US3604211A

US3604211A - Combined pulse jet and variable ram jet engine

Info

Publication number: US3604211A
Application number: US857497A
Authority: US
Inventors: John N Ghougasian
Original assignee: Individual
Current assignee: Individual
Priority date: 1969-09-12
Filing date: 1969-09-12
Publication date: 1971-09-14
Anticipated expiration: 1988-09-14

Abstract

A reaction thrust engine operating in a pulse jet mode to attain cruising speeds, is converted to ram jet operation by variations in the internal flow controlling geometry and location of the combustion zone. This is accomplished by an axially shiftable center body and a tubular check valve assembly.

Description

United States Patent John N. Ghougasian 666 West 188th St., New York, N.Y. 10040 [2]] App]. No. 857,497

[22] Filed Sept. 12, 1969 [45] Patented Sept. 14, 1971 Continuation-in-part of application Ser. No. 822,980, May 8, 1969, now Patent No. 3,533,239.

[72] Inventor [54] COMBINED PULSE JET AND VARIABLE RAM JET ENGINE v 7 Claims, 15 Drawing Figs.

[52] US. Cl.. 60/244 [5 1] Int. Cl F02k 7/06, .Q2JS.7

[50] Field of Search 60/244, 39.33

[56] References Cited UNITED STATES PATENTS 2,677,232 5/1954 Collins 60/244 2,745,248 5/1956 Winter 60/244 2,850,872 9/1958 Stockbarger. 60/244 3,078,660 2/1963 Hansel 60/244 .Primary Examiner-Douglas Hart Attorneys-Clarence A. OBrien and Harvey B. Jacobson ABSTRACT: A reaction thrust engine operating in a pulse jet mode to attain cruising speeds, is converted to ram jet operation by variations in the internal flow controlling geometry and location of the combustion zone. This is accomplished by an axially shiftable center body and a tubular check valve assembly.

PATENTED SEP 1 4 I97! SHEET 1 BF 5 AT Q Wan 3m PATENTED'SEPI 41911 sum 3 or 5 John N. Ghougasian INVIs'NI'OK.

PATENTEU SEPI 4:971

SHEET 5 OF 5 John N. Ghougas/an COMBINED PULSE JET AND VARIABLE RAM JET ENGINE This invention relates to propulsion engines of the reaction type and more particularly to a reaction jet engine having pulse jet and ram jet modes of operation similar to the combined pulse and ram jet engine disclosed in my prior copending application Ser. No. 822,980, filed May 8,1969, now U.S. Pat. No. 3,533,239, with respect to which the present application is a continuation-in-part.

In my prior copending application aforementioned, a reaction jet engine is disclosed having means for altering its internal geometry in order to accommodate both pulse jet and ram jet operation. In this previously disclosed engine, a movably mounted check valve was utilized for pulse jet operation which required retracting mechanism and protuberances on the engine housing for check valve storage during ram jet operation. An important object of the present invention therefore is to eliminate engine protuberances required for storing a retractable check valve assembly and the aerodynamic disadvantage inherent therein.

In one embodiment of the present invention, the intake geometry during pulse jet operation chokes any upstream movement of the combustion zone so as to confine it to a proper location within the engine housing. In other forms of the invention, a check valve assembly is rendered effective during pulse jet operation and ineffective during ram jet operation by altering the internal flow passage geometry. The flow passage geometry is altered either by axial shift of an intake center body between positions blocking inflow through the intake end for pulse jet operation and a position admitting inflow through the intake end for ram jet operation. Flow is directed radially through a tubular type of check valve bypassed during ram jet operation without any serious adverse effect on ram jet operation.

These together with other objects and advantages which will become subsequently apparent reside in the details of construction and operation as more fully hereinafter described and claimed, reference being had to the accompanying drawings forming a part hereof, wherein like numerals refer to like parts through, and in which:

FIG. I is a side sectional view through one form of jet engine construction in accordance with the present invention shown in a pulse jet mode of operation.

FIG. 2 is a side sectional view of the engine shown in FIG. 1 but in a ram jet mode of operation.

FIG. 3 is an enlarged transverse sectional view taken substantially through a plane indicated by section line 3-3 in FIG. 2.

FIG. 4 is a partial sectional view taken substantially through a plane indicated by section line 44 in FIG. 3.

