WO2012117152A1

WO2012117152A1 - A fuel feed system for an internal combustion engine and a method of operating such

Info

Publication number: WO2012117152A1
Application number: PCT/FI2012/050160
Authority: WO
Inventors: Sebastian LÖNNQVIST
Original assignee: Wärtsilä Finland Oy
Priority date: 2011-03-03
Filing date: 2012-02-20
Publication date: 2012-09-07
Also published as: FI20115212A; FI20115212A0

Abstract

A fuel feed system for an internal combustion engine and a method of operating thereof. To improve the handling of fuels of different grades the fuel feed system has been provided with closed cooling means for the return fuel oil.

Description

A FUEL FEED SYSTEM FOR AN INTERNAL COMBUSTION ENGINE AND A METHOD OF OPERATING SUCH

Technical field [0001] The present invention relates to a fuel feed system for an internal combustion engine in accordance with the preamble of claim 1 , and a method of operating a fuel feed system for an internal combustion engine in accordance with the preamble of the independent method claim. Background art

[0002] Internal combustion engines used in both marine and power plant installations have most often been designed for operation on marine diesel oil (MDO), sometimes also called marine diesel fuel (MDF) or light fuel oil (LFO). During the last decades the trend has been to move towards the use of heavy fuel oil (H FO) in addition to the two above mentioned options, as the HFO is a clearly cheaper option than the other more refined ones. However, most often the engines have to be designed for fuels of different grades, as the local legislation may require that a marine vessel cannot use HFO in port areas due to its sulfur emissions whereby MDO has to be used instead. Compared to operating an internal combustion engine on MDO/LFO the plant requires auxiliary equipment for handling heavy fuel oil, particularly for heating the fuel oil to a correct viscosity before the engine injection system. Most often, the same injection equipment is used for treating both fuels, whereby the viscosities of the fuels have to meet with the demands of the injection equipment. This means that the viscosities of the fuel oils have to be adjusted by either heating or cooling the oils. Since the heavy fuel oil has a clearly higher viscosity than that of the marine diesel oil or light fuel oil the heavy fuel oil is normally heated. For the same reason the marine diesel oil or light fuel is in some specific circumstances cooled.

[0003] A prior art fuel system for the handling of H FO comprises the following components and functions as follows. The marine or power plant installation has a storage tank for the storage of the fuel oil at a suitable temperature to ensure that the fuel oil can be pumped. The storage tank also requires heating in order to control and maintain correct viscosity of the fuel oil in the tank. A transfer system is designed to pump fuel oil from storage tank to a fuel oil treatment system. The fuel oil treatment system consists of a buffer tank, separator units and a day tank. The treatment system ensures that a sufficient amount of clean HFO is available for engine operation. The HFO is transferred from the storage tank to the buffer tank. The buffer tank is also called the settling tank. HFO separation takes place before the fuel oil is passed to the day tank. The day tank ensures normally fuel supply for at least 8 - 12 operating hours.

[0004] The fuel feed system supplies clean HFO from the day tank to the engine injection pumps. The fuel feed system comprises at least one or more pumps, a fuel oil heater (steam or electric), a filter unit and a viscometer to control the viscosity (with temperature adjustment). The viscometer follows the viscosity of the fuel oil so that it may be controlled and maintained at an appropriate level before the fuel oil is introduced to the engine. The fuel feed system may supply fuel oil to more than one engine. All the above discussed equipment/components of the fuel feed system have been positioned along the fuel supply line leading from the day tank towards the engine. However, there is also a return fuel line leading fuel from the engine to be introduced either back to the day tank or to the fuel supply line by means of a mixing tank positioned at a proper location upstream of the engine. The amount of recirculated return fuel is considerably high, as it may be up to 90 % of the fuel pumped to the engine. This return line is normally provided with means for cooling the fuel, if and when such operation is needed . The cooling means is needed particularly when shifting from the use of HFO to the use of LFO or MDF.

