EP4731881A1

EP4731881A1 - Turbocharging system, multi-cylinder piston engine and method of operating a multi-cylinder piston engine

Info

Publication number: EP4731881A1
Application number: EP23736406.2A
Authority: EP
Inventors: Martin AXELSSON
Original assignee: Wartsila Finland Oy
Current assignee: Wartsila Finland Oy
Priority date: 2023-06-20
Filing date: 2023-06-20
Publication date: 2026-04-29
Also published as: WO2024261369A1; KR20260003838A

Abstract

The turbocharging system for a multi-cylinder piston engine (20) comprises a high-pressure turbocharger (1) comprising a high-pressure turbine (3) and a high-pressure compressor (4), a first exhaust receiver (11) for connecting a first group of cylinders of the engine (20) to a first inlet (3A) of the high-pressure turbine (3), a second exhaust receiver (12) for connecting a second group of cylinders of the engine (20) to a second inlet (3B) of the high-pressure turbine (3), a low-pressure turbocharger (2) comprising a low-pressure turbine (5) and a low-pressure compressor (6), an air bypass duct (7) having a first end connected to the downstream side of the high-pressure compressor (4) and a second end connected between the high-pressure turbine (3) and the low-pressure turbine (5), and a controllable air bypass valve (8) for controlling flow in the air by-pass duct (7).

Description

Turbocharging system, multi-cylinder piston engine and method of operating a multi-cylinder piston engine

Technical field of the invention

The present invention concerns a turbocharging system for a multi-cylinder piston engine, as defined in claim 1. The invention also concerns a multi-cylinder piston engine and a method of operating a multi-cylinder piston engine.

Background of the invention

Many large piston engines, such as ship and power plant engines, utilize two- stage turbocharging. In two-stage turbocharging two turbochargers are connected in series. The turbochargers can be referred to as a high-pressure turbocharger and a low-pressure turbocharger. Each turbocharger comprises a turbine and a compressor driven by the turbine.

In a two-stage turbocharged engine exhaust gas from the engine is conducted first into the turbine of the high-pressure turbocharger and from the turbine of the high-pressure turbocharger into the turbine of the low-pressure turbocharger. The pressure of the intake air of the engine is raised from the ambient pressure to a first level by the compressor of the low-pressure turbocharger, and to a second, higher level by the compressor of the high-pressure turbocharger. With two-stage turbocharging, efficiency of the engine can be improved, since the energy of the exhaust gas is more effectively utilized. Two- stage turbocharging also enables higher intake air pressures. With higher intake air pressure, higher power output can be achieved for a given engine displacement, which enables use of smaller engines. In addition, higher intake air pressure allows earlier closing of intake valves during intake stroke (Miller cycle), which helps in reducing NOx emissions.

The operation of the turbochargers need to be carefully controlled to achieve efficient operation of the turbochargers and optimal air-fuel ratio in the cylinders of the engine.

It is particularly important to prevent surging and choking of the compressors of the turbochargers. Compressor surge refers to a phenomenon, where there is an oscillating air flow over the impeller blades of the compressor. Compressor choke refers to a phenomenon, where the compressor operates at its maximum flow and the air speed has reached sonic speed. Both the compressor surge and choke cause noise and wear and can damage the turbocharger.

Achieving of an optimal operating point and air-fuel ratio is challenging in particular in transient situations, where the engine load changes rapidly. Also at low engine loads, it is difficult to achieve good turbocharging efficiency.

For controlling the operation of the turbochargers, turbocharging systems can be provided with (exhaust gas) waste gates. A waste gate valve allows bypassing of a turbine of a turbocharger to control the pressure difference over the turbine. Turbocharged engines can also be provided with (air) bypass ducts and bypass valves. A bypass duct and valve allow directing part of pressurized intake air from an intake duct to an exhaust duct. Alternatively, or in addition, the intake side can be provided with one or more blow-off valves. In contrast to a bypass valve, a blow-off valve releases pressure from the intake duct, but does not conduct the air to the exhaust side. A large number of different valves needed for controlling the operation of the turbochargers can make a turbocharging system complex both in terms of the construction and the control logic. of the invention

An object of the invention is to provide an improved turbocharging system for a multi-cylinder piston engine. Other objects of the invention are to provide an improved multi-cylinder piston engine and a method of operating a multi-cylinder piston engine.

