GB2507968A

GB2507968A - Two-stage turbomachine with intermediate exhaust treatment component.

Info

Publication number: GB2507968A
Application number: GB1220479.8A
Authority: GB
Inventors: Timothy James William Proctor
Original assignee: Cummins Ltd
Current assignee: Cummins Ltd
Priority date: 2012-11-14
Filing date: 2012-11-14
Publication date: 2014-05-21
Also published as: GB201220479D0

Abstract

The turbo machine 60 comprises a first turbine 18 that receives exhaust gas from an engine outlet 22, an exhaust treatment component 42a downstream from the first turbine and a second turbine 32a that receives gas that has passed through both the first turbine and the treatment component. The first turbine may be part of a turbocharger 14 that provides air to the engine 12 at a boost pressure, whilst the second turbine may for part of an electric generator 30a. The treatment component may comprise a catalyst for exothermic and oxidation reactions. A method for reducing contaminants in exhaust gas from engines using a two-stage turbomachine is also claimed.

Description

Turbomachine The present invention relates to a two-stage turbomachine. Particularly, but not exclusively, the present invention relates to a two-stage turbomachine which includes a turbocharger and an electric generator and a method for operating the same.

Turbochargers are well known devices for supplying air to an inlet of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to an engine inlet manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.

The turbine stage of a typical turbocharger comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet volute arranged around the annular inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chamber communicate such that pressurised exhaust gas admitted to the inlet volute flows through the inlet passageway to the outlet passageway via the turbine chamber and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet so as to deflect gas flowing through the inlet. That is, gas flowing through the inlet flows through inlet passages (defined between adjacent vanes) which induce swirl in the gas flow, turning the flow direction towards the direction of rotation of the turbine wheel.

Turbines may be of a fixed or variable geometry type. Variable geometry turbines differ from fixed geometry turbines in that characteristics of the inlet (such as the inlets size) can be varied to optimise gas flow velocities over a range of mass flow rates so that the power output of the turbine can be varied to suit varying engine demands. For instance, when the volume of exhaust gas being delivered to the turbine is relatively low, the velocity of the gas reaching the turbine wheel is maintained at a level which ensures efficient turbine operation by reducing the size of the inlet using a variable geometry mechanism. Turbochargers provided with a variable geometry turbine are referred to as variable geometry turbochargers.

Nozzle vane arrangements in variable geometry turbochargers can take different forms. Two known types of variable geometry turbine are swing vane turbochargers and sliding nozzle turbochargers.

Generally, in swing vane turbochargers the inlet size (or flow size) of a turbocharger turbine is controlled by an array of movable vanes in the turbine inlet. Each vane can pivot about an axis extending across the inlet parallel to the turbocharger shaft and aligned with a point approximately half way along the vane length. A vane actuating mechanism is provided which is linked to each of the vanes and is displaceable in a manner which causes each of the vanes to move in unison, such a movement enabling the cross sectional area available for the incoming gas and the angle of approach of the gas to the turbine wheel to be controlled.

Generally, in sliding nozzle turbochargers the vanes are fixed to an axially movable wall that slides across the inlet. The axially movable wall moves towards a facing shroud plate in order to close down the inlet and in so doing the vanes pass through apertures in the shroud plate. Alternatively, the nozzle ring is fixed to a wall of the turbine and a shroud plate is moved over the vanes to vary the size of the inlet.

Another known approach to improving turbocharging efficiency and/or performance for an engine with a wide speed/load range is to provide a sequential two stage turbocharging system (which may also be referred to as a two-stage turbomachine or a compound turbo machine), comprising one relatively small high pressure (HP) turbocharger and another relatively large low pressure (LP) turbocharger. The turbochargers are arranged in series so that exhaust from the engine flows first through the smaller turbine of the HP turbocharger and then through the larger turbine of the LP turbocharger. A valve-controlled bypass path may be provided for allowing exhaust gas to bypass the HF turbine, for instance at high engine speeds and/or loads. Similarly, the compressors of the two turbochargers are also arranged in series, with air flowing first through the relatively large compressor of the LP turbocharger and then through the relatively small compressor of the HP turbocharger. Again, a valve controlled bypass may be provided to allow the inlet air to bypass the compressor of the HP turbocharger, for instance at high engine speeds and/or loads.

