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Sunday, October 23, 2011

Clearance Between Valve Head & Piston Measurement


Tools Required

1.      Sliding caliper
2.      Felt pen
3.      Sheet lead strips, width 5mm length 70 mm(appx).
4.      Adhesive tape
Legends

1.      Sealing surface, valve cage/cylinder head
2.      Sealing surface, valve /valve cage
3.      Steel seal ring
4.      Sealing surface , cylinder liner collar/supporting block
5.      Sheet lead strips
6.      Adhesive tape
7.      Valve head(Vlave disc)
8.      Valve cage
9.      M=Marking
10.  S=Clearance between valve head lower edge and piston reacess approx. 5 degree before/after charge exchange cycle TDC.

Clearance between valve and piston
M marking
General

All modifications to timing train, to the crank gear(connecting rid length, piston height), and especially reworking of the valve cage or valve cage seat in cylinder head etc. can have an effect on the clearance S between the opened valve at charge-exchange cycle TDC and the top edge of the piston crown. Whenever reworking is carried out at this points(1-5), or replacements are made, a check must be made on the clearance S.
Starting position

Rocker casing with rocker leaver/swings arm bearig has been removed. Piston has been turned to ignition TDC(valve closed).

Sequence of operation

1.      Release and unscrew all valve fastening nuts and take off all thrusts flanges. (with exhaust valves, shift the coolant plugs to the “ZU-CLOSED ” position).
2.      Make M mark on all vlaves with a felt pen. This marking must be at the outer edge of the valve cage and is an extension of the line drawn fom center of cylinder through the center of the valve.
3.      Proceed with further disassembly in accordance with work cards.

Lead strips
Lead strip attached in valve head
4.      Wind sheet lead strip around a marking tool, or something similar. Wind height about 8-9 mm.
5.      Clean and degrease valve head. Using adhesive tape, tape the prepared sheet-lead strip approx.25mm away from valve head end vertically above the marking M.
6.      Refit valves and rocker casing and tightened to the specified torque loadings.
7.      Turn the engine over once past the charge-exchage TDC, and then remove the rocker casting , remove the vlaves again.
8.      With slider caliper measure the deformed sheet lead strip thickness.
 
Caution:The measured thickness S is note to be less than 3.6mm (MAN B&W 9L 58/64)on any of the valves(engine cold).
As new dimension S-5.2 mm.
Note: Oversize seal ring are available for corrective purpose.

VIEW WHOLE PROCESS IN PHOTOS

Friday, October 21, 2011

Cylinder Head Installation Process


Tool Required

  1. Suspension device with 4 hexagonal screws
  2. Guide rod
  3. Protection cap
  4. Set sealing covers, Lifting gear
  5. Set open end wrenches

cyl.head installation precess
cylinder head installation process

Sequence of operation

  1. Clean sealing surface between cylinder head and cylinder liner. Should sealing surface be damaged, they must be refaced, using the special tool provided for.
Note: On refaced sealing surface, minimum gap between cylinder head and liner is 0.5 mm.

  1. Clean insert seal ring. Damaged ring or ring which have bean changed in height (compressed) must be replaced.
  2. Clean sealing surface on the charge air and exhaust connections. Check C-ring for damage and replace if necessary
  3. Slip protection caps over the cylinder head studs and using lifting gear, carefully lower the cylinder head onto the liner. When doing so, pay attention to the position of the locating pin in the locating groove.
  4. Tighten the cylinder head studs.
  5. Close quick coupling of exhaust manifold, tightened screws at specified torque.
  6. Connect charge air line, to accomplish this, back off nuts far enough so that the charge air line flange can be screwed on tightly to the cylinder head, without the nuts being seated.\
  7. Screw the thrust bolts until gap has been reached the dimension as specified for preloaded condition.
  8. Tighten nuts on the adjusting screws until gap has likewise reached dimension specified for preloaded condition.
  9. Re check measurements of all gaps correct if required and lock with counter nut or nut resp.
Note: Measurements are applying to cold engine. When mounting several cylinder heads, the tightening of the compensator is effected for each cylinder head consecutively, i.e., the process tightening in the prescribed sequence must be completed with one cylinder head before starting work on the next one.

