US3777731A

US3777731A - Fuel injection system in a distributor-type injection pump for internal combustion engine

Info

Publication number: US3777731A
Application number: US00332521A
Authority: US
Inventors: M Kobayashi; M Uchino
Original assignee: Diesel Kiki Co Ltd
Current assignee: Bosch Corp
Priority date: 1973-02-14
Filing date: 1973-02-14
Publication date: 1973-12-11
Anticipated expiration: 1990-12-11

Abstract

Fuel injection system in a distributor-type fuel injection pump for internal combustion engine in which fuel in the distributing section of the injection pump is associated with engine rotation speed by throttling the flow of fuel to improve the fuel injection characteristic for the low-speed region to satisfy more effectively the injection requirements imposed by the engine.

Description

United States Eatent [1 1 Kobayashi et a1.

[ 1 Dec. 11, 1973 FUEL INJECTION SYSTEM IN A DISTRIBUTOR-TYPE ENJECTHON PUMP FOR lNTERNAL COMBUSTHON ENGINE Inventors: Masayoshi Kobayasili; Mitsuo Uchino, both of Higashi, Japan Diesel Kiki Kabushiki Kaisha, Tokyo, Japan Filed: Feb. 14, 1973 Appl. No.: 332,521

Related U.S. Application Data Continuation of Ser. No. 122,621, March 1, 1971.

Assignee:

References Cited UNITED STATES PATENTS v Roosa 123/140 3,219,020 1l/1965 Roosa l/l 3,314,406 4/1967 Kemp 417/295 2,869,529 1/1959 Oxenfart 417/295 2,297,234 9/1942 Meiswinkel 417/295 2,465,784 3/1949 Berlyn 417/295 3,368,490 2/1968 Uirello 417/295 3,046,963 7/1962 Bessiere 417/295 Primary ExaminerLaurence M. Goodridge Assistant Examiner-Ronald B. M. Cox Attorney-Ross F. Hunt, Jr.

[ 5 7 ABSTRACT for the low-speed region to satisfy more effectively the injection requirements imposed by the engine.

,4 Claims, 7 Drawing Figures see so j 33c sie/ai ea sou sa e 35 3% 5 3a a7 a4 PATENTED DEC 1 I I973 SHEET 2 0f 3 NsANtm N- FIG.4

- FUEL'INJECTION SYSTEM 1N A DISTRIBUTOR-TYPE INJECTION PUMP FOR INTERNAL COMBUSTION ENGlNE This is a continuation, of application Ser. No. 122,621 filed Mar. 1, 1971'. I

This invention relates to a fuel injection system in a distributor-type fuel injection pump for internal combustion engine, in which a distributor and pumping member both rotates and axially reciprocates and pressurizes fuel oil for distribution to each of cylinders in a multi-cylinder engine, and a fuel injection system in a distributor-type fuel injection pump which adjusts the injection quantity by controlling the throttling action of fuel supply quantity.

The conventional distributor-type fuel injection pump, which is the basis of this'invention, will be explained in reference to H6. 1, which shows a conventional distributor-type fuel injection pump in a longitudinal cross sectional view, together with the first embodiment of the invention for comparative illustration and FIG. 2 is a performance diagram.

Fuel oil is drawn from a fuel tank 1 by an auxiliary feed pump 2 and delivered at low pressure through a filter 3 to a supply pump 5 built in the fuel injection pump. The-pump 5 is driven by a drive shaft 4 with a speed associated with the revolving speed of the engine, and supplies high-pressure fuel oil through fuel passage 6 to fuel inlet 7a in hydraulic head 7, wherein plunger8 is rotated by drive shaft 4 and is actuated'by the actions of plunger return spring 10 and cam device '9 to reciprocate within distributor barrel 11 and cut-off barrel l2. Cam device 9follows drive shaft 4. For each rotation, plunger 8 reciprocates a number of times equaling the number of cylinders in the engine. This plunger has inflow holes 8a, the number of which equals the numberof engine cylinders, and also an outflowhole 8b, all radially oriented, and an annular groove Be on its periphery. These holes and groove are communicated to pump operating chamber 13 through center hole 8d extending along the .axis of plunger 8; Distributor barrel has an inlet hole 11a corresponding to inflow holes 8a and also outlet holes 11b corresponding to outflow hole 812. The number of outlet holes 11b equals the number of engine cylinders. Cutoff barrel 12 has an overflow hole 12b corresponding to said annular groove 80 and communicating to barrel outer chamber provided on cut-off barrel 12. The opening'area of inlet hole 11a, communicating to fuel inlet 7a, is subjected to the throttling action performed by the notched portion of the piston-in throttling valve 15. This piston moves in the valve bore with an oiltight clearance, as actuated by the governor and adjusting lever, not shown. Outlet holes 11b are communicated through respective fuel passages, not shown, to fuel supply valves 16, from which the pressurized fuel oil is forwarded to injection nozzles 17. Barrel outer chamber 14 is communicated through passage 7b to pump chamber 18 wherein cam device 9 is located. Chamber'19, separated'from barrel outer chamber 14 by cut-off barrel 12, is communicated through passage 19a to said fuel inlet 7a. Cut-off barrel l2 floats in axial direction, as supported by spring 20 and positioned by the composite force consisting of the force of spring 20 and the pressure differential between chamber 19 and barrel outer chamber 14.

