Investigation Into Piston Failures Engineering Essay


The old extended treatments about the significance of the maps, composing, and failure of the mechanical parts of an engine have been rather controversial. Such probes and surveies gave rise to assorted inquiries. 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 constituents in remainder of the field constituents of automotive every bit good as of other industry. If engine is considered as the bosom of a auto so the Piston is regarded as its most of import portion. Assorted researched have been conducted so far suggesting new geometries, stuffs and fabricating techniques for engine Pistons which farther accounted for a changeless betterment for the past decennaries and demanded comprehensive analysis of the smallest inside informations. Despite of all these huge researches, there is a mass figure of Piston failures. There are assorted beginnings in harm mechanisms which country fundamentally related to temperature, wear and weariness. Further the fatigue amendss conspicuously include thermic weariness and mechanical weariness, which are either at high temperature or at room temperature.

This survey is non merely related to the causes, diagnosing and solutions of Piston failures but besides related to an overpowering and a specialist geographic expedition of the points of probe. Assorted interlinked facets and methods matching to the Piston failures or fatigues have to be considered to build a dependable, overpowering and consistent probe about engine Piston failures. Therefore, the undermentioned three basic attacks are employed to analyse the weariness mechanism.

Entire Life Approach,

Crack Initiation Approach, and

Crack Propagation Approach.

Assorted features such like the piston-materials, the piston-operations, the entire life attack or the other related operations classify lubrication operations as the nature of the engine Piston failure. The fatigue total-life attack corresponds to the Piston life method that analyzes the emphasiss damaging the Piston. In the similar context, the stuffs of the Piston focal point on the fatigue affair in which a Piston is most likely to damage or check due to the usage of the bad stuffs. Such clefts or amendss relate to the facets of material-life, usage-life, stress-life, and operation-life. Nevertheless, the cyclic fatigues are normally elastic where the material-stress life curve is used and the Piston weariness mechanism is fundamentally fictile distortion. The feature of the cleft induction related to the quandary of Piston weariness is normally employed to find the operation ‘s procedures act uponing the efficiency of the engine Piston.

Table of Contentss

Table of Contentss 3

1.Introduction to Walter pistons 4

1.1 Background on Walter pistons: 4

1.2 Purposes and aims: 6

1.3 Piston map 6

1.4 Piston Partss 8

2.Piston Design 11

2.1 Piston Shape probe: 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 fabrication: 15

2.5.1 The significance of silicone: 16

2.5.2 The types of aluminium metal: 16

2.5.3 Forged versus Cast: 18

Mentions 19

Introduction to Walter pistons

1.1 Background on Walter pistons:

Let ‘s analyze the historical background by reexamining the initial development of the Piston ring. During the crude times of world, the most of import end was to pull H2O more expeditiously. Agricola, the Renaissance writer published a comprehensive description, in 1556, about the Piston pumps used by the mineworkers to pull H2O. The writer pointed out the usage of metal phonograph record and leather washers in which the intent of utilizing the leather was to avoid clash on the pump rods. The rapid growing of the urban population at that clip gave rise to the demand for pumps to raise H2O. In 1582, the really first waterwheel-driven pump was set up in London. During the late 17th century, the terrible issues related to the excavation industry in England were observed with the agglomeration of H2O in to deeper shafts set frontward the chance to get down the usage of steam power for driving reciprocating engines. Ever since, the reciprocating engines have been invariably developing, and there is no predictable terminal towards their use and continued augmentation. During the old ages of their usage, one particular job has stimulated permeant developmental attempts, which is the escape of fluid in between the Piston and the cylinder dullard. This job is experienced in assorted types of machineries, burning engines, H2O pumps, hydraulic motors, air compressors, hydraulic pumps, and cylinders. Initially, the attempts were applied to overly contract down the spread nevertheless the efficiency of such a sealing method was found to be really low. It was considered impossible to develop a Piston and dullard with adequately little tolerances to determine low escape. This job was resolved by insulating the sealing map with the aid of a separate machine component that was called the Piston ring. The intent of the Piston ring was to better set the contact surface of the cylinder dullard or line drive. In 1774, the first effort was made to make such a ring, when rope wadding was incorporated in to a steam engine for geting a seal that enhanced the engine ‘s thermic efficiency to 1.4 per centum. This later followed the development of Piston pealing out of metal ( Dowson, 1998 ) . In 1860, Etienne Lenoir patented the internal burning ( IC ) engine working at much higher temperatures and force per unit areas in comparing to a steam engine. The usage of ferric stuff for the Piston rings was prompted with the development of this type of engine since it has the mechanical belongingss of bearing or digesting high temperatures. Ever since, the Piston rings are largely made up of dramatis personae Fe or steel. Since 1889, the go oning efforts to better Piston rings have derived the creative activity of assorted scientific documents ( Hersey, 1944 ) . There is yet important research involvement in developing our comprehension about the tribological phenomena taking topographic point among the Piston rings, the cylinder line drives and the Piston in mechanisms such as that of internal burning engines ( Andersson et al. , 2002 ) . Early on, the improved public presentation was experienced due to the betterment in the signifier and the decrease in the curliness of the cylinder dullard. Afterwards, the honing technique was developed to better the grade of surface raggedness. In the internal burning engines, the honing of cylinder dullard is being done for about a century where as the augmented plateau-honing technique is being employed for the last half century ( Suzuki, 1997 ) . A level land surface is created between the cross-hatched channels ‘ form through the tableland honing technique. The vale or channels fundamentally serve to retain oil. In order to diminish the sum of maintained oil for the intent of