FIG. 5 is a longitudinal sectional view through a second form of reaction jet engine constructed in accordance with the present invention.

FIG. 6 is a transverse sectional view taken substantially through a plane indicated by section line 6-6 in FIG. 5.

FIG. 7 is a longitudinal sectional view through a third form of jet engine constructed in accordance with the present invention.

FIG. 8 is a transverse sectional view taken substantially through a plane indicated by section line 8-8 in FIG. 7.

FIG. 9 is a longitudinal sectional view of the engine shown in FIG. 7 but in a ram jet mode of operation.

FIG. 10 is a longitudinal sectional view through a fourth form of jet engine shown in a pulse jet mode of operation.

FIG. 11 is a longitudinal sectional view through the engine shown in FIG. 10 but in a ram jet mode of operation.

FIG. 12 is a longitudinal sectional view through a fifth form of jet engine constructed in accordance with the present invention shown in a pulse jet mode operation.

FIG. I3 is a longitudinal sectional view through the engine shown in FIG. 12 but in a ram jet ofoperation.

FIG. 14 is a longitudinal sectional view through a sixth form of jet engine constructed in accordance with the present invention, shown in a pulse jet mode of operation.

FIG. 15 is a longitudinal sectional view through a jet engine as shown in FIG. 14, but in a ram jet mode operation.

Referring now to the drawings in detail, FIGS. 1 and 2 illustrate one example of a combined pulse and ram jet engine constructed in accordance with the present invention,

generally referred to by reference numeral 10. The engine includes a generally tubular housing 12 extending from an intake end 14 to an outwardly flaring exhaust end 16 from which thrust-producing gases are discharged. The housing 12 includes a forwardly convergingfrontsection 18 internally provided with radial struts 20 which support an axially fixed center body 22 having a streamlined nose portion 24 projecting forwardly from the intake end 14 of the housing. The center body also includes a forwardly converging section 26 within the front end section of the housing to define an annular intake passage 28 thereabout. The center body is also provided with a rearwardly converging conical section 30 having a downstream end on which a fuel injection nozzle assembly 32 may be mounted as well as fuel ignition means 34 located just downstream thereof.

The housing 12 is also provided with an annular cavity 36 that extends from the exhaust end 16 toward the front end section 18 so as to slidably mount an axially shiftable, tubular housing extension 38 provided with an outwardly flaring portion 40 adapted to be nested within the tail end section 42 of the fixed portion of the housing. Thus, the housing extension 38 will be axially shifted from an extended position as shown in FIG. 1 accommodating pulse jet operation of the engine to a retracted position as shown in FIG. 2 accommodating ram jet operation of the engine.

The housing also slidably mounts internally thereof, a noz- 216 member 44 which is internally formed with a convergent passage section 46 and a divergent passage section 48 intersecting at a throat portion 50. In the pulse jet mode of operation as shown in FIG. 1, the nozzle member 44 is in a position adjacent the front end section 18 of the housing so as to form a convergent extension of the intake passage about the rear conical section 30 of the center body. This geometry of the intake passage during pulse jet operation is thereby effective to choke any rearward extension of the combustion zone established by ignition of a fuel and air mixture just rearwardly of the ignition means 34 within the divergent passage section 48 of the nozzle member 44. Further, in the pulse jet mode of operation, the tubular housing extension 38 is in its extended position so as to more closely establish the geometry for resonance with the pulsating combustion frequency that characterizes operation of a pulse jet engine. The outwardly flaring section 40 of the housing extension will accommodate the inflow of air from the exhaust end of the extended housing between the combustion phases of the pulse jet cycles of operation while the flow area of the intake passage will be sufficient to supplement the inflow of air to the combustion zone from the exhaust end of the engine as well as to choke any forward extension of the combustion zone thereby eliminating the need for a check valve assembly.

During ram jet operation at the higher speeds at which pulse jet operation becomes inefficient, the nozzle member 44 is axially shifted to its rearmost position as illustrated in FIG. 2 while the housing extension 38 is retracted so as to shorten the effective length of the housing. The combustion zone is then relocated within the convergent flow section 46 of the nozzle member just rearwardly of the ignition means 34 which is suitable for ram jet combustion with the combustion products being directly conducted to the exhaust end as a continuation of the divergent passage section 48 of the nozzle member. The nozzle member in its rearmost position will offer no obstruction to the inflow of air from the intake end 14 of the engine hosing, the velocity of which is sufficient to compress the air and confine the combustion zone rearwardly of the center body 22. If desired, the axially fixed center body 22 may mount flow restricting means in order to match the inlet flow area to the exit nozzle flow area during pulse jet and ram jet operation respectively.

