WO2009058056A1

WO2009058056A1 - Internal combustion engine with exhaust gas recirculation

Info

Publication number: WO2009058056A1
Application number: PCT/SE2007/000971
Authority: WO
Inventors: Per Larsson; Kent Giselmo; Martin Bauer
Original assignee: Volvo Lastvagnar Ab
Priority date: 2007-11-01
Filing date: 2007-11-01
Publication date: 2009-05-07

Abstract

The invention relates to an internal combustion engine (1 ) having least one cylinder (2), an intake (3, 4) for the supply of air, an exhaust outlet (9, 10) for discharging exhaust gases, a further line (15) for recirculation of exhaust gases from said outlet (9, 10) to said intake (3, 4) for the reduction of harmful emissions from the engine (1). A controllable valve (16) is arranged in said further line (15). A turbocharger unit (11) comprises a first means (12) for absorbing energy from the exhaust gases and a compressor (14) with an inlet and an outlet for compressing air to said intake (3, 4) and a second means (22) for absorbing energy from the exhaust gases arranged downstream of said first means (12), for building a pressure in said outlet (9, 10) surmounting the pressure in said intake (3, 4). A power transmission (25) is arranged between said second means (22) and a crankshaft (24) associated with the engine (1). A charge air recirculation conduit (26) is arranged between the compressor (14) outlet and the compressor inlet and a valve (27) controls flow through the charge air recirculation conduit (26).

Description

TITLE

Internal combustion engine with exhaust gas recirculation

TECHNICAL FIELD

The present invention relates to an internal combustion engine with exhaust gas recirculation according to the preamble of the appended claim 1. The invention is particularly related to reducing harmful emissions from a diesel engine being provided with a system for recirculation of exhaust gases to the intake of the engine, a so called EGR system (Exhaust Gas Recirculation). The invention also relates to a method for a combustion engine according to the preamble of the appended claim 5.

BACKGROUND OF THE INVENTION

In connection with load-carrying vehicles powered by a diesel engine, there exists a general need for reducing, as much as possible, the emission of harmful pollutants from the engine exhaust gases. These emissions are primarily nitrogen oxide compounds (NO_x), carbon monoxide (CO) and hydrocarbons (HC). In order to reduce these emissions, various measures can be taken. For example, it is previously known that the design of the combustion chamber in the engine cylinder and the process of injecting the engine fuel can be adjusted so as to minimise the emissions. In those cases where the engine is equipped with a turbocharger, the emission of NO_x compounds may be reduced by cooling of the air fed into the engine (known as "intercooling").

In connection with petrol driven engines, the exhaust gases are normally purified by means of a catalytic converter located in the exhaust system. Because a diesel engine is operated with an air surplus, however, the three- way catalyst cannot be used for reduction of NO_x compounds from a diesel engine.

Due to increasing environmental requirements and expected future legislation demands, the need for reducing the emissions, primarily of NO_x compounds, from diesel engines has grown even stronger. In this context, it is previously known that the amount of NO_x compounds from a diesel engine can be reduced by equipping it with something known as an EGR system (Exhaust Gas Recirculation), by means of which a certain amount of exhaust gases can be returned from the engine exhaust to its intake. The amount of

NO_x compounds generated in a diesel engine is principally exponentially proportional to the temperature inside the combustion chamber, and, by using an EGR system, the local temperature during combustion can be lowered by dilution with exhaust gases (CO₂ and H₂O). This in turn leads to less creation of NO_x.

A diesel engine may be provided with an EGR system by connecting a separate line between the ordinary exhaust outlet of the engine and a point close to the fresh air intake of the engine. In this line, a controllable valve is arranged, this valve further being connected to a control unit. In dependence of the existing engine operating conditions, particularly regarding its rotational speed and its load, the control unit will determine the degree of opening of the valve, i.e. the amount of EGR gases to be recirculated to the engine air intake. A certain amount of EGR gases will then be fed from the engine exhaust side to its intake side, through the exhaust side pressure normally being higher than the intake side pressure, thus creating a natural "propulsion pressure" for the EGR gases.

In those cases where a diesel engine having an EGR system is utilised together with a turbocharger unit, a problem will be created by there being, for most of the operating points, a higher pressure after the turbocharger compressor (i.e. at the point of the engine intake manifold where the incoming fresh air is fed to the engine) than at the engine exhaust outlet. This in turn means that a recirculation of EGR gases will not be possible, as there will not be any natural propulsion pressure from the engine exhaust to its intake side. In this manner, no EGR gas flow can be injected into the engine.

In accordance with known art, this problem can be solved by providing the turbocharger with variable turbine geometry. One arrangement using this solution is shown in the patent document JP-08270454 A, disclosing a diesel engine having a turbocharger in turn comprising adjustable guide vanes, which, depending on the engine operating conditions, ican be adjusted to a certain position by means of a control unit. In this way, a sufficiently high pressure can be created on the engine exhaust side, whereby a required amount of EGR gases can be recirculated to the intake side.

