WO1983001277A1

WO1983001277A1 - Compact power plant with efficient heat cycle

Info

Publication number: WO1983001277A1
Application number: PCT/US1981/001336
Authority: WO
Inventors: Alexander Goloff; John M Clarke
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-10-02
Filing date: 1981-10-02
Publication date: 1983-04-14
Anticipated expiration: 1984-04-02

Abstract

Un groupe propulseur pouvant fonctionner sensiblement selon un cycle du type Atkinson comprend un compresseur (10) pour comprimer un milieu gazeux. Un mécanisme à déplacement positif (30) sert à recevoir le milieu gazeux comprimé provenant du compresseur et à ajouter de la chaleur jusqu'à une température limite dans un volume sensiblement maximum et à déplacer ensuite du mécanisme (30) le milieu gazeux comprimé chauffé. Une turbine (52) pouvant fonctionner en continu à une température égale ou inférieure à la température limite reçoit le milieu gazeux comprimé chauffé qui se détend pour fournir du travail utile et pour entraîner le mécanisme (30) et le compresseur (10). L'addition de chaleur dans le mécanisme (30) lorsque celui-ci présente son volume maximum permet au mécanisme (30) d'avoir une taille relativement réduite et permet d'obtenir un groupe propulseur compact qui fonctionne avec le rendement propre du cycle Atkinson.A propulsion unit capable of operating substantially according to an Atkinson type cycle comprises a compressor (10) for compressing a gaseous medium. A positive displacement mechanism (30) is used to receive the compressed gaseous medium from the compressor and to add heat to a limit temperature in a substantially maximum volume and then to move the heated compressed gaseous medium from the mechanism (30). A turbine (52) capable of operating continuously at a temperature equal to or lower than the limit temperature receives the heated compressed gaseous medium which expands to provide useful work and to drive the mechanism (30) and the compressor (10). The addition of heat in the mechanism (30) when it has its maximum volume allows the mechanism (30) to have a relatively small size and makes it possible to obtain a compact powertrain which operates with the inherent efficiency of the Atkinson cycle. .

Description

Compact Power Plant With Efficient Heat Cycle

Technical Field

This invention relates to a power plant of relatively small si2e and yet which can operate on what is basically an Atkinson cycle.

Background Art

Modern day engines or power plants generally operate on one of three major thermodynamic cycles. Most spark ignition, positive displacement engines, operate on the so-called "Otto" cycle while most_. com¬ pression ignition, positive displacement engines operate on the so-called "Diesel" cycle.

Turbine engines operate on the "Brayton" cycle. Each, of these cycles has a certain theoretical efficiency which is dependent upon design parameters associated with the particular mechanism involved. Eac of the foregoing cycles also has points of inef¬ ficiency which may arise out of either theoretical or practical considerations.

For example, in the case of the Otto or Diesel cycles, isentropic expansion, during which useful work is recovered from the working fluid, is halted in both cycles before attaining the lov/est cycle pressure. Addi- tional work could be harnessed from each such cycle if the gases were permitted to isentropically expand to the lowest cycle pressure, and thus increase overall efficiency.

In the case of the Brayton cycle, heat is added to the working fluid at a constant pressure and cycle efficiency is mathematically related to the ratio of the temperature of the working fluid at the time isentropic compression begins to the temperature of the working fluid at the time constant pressure heat addition begins. The smaller this ratio, the greater the cycle efficiency. Thus, it is desirable, in Brayton cycle machines, that the working fluid be at the highest possible temperature when constant pressure heat addition begins. Unfortunately, most Brayton cycle machines such as conventional turbines, require the flow of the working fluid through the machine at substantially steady state conditions. Thus, the physical characte istics of the material utilized in constructing various parts of a Brayton cycle machine such as a turbine becomes a limiting aspect on the maximum temperature that may be employed during the cycle. Given current state of the art metallurgy, without resort to exotic materials or cooling methods, maximum temperatures allowable in turbines are on the order of 1700 F. At temperatures appreciably in excess of 1700 F. , thermal expansion as well as growth due to centrifugal force may cause interference between the turbine blades and the housing which results in the destruction of the machine.

Nonetheless, Brayton cycle machines have the ability to isentropically expand the work fluid substantially down to the lowest cycle pressure and therefore provide increased efficiency in this area. As a result of the limitations of the Otto,

Diesel and Brayton cycles, proposals have been made whereby efficiencies not obtainable with any of above- mentioned cycles can be obtained by selected use of the best characteristics of the Brayton cycle and of the Otto or Diesel cycles. An example is a turbocompound internal combustion engine. Such an engine consists of a positive displacement mechanism such as a reciprocating or rotary engine operating on the Otto or Diesel cycle as desired. Exhaust gases from the positive displacement mechanism are not discharged directly to atomosphere as in conventional Otto or Diesel cycle engine operation, but rather, expanded further in turbines which drive a compressor which compresses the incoming gaseous medium and also add work to the output shaft thereby recovering work from ^• the exhausting gaseous medium. The initial expansion occurring in the positive displacement mechanism results in a reduction in the temperature of the working fluid to a sufficiently low level that it may enter the turbine to be expanded further therein without heating the turbine to an undesirable high temperature.