Another type of two-stage turbomachine is a two-stage turbomachine which includes a turbocharger and an electric generator. In this type of turbomachine the large low pressure (LP) turbocharger is replaced by a turboelectric generator. In this case the turbocharger and turboelectric generator are arranged in series such that that exhaust from the engine flows first through the turbine of the turbocharger and then through the turbine of the turboelectric generator. Due to the fact that there is only one turbocharger in this arrangement, air flows through the compressor of the turbocharger to an engine inlet. The turboelectric generator generates electrical energy which can be stored or utilised as desired.

In order to reduce the emission of various contaminants by an engine, it is known for engines to incorporate an exhaust after treatment component. The exhaust after treatment component is designed to remove contaminants from the exhaust gas.

Examples of contaminants which may be removed by an exhaust after treatment component include nitrogen oxides (NOr) and particulate matter emissions. Exhaust after treatment components are conventionally located downstream of any other engine components and conventionally located adjacent to the engine's outlet to the atmosphere. For example, in the case of engines including the previously discussed two-stage turbomachines, the exhaust after treatment component would be located downstream of the low pressure turbine or the turbine of the turboelectric generator respectively.

It is an object of the present invention to provide a two-stage turbomachine which provides an increase in turbocharging efficiency and/or an increase in the efficiency with which contaminants are removed from exhaust gas compared to known two-stage turbomachines. It is a further object of the present invention to provide a two-stage turbomachine which overcomes or mitigates at least one problem of known two-stage turbomachines whether mentioned above or otherwise. Finally, it is an object of the present invention to provide an alternative two-stage turbomachine.

According to a first aspect of the invention there is provided a two-stage turbomachine comprising a first turbine adapted to receive exhaust gas from an engine outlet, an exhaust gas treatment component for reducing the amount of at least one contaminant in the exhaust gas, the exhaust gas treatment component being adapted to receive exhaust gas which has passed through the first turbine; and a second turbine, the second turbine being adapted such that it can receive exhaust gas which has passed through the first turbine followed by the exhaust gas treatment component.

The second turbine may form part of an electric generator, the electric generator being adapted to convert rotation of the second turbine by the exhaust gas to electrical energy.

The exhaust gas treatment component may comprise a catalyst, the catalyst being located such that, in use, exhaust gas from the first turbine passing through the exhaust gas treatment component contacts the catalyst.

The catalyst may catalyse an exothermic reaction.

The catalyst may catalyse an oxidation reaction.

The first turbine may form part of a turbocharger which is adapted to provide air at a boost pressure to an engine inlet.

According to a second aspect of the present invention there is provided a method of reducing the amount of at least one contaminant in an exhaust gas produced by an engine using a two-stage turbomachine comprising a first turbine, an exhaust gas treatment component and a second turbine, the method comprising the first turbine receiving exhaust gas from an outlet of the engine, the exhaust gas treatment component receiving exhaust gas which has passed through the first turbine; and the second turbine being adapted such that it can receive exhaust gas which has passed through the first turbine followed by the exhaust gas treatment component.

The second turbine may form part of an electric generator, and the method further includes using the electric generator to convert rotation of the second turbine by the exhaust gas to electrical energy.

The exhaust gas treatment component may comprises a catalyst, the method further comprising contacting the exhaust gas passing through the exhaust gas treatment component with the catalyst.

The catalyst may catalyse an exothermic reaction.

The catalyst may catalyse an oxidation reaction.

The first turbine may form pad of a turbocharger which is adapted to provide air at a boost pressure to an engine inlet.

