JP2008111384A - Surplus exhaust energy recovery system for marine engines - Google Patents

Abstract

PROBLEM TO BE SOLVED: To recover surplus exhaust energy of a propulsion main engine, to remarkably improve the energy efficiency of a power generation auxiliary engine, and to dramatically improve the energy efficiency of the entire ship.
A propulsion main engine (1) having a supercharger (5), an auxiliary engine (10, 18) having a generator (12, 19) and capable of generating electric power, and a main engine The first hydraulic pump (21) connected to the supercharger, the second hydraulic pump (22) connected to the generator of the auxiliary engine, and the first hydraulic pump and the second hydraulic pump are interposed. And a hydraulic mechanism (20) for interlocking the first hydraulic pump and the second hydraulic pump by hydraulic pressure. The hydraulic pressure generated by the first hydraulic pump assists the rotational force of the generator by rotationally driving the second hydraulic pump when the propulsion main engine is heavily loaded. The hydraulic pressure generated by the second hydraulic pump assists the rotational force of the supercharger by rotating the first hydraulic pump when the main engine is started and / or when the load is low.
[Selection] Figure 1

Description

  The present invention relates to a surplus exhaust energy recovery system for a marine engine equipped with a supercharger.

  2. Description of the Related Art Conventionally, for example, a turbocharger equipped in a marine diesel engine has its turbine rotated by the exhaust energy of the exhaust gas of the engine, and the air supply density of the engine is increased by a compressor rotated by the turbine. Thus, the engine output is improved. However, even if the turbocharger is attached in this way to effectively use the engine exhaust energy, there is sufficient exhaust energy when the engine is under a high load, and it is necessary to use this excess exhaust energy without waste. There is a strong demand not only for improvement but also for environmental protection.

  In order to effectively use the surplus exhaust energy of this marine engine, there is one in which a generator is connected to a supercharger or an exhaust turbine, and this generator is rotated by the supercharger or the exhaust turbine to generate power ( For example, see Patent Documents 1 and 2). In this case, surplus exhaust energy of the engine is converted into electric energy and used for inboard equipment or the like.

  However, simply connecting the generator to the turbocharger or the exhaust turbine and using it as electric energy does not mean that the exhaust energy of the engine is sufficiently utilized, and the surplus exhaust energy of the engine is further increased. Effective use at the level is an urgent need not only for improving fuel efficiency but also for environmental protection.

For this reason, the applicant of the present application connects a hydraulic pump to the turbocharger of the engine, generates hydraulic pressure by the hydraulic pump to convert surplus exhaust energy of the engine into hydraulic pressure, and uses this hydraulic pressure as an energy source for various devices. Proposed to use. As one of the utilization devices, there is a device that causes a generator to be rotationally driven by the generated hydraulic pressure, thereby generating power (see Patent Document 3).
Japanese Utility Model Publication No. 61-200423 (Fig. 1-2) JP 2004-346803 A (FIG. 1) Japanese Patent Laying-Open No. 2006-242051 (FIG. 3)

  On the other hand, ships generally have an auxiliary engine for power generation separately from the main engine for propulsion so that sufficient in-board power can be secured regardless of the operating state of the main engine for propulsion. The power generation auxiliary engine is also equipped with a generator.

  Therefore, when a generator that is rotationally driven by the hydraulic pump described above is newly provided separately from the power generation auxiliary engine, a control device for instantaneously performing both the rotational speed control and the power generation load control only for the generator. Etc. are required and the cost is high. In addition, there is a problem that synchronization with the inboard power system becomes necessary, and the entire inboard power system becomes complicated.

  Further, when the propulsion main engine is started or when the output is low, the exhaust gas energy of the propulsion main engine is not sufficient, and the supercharging pressure of the supercharger is insufficient. For this reason, in addition to the supercharger that is rotationally driven by the exhaust gas of the main engine for propulsion, measures such as disposing an auxiliary blower to eliminate the insufficient supercharging pressure are taken.

  In response to such a shortage of the supercharging pressure at the start of the propulsion main engine or at a low output, in a conventional engine surplus exhaust energy recovery system that uses hydraulic pressure, a generator is used at the start of the engine or at a low output. It is disclosed that the turbocharger is driven to rotate by the hydraulic pressure generated thereby to compensate for the lack of the supercharging pressure (for details, see Patent Document 3). However, this surplus exhaust energy recovery system for an engine does not disclose any power source for rotationally driving the generator.

