Abstract:
In one particular embodiment, an internal combustion engine is provided. The engine comprises a block, a head, a piston, a combustion chamber defined by the block, the piston, and the head, and at least one thermoelectric device positioned between the combustion chamber and the head. In this particular embodiment, the thermoelectric device is in direct contact with the combustion chamber. In another particular embodiment, a cylinder head configured to sit atop a cylinder bank of an internal combustion engine is provided. The cylinder head comprises a cooling channel configured to receive cooling fluid, valve seats configured for receiving intake and exhaust valves, and thermoelectric devices positioned around the valve seats.

Description:
U.S. GOVERNMENT RIGHTS 
   This invention was made with government support under the terms of DE-FC26-04NT42280 awarded by the Department of Energy. The government may have certain rights in this invention. 

   TECHNICAL FIELD 
   The present disclosure relates to a thermoelectric system for generating electrical power. In particular, the present disclosure relates to thermoelectric devices disposed within the head of an internal combustion engine. 
   BACKGROUND 
   Internal combustion engines have become an integral component of many cultures throughout the world, providing a means of transportation and power generation while improving people&#39;s work productivity, generally. Over the years, researchers have improved many aspects of engine technology. Despite these many advances, unfortunately, engines only operate at about 50% efficiency or lower. 
   Poor engine efficiency is largely attributable to thermal energy lost during the combustion process. Much of this waste heat is conducted through various engine components and transferred to the environment, providing no useful work whatsoever. 
   In an effort to improve the efficiency of combustion engines, researchers have developed ways to convert some of the waste heat into useful energy. For example, some researchers have converted waste heat into useful electrical energy that can be used to supplement a portion of the engine&#39;s electrical loads. 
   One such way is disclosed in U.S. Pat. No. 6,029,620 to Zinke (“Zinke”). Zinke discloses an engine block containing thermoelectric materials that generate a direct current during operation and, in so doing, provides for at least some of the necessary engine cooling requirements and for at least some of the electric power requirements. Zinke discloses manufacturing internal combustion engines out of thermocouple-type materials. Zinke also discloses attaching thermoelectric modules to the exterior of an engine for minimizing the redesign of internal engine components. 
   Thermoelectric devices may either convert electrical energy into thermal energy or thermal energy into electrical energy. Early 19th century scientist Thomas Seebeck discovered the phenomenon of placing a temperature gradient across the junctions of two dissimilar conductors resulted in the flow of electrical current. 
   The engines disclosed in Zinke, unfortunately, fail in several respects. First, thermoelectric materials do not generally share the same material characteristics as the iron alloys used in engine block and head castings. As a result, an engine composed entirely of thermoelectric materials may exceed design limitations or fail to be robust enough for practical use. Additionally, the cost of thermoelectric materials is generally considerably higher than those of iron alloys. As a result, an engine composed entirely of thermoelectric materials would be prohibitively expensive. 
   Furthermore, Zinke fails to disclose precise locations for placing these thermoelectric materials. Zinke simply discloses either making an engine entirely out of thermoelectric materials or, in the alternative, generally attaching thermoelectric materials to the engine block. Simply attaching thermoelectric materials to an engine block, without anything further, fails to provide a practical solution for recovering waste heat. 
   The present disclosure is aimed at overcoming one or more of the shortcomings set forth above. 
   SUMMARY OF THE INVENTION 
   In one particular embodiment, an internal combustion engine is provided. The engine comprises a block, a head, a piston, a combustion chamber defined by the block, the piston, and the head, and at least one thermoelectric device positioned between the combustion chamber and the head. In this particular embodiment, the thermoelectric device is in direct contact with the combustion chamber. 
   In another particular embodiment, a cylinder head configured to sit atop a cylinder bank of an internal combustion engine is provided. The cylinder head comprises a cooling channel configured to receive cooling fluid, valve seats adapted to receive intake valves and exhaust valves, and thermoelectric devices positioned around the valve seats. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagrammatic illustration of a thermoelectric device arrangement according to an exemplary embodiment of the disclosure; 
       FIG. 2  is another diagrammatic illustration of a thermoelectric device arrangement according to an exemplary embodiment of the disclosure; 
       FIG. 3  is a cross-sectional view of the diagrammatic illustration of  FIG. 2 ; and 
       FIG. 4  is a cross-sectional view of a thermoelectric device according to one particular embodiment of the disclosure. 
   