Specific embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram showing the layout of a known twin turbocharger two-stage turbomachine; Figure 2 is a schematic diagram showing the layout of a twin turbocharger two-stage turbomachine in accordance with an embodiment of the present invention; and Figure 3 is a schematic diagram showing the layout of a two-stage turbomachine in accordance with a second embodiment of the present invention which includes a turboelectric generator and a turbocharger.

Figure 1 shows a known two-stage turbomachine 10. The turbomachine 10 is connected to an engine 12 and comprises first and second turbochargers 14 and 30.

The first turbocharger 14 is a relatively small, high pressure (HP) turbocharger. The second turbocharger 30 is a relatively large low pressure (LA) turbocharger. The HP turbocharger 14 and LP turbocharger 30 are connected in series. Due to the fact that the turbomachine 10 includes two turbochargers 14, 30, the turbomachine may be referred to as a twin turbocharger two-stage turbomachine or a twin turbocharger system.

The HP turbocharger 14 has a compressor 16 having a compressor wheel (not shown) and a turbine 18 having a turbine wheel (also not shown). The compressor wheel and turbine wheel are linked by a rotatable shaft 20.

The engine 12 has an engine exhaust outlet 22. An inlet to the turbine 18 of the HP turbocharger 14 is connected to the engine exhaust outlet 22 such that the turbine 18 is in fluid flow communication with the engine exhaust outlet 22. Exhaust gases produced by the engine 12 are provided to the turbine 18 via the engine exhaust outlet 22 and cause the turbine wheel within the turbine 18 to rotate. Rotation of the turbine wheel, and hence the attached shaft 20, results in the rotation of the compressor wheel within the compressor 16.

The compressor 16 is connected to an engine inlet 24 in a manner such that the compressor 16 is in fluid flow communication with an engine inlet 24.

The LP turbocharger 30 has a compressor 36 having a compressor wheel (not shown) and a turbine 32 having a turbine wheel (also not shown). The compressor wheel and turbine wheel are linked by a rotatable shaft 34.

An inlet to the turbine 32 of the LP turbocharger 30 is connected to an outlet of the turbine 18 of the HP turbocharger 14 such that the turbine 32 is in fluid flow communication with the turbine 18. Exhaust gases are provided to the turbine 18 via the engine exhaust outlet 22 then pass to the turbine 32 and cause the turbine wheel within the turbine 32 to rotate. Rotation of the turbine wheel within the turbine 32, and hence the attached shaft 34, results in the rotation of the compressor wheel within the compressor 36.

An outlet of the compressor 36 is connected to an inlet of the compressor 16 such that the compressor 36 is in fluid flow comrriunication with compressor 16. An inlet of the compressor 36 is connected to a fluid source 26. In this case, the fluid concerned is a gas, and more particularly, is air. The fluid source 26 in this case is the atmosphere.

As previously discussed, the compressor wheel within the compressor 16 is rotated via the shaft 20 due to the rotation of the turbine wheel within the turbine 18 caused by the passage of exhaust gas through the turbine 18. Similarly, the compressor wheel within the compressor 36 is rotated via the shaft 34 due to the rotation of the turbine wheel within the turbine 32 caused by the passage of exhaust gas through the turbine 32.

Rotation of the compressor wheel within the compressor 36 results in the compressor 36 supplying fluid from the fluid source 26 to the compressor 16. Rotation of the compressor wheel within the compressor 16 results in the compressor 16 supplying fluid from compressor 36 to the engine 12 via the engine inlet 24. The fluid supplied to the engine inlet 24 by the compressor 16 is at a higher pressure (also known as boost pressure) compared to that of the fluid at the fluid source 26. Furthermore, the fluid supplied to the compressor 16 by the compressor 36 is also at a higher pressure than that of the fluid at the fluid source 26.