A substantial disadvantage of this known arrangement relates to the fact that it will entail an impaired gas exchange in the engine. Consequently, there is a need for engine arrangements providing a flow of EGR gases without impaired gas exchange in a diesel engine being equipped with a turbocharger.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved arrangement for a combustion engine, particularly a diesel engine being equipped with an

EGR system and a turbocharger, providing an adequate propulsion pressure for the EGR gases, so as to achieve a reduction of the NO_x emissions of the engine. This object is achieved by an arrangement, the characteristics of which will be defined in the appended claim 1. This object is also achieved by a method, the characteristics of which will be defined in the appended claim 5 a recirculation conduit between the compressor outlet and the compressor inlet and a valve controlling flow through the recirculation conduit.

Preferred embodiments of the invention are described in the accompanying dependent claims.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the appended drawings below follows a more detailed description of embodiments of the invention cited as examples.

In the drawings:

Fig. 1 depicts in a schematical form an example embodiment according to the present invention. Fig. 2 depicts a diagram of the Exhaust Recirculation Valve (EGR) valve area as a function of Discharge Recirculation Valve (DRV) area @ 850rpm and 50% load according to an example embodiment of the present invention. Fig. 3 depicts a BSNO_x (Break Specific Oxides of Nitrogen) (engine out) as a function of DRV area @ 850rpm and 50% load according to an example embodiment of the present invention.

Fig. 4 depicts a pressure drop engine as a function of DRV area @ 850rpm and 50% load according to an example embodiment of the present invention.

PREFERRED EMBODIMENT Fig. 1 schematically illustrates an arrangement according to the present invention, which can be utilised particularly for a combustion engine of the diesel type. According to a preferred embodiment, the diesel engine 1 is intended for use in a load-carrying vehicle, and comprises six cylinders 2. The invention is however not limited to any specific number of cylinders or any specific cylinder configuration. In a way as such previously known, the engine 1 is equipped with an intake manifold 3, to which air is fed from the atmosphere via an intake conduit 4. The input air is then divided between the various cylinders 2. Furthermore, fuel is supplied to the cylinders 2 by a corresponding number of fuel injection devices 5 that are each connected to a central control unit 6 via an electrical connection 7. The control unit 6, which is preferably computer based, is in a known manner operative to control each fuel injection device 5 so as to supply, in each instant, an appropriate fuel/air mixture to the engine 1.

During operation of the engine 1 , the control unit 6 is operative to control the respective injection device 5 in such a way that the fuel/air mixture supplied to the engine 1 will be adapted, in each instant, to the current operating conditions. The fuel supply is hereby obtained in a generally known manner, i.e. in dependence of a multitude of parameters representative of the operating conditions of the engine 1 and the vehicle in question. For example, the control can be performed depending on the present throttle position and the rotational speed and load of the engine 1.

Each cylinder 2 is provided with an exhaust outlet 8. Together, the exhaust outlets 8 converge into an exhaust manifold 9, continuing into an exhaust pipe 10. This exhaust pipe runs via a turbocharger unit 11 , which as such is substantially conventional. Thus, the turbocharger unit 11 comprises a means for absorbing energy from the exhaust gases in the form of a turbine

12, which is arranged in the exhaust pipe 10 and is being rotated by the exhaust gases flowing through the exhaust pipe 10. As an alternative to the embodiment shown in the figure, which is designed in such a way that the exhaust outlets 8 join into a single exhaust pipe 10 (known as a "single inlet"), the exhaust outlets may be grouped into two groups, making the exhaust duct consist of two pipes leading to the turbine 12 (known as a "twin inlet" or a "double inlet"). The turbine 12 is arranged on a shaft 13, on which a compressor 14 is likewise arranged. The energy absorbed from the exhaust flow by the turbine 12 is in this way transferred to the compressor 14, which functions to compress the in-flowing air to the intake conduit 4 of the engine 1. In this manner, an increased fuel amount may be fed to the engine 1 , whereby its power output can be increased.

The engine 1 is further equipped with an arrangement for recirculation of a certain amount of exhaust gases to the intake side of the engine 1. According to what was discussed in the introduction, an EGR system

("Exhaust Gas Recirculation) is as such previously known. According to the embodiment, a further line in the form of an EGR line 15 is therefore connected to the exhaust pipe 9, at a point upstream of the turbine 12. The EGR line 15 debouches in the intake conduit 4, at a point upstream of the intake manifold 3 of the engine 1. Along the EGR line 15, a controllable valve

(EGR valve) 16 is arranged, connected to the control unit 6 via a further connection 17. According to what will be described in greater detail below, the control unit 6 is operative, in dependence of the present operating conditions, to adjust the valve 16 to a dosed, open or partially open position. Depending on the position of the valve 16, a corresponding amount of exhaust gases will thus be recirculated to the intake manifold 3 via the EGR line. Through the recirculation of these EGR gases to the intake manifold 3, a temperature reduction during the combustion is achieved in the respective cylinder 2, whereby the NO_x generation in the cylinder 2 is reduced.

