This invention provides improved control of emissions of an internal combustion engine, and is particularly concerned with ensuring a rapid commencement of treatment from a cold engine start. Aspects of the invention relate to a turbocharger, to an engine, to a vehicle and to a method.

Emissions control legislation requires that exhaust gases of an internal combustion engine be treated prior to discharge from the exhaust tail pipe. Typically a catalytic converter is included in the exhaust system downstream of the engine exhaust manifold, and comprises a catalyst for chemically converting undesirable products of combustion into relatively benign substances. Different kinds of catalytic converters are required for gasoline and diesel engines.

A feature of catalytic converters is that they require a minimum operating temperature before becoming effective. Heat is usually provided from the exhaust gas stream, and typically the catalyst becomes effective a few minutes after a cold start. Rapid commencement of catalytic treatment after cold start is desirable because during the engine warm-up phase undesirable products of combustion tend to be increased. Proposals for reducing the so- called 'light-off time have been numerous, and include electrical heating of the catalytic converter, and forcing immediate hot combustion conditions in the vehicle engine.

Internal combustion engines commonly include a turbocharger, which may comprise multiple stages and/or variable geometry turbine wheels. Various bypass ducts and associated control mechanisms are included. As a consequence the relative mass of turbochargers has increased in recent years, and is likely to increase further.

Turbochargers are immediately adjacent the engine exhaust manifold, and are accordingly subjected to very high thermal shock. Design parameters may for example specify a temperature increase from -40 °C (winter cold start) to 650 'Ό (normal running temperature) within a few minutes. A turbocharger housing must also accommodate structural loads, and be dimensionally stable so as to maintain close running clearances. Accordingly the material of choice for turbocharger housings is cast iron, which has a relatively high thermal mass. As a consequence exhaust gas passing through the turbocharger during the engine warm-up phase is cooled more rapidly by a complex turbocharger than by a relatively simple single stage turbocharger, with the consequence that light-off time of the catalyst may be increased.

The present invention provides a reduction in catalyst light-off time for an internal combustion engine having a multi-stage turbocharger.

According to an aspect of the invention there is provided a multi-stage turbocharger for an internal combustion engine, said turbocharger comprising a housing having an exhaust gas inlet, an exhaust gas outlet, and a plurality of successive turbines between said inlet and outlet wherein said turbocharger further includes a flow control valve between two successive turbines, said control valve being operable to bypass a downstream turbine on demand.

In such an arrangement, the control valve may be actuated to divert exhaust gas around a downstream turbine so as to avoid cooling of the exhaust gas as it passes through that portion of the turbocharger duct which is associated with the downstream turbine. In consequence less heat is lost to the turbocharger during the warm-up phase, and the temperature of exhaust gas reaching the catalytic converter is relatively increased. Thus the catalytic converter is heated more quickly and 'light-off time is reduced.

Such a control valve may appear similar to a conventional wastegate, but is quite different because all flow of exhaust gas is diverted. In a wastegate, which need not be further described here in detail, a portion of exhaust gas is diverted, but only an amount sufficient to prevent over pressure of the turbine stage. A further difference is that a wastegate is operable at relatively high flow rates, whereas the control valve of the invention is typically operable at low flow rates generally insufficient to spool-up the downstream turbine. A wastegate may be characterized as a relief valve whereas the invention relates to a diverter valve. It will be appreciated that the turbocharger mass associated with the first turbine is relatively small compared with the subsequent stage(s) because these subsequent stages are designed to operate at a lower pressure and are consequently physically larger, and have a correspondingly large heat absorption capacity. The invention takes advantage of this characteristic by bypassing the proportionately larger turbine(s) of the subsequent stage(s), and the correspondingly wider duct(s) which provide a large surface area for heat transfer to the turbocharger components. Some heat transfer may occur through the turbocharger housing from the vicinity of the first turbine stage to the subsequent turbine stage(s). Such heat soak is small compared with effect of passing exhaust gas directly through the subsequent stage(s), and it will be appreciated that the bypass is required only for a relatively short period after cold engine start - typically a few minutes depending upon configuration and packaging within a vehicle, and ambient conditions.

Typically the control valve comprises an electronically controlled actuator responsive to a control signal generated by an engine ECU (electronic control unit), which in turn is responsive to one or more input signals of temperature from the turbocharger, the catalytic converter and/or the exhaust system. The ECU may also be responsive to an input signal indicative of the ambient temperature. The ECU may provide for a maximum time from cold start for operation of said bypass. The ECU may also cease operation of said bypass in the event of an upstream turbine reaching a through flow limit, for example due to driver demand of increased engine speed, and in consequence increased exhaust gas flow.

The bypass may comprise a conduit of the turbocharger housing, and is preferably external to the housing so as to reduce heat loss thereto upon cold start. The conduit may be spaced from the turbocharger housing so far as is practicable, and may be insulated to further reduce said heat loss.

The benefit of the invention is maximized if the inlet to the bypass is immediately downstream of the first stage turbine wheel.

In an embodiment, a catalytic converter is provided downstream of the turbocharger housing, and may be connected thereto. In another embodiment, the bypass has an outlet immediately upstream of the catalytic converter so as to maximize heat transfer during the engine warm-up phase, and thus reduce light-off time to a minimum.

By 'immediately' we mean as close as practicable taking into account the installation and other design requirements.