Reducing the oil ingestion, the tendency of utilizing a land surface between the vales with higher bearing capacity was followed which farther signified an betterment in draging opposition. Therefore, the vales became narrower and shallower where as the land surface turned smoother ( Lenthall, 1997 ) . What forces lead towards the betterment of the operation and efficiency of Piston rings in Internal Combustion Engines, which are soon employed in the development of about 50 million cars per twelvemonth? The basic drive forces are decidedly rigorous environmental ordinances and concurrent demands for the augmentations such like reduced wear and loss of clash.

1.2 Purposes and aims:

This paper investigates the type of Piston ring wear and its patterned advance in relation to assorted operational parametric quantities. It will enable us better comprehend the clash behaviour and the erosion of Piston rings. The ultimate aim of this survey is to let increasing the life-time of the Piston rings, with some border of safety, with the aid of the turn overing contact fatigue life of the bearings.

This survey is carried out to back up the mechanical technology procedure digesting a extremely qualified categorization of the jobs of the engine block and its parts. Therefore, this survey must be able to enable a mechanical applied scientist to develop a general apprehensiveness about forestalling future engine Piston failures. This research will integrate to two facets so as to cover with the practical issues. This first facet will cover the account of the full construct of the assembly of Piston in relation to its environing parts. The 2nd facet will include the probe of the full construct environing engine Piston failures such like break, weariness, sealing, clash and other concerns.

1.3 Piston map

A monolithic figure of procedures in the universe need some input such as mechanical work, rotary motion to map and torsion. 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 signifier of heat as consequence of a thermodynamic procedure in an internal burning engine in order to bring forth the mechanical work, rotary motion and torsion. The 2nd common manner to transform energy in to mechanical work is to utilize an electric motor. The 3rd usual manner is to transform hydrostatic energy in to mechanical work with the aid of a hydrostatic transmittal of the radial Piston type, as in the Hagglunds hydraulic motor where the rotary motion and the torsion are generated by the hydrostatic force per unit area and flow. In 1957, the design of the radial Piston hydraulic motor was foremost introduced accompanied by a patent for an internal burning engine with radial Piston which uses Diesel fuel ( Bergstrom and Omnell, 1996 ) . Hagglunds put frontward this patent and thereby, the operating mechanism employed was the transition of energy from chemical to that of a hydrostatic transmittal.

