US2713857A - Manifolding system with four-barrel carburetor - Google Patents

Description

P. F. KEYDEL July 26, 1955 MANIFOLDING SYSTEM WITH FOUR-BARREL CARBURETOR 2 Sheets-Sheet 1 Filed Dec.

July 26, 1955 P. F. KEYDEL 2,713,857

MANIFOLDING SYSTEM WITH FOUR-BARREL CARBURETOR Filed Dec. 1, 1951 2 Sheets-Sheet 2 V gmenbr 5 fay/42% slimme 5 United States Patent 0 MANIFOLDIN G SYSTEM WITH FOUR-BARREL CARBURETOR Paul F. Keydel, Detroit, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware Application December 1, 1951, Serial No. 259,445

13 Claims. (Cl. 123-122) This invention relates to charge forming devices for engines and has a particular relation to an inlet manifold for use with a compound or other carburetor for supplying explosive mixtures to cylinders in V-type or other internal combustion engines.

It is proposed to increase the volumetric efiiciency of engines and to more nearly equalize the distribution of combustible mixture among the various cylinders thereof by employing a compound carburetor for supplying different combustible mixtures under different conditions of operation to the middle portion of a main distribution passage, the opposite ends of which are formed to provide ports for supplying the mixtures to a group of cylinders of the engine.

The compound carburetor has a primary mixture supply passage which supplies combustible mixture to the main distribution passage throughout all conditions of operation of the engine and has a secondary mixture supply passage which supplies combustible mixture to the main distribution passage under conditions of operation of the engine when the mixture supplied by the primary mixture supply passage is inadequate in quantity or improper in fuel and air ratio or both.

In order to improve the distribution of the fuel mixture supplied by the primary mixture supply passage it is proposed to locate the junction between the primary supply and main distribution passages nearer the middle of the main distribution passage than the junction between the secondary supply passage and the main distribution passage. Since the primary supply passage supplies to the main distribution passage all of the combustible mixture employed in starting the engine and in operating the engine until the engine is warm enough for normal operation, it is proposed to heat the primary mixture supply passage and the junction thereof with the main distribution passage. A heated charge therefore may be employed in starting the engine. To accomplish this purpose, the charge forming device is provided with exhaust or other suitable heating means for heating a wall of the main distribution passage opposite the junction of the main distribution passage with the primary mixture supply passage and the wall of the primary supply passage anterior to the junction referred to. If a carburetor having primary and secondary mixture supply passages is formed as a single heat conductive unit, then there may be some heating of the secondary mixture supply passage and the wall of the main distribution passage opposite the junction between the two passages. But this heating will not noticeably affect the volumetric efliciency of the engine under normal operating conditions. The heat supplied will be mostly absorbed by the mixture supplied by the primary mixture supply passage and little will be conducted to the secondary mixture supply passage. Also it is proposed to cut off the supply of heat to the heating means referred to after the engine is sufliciently warmed for normal operation. Since the secondary mixture supply passage does not supply mixture to the main distribution passage during the time when heat is supplied to the'heating 2,713,857 Patented July 26, 1955 means, there will be little heating of the secondary passage and the junction thereof with the main distribution passage when the secondary passage later is employed in supplying mixture to the main distribution passage.

In the present instance it is proposed to employ the charge forming device with a V-type engine in which the cylinders are divided into two groups of four cylinders each and in which one main distribution passage supplies mixture to each group of four cylinders. In such an arrangement, each main distribution passage is supplied with a primary mixture supply passage and a secondary mixture supply passage, the junctions thereof with the main distribution passages being located as previously described.

It is also proposed to employ liquid fuel collecting sumps in the walls of the main distribution passages, these being located directly opposite the primary mixture supply passages. These liquid fuel sumps are heated by the heat supplied to the junctions between the passages to prevent unvaporized liquid from being delivered to the cylinders of the engine.

Particularly with V-type eight cylinder engines it is proposed to employ a cross-over passage between the exhaust passages of one or more of the middle cylinders of each group of cylinders of the engine. This cross-over passage may be formed integrally with the inlet manifold unit or body in which the main distribution passages and the primary and secondary mixture supply passages are formed. The walls of the heating or cross-over passage and the main distribution passages are provided with fins for extending the surface thereof and these fins are constructed and arranged in such a way that uniform heating of the main distribution passages will be provided.

