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Title:
POWER GENERATOR FOR PISTON INSTRUMENTATION
Document Type and Number:
WIPO Patent Application WO/2016/069068
Kind Code:
A1
Abstract:
A piston assembly for monitoring at least one operating condition of an internal combustion engine and/or piston during use of the piston in the engine, for example during an engine test, is provided. The piston assembly includes an electronic instrumentation unit coupled to the piston for collecting data related to the operating conditions. Instead of a battery or wireless power transfer system, the piston assembly includes a thermoelectric module to provide energy to the electronic instrumentation unit. One side of the thermoelectric module is coupled to an undercrown surface of the pistons which is typically hot due its proximity to the combustion chamber. The opposite side of the thermoelectric module is cooled by a cooling fluid, such as a spray of cooling oil. The temperature flux at the thermoelectric module is converted into electrical energy and used to power the electronic instrumentation unit.

Inventors:
NEUENDORF, Jonathan, Owen (411 Pine Brae, Ann Arbor, MI, 48105, US)
GLADWIN, Ross (3216 Lockridge, Ann Arbor, MI, 48108, US)
GYETVAY, Patrick (9021 Carter, Allen Park, MI, 48101, US)
Application Number:
US2015/038773
Publication Date:
May 06, 2016
Filing Date:
July 01, 2015
Export Citation:
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Assignee:
FEDERAL-MOGUL CORPORATION (27300 West 11 Mile Road, Tower 300 9th Floor,Metro Office Comple, Southfield MI, 48034, US)
International Classes:
F02F3/00; G01K13/06; H01L35/30
Domestic Patent References:
WO2012079759A12012-06-21
WO2003065326A22003-08-07
Foreign References:
US20060137627A12006-06-29
JP2007077951A2007-03-29
US8843341B22014-09-23
Attorney, Agent or Firm:
STEARNS, Robert L. et al. (Dickinson Wright PLLC, 2600 W. Big Beaver RoadSuite 30, Troy MI, 48084-3312, US)
Download PDF:
Claims:
CLAIMS

What is claimed is:

1 , A piston assembly, comprising;

a piston including a combustion bow! surface for exposure to heat from a combustion chamber of an interns! combustion engine;

said piston including an undercrown surface facing opposite said combustion bowl surface for exposure to a cooling medium;

an electronic instrumentation unit coupled to said piston for monitoring at least one condition of said piston aiid or the engine; and

a thermoelectric module coupled to said electronic instrumentation unit and said undercrown surface of said piston for exposure to the cooling medium and the heat from the combustion chamber, wherein said thermoelectric module converts a temperature difference between the cooling medium and the heat from the combustion chamber into energy and transfers the energy to said electronic instrumentation unit

2, The piston assembly of claim 1, wherein said thermoelectric module Is disposed along a center axis of said piston.

3. The piston assembly of claim 1 further including a cooling medium supply for providing the cooling medium to said undercrown surface and said thermoelectric module.

4. The piston assembly of claim 3, wherein said cooling medium supply is a nozzle and said cooling medium is a spray of cooling fluid,

5. The piston assembly of claim 1 further including power management electronics disposed between said thermoelectric module and said electronic instrumentation unit for receiving the energy from said thermoelectric module and adjusting the energy before transmitting the energy from said thermoelectric module to said electronic instrumentation unit.

6. The piston assembly of claim I including a heat sink coupled to said thermoelectric module opposite said undercrown surface for exposure to the cooling medium,

7. The piston assembly of claim 1 including a power source separate from said thermoelectric module coupled to said electronic instrumentation unit for providing additional energy to said electronic instrumentation unit.

8. The piston assembly of claim 1 including a plurality of thermocouples disposed at a plurality of locations along said piston for measuring the temperature of said piston at each of the locations; and each of said thermocouples being coupled to said electronic instrumentation unit for providing the temperature at each of the locations to said electronic instrumentation unit.