Piston is an indispensable constituent 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 detonation of fuel mixture. Inside the cylinders, the Pistons move up and down with the aid of the burning of fuel. The fuel is foremost injected in to the cylinder and combusted afterwards which enables the Piston to travel in the downward way. The fuel is combusted at a really fast interval in every cylinder. The Piston executes four phases which are ( 1 ) consumption, ( 2 ) compaction, ( 3 ) ignition and ( 4 ) fumes, in order of their happening. In the first phase, a mixture of fuel and air or merely air enters in to the cylinder. Then the air passes in to the valve that opens and closes with the aid of the camshaft. In the 2nd phase, the compaction takes topographic point when the Piston moves up that compresses the air or the mixture of fuel and air. In the 3rd phase, 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 nowadays in the cylinder where as the ignition in a gasolene engine is done with the aid of a flicker stopper. In the 4th phase, the fumes occurs in which the burnt gases are forced out by the Piston traveling in the upward way. In 1959, the first paradigm of the hydraulic motor was tested in research lab, which involved new constructs of Piston assembly, which transmitted the digressive force bring forthing the torsion ( see Figure 2 ) .

Figure 1: The first paradigm of hydraulic motor by Hagglunds in 1959.

A long and narrow spread is normally employed in order to seal off the high hydrostatic force per unit area between Piston and cylinder dullard ( Ivantysyn and Ivantysynova, 2003 ) . The perfect design for hydrostatic pump and motor incorporates either a long or a short Piston usher. The benefits of a long Piston usher as opposed to a shorter one are reduced escape and the support for the side burden with decreased frictional loss. Furthermore, higher volumetric and mechanical efficiency is achieved through a long Piston usher in comparing to the volumetric and mechanical efficiency achieved through a short Piston usher.

1.4 Piston Partss

The Piston serves as the traveling terminal of the burning chamber where as the cylinder caput is the stationary terminal of the burning chamber. Normally, the Pistons are made up of a dramatis personae aluminium metal due to its improved thermic conduction and light weight. The potency of a stuff to behavior and transportation heat is defined as its thermic conduction.

A Piston includes the undermentioned parts.

Head: It is the top surface of the Piston that is near to the cylinder caput and is subjected to enormous forces and heat whiles the normal operation of the engine.

Pin dullard: It is a hollow capacity with in the side of the Piston, having the Piston pin, which is at an angle of 90 grades to the Piston travel.

Piston pin: It is a hollow shaft fall ining the linking rod ‘s little terminal with the Piston.

Piston Skirt: It is the portion of the Piston nearest to the crankshaft helping the Piston to aline as it goes in to the cylinder dullard. In order to take down the Piston mass and to give clearance for the rotating crankshaft counterbalances, some skirts have profiles cut into them.

Ringing channel: It is a concaved country positioned around the margin of the Piston which is employed for the intent of retaining a Piston ring.

Ringing lands: These are the two surfaces of the ring channel, parallel to each other, functioning as the sealing surface for the Piston ring.

Piston ring: It is an expandible split pealing supplying a seal between the cylinder wall and the Piston. The rings of the Piston are normally created from dramatis personae Fe since the dramatis personae Fe maintains the unity of its original form even under the influence of dynamic forces such as heat, burden, etc. The burning 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 constellation of the Piston ring vary harmonizing to the engine design and cylinder stuff. In general, the Piston rings in little engines consist of the compaction ring, the wiper ring, and the oil ring which are explained in the farther subdivisions in this paper.

Figure 2 – Piston Ringss

Figure 3 – Piston Ring Gap

Piston Design

2.1 Piston Shape probe:

There are three possible signifiers of the spread geometry for seal off force per unit area that the Walter pistons could hold, all of which play a alone function towards the escape. These three signifiers of spread geometry are provided in the cross subdivision provided in Figure 4.

Figure 4: For different Piston sealing designs, the cross subdivisions of spread geometries: Figure A illustrates a to the full bizarre spread design where as the Figure B illustrates a rectangular spread design, and the Figure C illustrates a homocentric spread design. Eccentricity is represented by the

dimension, ‘e ‘ .