  1. Reconnect all pipes which had been disconnected before.

      Wednesday, October 12, 2011

      Thermistors


      The term thermistor is a contraction of the words thermal and resistor. The name is usually applied to metal-oxide sensors fabricated in the form of droplets, bars, cylinders, rectangular flakes, and thick films. A thermistor belongs to the class of absolute-temperature sensors; that is, it can measure temperature that is referenced to an absolute-temperature scale.
      Thermistor
      All thermistors are divided into two groups: NTC (negative temperature coefficient) and PTC (positive temperature coefficient). Only the NTC thermistors are useful for precision temperature measurements.

      NTC Thermistor


      The NTC thermistors which are discussed herein are composed of metal oxides.  The most commonly used oxides are those of manganese, nickel, cobalt, iron, copper and titanium.  The fabrication of commercial NTC thermistors uses basic ceramics technology and continues today much as it has for decades.  In the basic process, a mixture of two or more metal oxide powders are combined with  suitable binders, are formed to a desired geometry, dried, and sintered at an elevated temperature.  By varying the types of oxides used, their relative  proportions, the sintering atmosphere, and the  sintering temperature, a wide range of resistivities and temperature coefficient characteristics can  be obtained.

      READ DETAILS ABOUT NTC THERMISTOR

      PTC Thermistor


      All metals may be called PTC materials, however, their temperature coefficients of resistivity (TCR) are quite low . An RTD as described earlier also has a small PTC. In contrast, ceramic PTC materials in a certain temperature range are characterized by a very large temperature dependence. The PTC thermistors are fab-ricated of polycrystalline ceramic substances, where the base compounds, usually barium titanate or solid solutions of barium and strontium titanate (highly resistive materials), are made semiconductive by the addition of dopants. Above the Curie temperature of a composite material, the ferroelectric properties change rapidly, re-sulting in a rise in resistance, often several orders of magnitude. 



      NTC Thermistors


      A conventional metal-oxide thermistor has a NTC; that is, its resistance decreases with the increase in temperature. The NTC thermistor’s resistance, as of any resistor, is determined by it physical dimensions and the material resistivity. The relationship between the resistance and temperature is highly nonlinear (Fig. below).
      Resistance vs Temperature curve of NTC thermistor
      Whenever high accuracy is required or the operating range is wide, the thermistor characteristics should not be taken directly from a manufacturer’s data sheet. Typical tolerances of the nominal resistance (at 25◦C) for mass-produced thermistors are rather wide: ±20% is quite common. Unless it was adjusted at the factory to a better tolerance, to reach a high accuracy each such thermistor needs to be individually calibrated over the operating temperature range. Manufacturers can trim a thermistor by grinding its body to a required dimension that directly controls the nominal value of resistance at a set temperature. This, however, increases cost. An alternative approach for an end user is to individually calibrate the thermistors. Calibration means that a thermistor has to be subjected to a precisely known temperature (a stirred water bath is often employed3) and its resistance is measured. This is repeated at several temperatures if a multipoint calibration is needed. Naturally, a thermistor calibration is as good as the accuracy of the reference thermometer used during the calibration. To measure the resistance of a thermistor, it is attached to a measurement circuit that passes through it electric current. Depending on the required accuracy and the production cost restrictions, a thermistor calibration can be based on the use of one of several known approximations (models) of its temperature response. When a thermistor is used as an absolute-temperature sensor, we assume that all of its characteristics are based on the so-called “zero-power resistance”, meaning that the electric current passing through a thermistor does not result in any temperature in-crease (self-heating) which may affect accuracy of measurement.