Each of the turbines 18, 32 has a respective bypass valve 38a, 38b. The bypass valves 38a, 38b each have a closed state in which substantially no exhaust gas flows through the bypass valve (and hence exhaust gas flows through the turbine to which the bypass valve is attached); and an open state in which exhaust gas flows through the bypass valve and hence bypasses the turbine to which the bypass valve is attached.

Either of the bypass valves 38a, 38b may be placed in the open state when required.

For example a bypass valve may be placed in the open state when the speed of the turbine to which the bypass valve is attached is too high or if the properties of the exhaust gas are not suitable for driving the turbine to which the bypass valve is attached.

The turbine 32 has an outlet 40 to which exhaust gas passes once it has passed through the turbine 32 via turbine 18 and the engine outlet 22. The turbine outlet 40 is connected to an exhaust after treatment component 42. The exhaust after treatment component 42 is designed to remove contaminants from exhaust gas which passes through it. Examples of contaminants which may be removed by an exhaust after treatment component include nitrogen oxides (NOr) and/or particulate matter emissions. The exhaust after treatment component 42 is located downstream (with respect to the direction of travel of the exhaust gas), in use, of both of the turbines 18, 32. The exhaust after treatment component 42 is connected to an outlet 44 of the two-stage turbomachine. The outlet 44 of the two-stage turbomachine allows exhaust gas from the two-stage turbine machine which has passed through the engine exhaust outlet 22, turbine 18, turbine 34 and exhaust after treatment component 42 to pass to the atmosphere.

It has been found that the turbo machine shown in Figure 1 has a turbocharging efficiency and/or an efficiency with which contaminants are removed from the exhaust gas by the exhaust after treatment component which is suboptimal.

Figure 2 shows a schematic diagram of a two-stage turbo machine 50 according to an embodiment of the present invention. Features of the turbomachine 50 which correspond to those of the turbomachine 10 shown in Figure 1 have been given the same numbering.

It can be seen that the turbomachine 50 shown in Figure 2 differs from the turbomachine 10 shown in Figure 1 in that the turbomachine 50 does not include an exhaust gas after treatment component. Instead, the turbomachine 50 includes an exhaust gas treatment component 42a. The exhaust gas treatment component 42a is connected between an outlet 28 of the turbine 18 and an inlet 28a of turbine 32.

It follows that the two-stage turbomachine 50 comprises a first turbine 14 adapted to receive exhaust gas from the exhaust outlet 22 of the engine 12. The two-stage turbomachine 50 further comprises an exhaust gas treatment component 42a for reducing the amount of at least one contaminant in the exhaust gas. The exhaust gas treatment component 42a is adapted to receive exhaust gas which has passed through the first turbine 18. The two-stage turbomachine 50 further comprises a second turbine 32 which is adapted such that it can receive exhaust gas which has passed through the first turbine 18 followed by the exhaust gas treatment component 42a.

The exhaust gas treatment component 42a may comprise a catalyst (not shown), the catalyst being located such that, in use, exhaust gas from the first turbine passing through the exhaust gas treatment component 42a contacts the catalyst.

The catalyst may catalyse an oxidation reaction. For example, the catalyst may promote the oxidation of hydrocarbons and/or carbon monoxide present in the exhaust.

In some embodiments the exhaust gas treatment component may include a catalyst which catalyses a reduction reaction. For example, the catalyst may catalyse the reduction of oxides of nitrogen (NO) in the exhaust. In the case of the use of a catalyst which catalyses an oxidation reaction, hydrocarbons and carbon monoxide are considered to be contaminants. In the case where the catalyst catalyses reduction reactions, nitrogen oxide (NO) may be considered to be a contaminant.

In some cases, particularly where the catalyst catalyses an oxidation reaction, the reaction catalysed by the catalyst may be an exothermic reaction.

It is known that certain catalysts require heating to a temperature above room temperature before they become effective at catalysing the desired reaction. The temperature at which (and above which) the catalyst becomes effective at catalysing the desired reaction may be referred to as the "light off" temperature of the catalyst.

