Patent Application
20140238364
The present invention relates to an apparatus for improving exhaust gas recirculation performance. In particular, the present invention relates to an apparatus for inducing exhaust gas recirculation flow into an intake manifold of an engine such as a turbocharged diesel engine to provide improved recirculated exhaust gas mixing and more consistent recirculated exhaust gas flow during engine operating transients.
In the field of vehicle emissions controls, it is well known that during certain operating states of the engine undesired combustion products such as oxides of nitrogen (“NOx”) may be minimized by introducing a portion of the exhaust gases leaving the engine's combustion chambers back into the engine's intake manifold. The recirculated exhaust gas dilutes the incoming fresh intake air, resulting in a mixture to the engine that provides two primary mechanisms for reducing NOx formation. The first mechanism is the mixture reducing the peak in-cylinder combustion temperatures where the exhaust gas acts as a heat sink. The second mechanism is the dilution of the fresh air stream, displacing some of the oxygen which would have otherwise been drawn into the combustion chamber. The lower oxygen content results in fewer constituent oxygen atoms that feed the creation of NOx and results in an overall reduction of NOx formation.
In conventional internal combustion engines, such as for example the engine 1 shown schematically in
A well known problem with exhaust gas recirculation systems is the tendency for recirculating exhaust gas flow from the exhaust to the intake manifold to decrease or even halt during certain engine operating conditions, i.e., when there exists an unfavorable pressure ratio between the exhaust and the intake lines, or low exhaust mass flow rate conditions are present. For example, in response to a sudden increase in engine power output demand, there may be too little exhaust gas flow available in the exhaust to supply the intake manifold with sufficient recirculated exhaust gas to match the sudden increase in oxygen and fuel being supplied to the engine's cylinders. In such situations, the lack of sufficient recirculated exhaust gas may result in an inability to adequately suppress NOx formation during the transient condition, and a corresponding potential to exceed NOx emissions requirements.
An additional problem some recirculating exhaust gas systems can experience is with the use of a turbocharger, in that the turbocharger may develop sufficient pressure in the intake manifold to effectively halt the flow of exhaust gas through the exhaust gas recirculation line to the intake, particularly during engine output demand transients.
Previous attempts to improve exhaust gas recirculation flow primarily have concentrated on building backpressure in the downstream exhaust piping, such as by at least partially closing a downstream exhaust brake valve located upstream or downstream of the turbine side of a turbocharger, or by using a costly variable geometry turbocharger whose vanes may be adjusted to reduce flow through the turbocharger and thus build backpressure. Such approaches increase the pressure differential across the exhaust gas recirculation line between the exhaust line and the intake manifold. However, even with the assistance of such exhaust line components, adequate exhaust gas recirculation flow to the intake manifold cannot be assured in many transient engine operating conditions.
In view of these and other problems in the prior art, it is an objective of the present invention to provide enhanced exhaust gas recirculation flow in all operating engine conditions, including in particular transient engine operating conditions. It is a further objective to provide improved mixing of recirculated exhaust gas and fresh air in these operating conditions.
These and other objectives are addressed by a novel arrangement of a venturi and pitot tube nozzle components which establishes conditions in which the Coand{hacek over (a)} effect may be employed to enhance exhaust gas recirculation flow under virtually any engine operating condition. These arrangements thereby help minimize the potential for exceeding emissions limitations during engine operating conditions, including transient conditions.
The Coand{hacek over (a)} effect is a phenomenon in which a fluid jet, such as a gas jet, flowing in the same type of fluid (e.g., a gas jet flowing in a gas, or a liquid jet flowing in a liquid) is deflected as it passes an adjacent convex surface, following the contour of the convex surface until the fluid jet flow separates from the surface. In effect, the high-velocity jet conforms to the convex surface because the fluid in which the high-velocity jet is flowing is not present between the high-velocity jet and the adjacent convex surface, i.e., the high-velocity jet's deflection “fills” the vacuum which would otherwise be created between the prevents high-velocity jet and the adjacent convex surface. The deflection toward the convex surface is accompanied by a drop in pressure and an increase in fluid velocity in the vicinity of the adjacent surface, in accordance with Bernoulli's fluid flow equations.