      Fabrication of NTC Thermistors

      Generally, the NTC thermistors can be classified into three major groups depending on the method by which they are fabricated.
      • The first group consists of bead-type thermistors. The beads may be bare or coated with glass epoxy (Fig.below), or encap-sulated into a metal jacket. All of these beads have platinum alloy lead wires which are sintered into the ceramic body. When fabricated, a small portion of a mixed metal oxide with a suitable binder is placed onto parallel leadwires, which are under slight tension. After the mixture has been allowed to dry or has been partly sintered, the strand of beads is removed from the supporting fixture and placed into a tubular fur-nace for the final sintering. The metal oxide shrinks onto the lead wires during this firing process and forms an intimate electrical bond. Then, the beads are individually cut from the strand and are given an appropriate coating. 
      Glass coated bead type thermistor
        • Another type of thermistor is a chip thermistor with surface contacts for the lead wires. Usually, the chips are fabricated by a tape-casting process, with subsequent screenprinting, spraying, painting, or vacuum metallization of the surface electrodes. The chips are either bladed or cut into the desired geometry. If desirable, the chips can be ground to meet the required tolerances. 
        Chip thermistor
        • The third type of thermistor is fabricated by the depositing semiconductive ma-terials on a suitable substrate, such as glass, alumina, silicon, and so forth. These thermistors are preferable for integrated sensors and for a special class of thermal infrared detectors. 

        Semiconductor materials fabricated conductor
        Among the metallized surface contact thermistors, flakes and uncoated chips are the least stable. A moderate stability may be obtained by epoxy coating. The bead type with lead wires sintered into the ceramic body permits operation at higher tem-peratures, up to 550◦C. The metallized surface contact thermistors usually are rated up to 150◦C. Whenever a fast response time is required, bead thermistors are prefer-able; however, they are more expensive than the chip type. Also, the bead thermistors are more difficult to trim to a desired nominal value. Trimming is usually performed by mechanical grinding of a thermistor at a selected temperature (usually 25◦C) to change its geometry and thus to bring its resistance to a specified value.
        While using the NTC thermistors, one must not overlook possible sources of error. One of them is aging, which for the low-quality sensors may be as large as +1%/year. A good environmental protection and preaging is a powerful method of sensor characteristic stabilizing. During preaging, the thermistor is maintained at +300◦C for at least 700 h. For better protection, it may be further encapsulated in a stainless-steel jacket and potted with epoxy.

        PTC Thermistors


        All metals may be called PTC materials, however, their temperature coefficients of resistivity (TCR) are quite low . An RTD as described earlier also has a small PTC. In contrast, ceramic PTC materials in a certain temperature range are characterized by a very large temperature dependence. The PTC thermistors are fab-ricated of polycrystalline ceramic substances, where the base compounds, usually barium titanate or solid solutions of barium and strontium titanate (highly resistive materials), are made semiconductive by the addition of dopants. Above the Curie temperature of a composite material, the ferroelectric properties change rapidly, re-sulting in a rise in resistance, often several orders of magnitude. 

        A typical transferfunction curve for the PTC thermistor is shown in Fig.(above), in a comparison with the NTC and RTD responses. The shape of the curve does not lend itself to an easy math- ematical approximation; therefore, manufacturers usually specify PTC thermistors by a set of numbers:
        1.  Zero power resistance, R25,at25◦C, where self-heating is negligibly small.
        2.  Minimum resistance Rm is the value on the curve where the thermistor changes  its TCR from positive to negative value (point m).
        3. Transition temperature Tτ is the temperature where resistance begins to changerapidly. It coincides approximately with the Curie point of the material. A typical range for the transition temperatures is from −30◦Cto +160◦C (Keystone Carbon Co.).
        4. hermal characteristics are specified by a thermal capacity, a dissipation constant δ (specified under given conditions of coupling to the environment), and a thermal time constant (defines speed response under specified conditions). 
        5. Maximum voltage Emax is the highest value the thermistor can withstand at any
          temperature.  
        It is important to understand that for the PTC thermistors, two factors play a key role: environmental temperature and a self-heating effect. Either one of these two factors shifts the thermistor’s operating point. The temperature sensitivity of the PTC thermistor is reflected in the volt–ampere characteristic of Fig. below. According to Ohm’s law, a regular resistor with a near-zero TCR has a linear characteristic. A NTC thermistor has a positive curvature of the volt–ampere dependence. An implication of the negative TCR is that if such a thermistor is connected to a hard voltage source,5 a self-heating due to Joule heat dissipation will result in resistance reduction. In turn, that will lead to a further increase in current and more heating. If the heat outflow from the NTC thermistor is restricted, a self-heating may eventually cause overheating and a catastrophic destruction of the device. Because of positive TCRs, metals do not overheat when connected to hard voltage sources and behave as self-limiting devices. For instance, a filament in an incandes-cent lamp does not burn out because the increase in its temperature results in an increase in resistance, which limits current. This self-limiting (self-regulating) effect is substantially enhanced in the PTC thermistors. The shape of the volt–ampere char acteristic indicates that in a relatively narrow temperature range, the PTC thermistor possesses a negative resistance; that is,