The turbocharging system according to the invention comprises a high-pressure turbocharger comprising a high-pressure turbine having at least a first inlet and a second inlet, and a high-pressure compressor mechanically connected to the high-pressure turbine to be driven by said high-pressure turbine, a first exhaust receiver for connecting a first group of cylinders of the engine to the first inlet of the high-pressure turbine, the first group of cylinders comprising at least one cylinder, a second exhaust receiver for connecting a second group of cylinders of the engine to the second inlet of the high-pressure turbine, the second group of cylinders comprising at least one cylinder, a low- pressure turbocharger comprising a low-pressure turbine and a low-pressure compressor mechanically connected to the low-pressure turbine to be driven by said low-pressure turbine, an air bypass duct having a first end connected to the downstream side of the high-pressure compressor and a second end connected between the high-pressure turbine and the low-pressure turbine, and a controllable air bypass valve for controlling flow in the air bypass duct.

The turbocharging system according to the invention thus combines pulse-type turbocharging with a bypass duct connecting the high-pressure side of the high-pressure compressor between the high-pressure turbine and the low- pressure turbine. The combination provides many synergistic effects. Pulsetype turbocharging provides good performance in case of frequent load changes and at low loads. This is important in particular if the engine is operated using lean or ultra-lean mixtures. A drawback of pulse-type turbocharging is that the exhaust system can become very complicated, especially if the engine type is provided in various different cylinder configurations. For instance, an eight-cylinder engine could need four exhaust receivers. If the turbocharging system was provided with exhaust waste gates over the high-pressure turbine, four waste gate valves would be needed. The air bypass duct eliminates the need for exhaust waste gates over the high-pressure turbine thus making the turbocharging system simpler. A similar bypass duct can be used regardless of the number of cylinders, and only a little customization is needed for adapting the turbocharging system for different engine configurations. The turbocharging system according to the invention also allows optimizing the operation of the turbochargers in different ambient conditions, where the ambient temperature and pressure varies.

According to an embodiment of the invention, the turbocharging system comprises an exhaust waste gate duct having a first end connected between the high-pressure turbine and the low-pressure turbine and a second end connected to the downstream side of the low-pressure turbine, and a controllable waste gate valve for controlling flow in the exhaust waste gate duct. Although the air bypass duct provides in most situations sufficient optimization of the operation of the turbochargers, by adding a waste gate duct over the low-pressure turbine, the change in the operating point of the low-pressure compressor can be minimized.

According to an embodiment of the invention, the turbocharging system comprises a pulse reduction duct connecting the first exhaust receiver to the second exhaust receiver and a controllable pulse reduction valve for controlling flow in the pulse reduction duct. With the pulse reduction duct and valve, advantages of both pulse-type turbocharging and constant pressure turbocharging can be combined. At low loads, the pressure pulses caused by the periodic opening of the exhaust valve of the engine can be utilized, whereas the operation of the turbocharger can be optimized at high loads by opening the pulse reduction valve.

According to an embodiment of the invention, each of the first exhaust receiver and the second exhaust receiver is configured to be connected to at most three cylinders of the engine. Depending on the number of cylinders, each exhaust receiver of the engine can be connected to one, two or three cylinders. This ensures sufficient amplitude of pressure pulses.

According to an embodiment of the invention, the turbocharging system comprises a third exhaust receiver for connecting a third group of cylinders of the engine to a third inlet of the high-pressure turbine, the third group of cylinders comprising at least one cylinder. The turbocharging system can comprise even further exhaust receivers, depending on the number of cylinders.

A multi-cylinder piston engine according to the invention comprises a turbocharging system defined above.

According to an embodiment of the invention, the engine comprises at least four cylinders. With a conventional turbocharging system, a four cylinder engine would require four exhaust waste gate valves. The turbocharging system according to the invention can thus provide benefits in particular in engines having at least four cylinders.

According to an embodiment of the invention, the engine is configured to be operable according to lean burn principle. The engine can be configured to be operable using homogenous charge compression ignition (HCCI) and/or reactivity controlled compression ignition (RCCI). Lean burn combustion requires large amount of air, and the invention can thus provide particular benefits in lean burn engines.

According to an embodiment of the invention, the engine is a dual-fuel or multifuel engine. Different fuels can require different combustion processes or adaptations to the combustion process. The engine cannot be optimized for a single fuel type, but the engine needs to be designed to allow the use of different fuels. The turbocharging system according to the invention provides a simple solution allowing the engine to be operated efficiently with different fuels.

According to an embodiment of the invention, the engine is operable using at least two different combustion processes. Different combustion processes pose an extra challenge to the turbocharging system, and the turbocharging system according to the invention enables efficient operation of the engine regardless of the combustion process used. The at least two combustion processes can include at least one combustion process based on a premixed charge and at least one combustion process based on a non-premixed charge. The combustion processes can include the Diesel process and Otto process and/or variants of the Diesel and Otto processes.

According to an embodiment of the invention, the rated speed of the engine is 200-1500 rpm. The turbocharging system is suitable in particular for mediumspeed engines.

According to an embodiment of the invention, the operation of the air bypass valve is configured to be controlled based on the pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load. The control of the air bypass valve has several purposes, such as releasing pressure in case of rapidly decreasing load to avoid too high pressure of the intake air, preventing compressor surge, and controlling the temperatures at the inlet and outlet of the low-pressure turbine.

A method of operating a multi-cylinder piston engine defined above comprises the steps of monitoring the pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load and controlling the operation of the air bypass valve based on the monitored pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load.

According to an embodiment of the invention, the turbocharging system comprises an exhaust waste gate duct having a first end connected between the high-pressure turbine and the low-pressure turbine and a second end connected to the downstream side of the low-pressure turbine, and a controllable waste gate valve for controlling flow in the exhaust waste gate duct, and the operation of the waste gate valve is controlled based on the monitored pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load. By controlling the operation of the waste gate valve, the effects of the air bypass valve can be stabilized and the operating point of the low-pressure turbine can be maintained optimal.

Brief description of the drawings

Embodiments of the invention will be described in more detail with reference to the accompanying drawings, in which

Fig. 1 shows schematically a turbocharging system and an engine according to an embodiment of the invention,

Fig. 2 shows schematically a turbocharging system and an engine according to another embodiment of the invention, and

Fig. 3 shows schematically a turbocharging system and an engine according to still another embodiment of the invention.

Detailed description of embodiments of the invention

Figures 1 to 3 show schematically an engine 20 and a turbocharging system according to different embodiments of the invention.

Turbocharging systems can be divided into constant pressure systems and pulse systems. In constant pressure systems, exhaust gas from each cylinder of the engine is collected into a common exhaust manifold, from which the exhaust gas is conducted into the turbine inlet of a turbocharger. The pressure of the exhaust gas conducted into the turbocharger thus remains relatively stable. Constant pressure turbocharging provides good turbine efficiency and allows simple exhaust manifold construction.

In pulse-type turbocharging, the exhaust gas is not collected into a common exhaust manifold, but exhaust gas from each cylinder is conducted directly into the turbine inlet of a turbocharger. This allows utilizing the pressure pulses caused by the periodic opening of the exhaust valves to drive the turbocharger. Pulse-type turbocharging provides good performance in case of frequent load changes and at low loads. On the other hand, more complex piping is needed on the exhaust side. In many cases, the benefits of pulse-type turbocharging can be achieved even if exhaust gas from two or more cylinders is collected into a common exhaust duct. This is the case if the time interval between the opening of the exhaust valves of the cylinders connected to the same exhaust duct is long enough to allow the pressure in the exhaust duct to drop before the exhaust valves of the next cylinder open.

Waste gate valves, bypass valves and blow-off valves are used for controlling the air-fuel ratio of the engine. The valves allow controlling of the pressure on the exhaust side and the intake side to achieve optimal operation of the turbochargers and optimal air-fuel ratio in the cylinders of the engine. The valves are important in particular in transient situations, where the engine load changes rapidly to prevent excessive charge pressures and damaging of the turbochargers. The valves are also used for maintaining the optimal operating point of the turbochargers in different ambient conditions and at different altitudes.

The engine 20 according to the invention is a multi-cylinder piston engine 20. The engine 20 thus comprises a plurality of cylinders 21 , preferably at least four cylinders 21 . The number of cylinders 21 could be, for instance, 4 to 20. In the embodiments of the figures, the engine 20 comprises six cylinders 21 . In the embodiments of the figures, the cylinders 21 are arranged in line. However, the engine 20 could also be a V-engine, where the cylinders 20 are arranged in two cylinder banks.

The engine 20 is a large piston engine. The cylinder diameter of the engine 20 is at least 150 mm and/or the rated output power of the engine 20 is at least 150 kW/cylinder. The engine 20 is preferably a four-stroke engine. The engine 20 can be a medium-speed engine having a rated speed of 200-1500 rpm. The engine 20 can be, for instance, a powerplant engine that is used at a powerplant for producing electricity, or a ship engine that is used as the main engine in the propulsion system of a ship or as an auxiliary engine producing electricity for electrical system of the ship. The engine 20 can be, in particular, an engine that is used in an application where rapid load changes regularly occur, and/or where the engine 20 is regularly operated at low load.

The engine 20 can be configured to be operable using any liquid or gaseous fuel. Preferably, the engine 20 is operable using at least one gaseous fuel. The expression “gaseous fuel” refers here to a fuel that is gaseous in atmospheric pressure and at a temperature of 20 °C. The gaseous fuel can be, for instance, hydrogen (H2) or methane-containing gas, such as natural gas, biogas or synthetic methane. The term “biogas” refers here to a gas that mainly consists of methane and which is obtained from renewable sources. The biogas can be produced for example from organic waste. Natural gas could be stored either as a liquefied gas (LNG) or compressed gas (CNG). Biogas or synthetic methane could be stored in a similar manner. The gaseous fuel could also be, for instance, ammonia or a mixture of two or more types of gaseous fuel. The engine 20 can be operable using two or more different types of gaseous fuel. The engine 20 could also be operable using liquid fuel, such as light fuel oil, heavy fuel oil or methanol. The engine 20 can thus be a gas engine, a dualfuel engine or a multi-fuel engine or an engine that is operable solely using liquid fuel.

The engine 20 can be configured to be operable using at least two different combustion processes. For instance, the engine 20 can be configured to be operable using the Diesel process or a variant of the Diesel process, and using the Otto process or a variant of the Otto process. The engine 20 can be configured to be operable using at least one combustion process that is based on a premixed charge and using at least one combustion process that is based on a non-premixed charge.

The engine 20 can have at least one operating mode, in which the engine 20 is configured to be operated according to lean burn principle. The engine 20 can thus be operated using lean mixtures, i.e. mixtures containing more air than is needed for complete combustion of the fuel. A stoichiometric mixture has just enough air for complete combustion of the fuel and an air-fuel equivalence ratio A = 1 .0. The engine 20 could be operated, for instance, using mixtures having an air-fuel equivalence ratio of at least 1 .5.

The engine 20 can have at least one operating mode, in which the engine 20 is configured to be operated using homogenous charge compression ignition (HCCI) or reactivity controlled compression ignition (RCCI). In the HCCI combustion, a homogenous mixture is compressed to the point of auto-ignition. At least a major part of the fuel is injected during the intake stroke to allow effective mixing of the fuel and air. In the RCCI combustion, different reactivities of two different types of fuel are utilized. Low-reactivity fuel is mixed with the air to form a homogenous mixture. The low-reactivity fuel is therefore injected at least mainly during the intake stroke. High-reactivity fuel is injected into the cylinder during the compression stroke to form a local mixture of low- and high- reactivity fuel. The mixture is ignited close to top dead center by injection of the high-reactivity fuel.

The engine 20 is provided with a two-stage turbocharging system. The turbocharging system thus comprises a high-pressure turbocharger 1 and a low- pressure turbocharger 2. The high-pressure turbocharger 1 comprises a high- pressure turbine 3 and a high-pressure compressor 4 mechanically connected to the high-pressure turbine 3 to be driven by the high-pressure turbine 3. Similarly, the low-pressure turbocharger 2 comprises a low-pressure turbine 5 and a low-pressure compressor 6 mechanically connected to the low-pressure turbine 5 to be driven by the low-pressure turbine 5.

The intake air (charge air) of the engine 20 is pressurized in two stages. In a first stage, the pressure of the intake air is raised from the ambient pressure to a first pressure level by the low-pressure compressor 6 of the low-pressure turbocharger 2. In a second stage, the pressure of the intake air is raised from the first pressure level to a second pressure level by the high-pressure compressor 4 of the high-pressure turbocharger 1 .

In the embodiments of the figures, the turbocharging system is provided with a first charge air cooler 16 that is arranged between the compressors 6, 4 of the low-pressure turbocharger 2 and the high-pressure turbocharger 1 , and with a second charge air cooler 17 that is arranged downstream from the high- pressure compressor 4 of the high-pressure turbocharger 1 .

From the second charge air cooler 16, the pressurized intake air is conveyed to the cylinders 21 of the engine 20 via an intake manifold (charge air receiver) 15.

Exhaust gas from the cylinders 21 of the engine 20 flows first into the high- pressure turbine 3 of the high-pressure turbocharger 1 and from there into the low-pressure turbine 4 of the low-pressure turbocharger 2.

The turbocharging system is a pulse-type turbocharging system. The pressure pulses caused by the periodic opening of the exhaust valve of the engine 20 are thus utilized in driving the turbochargers 1 , 2. For utilizing the pressure pulses, the exhaust gas is not collected into a common exhaust manifold, but the turbocharging system comprises at least two exhaust receivers 11 , 12. The high-pressure turbine 3 of the high-pressure turbocharger 1 comprises at least two inlets 3A, 3B. In the embodiments of the figures, the turbocharging system comprises a first exhaust receiver 11 , which connects a first group of cylinders of the engine 20 to a first inlet 3A of the high-pressure turbine 3, and a second exhaust receiver 12, which connects a second group of cylinders of the engine to a second inlet 3B of the high-pressure turbine 3. In the embodiments of the figures, each of the first group and second group of cylinders comprises three cylinders 21 .

Pulse-type turbocharging provides good performance in case of frequent load changes and at low loads. Because of the pressure pulses, the turbochargers provide a faster response in transient situations and allow quick acceleration. Pulse-type turbocharging is beneficial in particular if the engine 20 is operated using lean or ultra-lean mixtures, which require large amount of air.

The turbocharging system further comprises an air bypass duct 7. The air bypass duct 7 has a first end connected to the downstream side of the high- pressure compressor 4 and a second end connected between the high-pressure turbine 3 and the low-pressure turbine 5. The turbocharging system further comprises controllable air bypass valve 8 for controlling flow in the air bypass duct 7. Many turbocharging systems are provided with separate air bypass valves and air blow-off (waste gate) valves. In the turbocharging system according to the invention, the air bypass valve 8 implements the functions of both bypass and waste gate valves. In addition, the air bypass duct 7 and the air bypass valve 8 eliminate a need for a high-pressure exhaust waste gate.

A drawback of pulse-type turbocharging is that the exhaust system can become very complicated, especially if the engine model is provided in various cylinder configurations. For instance, an eight-cylinder engine could need four exhaust receivers. If the turbocharging system was provided with exhaust waste gates over the high-pressure turbine, four waste gate valves would be needed. With the air bypass duct 7 and air bypass valve 8 according to the invention, the turbocharging system can be made much simpler. A similar air bypass duct 7 can be used regardless of the number of cylinders, and only a little customization is needed for adapting the turbocharging system for different engine configurations. Although an exhaust waste gate is not necessary, in certain situations the operation of the turbocharging system can be further improved by providing the system with an exhaust waste gate duct 9 shown in figures 2 and 3. The exhaust waste gate duct 9 has a first end connected between the high-pressure turbine 3 and the low-pressure turbine 5 and a second end connected to the downstream side of the low-pressure turbine 5. The turbocharging system further comprises a controllable waste gate valve 10 for controlling flow in the exhaust waste gate duct 9.

Figure 3 shows a further embodiment of the invention. In the embodiment of figure 3, the turbocharging system is provided with a pulse reduction duct 13 connecting the first exhaust receiver 11 to the second exhaust receiver 12 and a controllable pulse reduction valve 14 for controlling flow in the pulse reduction duct 13. The pulse reduction duct 13 and pulse reduction valve 14 allow combining the advantages of pulse-type turbocharging and constant pressure turbocharging. At low loads, the pulse reduction valve 14 can be kept closed to utilize the pressure pulses. At high loads, the pulse reduction valve 14 can be opened to reduce pressure pulses and to optimize the operation of the turbochargers 1 , 2.

The exhaust receivers 11 , 12 can be configured according to the cylinder configuration of the engine 20. Preferably, the same number of cylinders is connected to each exhaust receiver. For instance, in-line engines with four or six cylinders can be provided with two exhaust receivers. An eight-cylinder in-line engine can be provided with four exhaust receivers and a nine-cylinder in-line engine can be provided with three exhaust receivers. The turbine 3 of the high- pressure turbocharger 1 can thus have three or four inlets. In V-engines, each cylinder bank can be provided with own high-pressure and low-pressure turbochargers. Alternatively, the cylinders of both banks can be connected to the same turbochargers. A V-engine with 16 cylinders could thus be provided with four exhaust receivers for each bank.

The air bypass valve 8 and the exhaust waste gate valve 10 can be, for instance, electrically or pneumatically controlled valves.

During operation of the engine 20, the pressure of the intake air and the temperature of the exhaust gas are monitored. The temperature of the exhaust gas can be monitored in different locations, such as on the upstream side of the turbine 3 of the high-pressure turbocharger 1 , between the turbines 3, 5 of the high-pressure and low-pressure turbochargers 1 , 2, and on the downstream side of the turbine 5 of the low-pressure turbocharger 2. Similarly, he pressure of the intake air can be monitored in different locations. For the operation of the engine 20, the most relevant pressure is the pressure in the intake manifold 15. However, it is also important to know the pressures on both on the upstream side and on the downstream side of the compressor 4, 6 of each of the turbochargers 1 , 2.

The operation of the air bypass valve 8 is controlled based on the pressure of the intake air and the temperature of the exhaust gas. If the turbocharging system comprises a waste gate valve 10, also the operation of the waste gate valve 10 is controlled based on the pressure of the intake air and the temperature of the exhaust gas.

The operation of the air bypass valve 8 and/or the waste gate valve 10 can be additionally controlled based on the engine load.

The control of the turbocharging system has several purposes. One of the goals is to control the lambda value, i.e. the air-fuel equivalence ratio A. It is not necessary to measure the lambda value directly, but it can be determined based on the pressure in the intake manifold 15 and the amount of fuel introduced into the cylinders 21 of the engine 20. Instead of or in addition to using the pressure of the intake air for controlling the operation of the bypass valve 8 and/or the waste gate valve 10, the control could be based on the measured lambda values.

If the pressure of the intake air in the intake manifold 15 is too high or raising too much, the air bypass valve 8 can be opened to decrease the pressure. As the air is released into the exhaust duct between the turbines 3, 5 of the high- pressure turbocharger 1 and the low-pressure turbocharger 2, the energy of the air is utilized in driving the low-pressure turbocharger 2. A need to reduce the pressure in the intake manifold 15 by releasing air to the exhaust side can occur, for instance, when the engine load suddenly drops.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims.

