A METHOD FOR OPERATION OF A HYDRAULICALLY ACTUATED FUEL PUMP FOR AN INTERNAL COMBUSTION ENGINE, AND A HYDRAULICALLY ACTUATED FUEL PUMP

Abstract: A hydraulically actuated fuel pump (1) for an internal combustion engine has a pump piston (39), which is driven forwards in a delivery stroke by means of an actuator piston (36). Between each injection sequence the pump piston (39) is returned to a fixed starting position by the action of the fuel pressure on the end surface (61, 62) of the pump piston in the pump chamber (52). The delivery volume of the fuel pump is electronically controlled by the supply of hydraulic fluid to the pressure chamber being cut off by the control valve (11) at the moment when the pump piston has delivered the desired volume. The pump cylinder (25) and the actuator cylinder (24) are separate units, which are mutually separated by an intermediate spacing member (26).

Description: A method for operation of a hydraulically actuated fuel pump for an internal combustion engine, and a hydraulically actuated fuel pump The present invention relates to a method for operation of a hydraulically actuated fuel pump for an internal combustion engine, particularly a two-stroke crosshead engine, having a fuel conduit for distribution of fuel to the fuel pumps at primer pump pressure and a high-pressure conduit for distribution to the fuel pumps of pressurized hydraulic fluid, which is different from the fuel, a pump piston in the fuel pump being driven forwards in a delivery stroke by means of an actuator piston in an actuator cylinder with simultaneous pressing out of fuel from a pump chamber and driven backwards in a return stroke with simultaneous filling of the pump chamber with fuel from the fuel conduit, and the actuator piston being controlled by an electronically actuated control valve, by which a pressure chamber in the actuator cylinder can be provided with hydraulic fluid from the high-pressure conduit or be connected to a drain. Such a method is known from the Applicant's Danish patent No. 151145, according to which the delivery volume of the fuel pump is controlled by rotation of the pump piston and the pump piston is connected with the actuator piston. At the end of the delivery stroke, the pump piston can be pulled back by the actuator piston with simultaneous refilling of the pump chamber by means of a compression spring that forces the actuator piston backwards. The pump has a complex design with many separate parts, many of which must be machined with great precision for the pump piston and the actuator piston to function correctly. From old times, a hydraulically actuated fuel pump is known (the Applicant's Danish patent No. 41046 from 1928) that uses fuel as hydraulic fluid and where the delivery volume of the pump in one embodiment is fed from a cam-controlled feed pump and in another embodiment is determined by a cam-controlled control valve, neither providing a specific starting position for the pump piston nor a specific delivery volume, because the camshaft drive of the control valve causes it to react differently depending on the immediate speed of the engine, as it is provided with more energy at high speed than at low speed. The filling of the pump chamber takes place either by means of the hydraulic pressure or by means of suction caused by the actuator piston and thus the pump piston being pushed backwards by a spring. US patent No. 3,516,395 shows a fuel pump, the filling of which is controlled by an electrovalve, and the actuation of the injection is controlled by a second valve in synchronism with the rotation of the crank- shaft. The fuel is used as hydraulic driving fluid. For many years, it has been impossible to use the fuel in large diesel engines as hydraulic fluid, because the properties of such fuel render it unsuitable for hydraulic purposes. The typical fuel is heavy fuel oil, which requires preheating and contains aggressive and abrasive compounds. The most recent engines can also use liquid gas as fuel, but such gas is also unsuitable as hydraulic fluid due to risks of explosion and formation of ice upon cooling. In the known hydraulically driven pumps from recent times, the fuel is kept entirely separate from the hydraulic fluid, and the dosage of fuel in these pumps takes place by rotation of the pump piston, so that the position of an oblique cut-off edge is changed in relation to a cut-off hole in the side wall of the pump cylinder. In addition to the above Danish patent, US patent publication No. 4 907 555 can also be mentioned as an example of such pumps . The control valve for the supply of high-pressure hydraulic fluid can in this case be either cam-actuated or electronically actuated. Furthermore, an electronically actuated control valve for a hydraulically driven fuel pump is known from the Applicant's Danish patent No. 170121, which control valve is precise and fast-acting. The object of the present invention is to simplify the hydraulically actuated fuel pump in respect of design, operation and maintenance and to obtain high pump reliability. In view of this, the method according to the invention is characterized in that the pump piston is returned to a fixed starting position between each injection sequence by the action of the fuel pressure on the end surface of the pump piston in the pump chamber, and that the delivery volume of the fuel pump is electronically controlled by the control valve cutting off the supply of hydraulic fluid to the pressure chamber at the moment when the pump piston has delivered the desired volume. Several advantages are obtained by having the primer pump pressure of the fuel return the pump piston to a fixed starting position and by controlling the delivery volume by cutting off the supply of hydraulic fluid at the exact moment when the pump has delivered the desired volume. The cut-off time can thus be adjusted up until the moment when the cutting off actually takes place. When the delivery volume is changed, the new setting is immediately active and can take effect on the cylinder in which injection takes place and on subsequent injections in other cylinders during the same revolution of the engine. This fast- acting, electronic control of pump delivery can provide injection sequences, which may be varied as desired from one continuous injection to a series of injections at close intervals, or so-called intermittent