        Monday, October 10, 2011

        Thermocouple Assemblies


        A complete thermocouple sensing assembly generally consists of one or more of the following: a sensing element assembly (the junction), a protective tube (ceramic or  metal jackets), a thermowell (for some critical applications, these are drilled solid bar stocks which are made to precise tolerances and are highly polished to inhibit corrosion), and terminations (contacts which may be in the form of a screw type, open type, plug and jack disconnect, military-standard-type connectors, etc.). Some typical thermocouple assemblies are shown in Fig.below.



        The wires may be left bare or given electrical isolators. For the high-temperature applications, the isolators may be of a fish-spine or ball ceramic type, which provide sufficient flexibility. If thermocouple wires are not electrically isolated, a measurement error may occur. In-sulation is affected adversely by moisture, abrasion, flexing, temperature extremes, chemical attack, and nuclear radiation. A good knowledge of particular limitations of insulating materials is essential for accurate and reliable measurement. Some insulations have a natural moisture resistance. Teflon, polyvinyl chloride (PVC), and some forms of polyimides are examples of this group. With the fiber-type insulations, moisture protection results from impregnating with substances such as wax, resins, or silicone compounds. It should be noted that only one-time exposure to ultraextreme temperatures cause evaporation of the impregnating materials and loss of protection. The moisture penetration is not confined to the sensing end of the assembly. For example, if a thermocouple passes through hot or cold zones, condensation may produce errors in the measurement, unless adequate moisture protection is provided. 
        The basic types of flexible insulation for elevated temperature usage are fiber glass, fibrous silica, and asbestos (which should be used with proper precaution due to health hazard). In addition, thermocouples must be protected from atmospheres that are not compatible with the alloys. Protecting tubes serve the double purpose of guarding the thermocouple against mechanical damage and interposing a shield between the wires and the environment. The protecting tubes can be made of carbon steels (up to 540◦C in oxidizing atmospheres), stainless steel (up to 870◦C), ferric stainless steel (AISI 400 series), and high-nickel alloys (Nichrome,6 Inconel,7 etc.) (up to 1150◦C in oxidizing atmospheres). Practically all base–metal thermocouple wires are annealed or given a “stabi-lizing heat treatment” by the manufacturer. Such treatment generally is considered sufficient, and seldom is it found advisable to further anneal the wire before testing or using. Although a new platinum and platinum–rhodium thermocouple wire as sold by some manufacturers is annealed already, it has become a regular practice in many laboratories to anneal all Type R, S, and B thermocouples, whether new or previ- ously used, before attempting an accurate calibration. This is accomplished usually by heating the thermocouple electrically in air. The entire thermocouple is supported between two binding posts, which should be close together, so that the tension in the wires and stretching while hot are kept at a minimum. The temperature of the wire is conveniently determined with an optical pyrometer. Most of the mechanical strains are relieved during the first few minutes of heating at 1400–1500◦C. Thin-film thermocouples are formed by bonding junctions of foil metals. They are available in a free-filament style with a removable carrier and in a matrix style with a sensor embedded in a thin laminated material. The foil having a thickness in the order of 5 µm (0.0002 in.) gives an extremely low mass and thermal capacity. Thin flat junctions may provide intimate thermal coupling with the measured surface. Foil thermocouples are very fast (a typical thermal time constant is 10 ms) and can be used with any standard interface electronic apparatuses. 
        While measuring temperature with sensors having small mass, thermal conduction through the connecting wires always must be taken into account. Because of a very large length-to-thickness ratio of the film thermocouples (on the order of 1000), heat loss via wires is usually negligibly small. To attach a film thermocouple to an object, several methods are generally used. Among them are various cements and flame or plasma-sprayed ceramic coatings. For ease of handling, the sensors often are supplied on a temporary carrier of polyimide film which is tough, flexible, and dimensionally stable. It is exceptionally heat resistant and inert. During the installation, the carrier can be easily peeled off or released by application of heat. The free foil sensors can be easily brushed into a thin layer, to produce an ungrounded junction. While selecting cements, care must be taken to avoid corrosive compounds. For instance, cements containing phosphoric acid are not recommended for use with thermocouples having copper in one arm.