In the turboco pound engine useful work from the power plant is taken from a shaft driven by both the positive displacement mechanism and turbine as opposed to a shaft driven solely by the turbine, but there have been proposals whereby useful work is recovered wholly from a turbine. One such proposal is disclosed by Wood in Applications of Thermodynamics, Addison-Wesley Publishing Company, Reading,

Massachusetts, 1969, at page 77 et seg. This proposal utilizes a combination of a free piston engine/compressor plus a turbine wherein heat addition occurs when the free piston engine/compressor is at minimum volume.

A cycle in which near-isentropic expansion occurs over the full maximum to minimum pressure ratio after combustion at constant volume is generally known as an "Atkinson" cycle. This cycle increases efficiency relative to Otto, Diesel or Brayton cycles having the same compression ratio.

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OMPI The thermal efficiency of such a power plant is quite high compared to the commonly used cycles. However, a variety of practical considerations, such as bulk of the apparatus, have not led to substantial practical use of such a prime mover.

Disclosure of the Invention

The present invention is directed to overcoming one or more of the problems set forth above. According. to the present invention there is provided a power plant for operation on what may be generally termed an Atkinson type cycle. The power plant includes a compressor for compressing a working fluid. A positive displacement mechanism receives the compressed working fluid from the compressor and adds heat thereto at substantially maximum volume and thereafter displaces the heated compressed medium from the mechanism. An expander is provided for receiving the heated, compressed medium and for expanding the . medium to recover useful work therefrom and to drive the compressor and the positive displacement mechanism. Another aspect of the invention contemplates the use of other components to further increase the efficiency of the power plant while generally operating on the Atkinson cycle, albeit somewhat modified due to the presence of such other components.

Other objects and advantages will become apparent from the following specification taken in connection with the accompanying drawings.

OMPI Brief Description of the Drawings

Fig. 1 is a diagrammatic view of a power plant made according to one embodiment of -the invention; and Fig. 2 is a plot of various power plant opera- tional cycles of pressure versus percent volume.

Best Mode for Carrying Out the Invention

An exemplary embodiment of a power plant made according to the invention is illustrated in Fig. 1 and is seen to include a compressor 10 having an inlet shown schematically at 12 and an outlet 14. A gaseous working medium, usually air, is admitted to the compres¬ sor 10 at the inlet 12 and is compressed therein to flow therefrom out of the outlet 14.

Optionally, but not necessarily, the compressor 10 may be provided with interstage cooling means 16 of a conventional nature to enhance system efficiency by assuring that compression of the working medium in the compressor 10 approximates isentropic compression as nearly as possible. The outlet 14 is in fluid communication with the interior of a combination recuperator-surge tank 18. The recuperator-surge tank 18 includes an outlet 20 through which the compressed working fluid may exit for purposes to be seen as well as an interior heat exchanger 22. The heat exchanger 22 has an inlet 24 for receiving heated exhaust gases from the power plant. Such gases, after being cooled in the recupera¬ tor-surge tank 18 exit the same via an outlet 26 from the heat exchanger 22. A positive displacement mechanism 30 has an inlet 32 connected to the outlet 20 of the recuperator- surge tank 18. As seen in Fig. 1, the positive dis¬ placement mechanism 30 is of the trochoidal type and includes an operating .chamber wall^* 34 provided with a single lobe 36 between the inlet port 32 and an outlet port 38. A two apexed rotor 40 is journalled for rota¬ tion and translation on a main shaft 42 and includes seals 44 on its apexes. The construction is generally that of a two stroke trochoidal engine, whether epitro- choidal or hypotrochoidal with the exception that ports 32 and 38. have been relocated about the periphery of the wall 34 as has a fuel injector 46 from their conventional locations. Specifically, in the mechanism 30 as illus¬ trated in Fig. 1,. heat addition is to take place when the components are in the maximum volume position as illustrated in Fig. 1. This is in contrast to the usual heat addition occurring at minimum, volume and this ac- counts for the relocation of the ports^' 32 and 38 and the fuel injector 46 from thosein a- typical two stroke tro¬ choidal engine.