The design A involves a solid Piston, holding the highest escape, flexing inside the cylinder bore so as to back up the side loads. The design B involves a slit Piston ring that is by and large employed in an internal burning engine where: the slit bears an about rectangular cross subdivision, the breadth is given by the circumferential spread dimension while the spread is defined by the clearance between the cylinder dullard and the Piston. Three times lower escape is portrayed in the design B in comparing to the escape shown in design A. However, in pattern, the spread is non unfastened in design B and adapts the signifier of a trap for diminishing the escape. Relatively to all the designs, the least escape is accomplishable through the design C that is the design of a closed spread mentioning to the un-slit Piston in which the Piston ring can make closely in order to be homocentric inside the cylinder dullard as explained by Skytte af Satra ( 2005 ) . The design C has the undermentioned benefits:

less escape,

escape, in the signifier of Poiseuille flow, that is independent of the place in the shot,

reduced piston-cost, and

lessening in the size of the Piston and thereby, decrease of the size of the whole hydraulic motor.

2.2 Piston Crown:

2.3 Piston Ringss:

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

2.3.1 Compression Ring

The top-most ring that is closest to the burning gases is the compaction ring. Therefore, it has to confront the highest operating temperature every bit good as the greatest sum of chemical corrosion. The 70 per centum of the heat of the burning chamber is transferred from the Piston to the cylinder wall by the compaction ring. Either taper-faced or barrel-faced compaction rings are used in most Briggs & A ; Stratton engines. A Piston pealing holding a taper angle of about 1 grade on the running surface is referred as the taper faced compaction ring. This taper offers a smooth wiping action so as to suppress any extra oil before it approaches the burning chamber.

A Piston pealing holding a curving running surface in order to offer consistent lubrication of the cylinder wall and the Piston is called the barrel faced compaction ring. This type of compaction ring besides offers a cuneus consequence so as to augment the oil distribution across the full shot of the Piston. Furthermore, the curving running surface decreased the likeliness of the interruption down of an oil movie caused by the extra force per unit area at the border of the ring or by the inordinate warp of the Piston during operation.

2.3.2 Wiper Ring

It is besides known as the Napier ring, scraper ring or back-up compaction ring. The wiper ring is the following ring on the Piston that is off from the caput of the cylinder caput. The wiper pealing offers a consistent oil movie thickness with the intent of lubricating the running surface of the compaction ring. Most scraper rings have a taper angle face in Briggs & A ; Stratton engines. The place of the tapering angle is at the way of the oil reservoir and therefore, it is able to make a wiping action when the Piston is traveling toward the crankshaft.

The taper angle offers contact directing the extra oil on the cylinder wall, for returning to the oil reservoir, through the oil ring. If a wiper ring is installed falsely with the tapering angle nearest to the compaction pealing so it causes inordinate oil ingestion. This is due to the ground that the scraper pealing rubs extra oil toward the burning chamber.

2.3.3 Oil Ring

Two thin tracks or running surfaces are included in an oil ring. Slots or holes that are cut into the radial centre of the ring enable the flow of extra oil to return to the oil reservoir. In general, the oil rings are in a individual piece that includes all of these characteristics. In order to delegate more radial force per unit area to the Piston ring, some one piece oil rings employ a spring expander which increases the unit force per unit area exercised at the wall of the cylinder.

The highest built-in force per unit area is experienced at the oil ring in comparing to all the three types of the rings on the Piston. A tree-piece oil pealing affecting two tracks and an expander is used in some Briggs & A ; Stratton engines. The oil rings are positioned at both sides of the expander. In general, the expander includes multiple Windowss or slots in order to direct the oil back to the Piston pealing channel. The little running surface of the thin tracks provides the built-in Piston ring force per unit area, the high unit force per unit area and the expander force per unit area which are utilized by the oil ring.

The burning chamber is sealed by the Piston rings, as they conduct heat to the wall of the cylinder and regulate the ingestion of the oil. The Piston ring is able to accomplish this with the aid of the built-in and applied force per unit area that is the internal spring force coercing the Piston ring to spread out on the footing of the design and belongingss of the stuff employed in doing the Piston. A important force is necessary for the built-in force per unit area, which is required to contract the ring in to a smaller diameter. The free or uncompressed Piston pealing spread determines the built-in force per unit area, which is the length in between the two terminals of an uncompressed Piston ring. Conventionally, the larger the spread of the uncompressed Piston ring, the greater force the ring of the Piston exerts while compressed with in the cylinder dullard.