In the drawings:

Figure 2 is a fragmentary end elevational view of an engine having a charge forming device embracing the principles of the invention.

Figure 1 is a plan View with parts thereof broken away to show the interior of the structure of the inlet manifold part of the charge forming device illustrated by Figure 2.

Figure 3 is a fragmentary plan view of the inlet manifold disclosed by Figure 1, with parts thereof broken away and shown in section at the junctions between the primary supply and main distribution passages of the manifold.

Figure 4 is a fragmentary vertical sectional view of the inlet manifold and the heating passage therefor taken substantially in the plane of line 44 of Figure 1.

Figure 5 is another vertical sectional view of the manifold illustrated by Figure 1. Figure 5 is taken along the broken dot and dash line on Figure 1 looking in the direction of the arrows thereon.

Figure 6 is a side elevational view of the manifold disclosed by Figure 1. Figure 6 shows certain parts of the manifold broken away across the heating, main distribution and supply passages of the manifold in order better to illustrate the interior construction of the manifold structure.

The engine 10 employed in practicing the invention is a V-type eight cylinder internal combustion engine especially applicable for the operation of motor vehicles. The engine 10 includes a block 11 having banks of cylinders 12 and 13 provided with heads 14 and 16 respectiveiy. The cylinders in the banks 12 and 13 are provided with exhaust manifolds 17 and 18 adapted to discharge the exhaust gas from the engine to the atmosphere through the exhaust pipes 19 and 21 respectively. The exhaust pipe 19 is adapted to be cut off by a thermostatically controlled heat control valve indicated at 20 which will open when the engine is warm to permit the exhaust gas to escape through the exhaust pipe 19.

The engine it) is provided With an inlet manifold 22 having outlet ports 23 adapted to be connected to inlet passages formed in the heads 14 and 16 and leading to the different cylinders of the engine 10. The manifold 22 is supplied with combustible mixtures for operating the engine by a compound carburetor indicated at 26. The manifold 22 is preferably formed of an integralcast body having formed therein a pair of main distribution passages 27 and 28. The main distribution passages 27 and 28 are approximately of equal length and cross-sectional area, extend in parallel relation to one another and the banks of cylinders of the engine 10, and the distribution passage 27 is arranged somewhat above the distribution passage 28 so that each passage may be readily employed for supplying mixtures to certain cylinders in each of the banks of cylinders of the engine. The distribution passage 27 has branch passages 29 and 31 extending in opposite directions from one end thereof for supplying an end cylinder of the bank 12 and a middle cylinder of the bank 13 respectively. The opposite end of the passage 27 is also branched at 32 and 33 for supplying the opposite end cylinder of the bank 12 and the remaining middle cylinder of the bank 13 respectively. The main distribution passage 28 is also provided with oppositely disposed branches 34 and 36 for supplying respectively a middle cylinder of the bank 12 and an end cylinder of the bank 13. The opposite end of the main distribution passage 28 is similarly branched at 37 and 38 for supplying respectively the remaining middle cylinder of the bank 11 and the remaining end cylinder of the bank 13. The branches 29, 31, 32, 33, 34, 36, 37 and 38 of the main distribution passages 27 and 28 are all of substantially equal length and cross-sectional area so as to provide substantially equal distribution of combustible mixture to all of the cylinders of the engine 10. It will be apparent that the main distribution passage 27 supplies two end cylinders of the bank 12 and two middle cylinders of the bank 13, while the main distribution passage 28 supplies two middle cylinders of the bank 12 and two end cylinders of the bank 13. The cylinders of the engine therefore are divided into two different groups, one group being supplied by the main distribution passage 27, and the other by the main distribution passage 28. Since the main distribution passage 27 is located somewhat above the main distribution passage 28, it will be apparent that the branch passages extending from each of the main disribution passages can cross over with respect to one another Without unduly distorting any of the passages. Also, it will be apparent that the main distribution passages 27 and 28 are curved to a slight extent toward one another and at the opposite ends thereof so as to permit the main distribution passages to join the branch passages at approximately the middle of the combined lengths of the branch passages.