14 through annular groove 12a The operation of the fuel injection pump with the foregoing arrangement'will be described. As plunger 8 is moved by cam device 9 in the directionof arrow B, that is on suction stroke while in rotation, one of inflow holes 8a indexes with inlet hole 11a to comminicate fuel inlet "7a to'center hole 8d through throttling passage 15a, inflow hole 8a and inlet hole 11a. Thus, fuel oil flows into pump operating chamber 13. As the plunger is similarly moved but in the direction of arrow A, that is, on discharge stroke, the communication between inlet hole 11a and inflow hole 8a becomes interrupted, so that the fuel oil in chamber 13 becomes pressurized. The pressurized fuel oil then flows toward injection nozzle 17 through outflow hole 8b, one of outlet holes 11c in register with hole 8b, and supply valve 16. While pressurization of the fuel oil in chamber 13 is continuing with the movement of plunger 8, annular groove 8c meets overflow hole 12b to bleed the pressure in chamber 13 to chamber 14.

When this bleeding occurs, fuel injection into the cylinder by nozzle 17 ceases. Further movement of plunger 8 in the same direction forces fuel oil into chamber 14 and, through passage 7b, into chamber 18. Since an overflow valve 21 of known type permits the fuel oil in chamber 18 to return to fuel tank 1, the pressure in chamber 18 is always kept at a constant pressure substantially below the discharging pressure in chamber, 13. Injection quantity is increased or decreased by varying the throttling opening by means of readily adjustable throttle valve 15.

In such construction the rate of dQ/dN, where dQ represents a change in injection quantity and dN a corresponding change in engine rotation speed for a given throttlling opening, takes a largervalue when the throttle valve is less open than when the valve is more open. One other thing to be noted is that dQ/dN is so related with the speed regulation 6 of a governor-controlled engine that the larger the value of dQ/dN, the smaller the value of 6. For engines used to drive vehicles, as small a value of 5 as possible is generally desirable for the high region in the speed range. The value of 8 for the low-speed region, however, should not be too small for the reason to be set forth later. In fuel injection pumps of the kind under consideration, the former desired'condition is met by using a centrifugal-type governor on the engine, but the latter condition has hitherto been left unsatisfied, if not neglected, so that dQ/dN is large in the low-speed region, resulting in too small a valve of 5. Thus, in the low-speed region a given stroke of accelerator lever brings about a large change in fuel injection quantity. In other words, engine speed becomes more sensitive to the displacement of the accelerator lever. Such high sensitivity is undesirable for satisfactory control of the engine in operation. Moreover, since the non-injection zone is necessarily closer to the stable idling zone, a rapid decelerating action would retard fuel injection to cause the engine to stall. The difficulties associated with a large value of dQ/dN in the low-speed region will be more specifically explained by referring to FIG. 2 in which are indicated relation of rotation speed, injection quantity and fuel supply pressure, wherein N stands for engine speed, Q for injection quantity, P for oil pressure and subscripts i and m for idling and maximum or full-load condition, respectively. Note that curve Pf, representing the oil pressure at fuel inlet 7a, is almost linearly proportional to engine speed N, while the injection quantities represented by Vi and Vm curves, one for idling and the other for fullload operation, are inversely proportional to engine speed N. The inclination of these curves is expressed as dQ/dN, which is the rate of change in injection quantity relative to speed. The reason why the proportionality is inverse will be seen in the fact that, for a given setting of the throttle valve, the rate of fuel oil inflow into pump operating chamber 13 is directly proportional to the registering or indexing time for inlet hole 11a and inflow hole 8a, and that the time required for these two holes to come into register is inversely proportional to engine speed N. Stated differently, for the throttle valve setting Vi for steady-state idling condition, fuel injection begins at Ns. At idling speed Ni, which is below Ns, injection quantity takes a value corresponding to Qi. As the speed falls further, injection quantity increases. Consider a rapid downward shift from the high-speed region, for example, from the throttle valve setting for Vm, to the other setting for Vi. In this case, injection quantity will fall rapidly to zero while engine speed will fall to Ns but gradually because of the inertia in the engine. Due to the resistance to fuel oil flow in the inlet passages and the inertial mass of fuel oil, the inflow of fuel oil for producing the injection quantity necessary for maintaining engine speed between N5 and Ni occurs only after a certain time lag. This is to say that, from the moment the throttle valve takes a position corresponding to Vi to the moment a steady-state relationship between N and Q is obtained for that position, there will be a certain lapse of time. Unless engine speed falls down to and below Ns within this time lag, the speed will keep on falling, because of the lack of injection quantity, until the engine halts by itself.

