This application is a continuation-in-part of applicant's copending application serial No. 879,969, filed February 21, 1978 and entitled "-Gas Turbine System"; and is also a continuation-in-part of applicant's, copending application serial No. 890,465, filed March 27, 1978 and entitled "Gas Turbine System". Each of these two parent applications in their entirety is hereby incorporated by reference in this application.

TECHNICAL FIELD

This invention relates to engines or prime movers and in a preferred embodiment to a constant pressure Brayton cycle engine using a positive dis- placement compressor.

BACKGROUND OF THE PRIOR ART

All previous methods of adapting the Brayton (or Joule or constant pressure) continuous cycle for prime mover service have centered around the classical gas turbine concept. A. gas turbine includes three devices: A dynamic (non-positive displacement) compressor of the axial or centrifugal type; a fuel combustor of the direct of indirect type; an output turbine (power wheel) of the impulse or reaction type. The gas turbine approach has many advantages at high power levels (central station power generation, aircraft engines, etc.), but suffers serious economic handicaps at the lower horsepower levels (up to about 300 h.p.) required by automobile service. The compressor and. turbine share a common shaft in the classical gas turbine engine. ^' Thus, the turbine (in a single shaft engine) must drive Its own compressor in additional to driving the load. The reason for this is that only a gas turbine is capable of the ultra-high r.p.m. required by the previously used, high speed, dynamic com- pressors. A simple single shaft, classical engine is shown in Fig. 1

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OMPI

and comprises a compressor 12, a turbine 14, a shaft 16 connecting the compressor and turbine and a combustion chamber 18. Fig. 2 shows two cases of different compress to-output power ratios (Pc/Po) . Fig. 2A shows a compress 20 and a turbine 22 connected together by a shaft 24. T arrangement shown in Fig. 2A has a compressor-to-output power ratio of 0.5. Thus, a 100 h.p. output requires a 0.5 x 100 = 50 h.p. compressor. It follows that the power turbine for this case must supply 50 + 100 - 150 h. Fig. 2B shows a compressor 26 and a turbine 28 connected togetherby a_s_haft~33. In Fig. 2B the compressor-to-output power ratio is 2.0. The 100 h.p. output power for this case requires a 200 h.p. compressor and a 100 + 200 = 300 p. ^" turbine.----Clea ly the overall equipmen - ^' size, ^{' '} for a given output shaft power, depends upon the required compressor ^' size. It can be shown (see "Aircraft Gas Turbines" C. W.

Smith John Wiley - page 43 or "Propulsion Systems" A. N.

Hosny, University of South Carolina Press - page 51) that the compressor to output power ratio can be simply stated as :

^■ where :

Pc/Pό - • Compressor To Output Horsepower Rati T3/T1 = Combustion To Inlet Temperature Rati

(OF, Absolute)

*7 „ = ^** Thermodynamic Efficiency of Turbine

. V7 = Thermodynamic Efficiency of Compress

P2/Pl ⁼ Compressor Outlet To Inlet Pressure _Ratio ^■ jj-» - Ratio of Heat Capacity At Constant

Pressure To That At Constant Volume

For a fixed ambient temperature (T* _j _) equation 1 states that the compressor- o-output power ratio depends only upon the pressure ratio adopted and the combustion temper ture, since the other parameters, * T, , andVare essentially

fixed. Equation 1 is plotted in Fig. 3 and provides a good overall picture of the classical problem. Anything above a compressor-to-output power ratio of 1.0 means a very large compressor indeed. The parameters from Fig. 3 can be transformed to practical values in order to illustrate the classic problem involved and the solution provided by this invention. Consider the case of 50 h.p. net output.