   DETAILED DESCRIPTION 
     FIG. 1  provides a diagrammatic illustration of thermoelectric devices  10  positioned within a cylinder head  1  of an internal combustion engine. 
   In the particular embodiment of  FIG. 1 , the engine is a reciprocating-piston internal combustion engine, with piston  60  that reciprocates within a cylinder  64  formed within engine block  2 . A combustion chamber  50  is defined by topside  63  of piston  60 , bottom side of head  1 , and cylinder  64  formed within engine block  2 . Combustion of a fuel and air mixture occurs within combustion chamber  50 , generating high temperatures as a result of the heat release associated with the combustion. Much of this heat is thermally transferred to head  1 , piston  60 , and block  2 . 
   The heat transferred to these components generally performs no useful work and consequently decreases the overall efficiency of the engine. In an effort to improve this efficiency, thermoelectric devices  10  are arranged within cylinder head  1 . These thermoelectric devices  10  convert some of this wasted heat energy into useful electrical energy, which can later be used to supplement the engine&#39;s electrical loads, for example. 
   As previously mentioned, electrical energy is produced from thermal energy under the phenomenon known as the Seebeck effect. 
   When a temperature gradient is imposed on a conductor under open circuit conditions—that is, no current is allowed to flow—a steady-state potential difference between the high- and low-temperature regions is created. In a closed circuit, on the other hand, electrical current will flow as long as the temperature gradient is maintained. The power density produced by this temperature gradient is proportional to the temperature gradient and defined by the following equation: 
   
     
       
         
           
             Q 
             ″ 
           
           = 
           
             
               λ 
               ⁢ 
               
                   
               
               ⁢ 
               Δ 
               ⁢ 
               
                   
               
               ⁢ 
               T 
             
             L 
           
         
       
     
   
   Q″ defines power density, or power per unit area. L defines the distance between hot surface  11  and cold surface  12  (see  FIG. 4 ) and A defines the thermal conductivity of thermoelectric device  10 . As can be seen, the larger the temperature gradient, the larger the power generated. 
   This disclosure proposes positioning thermoelectric devices  10  within head  1  of engine so that devices  10  are located between two areas with a large thermal gradient, such as between engine coolant  40  and combustion chamber  50 . Between these locations, a large temperature gradient is generally observed. In some instances, this temperature gradient may be as high as 650° C. 
   The Figure of Merit, ZT, of a material at a given temperature T is used to describe the material&#39;s performance or effectiveness when used in thermoelectric device  10 . The Figure of Merit is defined by the following equation: 
   
     
       
         
           
             Z 
             ⁢ 
             
                 
             
             ⁢ 
             T 
           
           = 
           
             
               
                 S 
                 2 
               
               ⁢ 
               T 
             
             
               
                 R 
                 ⁢ 
                 
                     
                 
                 ⁢ 
                 K 
               
               ⁢ 
               
                   
               
             
           
         
       
     
   