The object of this invention is to eliminate the drawbacks of the conventional distributor-type fuel injection pump described above, by providing a pump which satisfies therequirement on control of engine performance in the low-speed region with its dQ/dN' reduced to a proper valuein such a way as not to affect the engine performance characteristic in the high-speed region.

For avoiding engine stalling, it is generally proposed that the engine should I) take a longer time in slowing down to Ns, (2) make shorter thetime lag mentioned above, or (3) have its Ns set at a higher speed level. To raise Ns, reducing the dQ/dN alone or raising the idling level Ni alone is adequate. The dQ/dN reduction is simple but less preferable in view of engine performance. Therefore, the present invention takes the latter method, i.e., setting Ni at a higher level, and uses an additional and new throttling valve, by which the crosssection area of the throttled passage or opening is made to vary in proportion to engine speed within the lowspeed region under consideration so as to reduce the dQ/dN to a properly small value only for that low-speed region of the engine speed range.

The invention will be made clear by describing the example shown in the drawings, in which FIG. 3 is performance diagram for the pump according to the invention similar to FIG. 2, showing the same kind of relationship and FIG. 4, 5, 6 and 7 are cross sections of the essential parts of the first to forth embodiments of the invention.

In the conventional fuel-injection pump shown in FIG. 1, chamber 19, into which the supply pressure is applied from supply pump 5, is communicated to annular groove llb, provided in distributor barrel 1]. and communicating with fuel inflow holes 8a of plunger 8,

through oil passages 30 and 30a provided in hydraulic head 7 as shown in FIG. 4. A throttling valve, to be described later, is located in said communicating passages at is midway position. In other words, hydraulic head 7 has a valve hole 31 for holding the plunger 8. This hole 31 is made by drilling into the head in a direction normal to the plunger 8 and at a position opposite to the fuel inlet 7a across said plunger. Bottom section 31a of hole 31 so provided is communicated to chamber l9 through said passage 30. Valve hole 31 is providedwith a stepped portion to present large diameters, the larger portion being the inlet section, which is communicated to cam chamber 18 through passage 32. Piston 33, having an annular groove 33a at its middle portion and an axially extending hole 33b which opens out to said bottom section 31a, is fitted into hole 31, there being a radial hole in piston 33 which communicates said annular groove 33a to said center hole 33b. Annular groove 33a will meet the port of communicating passage 30 when piston 33 becomes outwardly displaced by a rise in fuel oil pressure, and, by so registering with said port, it communicates itself to fuel annular groove 11b of said barrel l1. Groove 33a is provided with a tapered notch 33d, by means of which it performs a throttling action when it increasingly registers with the port of passage 30a. Stated specifically, notch 33d varies the degree of communication gradually through the port when said registering movement is in progress. As fuel oil pressure rises further, piston 33 moves further outwardly and closes passages 30a ultimately by its forward end 33c. The other or rear end of the piston is formed into flange 332. Spring 34 urges the piston inwardly by pushing on its flange 33e against the stepped face of hole 31. Thus, flange 33e limits the inward stroke of the piston. The outward stroke is limited by the inner end face of bush 36, against which flange 33:: will bear. Bush 36 is threaded into hydraulic head 7, there being a shim 35 interposed between head 7 and the shoulder formed of bush 36, so that the limit on outward stroke of the piston can be adjusted by changing the thickness of shim 35. A threaded through hole is provided in the axis of bush 36, into which adjusting screw 37, complete with a locking nut, is run to provide a seat for the other end of said spring 34.

