GB2492994A - Exhaust gas recirculation arrangement for an internal combustion engine with sequential turbocharger - Google Patents

Abstract

Disclosed is an Exhaust Gas Recirculation (EGR) arrangement for an internal combustion engine 11 with sequential turbocharger, arrangement comprising a low pressure exhaust gas recirculation duct linking a position downstream of turbocharger turbines 13, 14 to a location between a first turbocharger compressor 25 and a second turbocharger compressor 26, i.e. a location downstream from one compressor and upstream of the other. The EGR duct may also have a branch to the turbocharger compressor inlet and a suitable flow control valve. A non-return valve may be provided in the EGR duct at a suitable location. A high pressure exhaust gas recirculation duct may also be provided.

Description

I

Exhaust Gas Recirculation For An I.C. Engine This invention relates to exhaust gas recirculation (FOR) for an internal combustion engine, typically a diesel engine for a motor vehicle. Aspects of the invention relate to an arrangement, to an engine, to a method and to a vehicle.

FOR is a technique used to reduce nitrogen oxide (NOx) emissions of an internal combustion engine, by selectively recirculating some exhaust gas to the engine inlet manifold. As a consequence the amount of excess oxygen is reduced, the peak combustion chamber temperature can be reduced, and thus the amount of NOx which is generated.

EOR is very common in motor vehicle engines in order to meet the requirements of emissions legislation. It does however have some less desirable characteristics. These may include reduced power because of less efficient combustion, and particularly in diesel engines the introduction of contaminants to the engine inlet manifold, which may cause early deterioration of engine components and lubrication oil.

Internal combustion engines are commonly turbocharged in order to maximize power output and efficiency, and this forced induction necessarily results in an increase in inlet manifold pressure, which may prevent or at least obstruct the in-flow of exhaust gas via the FOR duct.

One solution to this problem requires the exhaust manifold pressure to be increased, for example by partially closing the vanes of a variable geometry turbocharger, but this is not desirable because exhaust back pressure on the engine is also increased. Any restriction in the EOR duct exacerbates this problem.

FOR in diesel engines may require recirculation of up to 40% of the exhaust gas volume.

The inevitable consequence is that the proportion passing through the turbocharger is reduced to 6O%, which has the effect of reducing turbocharger performance and efficiency.

A relatively expensive solution, such as a variable geometry turbocharger may provide a partial solution to this selective reduction in exhaust gas flow.

It will also be understood that EGR utilizes exhaust gas from the exhaust manifold, which is very hot, and therefore less dense than ambient air admitted to the inlet manifold; such ambient air may be heated in the compressor side of the turbocharger, but is typically cooled via an air to air intercooler. This dense cooled air is mixed with relatively hot less dense exhaust gas during FOR, which means that a larger comparative volume of exhaust gas is required for a given effect. This circumstance can be ameliorated by including an exhaust gas cooler in the EOR duct, by which heat is rejected, typically to the engine coolant. The exhaust gas cooler imposes a flow restriction on gas flowing in the FOR duct, as noted above; the more effective the cooler, the greater the flow restriction which is imposed.

Packaging space must also be found to install such a cooler within the confines of an engine compartment, which is often difficult.

Conventional FOR raises the average temperature of the mixed gas in the inlet manifold, which is thereby made less dense. In consequence the oxygen content is reduced, and accordingly the power which can be generated in each combustion cycle is also reduced.

Conventional high pressure FOR provides little opportunity for good mixing of exhaust gas with inlet air, since the outlet of the FOR duct is typically very close to the inlet manifold.

Better mixing would allow closer control of combustion and more consistent combustion.

It will be appreciated from the foregoing that conventional FOR, whilst being a valuable technique, has many characteristics which are less than desirable.

It has been proposed to provide an exhaust gas recirculation (FOR) arrangement for an internal combustion diesel engine, the arrangement comprising a low pressure FOR duct between locations downstream of the turbocharger turbine stage and upstream of the turbocharger compressor stage.

In combination the relatively low positive pressure in the exhaust pipe in conjunction with the slight negative pressure of the air inlet duct allows exhaust gas to flow through the low pressure FOR duct to the inlet of the turbocharger compressor where it mixes with clean fresh air.

The turbocharger compressor and relatively long inlet duct allow good mixing of exhaust gas and fresh air so as to deliver a consistent well-mixed charge to the inlet manifold of the engine.

Turbochargers must be capable of operating at a wide range of engine speeds. In order to improve response at low engine speeds, when exhaust gas flow is also low, it has been proposed to provide two-stage series sequential turbocharging. At low engine speeds a relatively small primary turbine is provided to ensure more rapid spooling-up than would be possible with a large turbine. At higher engine speeds a larger second stage turbine is spooled up by increasing exhaust gas flow. This arrangement provides the benefits of a large turbocharger whilst minimizing turbo-lag.

