The present invention relates to a method to operate a marine propulsion system with a forward clutch and a reverse clutch in a trolling mode. The invention further com- prehends a control unit with means to operate the marine propulsion system accord- ing to said method and a marine propulsion system with such a control unit.

A trolling mode can be used for low speed maneuvers of a ship. Trolling is used for instance for fishing or for docking maneuvers in a harbor. In the trolling mode a fric- tion clutch of the propulsion system is operated in a slip mode, in order to achieve very low rotation speed at a propeller. Usually internal combustion engine-driven ma- rine propulsion systems are used with preferably one or two propellers. A corre- sponding propeller shaft is caused to rotate via a transmission downstream of the engine. While the engine speed is usually controllable to determine the cruising speed of the ship, the transmission is usually switchable with respect to the sense of rotation to select between forward and reverse. For this, such a transmission often comprises two clutches, one for forward and one for reverse direction. Each of the clutches is a friction clutch and adjustable with respect to its slip. A trolling speed is controllable by varying the clutch's slip. In the trolling mode, the engine speed usually remains constant in the range of the idling speed, while the driving speed of the ship is determined by varying the slip of the corresponding clutch. By this, it is possible to adjust the speed of the propeller shaft below the corresponding engine speed. The use of the trolling mode can be limited to a certain engine speed range. A limitation of the engine speed to a range suitable for trolling operation can be carried out by a speed limiting unit.

In DE 43 37 401 C1 a marine propulsion system with a pressure operated forward clutch and a pressure operated reverse clutch is described. This document further describes how the marine propulsion system can be controlled in a trolling mode, while a slippage of the corresponding clutch is controlled by means of hydraulic and electronic components. However, precise control of the trolling speed of the propeller shaft is difficult, especially in situations when the percentage of slippage of the corre- sponding clutch is very low or very high. The precision and the possibilities of speed control with such a marine propulsion system depend on the features and tempera- ture of the pressure medium and on the friction material used on the clutches. Due to physical phenomenon like the stick-slip phenomenon it is not possible to control the propeller speed in a range from 0 - 100 % of slippage of the corresponding clutch.

The purpose of the present invention is to avoid the disadvantages of prior art and to provide an improved method to control a marine propulsion system with a forward clutch and a reverse clutch in a trolling mode. In particular, the invention shall enable a precise control of the clutch slippage in a wide range, especially in the ranges of low and high slippage. Moreover a control unit shall be provided which is able to con- trol the operation of a marine propulsion system according to said method. It is a fu- ther purpose of the invention to provide a corresponding marine propulsion system.

These purposes are achieved by a method according to claim 1 , a control unit ac- cording to claim 7 and by a marine propulsion system according to claim 8. Preferred embodiments are claimed in the dependent claims.

The invention provides a method to operate a marine propulsion system in a trolling mode. The corresponding marine propulsion system comprises at least one propeller shaft which can be driven by an engine via a pressure operated forward clutch and a pressure operated reverse clutch. Preferably the two pressure operated clutches are part of a transmission of the marine propulsion system.

A forward pressure is applied to engage the forward clutch to a certain extent and at the same time a reverse pressure is applied to engage the reverse clutch to a certain extent. The term“at the same time” does not mean that both pressures are applied for the exact same duration. At the same time means that there is at least one period when the forward and the reverse pressures are applied together. It does not matter, if the application of the forward and the reverse pressures does not start or end at the very same instant. The forward and the reverse pressures can be high enough to engage the forward clutch and the reverse clutch at least to such an extent as to transfer a torque on both clutches. At least the one clutch, which causes the propeller shaft to rotate, is operated in a slipping state during this time. The value of the for- ward pressure is different from the value of the reverse pressure, so that the propeller shaft is caused to rotate by the pressure difference. This means, that the pressure difference between the forward pressure and the reverse pressure must be high enough to cause at least a slow rotation of the propeller shaft in one direction. In the trolling mode the drive torque is transferred via a slipping clutch to the propeller shaft.

In one case a forward pressure is applied to the forward clutch which causes the pro- peller shaft to rotate in forward direction and a reverse pressure, which is lower than the forward pressure, is applied to the reverse clutch. In this case the reverse clutch acts like a brake to modulate the slippage of the forward clutch. By doing so, the per- centage of slippage can be precisely controlled over the complete range of slippage at the forward clutch. This means, that the rotation speed of the propeller shaft and hence the speed of the ship can be precisely controlled.