  Furthermore, the above-described power generation auxiliary engine is required to be operated so that sufficient power generation is always performed according to the power status in the ship regardless of the load state of the propulsion main engine. For this reason, a constant operating state may be maintained even when the propulsion main engine is heavily loaded. Therefore, in order to improve the energy efficiency of the entire ship, it is extremely important to improve the energy efficiency of the auxiliary engine for power generation as well as recover the surplus exhaust energy of the main engine for propulsion described above. .

  The present invention has been made to solve such a problem, and it is possible to recover the surplus exhaust energy of the main engine for propulsion without increasing the cost and complication of the inboard power system, and for the auxiliary engine for power generation. It is an object of the present invention to provide a surplus exhaust energy recovery system for a marine engine that can remarkably improve the energy efficiency and can drastically improve the energy efficiency of the entire marine vessel.

  In order to solve the above-mentioned problem, the means adopted by the surplus exhaust energy recovery system for marine engines of the present invention includes a propulsion main engine having a supercharger that is rotationally driven by exhaust gas, and a generator. An auxiliary engine capable of generating electric power, a first hydraulic pump connected to the supercharger of the main engine for propulsion, a second hydraulic pump connected to the generator of the auxiliary engine, a first hydraulic pump, And a hydraulic mechanism that intervenes between the first and second hydraulic pumps by being hydraulically interposed between the two hydraulic pumps.

  In this way, the operation of the main engine for propulsion and the auxiliary engine for the generator is integrated into one system by two hydraulic pumps and a hydraulic mechanism that interlocks them by hydraulic pressure, so that surplus exhaust gas energy of the main engine for propulsion can be obtained. Can be used for power generation of the auxiliary engine for power generation via hydraulic pressure, and the fuel efficiency of the auxiliary engine for generator can be remarkably improved.

  Furthermore, the integration of the generator system eliminates the need for a control device for instantaneously performing both the rotation speed control and the power generation load control, thereby preventing an increase in cost and simplifying the entire power system in the ship. be able to. At the same time, the rotational force of the generator auxiliary engine can be used as an assist for the rotational force of the supercharger via hydraulic pressure. As a result, the energy efficiency of the entire ship can be dramatically improved.

  It is desirable that the hydraulic pressure generated by the first hydraulic pump assists the rotational force of the generator by rotationally driving the second hydraulic pump as a hydraulic motor when the propulsion main engine is heavily loaded. By doing so, surplus exhaust energy generated at the time of high load of the propulsion main engine can be recovered as the rotational driving force of the generator, improving the energy efficiency of the generator auxiliary engine, Energy efficiency can be further improved.

  It is desirable that the hydraulic pressure generated by the second hydraulic pump assists the rotational force of the supercharger by driving the first hydraulic pump as a hydraulic motor when the propulsion main engine is started or when the load is low. By doing so, it is possible to compensate for the supercharging pressure shortage of the turbocharger due to the exhaust energy shortage at the start of the propulsion main engine or at a low load, and it is possible to smoothly operate the propulsion main engine. . For this reason, the number and capacity of conventional auxiliary blowers can be reduced, and the apparatus can be further simplified.

  The hydraulic mechanism includes an engine load detecting means for detecting an engine load of the propulsion main engine, a supercharger rotation speed detecting means for detecting a rotation speed of the supercharger, and a scavenging gas for detecting a scavenging pressure of the propulsion main engine. A pressure detection means and a controller for controlling the operation of the first hydraulic pump, the controller including an engine load detected by the engine load detection means and / or a supercharger rotation speed detected by the supercharger rotation speed detection means; Alternatively, it is desirable to control the discharge amount of the first hydraulic pump based on any one or more of the scavenging pressure detected by the scavenging pressure detection means.

  By doing so, the discharge amount of the first hydraulic pump, and consequently the operation of the entire surplus exhaust energy recovery system of the marine engine, can be determined by the engine load of the main engine for propulsion, the rotational speed of the supercharger, the main engine for propulsion. Depending on the scavenging pressure, optimum automatic control can be performed.

  The auxiliary engine has a governing mechanism for controlling the auxiliary engine speed, and the second hydraulic pump is preferably rotated at a substantially constant speed by the operation of the governing mechanism. Thus, when the second hydraulic pump is rotated at a substantially constant speed by the governing mechanism of the auxiliary engine, for example, even when the generator is auxiliary driven by the hydraulic pressure generated by the first hydraulic pump, the electric power having a constant frequency is used. Can be generated. For this reason, problems such as synchronization with the inboard power system do not occur.

  The first hydraulic pump is preferably a variable displacement pump and connected to the rotating shaft of the supercharger via a transmission while maintaining a constant speed ratio. In this way, the discharge amount of the first hydraulic pump is controlled not to provide a speed adjustment mechanism between the pump and the supercharger but to control the pump as a variable displacement type, thereby reducing the discharge amount of the first hydraulic pump. Control can be performed with a simple mechanism.

  The second hydraulic pump is a variable displacement pump and is connected to the rotating shaft of the generator or to the crankshaft of the auxiliary engine to which the generator is connected while maintaining a constant speed ratio. It is desirable that In this way, the discharge amount of the second hydraulic pump is controlled as a variable displacement type pump instead of providing a speed adjustment mechanism between the pump and the generator or between the pump and the auxiliary engine to which the generator is connected. Thus, the discharge amount of the second hydraulic pump can be controlled with a simple mechanism.

  The surplus exhaust energy recovery system for an engine according to the present invention includes a propulsion main engine having a supercharger that is rotationally driven by exhaust gas, an auxiliary engine that can generate electric power by having a generator, and the propulsion main engine A first hydraulic pump connected to the turbocharger, a second hydraulic pump connected to the generator of the auxiliary engine, and a first hydraulic pressure interposed between the first hydraulic pump and the second hydraulic pump. A hydraulic mechanism that interlocks the pump and the second hydraulic pump is provided, so that the surplus exhaust energy of the main engine for propulsion can be recovered and the auxiliary engine for power generation can be recovered without increasing the cost and complication of the inboard power system. The energy efficiency can be remarkably improved, and an excellent effect that the energy efficiency of the entire ship can be dramatically improved is achieved.

  A best mode for carrying out a surplus exhaust energy recovery system for a marine engine according to the present invention will be described in detail with reference to FIGS.

  Reference numeral 1 in FIG. 1 shows a propulsion main diesel engine mounted on a ship as an example, and the propulsion main engine 1 is rotationally driven by exhaust gas to supercharge the supply air to the engine 1. The supercharger 5 is provided. The number of stages of the supercharger 5 is not necessarily limited to a single stage, and the number of turbochargers 5 is not limited to one, and a plurality of stages may be provided.

  On the other hand, an auxiliary engine 10 for power generation is provided separately from the main engine 1 for propulsion so that sufficient power can be secured in the ship regardless of the operating state of the main engine 1 for propulsion. A power generator 12 is connected to the power generation auxiliary engine 10, and the power generation auxiliary engine 10 rotates the power generator 12 to generate power for the onboard power system.

  This power generation auxiliary engine 10 has a governing mechanism 11, and the number of revolutions thereof is controlled to be substantially constant by the governing mechanism 11. Therefore, the generator 12 can perform power generation at a constant frequency synchronized with the inboard power system.

  The supercharger 5 includes a compressor 6 and a turbine 7, and the compressor 6 and the turbine 7 are connected by a rotating shaft 8. The compressor 6 side of the supercharger 5 is connected to the scavenging pipe 2 of the engine 1 via the intercooler 9. Further, the turbine 7 side of the supercharger 5 is connected to the exhaust receiver 3 of the engine 1. For this reason, the turbine 7 is rotationally driven by the exhaust gas of the engine 1, and the compressor 6 is rotated by the turbine 7. Thereby, the air supply density of the engine is increased and the engine output is improved. The supply air whose temperature has been increased by the compressor 6 is cooled by the intercooler 9, and the supply air density is further increased.

  A variable displacement first hydraulic pump 21 is connected to the rotating shaft 8 of the supercharger 5 via a transmission 16. In addition, a variable displacement second hydraulic pump 22 is directly connected to the generator 12 of the auxiliary engine 10.

  A shaft output meter (engine load detecting means) 41 for detecting the shaft output (engine load) W of the main engine 1 for propulsion, and a speed sensor (supercharger rotational speed detecting means) for detecting the rotational speed S of the supercharger 5 42 and a pressure sensor (scavenging pressure detecting means) 43 for detecting the scavenging pressure P of the main engine 1 for propulsion are provided. Further, either the shaft output W of the propulsion main engine 1 detected by the shaft output meter 41, or the rotational speed S of the supercharger 5 detected by the speed sensor 42, or the scavenging pressure P detected by the pressure sensor 43. Based on one or more, a controller 40 is provided for controlling the discharge amounts of the first and second hydraulic pumps 21 and 22, respectively.

  As shown in FIG. 2, the above-described two hydraulic pumps 21 and 22 are incorporated in the hydraulic mechanism 20. When the engine 1 is under a high load, for example, a load exceeding about 50%, the hydraulic pump 21 is rotationally driven through the transmission 16 by the rotary shaft 8 of the supercharger 5 described above, and is 5 MPa in the direction indicated by the solid line in the figure. A high pressure oil pressure of ˜40 MPa is generated. At this time, the transmission 16 operates as a speed reducer. The hydraulic oil discharged from the first hydraulic pump 21 rotates the second hydraulic pump 22 through the main oil passage 23 and returns to the first hydraulic pump 21 through the main oil passage 23 again. The second hydraulic pump 22 directly drives the generator 12 described above to rotate.

  In this case, the second hydraulic pump 22 operates as a hydraulic motor. As described above, since the auxiliary engine 10 has the governing mechanism 11 for controlling the rotational speed of the auxiliary engine 10, the second hydraulic pump 22 connected to the crankshaft of the auxiliary engine 10 via the generator 12 is The operation of the governing mechanism 11 rotates at a substantially constant speed. Thus, when the propulsion main engine 1 is under a high load, surplus exhaust energy of the engine 1 is used to assist the rotational force of the generator 12.

  As shown in FIG. 2, the second hydraulic pump 22 rotates the hydraulic pump 24 and supplies the hydraulic oil in the tank 25 to the main oil passage 23 via the check valves 26 and 27 as indicated by the broken line in the figure. When the discharge pressure of the first hydraulic pump 21 is too high, the hydraulic oil is released to the inlet side of the first hydraulic pump 21 via the relief valve 28. Part of the hydraulic oil in the main oil passage 23 returns to the tank 25 through the branch passage 32 and the switching valve 33. Two accumulators 34 and 35 smooth the hydraulic pressure of the main oil passage 23.

  On the other hand, when the propulsion main engine 1 is started or when the load is low, for example, when the load is about 50% or less, the auxiliary engine 10 is normally in a normal operation state, and the second hydraulic pump 22 is connected to the auxiliary engine 10. It is rotationally driven via the generator 12 to generate a high pressure hydraulic pressure of 5 MPa to 40 MPa in the direction indicated by the dashed line in the figure. The hydraulic oil discharged from the second hydraulic pump 22 rotates the first hydraulic pump 21 through the main oil passage 23 and returns to the second hydraulic pump 22 through the main oil passage 23 again.

  The first hydraulic pump 21 is increased in speed by the transmission 16 described above, and rotates the rotary shaft 8 of the supercharger 5 at high speed. The first hydraulic pump 21 operates as a hydraulic motor, and the transmission 16 operates as a speed increaser. When the discharge pressure of the second hydraulic pump 22 is too high, the hydraulic oil is released to the inlet side of the second hydraulic pump 22 via the relief valve 29. A part of the hydraulic oil in the main oil passage 23 returns to the tank 25 through the branch passage 31 and the switching valve 33.

  Further, when the propulsion main engine 1 is started or when the load is low, it is generally impossible to obtain sufficient exhaust energy that the supercharger 5 requires for supercharging. However, in this surplus exhaust energy recovery system of the engine, the supercharger 5 can perform air supply pressurization corresponding to the load of the main engine 1, and the propulsion main engine 1 can be operated smoothly.

  In the above-described surplus exhaust energy recovery system for marine engines, the variable displacement type second hydraulic pump 22 is directly connected to the generator 12 of the auxiliary engine 10, but the present invention is not limited to this. As shown in FIG. 3, the variable displacement type second hydraulic pump 22 may be connected to the generator 12 via the transmission 17. Also, as shown in FIG. 4, the variable displacement type second hydraulic pump 22 is connected to the crankshaft of the power generation auxiliary engine 18 to which the generator 19 is connected directly or via a transmission (not shown). May be connected.

  As described above, according to the surplus exhaust energy recovery system for marine engines of the present invention, the operation of the propulsion main engine 1 and the auxiliary generator engines 10 and 18 are performed by the two hydraulic pumps 21 and 22 and the hydraulic mechanism 20. The surplus exhaust gas energy of the propulsion main engine 1 can be used for power generation of the power generation auxiliary engines 10 and 18 via hydraulic pressure, and the generator auxiliary engine 10, The load of 18 can be reduced and the fuel consumption can be remarkably improved.

  In addition, the integration of such generators eliminates the need for devices for instantaneously performing the rotational speed control and power generation load control that are required when power is generated using a plurality of generators, resulting in high costs. Can be prevented, and the entire power system in the ship can be simplified. In addition, the rotational force of the generator auxiliary engines 10 and 18 can be used as an assist for the rotational force of the supercharger 5 via hydraulic pressure. As a result, the energy efficiency of the entire ship can be dramatically improved.

  In particular, the hydraulic pressure generated by the first hydraulic pump 21 rotates the second hydraulic pump 22 as a hydraulic motor to assist the rotational force of the generators 12 and 19 when the propulsion main engine 1 is at a high load. For this reason, surplus exhaust energy at the time of high load of the main engine 1 for propulsion can be collect | recovered as rotational drive force of the generators 12 and 19, and the energy efficiency of the auxiliary engines 10 and 18 for generators is improved.

  Further, the hydraulic pressure generated by the second hydraulic pump 22 assists the rotational force of the supercharger 5 by rotating the first hydraulic pump 21 as a hydraulic motor when the propulsion main engine 1 is started or when the load is low. . Therefore, when the propulsion main engine 1 is started or when the load is low, insufficient supercharging pressure of the supercharger 5 can be compensated, and the main engine 1 can be operated smoothly.

  At the same time, the discharge amount of the first hydraulic pump 21 is detected by the shaft output W of the propulsion main engine 1 detected by the shaft output meter 41, the rotational speed S of the supercharger 5 detected by the speed sensor 42, and the pressure sensor 43. The controller 40 controls based on the scavenging pressure P and the like. For this reason, the discharge amount of the first hydraulic pump 21 and the operation of the entire surplus exhaust energy recovery system of the marine engine can be automatically and optimally controlled according to the engine load W of the propulsion main engine 1 and the like.

  Further, the power generation auxiliary engines 10 and 18 have a governing mechanism 11 for controlling the rotational speed of the auxiliary engines 10 and 18, and the second hydraulic pump 22 is operated via the generator 12 by the operation of the governing mechanism 11. (See FIG. 1 and FIG. 3) or by the crankshaft of the auxiliary engine 18 (see FIG. 4). Therefore, when the generators 12 and 19 are auxiliary driven by the hydraulic pressure generated by the first hydraulic pump 21, electric power having a constant frequency can be generated. As a result, problems such as synchronization with the inboard power system do not occur.

  The power generation auxiliary engines 10 and 18 are generally equipped with the governing mechanism 11 conventionally. According to the surplus exhaust energy recovery system for the marine engine, the rotational speed control of the second hydraulic pump 22 is controlled. No new device is required. However, in order to prevent the generators 12 and 19 from over-rotating, it is necessary to constantly monitor the outputs of the auxiliary engines 10 and 18 so as not to be less than zero.

  Further, the first hydraulic pump 21 is a variable displacement pump, and is connected to the rotating shaft 8 of the supercharger 5 via the transmission 16 while maintaining a constant speed ratio. Therefore, the discharge amount of the first hydraulic pump 21 can be controlled with a simple mechanism. Similarly, the second hydraulic pump 22 is also a variable displacement pump, and directly or via the transmission 17 to the rotating shaft of the generator 12 or to the crankshaft of the auxiliary engine 18 to which the generator 19 is connected. They are connected while maintaining a constant speed ratio. Therefore, the discharge amount of the second hydraulic pump 22 can be controlled with a simple mechanism.