A suitable closure valve may be a resiliency loaded poppet valve biased to close the bypass, but adapted to open in the event of an increase in pressure due to the passage through the downstream turbine being closed. 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:

Figure 1 illustrates a dual stage turbocharger installation of a diesel engine of a vehicle, incorporating the bypass of the present invention.

A turbocharger 1 1 comprises first and second stage compressor wheels 12, 13, and first and second stage turbine wheels 14, 15. As will be understood, the first stage compressor wheel 12 and second stage turbine wheel 15 are designed for higher pressures, and are consequently physically small compared with their neighbours 13, 14. The corresponding passages in the turbocharger housing are also smaller. The turbocharger inlet 16 is supplied with fresh air via an air filter 17 and mass air flow meter 18. Compressed air passes via an intercooler 19 to an inlet manifold 21 of the engine 22.

The exhaust manifold 23 supplies the turbine stages of the turbocharger, which exhaust via a diesel oxidation catalyst 24 to an open exhaust tail pipe 25. The usual wastegate (not illustrated) may be provided within the turbocharger to avoid turbine over speeding, and other conventional internal passages may be provided, for example to accommodate surge within the compressor.

As described so far, the installation of Fig. 1 is conventional and is of course susceptible to conventional enhancement.

A conventional ECU 30 provides for control of the engine, and typically includes strategies for minimizing emissions whilst maximizing fuel economy. Many input signals may be conventionally provided, including temperature at various locations within the turbocharger, and upstream and downstream thereof. Temperature sensors 31 , 32 may be provided at the exhaust manifold and at the inlet to the catalyst 24. According to the invention a bypass 40 is provided from a location downstream of the first stage turbine 12, so as to bypass the second stage turbine 13, and direct exhaust gas to a location immediately upstream of the catalyst 24. The bypass includes a diverter valve 41 under the control of the ECU 30, and operable between a first condition, in which the bypass 40 is open and the passage to the second stage turbine 13 is closed, and a second condition in which the bypass 40 is closed, and the passage to the second stage turbine 13 is open.

In use, at engine cold start, the diverter valve ensures that engine exhaust stream is directed to the catalyst 24 rather than through the second stage turbine 13 and associated ducts of the turbocharger housing. As a result less heat is lost from the exhaust stream and light-off time of the catalyst is reduced. Within a short period, typically a few minutes, the temperature sensor 32 indicates to the ECU 30 that the catalyst is at operating temperature, at which time the ECU commands the diverter valve 41 to close the bypass 40 and open direct fluid communication between the first and second turbine wheels 12, 13.

It will be understood that the time of operation of the bypass 40 is dependent on ambient temperature of the catalyst at engine start, and an engine operation in the moments after engine start.

For example, the vehicle engine 22 may be allowed to fast idle after start, which may permit light-off of the catalyst prior to drive away.

Alternatively the vehicle may be driven away immediately, in which case the engine 22 will work more or less hard with a consequent affect upon light-off time. The diverter valve 41 will change state accordingly.

In a further alternative, the driver may demand immediate rapid acceleration of the vehicle, in which case the ECU 30 control strategy will cause the diverter valve 41 to change state so as to permit the exhaust stream to flow through the second stage turbine 13, and give full turbocharged effect. In this case the temperature and volume of the exhaust stream will increase very quickly, so as to ensure rapid light-off of the catalyst in spite of closure of the bypass 40. It will be understood that strategies for controlling the diverter valve 41 can be very numerous depending on the particular vehicle installation, the relative complication of the turbocharger installation, and the location and type of catalyst treatment of the exhaust stream. However the aim of the invention is to ensure a reduced light-off time as compared to an equivalent absent the bypass of the invention, particularly at cold start and low engine demand.

The nature of the bypass 40 is inevitably dictated by the available space within any particular vehicle installation, and the location of the catalyst 24. Heat transfer to the turbocharger should be minimized whilst locating of the bypass 40 inlet as close as possible to the outlet of the first stage turbine wheel. The bypass 40 should be as short a possible, whilst allowing unobstructed gas flow, and the outlet should be as close as possible to the inlet of the catalyst 24.

Keeping these factors in mind, the bypass could be defined by a duct which is substantially external to and spaced from the turbocharger housing. The duct may be insulated, both to prevent heat loss to the surroundings and to prevent heat transfer to the turbocharger. Another possibility is to provide a duct at the exterior side of the turbocharger casing, but cast therewith or otherwise integrally formed, such a duct may minimize heat transfer through the housing and in any event has a much reduced surface area as compared with a conventional low pressure turbine duct and the turbine components within the low pressure duct.

As described an actively controlled diverter valve is provided. Alternatively a passive valve, for example controlled by a bi-metallic temperature sensitive actuator, can be provided. The latter may have a characteristic whereby the bypass passage is open whenever the adjacent exhaust tract is below a predetermined temperature (engine cold) and closes automatically when exhaust gas temperature reaches a value associated with effective operation of the exhaust catalyst.

In a further alternative the bypass passage may be always open and sized to accommodate exhaust flow at engine idle speed when the downstream turbine is by-passed. At higher exhaust flow rates, the downstream turbine is operable, and the relatively small flow via the bypass passage can be accommodated by suitable turbine design. Such flow may in practice be very low unless internal pressure approaches the operating point of the wastegate for the downstream turbine. Other arrangements are possible bearing in mind the factors to be balanced in achieving a reduction in light-off time at an affordable cost.