Exhaust gas recirculation apparatus for diesel engine

A predetermined amount of exhaust gas recirculation is constantly provided through a first passageway (21) connected between an engine intake manifold (12) and an exhaust manifold (13). A vacuum actuated valve (19) constitutes a second passageway which is connected in parallel with the first passageway (21). The valve (19) is opened to allow exhaust gas recirculation through the second passageway except during idling and heavy load operation of an engine (11), thereby preventing excessive emission of smoke under these conditions.

BACKGROUND OF THE INVENTION 
The present invention relates to an improvement in an exhaust gas 
recirculation or EGR system for a diesel engine. 
It is well known to recirculate exhaust gas in a diesel engine back to the 
intake in order to reduce the concentration of toxic nitrogen oxides or 
NOx emitted therefrom. 
Such recirculation of engine exhaust tends to produce an increase in the 
emission of smoke and unburned hydrocarbon or HC. It is therefore 
necessary to minimize or virtually stop exhaust gas recirculation in a 
heavy load operating range of the engine wherein the emission of smoke is 
considerable. 
A conventional and typical system to meet the above demand employs means 
for sensing heavy load operation and cutting off the supply of control 
vacuum to a vacuum actuator associated with an EGR valve in the specific 
heavy load operating range, thereby interrupting the recirculation of 
engine exhaust. 
However, although the heavy load condition may be sensed during rapid 
starting or rapid acceleration of a vehicle to immediately cut off the 
control vacuum, the exhaust gas recirculation frequently fails to be 
immediately interrupted. The result is the emission of increased amounts 
of smoke and unburned HC and CO during rapid starting of the vehicle. 
Suppose that the accelerator pedal is depressed from a position 
corresponding to light load to the maximum depressed position for a quick 
start of the vehicle. Although this may be immediately responded to by 
specific means which sense the heavy load condition through the position 
of a control lever of a fuel injection pump operatively connected to the 
accelerator pedal, the control vacuum in the vacuum actuator (diaphragm 
assembly) of the EGR valve still remains therein. The EGR valve does not 
rapidly close but closes gradually as the control vacuum leaks from the 
vacuum actuator. 
Thus, despite the full depression of the accelerator pedal and the maximum 
volume of fuel injection, engine exhaust is recirculated back to the 
intake until the EGR valve fully closes. This is the cause of the sharp 
increase in the emission of smoke and unburned HC and CO. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an EGR system for a diesel 
engine which prevents increases in the smoke and HC and CO concentrations 
during rapid starting while preserving the effect of reducing NOx emission 
at and in the neighborhood of idling operation. 
An exhaust gas recirculation apparatus for an engine embodying the present 
invention has an intake manifold and an exhaust manifold, and is 
characterized by comprising a first exhaust recirculation passageway 
connected between the intake manifold and the exhaust manifold for 
constant exhaust gas recirculation, a second exhaust gas recirculation 
passageway connected between the intake manifold and the exhaust manifold, 
valve means disposed in the second exhaust gas recirculation passageway, 
sensor means for sensing an idling condition and a heavy load condition of 
the engine, and control means responsive to the sensor means for closing 
the valve means when the sensor means senses one of the idling condition 
and the heavy load condition and opening the valve means when the sensor 
means does not sense both the idling condition and the heavy load 
condition. 
In accordance with the present invention, a predetermined amount of exhaust 
gas recirculation is constantly provided through a first passageway 
connected between an engine intake manifold and an exhaust manifold. A 
vacuum actuated valve constitutes a second passageway which is connected 
in parallel with the first passageway. The valve is opened to allow 
exhaust gas recirculation through the second passageway except during 
idling and heavy load operation of the engine, thereby preventing 
excessive emission of smoke under these conditions. 
It is another object of the present invention to provide a generally 
improved exhaust gas recirculation apparatus. 
Other objects, together with the foregoing, are attained in the embodiment 
described in the following description and illustrated in the accompanying 
drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the exhaust gas recirculation apparatus of the present invention is 
susceptible of numerous physical embodiments, depending upon the 
environment and requirements of use, substantial numbers of the herein 
shown and described embodiment have been made, tested and used, and all 
have performed in an eminently satisfactory manner. 
Referring to FIG. 1, there is shown a diesel engine 11 having an intake 
manifold 12 and an exhaust manifold 13. A turbocharger 14 is associated 
with the engine 11 and has a turbine 16 driven for rotation by the engine 
exhaust and a compressor 17 operated by the turbine 16 to compress intake 
air. The turbocharger 14 thus supercharges air flowing to the intake 
manifold 12. 
The reference numeral 18 denotes an exhaust gas recirculation or EGR 
passageway which extends from the exhaust manifold 13 upstream of the 
turbine 16 to the intake manifold 12 down stream of the compressor 17. 
The EGR passageway 18 has therein in parallel relation an EGR valve 19 and 
a constant orifice 21 bypassing the EGR control valve 19 which constitute 
second and first passageways respectively. 
A control vacuum from a vacuum pump 22 is selectively communicated to the 
EGR valve 19 through an electromagnetic valve 23. 