The NO_x generation in the respective cylinder 2 is temperature-dependent and for this reason it is desirable to lower, as far as possible, the temperature of the gases (i.e. including air and recirculated EGR gases) coming in to the engine 1. For this reason, the EGR line 15 is provided with a cooler 18 functioning to cool the EGR gases recirculated to the intake manifold 3. To this end the cooler 18 includes a loop 19 through which a suitable coolant is circulated. Preferably, this coolant is the ordinary coolant for the engine 1, but air may also be used for this cooling or a separate cooler cooled by a liquid medium. By means of this cooler 18, the EGR gases can be cooled, which further contributes to reducing the generated amount of NO_x compounds. An additional cooler may be provided in the

EGR line 15, e.g. a so called pre-cooler.

The intake conduit 4 is also equipped with a cooler 20, (known as an "intercooler"), which is used for cooling the compressed air supplied by the compressor 14. This also contributes to a reduction of the amount of NO_x compounds generated in the engine 1. The intake air cooler 20 may be arranged for cooling by air or a coolant, which is schematically indicated at the reference number 21.

A second turbine 22 is used for absorbing energy from the exhaust gases.

The exhaust gases leaving the engine 1 and being passed through the first turbine 12 are thus also fed through the second turbine 22, which is then brought to rotate. For this purpose, the second turbine 22 is rotatably arranged on a further shaft 23. After having transferred part of their energy to the second turbine 22, the exhaust gases are conducted out to the atmosphere, for instance by way of a silencer and/or an after treatment system (not shown).

Furthermore, the second turbine 22 is connected to the output crankshaft 24 of the engine 1 , via a power transmission 25, not shown in any detail, which in the figure is schematically indicated by a dashed line. The power transmission 25 is preferably of a mechanical type including a gear transmission connecting the shaft 23 to the crankshaft 24. The power transmission is moreover provided with a gear reduction for conversion of the rotational speed of the second turbine 22 to a rotational speed suitable for the crankshaft 24. In this manner, power is transferred between the second turbine 22 and the crankshaft 24, i.e. a certain amount of energy in the combustion gases can be regained from the exhaust flow and be used as additional power to the crankshaft 24.

A system for an engine having a turbocharger unit arranged for extraction of a certain amount of energy from the exhaust gases and feeding this energy back to the engine crankshaft is normally called a "turbo compound" system. By using the two turbines 12, 22 that are arranged in series, a pressure is achieved, during operation of the engine 1 , on the exhaust side of the engine 1 , which is higher than the pressure on its intake side. In this way, a sufficient propulsion pressure is obtained for recirculation of EGR gases to the intake manifold 3, without deterioration of the efficiency of the engine 1. Hereby, part of the energy contained in the exhaust gases is utilised as additional power to the crankshaft 24 instead of being conducted out into the atmosphere and thus being lost.

According to the invention, a recirculation conduit 26 is provided between the outlet and the inlet of the compressor 14. A valve 27 is provided for controlling flow through the recirculation conduit 26. By recirculating a portion of the inlet air through the compressor 14, the pressure in the intake manifold

3 can be reduced sufficiently for allowing more EGR gas to pass through the EGR line 15 into the intake conduit 4. The valve 27 is controllable by means of the control unit 6 via the connection 17, either step by step or continuously variable between its end positions.

According to what has been explained above, the invention includes controllable valves 16, 27, in connection with the control unit 6. Depending on the current operating conditions of the engine 1 and the vehicle in general, the valves 16, 27 are used in such a manner that a certain amount of exhaust gases from the respective exhaust outlets 8 of the engine 1 are passed back to the intake manifold 3. According to a preferred embodiment, the valves 16, 27 consist of electronically controlled valves, which, by means of the control unit 6, can be controlled continuously between an open and a closed position. Through a certain setting of the aperture areas of the EGR valve 16 in the EGR conduit 15 and a Discharge Recirculation Valve (DRV) 27 in the charge air recirculation conduit 26, a correspondingly sized flow of

EGR gases to the intake manifold 3 is obtained.

For control of the valves 16, 27, the control unit 6 is arranged for determination of the rotational speed and load (torque) of the engine, and for calculation, depending on these parameters, of the desirable amount of EGR gases to be recirculated to the intake manifold. This amount of EGR gases is preferably determined in the control unit 6 by using a stored table giving the required amount of EGR gases as a function of the speed and load. Depending on the calculated value of the amount of EGR gases, the valves 16 and 27 are adjusted to a corresponding position through a signal from the control unit 6.