   S defines the Seebeck coefficient of thermoelectric device  10 , R defines the electrical resistance of thermoelectric device  10 , K defines the thermal conductance of the material, and T defines the temperature. The higher the Figure of Merit, the better the performance of thermoelectric device  10 . In some embodiments of the present disclosure, the Figure of Merit is at least three. Nanostructured boron carbide, for example, is a material that exhibits a Figure of Merit of at least three and at the temperatures commonly associated with internal combustion engine operation. 
   Now referring to  FIG. 4 , a particular embodiment of thermoelectric device  10  is shown. The reader should appreciate that the present disclosure is not limited to the particular thermoelectric device  10  shown in  FIGS. 1-4 . Instead, one skilled in the art would understand that several different types of thermoelectric devices  10  might alternatively be used to carry out the invention of the present disclosure. 
   The particular thermoelectric device  10  of  FIGS. 1-4  comprises two ceramic substrates that serve as a foundation and electrical insulation for P-type semiconductors  14  and N-type semiconductors  13 . These semiconductors  13  and  14  are connected electrically in series and thermally in parallel between the ceramics. The ceramic substrates also serve as insulation between the internal electrical elements. In this particular embodiment, a heat sink is in contact with hot side  11  and a cooler surface is in contact with cold side  12 . An electrically conductive material—such as conducting pads attached to P-type semiconductors  14  and N-type semiconductors  13 —maintain electrical connections inside device  10 . Solder or any other known fixing technique may be used at the connection joints to enhance the electrical connections and hold device  10  together. 
   In some embodiments, P-type semiconductors  14  comprise compounds or boron and/or silicon. N-type semiconductors  13 , on the other hand, may comprise SiC or SiGe, for example. 
   In some embodiments, electrical leads  70  to the device  10  are attached to pads on the hot side  11  of device  10 . Leads  70  may then be connected to a DC battery, DC loads, or a DC-AC inverter for powering any AC loads, for example. The reader should appreciate that as electrical power is generated, its application may go towards any useful means envisioned by one skilled in the art and is not limited to those listed above. 
   The particular embodiment of  FIG. 1  depicts thermoelectric devices  10  positioned within head  1  so that a hot side  11  of thermoelectric devices  10  faces combustion chamber  50 . The highest temperatures within an engine generally occur within combustion chamber  50 , and can be as high as 750° C. or higher. 
   Additionally and as further depicted in the particular embodiment of  FIG. 1 , cold side  12  of thermoelectric device  10  faces away from combustion chamber  50  and faces towards cooling channel  40 . In one particular embodiment, the cooling fluid in coolant channel  40  is engine jacket water that was just previously cooled by an engine cooler, such as a radiator. Because the electrical power generated by thermoelectric devices  10  is proportional to the temperature gradient, it is desirable to configure the engine&#39;s coolant system so that a cooler portion of the coolant flows through channel  40 . 
   In the particular embodiment of  FIG. 1 , thermoelectric device  10  is separated from coolant channel  40  by metallic interface  42 , which in this embodiment is integrally formed with head  1 . Metallic interface  42  provides a high-pressure boundary separating combustion chamber  50  from channel  40  while at the same time providing a barrier to prevent thermoelectric device  10  from directly contacting coolant within channel  40 . 
   In some instances, head  1  may be manufactured from a casting process. Cavities may be formed during the casting process to accommodate thermoelectric devices  10 . Alternatively, cavities may be machined within head  1  to accommodate thermoelectric devices  10 . Thermoelectric devices  10  may then be placed within the cavities so that devices  10  directly contact the combustion gases within combustion chamber  50 . The reader should appreciate that the precise method of manufacturing these cavities is not germane to the disclosed embodiments and that one skilled in the art would understand that several methods might exist for manufacturing head  1  with cavities for accommodating devices  10 . 
   During the manufacture of cylinder head  1 , metallic interface  10  may also be formed integral with head  1  so that a high-pressure boundary exists to isolate combustion chamber  50 —and thermoelectric devices  10 —from coolant channel  40 . 
   Now referring to  FIG. 2 ,  FIG. 2  provides a diagrammatic illustration of thermoelectric devices  10  positioned within an engine&#39;s head  1 . The particular embodiment of  FIG. 2  depicts four thermoelectric devices  10  positioned around two intake valves  20  and two exhaust valves  25 . Many engines have two intake valves  20  and two exhaust valves  25  per cylinder  64 . Although the particular embodiment of  FIG. 2  depicts two intake valves  20  and two exhaust valves  25  per cylinder  64 , the reader should appreciate that the present disclosure applies to engines with other valve configurations. In addition, although intake valves  20  and exhaust valves  25  are shown as having similar diameters, the reader should appreciate that many internal combustion engines have intake valves  20  and exhaust valves  25  with varying diameters and that the present disclosure would apply to these engines, as well. 