The forward pressure is adjusted higher than the reverse pressure, in order to affect forward rotation of the propeller shaft and a forward movement of the ship. Preferably the method can be applied in a low slippage range in order to avoid the negative stick-slip phenomenon. Therefore the forward pressure and the reverse pressure are determined to cause a slippage in the range of 0% up to 30% at the forward clutch. For this a pressure can be applied to the forward clutch, which would cause a slip- page of 35-45% without a simultaneous application of the reverse clutch. However, at the same time a pressure is applied to the reverse clutch to keep the slippage at the forward clutch at one specific required percentage below 30%, for example at 10%. Both clutches can be operated with these pressures for a certain time, for example as long as the ship is required to move with low speed in a docking maneuver.

The actual slippage at the forward and at the reverse clutch can be detected and de- termined for example by applying rotation speed sensors on the input side and on the output side of both clutches and by calculating the difference of the corresponding signals at each clutch in an electronic control unit.

Another specific advantage of the present invention can be achieved, if the forward clutch is operated in a high slippage range. For this, the forward pressure and the reverse pressure are determined to cause a slippage in the range of 70% up to 100% at the forward clutch. With conventional marine propulsion systems it is not possible to precisely control the output speed at the forward clutch in this slippage range. The reason for this is that it is necessary to overcome the inertia of the system and the viscous resistance of the water to start rotating the propeller. With conventional ma- rine propulsion systems there is a certain minimum pressure which corresponds to a minimum rotation speed of the propeller shaft. Under that minimum rotation speed it is not possible to control the rotation speed of the propeller shaft constantly and smoothly. With the method as proposed by the present invention it is possible to start the rotation of the propeller shaft and then it is possible to brake the output going un- der the minimum rotation speed which is obtainable with conventional systems.

The present invention enabled a precise control in a slippage range between 70% and 100% at the forward clutch. This means that the input rotation speed at the for- ward clutch is significantly higher than the output rotation speed. Hence, the propeller shaft of the marine propulsion system rotates with a significantly lower speed than the engine shaft, even under consideration of an additional transmission ratio in the drive train. The simultaneous application of pressure to the forward clutch and to the reverse clutch effects a significantly better and more precise control of the slippage at the forward clutch at slippage rates between 70% and 100%.

As an example the forward clutch can be engaged at 95% of slippage and then the pressure on the reverse clutch can be slightly increased. The reverse clutch now acts like a brake and affects a kind of a filter on the output rotation speed of the forward clutch, so that inconstancies and unsteadiness in the output rotation speed can be decreased or completely avoided.

All benefits and effects of the invention which have been described above for forward movement of a ship in a trolling mode can be used vice versa for reverse movement. Accordingly, for reverse movement of the ship the reverse pressure is higher than the forward pressure in order to effect a reverse movement of the propeller shaft. Preferably the inventive method is used for reverse movement in a low slippage range, wherein the forward pressure and the reverse pressure are determined to cause a slippage in the range of 0% up to 30% at the reverse clutch.

Another preferred application of the present invention is for reverse movement in a high slippage range, wherein the forward pressure and the reverse pressure are de- termined to cause a slippage in the range of 70% up to 100% at the reverse clutch which means a significantly higher input rotation speed at the reverse clutch corn- pared to the output rotation speed at the reverse clutch.

For both directions of ship movement, forward and reverse, a prefill pressure can be applied to the forward clutch and to the reverse clutch before applying said forward and reverse pressure. The prefill pressure can be as high as to cause a small torque transmission on both clutches. However, the prefill pressure shall be smaller corn- pared to the forward pressure which is applied to the forward clutch in order to rotate the propeller shaft forward in trolling mode. The prefill pressure shall not cause a ro- tation of the propeller shaft. A low torque transmission by said prefill pressure on both clutches can be used to hold the propeller shaft in one position.

A torque transmission via one of the clutches does not necessarily mean a simulta- neous rotation of an input or output element of the corresponding clutch. Instead a torque can be transmitted from the input element to the output element of the clutch without a rotation of one of the elements.