At the compressor inlet of a turbocharger, the induction effect at low engine speeds is low because the turbocharger compressor stage is sized for maximum engine speed, and thus maximum gas flow. In the case of exhaust gas recirculation from downstream of the turbine stage the relatively large compressor tends to restrict gas flow at low engine speeds because the relative pressure drop across the low pressure FOR duct is too small to induce sufficient EOR flow.

According to one aspect of the invention there is provided an exhaust gas recirculation (EOR) arrangement of an internal combustion engine having a series sequential turbocharger, and a low pressure EOR duct from a location downstream of the turbocharger turbine stages to an intermediate location upstream of one turbocharger compressor stage and downstream of another turbocharger compressor stage.

Such an arrangement can bypass the first stage of the compressor, and thus supply EGR gas immediately upstream of a subsequent compressor stage where the induction effect is more marked at low flow rates and low engine speeds.

Thus in a twin series sequential turbocharger, the (smaller) first stage turbine is effective at low exhaust gas flow rate, and accordingly the second stage compressor (which is driven by the first stage turbine). EGR gas is supplied immediately upstream of the second stage compressor, and is induced by the depression generated by the second stage compressor.

In an embodiment the low pressure FOR duct also extends to a location upstream of the turbocharger compressor inlet. Thus at higher engine speeds, when the first stage compressor becomes effective, EOR gas is drawn into the first stage compressor by virtue of the increased depression generated by the first stage. At such higher engine speeds, this arrangement avoids throttling of FOR gas by the second or subsequent compressor stage.

Control of exhaust gas flow may be benign or determined according to a control strategy incorporated in, for example, an engine electronic control unit (ECU).

In the first case, the gas flow path is determined by the pressure drop across the EGH duct.

At low engine speed/load, the first compressor stage is relatively ineffective so that flow is induced to the intermediate location, for example immediately in advance of a second compressor stage. At higher engine speeds, the first stage compressor becomes effective, and being larger may induce the majority of FOR flow.

In order to prevent back flow of pressurized gas from the intermediate location, the EGR duct may include a closure or non-return valve. This arrangement avoids such back flow when the first stage compressor is effective.

Valves may be included in the EGR duct to control flow through outlet branches thereof in a benign or active manner. For example a non-return valve may be an automatic reed valve or the like. Adjustable throttles, for example under the control of an engine ECU, may preferentially permit gas flow through an outlet branch of the FOR duct. Closure valves, for example poppet valves, may provide simple open/closed functionality.

The arrangement of the invention may also include a conventional high pressure FOR duct extending between the engine exhaust manifold, and a location downstream of the turbocharger compressor -typically the engine inlet manifold. This high pressure EOR duct may be cooled or uncooled, and permits effective EOR during an engine warm-up phase.

In an embodiment, a catalyst is provided immediately downstream of the turbocharger turbine. The catalyst may be a diesel oxidation catalyst of a diesel engine or a three-way catalyst of a gasoline engine, or other suitable catalyst device.

The exhaust stream may include particulates which if returned to the compressor inlet via a low pressure EGR duct may cause deterioration of the turbocharger compressor stage by abrasion. In an embodiment, a filter is included in the low pressure FOR pathway from the turbocharger outlet to the turbocharger inlet. This pathway comprises the exhaust tract and the low pressure EGR duct, and the filter may comprise for example a conventional diesel particle filter of a diesel engine.

The invention also provides a method of recirculating exhaust gas in an internal combustion engine having a series sequential turbocharger, the method comprising providing an exhaust gas recirculation duct from a location downstream of the turbocharger turbine stage to a location intermediate two adjacent stages of the turbocharger compressor, and recirculating exhaust gas through said duct.

The method may further include the steps of providing a branch of the EGR duct to an inlet location upstream of the turbocharger compressor, re-circulating EGR flow to the inlet location, and controlling the flow of FOR in multiple branches of the FOR duct.

The method may further comprise the steps of providing a catalyst in the exhaust gas stream for treatment thereof, and/or providing a particle filter in the low pressure FOR pathway, for example a diesel particle filter of a diesel engine exhaust system.

Within the scope of this application it is envisaged that the various aspects, embodiments, examples, features and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings may be taken independently or in any combination thereof. For example, features described in connection with one embodiment are applicable to all embodiments, unless there is incompatibility of features.

The present invention will now be described, by way of example only, with reference to the accompanying drawing, in which: Fig. 1 is a schematic showing an EOR system according to an embodiment of the invention.

With reference to Fig. 1, an internal combustion diesel engine 11 has an exhaust manifold 12 leading to a two stage exhaust turbocharger T comprising a high pressure turbine 13 and low pressure turbine 14.