9. The piston assembly of claim 1 s wherein said piston includes a crown portion presenting said combustion bowl surface and said undercrown surface;

said crown portion includes a plurality of ring grooves for maintaining piston rings; said piston includes a pair of skirt sections spaced from one another by pirs bosses, wherein said pin bosses define a pin bore for receiving a wrist pin;

said electronic instrumentation unit is mounted with fasteners on one of said pin bosses of said piston; said electronic instrumentation unit includes a ground and a power input for receiving the energy from said thermoelectric module;

said electronic instrumentation unit includes a data collector for obtaining the at least one condition of said piston or the engines storing data about the at least one condition, and transmitting the data to a computer;

said electronic instrumentation unit includes a memory for storing the data, a processors snd a radio for transmitting the data to the computer;

said thermoelectric module is thermally coupled to said undercrown surface at a center axis of said piston by a thermally conductive epoxy; and

further comprising:

a plurality of thermocouples disposed at a plurality of locations along said piston for measuring the temperature of said piston at each of the locations;

each of said thermocouples being coupled to said electronic instrumentation unit for providing the temperature at each of the locations to said electronic instrumentation unit;

a heat sink coupled to said thermoelectric module opposite said undercrown surface;

a power management electronics disposed between said thermoelectric module and said electronic instrumentation unit for receiving the energy from said thermoelectric module and adjusting the energy before transmitting the energy from said thermoelectric module to said electronic instrumentation unit; and

a cooling medium supply disposed adjacent said piston for providing the cooling medium to said undercrown surface and said heat sink and said thermoelectric module, wherein said cooling medium supply is a nozzle, and said cooling medium is a spray of cooling oil

10. A system for monitoring at least one condition of a piston and/or interna! combustion engine during use of the piston in the engine, comprising;

an electronic instrumentation unit for being coupled to a piston and monitoring at least one condition of the piston and/or the engine; and

a thermoelectric module coupled to said electronic instrumentation unit for being coupled to the piston and for converting a temperature difference between a cooling fluid and heat from a combustion chamber into energy and transferring the energy to the electronic instrumentation unit.

1 1. The system of daim !0} wherein the thermoelectric module is disposed along a center axis of the piston.

12. The system of claim 10 including a cooling medium supply for providing the cooling medium to the undercrown surface and the thermoelectric module,

13. The system of claim 10 including power management electronics disposed between the thermoelectric module and the electronic instrumentation unit for receiving the energy from the thermoelectric module and adjusting the energy before transmitting the energy from the thermoelectric module to the electronic instrumentation unit,

14. The system of claim 10 including a heat sink coupled to the thermoelectric module opposite the undercrown surface for exposure to the cooling medium.

15, A method for monitoring at least one condition of a piston and/or internal combustion engine during use of the piston in the engine, comprising the steps of: providing a piston assembly including a piston, the piston including a combustion bowl surface and an undercrown surface facing opposite the combustion bowl surface, the piston assembly including an electronic instrumentation unit coupled to the piston for monitoring at least one condition of the piston and/or engine, and the piston assembly including a thermoelectric module coupled to the electronic instrumentation unit and the undercrown surface of the piston;

exposing the combustion bow! surface to heat from the combustion chamber of the internal combustion engine;

applying a cooling medium to the thennoeleetric module;

converting a temperature difference between the cooling medium and the heat from the combustion chamber into energy using the thermoelectric module; and

transferring the energy from the thermoelectric module to the electronic instrumentation un .

16, The method of claim 15, wherein the step of providing the piston assembly includes disposing the thermoelectric module at a center axis of the piston.

17, The method of claim 15, wherein the step of applying the cooling medium to the thermoelectric module includes spraying a cooling fluid toward the thermoelectric module and the undercrown surface.

18, The method of claim 15, including adjusting the energy befbre transferring the energy from the thenTioelectnc module to the electronic instrumentation unit using power management electronics, 1 » The method of claim 15, wherein the piston assembly includes a heal sink coupled to the thermoelectric module opposite the lindercrown surface, and the step of applying the cooling medium to the thermoelectric module includes applying the cooling medium to the heat sink.

20, The method of claim 15, including collecting data about the at leas one condition of the piston and/or the engine using the electronic instrumentation unit and the energy provided by the thermoelectric module.

8

Description:
CROSS REFERENCE TO RELATED APPLICATIONS

fOOOlJ This U.S. patent application claims the benefit of U.S. provisional patent application serial number 62/072,520 filed October 30 } 2014, and U.S. utility patent application serial number 14/743,221 filed June 18, 2015, the entire content of which arc incorporated herein by reference.

