US2991054A

US2991054A - Idle compensator

Info

Publication number: US2991054A
Application number: US805241A
Authority: US
Inventors: Harold R Scibbe
Original assignee: Thompson Ramo Wooldridge Inc
Current assignee: Northrop Grumman Space and Mission Systems Corp
Priority date: 1959-04-09
Filing date: 1959-04-09
Publication date: 1961-07-04
Anticipated expiration: 1978-07-04

Description

July 4, 1961 H. R. SCIBBE IDLE COMPENSATOR 3 Sheets-Sheet 1 Filed April 9, 1959 Eran-far Hare/a A. .Sp/bbe R m k H. R. SCIBBE IDLE COMPENSATOR July 4, 1961 5 Sheets-Sheet 5 Filed April 9, 1959 @Sbm wkgmkmww uRiQ WMIIN .Ez'mfar Hero/a P $c/bbe nited The present invention relates to improvements in fuel supply systems for internal combustion engines, and more specifically to an improved mechanism for modifying a density signal and producing an altered vacuum pressure signal from an intake manifold of an engine such as may be used in a speed-density controlled fuel injection system for the engine to maintain smooth operation at idle speeds.

In an Otto-cycle or spark ignition engine, the air-fuel mixture must support a flame front, and the richness of the mixture is important to proper operation of the engine. Fuel supply control systems measure the air intake of an engine and supply a fuel rate accordingly. Speed density measuring systems are superior to mass density measuring systems for certain purposes and obtain a strong fuel control signal and are convenient and require less precise control hardware. In a fuel injection system, fuel can be controlled by directly measuring speed by driving the fuel pump at a speed which is a function of the speed of the engine, and thereby providing a pump output which is a function of engine speed.

Density is simply measured by measuring intake manifold pressure. 'In many engines, the speed-density signal does not give an accurate measurement of air intake of the engine at all operating conditions. For example, oertain engines at idle speeds do not pump air proportional to the engine speed.

The present invention contemplates the provision of a mechanism which provides a modified density signal when the engine is operated with a closed throttle, such as at idling, to provide a more optimum fuel delivery rate for improved operation. While the mechanism is primarily used in a speed density fuel control system, the principles of the invention may be utilized in a mass flow system.

Accordingly, an object of the invention is to provide an improved mechanism for providing a vacuum pressure signal which is a function of the pressure within the intake manifold, but which is different from the pressure within the intake manifold and will convert the modified pressure to a density signal for use in a density control system for controlling the fuel delivery of a pump in a fuel injection system.

Another object of the invention is to provide an improved fuel supply control mechanism for use in fuel supply systems employing a density signal which is adjustable to vary the fuel supply rate at idling speeds to a quantity either above or below the rate of fuel which normally would be supplied according to the vacuum pressure in the intake manifold.

Another object of the invention is to provide an improved device for providing a vacuum pressure sign-a1 of a much lower absolute pressure than that in the intake manifold of an engine and lower than that which can be obtained by a flow of air through a venturi into the intake manifold.

A further object of the invention is to provide a mechanism for supplying a vacuum signal from an intake manifold vacuum which is at a much lower absolute pressure than the vacuum in the intake manifold and which is provided with a signal trim adjustment for accurately varying the relationship between the intake manifold vacuum and the vacuum produced by the device.

Another object of the invention is to provide a mechanism for supplying a false intake manifold pressure sig- Patented July 4, 1961 na-l which is an adjustable function of intake manifold pressure and wherein the accuracy and sensitivity of the adjustment is improved.

Another object is to provide a device which will decrease the fuel supplied to an engine at closed throttle conditions to less than normally provided at closed throttle to reduce unburned fuel discharged from the engine and smogging effects.

Oth r objects and advantages will become more appar-' cut with the teaching of the principles of'the invention in the disclosure of the preferred embodiments in the speci-' fication, claims and drawings, in which:

FIGURE 1 is a vertical sectional view taken through an intake manifold incorporating a mechanism embodying the principles of the present invention, and illustrating the position of the parts when the engine is idling;

FIGURE 2 is a vertical sectional view similar to- FIG- URE 1, illustrating the position of the parts when the throttle valve is open;

FIGURE 3 is a vertical sectional view similar to FIG-

URES

1 and 2 and illustrating a modified embodiment of the invention;

FIGURE 4 is a schematic illustration, partially in sec-' tion, illustrating a fuel control system embodying the principles of the present invention and incorporating the mechanism of FIGURES 1 and 2; and,

FIGURE 5 is a graph illustrating the relationship of fuel delivery to manifold pressure in a fuel system embodying the principles of the invention.

