BACKGROUND ART: In a turbocompound engine (TC engine) power is transmitted from the power turbine of the TC unit via a gear mechanism down to the engine's crankshaft. This power is obtained by extracting the residual energy that remains in the engine's exhaust gases after these have passed through a turbo compressor for compressing the engine's charging air.

For engine braking, it is normal for an exhaust brake to be used. For a TC engine, the exhaust brake consists of some form of arrangement, normally a throttle valve that can throttle the exhaust flow, and it is placed downstream of the TC unit. When the valve is closed and the fuel injection ceases, power is transmitted instead from the crankshaft via a gear to the TC unit's power turbine. This power helps to increase the braking

effect as it is an energy loss, which is positive for braking. A problem arises with this, however. As a valve has been closed downstream of the TC unit, the density increases of the air in which the TC unit's power turbine is operating. This, of course, assists the braking, but it also gives rise to increased thermal and mechanical stresses. These increased stresses will depend upon the engine speed and will increase with increased engine speed. In order that the TC unit or other components do not break, these must be dimensioned for the increased stresses. This can lead to the construction being unnecessarily expensive, as it is made more complicated and as expensive heat- resistant material must be used.

With an exhaust brake, the braking effect increases for a given engine speed when the back pressure after the turbine increases. In order to obtain as good braking as possible, there should thus be as high back pressure as possible. For a TC engine this is particularly difficult, as the stresses that were mentioned above arise as a result of the back pressure increasing. For a given back pressure, the stresses also increase when the engine speed increases. In order that components do not break, they must be constructed so that they can withstand the stresses that arise at the maximal permitted engine brake speed. Alternatively, a lower back pressure can be selected. A lower back pressure can be presumed still to give acceptable braking performance at high engine speeds, but at low engine speeds the braking effect would be low. The situation is thus that in order to obtain a good braking effect at low engine speeds, high back pressure is required, which in turn leads to large forces at high engine speeds, or alternatively using low back pressures at a

high engine speed and obtaining a poor braking effect at low engine speeds.

DISCLOSURE OF INVENTION: An object of the invention is therefore to achieve an arrangement that makes possible rapid and effective regulation of the exhaust back pressure during engine braking.

This object is achieved by a means for braking according to the characterizing part of claim 1. By means of the solution according to the invention, it is possible to optimize the braking effect for all engine speeds without being forced to select expensive constructional solutions. At low engine speeds, a high back pressure is selected that gives acceptable braking effect and acceptable stresses. At high engine speeds, a lower back pressure is selected that gives acceptable stresses and acceptable braking effect. With this solution, the braking effect can thus be optimized for the selected constructional solution and the selected material over the whole range of engine speeds. This means in principle that for each engine speed there is a unique back pressure that gives maximal braking effect without leading to unacceptable stresses arising.

According to an advantageous embodiment of the invention, the exhaust pressure regulator is provided with means for adapting the exhaust braking pressure to the engine speed.

According to a variant of the arrangement according to the invention, the exhaust braking throttle comprises an exhaust valve placed in the exhaust system

downstream of the exhaust turbine and regulating a parallel bypass with the exhaust pressure regulator.

The exhaust pressure regulator consists suitably of a piston valve that comprises a first piston surface that is acted upon by the exhaust pressure when the exhaust braking throttle is closed, and a second opposing piston surface permanently connected to the said first piston surface, which second piston surface is acted upon by a control pressure.

BRIEF DESCRIPTION OF DRAWINGS: The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which Figure 1 is an outline diagram showing a first embodiment of an arrangement according to the invention, and Figure 2 shows schematically a second embodiment of the arrangement according to the invention.

MODES FOR CARRYING OUT THE INVENTION: The arrangement shown in the figures is intended to be used with an essentially conventional internal combustion engine of the turbocompound type (not shown), preferably incorporated in the drive unit of a heavy truck or a bus. The engine is advantageously of the direct-injection. diesel engine type in which a supercharger 10, with exhaust gas driven turbine 11 and compressor 13 arranged on the turbine shaft 12, are used for the compression and supply of combustion air.

The inlet air is supplied to the compressor 13 for compression, after which the compressed air can be cooled during its passage through a charging air cooler before it enters the engine's inlet manifold.

The engine's exhaust gases are collected in the conventional way in an exhaust collector to be taken to the supercharger's 10 turbine 11 for driving the compressor 13. The exhaust gases are then taken via a second exhaust turbine, which in the embodiment shown consists of an axial turbine 15, and an exhaust brake arrangement 16 to a silencer unit with optional exhaust gas filter equipment.

The axial turbine 15 is used in turbocompound engines to extract residual energy from the exhaust gases after their passage through the supercharger's turbine. The exhaust gases drive the power turbine at very high speeds, up to approximately 90,000 rpm at a normal engine speed, which for a diesel engine for heavy trucks involves an engine speed of approximately 1,500 - 2, 500 rpm. The torque that is obtained is transmitted to the crankshaft of the internal combustion engine via, among other things, a transmission 17 that gears down the speed, and a fluid coupling 18 that isolates the transmission 17 mechanically from the engine's crankshaft.

