Abstract:
A method for controlling the hybrid drive system of a water craft, comprising a combustion engine ( 1 ), an electric engine/generator ( 2 ), a drive train ( 5 ) having a propeller ( 7 ) and a storage battery ( 3 ), a control unit ( 8 ) and a clutch ( 4 ). In order to increase efficiency, during travel in the upper power range the electric engine ( 2 ) is additionally operated as a generator, and during travel in the middle and lower power ranges the electric engine ( 2 ) is operated with current from a storage battery ( 3 ). The drive train is designed so that the characteristic ( 20 ) thereof lies by a defined charging capacity below the point of minimum specific fuel consumption (e.g.  25 ), or is steeper than the characteristic ( 16 ) of a conventional construction.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a process for controlling a hybrid propulsion system for a watercraft, the system comprising at least a combustion engine, at least one electric machine acting as a generator and as a motor, a drive train with a propeller, a battery and a control unit, a clutch being provided between the combustion engine and the electric machine. 
         [0002]    Hybrid propulsion systems are used in small watercraft, Yachts and also in bigger ships. Known systems are units comprising a combustion engine with an electric engine, usually operable as a motor or a generator. There can also be a plurality of electric machines. The electric machine is arranged between the combustion engine and the drive train. The clutch enables the combustion engine to be linked to the electric machine, and therefore propulsion by both machines together. A further clutch can be provided between the electric machine and the drive train. 
         [0003]    In general, the problem with such propulsion systems is to accord the characteristic load/speed curve with the performance graph of the combustion engine. The characteristic load/speed curve is essentially that of the propeller (in the following the propeller curve), which does not fit well with the performance graph of the combustion engine. It passes throughout in regions of the performance graph with higher to high specific fuel consumption. This renders an efficient operation of the propeller difficult or even impossible. Usually, when selecting a combustion engine and a propeller, only the maximum speed of the vessel is considered. 
         [0004]    Such a system is described in WO 2010/115479 A1. The description mentions that travel at low speed is possible either with electric drive only or with drive by the combustion engine. In the latter case, the combustion engine not being sufficiently loaded, the surplus torque can be used for charging the battery via the electric machine operating as a generator. It says further that this is advantageous foremost when travelling at low propulsion load, as this brings the operating point of the combustion engine in its performance graph nearer to the region of low specific fuel consumption. Even then, the operating point is still far away from minimal consumption, the electric machine would have to be heavily oversized. This would not only increase its bulk, the electric machine would also generate a loading current that is too high for the battery and therefore damage the battery. It would also be oversized for electric travel. 
         [0005]    The description further states that, when travelling at medium to high speed of the vessel, the combustion engine operates at a comparatively high load and that the operating point of the combustion engine in the characteristic graph is in or near the region of minimum fuel consumption. As nothing is disclosed about the layout of the propeller, it is most likely to be conventional and the operating point is at best somehow nearer the region of minimal fuel consumption, but not in it. 
         [0006]    With conventional layout of the propeller, its characteristic load curve intersects the load hyperbola in the point of rated power. If the characteristic load curve of the propeller passes lower, the engine would overspeed. If it passes higher, the vessel would not reach the speed corresponding to the power of the engine, because the propeller only reaches a lower number of revolutions at that torque. The conventional layout is therefore considered as optimal and thus generally accepted. 
         [0007]    What is not considered in this divulgation is, that vessels are far from being operated in the operating point of rated power of the combustion engine and that the overall efficiency of the propulsion system for all operating states (which means over a realistic load collective) also depends on the efficiency at which the electric current is generated. 
         [0008]    With the foregoing in mind, it is the object of the invention to minimize fuel consumption and emissions of a specific propulsion system for all states of operation, and possibly also to increase the lifetime of the combustion engine. 