As shown on the drawings:

An intake manifold 6 supplies air to the cylinders of an engine and is provided with a throttle valve plate 7 supported on a control shaft 8 and mounted in the air intake flow passage 9 leading to the intake manifold 6. The density signal device 11 is connected to the intake manifold to transmit a vacuum pressure signal to a vacuum motor 12 which is connected to the signal device through a conduit 13, FIGURE 4.

Fuel is supplied to the intake manifold to obtain a desired air-fuel ratio for the cylinders of the engine 14 by injectors 16 which deliver fuel through the wall of the intake manifold opposite the intake passages leading to the individual cylinders. Fuel supply lines 17 are connected to the injectors at one end and receive fuel at the other end from a distributor 18. The distributor is driven directly from the engine by a shaft 19 in order to release fuel at the appropriate times to the injectors. Pressurized fuel is supplied to the distributor through a conduit 21 from a fuel pump 22 which in turn is driven as a function of engine speed through a shaft 23. The pump 22 is preferably a positive displacement type pump, and as will be appreciated by those skilled in the art, the pump may take various forms wherein the displacement is a function of the driving speed.

The output for the displacement of the pump is controllably varied by a pivotal control shaft 24. The pivotal position of the shaft 24 is controlled by crank arm 26. Movement of the crank arm in a counter-clockwise or clockwise direction, as illustrated in FIGURE 4, will make the pump deliver less or more fuel in accordance with the control signal received.

The crank arm 26 is pivoted by a control rod 27 pivotally connected by a pin 28 at a mid-point of a pivotal rocking link 29. The link is laterally displaced to move the control rod 27 by a shaft 31 pivotally connected at 32 to one end of the pivotal link 29, and reciprocated to increase the =fuel delivery to the engine under cold engine conditions. For this purpose, a mechanism which is responsive to the operating temperature of the engine may be connected to the shaft 31. As will be appreciated, other modifying signals could be directed to the shaft 31 to vary the fuel delivery to the engine.

The density signal is transmitted to the pump from the vacuum motor 12 which has an output shaft 33 pivotally connected at 34 to the pivotal link 29. This provides the primary control movement for the pump and the pivotal connection '32 will be stationary during normal engine operating conditions.

i The vacuum motor 12 has a main body or housing portion 36 which has a base 36a and is closed at one end with a cover 37. The cover is provided with a bearing sup port annular flange 38 for supporting the shaft 33. A diaphragm 39 sealingly extends across the inside of the housing and is clamped at its edges between flanges 36b and 37a which are part of the body 36 and the cover 37, respectively. A reciprocating plunger 40 is connected to the end of the shaft 33 and clamped to the diaphragm 39, so that it will be axially displaced with changes in vacuum pressure in the chamber 44 within the body 36. Within the chamber is a coil compression spring 41 which engages the bottom wall 36a of the body, and which engages the plunger 40. A shorter coil compression spring 42 is coaxial and within the outer spring and will be engaged by the plunger as it moves further to the left. The outer spring 41 determines the power range of operation for the engine at throttle positions ranging from full open to partly closed, and the combined rate of springs 41 and 42 governs cruising range and operates for the range of throttle positions from partly closed to fully closed. The vacuum signal is transmitted to the chamber 44 within the vacuum motor through a port 43 in the end which is connected to the conduit 13. The manifold pressure acting on the plunger and diaphragm results in a fuel delivery metering curve 46, as illustrated in FIGURE 5. The outer spring 41 is slightly preloaded to provide the flat portion 47 of the curve at wide open throttle position. The portion 48 of the curve is the power range fuel delivery, and the portion 49 is the cruising range fuel delivery. The characteristics of these curves are determined by the rates of the springs 41 and 42. The lower end 51 of the curve 46 is the idling range of fuel delivery. The curve will continue along the solid line 51 at low idling manifold pressures if no compensation is made in the intake manifold pressure signal. Where the air pumping of the engine is a non-linear function of the engine speed at idling speeds, and a decrease in air intake at idling speeds is caused, such as by valve timing, the fuel delivery to the engine must be decreased for smooth operation. The resultant curve is illustrated at '53. For some purposes, an increased fuel delivery is desirable with the fuel delivery curve indicated at 52.