The operation of the fuel-injection pump constructed as described above will next be explained.

During normal operation with a high fuel supply pressure, piston 33 stays pushed all the way against the force of spring 34, so that the port of passage 30a is closed by forward end 33c of the piston. Under this condition, the throttling action is performed by main throttle valve 15 alone. Rapidly closing the main throttle valve from full open position Vm to idling position Vi will quickly bring about a non-injection condition in the engine, and engine rotation speed will fall toward Ns, shown in FIG. 3. In the meantime, the falling speed reduces supply pressure Pf, so that piston 33 yields to the force of spring 34 and moves back inwardly, allowing its forward end 330 to open the port of passage 30a and thus communicating chamber 19 to annular groove 11b of barrel 11. This communication results in an additive supply of fuel oil to the injection pump to reduce said dQ/dN so as to resume fuel injection. At this time, engine rotation speed is at Nt, shown in FIG. '3. Suppose, at this juncture, that the extent of valve opening is constant at V4; then the supply of fuel oil would rapidly increase along the line V4 but the extent of valve opening will decrease in proportion to the changing fuel supply pressure determined by rotation speed, so that, with the falling rotation speed, tapered notch 33d gradually throttles in a manner represented by V4, V3 VO, thereby preventing engine speed under noninjection condition from falling to Ms. At VO the port of passage 30a'becomes closed and, thereafter, idling rotation speed is maintained according to the extent Vi of opening in main throttle valve 115. The effect of tapered notch 33d is such as to gradually change the I cross-section area of oil flow in step with changing rotation speed, so that, when the Q characteristicshifts from V! to Vi, smooth control of idling rotation speed is made possible because the shift is gradual. To alter the characteristic, changing the spring force shifts point 'Nt, changing the spring constant modifies the gradient of characteristic curves, and varying the shim thickness changes A Ni, without affecting the general injection characteristic. Oil leakage through the sliding clearance around piston 33 collects into passage 32 and returns to pump cam chamber 18. I

The auxiliary throttle valve according to the invention may be oriented in a direction parallel to plunger 8 and located inside cam chamber 18, as in the second embodiment of the invention shown in FIG. 5. ln the parallel position shown, the throttle valve does not constitute such a protrusion from the pump body as in the first embodiment, and minor oil leakage from it,if any, does not present problems. I

The third embodiment: In FIG. 1, showing the construction of a conventional pump, at point 41 in fuel passage 6 between supply pump 5 and fuel inlet 7a is provided a compensator 43.'This compensator consists essentially of a throttle valve and constructed as shown in FIG. 6. A cylindrical valve body 43 of the compensator 42, whose one end receives fuel oil supplied through the passage 6, FIG. 1, carries a piston 45 slidably in its bore, with a radial sliding clearance 44 around the piston a sholder of said bore forming a seat. Compression spring 46, interposed between piston 45 and tube joint 48 rigidly connected with body 43, urges piston 45 toward inlet side to close said seat and supply oil pressure Pf from the passage 6 acts on the piston end face 45a pressed against the stepped seat face provided in the bore of body 43, to overcome the force of a spring 4610 unseat the piston, thereby admitting fuel oil into passage 47 which opens out to chamber 49 provided between piston 45 and tube joint 48. Thus, the unseating of piston45 communicates fuel passage 6 to outlet hole 48a in tube joint 48 and hence to fuel inlet 7a in the fuel injection pump. As shown in FIG. 3, the throttle valve opening of the oil passage a is set by V4 larger than Vi and within the range of valve opening between V4 and Vi, throttling opening of the valvein compensator 42 varies in proportion to engine rotation speed to V4, V3, V2 and V1. If the opening of the throttle valve 15 is throttled from Vm to V4, the injection quantity decreases rapidly to zero, but, since Ns is now at a higher level Ni, such a rapid deceleration will not result in engine stalling. When the engine rotation speed falls'from Nt, pressure Pf acting on the piston end face 45a decreases, so that spring 46 pushes piston 45 closer to the seat, thereby restricting passage 47 gradually: when the speed has reached Ni, passage 47 will have been completely closed. Thereafter, the amount of fuel oil flowing into fuel inlet 7a, is that which leaks past valve body 43 and the piston end face 45a and radial clearance 44. This flow rate maintains the idling injection quantity Qi. For engine rotation speeds above Nr, the passage 47 is wide open fuel injection quantity is to be controlled by moving throttle valve 15. The characteristics of throttling action performed in compensator 42, including the relationship the beginning and terminating points of inclined curves and pump rotation speed, are to be'determined by properly selecting the spring constant and load of spring 46 and the positional or registering relationship between piston 45 and passage 47.

