geometry are provided in the cross section provided in Figure 4.

Figure 4: For different piston sealing designs, the cross sections of gap geometries: Figure A illustrates a fully eccentric gap design where as the Figure B illustrates a rectangular gap design, and the Figure C illustrates a concentric gap design. Eccentricity is represented by the

dimension, 'e'.

The design A involves a solid piston, having the highest leakage, bending inside the cylinder bore so as to support the side loads. The design B involves a slit piston ring that is generally employed in an internal combustion engine where: the slit bears an almost rectangular cross section, the width is given by the circumferential gap dimension while the gap is defined by the clearance between the cylinder bore and the piston. Three times lower leakage is portrayed in the design B in comparison to the leakage shown in design A. However, in practice, the gap is not open in design B and adapts the form of a trap for decreasing the leakage. Comparatively to all the designs, the least leakage is achievable through the design C that is the design of a closed gap referring to the un-slit piston in which the piston ring can reach closely in order to be concentric inside the cylinder bore as explained by Skytte af Sätra (2005). The design C has the following benefits:

less leakage,

leakage, in the form of Poiseuille flow, that is independent of the position in the stroke,

reduced piston-cost, and

decrease in the size of the piston and thereby, reduction of the size of the whole hydraulic motor.

2.2 Piston crown:

2.3 Piston Rings:

Piston rings consist of the compression ring, the wiper ring, and the oil ring.

2.3.1 Compression Ring

The top-most ring that is closest to the combustion gases is the compression ring. Thus, it has to face the highest operating temperature as well as the greatest amount of chemical corrosion. The 70 percent of the heat of the combustion chamber is transferred from the piston to the cylinder wall by the compression ring. Either taper-faced or barrel-faced compression rings are used in most Briggs & Stratton engines. A piston ring having a taper angle of about 1 degree on the running surface is referred as the taper faced compression ring. This taper offers a smooth wiping action so as to inhibit any excess oil before it approaches the combustion chamber.

A piston ring having a curved running surface in order to offer consistent lubrication of the cylinder wall and the piston is called the barrel faced compression ring. This type of compression ring also offers a wedge effect so as to augment the oil distribution across the full stroke of the piston. Moreover, the curved running surface decreased the likelihood of the break down of an oil film caused by the excess pressure at the edge of the ring or by the excessive deflection of the piston during operation.

2.3.2 Wiper Ring

It is also known as the Napier ring, scraper ring or back-up compression ring. The wiper ring is the next ring on the piston that is away from the head of the cylinder head. The wiper ring offers a consistent oil film thickness with the purpose of lubricating the running surface of the compression ring. Most scraper rings have a taper angle face in Briggs & Stratton engines. The position of the tapered angle is at the direction of the oil reservoir and thus, it is able to create a wiping action when the piston is moving toward the crankshaft.

The taper angle offers contact directing the excess oil on the cylinder wall, for reverting to the oil reservoir, through the oil ring. If a wiper ring is installed incorrectly with the tapered angle nearest to the compression ring then it causes excessive oil consumption. This is due to the reason that the scraper ring wipes excess oil toward the combustion chamber.

2.3.3 Oil Ring

Two thin rails or running surfaces are included in an oil ring. Slots or holes that are cut into the radial center of the ring enable the flow of excess oil to revert to the oil reservoir. In general, the oil rings are in a single piece that includes all of these features. In order to assign more radial pressure to the piston ring, some one piece oil rings employ a spring expander which increases the unit pressure exercised at the wall of the cylinder.

The highest inherent pressure is experienced at the oil ring in comparison to all the three types of the rings on the piston. A tree-piece oil ring involving two rails and an expander is used in some Briggs & Stratton engines. The oil rings are positioned at both sides of the expander. In general, the expander includes multiple windows or slots in order to direct the oil back to the piston ring groove. The small running surface of the thin rails provides the inherent piston ring pressure, the high unit pressure and the expander pressure which are utilized by the oil ring.