An important feature of the present invention is to provide a modified or false vacuum signal which will obtain fuel delivery in accordance with the curves 52 and 53, or curves at any location between this range, and which will permit a wider range in alteration of fuel delivery and obtain simplified and more accurate adjustment of modified fuel delivery at idling speeds.

As illustrated in FIGURES l and 2, the vacuum signal is obtained from the intake manifold 6 through a first passageway 54. This first passageway has a lower portion 55 leading into a chamber 56 which is part of the passageway and continuing with a port 59 leading from the chamber 56 to a passage 58 to which the conduit 13 is connected to transmit the vacuum signal to the vacuum motor 12.

To provide a modified signal to the vacuum motor, a second or a by-pass passageway 61 is provided. This passageway communicates between the intake manifold 6 and the passageway 58 leading to the conduit 13 connected to the vacuum motor. The by-pass-passageway 61 has a first portion 62 connecting to a second portion 63, which in turn leads to the common passageway 58 to which the conduit 13 leading to the vacuum motor 12 is connected.

At idle conditions, as illustrated by the relationship of the parts in FIGURE 1, the first passageway 54 is closed by a ball valve 64. At engine operating conditions of open throttle, the ball valve 64 is opened, as illustrated in FIGURE 2, and the pressure signal transmitted to the vacuum motor 12 is obtained directly through the first passageway 54 so that the signal received by the vacuum motor is equal to the manifold pressure.

The ball valve 64 is moved to closed position against a seat 65 formed where the lower portion '55 of the passageway enters the chamber 56. The ball is held against the seat by a ball operating plunger 66, which is slidably mounted in a threaded fitting 67 screwed into the chamber 56. The plunger is forced inwardly to move the ball valve 64 against its seat by a throttle crank arm 68 which is mounted on a shaft 72 mechanically connected to the throttle shaft 8, and to the linkage for operating the shaft. At the upper end of the arm 68 is a plunger operating screw 69 which is adjustably threaded into brackets 71 on the arm. The arm 68 and throttle valve 7 are shown at running positions of the engine in the cruising range and power range in FIGURE 2, and at idle position in FIG- URE 1. When the throttle plate is opened, the plunger 66 is released by the arm 68 moving in a clockwise direction, as shown in the drawings, and a coil spring 73 unseats the ball 64. When the operator closes the throttle, the plunger 66 seats the valve 64 to block the vacuum signal through the passageway 54. The signal then received by the vacuum motor is a signal in the upper portion 63 of the by-pass passageway 61.

In order to produce a very low false reference pressure which is a function of intake manifold pressure, an orifice member 74 is provided, having an orifice 76 which opens into the bypass passageway 63. The orifice member has inwardly tapering side walls 77 and at the end of the orifice 76 is an abruptly outwardly extending radial wall 78. Air will continually bleed through the orifice 76 into the portion 62 of the passageway 61.

The pressure signal is taken immediately downstream from the orifice 76 behind the radial wall 78 by an opening 79 which opens through the side wall of the lower portion 62 of the by-pass passageway. It has been discovered that a signal pressure taken at this location with the foregoing construction obtains a pres-sure signal which is considerably lower than the pressure in the intake manifold and considerably lower than a pressure which is obtainable by providing an air bleed venturi. With a venturi, the minimum pressure signal which can be obtained is 0.528 times the absolute manifold pressure, and the pressure signal obtainable by the mechanism of the present invention is considerably lower.

This lower pressure will result in a false lower pressure signal being transmitted to the vacuum motor 12, and the fuel supply rate will be decreased accordingly, along the general trend of the curve 53 of FIGURE 5.

In an optimum construction and operation of the mechanisms of FIGURES 1 and 2, the side wall 77 of the orifice 76 should converge. The conduit 62 should lead away from the orifice. The signal port 79 is preferably located close to the orifice. Modifications of these above requirements may in certain circumstances obtain satisfactory results, although the above arrangement obtains optimum performance for most uses. For example, the orifice member can be arranged to extend slightly into the passageway 62, with the port 79 arranged at one side of the orifice outlet.