        Thermocouple Circuits


        In the past, thermocouples were often used with a cold junction immersed into a reference melting ice bath to maintain its temperature at 0◦C (thus, the “cold” junction name). This presents serious limitations for many practical uses. The second and third thermoelectric laws allow for a simplified solution. A “cold” junction can be maintained at any temperature, including ambient, as long as that temperature is precisely known. Therefore, a “cold” junction is thermally coupled to an additional temperature sensor which does not require reference compensation. Usually, such a sensor is either thermo resistive or a semiconductor. Figure 16.16B shows the correct connection of a thermocouple to an electronic circuit. Both the “cold” junction and the reference sensor must be positioned in an intimate thermal coupling. Usually, they are imbedded in a chunk of copper. To avoid dry contact, thermally conductive grease or epoxy should be applied for better 
        Thermal tracking. A reference temperature detector in this example is a semiconductor sensor LM35DZ manufactured by National Semiconductor, Inc. The circuit has two outputs: one for the signal representing the Seebeck voltage Vp and the other for the reference signal Vr . The schematic illustrates that connections to the circuit board input terminals and then to the amplifier’s noninverting input and to the ground bus are made by the same type of wire (Cu). Both board terminals should be at the same temperature Tc; however, they do not necessarily have to be at the “cold” junction temperature. This is especially important for the remote measurements, where the circuit board temperature may be different from the reference “cold” junction temperature Tr . For computing the temperature from a thermocouple sensor, two signals are essentially required.
        The first is a thermocouple voltage Vp and the other is the reference sensor output voltage Vr . These two signals come from different types of sensor and therefore are characterized by different transfer functions. A thermopile in most cases may be considered linear with normalized sensitivity αp (V/K), whereas the reference sensor sensitivity is expressed according to its nature. For example, a thermistor’s sensitivity αr at the operating temperature T is governed by Eq. below and has dimension Δ/K. There are several practical ways of processing the output signals. The most precise method is to measure these signals separately, then compute the reference temperature Tr according to the reference sensor’s equation, and compute the gradient temperature Δ from a thermocouple voltage Vp as

        Δ =Tx −Tr = Vp/αp

        Sunday, October 9, 2011

        Thermocouple


        Thermocouple

        Thermoelectric contact sensors are called thermocouples because at least two dissimilar conductors and two junctions (couples) of these conductors are needed to make a practical sensor. A thermocouple is a passive sensor.
        Thermocouple
        Thermocouple

        For practical purposes, an application engineer must be concerned with three basic laws which establish the fundamental rules for proper connection of the thermocouples. It should be stressed, however, that an electronic interface circuit must always be connected to two identical conductors.

        Law No. 1: A thermoelectric current cannot be established in a homogeneous circuit by heat alone.

        Law No. 2: The algebraic sum of the thermoelectric forces in a circuit com-posed of any number and combination of dissimilar materials is zero if all junctions are at a uniform temperature.

        Law No. 3: If two junctions at temperatures T1 and T2 produce Seebeck voltage V2, and temperatures T2 and T3 produce voltage V1, then temperatures T1 and T3 will produce V3 =V1 +V2.

        Thermocouple Circuits

        In the past, thermocouples were often used with a cold junction immersed into a reference melting ice bath to maintain its temperature at 0◦C (thus, the “cold” junction name). This presents serious limitations for many practical uses. The second and third thermoelectric laws allow for a simplified solution. A “cold” junction can be maintained at any temperature, including ambient, as long as that temperature is precisely known. Therefore, a “cold” junction is thermally coupled to an additional temperature sensor which does not require reference compensation. Usually, such a sensor is either thermo resistive or a semiconductor. Figure.B shows the correct connection of a thermocouple to an electronic circuit. Both the “cold” junction and the reference sensor must be positioned in an intimate thermal coupling. Usually, they are imbedded in a chunk of copper. To avoid dry contact, thermally conductive grease or epoxy should be applied for better thermal tracking.