The data from Fig. 3 can now be translated to absolute com¬ pressor ratings and this is plotted in Fig. 4 Typical 0 materials in today's technology allow for a tempera¬ ture ratio ( ₃ /T ₁ of about 3. Thus, Fig. 4

. shows a marked sensitivity to pressure ratio. Absolute efficiency considerations dictate a pressure ratio in the range of 4:1 to 6:1. If a temperature ratio of 5 6 could be tolerated, Fig. 4 shows an insensitivity of compressor requirements to pressure ratio. If we assume an ambient temperature of 70°F, then a. clearer picture of the problem emerges and Is shown in Fig. 5. The steepness of the curve in the vicinity of 1000°F is the cause of the major problems as will become clear from the next curve. A careful review of- commercially available compressors of different types allows the compressor weight vs. horsepower curves of Fig. 6 to be drawn. The highr.p _. m. associated with classical .gas turbines limits severely the tolerable turbine inlet temperature. The relatively cheap materials that must be used for automobile service sets a limit of 1000-1500°F. Fig.. 5 dictates a compressor in the 50-100 h.p. class to supply a net output of 50 h.p. Consider now Fig÷ 6: A piston compressor is completely out of the question since it would weigh between 1500 and 2000 pounds. The Roots type -blower and sliding vane . type compressors are better but also are In the impractical range of 200-600 pounds to supply 50 h.p. ^" of shaft output power. Only the non-positive displacement compressor has a reasonable weight for the classical approach. It is this compressor weight factor alone - not cost nor effi¬ ciency but simply compressor weight that forces classical gas

_4_ turbines to use the non-positive displacement compressor to supply the massive amounts of compressed air that are required.

The penalty paid for achieving the high compressor h.p. to weight ratio for the classic gas turb is severe indeed:

1. The ultra-high r.p.m. is many times great than the automobile r.p.m.

2. The ultra-high r.p.m. of the hot turbine gives rise to excessive tensile stress due to centrifugal forces. This limits both usable materials and allowable inlet temperatures.

3. The compressor mass flow and output pressure vary with r.p.m. The fact that pressure varies means part-load efficienc is poor.

It is important to note that the output turbine requirem does not dictate a high r.p.m., per se; the high r.p.m. is required strictly to satisfy the compressor needs. I fact, there is no other method (electric motor drive, fo example) of achieving the high r.p.m. required.by the compressor.

This then is the state of affairs for gas tur in automobile service. Fuel economy dictates both a hig inlet temperature and a relatively high pressure ratio (about 4:1). Material limitations prevent high combusti temperatures, which in turn results in.both low efficien and high compressor requirements (Fig. 5), thus practica eliminating all but high speed, turbo-compressors (Fig. on a weight basis alone. It is important to note that jet aircraft, central station power, and other non-auto applications can afford to use more expensive materials devices and hence can utilize somewhat higher combustion temperatures. A jet aircraft turbine may cost $50,000 t

-5- build, but an automobile gas turbine must be limited to the $100-$200.class. As a result, gas turbines have found widespread service outside the automobile field only. The main thrust of gas turbine technology today is towards better and cheaper high temperature materials and assemblies, super-alloys, exotic cooling methods, ceramic turbine buckets, cermet technology, plasma coatings, etc. SUMMARY OF THE INVENTION

The present invention is a Brayton cycle engine method and apparatus using a positive displacement compressor. The Brayton cycle engine of the present invention includes a positive displacement compressor, an output wheel connected by a shaft to the compressor, a combustion chamber, and means for feeding a gas to the compressor, means for feeding compressed gas from the compressor to. he combustion chamber, means for burning fuel in the combustion chamber, means for feeding hot gas from the combustion chamber to the output wheel, and means ■for feeding exhaust gas from the output wheel. The compressor can be any suitable positive- displacement compressor such as a sliding vane compressor, a Roots . type blower (sometimes called a screw or gear compressor) ^' , a regenerative blower, or a liquid seal (Nash) ype blower. The output wheel can be any one of the above mentioned devices or in addition can be a dynamic turbine wheel. This removes the classic problems associated with high speed, non-positive displacement compressors: h gh cost and complexity, high stress on hot turbine buckets, low efficiency at part-load, etc. Use of a low speed, positive displacement compressor is made possible by the use of high combustion temperatures as shown in Fig. 5. These high combustion temperatures are

-6- basically made possible by either one of two methods:

1. Use of an intermittent burn-cool operatin cycle as described in copending _. applicati serial No. 879,969; and 2. Use of a bootstrap argument. This result from the basic understanding provided by Figs. .1 through 6. The adoption of a lower r.p.m. means a lower tensile stress on the hot turbine bucket. The reduced stress, permits an Increased temperature since high temperature creep is a main limitation. The increased temperature results in reduced compressor requirement (Fig. 5) which in turn makes positive dis placement compressors (low r.p.m.) feasib

The net result is a cheaper, more efficie and flexible implementation of the Brayto cycle.

