Engine Control Suitable for Exhaust Gas Recirculation Control

This invention relates to a method and apparatus suitable for controlling exhaust gas recirculation which allows recirculation of exhaust gases to be adjusted automatically with a change in atmospheric pressure.

Recirculation of exhaust gases has been employed in automotive engines to inhibit the formation and emission of oxides of nitrogen. In general, it is desired to vary the proportion of exhaust gases which are recirculated in accordance with changes in engine operating conditions; for example, in a diesel engine it is desired to increase recirculation as the throttle reduces fuel flow, to increase recirculation with engine speed, and to reduce recirculation with a decrease in the ambient atmospheric pressure. The exhaust gas recirculation valve is frequently operated by a diaphragm responsive to a subatmospheric control pressure, and various electrical, pneumatic and mechanical devices have been employed to establish the desired control pressure.

This invention provides a novel engine control method and apparatus in which, when used for controlling exhaust gas recirculation (EGR), a diaphragm operated EGR valve is positioned in accordance with a control pressure in a manner that allows recirculation of exhaust gases to be reduced automatically in response to a decrease in the ambient atmospheric pressure.

To provide automatic control of exhaust gas recirculation with ambient pressure, as set forth herein, a diaphragm operated EGR valve is opened to permit recirculation of exhaust gases in accordance with the difference between atmospheric pressure and a control pressure. The absolute value of the control pressure is measured and compared with a command pressure signal which would effect the recirculation desired for the then existing engine operating conditions, and the control pressure is adjusted to equal the command pressure signal. As the command pressure signal is varied with changes in engine speed, throttle position, and/or other engine operating conditions, the control pressure is changed to effect recirculation of the desired proportion of engine exhaust gases. Should the ambient atmospheric pressure change, as with a change in altitude, for example, the absolute value of the command pressure signal and thus of the control pressure would not change; however, the difference between atmospheric pressure and the control pressure would change, and the diaphragm operated EGR valve is thereby automatically repositioned to adjust the proportion of exhaust gases which are recirculated.

The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are evident from the accompanying drawing.

The sole FIGURE of the drawing is a schematic view of a diesel engine exhaust gas recirculation control system which operates in accordance with the invention.

Referring to the drawing, a conventional exhaust gas recirculation (EGR) control valve assembly 10 has a valve body 12 defining a portion of a recirculation passage 14 for recirculating exhaust gases from the engine exhaust passage to the engine intake passage. An EGR valve pintle 16 is positioned in passage 14 by a diaphragm 18 and is biased by a spring 20 to engage a valve seat 22. When a subatmospheric control pressure is applied to a fitting 24, diaphragm 18 responds to the difference between the control pressure above diaphragm 18 and the ambient atmospheric pressure below diaphragm 18 and lifts EGR valve pintle 16 against the bias of spring 20 to permit recirculation of exhaust gases through passage 14.

The control pressure is applied to fitting 24 by a conventional small vacuum follower unit 26 which is connected to a conventional duty cycle modulated solenoid unit 28. Solenoid unit 28 has a fitting 30 for admitting air at ambient atmospheric pressure and a fitting 32 connected to a source of subatmospheric pressure such as the vacuum pump on a diesel engine. Within solenoid valve unit 28, a coil 34 is energized according to a variable duty cycle to reciprocate an electromagnetically responsive valve member 36 against the bias of a spring 38. When coil 34 is energized, valve member 36 is pulled back against spring 38 to open subatmospheric pressure fitting 32 and close atmospheric pressure fitting 30, and when coil 34 is deenergized, spring 38 pushes valve member 36 to open atmospheric fitting 30 and close subatmospheric fitting 32. As the duty cycle of coil 34 is increased, valve member 36 spends a lesser proportion of time obstructing subatmospheric fitting 32 and a greater proportion of time obstructing atmospheric fitting 30; conversely, as the duty cycle of coil 34 is decreased, valve member 36 spends a lesser proportion of the time obstructing atmospheric fitting 30 and a greater proportion of the time obstructing subatmospheric fitting 32. Solenoid valve unit 28 thereby generates an operating pressure in a fitting 40, the absolute value of which decreases with an increase in the duty cycle of coil 34. When the duty cycle is increased, the reduced time available for air flow through atmospheric fitting 30 results in a decrease in the absolute value of the operating pressure in fitting 40; when the duty cycle is decreased, the reduced time available for flow through subatmospheric fitting 32 results in an increase in the absolute value of the operating pressure in fitting 40.