injection. Furthermore, the invention provides the advantage that the known mechanical elements for mechanical return and filling and for control of the delivery volume may be left out, leading to a number of options regarding fuel pump design. The commonly used fuel oil accum- ulators, also called shock absorbers, may also be left out. It is a further advantage that the return to the starting position can take place during a relatively long period of time because the injection as such takes place during a short period of the engine cycle. Nor is a specific rotational angle required between the pump piston and the pump cylinder or the actuator piston. The operational simplifications and the resulting mechanical simplifications substantially reduce the risk of pump failure . In addition, the method has the important effect that loss of energy in connection with the operation of the pump is minimized, partly because the pump piston and the consumption of high-pressure hydraulic fluid stop at the end of fuel delivery, instead of continuing with simultaneous pressure relief of the pump chamber, as is usually the case with the known pumps, and partly because the fixed starting position renders it possible to minimize the dead volume in the pressure chamber of the actuator and thus to minimize the volume of hydrau- lie fluid to be pressurized immediately after opening of the control valve. It is known that the fuel for an internal combustion engine can be mixed with water to reduce the formation of undesirable NO compounds during combus- tion. In the known fuel pumps it is a problem that the engine must either run with permanent mixing of a specified amount of water or be stopped and reset for operation without addition of water. The possibility of the method according to the invention of changing the delivery volume of the fuel pump as desired during operation can advantageously be applied so that the electronic control unit switches the pump for delivery of an additional volume at the same engine load, amounting to from 1 to 60 per cent of the volume of fuel at the engine load in question, and so that at the same time the fuel is mixed with water corresponding to the additional volume. This change to low-NO operating mode can readily be made while the engine is running, and the percentage of the volume of fuel constituted by the additional volume can be varied as desired during operation, so that the addition of water can be adjusted continuously according to the desired NO reduction and to the engine load and other operating conditions such as the temperature and humidity of the suction air. This possibility of sudden change to another operating mode is particularly advantageous when the engine is a propulsion engine in a ship that periodically sails in waters that are subject to restrictions on NO emissions . The invention further relates to a hydraulically actuated fuel pump for an internal combustion engine, particularly a two-stroke diesel engine, comprising a pump piston that is longitudinally displaceable in a pump cylinder and has a front end surface which together with the pump cylinder defines a pump chamber, and an actuator piston, which is located in extension of the pump piston and is longitudinally displaceable in an actuator cylinder and has a piston surface that is located in a pressure chamber, faces away from the pump piston and has a substantially larger area than the cross-sectional area of the pump piston, at least one fuel inlet channel with a non-return valve opening into the pump chamber, and at least one fuel outlet channel leading from the pump chamber to at least one discharge port . Since the setting of fuel delivery, both in terms of volume and time, when applying the method according to the invention, does not impose the usual restrictions on the design of the fuel pump, this can advantageously be simplified. The fuel pump according to the invention is characterized in that the pump cylinder and the actuator cylinder are separate units that are mutually separated by an intermediate spacing member, which is annular, and that the pump cylinder has a lower section protruding down into the spacing member, and an upper section protruding radially in relation to the lower section and mounted by means of cover studs. The two cylinders are manufactured as independent units, which are stacked upon assembly of the pump, separated by the intermediate member and with relatively rough mutual alignment of the cylinder axes and largely without considering whether the cylinder axes are mutually displaced in the radial direction. The fact that the two cylinders need not fit together very precisely provides a substantial simplification of the manufacture and mounting of the parts . The division of the pump casing into two cylinders and an intermediate member and the assembly by means of cover studs, i.e., longitudinal bolts, provide the advantage that the casing is relieved of the tensile stress occurring when the pump chamber is pressurized and acts on the upper wall in the chamber with an upward force. The intermediate member and the fixation between said member and the two cylinder units can be relatively slight, because the intermediate member only has to resist the compressive loads from the assembly. During the delivery stroke, the pistons are pushed towards each other by the forward pressure on the piston surface of the actuator piston, and during the return stroke they are kept close to each other by a relatively minor force from the backward pressure from the inflow of fuel in the pump chamber. Furthermore, during operation the two pistons can be mutually positioned by small radial displacements of the backward end surface of the pump piston in relation to a forward end surface at the actuator piston, thus automatically compensating for temperature and pressure variations between the individual parts of the pump, and reducing pump wear and improving reliability. The pistons can to a certain extent be controlled in the radial direction by elements located between the two pump cylinders, for example, pressure gauges which transmit data on the current pump chamber pressure to an electronic control unit can be located between the mutually facing end surfaces of the pump pistons. The modular built fuel pump with the two separate pump cylinders further provides an advantageous possibility of replacing only the actuator cylinder with pertaining piston with another unit, in which the diameter of the actuator piston is larger or smaller than the original unit in order to change the maximum injection pressure of the pump, which depends on the pressure of the hydraulic fluid in the high-pressure conduit and on the area ratio between the actuator piston and the pump piston. In a preferred embodiment, which fully profits from the advantages of the mutually independent pistons, the pump piston is exclusively guided in the radial direc- tion by the pertaining cylinder bore in the pump cylinder, and the actuator piston is exclusively guided in the radial direction by the cylinder bore in the pertaining actuator cylinder, and the two pistons are mutually freely adjustable in the radial direction. This embodiment comprises a minimum of elements, resulting in simple manufacture and high reliability. In one embodiment, the actuator cylinder is an annular unit with an axially through-going cylinder bore, and the fuel pump is mounted on the top surface of a distributor block, the cylinder bore of the actuator cylinder being located in immediate extension of a discharge port in the top surface of the distributor block, whereby the pressure chamber is radially defined by the actuator cylinder and axially defined by the piston surface of the actuator piston and the top surface of the distributor block, respectively. The axially through-going cylinder bore in the actuator cylinder renders the design of the actuator extremely simple in terms of manufacture, since it comprises only two main parts, viz., the annular bottomless cylinder unit and the cylindrical actuator piston. The pressure chamber of the actuator is created by mounting the fuel pump on the top surface of the distributor block, because this constitutes the axially defining bottom surface of the chamber. At the same time, the top surface of the distributor block can act as a stop to the downward movement of the two pistons and thus determine their starting positions. In this position the pressure chamber is largely emptied of hydraulic fluid. Since the electronically actuated control valve is typically mounted on or in the distributor block, so that there are only short channels leading up to the discharge port in the top surface of the distributor block, the volume of hydraulic fluid between the control valve and the actuator piston is optimally small, providing the advantage that the energy consumption for pressurization of this volume of fluid is negligible when the control valve opens for the pressure in the high-pressure conduit at the beginning of a delivery stroke. At the same time, the minimization of the dead volume in the pressure chamber serves to improve the fast-acting characteristic of the electronically actuated control valve, because the delay in time following actuation of the valve due to the required pressure build-up becomes negligible. The cover studs can fasten the tripartite pump casing to the top surface of the distributor block, which provides several advantages in connection with fuel pump inspection. When the cover studs are loosened, the pump cylinder with pertaining piston can be lifted out, possibly together with the intermediate member, while leaving the actuator cylinder with pertaining piston on the distributor block. While the pump is removed, the actuator prevents dirt from entering into the hydraulic system below. Since the majority of inspections will relate solely to the pump cylinder, which is exposed to the influence of the fuel, the possibility of lifting out the pump piston with pertaining cylinder is a major advantage both in terms of maintenance and security of the system. If the fuel pump is applied for injection of liquid gas, heating of the pump is unnecessary. Although the pump according to the invention is substantially more simple than the known pumps for heavy fuel oil, it will be an advantage when using heavy fuel oil that preheated oil can flow through the pump, also when the engine is in the ready-to-start mode. To achieve this, the outer surface of the lower section of the pump cylinder can be provided with an annular recess, which is pressure- sealed at the top and at the bottom in relation to the surrounding inner surface of the spacing member, and a fuel inlet in the wall of the intermediate member can lead into the annular chamber formed by the recess and the intermediate member, and a discharge channel with a pressure-reducing flow restriction can be continuously connected with the annular chamber. The fuel thus circulates from the inlet through the annular chamber and out through the discharge channel, the flow restriction of which prevents the pressure in the annular chamber from dropping, and the pre-heated fuel provides heating of the pump cylinder by circulation. The above advantages of leaving the actuator as a sort of loose dirt cover on the sensitive hydraulic parts in the distributor block can be further improved by the top surface of the actuator cylinder having an upright annular wall with a larger inner diameter than the cylinder bore in the actuator cylinder, and by positioning an upwardly closed, cup-shaped shield at the lower end of the pump piston at a distance from the lower surface of the pump cylinder, extending down around the annular wall. When the pump cylinder and piston are lifted off, the upright wall around the cylinder bore of the actuator prevents dirt particles from entering into the top of the actuator piston, from which position the continued operation could cause them to be moved out to the gap between the cylinder and the piston, resulting in damage to the sliding surface due to wear. The cup-shaped shield of the pump piston together with the wall provides protection against fuel leaking down onto the actuator piston during operation, because the volume of fuel that leaks through the annular gap between the pump piston and the cylinder lands on the top surface of the shield. Since the shield extends down around the wall, the fuel from the top surface of the shield only runs down on the outer surface of the wall that prevents the fuel from reaching the actuator piston. A fuel drain in the cavity between the actuator and the pump can prevent major volumes of fuel from gathering around the wall. The cup-shaped shield is located in the cavity created by the height of the intermediate