[0005] When running the engine with HFO the temperature of the HFO is high, of the order of 100 degrees or even more (sometimes a temperature of the order of 130 degrees is used), in order to ensure that the viscosity of the oil is low enough (of the order of 3 - 20 cSt) for the fuel pumps and nozzles. When approaching a harbor, for instance, it is desired (for environmental reasons) to shift from the use of HFO to the use of cleaner LFO or MDF. This changeover from the use of a high viscosity fuel oil to the use of a low viscosity fuel oil takes place such that gradually more and more low viscosity fuel oil is added to the heavy fuel oil between the day tanks and the mixing tanks. However, it has to be taken into account that the hot fuel mixture returning from the engine back to the mixing tank to be mixed with the fresh fuel mixture raises the temperature of the fresh fuel oil mixture - return fuel mixture so that the viscosity of the final mixture of the return fuel oil and the HFO - MDF/LFO mixture falls easily below acceptable limits. In such a case only a very small amount of fresh MDF/LFO at a time may be added to the HFO to be mixed with the return fuel oil in the mixing tank, whereby it takes a very long time to shift totally to the use of MDF/LFO. Therefore, the prior art recirculation or return fuel line has been provided with return fuel cooling means. Such cooling means is normally an ordinary heat exchanger where heat from the return fuel oil is taken to water circulation, either to fresh or sea water (like discussed in, for example, FI-B1 -1 10201 ). However, it has been learned lately that the cooling capacity of the fresh or sea water based heat exchanger is limited, especially in an environment where the water temperature is relatively high, for instance of the order of 25 - 30 degrees. Therefore, the time it takes to shift from the HFO to the MDF/LFO is directly proportional to the temperature of the cooling water. The higher is the water temperature, the longer it takes to perform the total shift from the H FO to the MDF/LFO. The reason why the shift from H FO to MDF/LFO should take place as quickly as possible is purely economical. The longer one is able to run with the cheap HFO the shorter time the expensive MDF/LFO need to be used. Thus, since the cooling capacity of the return fuel oil is limited and, for instance in marine use, a marine vessel has to run on MDO at a certain phase the preparations for changing over from HFO to MDO have to be initiated well in advance. And since the time the fuel changeover takes is not predictable, the changeover will almost always be started too early.

[0006] Thus, an object of the present invention is to introduce a novel means of controlling the cooling of the return fuel such that the cooling capacity of fresh water or sea water does not significantly hamper the cooling.

[0007] Another object of the present invention is to design a novel fuel feed system for allowing the use of different grades of fuel oil in as an economical manner as possible.

[0008] A further object of the present invention is to look into the problem in more detail, and try to find creative solutions to the problem.

Disclosure of the Invention

[0009] The above and other objects of the invention are met by a fuel feed system for an internal combustion engine, the fuel feed system comprising at least two tanks for different fuel oil grades; a fuel changeover valve; a feed pump; a mixing tank; a fuel oil supply line having a fuel oil heater, a temperature measuring unit, and a viscosity measuring unit; and a return fuel line from the engine to the mixing tank; the fuel oil supply line and the return fuel line forming a fuel flow path, the fuel flow path being provided with a fuel cooler, wherein the fuel cooler is a part of a closed cooling circuit that is provided, in addition to the fuel cooler, with at least a pump, and cooling means for cooling down the heat exchange medium circulating in the fuel cooling circuit.

[0010] The above and other objects of the invention are met by a method of operating a fuel feed system for an internal combustion engine, the fuel feed system comprising at least at least two tanks for different grades of fuel oil; a mixing tank; a fuel supply line; the supply line being provided with a fuel mixture heater, a temperature measuring unit and a viscosity measuring unit; and a return fuel line, the fuel supply line and the return fuel line forming a fuel flow path, the fuel flow path being further provided with a fuel cooler; the method comprising the steps of: taking fresh fuel oil from at least one of the tanks, introducing the fresh fuel oil into the mixing tank, mixing the fresh fuel oil with a return fuel recirculated from the engine to the mixing tank to form a fuel mixture, taking the fuel mixture to the engine by means of the supply line, measuring the temperature of the fuel mixture by the temperature measuring unit and the viscosity by the viscosity measuring unit, returning a part of the fuel mixture from the engine to the mixing tank by means of the return fuel line, and cooling the fuel oil returning from the engine by means of the fuel cooler arranged in the fuel flow path and the further steps of, when changing the fuel grade from a high viscosity fuel over to low viscosity fuel: circulating in the closed fuel cooling circuit comprising the fuel cooler heat exchange medium for ensuring a desired temperature gradient of the fuel mixture in the supply line, and cooling the circulating heat exchange medium by means of cooling means arranged in the closed fuel cooling circuit.

[001 1] Other characteristic features of the fuel feed system of the present invention and the method of operating thereof will become apparent from the appended dependent claims.