A fitting 42 on vacuum follower unit 26 receives the operating pressure from fitting 40 and applies it to the upper surface of a diaphragm 44. The lower surface of diaphragm 44 is exposed to the control pressure applied through a fitting 46 to fitting 24 on EGR valve assembly 10. When the force created by the control pressure below diaphragm 44 exceeds the force created by the operating pressure above diaphragm 44 and the bias of a spring 48, diaphragm 44 lifts away from the top 50 of a valve member 52; an orifice 54 through diaphragm 44 is then exposed, allowing the operating pressure above diaphragm 44 to reduce the control pressure below diaphragm 44. When the force created by the operating pressure above diaphragm 44 and the bias of spring 48 exceeds the force created by the control pressure below diaphragm 44, diaphragm 44 pushes a seat 56 surrounding orifice 54 against the top 50 of valve member 52; the flange 58 of valve member 52 is then disengaged from an annular valve seat 60 against the bias of a spring 62, allowing air at atmospheric pressure to enter from a fitting 64 and increase the control pressure below diaphragm 44.

Thus it may be seen that vacuum follower unit 26 causes the control pressure applied to EGR valve assembly 10 to follow the operating pressure created by solenoid unit 28. As the duty cycle of solenoid unit 28 is decreased to increase the operating pressure, vacuum follower unit 26 admits air from fitting 64 to increase the control pressure. As the duty cycle of solenoid unit 28 is increased to decrease the operating pressure, orifice 54 in vacuum follower unit 26 is opened to reduce the control pressure. The control pressure thus follows the operating pressure and is offset from the operating pressure an amount determined primarily by the bias of spring 48.

A transducer 66 measures the absolute value of the control pressure applied to EGR valve assembly 10 and generates a control pressure signal on line 68 which is indicative thereof. A conventional memory unit 70 generates a command pressure signal on line 72 indicative of the control pressure desired for the then existing combination of engine speed (RPM), throttle position, and/or other engine operating conditions. The control pressure signal and the command pressure signal are compared at 74, and any deviation is directed on line 76 to a duty cycle generator 78 which energizes coil 34. As is conventional, duty cycle generator 78 has proportional and integral functions which decrease the duty cycle to increase the control pressure when the control pressure signal is less than the command pressure signal and which increase the duty cycle to reduce the control pressure when the control pressure signal is greater than the command pressure signal. This closed loop pressure regulation accordingly minimizes any deviation of the actual control pressure from the desired control pressure, and EGR valve assembly 10 is positioned to provide the recirculation of exhaust gases desired for the then existing engine operating conditions.

The ambient atmospheric pressure decreases substantially when the engine is operated at higher altitudes, and the proportion of engine exhaust gases which are recirculated should be decreased accordingly. Such is accomplished automatically in this system because transducer 66 measures the absolute value of the control pressure while EGR valve assembly 10 positions valve pintle 16 in accordance with the difference between the absolute value of the control pressure and the absolute value of the atmospheric pressure. Thus as the atmospheric pressure applying an upward force on the bottom of diaphragm 18 decreases, spring 20 urges valve pintle 16 toward its seat 22 to reduce recirculation of exhaust gases. Accordingly, with this invention exhaust gas recirculation is adjusted automatically with changes in ambient atmospheric pressure, without any requirement for a separate transducer to measure the ambient atmospheric pressure.