Before the catalyst reaches the "light off" temperature, it will not be effectively catalysing the desired reaction and as such the amount of contaminants which reach the two-stage turbomachine outlet 44 (and hence are released into the atmosphere) is relatively high (compared to when the catalyst has reached its "light off" temperature).

This may particularly be a problem when the engine has only recently been started and consequently when the temperature of the exhaust system is low. It will be appreciated that over time the hot exhaust gases heat the exhaust system such that the catalyst reaches its "light off" temperature. However, before this time, as previously discussed, the catalyst will not be functioning in an optimum manner and hence the two-stage turbomachine will be releasing a large amount of contaminants into the atmosphere.

The applicant has found that a two-stage turbomachine 50 according to the present invention in which the exhaust gas treatment component 42a is located between turbines 18 and 32 reduces the time it takes from a cold engine start for the catalyst within the exhaust gas treatment component 42a to reach "light off" temperature when compared to the time taken by the same catalyst in an exhaust gas after treatment component 42 in the prior art two-stage turbomachine 10. This is because the second turbine 32 (i.e., the low pressure turbine) has thermal inertia which reduces the speed with which temperature of the turbomachine downstream of the turbine 32 increases in temperature. In other words, due to the fact that an increase in temperature of the turbomachine downstream of the turbine 32 will be caused by heat being transferred from the exhaust gas from the engine to the portion of the turbomachine downstream of the turbine 32, and due to the fact that the exhaust gas produced by the engine will have to transfer heat to the turbine 32 (and hence increase the temperature of the turbine 32) before the temperature of the portion downstream of the turbine increases in temperature, the turbine 32 increases the time it takes for the portion of the turbomachine downstream of the turbine 32 to raise in temperature by a predetermined amount (e.g., from a cold engine temperature to the light off" temperature of the catalyst).

As a result of locating the exhaust gas treatment component 42 upstream of the turbine 32 within the present invention, it will take the exhaust gas treatment component 42a less time to reach light off" temperature (i.e., such that the catalyst within the exhaust gas treatment component 42a reaches its "light off" temperature) from a cold engine start condition compared to the time taken by the exhaust gas after treatment

component 42 in the prior art turbomachine.

Due to the fact that the catalyst of the exhaust gas treatment component reaches "light off" temperature within the present invention in a shorter time than that of the prior art, the catalyst within the exhaust gas treatment component of the present invention will begin to promote at least one reaction which reduces the amount of contaminant in the exhaust gas before this would happen in the case of the prior art turbomachine.

Consequently, a turbomachine according to the present invention may release less contaminants into the atmosphere during start up of the engine to which the turbomachine is attached.

In some embodiments of the present invention the exhaust gas treatment component may include a catalyst which promotes a reaction within the exhaust gas which is exothermic. That is to say, the catalyst promotes a reaction within the exhaust gas which produces heat and can therefore increase the temperature of the exhaust gas.

In this situation the amount of energy within the exhaust gas downstream of the exhaust gas treatment component will be increased. Specifically, the reaction within the exhaust gas promoted by the catalyst of the exhaust gas treatment component will increase the amount of heat energy within the exhaust gas downstream of the exhaust gas treatment component.

By increasing the amount of energy within the exhaust gas downstream of the exhaust gas treatment component, this increases the amount of energy which can be recovered and turned into useful work by the low pressure turbine 32. This is advantageous when compared to the prior art turbomachine because within the prior art turbomachine, if the exhaust gas after treatment component 42 promotes an exothermic reaction within the exhaust gas, the additional heat energy produced by said exothermic reaction will be lost to the atmosphere via the outlet 44. To the contrary, a turbomachine according to the present invention has an exhaust gas treatment component which is upstream of the second turbine such that any heat energy generated by an exothermic reaction catalysed by the catalyst within the exhaust gas treatment component can potentially be recovered by the second turbine 32 which is downstream of the exhaust gas treatment component 42a.