The invention is not limited to use of a tro¬ choidal mechanism. The general configuration of a typical two stroke slant axis rotary mechanism could likewise be used. Similarly, a reciprocating mechanism. similar to a valved, two stroke engine could be utilized with fuel being injected when the piston reaches, bottom dead center. In any event, as seen in Fig. 1, the components are in a maximum volume position and at this time, both the inlet port 32 and the outlet port 38 are closed. Heat is added at this time through the burning of fuel and continued rotation and translation of the rotor 40 in the direction of an arrow 48 will maintain the inllet port 32 in isolation from the now opening outlet port 38. The heat addition at this time will cause a rapid increase in pressure of the working fluid and the working fluid will be displaced by the.rotor 40 from the mechanism out of the outlet port 38. It will be appreciated that the heat addition is substantially at constant volume in that at the time of fuel injection and com¬ bustion, the volume of the mechanism 30 will be sub- stantially constant at its maximum.

At the same time, compressed air from the com¬ pressor 10 via the recuperator-surge tank 18 will be admitted to the operating chamber of the mechanism 30 on the opposite side of the rotor 40. Once the rotor 40 reassumes the position illustrated in Fig. 1, heat will again be added and the cycle repeated.

Heated compressed working fluid exiting the port 38 is conducted to a receiver or surge tank 50 and then to a turbine, generally designated 52 which may or may not have multiple stages such as the multi¬ ple stages 54 and 56 illustrated. The expanding medium drives the turbine 52 and its output shaft 58. Useful work may be taken off of the rotating shaft 58. In addition, the shaft 58 is coupled to the main shaft 42 of the positive displacement mechanism 30 to drive the same. A similar coupling, such as shown at 60, pro¬ vides motive power for the compressor 10.

Within the turbine 52, the working medium is . expanded substantially to atmospheric pressure and exits the. same through an outlet 62 which is connected to the inlet 24 for the heat exchanger 22 and the re¬ cuperator-surge tank 18. The gas will ultimately exit via the outlet 26 but only after giving up any residual heat within the recuperator-surge tank 18 to incoming compressed working fluid received from the compressor 10.

At this point it should be observed that the recuperator-surge tank 18 provides a surge tank function

OMPI for the. reason that. the. volume, within .the positive displacement mechanism^' 30 in fluid communication with the inlet 32 thereto will vary at different points in the cycle. Consequently, undesirable pressure fluctua- tions that might occur due to this fact, between the outlet 14 of the compressor 10 and the positive dis¬ placement mechanism 30 are smoothed out by the surge tank function provided by the recuperator-surge tank 18. The same, sort of intermittent relation will occur as between gases being displaced from the mechanism 30 via the outlet^' 38 to the turbine 52. The surge tank 50 again provides, for smoothing of any pressure fluctua¬ tions. Those skilled in the art will also recognize that efficiency may be decreased due to blow down losses occurring upon the opening of the port 38 to the turbine 52. To avoid these blow down losses, the turbine 52 will be of the variable geometry type so that the inlet pressure of the turbine 5-2 is maintained at a value equal to the pressure of the working fluid as is dis¬ charged from the surge tank 50. This may be accom¬ plished, for example, by utilizing variable nozzles in the turbine 52 which are suitably controlled in the man- ner disclosed in the commonly assigned U. S. Patent

Application Ser. No. 941,485, filed September 11, 1978, in the name of Alexander Goloff and entitled "Method and Apparatus Avoiding Blow Down Losses in Compound Engines". It should further be noted that the amount of fuel added by the injector 46 is such as to increase the temperature of the working fluid preferably only to a value on the order of the maximum operating temperature of the turbine 52. In practice, the temperature of the working medium exiting the port^' 38 may be slightly in excess of the maximum operating temperature of the turbine 52 as there will be some heat loss incurred in flowing from the outlet 38 to- .the turbine 52.

Industrial Applicability

As alluded to earlier, the power plant operates generally on the so-called Atkinson cycle. Air at ambient temperature is compressed from point A to point B as shown in Fig. 2. This occurs in the compressor 10. Heat is then added at constant volume in the mechanism 30 as shown by the line BC such that the temperature of the heated, compressed working flui at point C is not sub¬ stantially in excess of the maximum working temperature of the turbine 52. Expansion of the working fluid then occurs in the turbine 52 and is shown, by the line CD while heat rejection is shown by the line DA. If a straight Brayton cycle operation were util¬ ized, heat addition would occur at a constant pressure along the line BC .

Where the compression step is performed substan¬ tially isothermally as, for example, through the use of interstage cooling in the compressor 10 as mentioned earlier, the resulting cycle operation is shown by the Figure AEFGA. Consequently, more power and greater effi¬ ciency will result as it can be readily determined that the area of AEFGA is larger than the area of ABCDA. A further increase in efficiency is obtained through the use of the recuperator-surge tank 18. Again, compression proceeds along the line AE while residual heat contained in the exhaust from the turbine 52 is added at the line AE' . This is followed by heat addi- tion in the positive displacement mechanism 30 at maxi¬ mum volume as shown by the line E'F' while expansion occurs along the line F'G'. Heat rejection for this cycle is shown by the line G'A.