[0012] The present invention, when solving at least one of the above-mentioned problems, also brings about a number of advantages, of which a few has been listed in the following:

• Quick shift from HFO to MDF/LFO

· Reduces the fuel expenses of the engine, or marine vessel or power plant

[0013] However, it should be understood that the listed advantages are only optional, whereby it depends on the way the invention is put into practice if one or more of the advantages were obtained. Brief Description of Drawing

[0014] In the following, the fuel feed system for an internal combustion engine and the method of operating the fuel feed system of the present invention are explained in more detail in reference to the accompanying Figures, of which

Figure 1 illustrates a prior art fuel feed system,

Figure 2 illustrates a fuel feed system disclosing a first way of controlling the temperature and viscosity of the fuel mixture in the fuel supply line,

Figure 3 illustrates a fuel feed system disclosing a second and third way of controlling the temperature and viscosity of the fuel mixture in the fuel supply line,

Figure 4 illustrates a fuel feed system in accordance with a first preferred embodiment of the present invention,

Figure 5 illustrates a fuel feed system in accordance with a second preferred embodiment of the present invention, and

Figure 6 illustrates a fuel feed system in accordance with a third preferred embodiment of the present invention.

Detailed Description of Drawing

[0015] Figure 1 illustrates a prior art fuel feed system. The fuel feed system of the internal combustion engine 2 comprises various tanks, pumps, filters, heating and cooling equipment and a number of valves (of which only a small part is shown). The fuel for the engine 2 is stored in tanks 4 and 8, i.e. one (tank 4) for light fuel oil (LFO) or marine diesel fuel (MDF) and another (tank 8) for the heavy fuel oil (HFO). Fuels are introduced to the tanks 4 and 8, which are sometimes, for instance in marine vessels, called day tanks, from larger fuel storage tanks (not shown). Fuel from the MDF/LFO tank is taken to a 3- way valve 6 to be either guided directly to the engine 2 when it is a question of pure MDF/LFO operations or to a 3- way valve 10. The 3- way valve 10 receives fuel from the HFO tank 8 when it is a question of operation on mere HFO, or from both tanks 4 and 8 when the fuel change is going on. The chosen fuel or fuel combination from the 3- way valve 10 is pumped by means of a feed pump 12 through a filtering unit 14 to the mixing tank 16, where the fresh fuel and return fuel i.e. fuel returning from the engine 2 along return line 18 are mixed. The fuel mixture is pumped by means of a circulation pump 20 to a fuel heater 22 that is used for adjusting the viscosity of the fuel in supply line 28 appropriate for the fuel injection means arranged in connection with the engine 2. The temperature of the fuel is measured at 24 by a temperature measuring unit and the viscosity at 26 by a viscosity measuring unit. The viscosity input of the measuring unit 26 is used for adjusting the heating at the heater 22. Normally steam is used as the heating medium in heater 22, though also electricity may be used, too. The fuel supply line 28 takes the viscosity adjusted fuel mixture to the engine 2. A part of the fuel mixture is returned from the engine 2 along line 30 to a 3- way valve 32 by means of which the return fuel may be guided either along return line 34 back to the MDF/LFO tank 4 or along return line 36 to a 3- way valve 38. By means of the 3- way valve 38 the return fuel may be guided along line 40 to the return fuel line 18 terminating to the mixing tank 16 or along line 42 to the return fuel cooler 44. Thus fuel supply line 28, and the return fuel lines 36 and 18 form a fuel flow path. Both the three-way valve 32 and the three-way valve 38 may be replaced with two separate valves, one in each outlet line. The cooling circuit, which is open to the cooling water source, comprises in addition to the return fuel cooler 44 an inlet line 46 from, for instance, the sea or a river and a pump 48 for introducing cooling water to the return fuel cooler 44, and a return water line 50 for taking the water back to the sea or the river. Sometimes this kind of a cooling arrangement has also been positioned in the return fuel line 34 upstream of the tank 4.