  The above-described surplus exhaust energy recovery system for marine engines is merely an example, and various modifications can be made based on the spirit of the present invention, and these are not excluded from the scope of the present invention. For example, the connection between the first pump and the supercharger and the second pump and the generator is not limited to those shown in FIGS. 1, 3, and 4, and can be directly driven in some manner. It is sufficient that they are connected to each other indirectly or indirectly. Further, the hydraulic mechanism shown in FIG. 2 is merely an example.

It is a block diagram which shows the surplus exhaust energy recovery system of the marine engine of this invention. It is a circuit diagram which shows an example of the hydraulic mechanism of FIG. It is a block diagram which shows the surplus exhaust energy recovery system of the marine engine different from FIG. FIG. 4 is a block diagram showing a surplus exhaust energy recovery system for a marine engine different from those shown in FIGS. 1 and 3.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Propulsion main engine 2 Scavenging pipe 3 Exhaust receiver 5 Supercharger 6 Compressor 7 Turbine 8 Rotating shaft 9 Intercooler 10 Power generation auxiliary engine 11 Governing mechanism 12 Power generator 16 Transmission 17 Transmission 18 Power generation auxiliary engine 19 Power generation Machine 20 Hydraulic mechanism 21 First hydraulic pump 22 Second hydraulic pump 23 Main oil path 24 Hydraulic pump 25 Tank 26, 27 Check valve 28 Relief valve 29 Relief valve 31 Branch path 32 Branch path 33 Switching valve 34, 35 Accumulator 40 Controller 41 Shaft output meter (engine load detection means)
42 Speed sensor (supercharger rotation speed detection means)
43 Pressure sensor (scavenging pressure detection means)

Claims (7)

  A propulsion main engine (1) having a supercharger (5) driven to rotate by exhaust gas, an auxiliary engine (10, 18) having a generator (12, 19) and capable of generating electricity; A first hydraulic pump (21) connected to the supercharger of the main engine for propulsion, a second hydraulic pump (22) connected to the generator of the auxiliary engine, the first hydraulic pump, and the A surplus exhaust energy recovery for a marine engine, comprising a hydraulic mechanism (20) interposed between the second hydraulic pump and interlocking the first hydraulic pump and the second hydraulic pump by hydraulic pressure. system.   The hydraulic pressure generated by the first hydraulic pump (21) is generated by rotating the second hydraulic pump (22) as a hydraulic motor when the propulsion main engine (1) is heavily loaded. The surplus exhaust energy recovery system for a marine engine according to claim 1, wherein:   The hydraulic pressure generated by the second hydraulic pump (22) is such that the supercharger is driven by rotating the first hydraulic pump (21) as a hydraulic motor when the propulsion main engine (1) is started or when the load is low. The surplus exhaust energy recovery system for a marine engine according to claim 1 or 2, wherein the rotational force of (5) is assisted.   The hydraulic mechanism (20) detects an engine load detecting means (41) for detecting an engine load (W) of the propulsion main engine (1) and a rotational speed (S) of the supercharger (5). A supercharger rotation speed detecting means (42), a scavenging pressure detecting means (43) for detecting the scavenging pressure (P) of the propulsion main engine, and a controller for controlling the operation of the first hydraulic pump (21) ( 40), and the controller detects the engine load detected by the engine load detection means and / or the supercharger rotation speed detected by the supercharger rotation speed detection means and / or the scavenging pressure detection means The surplus exhaust energy of the marine engine according to any one of claims 1 to 3, wherein a discharge amount of the first hydraulic pump (21) is controlled based on any one or more of the scavenging pressures. Formic recovery system.   The auxiliary engine (10, 18) has a governing mechanism (11) for controlling the auxiliary engine speed, and the second hydraulic pump (22) rotates at a substantially constant speed by the operation of the governing mechanism. The surplus exhaust energy recovery system for a marine engine according to any one of claims 1 to 4.   The first hydraulic pump (21) is a variable displacement pump and is connected to the rotating shaft (8) of the supercharger (5) via a transmission (16) while maintaining a constant speed ratio. The surplus exhaust energy recovery system for a marine engine according to any one of claims 1 to 5.   The second hydraulic pump (22) is a variable displacement pump and directly to the rotating shaft of the generator (12) or to the crankshaft of the auxiliary engine (10) to which the generator (19) is connected. The surplus exhaust energy recovery system for a marine engine according to any one of claims 1 to 6, wherein the system is connected via a transmission (17) while maintaining a constant speed ratio.