The middle parts of each of the main distribution passages 27 and 28 are adapted to be supplied with combustible mixtures by one of a pair of primary mixture supply passages 39 and one of a pair of secondary mixture supply passages 41. The primary and secondary mixture supply passages 39 and 41 terminate in the manifold 22 and originate in the compound carburetor 26 where the carburetor jets form separate mixtures for each of the pairs of passages 39 and 41. The primary supply passages 39 are controlled by an automatic choke valve mechanism indicated at 42 which is adapted to restrict the passages 39 anterior to the carburetor jets for the purpose of supplying a richer mixture during the starting of the engine than at other times during engine opertaion. The carburetor 26 also is provided with the usual butterfly-type throttle valves for each of the passages 39 and 41. The passages 39 may be provided with throttle valves secured to the same shaft and operating as a unit for controlling the supply of mixture through the primary mixture supply passages 39. The secondary mixture supply passages 41 also may be provided with butterfly-type throttle valves on the same shaft and which likewise operate as a unit within the carburetor 26 for controlling the supply of secondary mixture to the main distribution passages 27 and 28. The carburetor 26 is provided with a throttle operating mechanism 43 by which the throttle unit for controlling the passages 39 is correlated with the throttle unit for controlling the passages 41. The throttle unit for the passages 41 is held in closed position until the throttle unit for the passages 39 is opened to the desired extent and until the choke valve for the carburetor is fully opened. The throttle control device 43 operates the carburetor 26 in such a way that a combustible mixture will be supplied to the main distribution passages 27 and 28 through the primary mixture supply passages 39 during all conditions of operation of the engine 10 and through the secondary mixture supply passages 41 only during certain conditions of operation of the engine 10. Since the primary mixture supply passages 39 supply mixtures to the main distribution passages 27 and 28 under all operating conditions of the engine 10, it is considered preferable to locate the junction between the passages 39 and the passages 27 and 28 somewhat nearer the middle of the passages 27 and 28 than the junctions between the passages 41 and the passages 27 and 28. This will result in a somewhat better distribution of combustible mixture to all of the cylinders of the engine 10 during low speed operation of the engine 10 and during the time when mixture is being supplied to the main distribution passages 27 and 28 only through the primary supply passages 39.

Since the primary supply passages 39 are exclusively employed in supplying combustible mixtures to the main distribution passages 27 and 28 for starting the engine 10, it is considered desirable to heat the mixtures entering the engine through the passages 39. To accomplish this the manifold '22 is formed in such a way as to provide a cross-over heating passage indicated at 44. The passage 44 has ports 46 and 47 formed at the opposite ends thereof which are adapted to communicate with exhaust passages formed in the heads 14 and 16 and leading from the two middle cylinders of the banks 12 and 13. When the engine 10 is cold and the heat control valve 21 is closed, it will be apparent that all of the exhaust from the cylinders in the bank 12, which normally would be discharged from the manifold 17 through the exhaust pipe 19, will be compelled to flow in a reverse direction. The exhaust from the end cylinders of the bank 12 will flow into the exhaust manifold 17, then in a reverse direction through the exhaust passages for the two central cylinders of the bank 12 and from which all of the exhaust gas from all of the cylinders of the bank 12 will be discharged through the exhaust passage leading to the port 46. From the port 46 the exhaust gas from all of the cylinders of the bank 12 will flow through the heating passage 44, from the port 47 and into a passage in the bank 13 leading to the two middle cylinders of the bank 13. From the exhaust passage for the two middle cylinders of the bank 13 the exhaust gas from the bank 12 will be discharged into the exhaust manifold 18 with the exhaust gas from the cylinders of the bank 13. From the exhaust manifold 18 all of the exhaust gas from the cylinders of the banks 12 and 13 will be discharged from the engine through the ex haust pipe 21. When the engine is warm enough for normal operation and the heat control valve 21 opens, then the exhaust gas from the cylinders of the banks 12 and 13 will follow the paths of least resistance, which will be respectively through the exhaust manifolds 17 and the exhaust pipe 19 and the exhaust manifold 18 and the exhaust pipe 21. It will be apparent that under such conditions the fiow of exhaust gas through the passage 44 will be reduced to a minimum.