member, and this cavity has no other practical use than to provide room for the shield to participate in the movements of the pump piston and to create the aforementioned separation of the fuel and hydraulic systems. The tripartite construction thus provides the possibility of utilizing the cavity for other purposes. Since all the known elements for determining the pump delivery volume mechanically by rotation of the pump piston are replaced by volume adjustment through time-based control of the supply of hydraulic fluid to the actuator, minor variations in the delivery volumes of the pumps may occur in practice due to minor variations between the pumps in the rate of delivery of hydraulic fluid, when the control valve has connected the pressure chamber of the actuator to the high pressure conduit. It is possible to identify such variations in the delivery volumes of the pumps by continuous measuring of one or more parameters of the combustion in the cylinder, and then to compensate therefor by means of the electronic actuation of the control valve. Alternatively, the available room in the cavity may be used for designing the fuel pump so that the thickness of the wall of the cup-shaped shield decreases downwards and so that at least two sensors are mounted in the intermediate member at and on the opposite sides of the shield for detection of the immediate position of the pump piston. The two opposite sensors measure the position of the piston simultaneously, and the electronic control unit receiv- ing the measuring signals can therefore compensate for the variations in the measurements at one of the sensors which may occur due to the possibility of the piston of free positioning in the radial direction. The sensors are well-known and may, for example, be based on inductive measurement of the position. The electronic control of the delivery volume of the fuel pump by stopping the delivery stroke of the pump piston, that starts from a fixed, lower starting position, provides a favourable possibility of further improving pump reliability by the pump cylinder having a lower section that fits in a pressure-sealing manner around the pump piston, and a chamber section of a larger inner diameter than the lower section, the fuel inlet channel opening into the side wall of the chamber section, and at least two fuel outlet channels originating from the side wall of the chamber section. It is well-known from the prior-art pumps with fuel outlet channels and/or pressure relief holes which are opened or closed by an edge on the piston passing the channel or hole that the sudden changes in pressure at the passage of the control edge on the piston can cause cavitation and other erosion damage to the piston and cylinder. By providing the chamber section with a larger inner diameter than the lower pressure-sealing section according to this preferred embodiment of the invention, the cylinder surface of the pump piston moves at a radial distance within the side wall of the chamber section. The inlet and outlet channels for fuel end or begin in the side wall of the chamber section, which means that, continuously and regardless of the position of the piston, the fuel has full access to the channel openings in the pump chamber, because at the openings there is at least an annular area filled with fuel between the cylinder surface of the piston and the side wall of the chamber section. This eliminates the sudden, erosive pressure variations and improves fuel pump reliability. Furthermore, due to abrasive particles in the fuel, wear may occur at the edges of the channel openings. The location of the openings at a radial distance from the cylinder surface of the pump piston prevents damage due to wear at the edge of the hole from spreading to the piston in the form of scratches, which further improves reliability. In one embodiment, the increased diameter of the upper section of the pump cylinder is utilized for further simplification of fuel pump design. In this embodiment, the fuel outlet channels are connected with upward discharge ports, and the flanges for the pressure conduits leading to the fuel injectors are mounted on the top surface of the upper section of the pump cylinder. This facilitates mounting of the pressure conduits and allows a design of these without bends near their mounting position on the pumps. The ample space on the top surface of the pump cylinder renders it possible to mount up to four pressure conduits on the top surface. One embodiment allows the pump cylinder in the area above the chamber section to have a section of a smaller diameter, constituting an upwardly closed dampening chamber, and the pump piston to have an uppermost section of a height corresponding to the height of the dampening chamber and an outer diameter slightly smaller than the diameter of the dampening chamber. The dampening chamber improves fuel pump reliability because the risk of damage to the pump is reduced in the event of non-termination of the high pressure to the pressure chamber of the actuator, for example, due to a stuck control valve or a delay of or failure in the electronic switching signal to the control valve. If due to such failure the pump piston is driven all the way to the top of the pump chamber, it will continue up into the dampening chamber and together with the side wall of this closed chamber it will create an annular gap, through which the fuel in the dampening chamber is to be expelled with a simultaneous pressure increase in the dampening chamber, causing the movement of the piston to slow down substantially. The dampening chamber thus constitutes a hydraulic buffer which prevents the pump piston from hitting hard against the top of the chamber. The uppermost section of the pump piston takes up approximately the same volume as the dampening chamber, providing the advantage that when the piston is in its starting position, the total volume of fuel in the pump cylinder largely is not increased due to the addition of the dampening chamber. As the total volume of fuel must be pressurized so as to attain the opening pressure of the injector before initiation of the fuel injection, an energy saving is achieved by minimizing this volume, because the travel of the pump piston for pressurization is reduced, thereby minimizing the consumption of high- pressure hydraulic fluid. An operator advantage is achieved in that