OMPI The area of the cycle utilizing the recuperator- surge tank 18 shown at AEE'F'G'A is approximately equal to the straight Atkinson cycle area ABCDA so that the power of the two is approximately the same. However, efficiency is greater in the former case.

Of course, the recuperator-surge tank 18 may be utilized in a system wherein the compressor 10 is not provided with interstage cooling in which case it would follow the diagram ABE'F'G'A, which cycle will be more efficient than the straight Atkinson cycle ABCDA but less efficient than the modified Atkinson cycle AEE'F'G'A.

In all cases, a substantial, increase in pressure is realized by adding heat at constant volume rather than at constant pressure as in a conventional Brayton cycle. Of course, the amount of heat added at constant volume is limited by the maximum tolerable temperature within the turbine 52.

It will be appreciated that because heat is added at maximum volume of the positive displacement mechanism 30, and no expansion occurs therein, the positive dis¬ placement mechanism 30 as well as the compressor 10 must be driven by the turbine 52. At the same time, it will be recognized that the fact that no expansion occurs within the mechanism 30 enables the same to be of con- siderably smaller size than an otherwise identical mech¬ anism wherein expansion is taking place. Consequently, a power plant made according to the invention is endowed with the relatively high efficiencies associated with power plants operated generally on the Atkinson cycle and yet may be physically smaller than prior art Atkinson cycle mechanismsby reason of the unique addition of heat in the positive displacement mechanism when it is at maximum volume.

It will also be recognized that the power plant of the present invention operates with increased efficiency - li ¬

as well. As is well, known, in- turbine operation, gen¬ erally,, the higher the pressure ratio, the higher the efficiency of operation. When high pressure ratios are utilized, the temperature of the compressed medium exit- ing a compressor such as the compressor 10 approaches the temperature of the exhaust gas exiting a turbine such as the turbine 52. Consequently, heat cannot be effectively extracted from the exhaust. However, in the power plant of- the invention, it will be recognized that the compression ratio may be relatively small so that the temperature of the working fluid exiting the compressor 10 is well, below that of the working medium exiting the turbine 52. This^' enables heat to. be ex¬ tracted from the exhaust gas in the recuperator-surge tank 18 to increase efficiency.

At the same time, because heat is added at a con- ' stant volume, as opposed to constant pressure in the conventional Brayton cycle, the pressure ratio at which the turbine operates is at a level commensurate with that utilized in turbines where recuperators for ex¬ tracting heat from the exhaust are ineffective. In other words, not only the efficiency expected with high pressure ratios is obtained, but it is enhanced as well by means of recuperation due to the unique arrangement of compon- ents that provides for a low compression ratio taken with a high expansion ratio.

OMPI

Claims

1. A power plant for operation substantially on an Atkinson-type cycle comprising: a compressor (10) for compressing a gaseous medium; a positive displacement mechanism (30) for receiv¬ ing the compressed gaseous medium from the compressor and for adding heat (46) thereto up to a limiting tempera¬ ture at substantially maximum volume and for thereafter displacing the heated compressed gaseous medium from the mechanism; and a turbine (52) capable of sustained operation at or below said, limiting, temperature for receiving the heated compressed gaseous medium and expanding the same to provide useful work and to drive said mechanism and said compressor.

2. The power plant of claim 1 wherein said mechanism (30) is a rotary piston (40) mechanism.

3. The power plant of claim 1 wherein said mechanism (30) intermittently receives and displaces said gaseous medium, and further including surge tank means (18,50) for interconnecting said compressor (10) and said mechanism (30) and said turbine (52) and said mech¬ anism (30) .

4. The power plant of claim 1 further including a recuperator (18,22) interconnecting said compressor and said positive displacement mechanism, said recupera¬ tor further receiving the expanding gaseous medium from said turbine. .

5. The power plant of claim 4. wherein said posi¬ tive displacement mechanism (30) is an intermittently operable combustor, and said recuperator (18,22) further serves as a surge tank (18) .

6. A power plant for operation on a generally

Atkinson type cycle, comprising: a compressor (10) for compressing an oxygen containing gaseous medium; a positive displacement combustor (30) for re- ceiving the compressed medium from the compressor and for adding heat (46) thereto under substantially con¬ stant volume conditions by the burning of fuel at or about its maximum volume position and thereafter displac¬ ing the. heated compressed medium, from the combustion, and expanding means (52) for receiving the heated compressed medium from the combustor and for expanding the medium to recover useful work therefrom and to drive said compressor and said combustor.

7. The power plant of claim.6 wherein said expanding means, at least in part, includes a turbine (54,56).

OMPI