The exhaust brake arrangement 16 comprises a throttle 19 that can be moved between two end positions by means of a servo device 20, with the throttle rapidly changing between a completely open and a completely closed position. The exhaust brake arrangement comprises, in addition, a bypass 21 past the throttle 19, which bypass can be controlled by means of an exhaust brake regulator in the form of a piston valve 22 which is placed upstream of the throttle 19. A first piston surface 23 is acted upon by the exhaust pressure when the exhaust braking throttle is closed, with the piston surface 23 being pressed against the action of a helical spring 24, so that the bypass 21 is opened. A

second piston surface 25 is permanently connected to the piston surface 23 via a rod 26 and is mounted in a cylinder 27 in such a way that it can be moved.

A regulating air pressure acts against the piston surface 25 via a compressed air pipe 28 which is connected to a compressed air system in the vehicle which is used to generate power for auxiliary units in the vehicle, for example the braking system and system for pneumatic operation of the vehicle's gearbox. This compressed air system comprises, among other things, a compressor 29, an accumulator tank 30 and a valve housing 31.

As the second piston surface 25 of the piston valve 22 has a slightly smaller diameter than the first piston surface 23, the piston valve will be able to react during engine braking and open the bypass 21 past the exhaust braking throttle 16 in the event of an exhaust gas pressure acting against the first piston surface that is less than the pressure that is to be found in the compressed air pipe 29 and thus acts against the second piston surface 25. For example, the piston surface 23 can have a diameter of 90 mm while the piston surface 25 has a diameter of 84 mm, whereby the piston valve 22 can react to an exhaust gas pressure which is approximately 15% lower than the system pressure.

The valve unit 31 provides overpressure that can vary from the standby level 0.5 bar overpressure to a higher level that can be regulated in relation to the required engine braking effect. For this purpose, the valve unit is connected to an engine control unit 32, (see figure 2) which is arranged to regulate the higher level of overpressure with regard to various parameters, for

example information about the brake pedal pressure and ABS system, so that the braking power is optimized in relation to the operation of the engine and to the state of the road.

The regulation is carried out in the following way: The control pressure, Control. acts upon the piston surface 25 in the exhaust pressure regulator 22. By this means, a force arises which is transmitted to the piston surface 23 via the rod 26. This force attempts to hold back the gases that want to pass by the valve 23 and a back pressure, Pm is obtained.

A state of equilibrium gives: Pcontrol a constant The control pressure Pcontroi is adjustable and is adjusted by the valve unit 31. As the back pressure is directly proportional to the control pressure, the back pressure will be changed when the control pressure is changed.

Figure 2 shows a variant of the invention in which the exhaust brake regulator 16 is designed in a different way to that shown in figure 1. Thus the exhaust pipe is L-shaped and the piston valve 22 is set in the angle between the two parts of the pipe. In this case, the throttle 19 and the bypass 21 are not needed, as the changeover from normal operation to exhaust braking is carried out by the piston valve 22 being moved from an inner inactive position to an outer active position. In this position, the piston surface 23 blocks off the exhaust pipe with a pressure that is determined by the valve housing 31 and the engine control unit 40, so that excess pressure can leak past the piston surface 23. The variant of the invention shown in figure 2 is

somewhat cheaper to implement than the solution shown in figure 1, but unfortunately it results in a greater pressure loss in the exhaust pipe.

An advantage of the arrangement according to the invention is that the braking effect of the engine brake can be regulated. This means that it is possible to obtain different braking effects at different engine speeds. This adjustable braking effect can, for example, be used to reduce the fuel consumption and to increase driving comfort. These side effects are, of course, also obtained if the exhaust back pressure is regulated in an ordinary turbo engine or aspirating engine.

With the modern control units that are on current engines, this is possible to achieve, and mechanical components are available that are sufficiently quick to be able to achieve the required back pressure. In the example of the regulation system that is shown in the patent application, an indirect setting of the back pressure is carried out by a feed pressure to the EPG being varied. This feed pressure gives rise to a predetermined back pressure. An alternative way of controlling the back pressure is to mount a pressure sensor in the collector housing and to measure the pressure and regulate the feed pressure to the air throttle (the EPG) by. means of the control unit in such a way that the required back pressure is obtained. This method is, however, more complicated and more expensive than the indirect method. Practical tests have shown that for a normal engine. size for heavy trucks it is almost possible to achieve a constant braking moment over a very large range of engine speeds using the arrangement that is shown in the patent application. The invention is not to be regarded as being limited to the embodiments described above, a number of further variants and modifications being possible within the framework of the following patent claims.