Exhaust gas exiting the turbocharger passes through a diesel oxidation catalyst (DOC) 15 and a diesel particle filter (DPF) 16 to an open (unthrottled) exhaust pipe 17. The DOC 15 removes excess fuel and carbon monoxide from the exhaust stream; the DAF 16 removes carbonaceous particulate matter.

A fresh air inlet 21 is coupled to an air box/filter 22 having a mass air-flow meter 23. Air flows via low and high pressure turbocharger turbines 25, 26, and then via an air to air intercooler 27 to an engine inlet manifold 28.

Hot exhaust gas is routed via a high pressure FOR duct 30 from the exhaust manifold 12 to a location 32 upstream of the inlet manifold. FOR valve 31 controls the flow of FOR gas in this duct 30; a cooler 41 is provided for lowering the temperature of EOR gas in the duct 30.

Exhaust gas is also routed from a location 33 downstream of the diesel particle filter 16 to the air inlet via a low pressure FOR duct 35. This duct 35 has a closure valve 24 to control flow therein, and the duct 35 further includes a cooler 34 upstream of the valve 24. The DAF 16 ensures that particulates do not reach the compressor stage of the turbocharger, and cause deterioration thereof by for example abrasion.

The turbocharger has the usual waste gate (not shown) to prevent over speeding of the turbines 13, 14. A relief valve (not shown) may be included to prevent excess pressure (surge) at the inlet side.

The outlet of the valve 24 is connected to the turbocharger turbine stage via two routes.

A first branch 42 allows EGR gas to flow directly from the valve 24 to the upstream side of the first stage turbine 25, where it mixes with incoming fresh air. The mixed air/FOR gas is compressed in the turbocharger and delivered to the inlet manifold 28 via the intercooler 27.

A second branch 43 allows FOR gas to flow via a non-return valve 44 to a location between the first stage turbine 25 and the second stage turbine 26. Gas flowing via the second branch 43 is thus compressed only by the turbine 26 before passing to the inlet manifold via the intercooler 27.

A controller (not shown) receives inputs from sensors of the system illustrated in Fig, 1 so as to determine appropriate operation of the FOR valves in conjunction with the air and fuel supplied to the engine. In addition to an indication of mass air flow from sensor 23, sensor inputs may include fresh temperature; gas temperature upstream and downstream of the intercooler 27; pressure and temperature of the inlet manifold 28; pressure and temperature of the exhaust manifold 12; temperature upstream of the oxidation catalyst 15, upstream of the particle filter 16, at the inlet 33 of the low pressure EGR duct, and downstream of the cooler 34. Control strategies for emission control are highly complex, and need not be further described here; such strategies may be implemented in an engine electronic control unit (ECU) of the vehicle.

The arrangement illustrated in Fig. 1 provides two sources of EGR gas, namely a high pressure source from upstream of the turbocharger turbines, and a comparatively low pressure source having an nt low downstream of the turbocharger and between the compressor stages of the turbocharger. Suitable drops in pressure are required to ensure natural flow of EGR gas in the respective EGR ducts 30, 35. Back pressure exerted by the non-return valve 44 should be as low as practicable so as not to impede FOR flow.

At low engine loads and/or during engine warm-up, high pressure EGR is preferred because the DOG 15 is less able to deal with excess fuel during the warm-up phase. This arrangement provides good combustion consistency, but may for example require restriction of the exhaust gas flow path in order to ensure sufficient pressure drop, and thus flow to the inlet manifold 28. This phenomenon occurs because at the relative speed and load, the turbocharger compressor is more efficient than the turbine, and consequently inlet manifold pressure may prevent flow of EGR gas. One means of restricting exhaust gas flow is to close the turbine vanes of a variable geometry turbocharger, but this solution necessarily degrades turbocharger performance.

High pressure EGR gas is also relatively hot, and therefore less dense. A considerable volume is required to have an appreciable effect upon relatively cool air entering the inlet manifold, so that as much as 40% of exhaust gas may be recirculated via the EGR duct 30.

Furthermore exhaust gas recirculated via the duct 30 is not available to drive the turbines 13, 14, and thus the turbocharger potential is yet further reduced. The disadvantage of high pressure EGR can be tolerated during engine warm-up.

Once the engine is warm, low pressure EGR is desirable. Exhaust gas is somewhat cooler at the take-off point 33, and consequently more dense. The cooler 34 reduces gas temperature from about 30000 to about 1 50°C, thus ensuring a further increase in density.

Good mixing of fresh air and low pressure EGR gas is assured by the compressor side of the turbocharger, and the relatively long inlet duct. The temperature of the air/EGR gas mixture is raised in the compressor to about 130°C, but subsequently reduced by the intercooler 27 to about 40°C. Approximate maximum temperatures are quoted, and may of course vary according to the engine specification and ambient conditions.