[0016] The above described prior art fuel feed system works such that when operating on MDF or LFO the fuel is taken directly from the tank 4 via 3-way valve 6 to the engine 2, the HFO valve 10 being shut off. Naturally, there may be some safety filters, pumps and valves between the tank 4 and the engine 2, but such are not interesting in view of the present invention and has not, therefore, been shown. When using MDF/LFO the recirculation or return fuel is guided directly back to the MDF/LFO tank 4. When operating the engine 2 on HFO the MDF/LFO valve 6 is shut off and the HFO is taken via valve 10 to a feed pump 12 and fed by the feed pump 12 through a filtering unit 14 to mixing tank 16 where recirculating or return HFO is introduced and mixed with the fresh HFO. The return HFO raises the temperature of the fuel mixture whereafter the fuel mixture is pumped towards the engine 2 along a supply line 28 through heater 22 and through temperature and viscosity measuring units 24 and 26. In normal conditions the HFO needs to be heated by the heater 22 for adjusting the viscosity to match the requirements of the fuel injection equipment of the engine 2. However, in general the viscometer 26 and the heater 22 are designed to cooperate such that if the viscosity measuring unit, i.e. the viscometer 26 detects that the viscosity of the fuel tends to rise, the heater 22 is either turned on or its heating capacity is increased so that the viscosity remains within its preferred range. In a similar manner, if the viscosity tends to decrease below an allowable limit, the heating capacity is either reduced or the heater 22 is turned off.

[0017] When using HFO the prior art fuel feed system does not recirculate the HFO back to the HFO tank 8 but along a return path of its own back to the mixing tank 16. In other words, when recirculating the HFO the 3- way valve 32 guides the HFO towards the return fuel cooling unit. The cooling unit has two flow paths for the return fuel oil, one via the return fuel cooler 44 and another via the by-pass line 40. The cooling unit is put into use when the viscosity measuring unit 26 detects that the viscosity of the fuel is getting too low even if the heater 22 has been turned off. Normally the reduction in viscosity is due to adding some MDF/LFO by means of the 3- way valve 10 to the HFO flow i.e. when shifting from the use of HFO to the use of MDF/LFO. Anyway, when the cooling of the return fuel has been deemed necessary, pump 48 is turned on and water from the sea or the river is pumped to the return fuel cooler 44, and the fuel flow is switched by the 3- way valve 38 to line 42, i.e. to enter the return fuel cooler 44. The heat from the return fuel oil is absorbed by the water, which is thereafter guided back to its origin along the return water line 50.

[0018] Now, it is easy to understand that the more efficient the operation of the return fuel cooler 44 is, the more low viscosity LFO/MDF may be added in a certain time period by the 3- way valve 10 to the HFO. Also, the colder the sea or river water is, the more efficiently the return fuel cooler 44 works. However, when the power plant either in a marine vessel or on solid ground is in a location where cold water is not available, but water having a temperature exceeding, for instance, 25 degrees has to be used it is easy to understand that the return fuel cannot be cooled to as low a temperature as with water having a temperature below, let us say, 5 or 10 degrees. Since the return fuel temperature, and, naturally, viscosity, determines how much MDF/LFO may be added by the 3- way valve 10 to HFO in a certain time period, it is obvious that the less the temperature of the return fuel in the return fuel cooler 44 is reduced, the less MDF/LFO may be added, and the longer it takes to fully change over from HFO to LFO/MDO Therefore, to be able to stop using HFO (cheaper fuel) as late as possible the cooling system of the fuel recirculation system should be improved.

[0019] Figure 2 illustrates a fuel feed system disclosing a first, improved in view of the way discussed in Fig. 1 , way to control the temperature and viscosity of the fuel oil in the engine fuel supply line.

[0020] Main operation - running on HFO [0021] The PLC instructs the 3-way valve 6 to prevent the M D F/LFO from advancing to the engine 2, the 3-way changeover valve 10 to be in a position connecting the HFO tank 8 to the fuel feed system and the trace heating at 22 to be turned on. [0022] Switching from HFO to MDF/LFO

[0023] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to MDF/LFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the changeover valve 10 to start adding M DF/LFO to H FO . Oil temperature T_HFO in the HFO tank 8 and oil temperature T_MDF in the MDF tank 4 are measured at the start of the changeover and signaled to the PLC. Based on these signals the PLC calculates the way the changeover valve 10 adds the MDF/LFO to the HFO to end up with the fastest changeover time without exceeding the desired temperature gradient (preferably of the order of 2°C/min). I n such a case that the temperature gradient, based on signals from the temperature measuring unit 24 to the PLC, is exceeding 2°C/min, an alarm is given and the way the changeover valve 10 proportions the MDF/LFO is slowed down until the temperature gradient drops to or below 2°C/min. The above is continued until the MDF/LFO is the only fuel introduced into the fuel feed system. This information is received by the PLC from the limit switch at changeover valve 10.