The application of a low prefill pressure to both clutches improves the reactivity of the clutches. The transition from forward to reverse can be done very fast. The two clutches are ready to transmit the higher torque for rotating the propeller shaft very quickly, simply by increasing the pressure on the desired clutch up to said forward pressure. The pressure control can be done by means of a proportional valve, for example. The proposed method requires controlling the pressures applied to the for- ward and to the reverse clutch independently. Therefor a separate proportional valve for the pressure control of each of the two clutches is required. The present invention includes further a control unit for a marine propulsion system. The control unit is assigned and enabled to control the propulsion system according to a method as described above. For that, the control unit comprises means to oper- ate the marine propulsion system according to the method as described above. The means of the control unit may comprise an electronic control unit (ECU) with a pro- cessor, data storage (volatile or non-volatile) and interfaces to be connected to other components of the marine propulsion system or the ship. Said means further corn- prise application software running on the processor and executing instructions in or- der to operate the marine propulsion system according to the proposed method. The application software may be stored in said data storage together or separate to addi- tional data which is necessary to operate the marine propulsion system. The control unit may further comprise hydraulic components like a pump, filter, valves and hy- draulic connecting components. Above mentioned proportional valves may also be part of the control unit.

A corresponding marine propulsion system comprises an engine, a transmission, a propeller shaft which can be driven by the engine via the transmission and a control unit. At least one propeller shaft for each associated propeller is connected to the output side of the transmission. The transmission comprises a pressure operated forward clutch and a pressure operated reverse clutch. The control unit is connected to both of said clutches to control the pressure which is applied to each of the clutch- es. The clutches can be constructed as pressure operated multi-disc clutches. The clutches can be completely open (no torque is transmitted), completely engaged (no slip, input element rotates with same speed than the output element) or partially en- gaged (slipping state). A partial engagement of the clutch causes the clutch to slip and the input rotation speed at the clutch is different to the output rotation speed . The degree of slip can be altered at will, by adjusting the pressure which is applied to each clutch under consideration of the load on the propeller shaft.

The present invention allows stable trolling speed control in a wide range even with simple single acting cylinders. However, double acting or special cylinders can be used as well. For such an embodiment the forward clutch and the reverse clutch may comprise each one double acting pressure cylinder in order to enhance the precise adjustment of the required slippage in the clutches and the required rotation speed of the propeller shaft. Using double acting cylinders it is necessary to calculate the pressure difference between the two pressure chambers of each cylinder and to con- sider the different effective pressure surface on both sides of the corresponding pres- sure piston. The result of such a calculation can be used for the forward and reverse pressure which is applied to the forward clutch respectively to the reverse clutch.

A double acting pressure cylinder allows adjusting the corresponding clutch in any position between fully engaged and fully disengaged. In connection with the above- mentioned electronic control unit, it will be possible to optimize the applications through application software, without a change of any physical parts.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Fig. 1 shows a simplified schematic of a marine propulsion system according to present invention and

Fig. 2 shows graphs of an example of pressure and slip control according to the invention.

The schematic of Fig. 1 shows a marine propulsion system 1 with its essential ele- ments. The marine propulsion system 1 comprises an engine 2, a transmission 8 and a propeller shaft 6, which can be driven by the engine 2 via the transmission 8. A control unit 9 is connected by connections 10 to a pressure operated forward clutch 4 and to a pressure operated reverse clutch 5. Connections 10 symbolize hydraulic lines from corresponding hydraulic valves which are part of the control unit 9 and each one pressure chamber of the two clutches 4 and 5. In other embodiments the hydraulic valves may as well be positioned at or inside the transmission 8 near to the clutches 4, 5. In this case the hydraulic valves would be connected via electric con- nections to the control unit 9. The connection 11 between engine 2 and the control unit 9 is used to monitor and control the engine speed. The connection 11 can be realized as electric cable connection or as a wireless connection. Both clutches 4, 5 are friction clutches, namely pressure operated multi-disc clutches. Each of the clutches 4 and 5 comprises an inner disc carrier 12, 14 and an outer disc carrier 13, 15. The outer disc carriers 13 and 15 are connected by spur gears 16, 17 which are fixed to the outer disc carriers 13, 15 of the two clutches 4 and 5. Propeller 7 is fixed onto propeller shaft 6 which is permanently connected to the output side of the transmission 8. The transmission 8 comprises a housing. The forward clutch 4 and the reverse clutch 5 are arranged inside the housing of the transmission 8.