Due to the fact that the turbomachine according to the present invention may be capable of recovering additional energy from the exhaust gas (e.g., heat energy resulting from exothermic reactions catalysed by the catalyst within the exhaust gas treatment component) compared to prior art turbomachines and convert said recovered energy into useful work, the turbomachine according to the present invention may be said to have a greater turbocharging efficiency compared to prior art turbomachines.

Figure 3 shows a schematic representation of a two-stage turbomachine 60 in accordance with a further embodiment of the present invention. As before, features of the embodiment of the invention shown in Figure 3 which correspond to those in the embodiment shown in Figure 2 and the prior art in Figure 1 have been given the same numbering.

The two-stage turbomachine 60 shown in Figure 3 differs from the turbomachine 50 shown in Figure 2 in that the low pressure turbocharger 30 within the embodiment shown in Figure 2 has been replaced with a turboelectric generator 30a. The turboelectric generator comprises a turbine 32a which is connected downstream of the exhaust gas treatment component 42a. The turboelectric generator 30a also includes a shaft 34a which links a turbine wheel (not shown) of the turbine 32a to a transducer 62. In this case, the transducer 62 is an electric generator. The transducer 62 operates such that rotation of the turbine wheel of the turbine 32a causes rotation of the shaft 34a and results in the transducer 62 producing electrical power.

Due to the fact that the inlet 28a of the turbine 32a of the turboelectric generator 30a is in fluid flow communication with the outlet 28 of the turbine 18 of the turbocharger 14 (albeit via the exhaust gas treatment component), such that the turbine 32a of the turboelectric generator 30a is downstream of the turbine 18 of the turbocharger 14, the turbocharger 14 and turboelectric generator 30a may be said to be arranged in series.

In use, the exhaust gases from the engine pass through the turbine 18 of the first turbocharger 14 and then through the exhaust gas treatment component 42a to the turbine 32a of the turboelectric generator 30a. This causes the turbine 32a (and in particular a turbine wheel (not shown) of the turbine 32a) to rotate and consequently drive the transducer 62 via the shaft 34a.

The electric energy generated by the transducer 62 of the turboelectric generator 30a may be stored (for example in a battery) or utilised as appropriate. For example, the electric energy generated by the transducer 62 may be used to power control electronics or a motor.

It will be appreciated that, in common with the embodiment of the invention shown in Figure 2, the location of the exhaust gas treatment component 42a upstream of the turbine 32a of the turboelectric generator 30a may be advantageous in the case where the exhaust gas treatment component 42a includes a catalyst which promotes an exothermic reaction in the exhaust gas. This is because any exothermic reaction occurring in the exhaust gas due to the catalyst within the exhaust gas treatment component 42a will result in the amount of heat energy within the exhaust gas increasing downstream of the exhaust gas treatment component 42a. This may mean that the turbine 32a of the turboelectric generator 30a which is located downstream of the exhaust gas treatment component 42a may be able to recover at least some of the heat energy added to the exhaust gas due to exothermic reactions catalysed by the catalyst within the exhaust gas treatment component 42a. By recovering at least some of the heat energy from the exhaust gas, the turbine 32a of the turboelectric generator SOa may transfer more power to the transducer 62 such that the transducer 62 produces more useful work (i.e., more electrical energy) compared to the prior art example in which an exhaust gas after treatment component is located downstream of the turbine of the turboelectric generator.

Again, as previously discussed in relation to Figure 2, the location of the exhaust gas treatment component 42a upstream of the turbine of the turboelectric generator 30a may reduce the thermal inertia of the two-stage turbomachine upstream of the exhaust gas treatment component 42a such that (compared to prior art turbomachines) the exhaust gas treatment component 42a and any catalyst which forms part of the exhaust gas treatment component reaches "light off" temperature from a cold engine start in a shorter time. Consequently, a turbomachine according to the present invention may cause less contaminants to be released into the atmosphere during a cold engine start compared to a prior art turbomachine. It follows that a turbomachine according to the present invention may be more environmentally friendly compared to a

prior art turbomachine.

The turbine 18 of the turbocharger 14 within the two-stage turbomachine 60 is provided with a wastegate valve (or bypass valve) indicated generally by 38a. The wastegate valve 38a can be used to define a flow path which can be selectively opened or closed and which allows exhaust gas produced by the engine to substantially bypass the turbine 18 of the turbocharger 14. When the wastegate valve 38 is open exhaust gas may flow from the engine outlet 22 to the turbine outlet 28 such that substantially less exhaust gas passes the turbine wheel of the turbine 18 compared to when the wastegate valve 38 is closed. In some embodiments, substantially no exhaust gas may pass the turbine wheel within the turbine 18 when the wastegate valve 38 is open.

It will be appreciated that by opening the wastegate valve 38 (and therefore reducing the amount of exhaust gas which passes from the engine 12 to the turbine wheel of the turbine 18) the force exerted on the turbine wheel of the turbine 18 by exhaust gases from the engine is reduced. This results in a reduction in the speed of rotation of the turbine wheel, and hence a reduction in the speed of rotation of the compressor wheel of the compressor 16 (caused by the rotation of the turbine wheel). This reduction in the speed of rotation of the compressor wheel of the compressor 16 results in a reduction in the rate at which fluid which is transferred to the engine by the compressor 16 via the engine inlet 24.

Furthermore, opening the wastegate valve 38, by allowing the exhaust gas produced by the engine 12 to bypass the turbine 18 of the turbocharger 14 and therefore to pass directly to the turbine 32a of the turboelectric generator 30a will increase the proportion of the energy of the exhaust gas which is passed to the turbine 32a of the turboelectric generator 30a and therefore converted to electrical power by the transducer 36.

Some embodiments of the present invention may incorporate a second wastegate valve 38b which operates in substantially the same manner as the first wastegate valve 38a, except that it selectively allows at least some exhaust gas to bypass the turbine 32a of the turboelectric generator 30a.

Depending on the operating conditions of the engine 12 at least one of the first wastegate valve and the second wastegate valve (if present) may be controlled so as to provide a desired amount (or proportion) of power to the turbine 18 of the turbocharger 14 and/or to the turbine 32a of the turboelectric generator 30a.

It will be appreciated that the described embodiments of the invention include a turbine downstream of the exhaust gas treatment component which is either a turbine of a turbocharger or a turbine of a turboelectric generator. However, in other embodiments of the present invention, the turbine which is downstream of the exhaust gas treatment component may form part of any appropriate power turbine. That is to say that the turbine may form part of a device which converts the rotational motion of the turbine into any appropriate desired form of energy. For example, the transducer 62 which forms part of the turboelectric generator may be changed from a transducer which converts rotational motion into electrical energy to a transducer which converts rotational motion of the turbine wheel into any appropriate form of energy.

In the above examples, a turbine has been described as being the gas expander downstream of the exhaust gas treatment component which is used to convert energy in the exhaust gas to useable power (e.g. electric power or power for driving a compressor). However, a different type of gas expander may be used instead. For example, a suitable gas expander might be a turbine type expander, a piston type expander, a reciprocating type expander, a scroll or screw type expander, a vane type expander, a swash plate type expander, or the like. A turbine type expander is preferable in some embodiments of the invention due to its superior efficiency and relative compactness.

In some embodiments of the present invention the exhaust gas treatment component may not comprise a catalyst, but include any appropriate component which is capable of promoting a reaction which reduces the amount of at least one contaminant in the exhaust gas which passes through the exhaust gas treatment component.

It will be appreciated that it is within the scope of the invention for any of the turbines which form part of the described embodiments of the invention above to be a variable geometry turbine. Any appropriate variable geometry mechanism may be used.