The heating passage 44 is formed in the manifold 22 in such a way as to extend across the lower parts of the main distribution passages 27 and 28 and to expose the walls of the main distribution passages 27 and 28 to the heat in the exhaust gas that will flow through the passage 44 when the engine is cold. The heating passage 44 has the walls thereof arranged in such manner that the heating passage will extend across the lower parts of the main distribution passages 27 and 28 just below the juctions between the main distribution passages 27 and 28 and the primary mixture supply passages 39. The heating passage 44 spreads out from each of the ports 46 and 47 in such a way as to provide a broader more shallow passage in the region where the passage 44 extends beneath the main distribution passages 27 and 28. The broader part of the passage 44 is indicated by the numeral 48. Directly above the middle of the broader part 48 of the passage 44 and immediately opposite the junctions between the primary passages 39 and the main distribution passages 27 and 28, the lower walls 49 and 51 of the main distribution passages 27 and 28 are formed in such a Way as to provide depressions or liquid fuel collecting sumps indicated at 52 and 53 respectively. The upper surfaces of the lower walls 49 and 51 are extended for heat transfer purposes by the provision of fins 54 and 56. These fins are spaced from one another across the width of the sumps 52 and 53 respectively, project slightly above the upper extremities of the sumps 52 and 53 and extend in parallel relation to the passages 27 and 28 respectively.

It will be apparent that the heavier ends of the fuel in the mixture supplied by the passages 39 to the main distribution passages 27 and 28 will be collected in the sumps 52 and 53, particularly when the engine is cold. Thereafter the liquid collected in the sumps 52 and 53 will be vaporized by the heat transferred from the passage 44 through the walls 49 and 51 and through the extended surfaces thereof provided by the fins 54 and 56. It will be apparent from Figure 6 that the downward projections of the walls 49and 51 of the passages 27 and 28 respectively, which provide the sumps 52 and 53 respectively, terminate at the ends thereof between the oppositely disposed walls forming the side walls of the Wider part 43 of the heating passage 44.

From Figures 1 and 5 it will be apparent that from the port 46 the passage 44 not only increases in Width, but extends upwardly in such a Way as to discharge the exhaust gas from the bank of cylinders 12 into a pocket 62 and directly against the adjacent side wall 57 of the main distribution passage 27. The width of the passage 44- at the pocket 62 and adjacent the side wall 57 is great enough to heat the side wall 57 somewhat beyond the ends of the sump 52. It will be apparent that the main distribution passage 27 adjacent the junction thereof with the primary passage 39 has one side wall and the bottom wall thereof directly exposed to the exhaust gas supplied by the passage 44 throughout a distance exceeding the length of the sump 52. The wider part 48 of the passage 44 will direct exhaust gas directly against the side wall 58 of the main distribution passage 28 throughout the entire width of the passage section 48. Beyond the main distribution passage 28 the heating passage 44 extends upwardly along the opposite side wall 59 of the main distribution passage 28 in such a way as to provide an upwardly extending pocket indicated at 61. The pocket 61 is narrower than the corresponding pocket 62, which extends along the side wall 57 of the main distribution passage 27, but notwithstanding this it will be apparent that the exhaust gas within the passage 44 will be applied to one side wall 57 and the bottom wall 49 of the main distribution passage 27, whereas exhaust gas will be applied to two side walls 58 and 59 and the bottom wall 51 of the main distribution passage 28. The side wall 57 and the bottom wall 49 are therefore provided with outwardly projecting fins indicated at 63 to compensate for the greater surface of the walls of the main distribution passage 28 which is exposed to the exhaust gas within the passage 44.

In order to provide means for directly heating the primary supply passages 39 during the time when the engine is being started, the manifold 22 is formed in such a way as to provide passages 64 and 66 which extend to the upper surface of the manifold 22 from the pockets 62 and 61. respectively. In order to join the upper ends of the passages 64 and 66 to provide a bypass passage 67 for heating the primary mixture supply passages 39, the lower wall 68 of the carburetor 26, which is adapted to engage the upper wall 69 of the manifold 22, is channeled in such a way as to provide a sinuous passage indicated at 71. It will be noted from Figure 1 that the passage 71 extends around the side of the primary mixture supply passages 39, which is opposite the secondary mixture supply passages 41 and in the vicinity of and just below the idling passage 72 of the carburetor 22. It will be apparent that the exhaust gas which is trapped in the pocket 62 will flow through the by-pass passage 67 and will heat the mixture in the primary mixture supply passages and in the idling passage 72 before the mixture reaches the main distribution passages 27 and 28. It will also be apparent that the exhaust gas flowing through the passage 44 will heat the walls 57, 49, 58, 51 and 59 of the main distribution passages 27 and 28 in the vicinity of the liquid fuel collecting sumps 52 and 53 and at the junctions between the main distribution passages 27 and 28 and the primary mixture supply passages 39.