[0024] Running on MDF/LFO [0025] The PLC instructs the 3-way changeover valve 10 to direct only MDF/LFO to the fuel feed system, and the trace heating at 22 to be shut off after a certain time period when switching from HFO to MDF/LFO. [0026] Switching from MDF/LFO to HFO

[0027] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to HFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the changeover valve 10 to start adding HFO to MDF/LFO. Oil temperature THFO in the HFO tank 8 and oil temperature T_MDF in the MDF tank 4 are measured at the start of the changeover and signaled to the PLC. Based on these signals the PLC calculates the way the changeover valve 10 adds the HFO to the MDF/LFO to end up with the fastest changeover time without exceeding the desired temperature gradient (preferably of the order of 2°C/min). In such a case that the temperature gradient based on signal from the temperature measuring unit 24 is exceeding 2°C/min, an alarm is given and the way the changeover valve 10 proportions the HFO is slowed down until the temperature gradient drops to or below 2°C/min. The above is continued until the MDF/LFO is the only fuel introduced into the fuel feed system. This information is received by the PLC from the limit switch at changeover valve 10.

[0028] Figure 3 illustrates a fuel feed system disclosing a second, improved in view of the way discussed in Fig. 1 , way to control the temperature and viscosity of the fuel oil in the engine fuel supply line.

[0029] Main operation - running on HFO

[0030] The 3-way valve 6 prevents the MDF/LFO from advancing to the engine 2, the 3-way changeover valve 10 is in a position connecting the HFO tank 8 to the fuel feed system, the 3-way temperature control valve 68 allows the heat exchange medium to by-pass the return fuel cooler 44, and the trace heating at 22 is turned on.

[0031] Switching from HFO to MDF/LFO [0032] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to MDF/LFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the changeover valve 10 to start adding MDF/LFO to HFO and to control based on the signals from the temperature measuring unit 24 to the PLC, the temperature gradient of the fuel oil at 24 to 2°C/min until the fuel is changed entirely over to MDF/LFO. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. If the viscosity at 26 is less than V11 (for instance <4 cSt) and the temperature at 24 above T1 (for instance >45°C) the heat exchange medium pump/s 48 is/are started and the trace heating at 22 is shut off. The 3-way temperature control valve 68 maintains the temperature gradient at 2°C/min, with the aid of signals from the temperature measuring unit 24 to the PLC, until viscosity at 26 is VI2 (for instance 3 cSt) and maintains this viscosity. If temperature at 24 is above T1 while viscosity is VI2 the 3-way temperature control valve 68 maintains the temperature gradient at 2°C/min until temperature at 24 is T1 (for instance 45°C).

[0033] Running on MDF/LFO [0034] The 3-way changeover valve 1 0 directs only MDF/LFO to the fuel feed system, the 3-way temperature control valve 68 is controlling the viscosity at 26 to VI2 (for instance 3 cSt), and the trace heating at 22 is turned off.

[0035] Switching from MDF/LFO to HFO

[0036] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to HFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the trace heating at 22 to be turned on directly, the changeover valve 10 to start adding HFO to MDF/LFO and to control, based on the signals from the temperature measuring unit 24 to the PLC, the temperature gradient of the fuel oil at 24 to 2°C/min until the fuel is changed entirely over to HFO. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. The 3-way temperature control valve 68 maintains the viscosity at 26 at VI2 (for instance 3 cSt). If the viscosity at 26 is more than V11 (for instance >4 cSt) the heat exchange medium pump/s 48 is/are stopped.

[0037] Figure 3 illustrates also a fuel feed system disclosing a third, improved in view of the way discussed in Fig. 1 , way to control the temperature and viscosity of the fuel oil in the engine fuel supply line

[0038] Main operation - running on HFO [0039] The 3-way valve 6 prevents the MDF/LFO from advancing to the engine 2, the 3-way changeover valve 10 is in a position connecting the HFO tank 8 to the fuel feed system, the 3-way temperature control valve 68 allows heat exchange medium to by-pass the return fuel cooler 44, and the trace heating at 22 is turned on. [0040] Switching from HFO to MDF/LFO operation

[0041] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to MDF/LFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the changeover valve 10 to start adding MDF/LFO to HFO and to control, based on the signals from the temperature measuring unit 24 to the PLC, the temperature gradient of the fuel oil at 24 to 2°C/min until the fuel is changed entirely over to MDF/LFO. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. If the viscosity at 26 is less than VI1 (for instance <4 cSt) and the temperature at 24 above T1 (for instance >45°C) the trace heating at 22 is shut off. The 3-way temperature control valve 68 maintains the temperature gradient at 2°C/min, with the aid of signals from the temperature measuring unit 24 to the PLC, until viscosity at 26 is VI2 (for instance 3 cSt) and maintains this viscosity. If the temperature at 24 is above T1 while viscosity is VI2 (for instance 3 cSt) the 3-way temperature control valve 68 maintains the temperature gradient at 2°C/min until temperature at 24 is T1 (for instance 45°C).