FIGS. 3 and 4 illustrate the mechanism for synchronizing axial shift of the nozzle member 44 with the extension and retraction of the housing extension 38. Thus, the housing extension may be provided with a plurality of rack formations 52 circumferentially spaced from each other by equal amounts extending longitudinally from the forward end of the housing extension rearwardly in parallel-spaced relation to the longitudinal axis of the housing, within the annular cavity 36. Each rack formation 52 includes a predetermined axial length of radial outer rack teeth 54 and a shorter axial length of radially inner rack teeth 56. A recessed blank portion 58 is accordingly formed on the radially inner, forward end portion of each rack formation. The radially outer rack teeth 54 meshingly engage a rack gear pinion 60 as shown in FIG. 4 which is rotatably mounted by the housing 12 for rotation about a fixed axis. A second rack gear pinion 62 is rotatably mounted by the housing adjacent the same axial location for meshing engagement with the radially inner rack teeth 56 and with rack teeth 64 formed on the radially outer surface of the nozzle member 44. Power-operated means (not shown) may be drivingly connected to the rack gear pinion 60 so as to impart rotation thereto in reverse directions for axially shifting the housing extension between its extended and retracted positions as respectively shown in FIGS. 1 and 2. Axial movement of the housing extension 38 in this manner will produce simultaneous axial movement of the nozzle member 44 in the opposite axial direction when the housing extension is being displaced from its retracted position as shown in FIGS. 2 and 4, by virtue of the meshing relationship of the rack pinion 62 to the radially inner rack teeth 56 on the housing extension and the rack teeth 64 on the nozzle member. However, since the axial stroke of the nozzle member is shorter than that of the housing extension, after the nozzle member reaches its forwardmost position as shown in FIG. I, rearward axial movement of the housing extension will continue to the fully extended position without continued forward movement of the nozzle member since the radially inner rack teeth 56 will no longer be in mesh with the rack pinion 62. It will therefore also be apparent that when retracting movement is imparted to the housing extension 38, rearward movement will not be imparted to the nozzle member 44 until the housing extension has moved forwardly a certain distance bringing the radially inner rack teeth 56 into meshing engagement with the rack pinion 62.

FIG. illustrates a second type of jet engine 64 which is similar to the jet engine in the utilization of a nozzle member 66 rearwardly shiftable from a forwardmost pulse jet position as shown to a ram jet position as shown by dotted line. A center body 68 is associated with the engine 64 having a rear hollow section mounting a cylindrical type of check valve assembly 70 through which intake air may flow radially inwardly only from intake passage 72 and exit axially from the downstream end portion of the center body into the divergent flow section of the nozzle member 66 in its forwardmost position as shown in FIG. 5. In its forwardmost position, the nozzle member engages the downstream end portion he center body in order to confine inflow of air in a radially inward direction through the check valve assembly for pulse jet operation. Fuel is injected and ignited by a nozzle and spark plug assembly 75 during pulse jet operation. It will be apparent, that inflow of air from the intake passage 72 will effectively bypass the check valve assembly during ram jet operation when the nozzle member is in its rearmost position. Axial movement of the nozzle member is effected through any suitable gear means 73. Fuel is then injected by nozzles 77 in passage 72 to accommodate ram jet operation. Other aspects of engine operation are the same as that described in connection with engine 10.

FIG. 6 illustrates the details of the cylindrical check valve assembly 70. As shown, the check valve assembly includes an annular frame portion 110 at the downstream end interconnected with an annular frame portion 1 12 at the upstream end by means of a plurality of spacing bars 114. The spacing bars also slidably mount at circumferentially spaced locations, a plurality of elastically flexible, flap elements 116 which normally engage each other with a predetermined tension to close the axial gap in the intermediate portion of the housing occupied by the check valve assembly 102. In response to the inflow pressure of the air within passage 100, the flap elements 116 separate to permit radial inflow of air into axial flow passage of the housing 76. The closing bias of the check valve assembly is determined by the axial force exerted by springs 118 on an axially shiftable anchor element 120 transmitted to the elastically flexible elements I 16 at one axial end. The bias of springs 118 may be adjusted to change the opening characteristics of the check valve assembly by means of adjustment screw elements 122 on which the springs 118 are seated within I the end frame portion 122 as shown in FIG. 6.