Of course, as new high temperature materials, devices, and techniques became available they can be In¬ corporated into the present inventions. However, it is t be clearly understood that the present invention can be practiced with today's materials, devices, and technologi It is desirable to expand Fig. 6 to the 0-50 h range of interest to automobile service. This results in Fig. 7. However, since Roots blowers, sliding vane com¬ pressors, helical screw compressors, etc. have not been engineered with the present problem in mind, it is felt that improvements can be readily made to transform the data in Fig. 7 into the curves of Fig. 8. The basic idea to be conveyed is that virtually any type of positive displacement compressor can be used to implement the present invention.

It is an object of the present Invention to overcome the disadvantages of the Brayton cycle gas turbi

by providing a non-gas turbine implementation of the constant pressure Brayton cycle. It is another object of the invention to provide a Brayton cycle engine having a positive displacement compressor. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood by reference to the following detailed descrip-?. ti'on thereof, when read in conjunction with the attached drawings, wherein like reference numerals refer to like elements and wherein;

Fig. 1 is a partly diagramatic, partly schematic view of a prior art Brayton cycle gas turbine;

Figs. 2A and 2B are diagramatic views of a compressor-turbine-shaft combination; Fig. 3 is a graph of the ratio of compressor power to output shaft power vs. pressure ratio.

Fig. 4 is a graph of required compressor power (h.p.) vs. pressure ratio;

Fig. 5 is a graph of required compressor pox^er (h-P-) vs. combustion temperature (°F);

Fig. 6 is a graph of compressor weight (lbs.) which is compressor power (h.p.);

Fig. 7 is a graph of compressor weight (lbs.) vs. compressor power (h.p.); Fig. 8 is a graph of compressor weight (lbs.) vs. compressor power (h.p.);

Fig. 9 is a partly diagramatic, partly schematic view of an engine according to one embodiment of the present invention including a compressor, an output power wheel and a combustion chamber;

Fig. 10 is a partly diagramatic, partly schematic, view of an engine according to another embodiment of the present invention including a compressor, an output power wheel and an indirect combustion chamber for use in a closed cycle operation;

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Fig. 11 is a graph showing desirabilit vs. horsepower, speed, or torque;

Fig. 12 is a graph showing tensile strength vs. temperature for selected graphites; Fig. 13 is a graph showing vaporization rate vs. temperature for graphite;

Fig. 14 is a graph showing pressure and flow rate vs. r.p.m. for a dynamic turbo-compressor; and

Fig. 15 is a graph showing pressure and flow rate vs. r.p.m. for a positive displacement compressor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the drawings, Fig. 9 shows a substantially constant pressure Brayton cycle engine ^* 40 according to the present invention comprising a; positive displacement compressor 42 and an output power wheel 44 connected to the compressor by a shaft 41 and also connected to an output shaft 48. The engine 40 includes a combustion chamber 46 to which fuel is fed by a fuel line 47. Air is fed into the compressor 42 throug an 'air inlet 43 and compressed air is ^* fed from the com¬ pressor 42 to the combustion chamber 46 through an air line 49. Compressed, hot gas is fed from the combustion chamber 46 through a continuation of the gas line 49 to the output power wheel 44 from which the exhaust gas is fed to ambient through an exhaust line 45. The engine 40 is an open cycle engine as will be clearly understood by those skilled in the art.