The combustion chamber is sealed by the piston rings, as they conduct heat to the wall of the cylinder and govern the consumption of the oil. The piston ring is able to achieve this with the help of the inherent and applied pressure that is the internal spring force forcing the piston ring to expand on the basis of the design and properties of the material employed in making the piston. A significant force is necessary for the inherent pressure, which is required to contract the ring in to a smaller diameter. The free or uncompressed piston ring gap determines the inherent pressure, which is the length in between the two ends of an uncompressed piston ring. Conventionally, the larger the gap of the uncompressed piston ring, the greater force the ring of the piston exerts while compressed with in the cylinder bore.

The piston ring should be able to offer the expected and positive radial fit between the wall of the cylinder and the piston ring running surface in order to achieve an efficient seal. The inherent pressure of the piston ring attains the radial fit. The piston ring should also be able to sustain the seal over the piston ring lands.

Moreover, the combustion chamber is also sealed by the piston ring with the help of the applied pressure that is the pressure exerted to the piston ring from combustion gases, resulting in the expansion of the piston ring. There is a chamfered edge, in some piston rings, that is opposite to the running surface. The piston ring is twisted due to this chamfered edge if not influenced by the combustion gas pressures.

The other consideration of the piston ring design contact pressure of the cylinder wall, which is mostly based upon the elasticity of the material of the piston ring, the exposure to combustion gases, and the uncompressed piston ring gap. Cast iron is used in all piston rings in the Briggs & Stratton engines. The cast iron material easily adjusts to the wall of the cylinder. Moreover, the cast iron is coated conveniently with other materials in order to augment its durability. However, care should be taken in handling the piston rings since cast iron can be distorted easily.

2.4 Piston skirt:

2.5 Materials used for manufacturing:

In general, the pistons are constructed with the same material as used in making the engine block. This is because the piston is developed to bear the entire combustion process during the running state of the engine so that it is able to survive the high stresses and pressures. Thus, numerous pistons in use are created from ''hypoeutectic'' aluminum alloys such as the SAE 332 that includes 8.5 % to 10.5 % of silicone. Therefore, most of the pistons are developed from aluminum alloys that posses a rare gray cast iron formation. Hence, the aluminum alloy pistons are created by pouring the melted aluminum alloy in to moulds and then cooled down suddenly. The cast iron piston is different from the aluminum alloy piston in the sense that it has a high erosion resistance. The cast iron metal is not used now, in accordance to the latest technology of vehicles' development, for fulfilling various purposes associated with high speeding however it is still permitted to be used in the piston compressors. Due to the fact that the pure aluminum has a soft ground resistance, thus, it was not appropriate to use it as the only material for the piston formation that is why it should be mixed in the alloy. Where as the most of the latest ''eutectic'' alloy pistons include 11percent to 12 percent silicone, and ''hypereutectic'' alloy pistons include 12.5 percent to more than 16 percent silicone.

2.5.1 The significance of silicone:

Silicone augments high heat strength as well as lowers the coefficient expansion in order to get tighter tolerances when the temperatures change. The coefficient of thermal expansion for a hypereutectic piston is approximately 15 percent less than the coefficient of thermal expansion for standard F-132 alloy piston.

2.5.2 The types of aluminum alloy:

In general, the aluminum silicone alloys in use are classified into three basic categories, namely: eutectic, hypoeutectic, and hypereutectic. The saturation in aluminum in the eutectic type takes place when 12 percent silicone level is achieved where as the aluminum that has silicone levels less than 12 percent is called hypoeutectic. In hypoeutectic type, the silicone is dissolved into the aluminum matrix. Moreover, the aluminum that has silicone levels more than 12 percent are called hypereutectic. Aluminum having 16 percent silicone includes 12 percent dissolved silicone and 4 percent primary silicone crystals.

Piston formation through different alloy categories varies in accordance to its own characteristics that can be utilized for various purposes in many fields. In general, hypereutectic pistons include approximately 9 percent silicone whereas most of the eutectic type pistons include silicone varying from 11 percent to 12 percent. In the context of stability and power, the eutectic alloys ascertain high strength and low cost. Silicone content in hypereutectic pistons is more than 12 percent. In addition to the high power and strong metallic characteristics, seizure resistance and efficiency, the hypereutectic type piston will take care of the groove wear through augmenting it with a high thermal resistance in the head area.