The decrease in fluid pressure resulting from the Coand{hacek over (a)} effect may be used to augment a pressure differential and/or to increase a mass flow rate in another fluid. For example, in aircraft applications a wing's lift may be increased by discharging the exhaust gases from a jet engine over the top surface of a wing so that high velocity exhaust gas jet flow deflected along the convex wing upper surface experiences a decrease in pressure, effectively increasing the pressure differential between the lower and upper surfaces of a wing. In other applications, the decreased pressure and increased velocity in the high-velocity jet at the point at which it must follow an adjacent convex surface may be used to enhance the mass flow rate of an adjacent fluid stream by: (i) increasing the differential pressure between an upstream point at which the adjacent flow stream enters the flow channel and the point where the Coand{hacek over (a)} effect is being generated, and (ii) increasing the lateral dispersion of the adjacent fluid into the high-velocity fluid by physical entrainment in the high Coand{hacek over (a)} flow region. This high velocity entrainment also helps promote more thorough mixing of the fluid streams.
In an embodiment of the present invention, an exhaust gas recirculation line having the general shape of a pitot tube is located with an opening of the pitot tube axially aligned with the center of an outer tube of a venturi manifold though which relatively high pressure fresh air flows. Preferably, the outlet of the pitot tube is facing in the downstream flow direction and is axially located in the vicinity of an entrance of a necked-down region of the outer tube. The transition between a decreasing-diameter region of the outer tube and a necked-down, relatively-straight region of the outer tube provides a convex wall surface on the inside of the outer tube. As the fresh air flowing in this area conforms to the convex surface, the Coand{hacek over (a)} effect results in decreased pressure and increased velocity in the fresh air. Further, by locating the outlet of the pitot tube at the outer tube center line in the transition region between the widest and narrowest regions of the outer tube (i.e., in the converging flow region), the outer surface of the pitot tube effectively reduces the cross-sectional flow area for the fresh air in the outer tube, thereby further increasing the fresh air velocity and lowering the fresh air pressure for a given fresh air mass flow rate.
The relationships between the parameters of the present invention's flow enhancement may be varied as needed for the application, as long as suitable flow performance is maintained. For example, in the prior art the “velocity ratio” between the velocity of the recirculating exhaust gas being injected and the fresh intake air upstream of the exhaust gas injection point had typically been on the order of approximately 1.5. With the enhancement associated with the application of the Coand{hacek over (a)} effect in the present invention, velocity ratios of approximately 2.0 to 5.0 result, allowing better mixing efficiency, increased exhaust gas flow and prevention of back flow in the exhaust gas conduit.
The parameters affecting the velocity ratio include inner diameter D1 of the fresh air tube upstream of the recirculating exhaust gas injection point, the inner diameter D2 of the narrowed portion of the tube, the angle of convergence a of the converging portion of the tube (D1, D2 and a thereby defining the length l of the converging portion), the depth of insertion x and the outer diameter d1 of the exhaust gas injection conduit into the converging portion of the tube, and the mass flow rate of the recirculating exhaust gas m1 and the fresh air m2. Taken together, these variables define annular area A between the injection end of the recirculating exhaust gas conduit and the laterally adjacent converging tube wall (i.e., A=f(D1, D2, d1, x, a), and the resulting velocity ratio VR (i.e., VR=f(D1, D2, d1, x, a, m1, m2).
With this arrangement of the exhaust gas recirculation pitot tube and the converging fresh air outer tube, the significantly reduced pressure in the vicinity of the outlet of the pitot tube presents a substantially enhanced pressure differential between the exhaust gas line and fresh air outer tube. This increased pressure difference serves to significantly increase the mass flow rate and velocity of the exhaust gas extracted from the pitot tube outlet, even during certain engine operating conditions in which the pressure ratio between the exhaust and intake manifolds is typically unfavorable.