The EGR valve 19 is furnished with a diaphragm assembly 24 having a 
flexible diaphragm member 26. The diaphragm 26 is connected to one end of 
a valve rod 27 so as to pull it up against the action of a spring 28 in 
accordance with the control vacuum in a vacuum chamber 29 of the assembly 
24. 
The diaphragm 26 of the assembly 24 defines at its other side an 
atmospheric chamber 31 which communicates with the atmosphere. 
The constant orifice 21 serves to constantly recirculate a small volume of 
engine exhaust gas through the EGR passageway 18. 
The constant orifice 21 causes exhaust gas recirculation even in a very 
light load range (idling and its vicinity) and a heavy load range thereby 
reducing the concentration of nigrogen oxides or NOx in the exhaust gas. 
It will be understood that the EGR rate through the constant orifice 21 is 
controlled accurately despite the fact that it is preselected to be 
relatively small to avoid emission of smoke in the heavy load range. 
Should an EGR valve be employed to control such a small rate of exhaust gas 
flow, strict control over the valve precision and operating 
characteristics would be required at the stage of production. The control 
with the constant orifice 21 enables far higher productivity because it 
needs precision only in the effective cross-sectional area of the constant 
orifice 21. 
Operation of the electromagnetic valve 23 is controlled by a pair of 
switches 32 and 33 each of which is opened and closed depending on the 
position of a control lever 34 of a fuel injection pump 36. The control 
lever 36 is operatively connected to an accelerator pedal (not shown). 
As best shown in FIG. 2, the switch 32 is a limit switch responsive to a 
very light load condition. The switch 32 is opened when the control lever 
34 has an angular position within the range A between the vicinity of the 
idling opening (high idling opening somewhat larger than the idling 
opening) and the full closed position (idling opening), otherwise 
remaining closed. 
The second sitch 33 is opened when the control lever 34 is within the range 
B between medium and heavy load ranges approximate to the full open 
position and the heavy load range. In the light and medium load ranges 
other than those mentioned, the switch 33 is closed. 
The switches 32 and 33 are connected in series between the electromagnetic 
valve 23 and a power supply or battery 37. The valve 23 is disposed in a 
conduit 38 which communicates vacuum from the vacuum pump 22 to the vacuum 
chamber 29 of the diaphragm assembly 24. When both of the switches 32 and 
33 are closed, the valve 23 is opened to pass the vacuum from the vacuum 
pump 22 to the EGR valve 19 for opening the valve 19 and increasing the 
amount of exhaust gas recirculation. When either one of the switches 32 
and 33 is opened, the valve 23 introduces atmospheric pressure into the 
chamber 29 to close the valve 19. 
The exhaust gas recirculation apparatus thus constructed operates as 
follows. 
During idling of the engine, the switch 32 is opened to cause the valve 23 
to cut off the supply of vacuum while introducing atmospheric pressure 
into the diaphragm assembly 24. As a result, the EGR valve is brought to 
its full closed position. 
Under this condition, recirculation of exhaust gas takes place only through 
the constant orifice 21 of the EGR passageway 18. 
In low and medium load ranges of the engine 11, the control lever 34 of the 
fuel injection pump 36 is between the switches 32 and 33 and remains clear 
thereof. The switches 32 and 33 are thus closed causing the valve 23 to 
admit vacuum from the pump 22 to the assembly 24. 
In this case, the EGR valve 19 is in its full open position so that the EGR 
rate becomes the sum of the flow rate through the EGR valve 19 and that 
through the constant orifice 21. 
In the heavy load range of the engine, the switch 33 is opened and the EGR 
valve 19 is again closed to permit exhaust gas recirculation only through 
the constant orifice 21. 
When the accelerator pedal is rapidly depressed from its full closed 
position to its full open position to abruptly accelerate the engine, the 
switch 32 is first opened and then the switches 32 and 33 are both closed 
for a moment whereupon the switch 33 is opened. 
Accordingly, the diaphragm assembly 24 of the EGR valve 19 receives 
atmospheric pressure, then momentary vacuum, and again atmospheric 
pressure. 
Despite the action of the vacuum, the EGR valve 19 remains closed during 
the rapid acceleration because such momentary and, additionally, delayed 
action of the vacuum cannot produce a force large enough to open the EGR 
valve 19. 
It will be appreciated that, during rapid starting, the exhaust gas 
recirculation rate is small and is regulated by the constant orifice 21 
and therefore positively prevents generation of smoke. 
FIG. 3 is a plot demonstrating the relationship between the effective 
cross-sectional area of the EGR passageway 18 (total area of the EGR valve 
19 and constant orifice 21) and the position of the accelerator pedal. 
In summary, an EGR system according to the present invention prevents a 
control vacuum from remaining in a diaphragm assembly during rapid 
starting or acceleration and thereby allows and EGR valve to be kept 
closed even under such conditions. This avoids emission of smoke and 
increases in the HC and CO concentrations in the event of rapid starting. 
During idling, the NOx concentration in the engine exhaust can be reduced 
without degrading the operation of the engine because a constant orifice 
regulates the exhaust gas recirculation. 
It will be apparent that a system of the present invention is similarly 
applicable to engines without turbochargers. 
Various other modifications will becomes possible for those skilled in the 
art after receiving the teachings of the present disclosure without 
departing from the scope thereof.