GB2106191A

GB2106191A - A turbocompressor for supercharging a heat engine

Info

Publication number: GB2106191A
Application number: GB08128236A
Authority: GB
Inventors: Jacques Delesalle
Original assignee: Alsacienne de Constructions Mecaniques SA
Current assignee: Alsacienne de Constructions Mecaniques SA
Priority date: 1980-08-01
Filing date: 1981-09-18
Publication date: 1983-04-07
Also published as: DE3132134C2; FR2487913A1; FR2487913B1; CH642720A5; DE3132134A1

Abstract

The turbine 10 of the turbocompressor is supplied with exhaust gases from the engine 4 by means of a distributor 12 having a ring of stationary guide vanes, a jet of air under pressure being blown selectively through orifices 38 between the guide vanes to establish a fluid barrage for constricting the effective flow cross-section of the gas stream between the vanes towards the turbine rotor 18. <IMAGE>

Description

SPECIFICATION A method for improving the operation of a turbocompressor for supercharging a heat engine and a turbocompressor for the application of said method This invention relates to supercharged heat engines and especially to Diesel engines supercharged by a turbocompressor, the turbine of which is driven by the exhaust gases of the engine.

It is an acknowledged fact that the progress achieved in the supercharging of heat engines has brought about a rapid upward trend in their performances. Turbocompressors of current types attain a pressure ratio of 4.5 to 5, thus permitting engine mep (mean effective pressure) values of about 25 bar and it is possible to mount two turbocompressors in series, thus permitting mep values of 33 and over.

Unfortunately, these turbocompressors are designed for operation near the nominal point (maximum power of the engine) and give disappointing results at low engine loads.

Furthermore, a problem of general principle arises: the driving energy contained in the exhaust gases which actuate the turbine depends on the product of flow rate x temperature, namely the power of the engine. The supercharge pressure is thus approximately proportional to the power of the engine. In point of fact, said pressure should be proportional to the torque delivered by the engine and not to its power in order that the engine should operate at approximately constant excess air.

The result thereby achieved is that, when the engine is operated at variable speed for the propulsion of a vehicle, for example, the torque available at low engine speeds is fairly low and out of proportion to the nominal performances of the engine.

For example, in the case of a nominal mep of over 25 bar, it is impossible to engage the propeller of a boat when stationary without stalling the engine unless recourse is had to some expedient whereas the performances are remarkable at full speed.

It is known that the turbine of a turbocompressor essentially comprises an exhaust gas distributor consisting of a ring of stationary vanes, the function of which is to direct the gases onto the blades of the turbine rotor. The compressor of the turbocompressor essentially comprises a blade-type rotor coupled to the turbine shaft and a supercharging air diffuser.

In order to overcome the disadvantage mentioned in the foregoing, it would be necessary to permit adaptation of the dimensions of turbocompressors (gas distributors and air diffusers) to the operating parameters of the engine, especially to the value of torque to be produced by the engine.

A number of solutions have been proposed and applied as follows: - the use of only one sector of the gas distributor at low loads, but this introduces a loss by ventilation; - the use of a number of turbocompressors in parallel, only one-quarter or one-half of this number being employed at low loads, but this is a cumbersome arrangement and the valves required are of complex technology and difficult to actuate in synchronism; - the use of a turbocompressor having a mechanically variable geometry: the distributor vanes and in some cases the diffuser vanes as well are orientable as is the practice in certain industrial and aeronautical turbomachines but at the cost of mechanical complication.

These various solutions basically involve a reduction in gas-flow cross-section within the turbocompressor or turbocompressors at low engine loads but these are complicated and costly mechanical solutions.

The present invention makes it possible in a turbocompressor to carry out selective reduction of the gas-flow cross-section by producing a fluidic action on the gas stream without entailing the need for any additional moving mechanical part.