In the forms of the invention hereinbefore described, internal engine geometry is altered in order to accommodate pulse jet and ram jet operations by axial shift of an internal nozzle and corresponding changes in the axial length of the engine housing involving relative axial relocation of the combustion zone. FIGS. 7, 8 and 9 show an engine 174 wherein the internal engine geometry is altered by axial shift of an intake center body 176 relative to a fixed exhaust portion between a forward pulse jet position shown in FIG. 7 and a rear ram jet position shown in FIG. 9. In the forward position, the center body blocks inflow through the forwardly converging intake portion 178 of housing 177 so that a restricted inflow occurs through orifices 180 formed in the portion of the center body projecting from the intake end of the housing. Flow from the center body to the combustion zone in the housing is conducted radially outwardly through a one-way check valve assembly 182 which is similar in construction to the check valve assembly 70 of FIG. 5 except for the radial direction in which flow is blocked. Wen the center body is shifted to its rearward position shown in FIG. 9 by any suitable mechanism (omitted from FIGS. 7 and 9 for sake of clarity), the intake end of the housing is opened causing a rearward shift of the combustion zone under ram jet operation as most of the inflowing air bypasses the check valve assembly 182 which is then rendered ineffective. The flame produced in the combustion zone is confined to a downstream location by a flame holder assembly 184 axially shiftable relative to the fixed exhaust portion by its mounting the center body 176. Fuel is injected during pulse and ram jet operation through nozzles 186 mounted by the support 188 for the center body.

FIGS. 10 and 11 illustrate yet another type of jet engine constructed in accordance with the present invention wherein the internal engine geometry is also altered by axial shift of the center body at the intake end portion of the engine housing. Thus, the jet engine generally referred to by reference numeral 74 in FIGS. 10 and l l is provided with a tubular housing 76-of axially fixed length as in the case of engine 174 in FIGS. 7, 8 and 9. The housing is provided with a forwardly converging intake portion 78 connected to a cylindrical intermediate portion 80. An internally converging passage portion 82 of the housing extends from the intermediate portion 80 to an inter nally divergent exhaust portion 84. The housing adjacent the portion 78 is provided with support structure 86 for movably mounting a center body 88 as well as to fixedly mount a fuel injecting nozzle assembly in surrounding relation to the rearwardly converging section 92 of the center body. The forwardly converging section 94 of the center body in its forwardmost position as illustrated in FIG. 10, projects from and closes the forward end portion 78 of the housing so as to block any inflow air therethrough during pulse jet operation.

The engine housing 76 slidably mounts in coaxial relation thereto, a tubular shroud 96 having a radial inwardly projecting slide bearing portion 98 rearwardly terminating an annular fuel carburetion passage established about the forward end portion 78 of the housing and the intermediate portion 80. The tubular shroud 96 is displaceable from a forwardmost position as shown in FIG. axially abutting a cylindrical check valve assembly 102 similar in construction and purpose to the check valve assembly 70 described in connection with FIG. 6. In the forwardmost position of the tubular shroud 96 as shown in FIG. 10, the slide bearing 98 externally covers and thereby blocks flow of carbureted fuel through orifice nozzles 104 into the axial flow passage of the housing 76 just downstream of the check valve assembly 102. However, in the rearmost position of the tubular shroud as shown in FIG. 11, the annular slide bearing 98 in engagement with the abutment 106, uncovers the fuel mixture injection nozzles 104 through which fluid communication is established between the intake passage 100 and the axial flow passage of the housing 76. Fuel injection nozzles 108 are mounted within the carburetion passage 100 for continuous supply of fuel mixing with the inflowing air during both pulse jet operation and ram jet operation.