[0042] Running on MDF/LFO [0043] The PLC instructs the 3-way changeover valve 10 to direct only MDF/LFO to the fuel feed system, the 3-way temperature control valve 68 to control the viscosity at 26 to VI2 (for instance 3 cSt), and the trace heating at 22 to be turned off. [0044] Switching from MDF/LFO to HFO

[0045] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to HFO position and thus giving a signal to the PLC. Consequently, the PLC instructs the trace heating at 22 to be turned on directly, the changeover valve 10 to start adding HFO to MDF/LFO and to control, based on the signals from the temperature measuring unit 24 to the PLC, the temperature gradient of the fuel oil at 24 to 2°C/min until the fuel is changed entirely over to HFO. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. The 3-way temperature control valve 68 maintains the viscosity at 26 at VI2 (for instance 3cSt).

[0046] Referring to Fig. 3 it has to be understood that the return fuel valve that has been shown in Fig. 2 by reference numeral 38 has here been replaced with two separate valves, one in the line leading to the return oil cooler 44 and another in the line 40 by-passing the cooler 44. Thus it is clear that the same function i.e. guiding the return oil either to the cooler or to by-pass the cooler may be performed by both valve options. [0047] All the fuel feed systems discussed in connection with Figures 1 - 3 are applicable, as they include means for controlling the temperature and viscosity of the fuel oil mixture in the engine supply line 28 during the fuel changeover phase. However, the efficiency of each control system discussed above is dependent on the temperature of the fresh water used for cooling the return fuel oil. And since the temperature of the cooling water may, for instance in marine applications, change unexpectedly when approaching the coast, the user has to take such a change in account and start the fuel changeover so early that the return fuel cooling by the warmest available cooling water results in completion of the fuel changeover in time. [0048] To overcome the above discussed problem a novel means for cooling the return oil and for controlling the cooling has been taken into use. The novel means, for instance, allows the user to start the fuel changeover at a certain time period before the fuel changeover needs to be performed irrespective of the temperature of the fresh water used for cooling. [0049] Figure 4 illustrates a fuel feed system in accordance with a first preferred embodiment of the present invention. The most significant change when compared to the prior art fuel feed system can be seen in the left hand side lower corner of the Figure where a 3- way valve 52, a water inlet line 54, an additional heat exchanger 58 (a relatively low efficiency one), a water outlet line 56 and a chiller unit 60 have been arranged. Additionally, the return fuel cooling circuit is not any more open to the sea or the river or any other source of cooling water but forms a closed circuit of its own. The chiller unit 60 comprises an efficient chilling circuit for cooling the heat exchange medium that is used to cool down the return fuel oil in the return fuel cooler 44. The chilling circuit comprises a compressor 62, a condenser 64, an expansion valve and an evaporator 66 that is arranged in series with the heat exchanger 58 in the cooling circuit for the return fuel oil. In the chilling circuit 60 an ordinary refrigerant (like for instance R407C or R134a) is preferably used as the heat exchange medium.

[0050] Further, a 3-way valve 68 has been arranged in the return fuel cooling circuit 46, 50 between the pump/s 48 and the return fuel cooler 44. The 3- way valve 68 may be used to direct the flow of the heat exchange medium to the return fuel cooler 44 or to by-pass the return fuel cooler 44. It is also possible that the valve 68 may direct a part of the heat exchange medium flow to the cooler 44 and a part to the by-pass line. Thus the function of the 3- way valve 68 is to control the temperature of the circulating oil, whereby the valve 68 is called a temperature control valve.