the actuator piston has a recess close to its lower surface, and that a stop mounted in the wall of the actuator cylinder protrudes into the recess. The stop prevents the actuator piston from falling out of the cylinder when the cylinder is lifted. This feature is particularly important when the actuator is dismantled after a long period of operation if performed by inexperienced staff who may not be aware that the actuator cylinder has no bottom. An example of a preferred embodiment of the invention will now be described in further detail below with reference to the schematic drawing, in which Fig. 1 shows a side view of an outline of a fuel pump according to the invention mounted at a cylinder in an internal combustion engine, Fig. 2 shows a longitudinal sectional view of the fuel pump in Fig. 1, and Fig. 3 shows the fuel pump from above. A fuel pump 1 is mounted on the top surface of a distributor block 2, which is supported by a console 3. The console is connected to a high-pressure conduit for hydraulic fluid, which is supplied with hydraulic fluid from a pump station, not shown, at a pressure in the range, for example, from 125 to 325 bar. The pressure may be fixed, but is preferably adjustable in relation to the engine load. The pump station may be supplied from a storage tank, and the hydraulic fluid may, for example, be a standard hydraulic oil, but preferably the lubricating oil of the engine is used as hydraulic fluid, and the system is fed from the oil sump of the engine . The internal combustion engine may be a medium- speed four-stroke engine, but is typically a low-speed two-stroke crosshead engine, which may be a propulsion engine in a ship or a stationary prime mover of a power plant . The engine can be designed in various sizes with outputs per cylinder from 400 kW to 5500 kW and it may have speeds at full load in the range, for example, from 200 rpm for the smallest engines down to, for example, 60 rpm for the largest engines. At a specific fuel consumption in the interval typically from 160 g/kWh to 185 g/kWh, the required volume of fuel per injection sequence at full load may vary from about 6 g for the smallest engines to about 250 g for the largest ones. The fuel pump according to the invention is particularly suitable for injection of large volumes, for example, volumes of more than 30 g of fuel per sequence at full load. The high pressure conduit from the pump station is mounted on the side of the console 3, which on its top surface supports the distributor block 2. A pressure conduit 4 extends from the block to the actuator of the exhaust valve. Several, such as three, pressure pipes 5 extend from the fuel pump 1 to injectors 6, which inject the fuel into the combustion chamber of an engine cylinder 7. The lower flanges 8 of the pressure pipes are bolted to the pump 1. Each cylinder of the engine is associated with an electronic control unit 9, which receives general synchronizing and control signals through wires 10 and transmits electronic control signals to, i.a., a control valve 11 through a wire 12. The control unit can receive signals on the immediate position of the control valve through a wire 13 from the control valve. The cylinders may have a control unit 9 each or several cylinders may be associated with the same control unit. The control unit may also receive signals from an overall control unit common to all the cylinders. In the console, a channel 14 branching off from the high-pressure conduit passes the pressurized hydraulic fluid to a high-pressure port on the control valve 11. The channel 14 is provided with a number of fluid accumulators 15 that deliver most of the fluid volume when the control valve opens and are post-fed from the high-pressure conduit while the control valve is closed. Via a relatively short channel 16 in the distributor block, a control port on the control valve is connected to a discharge port on the top surface 17 of the block. The control valve also has a tank port which is connected to a return conduit for used hydraulic fluid via a channel 18. The pressure in the return conduit can range from atmospheric pressure to a few bars of overpressure . When the fuel pump is to initiate fuel delivery, a control signal from the control unit 9 actuates the control valve 11 to the position in which the high- pressure port is connected to the control port, so that the high-pressure fluid has free access to the discharge port through the channel 16. When the delivery stroke of the pump is to be stopped, the control valve is actuated to the position in which the tank port is connected to the control port, whereby the high pressure in the channel 16 is drained off. Accordingly, the control valve 11 has at least three ports and at least two positions. It may have a fast-acting pilot slider, which hydraulically sets a main slider. The control valve may, for example, be of the type described in the Applicant's Danish patent No. 170121. The top surface of the fuel pump 1 is elevated by means of the console 3 to about the same height as the cover 19 of the engine cylinder, so that the pressure pipes 5 are of short length, and thus of an advantageously small volume to be pressurized at initiation of each injection. The fuel pump is kept supplied with fuel from a fuel conduit 20, which distributes the fuel to the pumps by means of a primer pump at an overpressure typically in the interval from 4 to 15 bar, typically an overpressure of 8 bar. A branch conduit 21 extends from the fuel conduit to a fuel inlet 22 shown in Fig. 2. The pump is furthermore connected to a drain conduit 23 that removes fuel leakages. The fuel pump comprises two cylinder units, viz., an actuator cylinder 24 and a pump cylinder 25, which are mutually separated by an intermediate member 26, the lower surface of which abuts on an upward annular surface on the top surface of the actuator cylinder, and the top surface of which abuts on a downward annular surface on the pump cylinder. The intermediate member is annular and substantially cylindrical and has a relatively small wall thickness that is less than half the wall thicknesses of the two cylinders . The intermediate member is fastened to the two cylinders by means of small machine screws 27 so as to hold together the three parts of the casing when the pump is moved. The pump cylinder 25 has a substantially circularly cylindrical lower section 