Automotive turbocharger systems

This application is a continuation of U.S. patent application Ser. No. 10/570,877, filed Oct. 25, 2006, under 35 U.S.C. § 371 claiming priority to Great Britain application ser. no. 0320986.3, filed Sep. 8, 2003, and international application number PCT/GB2004/003809, filed Sep. 6, 2004, the entirety of each is hereby incorporated by reference herein.

The present invention relates to turbocharger systems for automotive engines.

Turbochargers are of course well known devices which include a compressor or blower wheel, typically an impeller, which is situated in an engine inlet duct and is connected to an exhaust turbine, which is situated in the engine exhaust duct and arranged to be rotated at high speed by the engine exhaust gases. Rotation of the exhaust turbine results in rotation of the blower wheel which produces a boost pressure, that is to say it increases the pressure in the inlet duct to a superatmospheric value. The result of this increased inlet pressure is that a greater amount of air is admitted into each cylinder of the engine during the induction stroke of the pistons in the cylinders, which results in an increased power output from the engine.

The power absorbed from the exhaust gases by a turbocharger exhaust turbine is proportional to the cube of the speed of the exhaust gases, which means that although the blower wheel rotates very rapidly and thus produces a substantial boost pressure at high engine speed, it does not rotate at all or only at negligible speed at low engine speed. This means that no boost pressure is available at a time when maximum engine power is frequently needed, i.e. when accelerating rapidly from engine idle speed.

One way of overcoming this problem is to increase the speed of the exhaust gases past the exhaust turbine. This can be done by providing guide vanes of variable pitch in the exhaust duct to enable the local exhaust gas speed to be increased and thus the power output of the turbine wheel to be increased, even at low engine speed. However, such a construction is complex and expensive and subject to failure as a result of lubrication problems. Simply making the turbocharger physically smaller, thereby increasing the exhaust velocity through it, would substantially improve the characteristics of the turbocharger at low engine speeds, but at high engine speeds the exhaust turbine would constitute an unacceptable flow restriction for the exhaust gases and would be liable to failure as a result of being driven at an unacceptably high speed.

It has been proposed that an automotive engine be provided with a turbocharger system comprising two turbochargers, one relatively small and the other relatively large. The two blower wheels are provided in series in the engine inlet duct and the two exhaust turbines are provided in series in the exhaust duct. Since the small turbocharger is inappropriate at high engine speeds and would be liable to failure if used at such speeds, the smaller exhaust turbine and the smaller blower wheel are provided with respective bypass passages incorporating respective shut-off valves operated under the control of the engine management system.

The operation of such a system is supposed to be as follows: The two bypass valves are shut at low engine speeds. The relatively small volume of exhaust gas flows through the exhaust turbine of the smaller turbocharger at a substantial speed due to the relatively small dimension of the duct in which the turbine is situated. The smaller exhaust turbine is thus rotated at a substantial speed and this rotation is transmitted to the smaller blower wheel, which thus creates a significant boost pressure in the inlet duct. The exhaust gas also flows through the exhaust turbine of the larger turbocharger, but at a significantly lower speed due to its greater size. The larger exhaust turbine is thus rotated very slowly, if at all, and the larger blower wheel thus plays effectively no part in the creation of the boost pressure. As the engine speed and/or load rises, the engine management system opens the two bypass valves. The exhaust gas now flows through the passage bypassing the smaller exhaust turbine and then flows through the larger exhaust turbine where it now reaches a substantial speed due to the increased flow rate of exhaust gas. The larger exhaust turbine is thus rotated at high speed and this rotation is transmitted to the larger blower wheel, which creates a boost pressure in the inlet duct. The bypass duct around the smaller blower wheel has larger flow area than that of the smaller blower and thus does not constitute an unacceptable flow restriction in the inlet duct.

Accordingly, such a composite turbocharger system should provide a solution to the problem of inadequate boost pressure at low engine speeds. However, it is found in practice that it does not do so and tests have indicated that an engine fitted with such a turbocharger system has a power output of only about two-thirds of that which would be expected at low engine speeds.