No means is provided for directly heating the secondary mixture supply passages 41, nor the junctions between the passages and the main distribution passages 27 and 28. Although some heat may be transferred through the walls of the manifold between the passages, it will be apparent that the mixture supplied by the secondary mixture supply passages 41 will not be heated to any great extent, either within the passages 41 or at the junctions between the passages 41 and the main distribution passages 27 and 28. The reason for this is that under ordinary operating conditions in the engine 10 no mixture will be supplied by the secondary mixture supply passages 41 until after the heat control valve 21 has been opened. Consequently when mixture is supplied by the secondary mixture supply passages 41, there will be little exhaust gas actually supplied to the manifold 22 through the heating passage 44.

I claim:

1. An inlet manifold for engines comprising a body having a pair of main distribution passages formed therein and having branch passages at the opposite ends of said main distribution passages extending in opposite directions therefrom, primary and secondary mixtures supply passages formed in said body and connected to said main distribution passages intermediate the ends of said main distribution passages, and walls forming an exhaust gas heating passage extending transversely across said main distribution passages on the sides of said main distribution passages opposite said mixture supply passages and in position to heat the walls of said main distribution passages directly opposite said primary mixture supply passages to a greater extent than the Walls of said main distribution passages opposite said secondary mixture supply passages.

2. An inlet manifold for engines comprising a body having a pair of main distribution passages formed therein and branch passages for said main distribution passages extending opposite directions from the opposite ends of said main distribution passages, said main distribution passages beng arranged in side by side relation with one passage above the other, a heating passage formed in said body and extending transversely across the lower part of said main distribution passages intermediate the ends thereof, said treating passage being formed around the lower walls of said main distribution passages to expose two side walls and the bottom wall of the lower of said main distribution passages and the bottom wall and a side wall of the other of said main distribution passages, and fin means formed on the side wall and bottom wall of said upper main distribution passage for equalizing the heat transfer rate through said walls of said main distribution passages exposed to heating fluid in said heating passage.

3. An inlet manifold for engines comprising a body having main distribution passages formed therein in parallel relation to one another, said body being formed to provide branch passages extending in opposite directions from opposite ends of said main distribution passages for supplying combustible mixture from said main distribution passages to different groups of cylinders of an engine, a primary mixture supply passage and a secondary mixture supply passage formed in said body and communicating with each of said main distribution passages intermediate the ends thereof, a liquid fuel collecting sump formed in a wall of each of said main distribution passages in opposed relation to said primary mixture supply passages, and a heating passage formed in said body and extending transversely across said main distribution passages and exposing the walls of said main distribution passages and said liquid fuel sumps to the heating fluid supplied by said heating passage.

4. An inlet manifold for engines comprising a body having a pair of main distribution passages formed therein for supplying combustible mixtures to different groups of cylinders of an engine, said main distribution passages having branch passages formed in said body and extending in opposite directions from the ends of said main distribution passages, primary and secondary mixture sup ply passages formed in said body and communicating with each of said main distribution passages, liquid fuel sumps formed in the walls of said main distribution passages in opposite relation to said primary mixture supply passages, fins formed in the bottom walls of said main distribution passages in spaced relation to one another and T extending in parallel relation to said main distribution passages and dividing said sumps into a plurality of parallel compartments, and a heating passage formed in said body and extending across said walls of said main distribution passages opposite said fins said sumps formed in said walls and limited in extent substantially to the parts of said walls containing said fins and said sumps.

5. An inlet manifold for engines comprising a pair of main distribution passages for supplying combustible combustible mixtures to different groups of cylinders of an engine, a primary and a secondary mixture supply passage communicating with each of said main distribution passages intermediate the ends thereof, means controlling the supply of combustible mixtures by said primary mixture supply passages under all conditions of operation of said engine, means preventing the supply of combustible mixtures through said secondary mixture supply passages to said main distribution passages under certain conditions of operation of said engine, a heating passage formed in. said manifold and exposing to the heating fluid therein the walls of said main distribution passages adjacent said primary and secondary supply passages, said walls of said main distribution passages opposite said primary supply passages being exposed to said heating passage to a greater extent than the walls of said main distribution passages opposite said secondary supply passages, and tin means formed upon the walls of said main distribution passages in opposed relation to said heating passage.