Fig. ^" 10 shows a closed cycle engine 50 accordi to another embodiment of the present invention. The engi 50 includes a positive displacement compressor 52 and an output power wheel 54 connected to the compressor by a shaft 56. The output power wheel 54 is also connected to an output shaft 60. The engine 50 also includes an Indirect combustion chamber 58 into which air and fuel are fed by lines 68 and 70 respectively, and from which

_9_ the exhaust gas is fed to ambient through, an exhaust line 72. Compressed gas from the compressor 54 is fed by a line 62 from the compressor to a heat exchanger 74 (such as a coil) in the indirect combustion chamber 58 and from there to the output power wheel 54. The exhaust from the output wheel 54 is fed by a line 64 back to the compressor 52. The exhaust gas from the output power wheel 54 is heat exchanged by a heat ex¬ changer. 66 with the gas fed from the compressor 52 to the combustion chamber 58. The various advantages of the closed cycle are described- in the parent applications . incorporated herein by reference above. It is to be noted that in the open cycle embodiment of Fig. 9, the hot gas fed to the output power wheel is the products of combustion of the burning fuel in air. In the closed cycle embodiment of Fig. 10, on the other hand, the hot gas fed to the output power wheel is whatever is chosen for the working gas, such as nitrogen, neon, carbon dioxide, etc. , According to the present invention, the engines 40 in Fig. 9 and 50 in Fig. 10 can use for the compressor any positive displacement compressor such as, for example: (1) a sliding vane compressor, (2) a Roots type blower (sometimes called a screw or gear compressor), (3) a regenerative blower, or (4) a liquid- seal (Nash) type blower.

The output power wheel in each embodiment can also use any one of the above listed types of devices ^* and in addition can use a dynamic turbine wheel. Thus, 20 different combinations are preferred and each has Its own special application. For example:

1. Rotary vane/Ro ary vane - is preferred for small car automotive service;

2. Rotary vane/Roots blower - is preferred for truck service;

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3. Roots blower/Roots blower ^* - is preferred for low speed, high torque service (farm • tractor, etc.)

In the above, the first name refers to the compressor and the second to the output power wheel or motor. Thus, a rotary vane/roots blower configuration means, a rotary vane compressor and a roots blower run backwards as a motor. It should be noted that compresso run backwards act as efficient motors, see, for example, "Pneumatics and Hydraulics" by H. L. Steward (Audel and Co _. .) illustrate this point. The GastManufacturing offer sliding vane motors for sale in the 1-10 h.p. class (Models 16AM-FCC-1)

These configurations result in a desirability curve vs. horsepower, and/or speed and/or torque as depicted in Fig. 11.

One preferred specific embodiment is to use a sliding vane motor (output power wheel) with the vanes being made of graphite. This offers several advantages.. Graphite is a good lubricant in itself and .thus the • wear on the vanes is minimized-. Second, graphite is know to increase in tensile strength as temperature is increa to about 4500°F as shown in Fig. 12. Thus, this feature allows combustion temperatures up to about 4500?F to be used In the expansion output wheel. Fig. 12 is a graph ultimate tensile strength vs. temperature for selected ^* graphites. All specimens were tested in the direction of major anisotropy. (1) Petroleum coke base, fine grain, ^' extruded, d = 1.67; (2) lampblack base, molded, d = 1.50; (3) petroleum coke base, medium grain, extruded, d = 1.5 (4) petroleum coke base, fine grain, molded, d = 1.75. Graphite is known to have a relatively high vapor pressure at high temperatures. This will result in a vaporization rate. Assuming 100,000 miles of servi life, and 50 miles/hour average speed, then 7 x 10° sec.

BU

-11- of service life is required. If 107 _o of the graphite sur¬ face is allowed to vaporize during this period, then a vaporization rate of about 10~ ⁸ gram/cm ² sec. is tolerable. From Fig. 13 (which is the free vaporization rate of graphite) it is seen that an average temperature of about 3600°F (2200°K) is tolerable. While this temperature greatly exceeds present day gas turbine capabilities, it can be extended ^* further by coating the graphite vanes with a suitable high temperature metal, alloy, or ceramic. In this case, vaporization will occur (if at all), only at the tiny exposed area where the vane rubs the housing- While the above discussion has been with respect to using graphite for the sliding vanes in the motor or output power wheel of the present invention, it is also desirable to use graphite for the sliding vanes in a sliding vane type compressor. Graphite has the advantages that it wears uniformly and conforms by the wear process to the desired shape and that it also provides self lubrication, is relatively inexpensive and is easy to machine, etc.