Each outer disc carrier 13, 15 is the respective input element at both clutches 4 and 5, whereas the inner disc carriers 12, 14 are the corresponding output elements. The outer disc carrier 13 of the forward clutch 4 is permanently connected to a drive shaft 3 of the engine 2. The outer disc carrier 15 of the reverse clutch 5 is driven by the outer disc carrier 13 of the forward clutch 4 by means of spur gears 16 and 17. Each of the inner disc carriers 12, 14 is permanently connected to a corresponding drive gear 18, 19. Both drive gears 18 and 19 are constantly meshing with driven gear 20 which is fixed to propeller shaft 6.

Fig. 2 shows curves of the forward pressure PFWD, reverse pressure PREV, percentage of slip in the forward clutch and the pressure difference delta P between forward and reverse pressure, during the same time period.

The two upper graphs in Fig. 2 show the forward pressure PFWD which is applied to the forward clutch and the reverse pressure PREV which is applied to the reverse clutch over a certain time period from to to tio.

The next graph shows the percentage of slip (SLI PFWD) in the forward clutch in the same time period from points in time to to tio.

The lowest graph shows the pressure difference delta P between the forward pres- sure and the reverse pressure. The curve of the pressure difference delta P is ap- proximately congruent to the curve of to the rotation speed of the propeller shaft, pro- vided that the load on the propeller stays nearly constant during the considered time. At a starting time to there is no forward pressure PFWD applied to the forward clutch and no reverse pressure PREV is applied to the reverse clutch. Hence, both clutches are completely disengaged and no torque is transmitted. The slip in the forward clutch is at 100%.

At the time ti a prefill pressure is applied to the forward clutch and to the reverse clutch. This causes an equal low torque on both clutches and a hold of the propeller shaft. The prefill pressure at both clutches makes them ready to transmit a higher torque very quickly, as soon as the pressure is further increased.

At the time t2 the forward pressure PFWD is further increased, while the reverse pres- sure PREV stays at the low prefill pressure level. This causes the forward clutch to be more engaged and to rotate the propeller shaft to a certain degree in forward direc- tion. Hence, the percentage of slip in the forward clutch changes to about 70%.

These conditions are kept until the time t3.

Between the times t3 and t ₄ the reverse pressure is slightly increased, so that the re- verse clutch is engaged a little bit more and acts like a brake against the driving force of the partially closed forward clutch. As a result the rotation speed of the propeller shaft decreases and the percentage of slip in the forward clutch increases corre- spondingly to about 85%. With such pressure adjustment the propeller shaft could be driven for a longer time constantly at 85% of slippage in the forward clutch. This ena- bles trolling maneuvers at a very low but constant speed, what is not possible with conventional methods of operation of such a propulsion system.

Between the times t ₄ and tsthe system is operated at the conditions before the time t3. At ts the forward pressure is raised significantly, while the reverse pressure stays constant, so that the propeller shaft is further accelerated and the percentage of slip in the forward clutch is lowered to about 15%. However, in this low slippage range a stick-slip effect is caused which can be seen at the zigzag curve of the corresponding curve of SLIPFWD. In order to eliminate the stick-slip phenomenon the forward pres- sure and the reverse pressure are raised slightly after the time tz. Again the reverse clutch is engaged a little bit more and acts like a brake against the driving force of the partially closed forward clutch. This stops the stick -slip phenomenon while the per- centage of slip in the forward clutch stays constant at about 15%.

The forward pressure is lowered to a certain extent after the time te, while the reverse pressure is reduced to zero after the time te. This causes the percentage of slip in the forward clutch to change close to zero. However, in this very low slippage a stick-slip phenomenon occurs again in the forward clutch. The stick-slip phenomenon contin- ues until the time t9, when the forward pressure is raised to a level of 100, which cor- responds to a completely engaged forward clutch. With the forward clutch being completely engaged, the percentage of slip in the forward clutch logically is zero.

In accordance with the proposed new method the described stick -slip phenomenon between the times t6 and 17 and between the times te and t9 can be completely avoid- ed, if the forward pressure PFWD and the reverse pressure PREV are both raised imme- diately at time t6 to the higher level as can be seen in the graphs by dotted lines. So the reverse clutch can act as a counter-brake against the driving torque which is transferred via the forward clutch during this period.

Reference Numeral marine propulsion system

engine

drive shaft

forward clutch

reverse clutch

propeller shaft

propeller

transmission

control unit

connection

connection

inner disc carrier

outer disc carrier

inner disc carrier

outer disc carrier

spur gear

spur gear

drive gear

drive gear

driven gear