In connection with recirculation of EGR gases, the recirculated amount has to be adapted concerning predetermined limit levels for soot and carbon monoxide released with the exhaust gases. As the EGR recirculation reduces the amount of air available at the engine intake, it must be ascertained that the amount of air is sufficient with regard to said limit levels.

By virtue of the valves 16 and 27 discussed above, that can be continuously adjusted to a desired position, a very large variation of the aperture area of the valve 16 is allowed between different extremes in the operating conditions of the engine 1. For the arrangement according to the embodiment, where energy is transferred from the second turbine 22 to the crankshaft 24, this large variation is required, as the pressure differential between the exhaust side and the intake side varies to a high degree as a function of the engine load. As a comparison it can be mentioned that this pressure differential varies considerably more than by previously known systems comprising turbocharger units with variable turbine geometry. By means of the control according to the invention of the valves 16, 27, a correct flow of EGR gases can be ascertained, which is substantially independent of the operating conditions of the engine 1.

Besides the control of the valve 16 as a function of the speed and load of the engine 1 , the control unit 6 may also be functional to control the valves 16, 27 in dependence of other parameters. For example, transients can be taken into account to avoid unwanted puffs of smoke in the engine 1 exhausts.

For an embodiment of the invention, the flow area relation of the DRV 27 and the EGR valve 16 may for example be regulated to a proportion between 1 :80 and 1 :2. In another example embodiment the flow area relation of the DRV 27 and the EGR valve 16 may for instance be regulated to a proportion between 0-1.

Figure 2 illustrates the EGR valve 16 area as a function of DRV (Discharge Recirculation Valve) 27 area @ 850rpm and 50% load of the engine. From the diagram in figure 2 it is illustrated the necessary opening area of EGR valve 16 at different areas of the DRV 27. In case DRV 27 is closed it is necessary in this example embodiment to open the EGR valve 16 with 15 cm², which is its maximum opening valve area in this embodiment. As is illustrated from figure 2, the more we open up the DRV 27 the more the EGR valve 16 may be closed with constant NO_x, see figure 3. This is true up to opening area 1 cm2 for the DRV 27, going beyond 1 cm2 of the DRV 27 the NO_x is decreased.

According to figure 4, it is apparent the pressure drop is increasing the more the DRV 27 is open. The invention is not limited to the embodiment described above, but can be varied within the scope of the appended patent claims.

Claims

1. An internal combustion engine (1) having least one cylinder (2), an intake (3, 4) for the supply of air, an exhaust outlet (9, 10) for discharging exhaust gases, a further line (15) for recirculation of exhaust gases from said outlet (9, 10) to said intake (3, 4) for the reduction of harmful emissions from the engine (1 ), a controllable valve (16) arranged in said further line (15), a turbocharger unit (11 ) comprising a first means (12) for absorbing energy from the exhaust gases and a compressor (14) with an inlet and an outlet for compressing air to said intake (3, 4) and a second means (22) for absorbing energy from the exhaust gases arranged downstream of said first means (12), for building a pressure in said outlet (9, 10) surmounting the pressure in said intake (3, 4), and a power transmission (25) between said second means (22) and a crankshaft (24) associated with the engine (1), characterized in a charge air recirculation conduit (26) between the compressor (14) outlet and the compressor inlet, and a valve (27) for controlling flow through the charge air recirculation conduit (26).

2. An engine according to claim 1 , characterized in that it is a diesel engine.

3. An engine according to any one of the preceding claims, characterized in that said first means (12) and said second means (22) for absorbing energy from the exhaust gases in the outlet (9, 10) consist of exhaust-powered turbines.

4. An engine according to any one of the preceding claims, characterized in that said further line (15) is connected to said outlet (9, 10) at a point upstream of said first means (12) for absorbing energy from the exhaust gases.

5. A method for reduction of harmful emissions from a combustion engine

(1), comprising: absorption of energy from exhaust gases coming from the engine (1 ) by means of a turbocharger unit (11), further absorption of energy from the exhaust gases downstream said turbocharger unit (11 ), whereby a pressure is built in said outlet (9, 10), which surmounts the pressure in said intake (3, 4), transmission of force from said means (22) of said further absorption of energy from the exhaust gases downstream said turbocharger unit (11 ), to a crankshaft (24) associated with the engine (1 ). compressing charge air in a compressor (14), and recirculation of exhaust gases from the engine (1) via control of a valve (16) arranged in a line (15) that connects an outlet (9, 10) of the engine (1) with an intake (3, 4) of the engine (1), characterized by a recirculation of compressed air from an outlet of the compressor to an inlet of the compressor such that compressed gas from the outlet of the compressor comprises a mixture of charge air and recirculated compressed gas.