28 and an upper section 29 having a substantially larger outer diameter than the lower section. The downward annular surface for abutment on the intermediate member 26 is formed by the lower surface of the radially protruding part of the upper section and is positioned immediately outside the lower section 28, so that said section is pressure-sealed at the top and at the bottom of its cylindrical outer surface in relation to the inner surface of the inter- mediate member by means of two 0 rings 29 positioned in grooves in the outer surface of the lower section. Between these two axially separated pressure seals, the outer surface of the lower section has a recess which together with the inner surface of the intermedi- ate member defines an annular chamber 30, which communicates partly with the fuel supply 22 and partly with the discharge channel 31 leading from the annular chamber to a circulation conduit, not shown, for pre-heated fuel. A throttle means 32 is located at the connection of the circulation conduit and is provided with a flow restriction which restricts the circulating volume of fuel. At the mounting on the top surface 17 of the distributor block, the fuel pump is positioned so that the actuator cylinder is located around the discharge port of the channel 16, and the pump cylinder is fastened to the distributor block by means of cover studs 33 that are inserted into through holes 34 in the protruding part of the upper section 29 of the cylinder. The actuator cylinder 24 is an annular unit with an axially through-going cylinder bore 35 receiving a pressure-sealing and longitudinally displaceable actuator piston 36. At its lower edge the actuator piston is machined to have a slightly smaller diameter, so that it has an annular recess 37, into which a stop 38 protrudes in the form of the end of a screw, which is inserted through a bore extending obliquely through the cylinder wall. The front section of the screw fits into the pertaining bore in a pressure-sealing manner, and in addition thereto a pressure-sealing gasket is positioned below the head of the screw. When removing the screw the actuator piston can be moved downwards out of the cylinder. Together with the cylinder bore and the top surface 17, the lower surface 59 of the actuator piston defines a pressure chamber 60, which is approximately empty in the starting position shown, because the lower surface abuts on the top surface 17. A longitudinally displaceable pump piston 39 is inserted in the pump cylinder, which in a lower section 40 fits in a pressure-sealing manner around the pump piston. Via a conical section, the lower section continues into a superjacent chamber section 41 having a larger inner diameter than the lower section 40, and at the top of the chamber section the cylinder bore continues into a superjacent dampening chamber 42. The actuator piston has a plane upward end surface 43 perpendicular to the longitudinal axis of the actuator, and the pump piston has a downward end surface 44 which abuts on the end surface 43 in the starting position of the pistons shown. The two end surfaces can move independently of each other in the radial direction under the influence of the guide forces between the pump piston 39 and the cylinder section 40 and between the actuator piston 36 and the cylinder bore 35. It should 5 be understood that the upward and downward directions indicated are used for the sake of clarity and apply to the mounting of the fuel pump shown. Other methods of mounting, such as sidewards mounting on a vertical surface, are also possible. Directional indicators could 10 also be forward and backward or front and back, forward and front indicating the direction of movement of the pump piston during the active delivery stroke. At the top of the actuator cylinder 24 in the cavity 45 created by the intermediate member 26, an 15 upright, annular wall 46 is mounted having a larger inner diameter than the cylinder bore 35. At the lower end of the pump piston, an upwardly closed cup-shaped shield 47 is mounted, the side wall 48 of which extends downwards and passes down around the outer surface of 20 the wall 46. The shield and the wall prevent impurities, such as leaked fuel and dirt particles, from penetrating into the top surface of the actuator piston. Instead of the mounting of the shield 47 shown in the drawing, where it has been pressed up on a conical guide surface 25 by means of a nut screwed onto the end of the pump piston, the shield 47 can be shrink fitted onto a cylindrical guide surface on the pump piston. A drain opening, not shown, near the top surface of the actuator cylinder connects the cavity 45 with the drain conduit 30 23. The thickness of the side wall 48 of the shield decreases downwards. In the intermediate member 26, two sensors 49 are mounted on diametrically opposite sides of the side wall 48 and transmit signals to the control 35 unit 9 through wires 50. The signals may, for example, be a voltage, the size of which depends on the distance between the end of the sensor and the outer surface of the side wall 48. As this wall is inclined, the distance grows when the pump piston moves upwards, and the sensors thus provide the control unit with feed-back about the immediate position of the piston. A fuel inlet channel 51 connects the pump chamber 52 with the annular chamber 30 through the discharge channel 31. In the flow path between the discharge channel 31 and the pump chamber, a non-return valve 53 is located, which only allows a flow of fuel into the pump chamber. When the pressure in the pump chamber exceeds the primer pump pressure in the annular chamber 30 the non-return valve closes, and it reopens and allows an inflow of fuel at primer pump pressure, once the pressure in the pump chamber drops again on termination of the delivery stroke of the pump piston. The nonreturn valve is screwed into a sideward bore 54 in the upper section 29, providing the advantage that the non- return valve does not take up room on the top surface of the pump cylinder. Three fuel outlet channels 55 originate from the chamber section 41 and end in upward discharge ports 56 at pressure pipe connections for the injectors 6. Each channel 55 comprises a radial bore, the outer end of which is blocked by an element 57 screwed into it, and an axial bore. As shown in Fig. 3, the three radial bores and the bore 51, 54 can be