         
        READ DETAILS

        Thermocouple Assemblies

        A complete thermocouple sensing assembly generally consists of one or more of the following: a sensing element assembly (the junction), a protective tube (ceramic or  metal jackets), a thermowell (for some critical applications, these are drilled solid bar stocks which are made to precise tolerances and are highly polished to inhibit corrosion), and terminations (contacts which may be in the form of a screw type, open type, plug and jack disconnect, military-standard-type connectors, etc.). Some typical thermocouple assemblies are shown in Fig. below.


        Basic Laws of Thermocouple


        For practical purposes, an application engineer must be concerned with three basic laws which establish the fundamental rules for proper connection of the thermocouples. It should be stressed, however, that an electronic interface circuit must always be connected to two identical conductors.

        Law No. 1: A thermoelectric current cannot be established in a homogeneous circuit by heat alone.

        1st law

        This law provides that a nonhomogeneous material is required for the generation of the Seebeck potential. If a conductor is homogeneous, regardless of the temper- ature distribution along its length, the resulting voltage is zero. The junctions of two dissimilar conductors provide a condition for voltage generation.

        Law No. 2: The algebraic sum of the thermoelectric forces in a circuit com-posed of any number and combination of dissimilar materials is zero if all junctions are at a uniform temperature.

        2nd law

        The law provides that an additional material C can be inserted into any arm of the thermoelectric loop without affecting the resulting voltage V1 as long as both additional joints are at the same temperature (T3 in Fig. 16.15A). There is no limitation on the number of inserted conductors, as long as both contacts for each insertion are at the same temperature. This implies that an interface circuit must be attached in such a manner as to assure a uniform temperature for both contacts. Another consequence of the law is that thermoelectric joints may be formed by any technique, even if an additional intermediate material is involved (such as solder). The joints may be formed by welding, soldering, twisting, fusion, and so on without affecting the accuracy of the Seebeck voltage. The law also provides a rule of additive materials (Fig. 16.15B): If thermoelectric voltages (V1 and V2) of two conductors (B and C) with respect to a reference conductor (A) are known, the voltage of a combination of these two conductors is the algebraic sum of their voltages against the reference conductor.

        Law No. 3: If two junctions at temperatures T1 and T2 produce Seebeck voltage V2, and temperatures T2 and T3 produce voltage V1, then temperatures T1 and T3 will produce V3 =V1 +V2 .

        3rd law

        This is sometimes called the law of intermediate temperatures. The law allows us to calibrate a thermocouple at one temperature interval and then to use it at another interval. It also provides that extension wires of the same combination may be inserted into the loop without affecting the accuracy.

        Laws 1–3 provide for numerous practical circuits where thermocouples can be used in a great variety of combinations. They can be arranged to measure the average temperature of an object, to measure the differential temperature between two objects, and to use other than thermocouple sensors for the reference junctions and so forth. It should be noted that thermoelectric voltage is quite small and the sensors, especially with long connecting wires, are susceptible to various transmitted interferences. A general guideline for the noise reduction can be found in Section 5.9 of Chapter 5. To increase the output signal, several thermocouples may be connected in series, while all reference junctions and all measuring junctions are maintained at the respective temperatures. Such an arrangement is called a thermopile (like piling up several thermocouples). Traditionally, the reference junctions are called cold and the measuring junctions are called hot. Figure 16.16A shows an equivalent circuit for a thermocouple and a thermopile. It consists of a voltage source and a serial resistor. The voltage sources represent the hot (eb) and cold (ec) Seebeck potentials and the combined voltage Vp has a magnitude which is function of a temperature differential. The terminals of the circuit are assumed to be fabricated of the same material—iron in this example.

        Temperature Sensors


        Taking a temperature essentially requires the transmission of a small portion of the object’s thermal energy to the sensor, whose function is to convert that energy into an electrical signal. When a contact sensor (probe) is placed inside or on the object, heat conduction takes place through the interface between the object and the probe. The sensing element in the probe warms up or cools down; that is, it exchanges heat with the object. The same happens when heat is transferred by means of radiation: thermal energy in the form of infrared light is either absorbed by the sensor or liberated from it depending on the object’s temperature and the optical coupling. Any sensor, no matter how small, will disturb the measurement site and thus cause some error in temperature measurement. This applies to any method of sensing: conductive, convective, and radiative. Thus, it is an engineering task to minimize the error by an appropriate sensor design and a correct measurement technique.