The method in accordance with the invention consists in blowing a jet of air under pressure between the stationary vanes of the turbine gas distributor in a substantially radial direction towards the gas stream in order to establish a fluidic barrage which has the effect of constricting the effective flow cross-section of the gas stream between the vanes towards the turbine rotor.

Preferably, a jet of air is blown between each pair of stationary vanes of the distributor.

By means of this method, the flow crosssection of the hot gas stream is constricted by the air jets which impinge upon this latter substantially at right angles.

A turbocompressor in accordance with the invention for supercharging a heat engine is essentially distinguished by the fact that provision is made within the gas distributor casing and between the stationary distributor vanes for airblowing slots or orifices which open substantially in a radial direction into the cross-section of the flow of exhaust gases between the stationary vanes. A further distinctive feature lies in the fact that said slots are connected externally of the distributor to an air distribution duct which communicates with a source of air under pressure.

The result thereby achieved is that the flow crosssection of exhaust gases within the turbine can be reduced by the screen of fluid blown radially through said slots or orifices.

The air discharged through the blowing slots can be supplied from an external source of compressed air since the necessary values of flow rate and pressure are not very high.

In a preferred embodiment of the invention, however, the air supply is drawn from the engine supercharge circuit downstream of the compressor of the turbocompressor.

This arrangement is of considerable interest since it combines and cumulates the advantageous effects of the system for reducing the gas-flow cross-section and the known system for diverting part of the supercharging air to the turbine of the turbocompressor.

The operating principle of this system, commonly known as a "bypass flow system", consists in withdrawing air from the discharge side of the air compressor and conveying it directly to the turbine inlet. A device which is often designated as "AP valve" adjusts the bypass-air flow rate as a function of certain parameters which are related to the engine. The engine is thus bypassed by a small quantity of air representing approximately 0 to 50% of the total air delivery.

The bypass produces the following action: when the bypass is open, the turbine of the turbocompressor admits gas at a higher flow rate and at a lower temperature by reason of dilution with the bypass air. When the bypass flow crosssection is increased, there is an increase in energy in the turbine and therefore in the compressor, with the result that the air pressure rises.

Above a given cross-sectional area of the bypass, the temperature drop becomes excessive and the energy in the turbine decreases. An optimum value therefore exists. As a general rule, it is necessary to close the AP valve at 100% of the nominal load and to open it to the full extent at 35% (the valve operation in fact takes place in a progressive manner). It is apparent that the bypass airflow rate has the highest value at low loads.

If this bypass air is employed in order to produce a fluidic reduction in the turbine crosssection, two effects are added and result in an increase in supercharge pressure. In point of fact, it is precisely at low loads that this result is expected and that bypass air is available for supplying the blowing slots.

The above-mentioned combination of blowing and bypassing of supercharge air can be completed by interposing a combustion chamber in the exhaust gas circuit, said combustion chamber being placed upstream of the turbine.

It will readily be apparent that, in all cases, an airflow regulating device is interposed in the pipe for supplying air under pressure to the air-biowing slots.

An air-blowing system in accordance with the invention can also offer advantageous resuits in the case of a gas distributor provided with orientable vanes since, in this case, the air jets blown through the slots are capable of reducing marginal leakages around the vanes.

A more complete understanding of the invention will be gained from the following description and from the accompanying drawings in which a number of embodiments of the invention are illustrated by way of example without any limitation being implied, and in which: Fig. 1 is a schematic axial sectional view which illustrates a turbocompressor in accordance with the invention and shows the connection between this latter and a heat engine; - Fig. 2 is a radial sectional view of the stationary-vane distributor of the turbocompressor turbine, this view being taken substantially along the plane lI-lI of Fig. 1; - Fig. 3 is a developed partial view of the distributor; - Fig. 4 is a schematic representation of the constriction of the gas stream within the distributor;; - Fig. 5 is a partial sectional view to a larger scale showing the distributor and the rotor of the turbine; - Figs. 6, 7 and 8 illustrate the combination of the air-blowing system for constricting the gas stream with the system for diverting supercharge air to the turbine of the turbocompressor.