[0051] Basically the above described cooling/chilling system works as follows. The 3-way valve 52 (preceded preferably by a pump, not shown) is installed upstream of the condenser 64 of the chiller unit 60 to direct the cooling water flow either to the condenser 64 or to the low efficiency heat exchanger 58 along water inlet line 54. The water returning from the low efficiency heat exchanger 58 is returned along line 56 either directly to its original source as shown in Figure 4. The position of the 3-way valve 52 is controlled by a programmable logic controller (PLC) based on signals received from the fuel oil temperature measuring unit 24, the viscosity measuring unit 26 and the position of the temperature control valve 68. [0052] When the chilling system is needed it functions as follows. The 3-way valve 52 directs water to the condenser 64 to which refrigerant as a superheated vapor is pumped at a high pressure by the compressor 62. The refrigerant, being at a high pressure and at a high temperature is cooled in the condenser by transferring the heat of the refrigerant to water so that the refrigerant condenses into a liquid. Thereafter the pressure of the cooled refrigerant is relieved by means of an expansion valve where the refrigerant is partially evaporated. The partial evaporation reduces the temperature of the liquid - vapor refrigerant mixture to where it is colder than the temperature of the enclosed space to be refrigerated. The cold mixture is taken to the evaporator 66 where it cools down the temperature of the heat exchange medium running in the return fuel oil cooling circuit 46, 50 much more efficiently than in the low efficiency heat exchanger 58. At the same time the cold refrigerant mixture is warmed such that it evaporates and turns into vapor form, and is taken back to the compressor. [0053] When running on HFO, the 3-way valve 52 directs the water flow along line 54 to the low efficiency heat exchanger 58 the purpose of which is to cool down the heat exchange medium running in the closed return fuel cooling circuit 46, 50 after the heat exchange medium has been heated by the return fuel oil in the return fuel cooler 44. By cooling heat exchange medium in the return fuel cooling circuit 46, 50 it is avoided that the temperature and pressure is increasing in the refrigerant circuit of the chiller unit 60 whereby it is also avoided that the safety valve opens and reduces the amount of refrigerant in the chilling circuit.

[0054] When changing from HFO to MDF/LFO the heat exchange medium pump/s 48 is/are started by the PLC based on signals concerning the measured fuel oil temperature at 24 and viscosity at 26. When the 3-way valve 52 directs water in the low efficiency heat exchanger 58, a too early start of the chiller unit 60 is avoided, as the closed return fuel cooling circuit 46, 50 is cooled by fresh or sea water. When the cooling capacity of the return fuel cooling circuit 46, 50 by means of fresh or sea water running via the low efficiency heat exchanger 58 is not sufficient, determined by measuring fuel oil temperature at 24, and viscosity at 26 and by detecting the position of the temperature control valve 68, the PLC switches the 3-way valve 52 to direct the water flow towards the condenser 64 and starts the chiller unit 60 i.e. the compressor 62 thereof. [0055] In the following the operation of a fuel feed system in accordance with the first preferred embodiment of the present invention is discussed in more detail.

[0056] Main operation - running on HFO

[0057] The 3-way valve 6 is closed blocking the MDF/LFO flow from the tank 4 onwards either totally or allowing MDF/LFO to reach the 3-way valve 10, and the 3-way changeover valve 10 is in H FO position whereby only HFO is introduced in the fuel feed system. The first 3-way return fuel valve 32 directs the return fuel oil flow towards the return fuel cooling circuit along the return fuel line 36 so that the return fuel oil is finally introduced from return line 18 into the mixing tank 16. The second 3-way return fuel valve 38 is in a position allowing the return fuel oil to by-pass the return fuel cooler 44 along the by-pass line 40. The temperature control valve 68 is in a position allowing the heat exchange medium in the return fuel cooling circuit 46, 50 to by-pass the return fuel cooler 44. The 3-way valve 52 is in a position allowing water from the cooling water source to flow along line 54 to the heat exchanger 58. The trace heating by means of the heater 22 is turned on.

[0058] Switching from HFO to MDF/LFO operation

[0059] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally for instance by turning the HFO - MDF/LFO switch to MDF/LFO position and thus giving a signal to the PLC. The PLC instructs MDF/LFO valve 6 to allow the fuel to flow to valve 10, and the changeover valve 10 to start adding MDF/LFO to HFO. The valve 10 is controlled by the PLC to adjust the proportions of the two fuels such that a desired temperature gradient, i .e. the temperature change rate is reached. A preferred temperature gradient range is 1 - 5°C/min; a more preferred gradient is 2°C/min. The actual temperature rate change is calculated by the PLC from the information signals received from the temperature measuring unit 24 in the fuel supply line 28. The changeover valve 10 control is continued until it is in a position that takes all the fuel from the MDF/LFO tank 4. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. [0060] The temperature gradient of the fuel is maintained in the following manner. The viscosity readings or signals of the viscosity measuring unit, i.e. the viscometer 26 and the temperature values or signals of the temperature measuring unit 24 are recorded by the PLC. If the viscosity of the fuel at 26 is below a set value V11 (for instance <4 cSt) and the temperature of the fuel above a set value T1 (for example >45°C) the second 3-way return fuel valve 38 is adjusted to direct a part of or all of the return fuel oil flow to the return fuel cooler 44, the heat exchange medium pump/s 48 is/are started and the trace heating at 22 is shut off. The 3-way temperature control valve 68 controls the temperature gradient to 2°C/min with the help of signals from the temperature measuring unit 24 to the PLC and from the PLC to the control valve 68 until a borderline viscosity value of VI2 (for instance 3 cSt) is reached, whereafter the viscosity is maintained. If the temperature at 24 is above T1 when viscosity is VI2, the 3-way valve 68 continues to control the temperature gradient to 2°C/min until temperature at 24 is T1 .