In addition difficulties are encountered in controlling operation of the individual turbochargers and in particular airflow. For example the larger turbocharger has a turbine bypass valve (for bypassing the larger turbine in an overboost or overspeed condition) and control of the smaller turbine bypass and larger turbine bypass must be achieved without competition between the control strategies. Yet a further problem is that the smaller compressor can act as a restriction on airflow from the larger compressor whilst producing no pressure rise at higher engine speeds/loads.

It is, therefore, the object of the invention to provide a turbocharger system of the type incorporating two turbochargers which does provide a substantial boost pressure at substantially all engine speeds and enables the engine to produce a significantly enhanced power output at low engine speeds.

According to the present invention, a turbocharger system for an automotive engine comprises an air inlet duct, an exhaust gas duct and first and second turbochargers, the first turbocharger being substantially smaller than the second turbocharger, each turbocharger including an exhaust turbine situated in the exhaust duct and a blower wheel situated in the inlet duct, a bypass duct being connected to the exhaust duct on each side of the exhaust turbine of the first turbocharger, the bypass duct including a selectively operable butterfly shut-off valve including a valve flap pivotally mounted within a housing, the internal wall of the housing carrying two oppositely directed semi-annular sealing surfaces extending transversely to the direction of the exhaust gas flow, the valve flap being movable between an open position in which the bypass duct is substantially unrestricted and a closed position in which it is in sealing engagement with the two sealing surfaces.

Exhaustive tests on the known turbocharger system including two turbochargers have revealed that the reason why it does not produce a satisfactory boost pressure at low engine speeds is that the bypass valve is inherently leaky and a substantial proportion of the exhaust gas thus flows through the bypass passage and not through the smaller exhaust turbine, even when the bypass valve is nominally closed. Although numerous different types of shut-off valve are known, the high pressures and temperatures and aggressive conditions which prevail in an automotive exhaust duct mean that one type of valve that is practicable is a butterfly valve. However, in order to avoid the valve flap becoming jammed against the wall of the housing, particularly as a result of the differential thermal expansion which occurs, it is, as a matter of practice, necessary to make the valve flap significantly smaller than the housing in which it is pivotally accommodated. This means that there is in practice a significant gap between the internal wall of the housing and the outer edge of the valve flap, when the valve is closed. This gap constitutes the leakage path through which a significant proportion of the exhaust gas escapes and thus does no work in the exhaust turbine.

It has thus been appreciated that what is needed is to substantially improve the gas tightness of the bypass valve, when closed, and this is achieved by the two semi-annular sealing surfaces in the present invention. These two sealing surfaces will in practice be offset in the housing in the direction of exhaust gas flow through it by a distance substantially equal to the thickness of the valve flap. Thus when the valve is closed, a seal is created not between the outer edge surface of the valve flap and the inner surface of the valve housing, as previously, but between the outer portion of one flat surface of one half of the valve flap and one of the sealing surfaces and between the outer portion of the other flat surface of the other half of the valve flap and the other of the sealing surfaces.

In one embodiment, two semi-annular sealing projections are provided on the internal surface of the housing, opposite side surfaces of which constitute respective sealing surfaces. Alternatively, the interior surface of the bypass valve housing may be effectively smoothly continuous throughout with the exception of two discontinuities at which the respective sealing surfaces are defined. In this latter embodiment, the two portions of the gas flow passage through the housing on opposite sides of the valve flap are effectively slightly offset from one another in a direction transverse to the direction of gas flow through it, whereby the two opposed sealing surfaces are afforded at the discontinuities, that is to say at the positions where the offset portions of the flow passage merge into one another. The flow passage through the housing may of course be of any shape conventional with butterfly valves, e.g. circular or rectangular.

The provision of the opposed sealing surfaces with which the valve flap co-operates in the closed position results in the valve forming a very effective seal. Little or no exhaust gas thus leaks through the bypass passage when the valve is closed which results in substantially all of the exhaust gas flow flowing past the turbine wheel of the smaller turbocharger at low engine speeds, whereby the blower wheel of the smaller turbocharger may produce a substantial boost pressure in the air inlet duct. The power output of the engine is therefore substantially increased at low engine speeds by comparison with engines with dual turbocharger systems of known type.

The present invention also embraces an automotive engine including a turbocharger system of the type referred to above. Further aspects of the invention are set out in the claims.

Further features and details of the invention will be apparent from the following description of one specific embodiment which is given by way of example with reference to the accompanying drawings, in which: