On such an internal combustion engine with two-stage turbocharging disclosed in DE 195 14 572 A1, a high-pressure stage and a low-pressure stage are arranged in series in a turbocharger in the lower speed range of the internal combustion engine. The exhaust initially flows through the high- pressure turbine and then through the low-pressure turbine. The turbocharging air is first compressed by the low-pressure compressor and then by the high-pressure compressor and fed, after cooling in a heat exchanger, to the fresh air side of the internal combustion engine. As the rotational speed of the internal combustion engine increases, a changeover can be made to single-stage compression exclusively in the low-pressure compressor in that the high-pressure turbine is completely bypassed by means of an exhaust-side pipe switch and, appropriately, the high-pressure compressor can be fully bypassed via a turbocharging-air-side pipe switch.

A disadvantage of such changeover turbocharging can be seen by the fact that in the event of frequently desired load and speed changes of the internal combustion engine, very often a changeover must be made between one- stage and two-stage mode of operation of the turbocharging unit.

Consequently, there may be a loss of travelling comfort, i. e. unsteady acceleration and braking power response.

A further internal combustion engine according to the preamble is disclosed in DE 39 03 563 C1. Here, too, a changeover is provided from two-stage to one-stage turbocharging. The changeover is effected by means of a pipe switch arranged between the outlet side and the high-pressure turbine. Thus losses in travelling comfort may occur here also.

The invention is based on the problem of providing an internal combustion engine in accordance with the preamble of claim 1, which responds to fast load and speed changes without unsteady acceleration and braking power response. The turbocharging pressure is intended, in the case of acceleration -i. e. when the vehicle is to be accelerated-to build up rapidly and be capable of being adapted infinitely and variably to the engine requirements.

This problem is solved by the characteristics of the independent claims.

Through the characteristics according to the invention, specifically the following is achieved: Because there is a continuously flow through the high-pressure turbine at least to a certain extent, and this flow circulates, it is ensured that in the case of acceleration a minimum turbocharging pressure exists and, in particular, the rotational speed of the HP rotor is at a favourable initial level. Furthermore, the individual exhaust mass flows can be supplied to the high-pressure turbine, the low-pressure turbine or the fresh air side by the arrangements according to the invention with the help of the central processing unit and the pipe switch to the extent desired in each case, so that an optimisation of the mode of operation of the engine can be made with respect to minimum fuel consumption and/or minimum polluant emission.

With corresponding load and increasing rotational speed of the engine a fast response of the high-pressure turbine is thus ensured in that the expansion work is shifted in the direction of the high-pressure turbine, i. e. through extensive closure of the bypass channel by means of pipe switch the largest portion of the exhaust flow is fed to the high-pressure turbine. If, at low load and with small exhaust mass flows, a low-consumption, small load and, above all, exhaust counter pressures are desired in this operating range, the expansion work of the exhaust can for the greater part take place in the low- pressure turbine and possibly by appropriate positioning of the pipe switch via the exhaust return, independently of the rotational speed of the engine by opening the bypass channel.

Linked to an motor electronic system, which records the operating characteristics of the engine, such as rotational speeds, mass flows, turbocharging pressures and turbocharging air temperatures, the pipe switches can be controlled for a mode of operation that minimises consumption or polluants at any operating point of the engine. As a rule, a trade-off is required between minimal consumption and minimal pollutants, Depending on the ambient conditions, load state and rotational speed, a target-optimised splitting of the exhaust mass flow is made to the fresh air side, the high-pressure turbine and the low-pressure turbine.

Further advantages can be seen in the fact that, due to the possible distribution of the exhaust flow, the operating lines run in the high-pressure and low-pressure compressor performance characteristics so that on the one hand a high compressor efficiency is reached, and on the other pumping is virtually excluded under extreme conditions.

Apart from the previously described first main idea of the invention, according to a second main idea of the invention the following is also possible:

A bypass channel which connects the internal combustion engine to the inlet side of the low-pressure turbine is not absolutely required. Rather, one of the two turbines-preferably the high-pressure turbine-can also be designed with a corresponding variable turbine geometry, above all with a distributor with adjustable vanes. If, for example, the high-pressure turbine is provided with such a distributor, although the entire mass flow goes through the high- pressure turbine, the rate of this mass flow can be throttled to a greater or less extent.

Additionally, a bypass pipe can be provided, with which the high-pressure turbine can be bypassed, and which has a pipe switch. In this case, too, the distributor is always a little open so that at least a minimum exhaust mass flow flows reliably through the high-pressure turbine so that always at least a minimum turbocharging pressure is present and, in particular, the rotational speed of the HP rotor is at a favorable initial level. By means of the pipe switch, however, there is an addition possibility of control.

At any rate, on the use of one of the two main ideas the advantage is achieved that different operating parameters of the internal combustion engine can be dealt with in a very sensitive way.

Preferred embodiments of the invention are expiained in the following sections with reference to the attached drawing. It shows: Fig. 1a a cycle diagram of the exhaust and fresh-air flow of a two-stage turbocharged diesel internal combustion engine with bypass run in pairs, Fig. 1 b a cycle diagram of the exhaust flow of a two-stage turbocharged diesel internal combustion engine with common bypass run,

Fig. 2 a cycle diagram of the exhaust flow of a two-stage turbocharged diesel internal combustion engine with bypass run in pairs for twin-flow low-pressure turbines, Fig. 3 a cycle diagram of the exhaust and fresh-air flow according to Fig. 1 a with low-pressure bypass unit, Fig. 4 a cycle diagram of the exhaust and fresh-air flow of a two-stage turbocharged diesel internal combustion engine in V shape.

Figs. 5 and 6 further cycle diagrams in which turbines with a variable geometry are used as high-pressure turbines.

Fig. 7 a cycle diagram disclosing a particular bypass arrangement.

The six-cylinder diesel internal combustion engine 10 in series mode of construction shown in Fig. 1 is turbocharged in two stages via a turbocharger unit. For this purpose a high-pressure stage 20 is arranged ahead of a single- flow low-pressure stage 30. Via the compressors 22 and 32 driven by the high-pressure turbine 21 and low-pressure turbine 31, fresh air is compressed, cooled down in the two turbocharging-air coolers 40, mixed to a certain percentage (_0) with exhaust from an exhaust return flow 50 and fed to the fresh air side 11 of the engine 10. The rotor diameter of the low- pressure turbine 32 is larger than that of the high-pressure turbine 21, with the rotor diameter ratio dL. ND/dL, HD being 1.2 to 1.8 between low-pressure and high-pressure turbine. The two flows 23a, b of the twin-flow high-pressure turbine 21 are each connected on the inlet side via a separate pipe 60,61 with the exhaust side 12 of the engine. On the outlet side the flows 23a, b are connected via outlet-side pipes 63,64 to a common pipe 62, which in turn is connected on the inlet side to the single-flow low-pressure turbine 31. One of the two turbocharging air coolers can, of course, also be omitted.

For optimum adaptation of the turbocharger unit to the operating conditions of the engine 10, a bypass channel 24a and 24b is provided in symmetrical arrangement for each flow 23a, b of the high-pressure turbine 21. Each of these branches off the separate pipe 60 or 61 designed as exhaust elbow, bypass the high-pressure turbine 20 and flow into the common pipe 62 for the same supply to the single-flow low-pressure turbine 30. Each bypass channel 24a, b is provided with a pipe switch 70 or 71 arranged downstream of the branch. These can be integrated in the exhaust elbow or in the housing of the high-pressure turbine and can be designed as slide, valve or flap or similar element and controlled by a CPU both singly and jointly.

In addition, exhaust return pipes 50 are connected, leaving to the fresh air side 11 respectively behind compressor 22. The returned quantity of exhaust can, however, also be fed to any other point of the fresh air side. By means of the pipe switch 70 on the one hand the bypass channel 24a can be closed and, on the other, with opened bypass channel 24a partial flows distributed in the required ratio to the low-pressure turbine 30 and exhaust return pipe 50 (exhaust gas return rate 2 O). Furthermore, for control of the pipe switches 70,71 and 50 as a function of the operating characteristic variables a1-n the pipe switches 70,71 and 50 are connected to an electronic motor control 80, which ensures an optimum distribution of the exhaust mass flow for operation.

Through the possible adjustment of different bypass rates24a, b, an additional degree of freedom is obtained for the distribution of the entire exhaust mass.

An alternative embodiment of the internal combustion engine10 is shown in Fig. 1 b; this differs from the variant according to Fig. 1a in the design of the turbocharger unit. In this case the outlet side connection of the high-pressure turbine 21 is provided to the common pipe 62 downstream of the mouth point 63 of the two bypass channels 24a, b, whereas this is designed upstream according to Fig. 1a.

A third variant of the internal combustion engine 10 is represented in Fig. 2.

Here, the low-pressure turbine 30 is designed to be double flow. The two channels 33a, b of the low-pressure turbine 31 are each supplied from a separate pipe 62a and 62b and so an uneven admission to the low-pressure turbine is possible. Thus the bypass channels 24a, b are also each allocated a flow 33a and 33b and, like the flows 23a, b of the high-pressure turbine 21 are each connected separately from each other to the separate pipes 62a and 62b.

The internal combustion engine that can be seen in Fig. 3 has a low-pressure turbine 31 provided with a bypass unit 34, which is controllable by means of a pipe switch 72 for optimisation of the precompression as a function of the operating characteristics a1-n. This is particularly interesting for applications (passenger cars) in which, for example, because of construction space problems cooling of the compressor air between high-pressure 22 and low- pressure compressor 32 has to be dispensed with. Through this, the precompression can be limited in the area of the rated power of the engine 10 by the low-pressure stage 30 to a desired extent.

Through the bypass piping 34 with pipe switch 72 it is possible to use a very small low-pressure 31. This makes possible higher braking powers in engine overrun. In addition, the acceleration response of the engine can be improved by the said measure. Furthermore, the turbocharging and exhaust counterpressure can be further reduced in certain operating ranges. This additionally increases the efficiency of the internal combustion engine.

Fig. 4 shows a fifth embodiment of the internal combustion engine 10, which in this case is of the V8 type. Each cylinder bank 13a, b is allocated a separate high-pressure stage 20. The single-flow high-pressure turbines 21 are provided with a bypass channel 24 including pipe switch 70. On the exhaust side both high-pressure turbines 21 are connected to the inlet of the

joint low-pressure turbine 31. Through the possible setting of different bypass rates of the two high-pressure stages 20, here, too, a further degree of freedom is obtained for the distribution of the entire exhaust mass. By means of the pipe switch 70, as described before a splitting of the exhaust flow is possible to the high-pressure turbine 21, low-pressure turbine 31 and exhaust return 50.

Fundamentally, any turbine can be designed to be single flow, double flow or with variable turbine geometry, especially with a distributor with ajustable vanes.

The diagram shown in figure 7 is similar to diagram of figure 3. It comprises, however, a bypass line 86 which bypasses the high pressure compressor.

Further, it comprises a pipe switch 87. The said embodiment has proven particularly useful with diesel engines in view of significant emprovements with regard to motor efficiency, fuel consumption and emissions in the upper speed range. The mechanical effort as compared with the result obtained is relatively low.

List of reference numbers 10 Diesel internal combustion engine 11 Fresh air side 12 Exhaust side 13a, b Cylinder bank 20 High-pressure stage 21 High-pressure turbine 22 High-pressure compressor 23a,b Flow 24,24a, b Bypass channel 30 Low-pressure stage 31 Low-pressure turbine 32 Low-pressure compressor 33a,b Flow 34 Bypass unit 40 Turbocharging air cooler 50 Exhaust return 60,61,62,62a, b Pipe 63,63a, b Mouth point 70,71,72 Pipe switch 80 Motor control 86 bypass line 87 pipe switch