Claims

Claims:

1 . A turbocharging system for a multi-cylinder piston engine (20), the turbocharging system comprising

- a high-pressure turbocharger (1 ) comprising a high-pressure turbine (3) having at least a first inlet (3A) and a second inlet (3B), and a high- pressure compressor (4) mechanically connected to the high-pressure turbine (2) to be driven by said high-pressure turbine (3),

- a first exhaust receiver (11 ) for connecting a first group of cylinders of the engine (20) to the first inlet (3A) of the high-pressure turbine (3), the first group of cylinders comprising at least one cylinder (21 ),

- a second exhaust receiver (12) for connecting a second group of cylinders of the engine to the second inlet (3B) of the high-pressure turbine (3), the second group of cylinders comprising at least one cylinder (21 ),

- a low-pressure turbocharger (2) comprising a low-pressure turbine (5) and a low-pressure compressor (6) mechanically connected to the low-pressure turbine (5) to be driven by said low-pressure turbine (5),

- an air bypass duct (7) having a first end connected to the downstream side of the high-pressure compressor (4) and a second end connected between the high-pressure turbine (3) and the low-pressure turbine (5), and

- a controllable air bypass valve (8) for controlling flow in the air bypass duct (7).

2. A turbocharging system according to claim 1 , wherein the turbocharging system comprises an exhaust waste gate duct (9) having a first end connected between the high-pressure turbine (3) and the low-pressure turbine (5) and a second end connected to the downstream side of the low- pressure turbine (5), and a controllable waste gate valve (10) for controlling flow in the exhaust waste gate duct (9).

3. A turbocharging system according to claim 1 or 2, wherein the turbocharging system comprises a pulse reduction duct (13) connecting the first exhaust receiver (11 ) to the second exhaust receiver (12) and a controllable pulse reduction valve (14) for controlling flow in the pulse reduction duct (13).

4. A turbocharging system according to any of claims 1-3, wherein each of the first exhaust receiver (11 ) and the second exhaust receiver (12) is configured to be connected to at most three cylinders (21 ) of the engine (20).

5. A turbocharging system according to any of the preceding claims, wherein the turbocharging system comprises a third exhaust receiver for connecting a third group of cylinders of the engine (20) to a third inlet of the high-pressure turbine (3), the third group of cylinders comprising at least one cylinder (21 ).

6. A multi-cylinder piston engine (20) comprising a turbocharging system according to any of the preceding claims.

7. A multi-cylinder piston engine (20) according to claim 6, wherein the engine comprises at least four cylinders (21 ).

8. A multi-cylinder piston engine (20) according to claim 6 or 7, wherein the engine (20) is configured to be operable according to lean burn principle.

9. A multi-cylinder piston engine (20) according to any of claims 6-8, wherein the engine (20) is configured to be operable using homogenous charge compression ignition (HCCI) and/or reactivity controlled compression ignition (RCCI).

10. A multi-cylinder piston engine (20) according to any of claims 6-9, wherein the engine (20) is a dual-fuel or multi-fuel engine (20).

11. A multi-cylinder piston engine (20) according to any of claims 6-10, wherein the engine (20) is operable using at least two different combustion processes.

12. A multi-cylinder piston engine (20) according to any of claims 6-11 , wherein the rated speed of the engine (20) is 200-1500 rpm.

13. A multi-cylinder piston engine (20) according to any of claims 6-12, wherein the operation of the air bypass valve (8) is configured to be controlled based on the pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load.

14. A method of operating a multi-cylinder piston engine (20) according to any of claims 6-13, the method comprising the steps of monitoring the pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load and controlling the operation of the air bypass valve (8) based on the monitored pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load.

15. A method according to claim 14, wherein the engine (20) comprises a turbocharging system according to claim 2, and the operation of the waste gate valve (10) is controlled based on the monitored pressure of the intake air, temperature of the exhaust gas, air-fuel equivalence ratio and/or engine load.