A further benefit of the use of the Coand{hacek over (a)} effect is to enhance the lateral migration of the exhaust gas extracted from the pitot tube, where the decrease in pressure in the fresh air as it flows long the convex outer wall surface effectively draws the exhaust gas laterally toward the tube wall. This effect provides a much greater homogeneity in the recirculated exhaust gas fresh air mixture flowing toward the combustion chambers, and a corresponding cross-sectional velocity profile in the outer tube in which the typical “V” distribution from the center outward is flattened. The enhanced mixing also occurs in a much shorter distance than in conventional exhaust gas recirculation systems which simply discharge exhaust gas directly into the engine fresh air intake line. The lateral mixing of the exhaust gas with fresh air may be further enhanced by providing a divergent outer tube region downstream of the outer tube convergent flow region.
The improved homogeneity of the fresh air/recirculated exhaust gas mixture entering the engine's combustion chambers further enhances control of NOx formation in the combustion chamber by substantially reducing localized regions of over- and under-supply of recirculated exhaust gas in the combustion chamber.
Previous systems using a pitot tube to inject exhaust gas into the fresh air charge tube used a straight pitot tube in a manner which primarily only provided an exhaust gas entry point in the fresh air flow at or near a venturi section, resulting in the exhaust gas being swept along in a relatively homogeneous flow inside the fresh air column for a substantial downstream distance. Accordingly, in order to try to avoid an inhomogeneous air/exhaust gas mixture entering the engine cylinders (potentially increasing NOx emissions), previous designs required an undesirably long intake tract downstream of the exhaust gas injection point in order to provide sufficient gas mixing. Such long intakes are difficult for designers to accommodate in the highly space-constrained engine compartments of modern vehicles.
In order to address the lack of homogeneous mixture in the above-described previous design, it has been known to provide multiple small exhaust gas inlet tubes clustered in the fresh air charge tube to provide enhanced mixing at a shorter distance from the exhaust gas injection point. However, these designs suffer from the disadvantage of not providing a pressure gradient sufficiently high to ensure adequate exhaust gas recirculation flow in all engine operating conditions.
In contrast to these previous designs, the arrangement of the venturi and pitot tube to generate a Coand{hacek over (a)} effect-driven enhancement of exhaust gas extraction from the pitot tube outlet results in significantly greater homogeneity in the mixed fresh air/exhaust gas flow, and achieves a higher level of this homogeneity in a considerably shorter distance. This higher performance allows the designer much greater flexibility in engine systems layout, as excessively long downstream intake passages are no longer required to ensure a sufficiently well mixed charge arrives in the engine cylinders.
Variations on the above embodiment to achieve equivalent functional arrangements are possible. For example, rather than providing an outer tube which has to be contoured to achieve the converging, convex and diverging regions, an appropriately-shaped tubular insert may be placed inside a constant-section outer tube.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.
a is a schematic illustration of an exhaust gas recirculation system arrangement in accordance with an embodiment of the present invention.
b is a schematic illustration of an exhaust gas recirculation system arrangement in accordance with an embodiment of the present invention.
a-4c are illustrations of an example gas velocity distribution in the exhaust gas recirculation system arrangement of
a-5b are an illustration of an example distribution of exhaust gas and fresh air mixture as a function of location and distance from the conduit wall in an exhaust gas recirculation system arrangement similar to that illustrated in
a is a simplified schematic cross-sectional illustration of an embodiment of the present invention.
The outlet 203 is positioned directly in the vicinity of the narrowest portion 221 of a converging portion 222 of the fresh air intake 220. The converging portion 222 is followed by an approximately parallel-walled, constant cross-section portion 223 (also referred to as the “throat”), which in turn is followed by a diverging portion 224. This generally venturi-shaped portion of the fresh air intake 220, combined with the careful location of the pitot tube-shaped outlet 203 directly adjacent to the narrowest portion 221 of the converging section 222. In an example embodiment, the venturi throat may have a diameter of 44.5 mm, the pitot tube may have an outer diameter of 38.1 mm, and the venturi is located at distance of 2.4 mm from the throat to obtain Coand{hacek over (a)}-driven enhancement of the exhaust gas flow from the pitot tube outlet 223.