       SUMMARY OF THE INVENTION 
       [0009]    By controlling the generator, the joint load of the propeller and the generator is adjusted so as to bring the operating points of the combustion engine in the characteristic graph to coincide with the very points of minimum specific fuel consumption at the prevailing rotational speed of the engine. In this way, the combustion engine operates at maximum efficiency (thus with minimum specific fuel consumption) in a wide range. Thus, the electric current so generated is “cheap”. 
         [0010]    The cheap electric current so generated is available for electric propulsion in a wide power range, for assisting or replacing the combustion engine, indirectly achieving further fuel economy. This can entail further advantages. In the low power range, the specific fuel consumption of a combustion engine is particularly high. Solely electric propulsion in the low power range is more efficient thanks to the cheap electric current generated at highest efficiency. It is therefore economically advantageous if the electric machine assists the combustion engine in a wide lower range of power. 
         [0011]    This assistance is also helpful in the medium range of power, in two situations: First, if the combustion engine is supercharged for bridging the “turbo-hole” entailed by delayed response of the turbocharger. The characteristic graph of such a combustion engine oftentimes has a depression in the middle range, where the distance to the propeller characteristic curve is small. This means that there is little power or torque, as the case may be, available for acceleration. This low agility can be remedied by the assistance of the electric machine. 
         [0012]    Second, it is helpful with hydroplane boats during transition from displacement travel to planing travel. In the first state, acceleration against the high travelling resistance is enhanced by the assistance of the electric machine. When the boat then has risen into planing travel, resistance and torque demand is much lower. 
         [0013]    The assistance of the electric machine can even be used for increasing the travelling speed of the vessel beyond the speed attainable with the combustion engine alone (“boost”). Thereby the lower top speed due to the layout of the propeller—also according to the invention—with drive by the combustion engine alone, can be compensated. 
         [0014]    In an advantageous embodiment of the invention the characteristic curve of the drive train (essentially the characteristic curve of the propeller, taking account of a reduction gear, if any) and the characteristic graph of the combustion engine (represented in a diagram of torque or power over the number of revolutions, containing also the the so-called shell curves joining the points of constant specific fuel consumption) are stored in the memory of the control unit, and adjustment of the joint load for bringing the operating point to the position of minimum specific fuel consumption. Thus, the operating point of the combustion engine at a given rotational speed is raised until the point of lowest fuel consumption is reached. 
         [0015]    In a further development of the process, the load or torque consumed by the electric machine is limited. This can be achieved in different ways: Either by a limiting value stored in the memory of the control unit, or by suitable layout of the drive train, e.g. the propeller, as disclosed below. 
         [0016]    The proposed process results, with a realistic load collective, in a very considerably reduced fuel consumption (by up to 40%), and accordingly in lower emissions. Minimum specific fuel consumption is entailed by complete combustion, which also produces less polluting substances (CO, unburnt hydrocarbons, soot). This success surprising as is seems absurd at first sight to lower fuel consumption of a combustion engine by increasing the load. 
         [0017]    The invention also relates to a propulsion system of a naval vessel, the layout of which improves, or even perfects, the inventive process. The drive train is laid out in such a way that its characteristic curve is situated under the point of minimum specific fuel consumption by a distance corresponding to a defined generating power at the prevailing number of revolutions. The generating power is given by the electric machine operated as a generator and limited by the battery. The characteristic curve of a drive train of this layout is nearer to the operating points of lowest specific fuel consumption than that of a conventional drive train, thus increases the efficiency. The actual operating point of minimum specific fuel consumption is then reached by the process according to the invention (control of the electric machine working as a generator). 
         [0018]    If the layout of the propeller is according to the state of the art, its characteristic curve is far under the region of lowest specific fuel consumption. In order to reach a point of minimum specific fuel consumption from there, the generator would have to be laid out for a huge torque/power. This would entail big bulk and would produce a charging current far too high, and therefore damaging, for the battery. 