Another.method of causing a major reduction in the compressor size and weight is to utilize the closed cycle in Fig. 10. This technique does not change ^" the compressor horsepower requirement. This fact is evident from equation 1. The compressor-to-output power ratio depends only upon the pressure ratio and not the absolute gas pressure. However, at higher total pressures made possible by the closed gas system, the compressor "swept ^" - out volume" becomes less due to the higher gas density. As a result, the key item (the compressor weight) is reduced. This allows for an extension of this invention to higher power levels. Thus, for example, the vane/vane desirability curve can now take the dashed position shown in Fig. 12.

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It should be noted that a turbine does not require the same high tip speed (high r.p.m.) as does the turbo-compressor. Thus, combinations like vane/turb and roots blower/turbine are useful configurations. Whil c the main thrust of this invention is the automobile, it is to be understood that the engine of this invention is also applicable to trucks, locomotives, central statio power, remote station power generation, emergency and stand-by power generators, aircraft, etc. 10 The present invention can be used with multipl shaft arrangements, multiple stage compressors, multiple stage expansion stages (motor output), interstage cooling re-heat, and various techniques of heat exchange. ^" The Intermittent burn-cool operation cycle of parent applicat -15 serial No. 879,969, the throttle control invention of par application serial No. 890,465, the various flywheels and compressed gas surge tanks, etc. taught in said parent applications are also applicable in this invention.

One major advantage of the present invention 20 over the classical gas turbine for automotive service is in *the matching of r.p.m. The wheels of an automobile rotate at a maximum of about 1500 r.p.m. Thus, complicat speed reducers are required to match a gas turbine, at 50,000 r.p.m., to an automobile. The r-.p.m. of the prese 25 invention can be selected to closely match that of the au mobile by proper selection of compressor and power wheel diameters.

Another major advantage of this invention over the prior art is the efficiency at part-load. In fact, 0 the present teachings maintain efficiency essentially dow to 0 r.p.m. This statement can best be visualized with the aid of Figs. 14 and 15. The classical turbo-compress relies upon high impeller tip speed to provide both mass flox? and pressure. Thus, a reduction in r.p.m. causes 5 both mass flow rate and pressure to be reduced -as shown i

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Figs. 14. The flow rate reductionat low r.p.m. is acceptable since low r.p.m. (part-load) requires only a portion of the air mass flow. However,- efficiency considerations insist upon a high gas pressure, in fact, optimal conditions require a narrow band of gas, pressures (pressure ratio in 4-6 range). Thus, the low r.p.m. drops the pressure ratio and introduces the well-known part-load inefficiency. This is not true for the positive ^' displacement compressor of the present invention as shown 0 in Fig. 15. The compressor outlet pressure does* not depend upon speed but only upon the geometry of the com¬ pressor apparatus. The flow rate is a direct linear function of the r.p.m. This is the ideal state of affairs for part-load. As more fuel is pumped to the combustor, 5 the power output wheel (sliding vane, turbine, more output power and higher r.p.m. The higher r.p.m. provides more compressed gas, as it should, at the ^■ same pressure and thus the efficiency, and combustion temperature conditions do not change. 0 The following chart may be found useful:

TYPICAL COMMERCIAL UNITS

Horsepower Weight Model No.

Room Temp. 2500° F* (pounds)

4.0 20.0 21.0 6AM-FRV-23A 10.0 ^' 50.0 80.0 16AM-FRV-13 40.0 200.0 600.0 RAI-88

The invention has been described in detail with particular reference to the preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention as described hereinabove and as defined in the appended claims.