evenly distributed in the circumferential direction of the cylinder. In connection with the connection of the pressure pipe, a non-return valve can be positioned in the pressure pipe, if desired, so as to prevent a flow of fuel back to the pump chamber after termination of the delivery stroke. Furthermore, there can be an axial, central bore 58 that leads up from the top of the dampening chamber 42. The bore 58 is blocked by an element 57, but may, if desired, serve for insertion of a lifting tool, which can be screwed onto the top of the pump piston, so that said piston can be lifted out together with the pump cylinder. The pump piston has a stepped front end surface having an annular, radially outermost section 61 and a central, radially innermost section 62, upwardly displaced in relation to the section 61. Between the two sections, the pump piston thus has an uppermost section 63, the height of which corresponds to the height of the dampening chamber 42, and the outer diameter of which is slightly smaller than the inner diameter of the dampening chamber . The fuel pump operates as follows: When the control valve 11 keeps the pressure chamber 60 in the actuator in connection with the drain, i.e., the tank port of the valve, the fluid pressure in the pressure chamber is low, such as an overpressure of 0.5 bar. In the present context, a drain means a general possibility of draining off used hydraulic fluid. Therefore, the drain does not necessarily have atmospheric pressure. The drain connected to the tank port of the valve can be pressurized to a minor overpressure, for example the aforementioned 0.5 bar. While the chamber is connected to the drain, the non-return valve 53 is open and allows fuel at primer pump pressure to flow into the pump chamber 52, so that the pressure in said chamber is, for example, 8 bar of overpressure. When the drain pressure is set above atmospheric pressure, the ratio between the areas of the actuator piston and the pump piston must be smaller than the ratio between the primer pump pressure in the pump chamber and the drain pressure in the pressure chamber of the actuator, for the downward force on the pump piston to exceed the upward force on the actuator piston. In the embodiment shown, the ratio between the piston areas is 4:1, while the ratio between the absolute values of pressure is about 6:1. As long as the control valve 11 is in the position mentioned, the pump piston and the actuator piston are pushed downwards by the pressure in the pump chamber and are therefore moved back to the starting position shown, the pistons abutting each other and the lower surface 59 of the actuator piston abutting the top surface 17 of the distributor block. When the control valve is actuated to connect its high-pressure port to the channel 16, the pressure in the pressure chamber 60 suddenly increases to the pressure in the high-pressure conduit, for example, to 250 bar. The actuator piston is pushed forward by this pressure and drives the pump piston into the pump chamber 52, in which the pressure increase causes the non-return valve 53 to close and the pressure in the pressure pipes 5 to increase. After a short travel of the pistons, the pressure of the fuel in the pipes 5 reaches the opening pressure of the injectors, which may, for example, be 400 bar, and the fuel injection commences . When the control unit 9 causes the injection to be interrupted and transmits an actuating signal for switching of the control valve 11, the channel 16 is reconnected to the tank port, and the pressure in the pressure chamber drops rapidly to the drain pressure with a simultaneous drop of pressure in the pump chamber to the primer pump pressure, and the pump piston stops. Subsequently, the non-return valve 53 opens, and the fuel starts flowing into the pressure chamber again, thereby pushing the pump piston and the actuator piston back to their starting positions, whereupon the process can be repeated at the subsequent injection sequence. It is worth noticing that, during the return movement, the pump chamber only needs to be supplied with a volume of fuel corresponding to the volume injected at the preceding sequence, and that injection in a two-stroke engine typically takes place during 1/15 of a rotation of the engine, whereupon 14/15 of the rotation of the engine remains for returning the pistons, or for effecting injection into other cylinders by means of the same pump. Even with relatively small resulting, downward forces on the pistons, there is ample time for the pistons to return to their fixed starting positions. Naturally, a number of variations of the above embodiment can be made. For example, it is possible to design the actuator cylinder with a closed bottom by providing the cylinder with a supply channel to the pressure chamber. It is also possible without further measures to provide the top surface of the actuator piston with a different design, such as stepped, and an upright stud on the actuator piston may support the cup- shaped shield 47, to the effect that the lower surface of the pump piston abuts loosely on the top surface of the shield. The pump piston can be designed without the shield, when signals on position are not desired. It is possible, but not desirable, to design the fuel pump with a casing which is able to absorb tensile stress in the conventional manner. Furthermore, the two pump cylinders may be integral without an actual cavity, the cylinder bore of the actuator continuing, for example, stepwise or conically, into the cylinder bore of the pump, resulting in a fuel pump of short length comprising neither an upright inner wall nor an inner shield. If the pump is to deliver fuel to four injectors, the flow restriction can be located in a sideward bore like the non-return valve. Moreover, the discharge ports 56 can be sidewards, so that the pressure pipes 5 are mounted on the side wall of the pump. Also, the fuel pump can be mounted with the actuator cylinder being located above the pump cylinder, and at the same time the annular wall 46 and the cup- shaped shield 47 can be omitted, since there is no possibility of any leakage of fuel oil to run down and mix with leaked hydraulic fluid, which is typically drained off to the oil sump of the diesel engine. The fuel pump can also be positioned below or beside the distributor block or elsewhere, such as on the cylinder cover 19.