During pulse jet operation, the forward end portion 78 of the housing is closed by the center body 88 so that inflow of air occurs exclusively through the fuel carburetion passage 100 within which fuel is mixed with the air. Since the tubular shroud 96 is in its forwardmost position during pulse jet operation as shown in FIG. 10, maximum inflow of air into the passage 100 is accommodated for proper fuel carburetion. Also, since the slide bearing 98 abuts the downstream end portion of the check valve assembly 102, the fuel mixture passes radially inwardly through the check valve assembly and enters the combustion zone within the housing 76. Operation may be initiated by ignition of high-pressure air and fuel injected into the housing through the fuel nozzle assembly 90. Once the fuel mixture is initially ignited, pulse jet operation may ensue by supply of the carbureted fuel mixture from passage 100 through the check valve assembly which blocks flow of combustion products radially outwardly into the passage 100 while outflow of combustion products from the housing at the forward end portion 78 is blocked by the center body 88.

During ram jet operation, the center body and the tubular shroud 96 are shifted to their rearmost position as shown in FIG. 9, by any suitable means, thereby accommodating substantial axial inflow into the flow passage of the housing 76 through the forward end portion 78 and rearward relocation of the combustion zone. Inflow continues into the passage 100 for fuel carburetion despite the reduced inflow area at its forward end portion 124 because of the high velocities at which the engine is propelled during ram jet operation. Further, the compression of the air mixing with the fuel within passage 100 causes ignition in the annular zone 126 forwardly of the slide bearing 98. The ignited fuel mixture within the annular zone 126 is therefore effective through the orifice nozzles 104 uncovered by the slide bearing 98 during rarn jet operation to ignite the fuel mixture within the combustion zone of the housing 76 just downstream of the check valve assembly 102. Fuel for mixing with the axial inflow of air during rarn jet operation is supplied by the fuel-injecting nozzle assembly 90. Further, it will be apparent that during ram jet operation, the pressure of the fluid within the housing will maintain the check valve assembly 102 closed.

FIGS. 12 and 13 show a jet engine 128 similar to the jet engines 174 and illustrated in FIGS. 7, 8, 9, and 10 and 1 1 in that a tubular housing 130 of fixed length is utilized with the internal geometry being altered by an axially shiftable center body 132. A tubular shroud 134 is also utilized in order to enclose an annular intake passage 136. The tubular shroud 134 is however flxedly mounted on the tubular housing 130 and is located generally downstream of the center body 132. Enclosed within the tubular shroud is a cylindrical type of check valve assembly 138 similar in construction and function to the check valve assembly 102 in jet engine 74 or check valve assembly 182 of engine 174 except for the direction of flow. The check valve assembly 138 controls one-way radial inflow of air from the intake passage 136 to the axial flow passage of the housing 130 just upstream of a plurality of fuel injection nozzles 140 mounted by the cylindrical intermediate section 142 of the housing located upstream of the exhaust tail end portion 144. The ram intake portion 146 of the housing within which the axially shiftable center body 132 is mounted, is preceded by an intake compression stage portion 148 of the housing within which velocity-to-pressure recovery occurs during ram jet operation.

As shown in FIG. 12, during pulse jet operation, the center body is in its forwardmost position blocking inflow through the intake portion 146 of the housing. Inflow of air therefore is received only through the tubular shroud 134 and passes radially inwardly from the intake passage 136 through the check valve assembly 138 to the combustion zone. A continuously energized flow plug device 150 is mounted in a cavity at the downstream end of the center body 132 in order to maintain combustion just downstream of the check valve assembly 138.

The fuel injection nozzle assembly 152 mounted in surrounding relation to the center body is utilized during ram jet operation when the center body is displaced to its rearmost position as illustrated in FIG. 13 opening the intake section 146 of the housing. The axial inflow of air compressed because of its high velocity within the compression stage portion 148, is at a substantially higher pressure than the air within the intake passage 136, to effectively close the check valve assembly during ram jet operation as in the case of the jet engine 74.

