Internal combustion engine with at least two cylinder banks

In a multi-cylinder internal combustion engine having at least two cylinder banks each with an exhaust system including a catalytic converter, wherein one of the cylinder banks can be deactivated when the engine operates only under partial load, the exhaust systems have exhaust pipe sections which are interconnected by a cross-over structure disposed upstream of the catalytic converters so that exhaust gas can flow also through the catalytic converter of the deactivated cylinder bank, and a length compensating element is disposed in at least one of the exhaust pipe sections and the cross-over structure.

BACKGROUND OF THE INVENTION
 The invention relates to a multi-cylinder internal combustion engine with
 at least two cylinder banks each of which includes an exhaust system with
 a catalytic converter wherein one of the cylinder banks can be made
 inoperative during partial load operation of the engine.
 DE 196 11 363 C1 discloses a multi-cylinder internal combustion engine
 including two cylinder banks each having several cylinders wherein one of
 the cylinder banks is always operative while the other can be inactivated
 during partial load operation. Each cylinder bank is provided with an
 exhaust system including a catalytic converter. Downstream of the
 catalytic converters, the separate exhaust gas systems are joined in a
 common exhaust pipe section, which is then again split to feed two
 mufflers through which the exhaust gas is discharged to the atmosphere.
 During partial load operation, a vacuum is generated in the exhaust system
 associated with the inactivated cylinder bank upstream of the respective
 catalytic converter so that the exhaust gas of the operational cylinder
 bank first flows through its associated catalytic converter. Then however
 it is returned, by way of the common exhaust pipe section and the
 catalytic converter of the inactivated cylinder bank, to the cylinders of
 the deactivated cylinder bank. In this way, the catalytic converter of the
 deactivated cylinder bank is maintained at operating temperature also
 during partial load operation so that the exhaust gas emissions are
 maintained at minimum level when the inactivated cylinder bank is
 reactivated.
 It is the object of the present invention to reliably maintain the
 catalytic converter of the cylinder bank, while inactivated during partial
 load operation, at operation temperature in a simple manner, specifically
 without the need for generating a vacuum upstream of the catalytic
 converter of the inactivated cylinder bank.
 SUMMARY OF THE INVENTION
 In a multi-cylinder internal combustion engine having at least two cylinder
 banks each with an exhaust system including a catalytic converter, wherein
 one of the cylinder banks can be deactivated when the engine operates only
 under partial load, the exhaust systems are interconnected by a cross-over
 structure disposed upstream of the catalytic converters so that exhaust
 gas can flow also through the catalytic converter of the deactivated
 cylinder bank and a length compensating element is disposed in at least
 one of the exhaust pipe sections extending between the cylinder banks and
 the cross-over structure and the cross-over structure.
 The cross-over structure interconnecting the exhaust systems of the
 different cylinder banks upstream of the catalytic converters provides for
 heating of all the catalytic converters also during partial load operation
 of the engine. Part of the exhaust gas of the operational cylinder bank or
 banks is diverted, by way of the cross-over structure, to the catalytic
 converter of the exhaust system of the deactivated cylinder bank. All
 catalytic converters are therefore maintained operational. As a result,
 exhaust emissions will not increase when the deactivated cylinder bank is
 reactivated.
 The heating of the catalytic converter of the exhaust system of the
 deactivated cylinder bank is achieved without a need for vacuum generating
 means since the cross-over structure is arranged upstream of the catalytic
 converters. With this arrangement, a part of the exhaust gas flows
 automatically through the catalytic converter of the deactivated exhaust
 system simply because of the lower flow resistance of this parallel
 passage.
 In order to accommodate thermally caused differential length changes of the
 exhaust pipes between the cylinder banks and the cross-over structure at
 least one of the exhaust pipe sections upstream of the catalytic
 converters includes a length compensating element. Different component
 temperatures and thermally caused differences in the component length can
 be accommodated by the length compensating element or elements. The high
 temperature exhaust gas flows through the exhaust pipe section of the
 operative cylinder bank upstream of the crossover pipe section and also
 through the cross-over pipe section so that these pipe structures are hot
 and thermally expanded. The upstream pipe section of the deactivated
 cylinder bank, however, assumes a substantially lower temperature. As a
 result, the different pipe sections expand to different degrees. The
 length difference between the pipe sections is accommodated by the length
 compensating element, so that cracks and breaks which could make the
 engine inoperational are avoided.
 With the incorporation of a length compensating element in the cross-over
 pipe section interconnecting the exhaust systems, the cross-over structure
 may be arranged in close proximity to the engine upstream of the catalytic
 converters so that both catalytic converters are heated by the exhaust gas
 even when one cylinder bank is deactivated. The differential expansions of
 the exhaust pipes are accommodated by the length compensating element.
 There may be only a single length compensating element arranged in one of
 the exhaust pipe sections between the cylinder banks and the cross-over
 pipe structure or in the cross-over pipe structure. However, preferably
 several such length compensating elements are provided, that is preferably
 one in each exhaust pipe section upstream of the cross-over pipe
 structure. The length compensating elements can accommodate pull
 (contraction) as well as push (expansion) forces on the pipes.
 In one embodiment of the invention, there is additionally provided a vacuum
 generating means, preferably an exhaust gas re-circulation arrangement
 with a conduit extending between the exhaust pipe section of the
 deactivated cylinder bank and the intake duct of the operating cylinder
 bank. The exhaust gas re-circulation arrangement has the advantage that
 additional catalytic converters arranged close to the respective cylinder
 bank or oxygen sensors can be maintained at operating temperature by the
 re-circulated exhaust gas.
 Another advantage of this arrangement is that thermal energy is drained
 from the re-circulated exhaust gas that is, the exhaust gas is cooled
 whereby the density of the re-circulated exhaust gas is increased and
 higher exhaust gas re-circulation rates and a reduction in exhaust gas
 emissions can be achieved.
 Further advantages and embodiments will be described below on the basis of
 the accompanying drawings.