6. A charge forming device for engines comprising a pair of main distribution passages, primary mixtures supply passages connected to said main distribution passages for supplying combustible mixtures to said main distribution passages under certain conditions of operation of said engine, secondary mixture supply passages connected to said main distribution passages for supplying combustible mixtures to said main distribution passages under other conditions of operations of said engine, a heating fiuid passage formed in said device and extending transversely across said main distribution passages in opposed relation to said primary mixture supply passages, and a by-pass passage formed in said device on the side of said primary mixture supply passages opposite said secondary mixture supply passage and having the opposite ends thereof connected to said heating passage on opposite sides of said main distribution passages, said secondary mixture supply passages having wall surfaces in directly opposed relation to said by-pass passage and directly exposed to the atmosphere surrounding said device.

7. A charge forming device for engines comprising a body having a pair of main distribution passages formed therein, primary and secondary mixture supply passages for supplying combustible mixtures to said main distribution passages and communicating with each of said main distribution passages intermediate the ends thereof, a heating passage formed in said body and extending transversely across said primary mixture supply passages on the side of said primary mixture supply passages opposite said secondary mixture supply passages, said secondary mixture supply passages having wall surfaces in directly opposed relation to said heating passage and directly exposed to the atmosphere surrounding said device, and means for supplying heating fiuid to said heating passage.

8. A charge forming device for engines comprising a body having main distribution passages formed therein for supplying combustible mixtures to different groups of cylinders of an engine, primary and secondary mixture supply passages formed in said body and communi- 3 eating with each of said main distribution passages intermediate the ends of said main distribution passages, and means for heating sai'd main distribution passages opposite said primary mixture supply passages to a greater extent than said main distribution passages opposite said secondary mixture supply passages.

9. A charge forming device for engines comprising a body having a pair of main distribution passages formed therein for supplying combustible mixtures to different groups of cylinders of an engine, primary and secondary mixture supply passages formed in said body and connected to said main distribution passages intermediate the ends of said main distribution passages, and means for heating said primary mixture supply passages to a greater extent than said secondary mixture supply passages.

10. A charge forming device for engines comprising a body having a plurality of parallel main distribution passages for supplying combustible mixtures to different groups of cylinders of an engine, a primary mixture supply passage communicating with each of said main distribution passages for supplying combustible mixtures to said main distribution passages under certain conditions of operation of said engine, a secondary mixture supply passage for eachof said main distribution passages for supplying combustible mixtures to said main distribution passages under dilferent conditions of operation of said engine, and a heating passage formed in said body adjacent said primary mixture supply passages and remote from said secondary mixture supply passages.

11. An inlet manifold for engines comprising a body having apain of parallel main distribution passages formed therein, a primary and a secondary mixture supply passage formed in. said body and communicating with each of said main distribution passages intermediate the ends of said main distribution passages, said mixture supply passages being formed in said body with the junctions between said primary mixture supply passages and said main distribution passages being nearer the middle of said main distribution passages than the junctions between said secondary mixture supply. passages and said main distribution passages.

12. A charge forming device for engines comprising a body having a pair of main distribution passages formed therein for supplying combustible mixtures to different groups of cylinders. of an engine, a primary and a secondary mixture supply passage communicating with each of said main distribution passages intermediate the ends of said main distribution passages, said primary mixture supply passages having junctions with said main distribution passages nearer the middle of said main distributionthan the junctions between said secondary mixture supply passages and said main distribution passages, and means for heating said junctions between said primary mixture supply passages and said main distribution passages to a greater extent than said junctions between said secondary mixture supply passages and said main distribution passages.

13. A charge forming device for engines comprising a body having a pair of main distribution passages formed therein for supplying combustible mixtures to diflerent groups of cylinders of an engine, primary and secondary mixture supply passages communicating with each of said main distribution passages intermediate the ends of said main distribution passages, the junctions between said pri- 10 mary mixture supply passages and said main distribution passages being nearer the middle of said main distribution passages than the junctions between the said secondary mixture supply passages and said main distribution passages, and means for differently heating said primary and secondary mixture supply passages.

References Cited in the file of this patent UNITED STATES PATENTS 2,135,628 Smith Nov. 8, 1938 2,192,067 Betry Feb. 27, 1940 2,282,311 Ericson et al May 12, 1942 2,324,592 Olson July 20, 1943 2,420,925 Wirth May 20, 1947 2,434,192 Braun Jan. 6, 1948 2,523,611 Clayton Sept. 26, 1950 2,603,199 Moseley July 15, 1952 2,640,471 Haltenberger June 2, 1953

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