The Piston ring should be able to offer the expected and positive radial tantrum between the wall of the cylinder and the Piston pealing running surface in order to accomplish an efficient seal. The built-in force per unit area of the Piston ring attains the radial tantrum. The Piston ring should besides be able to prolong the seal over the Piston pealing lands.

Furthermore, the burning chamber is besides sealed by the Piston pealing with the aid of the applied force per unit area that is the force per unit area exerted to the Piston pealing from burning gases, ensuing in the enlargement of the Piston ring. There is a chamfered border, in some Piston rings, that is opposite to the running surface. The Piston ring is twisted due to this chamfered border if non influenced by the burning gas force per unit areas.

The other consideration of the Piston pealing design contact force per unit area of the cylinder wall, which is largely based upon the snap of the stuff of the Piston ring, the exposure to burning gases, and the uncompressed Piston pealing spread. Cast Fe is used in all Piston rings in the Briggs & A ; Stratton engines. The dramatis personae Fe stuff easy adjusts to the wall of the cylinder. Furthermore, the dramatis personae Fe is coated handily with other stuffs in order to augment its lastingness. However, attention should be taken in managing the Piston rings since dramatis personae Fe can be distorted easy.

2.4 Piston skirt:

2.5 Materials used for fabrication:

In general, the Pistons are constructed with the same stuff as used in doing the engine block. This is because the Piston is developed to bear the full burning procedure during the running province of the engine so that it is able to last the high emphasiss and force per unit areas. Therefore, legion Pistons in usage are created from ”hypoeutectic ” aluminium metals such as the SAE 332 that includes 8.5 % to 10.5 % of silicone. Therefore, most of the Pistons are developed from aluminium metals that posses a rare grey dramatis personae Fe formation. Hence, the aluminium metal Pistons are created by pouring the melted aluminium metal in to molds and so cooled down all of a sudden. The dramatis personae Fe Piston is different from the aluminium metal Piston in the sense that it has a high eroding opposition. The dramatis personae Fe metal is non used now, in conformity to the latest engineering of vehicles ‘ development, for carry throughing assorted intents associated with high rushing nevertheless it is still permitted to be used in the Piston compressors. Due to the fact that the pure aluminium has a soft land opposition, therefore, it was non appropriate to utilize it as the lone stuff for the Piston formation that is why it should be mixed in the metal. Where as the most of the latest ”eutectic ” metal Pistons include 11percent to 12 per centum silicone, and ”hypereutectic ” metal Pistons include 12.5 per centum to more than 16 per centum silicone.

2.5.1 The significance of silicone:

Silicone augments high heat strength every bit good as lowers the coefficient enlargement in order to acquire tighter tolerances when the temperatures change. The coefficient of thermic enlargement for a hypereutectic Piston is about 15 per centum less than the coefficient of thermic enlargement for standard F-132 metal Piston.

2.5.2 The types of aluminium metal:

In general, the aluminium silicone metal in usage are classified into three basic classs, viz. : eutectic, hypoeutectic, and hypereutectic. The impregnation in aluminium in the eutectic type takes topographic point when 12 per centum silicone degree is achieved where as the aluminium that has silicone degrees less than 12 per centum is called hypoeutectic. In hypoeutectic type, the silicone is dissolved into the aluminium matrix. Furthermore, the aluminium that has silicone degrees more than 12 per centum are called hypereutectic. Aluminum holding 16 per centum silicone includes 12 per centum dissolved silicone and 4 per centum primary silicone crystals.

Piston formation through different metal classs varies in conformity to its ain features that can be utilized for assorted intents in many Fieldss. In general, hypereutectic Pistons include about 9 per centum silicone whereas most of the eutectic type Pistons include silicone changing from 11 per centum to 12 per centum. In the context of stableness and power, the eutectic metals ascertain high strength and low cost. Silicone content in hypereutectic Pistons is more than 12 per centum. In add-on to the high power and strong metallic features, ictus opposition and efficiency, the hypereutectic type Piston will take attention of the channel wear through augmenting it with a high thermic opposition in the head country. Cast Aluminum