The schematic view of Fig. 1 shows a turbocompressor 2 which feeds supercharge air to a Diesel engine 4. The only parts of this engine which are shown in the drawings are the air intake manifold 6 and the exhaust gas manifold 8.

In accordance with conventional practice, the turbine 10 of the turbocompressor comprises an exhaust gas distributor 1 2 provided with a ring 14 of stationary vanes for directing the exhaust gases towards the blades 1 6 of the turbine rotor 1 8. The exhaust gases discharged from the manifold 8 pass into the turbine through the turbine inlet 20 and are finally discharged to the atmosphere through the outlet duct 22 after having worked within the turbine.

The turbine rotor 1 8 is keyed on a shaft 24 on which is also keyed the rotor 26 of the compressor 27, said rotor being adapted to carry blades 28.

Atmospheric air 30 (or air derived from another compressor stage) is sucked into the compressor inlet 32 and discharged through the duct 34 and through the compressor outlet 36 to the supercharge-air intake manifold 6 of the engine.

In accordance with the invention, air-biowing slots or orifices 38 are formed in the casing 39 of the distributor 1 2 between the stationary vanes 1 4. As shown more clearly in Figs. 2 and 3, said slots or orifices open in a radial or substantially radial direction into the exhaust gas flow cross section between the stationary vanes.

The slots 38 are connected externally of the distributor to an air distribution pipe 40 which communicates with the source 42 of air under pressure with interposition of a valve 41 for regulating the air flow rate.

The effect produced by fluidic constriction of the gas-flow cross-section is illustrated diagrammatically in Fig. 4. The arrows 44 represent the path followed by the exhaust gases between two stationary vanes of the distributor, the arrow 46 represents the air jet blown through a slot 38 and the line 48 represents the fluidic screen which constricts the cross-section of the gas stream as indicated by the arrows 44'.

The slots or orifices 38 can be oriented substantially at right angles to the gas stream or may be inciined downstream or upstream with respect to the direction of flow, up to an angle of 300, for example. As illustrated in Figs. 2 and 3, a slot 38 is provided between each pair of vanes 14.

By way of example, the slots open into the distributor in a substantially radial direction, for example in the vicinity of the throat of the nozzle formed by two adjacent vanes.

As shown more clearly in Fig. 5, the slots 38 are formed in the outer ring 39 on which the vanes of the distributor 12 are fixed. Said slots 38 are supplied from an air distribution duct consisting of an annular channel 50 cut in the outer frame 52 of the turbine. A bore 54 in the frame 52 serves to connect the channel 50 to the pipe 40 for admission of air under pressure.

Slots 38 for blowing air can be provided in the outer ring 39 of the distributor instead of the slots 38' formed in the inner ring 39' on which the vanes are fixed. Alternatively, provision can be made for both the slots 38 and the slots 38' as shown in Fig. 5.

The slots 38' can be supplied from an annular channel 50' which is connected to the pipe 40 for the admission of air under pressure by means of ducts 54' formed in the central body 52' of the turbine.

In order to gain a clearer understanding of the invention, Fig. 1 shows the supply of air-blowing slots 38 (and/or 38') by means of an external source of air 42.

In this case, the operation of the installation is extremely simple: when the engine is at full load, the valve 41 is closed and the turbine operates normally with the full cross-section of the exhaust gas stream, with its nominal performances. At low engine loads, the valve 41 is opened to a progressively greater extend in order to reduce the flow cross-section of the gas stream under the action of the air jets blown through the slots. It is thus possible to obtain a result which is equivalent to a reduction in cross-section of approximately 50%, thus making it possible to improve or restore satisfactory supercharging of the engine at all speeds and therefore mitigates or overcomes the disadvantages recalled at the beginning of this description.As will readily be apparent, the valve 41 can be controlled automatically by a servomechanism in dependence on the operating parameters of the engine.