BACKGROUND OF THE INVENTION

! . Field of the Invention

{0002J This Invention relates generally to systems for monitoring piston conditions during engine testing, and more specifically to systems for providing energy to devices which monitor the piston conditions in the engine.

2. Related Art

|Θ003| Manufacturers of internal combustion engines typically run tests simulating operation of the engine before bri ging the engine to market. Each component of the engine is evaluated for performance and durability purposes. Such tests include running the engine for long durations of time s for example multiple weeks or months, and monitoring the performance of the engine components throughout the entire test duration. For example, the piston should be monitored to confirm that the piston and engine maintain an acceptable operating temperature throughout the entire test duration,

[O 04J However, evaluating the internal engine components, such as the piston, is typically very difficult since these components are not readily accessible and operate in extreme environments. To properly evaluate such components, many engine manufacturers will make substantial changes to the engine block to accommodate various linkages and transmitting systems. Such modifications are not only costly and time consuming but also undesirable because the engines being tested are in high demand and are costly to produce, especially during the very early development phase.

|§O0S] Another approach some engine manufacturers use is to attach electronic instrumentation . , such as a data collector assembly, with a wireless transmitter/receiver to the engine component. In addition to the wireless transmitter, such data collector assemblies typically include a battery, a plurality of sensors, at least one processor, and a buffer memory which is configured to only store data until the data is transmitted via the wireless transmitter to a remote computer. Shortly before the engine test begins, a person must manually activate the data collector assembly. During the engine test, a substantial amount of the electrical power from the battery is used by the wireless transmitter/receiver to transmit the data to the remote computer.

|0006] Yet another system and method used to monitor engine components, such as the piston, is disclosed in U.S. Patent No. 8,843,341 to Federal-Mogul Corporation. This system includes a data collector assembly for collecting and storing data related to at least one condition of the piston during an engine test The data collector assembly also includes a sensor for sensing a condition of the internal engine component and for generating a signal corresponding to the sensed condition, and an analog to digital converter for converting the signal into a digital signal. The data collector assembly also includes a memory for storing data and a clock. The data collector assembly additionally includes a processor which is in electrical communication with the analog to digital converter, the memory and the clock. The processor is configured to process the digital signal and store data corresponding to the digital signal on the memory. At least one power source, such as a battery, is electrically connected to the sensor, the analog to digital converter, the memory, the processor and the clock. During the engine test, the sensor, the analog to digital converter, the memory, the processor and the clock are configured to be substantially the only

7 devices thai draw electricity from the power source. This data collector assembly is advantageous as compared to other known data collector assemblies used to monitor engine components because it lacks a power hogging wireless transmitter/receiver, thereby allowing for a smaller power source to be employed. In addition to providing for packaging advantages, the smaller power source, such as the battery, is typically more reliable than larger power sources when operating in extreme environments, such as those found within an internal combustion engine.

0ΘΘ7] Although the system disclosed in U,S, Patent No. 8,843,341 has numerous advantages, the size of the battery used to provide power to the data collector assembly typically limits the engine test duration. As an alternative to the battery, a wireless power transfer system could be used to provide power to the data collector assembly. However, such wireless transfer systems require significant and costly modifications to the engine block, which is not desirable to engine manufacturers,

SUMMARY OF THE INVENTION

|0008] One aspect of the invention provides an improved piston assembly for monitoring at least one operating condition of a piston and/or an internal combustion engine while using the piston in the engine. The piston includes a combustion bow! surface for exposure to heat from a combustion chamber of the engine, and an undercrown surface facing opposite the combustion bowl surface for exposure to a cooling medium. An electronic instrumentation unit is coupled to the piston for monitoring the at least one condition of the piston and or the engine. A thermoelectric module is coupled to the electronic instrumentation unit and the undercrown surface of the piston for exposure to the cooling medium and the heat from the combustion chamber. The thermoelectric module converts a temperature difference between the cooling medium and the heat from the combustion chamber into energy and transfers the energy to the electronic instrumentation unit. |00Θ9] Another aspect of the invention provides a system for monitoring at least one condition of a piston and/or interna! combustion engine during use of the piston in the engine. The system includes an electronic instrumentation unit for being coupled to a piston and monitoring the at least one condition of the piston and/or the engine. A thermoelectric module is coupled to the electronic instrumentation unit for being coupled to the piston and for converting a temperature difference between a cooling fluid and heat from a combustion chamber into energy and transferring the energy to the electronic instrumentation unit,

[0O1OJ Yet another aspect of the invention provides a method for monitoring at least one condition of a piston and/or internal combustion engine during use of the piston in the engine. The method includes providing a piston assembly including a piston. The piston includes a combustion bowl surface and an undercrown surface facing opposite the combustion bowl surface. The piston assembly also includes an electronic instrumentation unit coupled to the piston for monitoring the at least one condition of the piston and/or engine, and a thermoelectric module coupled to the electronic instramentation unit and the undercrown surface of the piston, The method further includes exposing the combustion bowl surface to heat from the combustion chamber of the internal combustion engine; applying a cooling medium to the thermoelectric module; converting a temperature difference between the cooling medium and the heat from the combustion chamber into energy using the thermoelectric module; and transferring the energy from the thermoelectric module to the electronic instrumentation unit,

0011] Numerous advantages are provided by the piston assembly, system and method of the present invention. Since portions of the piston are continuously heated by the combustion chamber while other portions are continuously sprayed with cooling fluid, the thermoelectric module is able to use the constant heat flux to provide electrical energy to the electronic instrumentation unit throughout the entirety of an engine test duration. The test duration is no longer limited by the amount of available energy. In addition, the thermoelectric module is a solid state design having a durable construction and no moving parts or components external to the piston are required. Thus, the thermoelectric module allows for operation at much faster engine speeds and higher temperatures. Furthermore, the dimensions, oiass s and location of the thermoelectric module can be selected so that the thermoelectric module does not interfere with or hinder the operation of the piston or engine being tested,

BRIEF DESCRIPTION OF THE DRAWINGS

[0§!2 Other advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein;

|0013| Figure 1 is a perspective view of an improved monitoring assembly including an electronic instrumentation unit and a thermoelectric module coupled to a piston according to an example embodiment of the invention;

|O0i4J Figure 2 is another perspective view of the improved monitoring assembly coupled to the piston according to the example embodiment;

|0 151 Figure 3 is a cross-sectional view of the piston of the example embodiment showing cooling oil being sprayed onto the thermoelectric module;

10016] Figure 4 illustrates transferring electrical energy from the thermoelectric module to the electronic instrumentation unit according to one example embodiment; and

|0O17J Figure 5 illustrates transferring electrical energy from the thermoelectric module to the electronic Instrumentation unit according to another example embodiment. DESCRIPTION OF AN ENABLING EMBODIMENT

|00ISJ The invention provides an improved assembly for monitoring at least one engine operating condition, such as temperature of a piston 20, during use of the piston 20 in the internal combustion engine. Typically,, the assembly is used m a test engine operated by an engine manufacturer, so that the manufacturer can evaluate the performance and durability of the piston 20, before bringing the engine to market. The assembly includes an electronic instrumentation unit 22 for monitoring the operating conditions of the engine and/or piston 20, and a thermoelectric module 24 for providing electrical energy to the electronic instrumentation unit 22 throughout the entire engine test.

|0019j Any type of piston 20 can be monitored using the assembly of the present invention. In the example embodiment shown in Figures 1-3, the piston 20 comprises a single-piece cast design including a crown portion 26 presenting a combustion bowl surface 28 and an oppositely facing undercrown surface 30, During engine operation, the combustion bowl surface 28 is directly exposed to high temperature gases in a combustion chamber, and heat from the combustion chamber travels through the crown portion 26 to the undercrown surface 30, To reduce the temperature of the crown portion 26, a cooling medium supply 56 provides a cooling medium 32 to the undercrown surface 30, In the example embodiment, the cooling medium supply is a nozzle, and the cooling medium 32 is a liquid, such as cooling oil, which is continuously sprayed onto the undercrown surface 30. The crown portion 26 also includes a ring belt 34 including plurality of ring grooves 36 for maintaining piston rings (not shown),

1002 Of The piston 20 further includes a lower portion 3S depending from the ring belt 34, The lower portion 38 includes a pair of skirt sections 40 spaced from one another by pin bosses 42, Each pin boss 42 defines a pin bore 44 for receiving a wrist pin (not shown) which is coupled to a connecting rod (not shown). Although the piston 20 of Figures 1-3 is a single-piece cast design, the piston 20 can comprise other designs. For example, the crown portion 26 and the lower portion 3S could be welded or otherwise connected together.

[§021] As shown in Figures 1-3, the thermoelectric module 24, also referred to as a thermoelectric generator, is thermally coupled to a portion of the piston 20 exposed to a temperature flux, which can be any portion of the piston 2© that provides heat and is also exposed to a cooling medium, such as the cooling fluid 32, In the example embodiment, the thermoelectric module 24 is coupled to the undercrown surface 30 along a center axis of the piston 20. Thus, heat from the combustion chamber travels through the crown portion 26 to the thermoelectric module 24, white the cooling fluid 32 is sprayed onto the undercrown surface 39 and also onto the thermoelectric module 24, as shown in Figure 3. The thermoelectric module 24 converts this temperature difference directly into electric energy according to a phenomenon known as the Seebeck effect, or thermoelectric effect. The temperature difference at the thermoelectric module 24 is present throughout the entire engine test, no matter how long the duration of the test. Thus, the thermoelectric module 24 is able to generate electrical energy throughout the entire duration of the test, and the duration of the test is not limited by the amount of energy available for the electronic instrumentation unit 22.

[00221 Various different types of thermoelectric modules 24 can be employed.

In the example embodiment, the dimensions and mass of the Ihermoelectric module 24 are selected such that the thermoelectric module 24 does not hinder the operation of the piston 20 or performance of the internal combustion engine heing tested. Also, in the example embodiment, the thermoelectric module 24 is conveniently located at the center axis and along the undercrown surface 3D of the piston 20, Thus, the thermoelectric module 24 allows the piston 20 to stay axially balanced and does not interfere with the wrist pin or connecting rod. In addition, various different methods can be used to couple the thermoelectric module 24 to the piston 20. However, in each case, the thermoelectric module 24 should be thermally coupled, so that the thermoelectric module 24 is not insulated, and so that heat from the piston 20 is not lost. In the example embodiment, the thermoelectric module 24 is coupled to the piston 20 using a thermally conductive epoxy to keep the temperature of the thermoelectric module 24 at or close to the temperature of the undercrown surface 30 of the piston 2Θ, In addition, although not shown, the assembly could include a plurality of the thermoelectric modules 24 connected in series, The assembly could also include a least one other type of power source, for example a battery, in addition to the thermoelectric module 24.

|ΘΘ231 The feemioelectric module 24 is electrically connected to the electronic instrumentation unit 22, and the electrical energy provided by the thermoelectric module 24 is transferred to the electronic instrumentation unit 22 to power the electronic instrumentation unit 22 throughout the entire engine test, In the example embodiment, the thermoelectric module 24 is electrically connected to the electronic instrumentation u it 22 by a plurality of wires 46 s which are attached to the piston 20 with epoxy or glue and conveniently located so as to not interfere with the performance of the piston 20. Since the temperature flux at the thermoelectric module 24 is present throughout the entire duration of the engine test, the electrical energy can also be supplied to the electronic instrumentation unit 22 throughout the entire duration of the test.

[ΘΘ24] The electronic instrumentation unit 22 can comprise various different designs which are capable of monitoring at least one condition of the piston 20 and/or the engine, The electronic instrumentation unit 22 includes a ground and a power input for receiving the energy from the thermoelectric module 24, typically via the wires 46, The electronic instrumentation unit 22 also includes a data collector for sensing at least one

S condition of the piston 20 and/or the piston, such as the temperature of the piston, storing the data, and transmitting the data to an external computer for analysis by the engine manufacturer. An example data collector is described in U.S. Patent No. 8,843,341 f which is incorporated herein by reference in its entirely.

f0f)25J In the example embodiment shown in Figures 1-3, the electronic instrumentation unit 22 is connected to a plurality of thermocouples 48 disposed at various locations on the piston 20 to measure the temperature at the various locations. The thermocouples 48 provide the temperature of the piston 20 at the various locations to the electronic instrumentation unit 22. The electronic instrumentation unit 22 of the example embodiment is also provided in the form of a board mounted outwardly of one of the pin bosses 42 of the piston 20. Holes are drilled through the board and piston 0, and fasteners, such as standoff cylinders SO are inserted through the holes to secure the board to the piston 20, However, the electronic instrumentation unit 22 could alternatively include multiple boards, could be located along another portion of the piston 20, and could be coupled to the piston 20 using other methods, in the example embodiment, the electronic instrumentation unit 22 also includes a memory for storing the data, a processor, and a radio for transmitting the data to an external computer for evaluation by the engine manufacturer.