The forth embodiment: In this embodiment acompensator 42 is directly mounted on the hydraulic head 7 and parallel to the throttle'valve 15, which is set for Vi whereas, in the third embodiment, the compensator 42 is in series with the throttle valve 15, which is set for V4. The compensator, as shown in FIG. 7 comprises a valve body 50, in which a piston 52 is slidably carried and is urged by spring 51 acting on one end face of it while the other end of it is acted on by the pressure Pf in the bore of throttle valve 15 communicating to fuel inlet 7a through communicating passage 55. The piston 52 has a recess 52b of an annular groovej52a on its sliding surface, which groove is communicated in place with a passage 50a of the valve body 50 and at center an axial hole 52c which is communicated with said recess 52b. The hole 50a of the valvebody 50 communicates through a passage 53 provided in the hydraulic head 7 and an annular recess 54 formed behind the passage 15a of throttle valve 15 to fuel inlet hole 11a in the injection pump. The hole 520 is communicated through the passage 55 to the fuel inlet 7a connected with the passage 6 to the fuel supply pump 5.

i The construction of the forth embodiment having been described, its function will be explained with reference to FIG. 3. Rapid deceleration is effected and engine rotation speed falls subsequently to reduce the pressure Pf. At Nt of the engine rotation speed, the spring 51 will have pushed out piston 52 against the pressure Pf to'communicate the recess 52b of the annular groove 52a with the passage 50a. The fuel oil flows through

passages

55 and 52c, annular groove 52a, recess 52b,

passages

50a and 53 and annular groove 54 thereby introducing an additional oil flow to the inlet holella. This prevents the engine rotation-speed from dropping rapidly. lower than Ns. While the rotation speed is falling, the pressure Pf too falls, allowing piston 52 to be pushed out far enough to interrupt said passage so that opening degree Vi of the throttle valve 15 maintains the idling speed of Ni.

It will be seen from the foregoing description that, according to this invention, a simple structural modification of the conventional distributor-type injection pump alters its fuel oil throttling action in-such a way as to enable the pump to meet the engine requirement for fuel injection in the low-speed region by its injection quantity characteristic, by relating the rate of fuel oil inflow in the pump operating chamber to the rotational speed of the engine, to suit said requirement.

Many variations may be effected without departing from the spirit of the invention. It is to be understood that these, together with other variations in details, are anticipated by the appended claims.

What is claimed is:

l. A distribution-type fuel injection pump for an internal combustion engine comprising a plurality of fuel supply valves corresponding in number to the number of engine cylinders, a plunger for distributing fuel under pressure to said fuel supply valves, a supply pump for feeding fuel to said plunger, a fuel supply passage connecting said supply pump to the inlet of said plunger, and a main throttle valve, the opening of which is regulated by a governor, mounted on said fuel supply passage so that said plunger and said supply pump are actuated in relation to the rotational speed of the engine and said supply pump feeds fuel to said plunger through said main throttle valve under a fuel feed pressure proportional to engine speed, characterized by an auxiliary throttle valve in communication with said supply pump and said plunger which opens responsive to the rotational speed of the engine decreasing to a preselected value and hence to said fuel feed pressure decreasing to a corresponding predetermined pressure, said auxiliary throttle valve having a pressure receiving surface against which said fuel feed pressure from said supply pump acts in a direction so as to close the valve passage for pressures above said predetermined pressure, and including a spring which acts to oppose said fuel feed pressure so as to tend to open the valve passage for pressures above said predetermined pressure, said auxiliary throttle valve being constructed so that the opening provided thereby is decreased through the action of said spring in response to a decrease in said fuel feed pressure below said predetermined pressure at which said auxiliary throttle valve opens so that the amount of fuel passing through the passage of said auxiliary throttle valve is regulated within the range from maximum and minimum in re sponse to a change in engine speed from preselected to "idling."