         [0019]    Preferably, the layout of the characteristic curve of the drive train according to the invention is achieved by a propeller characteristic curve which is steeper than the characteristic curve of a propeller according to the state of the art, the latter leading up to the point of rated power. The steeper curve is already nearer the points of lowest specific fuel consumption of the combustion engine. It intersects the full load curve of the combustion engine in the point where the full load curve intersects the load hyperbola, or at lower number of revolutions. If the full load curve meets this intersection point, the load consumption of the drive train equals the rated power (but with a higher torque and at lower rotational speed). Comparison with the state of the art where layout is based on the point of rated power shows that with identical combustion engines with the layout according to the invention only a lower rotational speed can be reached. This seems a drawback at first sight, and it would be if travelling at top speed most of the time. But this is unrealistic. If top speed is required for a short period, the electric machine can assist, according to the inventive process, this is called “boost”, again with the cheap current from the battery. 
         [0020]    The characteristic curve of the drive train is largely determined by layout or shape of the propeller. Its diameter may be greater than that of a conventional propeller for instance, or the pitch of its blades may be increased. An increase of the diameter results in a higher efficiency of the propeller (according to the Cordier-diagram, see Otto Cordier, “Ähnlichkeitsbedingungen für Strömungsmaschinen”, BWK Zeitschrift Nr. 10/1953, pages 337-340) and therewith in a further reduction of fuel consumption. 
         [0021]    In an optimal layout the ratio powers of the electric machine in driving mode and of the combustion engine range between 1:3 and 1:8, preferably between 1:4 and 1:5. The electric machine then is more powerful than customary, but thanks to the invention the points of lowest specific fuel consumption can be reached in a broad range. By the battery being charged also in the higher range of power, it is fully charged most of the time, justifying a larger electric motor or its operation for a longer time. 
         [0022]    In a further advantageous development and layout, the electric machine produces a higher torque at lower revs of the drive train, reaching the torque required by the drive train at a number of revolutions as high as possible. This provides sufficient torque in a wider lower range of power (when travelling without the combustion engine) and in the adjacent range, when the combustion engine needs assistance for acceleration, the adjacent range reaching further up to the middle range. 
         [0023]    If bulk is a matter of concern, or if the range in which the electric machine provides a higher torque is expected to be as wide as possible, a further development consists in achieving the high torque of the electric machine over a wide range by means of a reduction gearbox. This higher torque then is the torque of the drive shaft of the reduction gear. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    In the following, the invention will be described and commented in conjunction with the drawings, in which: 
           [0025]      FIG. 1  is a schematic representation of a propulsion system according to the state of the art and also of the present invention; 
           [0026]      FIG. 2  is a variant of the system of  FIG. 1 ; 
           [0027]      FIG. 3  is a characteristic graph of a propulsion system according to the state of the art; 
           [0028]      FIG. 4  is the characteristic graph of  FIG. 3 , a characteristic curve of a propeller according to the invention being added; 
           [0029]      FIG. 5  is the complete characteristic graph of a propulsion system according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    In  FIG. 1 , a combustion engine has the reference number  1 , an electric machine the reference number  2  and a battery reference number  3 . The electric machine  2  is suitable for being operated as and engine and as a generator, or there are two electric machines, one being an engine, the other being a generator. A first separating clutch  4  is ranged between the combustion engine  1  and the electric machine  2 , and a second separating clutch is ranged between the electric machine  2  and a drive train  5 . The outermost end of the drive train  5  bears a propeller  7 . A control unit  8  manages the combustion engine  1 , the electric machine  2  and charging of the battery  3 . 
         [0031]      FIG. 2  is a variant, distinguished from  FIG. 1  by a high-revving electric machine  9  driving the drive train  5  via a reduction gearbox  10 . The electric machine  9  can be for driving only and the generator disposed in a different manner. The high-revving motor  9  and the reduction gearbox  10  provide a high torque over a wide range of rotational speed, allowing travelling with electric drive only, the combustion engine being stopped and the coupling opened. 