|00261 Figures 4 and 5 illustrate two example embodiments for generating and transferring the electric energy from the thermoelectric module 24 to the electronic instrumentation unit 22. Figure 4 shows a first embodiment wherein one side of the thennoelectric module 24 is in direct contact with the heated piston 20, and the other side is in direct contact with the spray of cooling fluid 32. In addition, each side of the thermoelectric module 24 is in electrical contact with the power and ground of the electronic instrumentation unit 22. The thermoelectric module 24 converts the temperature difference (ΔΤ) between the heated piston 20 and the cooling fluid 32 to electrical energy (ΔΥ), and then transmits the electrical energy directly to the electronic instrumentation unit 22,

|00271 Figure 5 shows a second embodiment for generating and transferring the electric energy from the thermoelectric module 24 to the electronic instrumentation unit 22. In this embodiment, one side of the thermoelectric module 24 is still In direct contact with the heated piston 211. However, the opposite side of the thermoelectric module 24 is connected to a separate heat sink 52, and the heat sink 52 is in direct contact with the spray of cooling fluid 32. The thermoelectric module 24 converts the temperature difference (ΔΤ) between the healed piston 20 and the heat sink 52 to electrical energy (ΔΥ).

f Q02SJ Also in the embodiment of Figure 5, power management electronics 54 are disposed between the thermoelectric module 24 and the electronic instrumentation unit 22, and each side of the thermoelectric module 24 is In electrical contact with the power and ground of the power management electronics 34. Thus, instead of transmitting the electrical energy directly to the electronic instrumentation unit 22, the thermoelectric module 24 transmits the electrical energy to the power and ground of the power management electronics 54. The power management electronics 54 is typically used in cases where the electrical energy (ΔΥ) provided by the thermoelectric module 24 is not sufficient or is not in the form required by the electronic instrumentation unit 22. For example, the power management electronics 54 can increase the voltage and reduce the current prior to transmitting the electrical energy to the electronic instrumentation unit 22. Various different types of power management electronics 54 can be used. In the example embodiment, however, the power management electronics 54 includes a DC-DC converter which boosts the voltage, such that the voltage provided to the electronic instrumentation unit 22 is at the preferred level of 2 to 5 volts. (00291 Another aspect of the invention provides an improved system for monitoring the at least one condition of the piston 20 and or the engine during use in the interna! combustion engine. The system includes the electronic instrumentation unit 22 t the thermoelectric module 24, the optional heat sink 52, and the optional power management electronics 54 described above. The system also includes the cooling medium supply 56 for providing the cooling medium 32 to the wsdercrown surface 30 and the thermoelectric module 24,

|0030] Yet another aspect of the invention provides an improved method for monitoring the at least one condition of the piston 20 and or the engine during use in the internal combustion engine using the electronic instrumentation unit 22, the thermoelectric module 24, the optional heat sink 52, and the optional power management electronics 54 described above.

|0031J The method first includes providing the piston assembly, which preferably includes disposing the thermoelectric module 24 at the center axis of the piston 20. The method then includes exposing the combustion bowl surface 28 to heat from the combustion chamber of the engine; and applying the cooling medium 32 to the diermoelectrie module 24, for example by spraying a cooling fluid toward the thermoelectric module 24 and the undererown surface 30, The step of applying the cooling medium 32 to the thermoelectric module 24 can also include applying the cooling medium 32 to the heat sink 52.

|0Θ32| The method further includes converting the temperature difference between the cooling medium 32 and the heat from the combustion chamber into energy using the thermoelectric module 24; and transferring the energy from the thermoelectric module 24 to the electronic instrumentation unit 22. In one embodiment, the method includes adjusting the energy before transferring the energy from the thermoelectric module 24 to the electronic instrumentation unit 22 using the power management electronics 54, Finally, the method includes collecting the data about the at least one condition of the piston 20 and/or the engine using the electronic instrumentation unit 22 and the energy provided by the thermoelectric module 24.

[0 133| Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise thaji as specifically described while within the scope of the following claims.