DESCRIPTION OF PREFERRED EMBODIMENTS
 As shown in FIG. 1, an internal combustion engine 1 comprises two cylinder
 banks 2, 3 each including several cylinders. Each cylinder bank 2, 3 is
 provided with an exhaust system 4, 5 and each exhaust system 4, 5 includes
 pre-catalytic converters 17, 18 with lambda and control sensors arranged
 up-stream and downstream thereof, a main catalytic converter 6, 7 and a
 muffler 19, 20. Each of the cylinder banks 2, 3 includes the same number
 of cylinders. In the embodiment shown in the drawings, each of the
 cylinder banks 2, 3 includes six cylinders.
 The cylinder bank 2 is always in operation; the cylinder bank 3 can be
 deactivated when the engine is in a partial load operating mode in order
 to reduce exhaust emissions. Under partial load operation only the first
 cylinder bank 2 is operative and the exhaust gas therefrom is discharged
 through the exhaust system 4.
 Both exhaust systems 4, 5 are interconnected upstream of the catalytic
 converters 6, 7 so that, during partial load operation when the cylinder
 bank 3 is deactivated, exhaust gas of the cylinder bank 2 is discharged
 partially also through the exhaust system 5 of the deactivated cylinder
 bank 3. The exhaust systems 4, 5 include, adjacent the engine, exhaust
 pipe sections 9, 10 which are joined immediately upstream of the catalytic
 converters 6, 7 by a cross-over pipe section 8. During partial load
 operation when the cylinder bank 3 is deactivated, a part of the exhaust
 gas flows from the exhaust system 4 by way of the cross-over pipe section
 8 to the exhaust system 5 and through the catalytic converters 7 of the
 deactivated cylinder bank 3. In this way the catalytic converter 7 is
 maintained at operating temperature.
 Instead of providing a cross-over pipe section, the exhaust gas systems 4,
 5 may be combined in a common duct portion upstream of the catalytic
 converter 6, 7.
 Downstream of the catalytic converters 6, 7, the exhaust systems 4, 5 are
 combined in a common exhaust structure 21. The exhaust duct structure 21
 may then again be divided to supply the exhaust gas to two separate
 mufflers 19, 20. The common exhaust duct structure 21, however, is not
 needed. The exhaust systems may remain separate downstream of the
 catalytic converters.
 The pipe section 10 upstream of the catalytic converter 7 of the second
 exhaust system includes a pipe length compensation element 12 by which
 length changes of the pipes caused by different heat exposure of the
 exhaust systems 4, 5, particularly during partial load engine operation,
 can be accommodated. The length compensation element 8 can accommodate
 compression and expansion forces to compensate for component expansions
 and contractions.
 The cylinder bank 3 which can be deactivated, is provided with a vacuum
 generating means 14 which, in the embodiment shown, is an exhaust gas
 re-circulation system 15. By way of the exhaust gas re-circulation system
 15, the exhaust pipe section 10 of the cylinder bank 2 which can be
 deactivated, is in communication with the intake duct of the cylinder bank
 2, which is always operative during engine operation. The vacuum
 generating means 14 generates a low pressure in the exhaust pipe section
 10 upstream of the pre-catalytic converters 18. As a result, during
 partial load engine operation when the cylinder bank 3 is deactivated, a
 partial exhaust gas stream is diverted from the exhaust system 4 of the
 cylinder bank 2 and flows, by way of the cross-over pipe section 8,
 through the pipe section 10 and through the exhaust gas re-circulation
 conduit 15 to the intake duct of the cylinder bank 2 as indicated by the
 arrows 22. The re-circulation of the exhaust gas keeps the pre-catalytic
 converters 18 in the pipe section 10 and the associated .lambda. and
 control sensors at operating temperature. At the same time, the
 re-circulated exhaust gas is cooled whereby its density is reduced so that
 the exhaust gas re-circulation rate can be increased.
 The exhaust gas re-circulation conduit 15 includes a control valve 16 by
 which the exhaust gas re-circulation conduit 15 can be closed when exhaust
 gas re-circulation is not needed or desired for the operation of the
 engine.
 FIG. 2 shows an arrangement similar to that shown in FIG. 1, wherein the
 internal combustion engine 1 also has two cylinder banks 2, 3 each with an
 exhaust system 4, 5 including catalytic converters 6, 7. Again, the
 exhaust systems 4, 5 are interconnected upstream of the catalytic
 converters 6, 7 in the area of the exhaust pipe sections 9, 10 by way of a
 pipe section 8. In this arrangement, each of the exhaust pipe sections 9,
 10 of the two exhaust systems 4, 5 includes a length compensation element
 11, 12.
 FIG. 3 shows another embodiment wherein one length compensation element 13
 is used which is disposed in the cross-over pipe section 8 between the two
 exhaust systems 4, 5 of the cylinder banks 2, 3.
 However, it may be advisable to provide a length compensation element in
 each of the exhaust pipe sections 9, 10 and another one in the cross-over
 pipe section 8.