As the flow of fresh air in fresh air intake 220 passing through intake flow control device 250 encounters the portion of the exhaust gas recirculation conduit 202 within the fresh air intake 220, the flow of fresh air accelerates to a higher velocity as this portion of the conduit 202 effectively reduces the available cross-sectional flow area within the fresh air intake 220 (as one of ordinary skill will recognize, the decrease in flow area requires a proportionate increase in flow velocity in order to maintain the mass flow rate of the incoming fresh air). The increase in fresh air velocity is accompanied by a commensurate decrease in pressure in the fresh air, per Bernoulli's well-known flow equations. As the fresh air passes further down the intake 220 into the converging portion 222, velocity is further increased and pressure is decreased due to the decrease in cross-sectional flow area. The amount of reduction in cross-section preferably is optimized for each engine application, preferably such that the length of the converging portion and the diameter of the intake 220 at the narrowest portion 221 together provide the greatest acceleration of the fresh air without creating a choke point which undesirably raises back-pressure in the intake tube.
As the now-faster-moving fresh air flow near the inner wall of the intake 220 passes the narrowest part of the converging portion 222, the flow is presented with a convex-shaped wall surface as it passes from upstream to downstream of narrowest portion 221. As the fresh air flow passes in the region of the convex wall, in the absence of another fluid between the fresh air and the convex wall, the Coand{hacek over (a)} effect causes the fresh air flow to be deflected toward the walls of the narrow constant cross-section portion 223, further reducing the pressure in the fresh air flow (and thereby increasing the fresh air flow velocity) as the fresh air flow must expand from its flow stream toward the downstream portion of the convex wall surface.
The cumulative effect of these several fresh air flow velocity increases and pressure decreases is to present a particularly low pressure area in the region at, and immediately downstream of, the outlet 203 of the pitot tube-shaped end of exhaust gas recirculation conduit 202. This localized low pressure region acts to greatly assist in the extraction of recirculating exhaust gas flow from the conduit by increasing the pressure differential between the exhaust manifold 210 and the fresh air manifold 220. The relatively high pressure difference between these manifolds caused by the local pressure decrease at outlet 203 maintains the strong positive flow of recirculated exhaust gas in virtually all engine operating conditions of interest, even conditions which previously might have resulted in low or no recirculation of exhaust gas and exceeding emissions limits.
In addition to helping aid extraction of recirculated exhaust gas from the exhaust gas recirculation conduit 202, the location of the outlet 203 at or directly upstream of the convex wall portion of the converging portion 222 to take advantage of the Coand{hacek over (a)} effect enhances the lateral migration of the exhaust gas extracted from the pitot tube. The Coand{hacek over (a)} effect, which draws the fresh air toward the wall of the passage and thereby creates a lower pressure area at the inner annulus of the fresh air passing by outlet 203, effectively draws the exhaust gas emerging from the outlet 203 laterally toward the tube wall. This radially-outward-drawing of the exhaust gas provides greater mixing and homogeneity in the exhaust gas/fresh air mixture flowing toward the combustion chambers, and does so in a much shorter distance than in conventional exhaust gas recirculation systems which simply discharge exhaust gas directly into the engine fresh air intake line. This effect may be further enhanced by providing a divergent outer tube region downstream of the outer tube convergent flow region, the lateral mixing of the exhaust gas with fresh air is further enhanced.
Illustrated in the
However, by use of the present invention the extraction of recirculated exhaust gas into the fresh air intake is assured, due to the pressure reductions in the fresh air flow caused by the venturi and Coand{hacek over (a)} effects. In this example, the pressure seen by the exhaust gas at the tip of the EGR conduit 302 (i.e., at outlet 303) is 296581 Pa, or 1152 Pa lower than the pressure at EGR recirculation conduit inlet 304. This pressure differential provides for reliable extraction of recirculated exhaust gas from the EGR recirculation conduit 302, despite the greater pressure in the fresh air inlet tube 320. Moreover, as the fresh air encounters the portion of the EGR conduit 302 within the fresh air inlet tube 320, the reduction in flow cross-sectional area also reduces the pressure of the fresh air, such that it can support the maintenance of the desired low pressure at the EGR conduit outlet 303. This pressure reduction in response to the fresh air velocity increase may be observed in the
a-4c are an illustration of the velocity distribution of the fresh air and recirculated exhaust gases in the exhaust gas recirculation system arrangement of
By the end of the narrowest flow section (point 412), the flow velocity just inside the wall is on the order of 50 m/s, while the velocity in the center of the cross-section at point 412 is on the order of 20 m/s, indicating that the fresh air and recirculated exhaust gas is mixed to an even greater extent. By the time the mixed gas flow reaches intake point 408, the velocity distribution is nearly uniform across the flow section, varying from approximately 10 m/s to 30 m/s all of the way across the cross-section of the tube 420. Such a thorough mixing of the recirculated exhaust gas in the fresh intake air helps ensure an essentially homogeneous distribution of recirculated exhaust gas in the air admitted into the engine's combustion chambers, minimizing the potential for “hot spots” during fuel combustion in the chamber and undesired localized high concentrations of NOx.