To reduce the decrease in fuel delivery at idling provided by the above described mechanism, a signal trim mechanism is provided which bleeds air into the by-pass passageway upstream of the orifice. A bleed passageway 80 connects to the second portion 63 of the by-pass passageway, and the amount of atmospheric air bled into the by-pass passageway is regulated by a needle valve 81, coacting with a valve seat 82. The needle valve is carried on an' adjustable stem 83, and is adjustably controlled by a knurled adjusting knob 84 on the stem. A coil compression spring 87 bears against a flange 86 on the stem to lock in its adjusted position to prevent vibrational dislocation. The bleed passageway 80 and the second portion 63 of the passageway 61 are larger in cross-sectional area than the port 79, so that the pressure signal delivered to the vacuum motor can be raised above the absolute pressure in the intake manifold. At pressures above the intake manifold pressure, fuel delivery curves of the nature of broken line 52 of FIGURE 5, will be obtained.

In the modified form illustrated in FIGURE 3, the bleed passage 80 is connected to a conduit 88 which connects to the intake manifold. This opens the passageway 80 to a source of pressure lower than atmospheric pressure. This enhances the accuracy of the adjustment of the needle valve 81.

For example, if the absolute pressure in the intake manifold is at fifteen inches of mercury, and the absolute pressure in the passageway 61 is five inches of mercury, an atmospheric pressure signal of approximately thirty inches of mercury can be bled into the passageway 61 by the needle valve with the arrangement of FIGURES 1 and 2. With the arrangement of FIGURE 3, a mixing signal of fifteen inches of mercury can be bled in through i the needle valve, thus reducing the pressure differential between the pressure in the passageway 61 and the pressure which is bled in to trim the signal. This, of course, obtains improved control.

In operation, when the engine is running at open throttle conditions, as illustrated in FIGURE .2, a pressure signal is obtained from the intake manifold 6 through a first passageway 54 and transmitted to the vacuum motor 12. The vacuum motor reciprocates the shaft 33, FIGURE 4, to pivot the link 29 and rock the crank arm 26 to thereby regulate the output of the fuel pump 22. When the operator closes the throttle valve 7, the throttle arm 68 engages the plunger 66 to close the ball check valve 64 and block the vacuum pressure signal to the vacuum motor 12. The signal is then received through the bypass passageway 61 which has a much lower pressure than in the intake manifold 6. The lower pressure is obtained by atmospheric air bleeding into the first portion 62 of the bypass passageway through the tapered orifice 76, with the pressure signal being taken off through a port 79 directly behind the orifice. To increase the false signal pressure obtained from the by-pass passageway, a signal trim valve 81 is provided which is adjustable over a range to obtain pressures either lower or higher than the intake manifold pressure, and thereby obtain a range of fuel supply at idling pressures between the lines 52 and 53 of FIGURE 5.

Thus, I have provided an improved density sensing mechanism for speed density controlled fuel injection systerns, which meets the objectives and advantages hereinbefore set forth. The mechanism obtains smooth operation at idle speeds. The mechanism is well adapted to use for trucks and automobiles and serves to obtain fuel reduction when the vehicles are coasting, at closed throttle thereby reducing smogging.

I have, in the drawings and specification, presented a detailed disclosure of the preferred embodiments of my invention, and it is to be understood that I do not intend to limit the invention to the specific form disclosed, but intend to cover all modifications, changes and alternative constructions and methods falling within the scope of the principles taught by my invention.

I claim as my invention:

1. A fuel supply system for an Otto-cycle engine comprising in combination a positive displacement fuel pump, means for driving the pump as a function of engine speed, a distributor connected to receive the discharge of the pump, means for driving the distributor at a multiple of engine speeds, individual fuel injectors connected to receive metered fuel increments released by said distributor and adapted for connection to the intake manifold of the engine, a control member connected to the pump to control its output, a vacuum motor for supplying a displacement signal as a function of a vacuum signal and connected to said control member, a vacuum conduit connected to said vacuum motor and adapted for connection to the intake manifold of the engine, a by-pass conduit having a first portion adapted forconnection to the intake manifold of an engine and having a second portion connected to the vacuum motor, an air bleed orifice member having inwardly tapered side walls and communicating between atmospheric pressure and the first portion of the by-pass conduit with the tapered side walls facing atmospheric pressure and having a radial wall leading away from said orifice and facing said first portion of the bypass conduit, a pressure take-off port communicating with the first portion of the by-pass conduit immediately downstream of said radial wall and communicating with the second portion of the by-pass conduit, a trim orifice communicating with the second portion of the bypass conduit for bleeding trim fluid into said second portion to raise the pressure therein, a trim valve connected to the trim orifice to adjust the size and the fluid bled into said second portion of the by-pass conduit, a valve in said vacuum conduit, and a valve operator for operation with a throttle valve leading to the intake manifold and positioned to close said valve in the vacuum passage when the throttle valve is closed so that the signal from the by-pass passage alone is received by the vacuum motor.

2. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control the fuel delivery to the engine, comprising a pressure responsive control device for controlling fuel de livery to an engine as a function of a pressure signal, means defining a vacuum passage connected to said control device and adapted for connection to an intake manifold for the engine, means defining a by-pass passage connected to said control device and adapted for connection to the intake manifold to send a modulated pressure signal to the control device, an idle stop valve connected in said vacuum passage to close the passage during idling of the engine, and means for producing an idle signal pressure in said by-p ass passage less than 0.528 of the pressure in said intake manifold so that the idle signal pressure sent to the control device will obtain a lower fuel delivery than would be obtained with idle pressures in the intake manifold to compensate for non-linear air consumption at idle speeds.

3. A device for fuel control systems to control a fuel delivery to an engine as a function of intake manifold vacuum comprising a pressure responsive control device for controlling fuel flow to an engine as a function of a pressure signal, means defining a first passage for connection to the intake manifold of the engine, means defining a flow restricting orifice opening to a source of fluid at higher pressure and to said first passage for bleeding fluid into said first passage, and means defining a second passage connected to said pressure responsive control device and connected to said first passage immediately downstream of said orifice to produce pressures in said second passage which are a function of manifold pressures but which are considerably lower than manifold pressures for control of fuel delivery to the engine,

4. A device for fuel control systems controlling fuel delivery to an engine as a function of intake manifold pressure comprising a pressure responsive control device for controlling fuel flow to an engine as a function of a pressure signal, means defining a first passage for connection to the intake manifold of the engine, means defining an inwardly tapered flow orifice with walls extending abruptly outwardly at the end of said tapered orifice and leading into said first passage with said first passage extending away from said orifice, and means defining a second passage connected to said pressure responsive control device and connected to said first passage immediately downstream of said orifice to produce pressures in said second passage which are a function of said manifold pressures but are considerably lower than manifold pressures for control of fuel delivery to the engine.

5. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control fuel delivery to the engine comprising a pressure responsive control device for controlling fuel delivery to an engine as a function of a pressure signal, means defining a vacuum passage connected to the control device and adapted for connection to an intake manifold of the engine, means defining a by-pass passage with a first portion and a second portion, said first portion adapted for connection to the intake manifold and said second portion connected to the control device, an idle stop valve connected in said vacuum passage to close the passage during idling of the engine, means defining an inwardly tapering flow orifice with walls extending abruptly outwardly at the end of said tapered orifice and leading into said first portion of said by-pass passage with said first portion extending away from said orifice, said second portion of the by-pass passage connected to said first portion immediately downstream of said orifice, and an adjustable trim valve connecting between a source of gas at higher pressure and said second portion of the by-pass passage upstream of said flow orifice so that the pressure signal delivered to the control device may be adjustably varied from the signal in the intake manifold.

6. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control fuel delivery to the engine comprising a pressure responsive control device for controlling fuel delivery to an engine as a function of a pressure signal, means defining a vacuum passage connected to the control device and adapted for connection to an intake manifold of the engine, means defining a by-pass passage with a first portion and a second portion, said first portion adapted for connection to the intake manifold and said second portion connected to the control device, an idle stop valve connected in said vacuum passage to close the passage during idling of the engine, means defining an inwardly tapering flow orifice with walls extending abruptly outwardly at the end of said tapered orifice and leading into said first portion of said by-pass passage with said first portion extending away from said orifice, said second portion of the by-pass passage connected to said first portion immediately downstream of said orifice, and an adjustable trim valve connecting between a source of gas at higher pressure and said second portion of the by-pass passageway upstream of said flow orifice so that the pressure signal delivered to the control device may be adjustably varied from the signal in the intake manifold, said trim valve having a larger flow passage than the passage of said second portion of the by-pass passage between the orifice and trim valve so that the pressure signal delivered to the control device may be higher than manifold pressure.

7. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control fuel delivery to the engine comprising a pressure responsive control device for controlling fuel delivery to an engine as a function of a pressure signal, means defining a vacuum passage connected to the control device and adapted for connection to an intake manifold of the engine, means defining a by-pass passage with a first portion and a second portion, said first portion adapted for connection to the intake manifold and said second portion connected to the control device, an idle stop valve connected in said vacuum passage to close the passage during idling of the engine, means defining an inwardly tapering flow orifice with walls extending abruptly outwardly at the end of said tapered orifice and leading into said first portion of said by-pass passage With said first portion extending away from said orifice, said second portion of the by-pass passage connected to said first portion immediately downstream of said orifice, an adjustable trim valve connecting between a source of gas at higher pressure and said second portion of the by-pass passage upstream of said flow orifice so that the pressure signal delivered to the control device may be adjustably varied from the signal in the intake manifold, and valve operating means connected to said idle stop valve and automatically closing the valve with closing of a throttle valve leading to the intake manifold to idle position.

8. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control the fuel supply to the engine and for modifying the vacuum signal comprising in combination a pressure responsive control device producing a displacement signal as a function of a pressure signal, means defining a vacuum passage connected to said control device and connected to the intake manifold of the engine, means defining a by-pass passage having a first portion connected to the intake manifold and a second portion connected to said control device for sending a compensated signal to the control device, valve means in the vacuum passage for closing the passage so that the control device will receive a signal solely from the bypass passage, means connected to said first portion of the by-pass passage for lowering the pressure therein to an absolute pressure lower than the pressure in the intake manifold as a function of the pressure in the intake manifold so that a lowered pressure signal will be received by said second portion of the by-pass passage, a trim orifice opening into said second portion of the by-pass passage, a trim valve connected to control fluid bled into said second passage through said trim orifice, and conduit means connected between said trim orifice and the intake manifold whereby the pressure supplied to the trim orifice is less than atmospheric pressure.

9. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control the fuel supply to the engine and for modifying the vacuum signal comprising in combination a pressure responsive control device producing a displacement signal as a function of a pressure signal, means defining a vacuum passage connected to said control device and connected to the intake manifold of the engine, means defining a by-pass passage having a first portion connected to the intake manifold and a second portion connected to said control device for sending a compensated signal to the control device, valve means in the vacuum passage for closing the passage so that the control device will receive a signal solely from the bypass passage, an air bleed orifice opening into said first portion of the by-pass passage, said second portion connected to said first portion immediately downstream of said air bleed orifice so that a pressure signal will be received by the second portion which is lower than the pressure of the intake manifold, a trim orifice opening into the second portion of the by-pass passage, a trim valve connected to control the size of the opening of the trim orifice, and conduit means connected between the trim orifice and the intake manifold so that the pressure supplied to the trim orifice will be less than atmospheric pressure.

10. An idle compensation device for fuel control systems for measuring the intake manifold vacuum of an engine to control the fuel supply to the engine and for modifying the vacuum signal comprising in combination a pressure responsive control device producing a displacement signal as a function of a pressure signal, means defining a vacuum passage connected to said control device and connected to the intake manifold of the engine, means defining a by-pass passage having a first portion connected to the intake manifold and a second portion connected to said control device for sending a compensated signal to the control device, valve means in the vacuum passage for closing the passage so that the control device will receive a signal solely from the by-pass passage, an air bleed orifice opening into said first portion of the by-pass passage, said second portion connected to said first portion immediately downstream of said air bleed orifice so that a pressure signal will be received by the second portion which is lower than the pressure of the intake manifold, a trim orifice opening into the second portion of the by-pass passage, a trim valve connected to control the size of the opening of the trim orifice, and means connected to said trim orifice for supplying a trim fluid at a pressure less than atmospheric pressure for improved control of the pressure in the second portion of the by-pass conduit with adjustment of the trim valve.

11. In a speed density fuel control system for a fuel injection supply for an Otto-cycle engine, a mechanism 10 for supplying -a compensated density signal at idling speeds comprising a first conduit for connection to the intake manifold of the engine, a second conduit for transmitting the compensated pressure signal, an air bleed orifice communicating between atmospheric air and the first conduit, a radially extending wall immediately downstream of the air bleed orifice, and a pressure take-01f port communicating with the first conduit immediately downstream of said orifice and communicating with the second con- 10 duit.

References Cited in the file of this patent UNITED STATES PATENTS Dolza et al. Mar. 24, 1959 McDuflie July 7, 1959 Huse July 7, 1959