A jet engine 154 also similar to the jet engine 74 is shown in FIGS. 14 and 15. In FIG. 14, the axially shiftable center body 156 in its forward position closes the forward intake end portion 158 of the tubular housing 160 so that inflow occurs only through the tubular shroud 162 and the cylindrical check valve assembly 164 to the axial flow passage of the housing. Fuel is injected through the nozzles 166 just downstream of the check valve assembly during pulse and ram jet operation as in the case of the jet engine 128 of FIGS. 12 and 13. During ram jet operation, the center body 156 is axially shifted to its rearmost position as shown in FIG. 15 opening the forward end portion 158 of the housing so as to receive an axial inflow of air closing the check valve assembly 164 as in the case of the jet engines 74 and 128. A pivotaily retractable flame holder 168 is mounted on the rear section of the center body 156 as in the case of engine 174 in order to insure that the combustion zone does not move upstream to the intake portion during ram jet operation. The direction of flow through the check valve assembly 164 is reversed from that of valve assembly 182 of engine 174 so that valve assembly 164 is axially fixed relative to the shiftable center body. In flow during pulse jet operation is therefore conducted through an outer passage from an annular inlet in engine 154 rather than a radially inner passage from a perforated inlet portion as in engine 176. It will, of course, be appreciated that the foregoing variants of

engines

154 and 176 could be interchanged. Also, fuel injection nozzles 170 are mounted on the center body support 171 for injection of fuel during ram jet operation. In the jet engine 154, the center body projects forwardly from the forward end portion 158 of the engine housing unlike the arrangement illustrated in FIGS. 12 and 13 so that shock waves 172 as shown in FIG. 14 will occur forwardly of the tubular shroud 162 within which fuel carburetion occurs. The nozzles 166 will be operative to effect flame injection into the combustion zone during ram jet operation.

The foregoing is considered as illustrative only of the principles of the invention. Further, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and operation shown and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the invention.

What is claimed as new is as follows:

1. A combined pulse and ram jet engine comprising a tubular housing having an intake end portion and an exhaust end portion, means mounted within the housing for establishing a combustion zone sustaining combustion f a fuel and air entering at least one of said end portions of the housing, internal control means mounted by the housing for movement between pulse jet and ram jet positions to simultaneously change the inflow geometry of said one of the end portions and the axial location of said combustion zone within the housing, restricted inlet means through which an axial inflow of air is conducted from the intake end portion in the pulse jet position of the internal control means and tubular valve means through which said axial inflow of air is conducted radially in one direction relative to the internal control means into the combustion zone.

2. A combined pulse and ram jet engine comprising a tubular housing having an intake end portion and an exhaust end portion, means mounted within the housing for establishing a combustion zone sustaining combustion of a fuel and air entering at least one of said end portions of the housing, means movably mounted by the housing for simultaneously changing the inflow geometry of said one of the end portions and the axial location of said combustion zone within the housing, said movably mounted means comprising an axially shiftable center body displaceable between a pulse jet position blocking the intake end portion of the housing and a ram jet position opening said intake end portion, and a one-way check valve mounted on the center body through which flow of the air is conducted radially.

3. A combined pulse and ram jet engine comprising a tubular housing having an intake end portion and an exhaust end portion, means mounted within the housing for establishing a combustion zone sustaining combustion of a fuel and air entering at least one of said end portions of the housing, means movably mounted by the housing for simultaneously changing the inflow geometry of said one of the end portions and the axial location of said combustion zone within the housing, said movably mounted means comprising an axially shiftable center body displaceable between a pulse jet position blocking the intake end portion of the housing and a ram jet position opening said intake end portion, a one-way check valve mounted upstream of the combustion zone through which air is conducted during pulse jet operation, the check valve being tubular in shape and conducting air radially outward from the center body.

4. The combination of claim 1 including flame holder means mounted on the center body.

5. A combined pulse and ram jet engine comprising a tubular housing having an intake end portion and an exhaust nozzle end portion, axially shiftable means mounted adjacent the intake end portion for displacement between pulse jet and ram jet positions, flame holder means mounted on and movable with the axially shiftable means for controlling the location of a combustion zone within the housing, tubular valve means operatively mounted within the housing upstream of the flame holder means for radially conducting an inflow of air to the combustion zone only in the pulse jet position of the axially shiftable means and restricted inlet means for conducting an axial inflow of air to the valve means in the pulse jet position of the axially shiftable means, an inflow of air being conducted from the intake end portion of the housing in bypass relation to the valve means in the ram jet position of the axially shiftably means.

6. The combination of claim 2 including perforated inlet means through which said inflow of air is conducted in the pulse jet position of the axially shiftable means.

7. The combination of claim 1 wherein the internal control means includes a movable flame holder controlling the axial location of the combustion zone.