   In the particular embodiment disclosed in  FIG. 2 , thermoelectric devices  10  have a general “T” shape. This particular shape—when oriented according to FIG.  2 —increases the surface area of thermoelectric devices  10  that is in contact with combustion chamber  50 . This may allow for devices  10  to convert more waste heat to electrical energy. Fuel injector  30  (or spark plug) may be centrally located within cylinder  64 , which when in the presence of two intake valves  20  and two exhaust valves  25 , justifies the T-shape nature of the four thermoelectric devices  10 . 
   Now referring to  FIG. 3 ,  FIG. 3  provides a cross-sectional view-along line I-I—of part of engine head  1  that is depicted in  FIG. 2 . As can be seen in the particular embodiment of  FIG. 3 , thermoelectric devices  10  have a hot side  11  and a cold side  12 . In this particular embodiment, hot side  11  is in direct contact with combustion chamber  50  while cold side  12  faces cooling chamber  40 . As can further be seen, a metallic interface  42  exists to separate cooling channel  40  from device  10 . In the particular embodiment of  FIG. 3 , metallic interface  42  is integrally formed with head  1 . 
   Now referring to  FIG. 4 , a particular embodiment of thermoelectric device  10  is shown. 
   A typical thermoelectric device  10  comprises of two ceramic substrates that serve as a foundation and electrical insulation for P-type  14  and N-type  13  semiconductors. Semiconductors  14  and  13  are connected electrically in series and thermally in parallel between the ceramics. The ceramic substrates may also serve as insulation between the internal electrical elements and a heat sink that may be in contact with hot side  11  as well as a cooler object against cold side  12 . The electrical connections between P-type  14  and N-type  13  semiconductors may be achieved by the use of metallic leads  70 —or tabs—which may comprise nickel or chromium. Nickel, for example, is a material with suitable conductivity and oxidation resistance. 
   In some particular embodiments, metallic leads  70  may be connected to the ends of each semiconductor  13  or  14  leg by a conductive material that is applied at room temperature. When set, the conductive material may be capable of withstanding the high temperatures associated with engine combustion. 
   The electrical power developed by the thermoelectric device  10  may then be transferred to the point of use by wires (not shown)or any other type of electrical conductor known in the art. Referring to  FIG. 4 , a first wire may connect the left-most semiconductor to the point of use and a second wire may connect the right-most semiconductor to the point of use, which may be an electrical battery or load. As a temperature gradient is viewed across device  10 , an electrical potential will be generated and seen across the first and second wires. 
   INDUSTRIAL APPLICABILITY 
   The present disclosure provides a system and method for recovering waste heat from an internal combustion engine for converting it to useful electrical energy. Internal combustion engines convert chemical energy into useful work by the combustion of a fuel and air mixture. 
   Referring to the particular embodiment of  FIG. 1 , during combustion of a fuel and air mixture within combustion chamber  50 , heat is released causing the temperature within chamber  50  to rise. In some instances, the temperature may be as high as 750° C. The combustion gases are then used to drive piston  60  and connecting rod  62  down (as seen in FIG.  1 )—thus rotating a crankshaft (not shown) for the purpose of performing mechanical work. 
   Unfortunately, not all of the combusted fuel and air is converted into useful mechanical work. Some of the heat from the combustion process is thermally transferred to various engine components, such as head  1 , block  2 , and the exhaust system (not shown). Much of the thermal energy is wasted as it transfers to the environment. 
   The disclosed system transfers some of this thermal energy to hot side  11  of device  10 . In one particular embodiment, hot side  111  of device  10  is in direct contact with combustion chamber  50 , thus being exposed to the high temperatures resultant from the combustion process. 
   At the same time, engine coolant flows through channel  40 . This relatively cool coolant is in close proximity to cold side  12  ceramic of device  10  and is generally cooler than hot side  11 . In some embodiments, this coolant may have just exited the engine&#39;s jacket-water cooler or radiator. As a result, a temperature gradient is imposed across device  10 . 
   As long as this temperature gradient is maintained, electrical current will flow. This electrical current may then be used to supplement a vehicle&#39;s electrical loads, charge a battery, or perform any other function requiring electricity. 
   In one particular embodiment, the electrical energy generated is used support the electrical load of a hybrid machine. Hybrid vehicles and machines typically have a combustion engine and electric motor mechanically linked to a drive train for providing propulsion. In this particular embodiment, the electrical energy generated by device  10  would help power an electric motor, which when mechanically linked to a drive train, provides propulsion to the machine. 
   It will be apparent to those skilled in the art that various modifications and variations can be made with respect to the embodiments disclosed herein without departing from the scope of the disclosure. Other embodiments of the disclosed invention will be apparent to those skilled in the art from consideration of the specification and practice of the materials disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.