        Thermocouple
        Thermistor

        When a temperature sensor responds, two basic methods of the signal processing can be employed: equilibrium and predictive. In the equilibrium method, a temper-ature measurement is complete when no significant thermal gradient exists between the measured surface and the sensing element inside the probe. In the predictive method, the equilibrium is not reached during the measurement time. It is determined beforehand, through the rate of the sensor’s temperature change. After the initial probe placement, reaching a thermal equilibrium between the object and the sensor may be a slow process, especially if the contact area is dry.
        A typical contact temperature sensor consists of the following components.
        1.  A sensing element: a material which is responsive to the change in its own tem- perature. A good element should have low specific heat, small mass, high thermal conductivity, and strong and predictable temperature sensitivity.
        2.  The contacts are the conductive pads or wires which interface between the sensing element and the external electronic circuit. The contacts should have the lowest possible thermal conductivity and electrical resistance. Also, they are often used to support the sensor.
        3.  A protective envelope is either a sheath or coating which physically separates a sensing element from the environment. A good envelope must have low thermal resistance (high thermal conductivity) and high electrical isolation properties. It must be impermeable to moisture and other factors which may spuriously affect the sensing element. 
        All temperature sensors can be divided into two classes: the absolute sensors and the relative sensors. An absolute temperature sensor measures temperature which is referenced to the absolute zero or any other point on a temperature scale, such as 0◦C (273.15◦ K), 25◦C, and so forth. The examples of the absolute sensors are thermistors and resistance temperature detectors (RTDs). A relative sensor measures the temperature difference between two objects where one object is called a reference. An example of a relative sensor is a thermocouple.

        Classification of temperature sensors

        1. Thermoresistive Sensors: Thermoresistive sensors can be classified as belows
          2. Thermoelectric Contact Sensors

          3. Semiconductor P-N Junction Sensors

          4. Optical Temperature Sensors: Optical sensor can be classified as belows
          • Fluoroptic Sensors
          • Interferometric Sensors
          • Thermochromic Solution Sensor
          5. Acoustic Temperature Sensor

          6. Piezoelectric Temperature Sensors  
           
          Only the sensors used in HFO power plant are discussed briefly.

          Saturday, October 1, 2011

          Cylinder Liner Installtion Process


          Tools Required

          1. Puller Screw
          2. Support Ring
          3. Steel type seal Ring
          4. Check Screw
          5. Locating Collar
          6. Fixing Screw
          Starting position

          Cylinder liner cleaned thoroughly inside and outside, liner bore measured. The liner should be replaced if there are deep scores in the runner surface or if the wear limit heas been reached, especially in the range of top ring travel in TDC, or if liner ovality has reached the admissible limit. Lube oil passages blown out, sealing surface cleaned an unmarred. Cylinder liner positioned in the lowered support ting.

          Cylinder liner inserted into engine block
          Cylinder liner installation process
          Sequence of operation

          1. Critically check the seal ring. If necessary, apply new seal rings, coated with acid free grease and slipped over the liner from below. When inserting into the ring grooves, note that the ring should be evenly tensioned round the circumference and that they must not be twisted.
          2. Carefully pull the support ring from below over the cylinder liner, paying attention to seal ring.
          3. Mount traverse onto support ring and turn home the puller screw until the cylinder liner rests firmly in the support ring.
          4. Insert the support ring with cylinder liner with care into the crankcase & cylinder block, paying attention to the correct position of the support ring in relation to the locating collar.
          5. Fit two hexagonal screws.
          6. Detach traverse and two eye bolts, and again measure the liner bore.
          7. After completion of assembly work, unscrew the check screw and with the cooling water pressure being present, check the seal ring for leaks.

          Note: If new cylinder liners have been installed, they must be run in according to instructions.

            VIEW WHOLE PROCESS IN PHOTOS

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