Normally, the Pistons are formed from aluminium metals and on occasion, they are constructed utilizing the grey dramatis personae Fe. Therefore, liquefied aluminium is cascaded into a cast and so it is cooled down to organize the needed form. The dramatis personae aluminium Piston bears a crystalline construction that is comparatively weaker than that of a bad Piston. In the cast, steel sets are inserted during projecting the Piston in order to regulate the enlargement of the skirt country that is to be made parallel to the carpus pin enabling the piston-to-cylinder-wall-clearance to be fixed close and therefore, a quieter engine is achieved with less Piston smack. Forged Aluminum

In this type, a solid aluminium bullet is compressed with huge sum of force per unit area really quickly into a dice. After which the subsequent forging is machined to determine. Forged Pistons bear a grain construction as opposed to the crystalline construction of the dramatis personae aluminium due to which they become much stronger and are able to digest more. Forged Pistons were employed in rushing engines and in other high public presentation machines for many old ages nevertheless they have recently fallen out of favour due to their comparatively heavy weight. In add-on to this, there is a drawback of the bad Pistons, doing the engine noisier due to the wider Piston to palisade clearances needed by their steel sets ‘ deficit as a consequence of more Piston smack. Therefore, the bad Pistons are recommended by the experts for the use in rushing autos or vehicles. Hypereutectic Cast Aluminum

The hypereutectic dramatis personae aluminium is a via media between the dramatis personae and the bad Pistons. This type is defined as casting by using high force per unit area. It is about every bit strong as bad Pistons. The hypereutectic dramatis personae aluminium Piston is relatively lighter in weight. It has steel enlargement sets but unluckily, it is really expensive.

Due to the increased Piston temperatures, the demand for equal or augmented fatigue strength is really hard to be satisfied. New alloys holding more Si content and Cu content in them along with other debasing elements, have been observed to fulfill these demands ( Joyce, Styles and Reed, 2003 ) . The metal matrix complexs are already under use and besides in probe ( Payri, Benajes, Margot and Gil, 2003 ) . In future, farther betterments of the stuffs belongingss may be possible. New engineerings such as PM are besides found to be promising since its constituents portray first-class strength belongingss. PM has an equal potency for future development. However, these fluctuations must see that an efficient transportation of heat from the Piston to the cylinder line drive every bit good as to the oil is necessary. Under the development are other engineerings and die-casting procedures ( Vijaya, Krishna, Prabhakar, and Gowri, 1996 ; Nakajima, Otaka, Kashimura, Sakuma and Tanaka, 1996 ) . The formation of new stuffs and the development of treating engineerings with better public presentation under high temperature and weariness would be helpful in deciding the assorted issues of weariness amendss which will be discussed farther in this paper.

2.5.3 Forged versus Cast:

As discussed antecedently under the Piston fabrication operations that one time the Piston is developed utilizing the Fe cast metal specifically when it is all of a sudden transferred in to molds accomplishing its concluding form. A forging procedure is employed in applications necessitating stronger Pistons. Bing still hot and semi-solid, the unsmooth casting is placed in a dice set during the forging procedure. After which the hydraulic imperativeness is employed to put the unsmooth bullet under utmost force per unit area. Hence, this engineering allows the forging procedure to guarantee that a tighter metal is used which will develop a stronger stuff. In add-on to this, the true determination about the Piston stuffs relates back to its monetary value and certainly, the low cost Pistons are preferred. This infers that most of the present metal equipments are developed through casting. Therefore, a dramatis personae Piston is created by cascading liquefied aluminum-silicone metals into a cast.

Normally, a bad Piston is created under peculiar conditions which can be removed with easiness. Basically, the forging procedure selects a block of note metal and indents the Piston form from a dice. The hypereutectic type Pistons are a bit more than a bullet of the die-cast holding high silicone content. Due to this surfaces become harder and shinier and besides, their enlargement belongingss are changed, enabling a maker to transport out tighter Piston to cylinder wall clearances. In add-on to this, the bad Pistons are relatively better but are really expensive. Therefore, a high imperativeness is required which packs the aluminium into an intricate cast through utmost force per unit area. The most important benefits of the bad Pistons are portrayed through improved strength, more predictable enlargement belongingss, harder surfaces, and about, no porousness. A bad Piston besides ascertains extremely qualified production belongingss with less skirt.