         [0032]      FIG. 3  shows the characteristic graph of a combustion engine, here a diesel engine. The graph of a spark-ignited engine is somewhat similar, the curves differing in detail. On the abscissa, the rotational speed of the combustion engine  1  (which is—see FIG.  1 —identical with that of the electric machine  2 ) is represented, and on the ordinate the torque. The area of the characteristic graph is limited by the full-load curve  12  on top, this curve descending with lower rotational speed ( 13 ) and at high rotational speed continues as the rated power hyperbola  14 , according to the rated power. In the characteristic graph, the curves of constant specific fuel consumption are represented in a way similar to the isohypses in a geographical map. The specific fuel consumption is specified in grams per horsepower and hour. The curves of constant specific fuel consumption are referred to as conchoidal curves. It can be seen that the specific fuel consumption is a minimum in the area within the curve marked as “210”. With other rotational speeds, there is also a relative minimum, corresponding each time to a maximum efficiency. 
         [0033]    Further, a propeller characteristic curve  16  of a propeller according the state of the art is shown as a broken line, being the characteristic curve of the drive train. It ( 16 ) ends in the point of rated power  15  at the intersection with the hyperbola  14  which represents the rated power of the combustion engine  1 . It can be seen that when travelling with the combustion engine only operating (all points of operation being on the characteristic curve of the propeller), the characteristic curve of the propeller is throughout in regions of high specific fuel consumption (low efficiency). Slow travel, a state of operation prevailing during most of the lifetime, or idling is particularly inefficient. It is therefore desirable, to be able to use electric drive in a wide operating range. 
         [0034]    In  FIG. 4 , the full load curve of an electric motor/generator is shown as a semi-dotted line  17 . Thanks to the invention, the electric motor/generator improves the efficiency of the entire system over the entire operating range. The electric motor can in cooperation with the combustion engine power the drive train (this is called “parallel hybrid”), see  FIG. 1  and  FIG. 2 , as the case may be. It is important that the electric motor provides a high torque in the lower range and keeps the power constant over the rotational speed above its maximum power. This increases the range of operation with electric drive only, and the combustion does not operate in the range of high specific fuel consumption. This already is a measure to reduce fuel consumption. Further measures are to follow. 
         [0035]    An important measure according to the invention consists in lowering the operating rotational speed of the combustion engine, in other words to deliver a demanded torque at a lower rotational speed. By this, charging the battery with maximum efficiency of the combustion engine is achieved in a wide operating range. Lowering the rotational speed of the combustion engine is achieved by using an accordingly designed propeller  7  (e.g. a larger diameter), according to an aspect of the present invention. This lowers the operating point of maximum torque  21  in the characteristic graph towards a lower rotational speed (from 2600 to 2300 rpm). The accordingly designed propeller  7  can be designed with a larger diameter (thus also lowering losses according to the Cordier-diagram) or with blades of higher pitch, or other design measures. 
         [0036]      FIG. 4  shows the characteristic curve  20  of the drive train (essentially the propeller characteristic curve), modified according to the invention; it intersects the full load curve in point  21 , at a lower rotational speed than the characteristic curve  16  of a propeller according to the state of the art, see point  15 . By this, the operating points are nearer the respective points of minimum specific fuel consumption. It is important for the generation of energy to be described below that the characteristic curve of the propeller is slightly below or in the region of highest efficiency. 
         [0037]    Lowering the rated power rotational speed according to the characteristic curve  20  of the propeller brings this characteristic curve closer to the descending branch  13  of the full load curve. This entails a lower zorque (agility) available for acceleration. With combustion engines with a significant “turbo-hole” this may lead to the vessel being unable to reach the rated power rotational speed, it would be unable to accelerate the vessel in the “turbo-hole”. The additional torque provided by the electric machine helps the combustion engine and thus the original agility with the characteristic curve  16  of the propeller is achieved again. The full load curve  22  with joint drive by the electric motor and the combustion engine is shown in  FIG. 4 , only a fraction of the torque of the electric motor being added. 