In the case of a turbocompressor in accordance with the invention, the most advantageous solution, however, consists in combining the system of fluidic reduction of flow cross-section within the turbine with the bypass flow system in which part of the supercharged air is returned to the turbine, thereby cumulating the advantages of both systems.

A number of variants of a combination of this type are illustrated in the schematic Figs. 6, 7 and 8 in which only the air circuits and the exhaust gas circuits are shown.

In these figures, the turbocompressor is represented schematically by its turbine 10 (with its exhaust gas inlet 20 and its outlet 22 for discharge to the atmosphere) and by its compressor 27 (with its air inlet 32 and its supercharge air outlet 36). The air-blowing slots 38 between the stationary vanes of the turbine are supplied via the pipe 40 which is controlled by the valve 41. The engine 4 with its supercharge air intake 6 and its exhaust gas manifold 8 are also illustrated.

In the embodiment shown in Fig. 6, the pipe 40 is supplied with air under pressure by a bypass pipe 56 which is connected to the enginesupercharging air delivery circuit or in other words to the duct 6 between the compressor outlet 36 and the air intake of the engine. A portion of the supercharge air which is controlled by opening of the valve 41 to a greater or lesser extent is thus withdrawn for feeding the air-blowing slots 38 of the turbine.

Depending on the amount of opening of the valve 41 , the air withdrawal can represent between 0 and about 50% of the total air delivery.

At low engine loads, when the valve 41 is in the fully open position, the slots 38 blow powerful air jets which reduce the effective gas-flow crosssection. At the same time, however, by reason of the flow of air through the slots, the turbine receives a higher total delivery of gas at a lower temperature by reason of dilution with the bypass air. These two effects are cumulative, thus increasing the energy in the turbine and therefore in the compressor, that is to say producing a cumulative increase in the supercharge pressure.

In other words, in accordance with the invention, the bypass air is employed for two purposes in order to obtain an increase in the supercharge pressure.

During operation, the valve 41 is completely closed at full engine load and is completely open at approximately 35% of the load, the valve operation being of course progressive.

The alternative embodiment shown in Fig. 7 is similar to that of Fig. 6 apart from the fact that a second bypass pipe 58 is provided between the pipe 40 for supplying air to the blowing slots 38 and the inlet 20 for admission of the exhaust gases into the turbine 10. In accordance with this arrangement, part of the diverted supercharge air or "bypass air" is reintroduced and mixed with the exhaust gas within the turbine in order to increase the energy in the turbine. This arrangement may prove advantageous when the cross-sectional area of the blowing slots 38 is insufficient to allow the total bypass supercharge air flow to pass through. As can readily be understood, a valve (not shown) could also be provided in the pipe 58 in order to regulate the distribution of air between the slots and the turbine inlet.

Finally, Fig. 8 shows an arrangement derived from that of Fig. 7 in which a combustion chamber 60 is interposed upstream of the inlet 20 of the turbine 10 in order to reheat the mixture of exhaust gases discharged from the exhaust manifold 8 and the bypass supercharge air delivered by the second bypass duct 8. This permits an even greater improvement in the performances of the turbine at low engine loads.

Claims

1. A method for selectively reducing the flow cross-section of gases within a distributor having stationary vanes of the turbine of a turbocompressor for supercharging a heat engine, the said turbine being supplied with exhaust gases from the engine, the said method being characterized in that it consists in blowing a jet of air under pressure between the said stationary vanes towards the gas stream in order to establish a fluidic barrage for constricting the effective flow cross-section of the gas stream between the vanes towards the turbine rotor.

2. A method in accordance with claim 1, characterized in that it consists in blowing an air jet between each pair of stationary vanes of the distributor.

3. A method in accordance with claim 1 or claim 2, characterized in that the air jets are supplied from an external source of air under pressure.