2.5.2.1 Cast Aluminum

Usually, the pistons are formed from aluminum alloys and occasionally, they are constructed using the gray cast iron. Therefore, molten aluminum is cascaded into a mold and then it is cooled down to form the required shape. The cast aluminum piston bears a crystalline structure that is relatively weaker than that of a forged piston. In the mold, steel bands are inserted during casting the piston in order to govern the expansion of the skirt area that is to be made parallel to the wrist pin enabling the piston-to-cylinder-wall-clearance to be fixed nearer and thus, a quieter engine is achieved with less piston slap.

2.5.2.2 Forged Aluminum

In this type, a solid aluminum slug is compressed with immense amount of pressure very rapidly into a die. After which the subsequent forging is machined to shape. Forged pistons bear a grain structure as opposed to the crystalline structure of the cast aluminum due to which they become much stronger and are able to tolerate more. Forged pistons were employed in racing engines and in other high performance machines for many years however they have lately fallen out of favor due to their comparatively heavy weight. In addition to this, there is a drawback of the forged pistons, making the engine noisier due to the wider piston to wall clearances needed by their steel bands' shortage as a result of more piston slap. Thus, the forged pistons are recommended by the experts for the usage in racing cars or vehicles.

2.5.2.3 Hypereutectic Cast Aluminum

The hypereutectic cast aluminum is a compromise between the cast and the forged pistons. This type is defined as casting by employing high pressure. It is almost as strong as forged pistons. The hypereutectic cast aluminum piston is comparatively lighter in weight. It has steel expansion bands but unfortunately, it is very expensive.

Due to the increased piston temperatures, the requirement for equal or augmented fatigue strength is very difficult to be satisfied. New alloys having more Si content and Cu content in them along with other alloying elements, have been observed to satisfy these requirements (Joyce, Styles and Reed, 2003). The metal matrix composites are already under usage and also in investigation (Payri, Benajes, Margot and Gil, 2003). In future, further improvements of the materials properties may be possible. New technologies such as PM are also found to be promising since its components portray excellent strength properties. PM has an adequate potential for future development. Nevertheless, these variations must consider that an efficient transfer of heat from the piston to the cylinder liner as well as to the oil is necessary. Under the development are other technologies and die-casting processes (Vijaya, Krishna, Prabhakar, and Gowri, 1996; Nakajima, Otaka, Kashimura, Sakuma and Tanaka, 1996). The formation of new materials and the development of processing technologies with better performance under high temperature and fatigue would be helpful in resolving the various issues of fatigue damages which will be discussed further in this paper.

2.5.3 Forged versus Cast:

As discussed previously under the piston manufacturing operations that once the piston is developed using the iron cast alloy specifically when it is suddenly transferred in to moulds achieving its final shape. A forging process is employed in applications requiring stronger pistons. Being still hot and semi-solid, the rough casting is placed in a die set during the forging process. After which the hydraulic press is employed to place the rough slug under extreme pressure. Hence, this technology allows the forging process to assure that a tighter metal is used which will develop a stronger material. In addition to this, the true decision about the piston materials relates back to its price and surely, the low cost pistons are preferred. This infers that most of the present metal equipments are developed through casting. Thus, a cast piston is created by cascading molten aluminum-silicone alloys into a mold.

Normally, a forged piston is created under particular conditions which can be removed with ease. Basically, the forging process selects a block of billet alloy and indents the piston shape from a die. The hypereutectic type pistons are a bit more than a slug of the die-cast having high silicone content. Due to this surfaces become harder and shinier and also, their expansion properties are changed, enabling a manufacturer to carry out tighter piston to cylinder wall clearances. In addition to this, the forged pistons are comparatively better but are very expensive. Therefore, a high press is required which packs the aluminum into an intricate mold through extreme pressure. The most significant benefits of the forged pistons are portrayed through improved strength, more predictable expansion properties, harder surfaces, and nearly, no porosity. A forged piston also ascertains highly qualified production properties with less skirt.

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