         [0038]    The efficiency of the generation of the electric current is essential for the economy of electric propulsion. According to the invention, it is generated in points of operation with minimum fuel consumption. In this way, the efficiency when travelling at idling speed (with propulsion by the so generated electric current) is improved by up to 40 percent, losses in the generator, inverter and the battery charging system being taken into account. The efficiency would be particularly low, if the battery were charged by the combustion engine operating at low load. 
         [0039]      FIG. 5  shows how the battery is charged with highest efficiency, resulting in “cheap” electric current. By the lowered points of operation the combustion engine already operates in regions of lower specific fuel consumption, see curve  20 . But it is still at a distance from the curve linking points of minimum specific fuel consumption at various rotational speeds  24 . The operating point of the combustion engine is brought into this curve  24  by driving the generator and in this way generating cheap electricity. 
         [0040]    As an example in the characteristic graph: At 1800 revolutions (rpm) the torque consumed by the drive train  5  in the point of operation  26  on the characteristic curve of the drive train 275 Nm (Newtonmeter). The point of operation of minimum specific fuel consumption  25  is situated at 350 Nm, thus by 75 Nm higher. In order to reach that point, the electric machine operating as a generator is controlled so as to use these 75 Nm for the generation of electric current. This corresponds to a charging current which, due to electric properties of the machine and the battery, has limits and an optimum. This maximum and optimum is considered in the layout of the drive train. 
         [0041]    Due to the propeller characteristic curve  20  being closer to the curve  24  than the characteristic curve  16  of the propeller according to the state of the art, the torque to be consumed by the generator is limited. It is much lower than it would be with the characteristic curve  16 . Reaching the points of minimum consumption with a propeller according to the state of the art would require an enormous electric motor/generator. 
         [0042]    In the characteristic graph in  FIG. 5 , the region in which electric current is generated in addition to propelling is hatched and circumscribed by the characteristic curve  20  of the drive train, by the curve linking points of minimum specific fuel consumption at various rotational speeds  24 , by a small part  12 ′ of the full load curve  12  and by a line  27  steeply descending to the characteristic curve  20 . The line  27  meets the characteristic curve  20  somewhat higher than point  28  at the intersection of the full load curve  17  of the electric engine and the characteristic curve  20  of the drive train. Up to this point, propulsion with only electric drive is possible. 
         [0043]    This additional charging power (higher torque at the same rotational speed) entails an increase of the efficiency of the combustion engine which improves the overall efficiency of propulsion directly and indirectly. Directly, because the combustion engine operates in its point of operation of maximum efficiency and indirectly, because the efficiency of the generation of electric current—and therewith of the combustion engine—also influences the efficiency of the electric machine operating as a motor. This, because the electricity is generated in the point of maximum efficiency of the combustion engine. With the so generated cheap electricity it is also economically sensible to operate the electric machine as a motor doubling the combustion engine also in other regions of operation. 
         [0044]    A windfall advantage of charging the battery also with high load is an increased freedom in dimensioning the electric machine and the battery. For instance, the size of the combustion engine can be reduced and that of the electric machine increased, leaving the capacity of the battery unchanged. In this way, the ration of the rated powers of the combustion engine and of the electric machine can be in a region of 1:3 to 1:10, preferably 1:4 to 1:6. 
         [0045]    By this layout, also the emissions can be reduced in manifold ways: The reduced fuel consumption also means a reduced quantity of exhaust gas. As higher efficiency is essentially a consequence of complete combustion, the content in CO, HC and particles of the exhaust gas is dramatically reduced. A further reduction is due to the reduced operating time of the combustion engine. If, for instance, the electric motor is operated in the lower load range during 500 hours, the energy it needs can be generated and stored in a much shorter time. The absolute value of the reduction depends, as previously said, on the load collectif (share of low-load operation), of the characteristic graph of the particular combustion engine and on the charging system of the battery. Further, servicing intervals and lifetime are increased in this way.