This well mixed inlet stream facilitates good combustion efficiency and low combustion noise through improved distribution of the air/EGR mixture in each cylinder. The low inlet temperature of such a stream ensures that oxygen density, is raised as compared with conventional high pressure FOR. Accordingly the capacity of the engine to control NOx emissions is improved, particularly at altitude where the volume percentage of oxygen is reduced owing to low ambient air pressure.

Low pressure EGR may be used to raise the temperature of incoming air to a desired value.

For example a mixed gas at entry to the compressor 25 may have a target temperature of about 60t.

A humidity sensor (not shown) i-nay be provided at the compressor stage inlet. It will be appreciated that the relatively cool EGEI gas flowing through duct 35 may have the capability of producing hydrochloric, nitric and/or sulphuric acid if in the presence of a high air moisture content. The humidity sensor provides a control input which can be utilized by the controller to reduce or inhibit input of low pressure EGR gas in such circumstances.

The schematic of Fig. 1 illustrates a two-stage series sequential turbocharger, but this invention is applicable to a multi stage turbocharger having three or more stages. The EGR duct may be further branched to each intermediate location of the compressor stages with suitable valving or throttling of the kind described herein to promote a desirable flow regime throughout the engine speed/load range.

The invention is also applicable to turbocharged gasoline engines.

Claims (1)

1. <claim-text>Claims 1. An exhaust gas recirculation (EGR) arrangement for an internal combustion engine having a series sequential turbocharger, the arrangement comprising an EGR duct from a location downstream of the turbocharger turbine stages to an intermediate location upstream of one turbocharger compressor stage and downstream of another turbocharger compressor stage.</claim-text> <claim-text>2. An arrangement according to claim 1, wherein said low pressure EGR duct extends to an inlet location upstream of the turbocharger compressor.</claim-text> <claim-text>3. An arrangement according to claim 2, wherein said EGR duct includes a first closure valve immediately upstream of said intermediate location.</claim-text> <claim-text>4. An arrangement according to claim 3, wherein said first closure valve is a non-return ]5 valve.</claim-text> <claim-text>5. An arrangement according to claim 4, wherein said non-return valve is an automatic reed valve.</claim-text> <claim-text>6. An arrangement according to any of claims 2-5, wherein said EGR duct further includes a second closure valve immediately upstream of said inlet location.</claim-text> <claim-text>7. An arrangement according to claim 6, wherein first and second closure valves comprise adjustable throttles.</claim-text> <claim-text>8. An arrangement according to claim 4 or claim 5, and further including an adjustable throttle immediately adjacent said non-return valve.</claim-text> <claim-text>9. An arrangement according to any of claims 2-8, wherein said EGR duct comprises a first branch extending from said location downstream of the particle filter to a junction, a second branch extending from said junction to the inlet location, and a third branch extending from said junction to the intermediate location.</claim-text> <claim-text>10. An arrangement according to claim 9, wherein said first branch includes an exhaust gas cooler.</claim-text> <claim-text>11. An arrangement according to any preceding claim, and further including a high pressure EGR duct extending between the engine exhaust manifold and a location downstream of said turbocharger compressor.</claim-text> <claim-text>12. An arrangement according to any preceding claim, wherein said engine comprises a two-stage series sequential turbocharger.</claim-text> <claim-text>13. An arrangement according to any preceding claim, and further including a catalytic converter in the exhaust gas stream.</claim-text> <claim-text>14. An arrangement according to any preceding claim, and further including a particle filter in the low pressure EGR pathway.</claim-text> <claim-text>15. A method of recirculating exhaust gas in an internal combustion engine having a series sequential turbocharger, the method comprising recirculating exhaust gas from a location downstream of the turbocharger turbine stages to a location intermediate two adjacent stages of the turbocharger compressor.</claim-text> <claim-text>16. The method of claim 15, wherein said duct is branched to an inlet location upstream of the turbocharger compressor inlet, and said method includes the step of recirculating exhaust gas to said inlet location.</claim-text> <claim-text>17. The method of claim 16, and further including the step of controlling flow of exhaust gas in said duct using one or more valves of one or more of said branches.</claim-text> <claim-text>18. The method of any of claims 15-17, and including the step of filtering exhaust gas in the low pressure EGR pathway.</claim-text> <claim-text>19. An engine or a vehicle having an arrangement as claimed in any of claims 1 to 14 or adapted to use a method as claimed in any of claims 15 to 18.</claim-text> <claim-text>20. An arrangement, a method, an apparatus or a vehicle constructed and/or arranged substantially as described herein with reference to the accompanying drawing.</claim-text>