The two-stage turbomachines according to the present invention discussed above include a relatively low pressure turbine downstream of a relatively high pressure turbine. In some embodiments of the invention, there may be a plurality of relatively low pressure turbines (each of which may form part of either a power turbine or a turbocharger) connected in parallel to one another and each connected to the outlet of the relatively high pressure turbine, such that they are downstream of the relatively high pressure turbine.

In some embodiments of the present invention, at least one of the turbines (but preferably a turbine upstream of the exhaust gas treatment component) may comprise a variable geometry mechanism which may be controlled such that, if required, the variable geometry mechanism can be closed, to thereby reduce the size of the inlet to the turbine, and hence increase the pressure of the gas upstream of the inlet. The increase in pressure of the gas increases the temperature of the gas. Increasing the temperature of the gas in this way may be referred to as exhaust gas heating. It follows that a turbine of a turbomachine according to the present invention may include a variable geometry mechanism which may be controlled to heat the exhaust gas via exhaust gas heating. Heating the exhaust gas will result in heating of the exhaust gas treatment component. Hence, if the turbomachine comprises a variable geometry mechanism which is controlled such that in a clod engine start condition the variable geometry mechanism is closed to cause exhaust gas heating, any catalyst which forms part of the exhaust gas treatment component will reach light off' temperature from a cold engine start in a shorter time (compared to a case in the absence of exhaust gas heating). The variable geometry mechanism may also be controlled to increase the temperature of the exhaust gas and therefore the exhaust gas treatment component at any appropriate time during operation of the turbomachine, for example, if the temperature of the exhaust gas treatment component becomes too low.

Other possible modifications and applications of the invention will be readily apparent to the appropriately skilled person.

Claims

CLAIMS: 1. A two-stage turbomachine comprising: a first turbine adapted to receive exhaust gas from an engine outlet, an exhaust gas treatment component for reducing the amount of at least one contaminant in the exhaust gas, the exhaust gas treatment component being adapted to receive exhaust gas which has passed through the first turbine; and a second turbine, the second turbine being adapted such that it can receive exhaust gas which has passed through the first turbine followed by the exhaust gas treatment component.
2. A two-stage turbomachine according to claim 1, wherein the second turbine forms part of an electric generator, the electric generator being adapted to convert rotation of the second turbine by the exhaust gas to electrical energy.
3. A two-stage turbomachine according to either of claims 1 or 2, wherein the exhaust gas treatment component comprises a catalyst, the catalyst being located such that, in use, exhaust gas from the first turbine passing through the exhaust gas treatment component contacts the catalyst.
4. A two-stage turbomachine according to claim 3, wherein the catalyst catalyses an exothermic reaction.
5. A two-stage turbomachine according to either claim 3 or claim 4, wherein the catalyst catalyses an oxidation reaction.
6. A two-stage turbomachine according to any preceding claim, wherein the first turbine forms part of a turbocharger which is adapted to provide air at a boost pressure to an engine inlet.
7. A method of reducing the amount of at least one contaminant in an exhaust gas produced by an engine using a two-stage turbomachine comprising a first turbine, an exhaust gas treatment component and a second turbine, the method comprising: the first turbine receiving exhaust gas from an outlet of the engine, the exhaust gas treatment component receiving exhaust gas which has passed through the first turbine; and the second turbine being adapted such that it can receive exhaust gas which has passed through the first turbine followed by the exhaust gas treatment component.
8. A method according to claim 7, wherein the second turbine forms part of an electric generator, and the method further includes using the electric generator to convert rotation of the second turbine by the exhaust gas to electrical energy.
9. A method according to either of claims 7 or8, wherein the exhaust gas treatment component comprises a catalyst, the method further comprising contacting the exhaust gas passing through the exhaust gas treatment component with the catalyst.
10. A method according to claim 9, wherein the catalyst catalyses an exothermic reaction.
11. A method according to either claim 9 or claim 10, wherein the catalyst catalyses an oxidation reaction.
12. A method according to any preceding claim, wherein the first turbine forms part of a turbocharger which is adapted to provide air at a boost pressure to an engine inlet.