Claims: P A T E N T C L A I M S 1. A method for operation of a hydraulically actuated fuel pump (1) for an internal combustion engine, particularly a two-stroke crosshead engine, having a fuel conduit (20) for distribution of fuel to the fuel pumps at primer pump pressure and a high- pressure conduit for distribution to the fuel pumps of a pressurized hydraulic fluid, which is different from the fuel, a pump piston (39) in the fuel pump being driven forwards in a delivery stroke by means of an actuator piston (36) in an actuator cylinder (24) with simultaneous pressing out of fuel from a pump chamber (52) and driven backwards in a return stroke with simultaneous filling of the pump chamber with fuel from the fuel conduit, and the actuator piston being controlled by an electronically actuated control valve (11) , by which a pressure chamber in the actuator cylinder can be provided with hydraulic fluid from the high-pressure conduit or be connected to a drain, c h a r a c t e r i z e d in that the pump piston (39) is returned to a fixed starting position between each injection sequence by the action of the fuel pressure on the end surface (61, 62) of the pump piston in the pump chamber (52) , and that the delivery volume of the fuel pump is electronically controlled by the control valve (11) cutting off the supply of hydraulic fluid to the pressure chamber at the moment when the pump piston has delivered the desired volume. 2. A method according to claim 1, c h a r a c - t e r i z e d in that an electronic control unit (9) switches the fuel pump to deliver an additional volume at the same engine load, amounting to from 1 to 60 per cent of the volume of fuel at the engine load in question, and that at the same time the fuel is mixed with water corresponding to the additional volume. 3. A hydraulically actuated fuel pump for an internal combustion engine, particularly a two-stroke diesel engine, comprising a pump piston (39) that is longitudinally displaceable in a pump cylinder (29) and has a front end surface which together with the pump cylinder defines a pump chamber (52) , and an actuator piston (36) , which is located in extension of the pump piston and is longitudinally displaceable in an actuator cylinder (24) , and has a piston surface (59) that is located in a pressure chamber, faces away from the pump piston and has a substantially larger area than the cross-sectional area of the pump piston, the pump piston being loose in relation to the actuator piston, at least one fuel inlet channel (51) with a non-return valve (53) opening into the pump chamber, and at least one fuel outlet channel (55) leading from the pump chamber to at least one discharge port, c h a r a c t e r i z e d in that the pump cylinder (25) and the actuator cylinder (24) are separate units that are mutually separated by an annular intermediate spacing member (26) , and that the pump cylinder has a lower section (28) protruding down into the spacing member, and an upper section (29) protruding radially in relation to the lower section and mounted by means of cover studs (33) . 4. A hydraulically actuated fuel pump according to claim 3, c h a r a c t e r i z e d in that the pump piston (39) is exclusively guided in the radial direction by the pertaining cylinder bore in the pump cylinder (25) , that the actuator piston (36) is exclusively guided in the radial direction by the cylinder bore (35) in the pertaining actuator cylinder, and that the two pistons are mutually freely adjustable in the radial direction. 5. A hydraulically actuated fuel pump according to claim 3 or 4, c h a r a c t e r i z e d in that the actuator cylinder (24) is an annular unit with an axially through-going cylinder bore (35) , and that the fuel pump is mounted on the top surface (17) of a distributor block (2) , the cylinder bore (35) of the actuator cylinder being located in immediate extension of a discharge port in the top surface of the distributor block, whereby the pressure chamber is radially defined by the actuator cylinder and axially defined by the piston surface of the actuator piston and the top surface of the distributor block, respectively. 6. A hydraulically actuated fuel pump according to any one of claims 3-5, c h a r a c t e r i z e d in that the outer surface of the lower section (28) of the cylinder is provided with an annular recess, which is pressure-sealed at the top and at the bottom in relation to the surrounding inner surface of the intermediate member (26) , that a fuel inlet (22) in the wall of the intermediate member leads into the annular chamber (30) formed by the recess and the intermediate member, and that a discharge channel (31) with a pressure-reducing flow restriction is continuously connected to the annular chamber. 7. A hydraulically actuated fuel pump according to any one of claims 3-6, c h a r a c t e r i z e d in that on its top surface the actuator cylinder (24) has an upright, annular wall (46) with a larger inner diameter than the cylinder bore in the actuator cylinder, and that an upwardly closed, cup-shaped shield (47) is positioned at the lower end of the pump piston (39) at a distance from the lower surface of the pump cylinder extending down around the annular wall . 8. A hydraulically actuated fuel pump according to claim 7, c h a r a c t e r i z e d in that the thickness of the wall of the cup-shaped shield (47) decreases downwards, and that at least two sensors (49) are mounted in the intermediate member at and on the opposite sides of the shield for detection of the immediate position of the pump piston (39) . 9. A hydraulically actuated fuel pump according to any one of claims 3-8, c h a r a c t e r i z e d in that the pump cylinder has a lower section (40) that fits in a pressure-sealing manner around the pump piston (39) , and a chamber section (41) of a larger inner diameter than the lower section, that the fuel inlet channel (51) opens into the side wall of the chamber section, and that at least two fuel outlet channels (55) originate from the side wall of the chamber section. 10. A hydraulically actuated fuel pump according to claim 9, c h a r a c t e r i z e d in that the fuel outlet channels (55) are connected to upward discharge ports (56) , and that the flanges (8) for the pressure pipes (5) leading to the fuel injectors (6) are mounted on the top surface of the upper section of the pump cylinder. 11. A hydraulically actuated fuel pump according to claim 9 or 10, c h a r a c t e r i z e d in that in the area above the chamber section (41) the pump cylinder (25) has a section of a smaller diameter, constituting an upwardly closed dampening chamber (42) , and that the pump piston has an uppermost section (63) of a height corresponding to the height of the dampening chamber and an outer diameter slightly smaller than the diameter of the dampening chamber . 12. A hydraulically actuated fuel pump according to any one of claims 3-11, c h a r a c t e r i z e d in that the actuator piston (36) has a recess (37) close to its lower surface, and that a stop (38) mounted in the wall of the actuator cylinder protrudes into the recess .

Inventors: CENKER POUL, LINDQUIST HENNING, CENKER, POUL, LINDQUIST, HENNING
Applicants: CENKER POUL, LINDQUIST HENNING, MAN B & W DIESEL GMBH, CENKER, POUL, LINDQUIST, HENNING, MAN B & W DIESEL A/S
Application year: 1998
Main classification: F02M59
IPC codes: F01L9, F02M43, F02M47, F02M59