2. A distribution-type fuel injection pump for an internal combustion engine according to claim 1 characterized in that said auxiliary throttle valve is arranged in parallel with said main throttle valve.

3, A distribution-type fuelinjection pump for an internal combustion engine according to claim 1 characterized in that said auxiliary throttle valve is arranged in series with said main throttle valve between said main throttle valve and said supply pump.

4. A fuel injection system for a distribution-type fuel injection pump for an internal combustion engine including a rotating, reciprocating plunger for distributively supplying fuel underpressure to the cylinders of the engine in succession, the movement of said plunger being related to the rotational speed of the engine; a main fuel supply passage for feeding fuel to said plunger under a pressure which varies in direct proportion to the rotational speed of the engine; a main throttle valve located in said main fuel supply passage; an auxiliary fuel supply passage in parallel with said main fuel supply passage; and an auxiliary piston-type throttle valve disposed in said auxiliary fuel supply passage responsive to the said pressure of said fuel in a predetermined engine rotational speed range, said throttle valve opening when the rotational speed decreases to a speed corresponding to the upper limit of said range and including a slant face which slants in the direction of movement of the valve so that the amount of fuel passing through said auxiliary fuel supply passage is reduced in response to a further decrease in the rotational speed of the engine below said upper limit.

Claims

1. A distribution-type fuel injection pump for an internal combustion engine comprising a plurality of fuel supply valves corresponding in number to the number of engine cylinders, a plunger for distributing fuel under pressure to said fuel supply valves, a supply pump for feeding fuel to said plunger, a fuel supply passage connecting said supply pump to the inlet of said plunger, and a main throttle valve, the opening of which is regulated by a governor, mounted on said fuel supply passage so that said plunger and said supply pump are actuated in relation to the rotational speed of the engine and said supply pump feeds fuel to said plunger through said main throttle valve under a fuel feed pressure proportional to engine speed, characterized by an auxiliary throttle valve in communication with said supply pump and said plunger which opens responsive to the rotational speed of the engine decreasing to a preselected value and hence to said fuel feed pressure decreasing to a corresponding predetermined pressure, said auxiliary throttle valve having a pressure receiving surface against which said fuel feed pressure from said supply pump acts in a direction so as to close the valve passage for pressures above said predetermined pressure, and including a spring which acts to oppose said fuel feed pressure so as to tend to open the valve passage for pressures above said predetermined pressure, said auxiliary throttle valve being constructed so that the opening provided thereby is decreased through the action of said spring in response to a decrease in said fuel feed pressure below said predetermined pressure at which said auxiliary throttle valve opens so that the amount of fuel passing through the passage of said auxiliary throttle valve is regulated within the range from maximum and minimum in response to a change in engine speed from ''''preselected'''' to ''''idling.''''

3. A distribution-type fuel injection pump for an internal combustion engine according to claim 1 characterized in that said auxiliary throttle valve is arranged in series with said main throttle valve between said main throttle valve and said supply pump.

4. A fuel injection system for a distribution-type fuel injection pump for an internal combustion engine including a rotating, reciprocating plunger for distributively supplying fuel under pressure to the cylinders of the engine in succession, the movement of said plunger being related to the rotational speed of the engine; a main fuel supply passage for feeding fuel to said plunger under a pressure which varies in direct proportion to the rotational speed of the engine; a main throttle valve located in said main fuel supply passage; an auxiliary fuel supply passage in parallel with said main fuel supply passage; and an auxiliary piston-type throttle valve disposed in said auxiliary fuel supply passage responsive to the said pressure of said fuel in a predetermined engine rotational speed range, said throttle valve opening when the rotational speed decreases to a speed corresponding to the upper limit of said range and including a slant face which slants in the direction of movement of the valve so that the amount of fuel passing through said auxiliary fuel supply passage is reduced in response to a further decrease in the rotational speed of the engine below said upper limit.