An example of the thorough mixture of gases in a short distance is illustrated in
During operation of the internal combustion engine, the exhaust gas recirculation enhancing apparatus may operate in a generally autonomous manner, with the accelerated, decreased pressure fresh air flow serving to help draw exhaust gas from the exhaust gas recirculation conduit even when an unfavorable pressure ratio is present. The exhaust gas recirculation arrangement may be further operated in coordination with additional vehicle components, such as a mechatronic exhaust brake, a variable geometry exhaust turbocharger, a EGR conduit flow control valve and a pressurized air injection boost system (a “PBS” system) to more rapidly and/or more precisely control the rate and timing of recirculated exhaust gas injection into the fresh air stream. For example an electronic control unit (“ECU”) assigned to the vehicle engine executes a control program in which a variety of sensor inputs, such as engine torque demand, engine speed, exhaust gas temperature, intake air mass flow rate, pressure and/or temperature, exhaust flow control device position, exhaust gas treatment device operating condition (e.g., needing regeneration), exhaust oxygen content sensor, fresh air intake throttle plate position, etc., are used to define a target exhaust gas recirculation flow rate into the engine fresh air intake manifold, and actuation of the various control devices in the intake and exhaust systems to provide the target amount of recirculated exhaust gas to optimize emissions for the present combustion cycle. The EGR flow then may be iteratively refined via closed-loop monitoring of exhaust emissions (such as by an exhaust oxygen sensor) to maintain the recirculated exhaust gas concentration at a level which minimizes emissions.
With a sufficiently high speed processor, the ECU may be able to preemptively increase EGR flow to respond to an anticipated torque demand increase to more quickly match emissions targets. For example, the system may operate the system actuators to increase exhaust gas flow in response to receiving a sudden increase in driver torque demand signaled through the driver's throttle pedal, rather than waiting for the ECU to see a change in the exhaust oxygen concentration, or operate actuators in anticipation of a torque demand increase based on input from a GPS device signaling an upcoming road gradient.
The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. For example, while the foregoing disclosure refers to the exhaust gas recirculation conduit in which the outlet region is formed in a pitot tube-shape, other configurations which are suitable for enhanced exhaust gas extraction from the EGR conduit would be acceptable. A large number of alternative embodiments are also envisioned, both to permit tailoring of the EGR injection arrangements to suit the individual engine and/or vehicle arrangements, and to suit individual vehicle equipment specification, such as the presence or absence of an exhaust gas back pressure-generating valve downstream in the exhaust line to enhance or alter the pressure difference between the exhaust and intake manifolds. For example, a lower flow-restricting venturi/Coand{hacek over (a)} effect arrangement may be provided in the intake which, while on its own is not sufficient to cover all desired EGR injection flow regimes, nonetheless may be teamed with a mechatronic exhaust brake valve to provide sufficient EGR flow in all operating conditions of concern, thereby minimizing flow restrictions in both the intake and exhaust manifolds while achieving the desired EGR mixing and minimization of exhaust emissions. Configuration alternatives may include integration of the exhaust gas recirculation and venturi section into an integrated fresh air intake module to minimize system packaging space and facilitate easy integration into new engine designs and/or retrofitting of older vehicles with the present invention. Such a module may also include an intake throttle valve and/or a pneumatic boost system (“PBS”) compressed air injection apparatus, as schematically indicated by the dashed line in