4. A method in accordance with claim 1 or claim 2, characterized in that the air under pressure which feeds the air jets is withdrawn as a bypass off the air discharge of the supercharge compressor which is driven by the turbine aforesaid and delivers supercharge air to the engine.

5. A method in accordance with any one of claims 1 to 4, characterized in that the flow rate of air fed to the air jets is selectively adjustable as a function of the operating parameters of the engine, especially as a function of the engine load.

6. A supercharging turbocompressor for a heat engine, comprising a turbine supplied with exhaust gases from the engine and adapted to drive a compressor which delivers supercharge air to the engine, the said turbine being provided with a distributor having stationary vanes and with a rotor whose shaft is coupled to the compressor, the said turbocompressor being characterized in that provision is made within the distributor casing and between the stationary vanes for air-blowing slots or orifices which open substantially in a radial direction into the flow cross-section of exhaust gases between the stationary vanes, and that the said slots are connected externally of the distributor to an air distribution duct which communicates with a source of air under pressure so that the flow cross-section of exhaust gases within the turbine can be reduced by the screen of fluid blown radially through said slots or orifices.

7. A turbocompressor in accordance with claim 6, characterized in that means for regulating the air flow through the slots or orifices aforesaid are provided between the air distribution duct and the source.

8. A turbocompressor in accordance with claim 6 or claim 7, characterized in that provision is made for an air-blowing slot or orifice which opens in a substantially radial direction between each pair of distributor vanes.

9. A turbocompressor in accordance with claim 8, characterized in that the air-blowing slots or orifices open within the interior of the distributor in the vicinity of the throat of the nozzle formed by two adjacent vanes.

1 0. A turbocompressor in accordance with any one of claims 6 to 9, characterized in that the airblowing slots or orifices are formed in either the inner ring or outer ring, or in both rings to which the stationary distributor vanes are attached.

11. A turbocompressor in accordance with any one of claims 1 to 10, characterized in that the air distribution duct aforesaid is constituted by at least one annular channel or collector formed in a frame which surrounds the distributor, the outer ends of all the slots or orifices aforesaid being so arranged as to open into the said channel.

1 2. A turbocompressor in accordance with any one of claims 1 to 11, characterized in that the slots or orifices are oriented substantially at right angles to the direction of flow of the gas stream between the distributor vanes.

1 3. A turbocompressor in accordance with any one of claims 1 to 12, characterized in that the source of air under pressure supplied to the airblowing slots consists of an external air source.

14. A turbocompressor in accordance with any one of claims 1 to 12, characterized in that the source of air under pressure supplied to the airblowing slots is constituted by a withdrawal of engine-supercharging air from the outlet of the compressor which is driven by the turbine.

1 5. A turbocompressor in accordance with claim 14, characterized in that it comprises an air bypass pipe interposed between the compressor outlet and the duct for distribution of air to the slots or orifices aforesaid and that an airflow regulating device is mounted on the said bypass pipe.

1 6. A turbocompressor in accordance with claim 1 5, characterized in that it further comprises an additional bypass pipe connected between the first bypass pipe aforesaid and the pipe for the admission of exhaust gases into the turbine.

17. A turbocompressor in accordance with claim 16, characterized in that the additional bypass pipe is connected to the first bypass pipe downstream of the airflow regulating device.

1 8. A turbocompressor in accordance with claim 17, characterized in that a combustion chamber for reheating the exhaust gases and if necessary the bypass air derived from the compressor is mounted upstream of the inlet for admission of gases into the turbine.

1 9. A turbocompressor in accordance with any one of claims 7 to 18, characterized in that the device for regulating the flow of air through the slots is actuated by a servomechanism which is responsive to one of the operating parameters of the engine and especially the engine load.

20. A method for selectively reducing the flow cross-section of gases within the turbine distributor of a turbocompressor for supercharging a heat engine, substantially as hereinbefore described.

21. A turbocompressor for supercharging a heat engine in accordance with the said method, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.