[0061] If the viscosity gets below VI2 (for instance <3 cSt) or the temperature remains above T1 and the limit switch of the temperature control valve 68 signals PLC that the valve 68 is continuously in a full cooling position (full heat exchange medium flow into the cooler 44), the PLC instructs the 3-way valve 52 to direct water flow from the cooling water source to the condenser 64 and the chiller unit 60 to start. In other words, the low efficiency heat exchanger 58 does not cool the hear exchange medium in the return fuel cooling circuit 46, 50 efficiently enough. The 3-way valve 68 is thus again used to control the temperature gradient to 2°C/min (with the help of temperature input from 24 to the PLC) until viscosity value of VI2 (for instance 3 cSt) is reached and to maintain this viscosity. If the fuel oil temperature at 24 exceeds T1 (for instance >45°C) when the viscosity is VI2, then the 3-way valve 68 maintains the temperature gradient at 2°C/min until the fuel oil temperature is T1 .

[0062] Running on MDF/LFO

[0063] The 3-way valve 6 and the 3-way changeover valve 10 are in a position allowing the MDF/LFO to enter the fuel feed system. The 3-way valve 38 is in a position directing the return fuel oil to the return fuel cooler 44, the 3-way valve 52 is in a position directing the water flow to the condenser 64, and the 3-way temperature control valve 68 is controlling the viscosity of the return fuel oil to VI2 (for instance 3 cSt). The trace heating at 22 is naturally shut off.

[0064] Switching from MDF/LFO to HFO operation

[0065] The fuel change operation is switched on by a user in the engine control room (ECR) or at the bridge or locally, for instance by turning the HFO - MDF/LFO switch to HFO position and thus giving a signal to the PLC. The PLC turns the trace heating at 22 on directly, and instructs the changeover valve 10 to start adding HFO to MDF/LFO and to adjust, based on signals from the temperature measuring unit 24 to the PLC, the temperature gradient of the fuel oil at 24 to 2°C/min until the fuel is changed entirely over to HFO. For instance, the changeover valve 10 may be provided with a limit switch from the position of which the PLC determines that the valve is continuously fully open. The 3-way temperature control valve 68 controls the viscosity at 26 to VI2 (for instance 3 cSt). If the viscosity VI2 at 26 rises above VI2 (for instance >3 cSt) the chiller unit 60 is stopped and the 3-way valve 52 is turned to direct the water along line 54 to the low temperature heat exchanger 58. If the viscosity VI2 at 26 rises above V11 (for instance >4 cSt), heat exchange medium pump/s 48 is/are stopped and the 3-way valve 38 is turned to allow return fuel oil to by-pass the return fuel cooler 44.

[0066] Figure 5 illustrates a fuel feed system in accordance with a second preferred embodiment of the present invention. The starting point of this embodiment is the embodiment discussed in Figure 4. In the present embodiment an air cooled heat exchanger 158 is installed in the return fuel cooling circuit 46, 50 to replace the water cooled low efficiency heat exchanger 58 of Figure 4. The operation of the air cooled heat exchanger 158 is controlled by the PLC based on the temperature of the heat exchange medium at a temperature measuring unit 160 in the return fuel cooling circuit 46, 50, fuel oil temperature measured at 24, viscosity measured at 26 and the position of the 3-way temperature control valve 68.

[0067] When running on HFO, the PLC controls the cooling capacity of the air cooled heat exchanger 158 by the signals received from the temperature measuring unit 160 in the return fuel cooling circuit 46, 50 and its purpose is to cool down the heat exchange medium in the return fuel cooling circuit 46, 50 that is being heated by means of the fuel oil heat exchanger 44. By cooling the heat exchange medium in the return fuel cooling circuit it is avoided that the temperature and pressure increases in the refrigerant circuit of the chiller unit 60 whereby it is also avoided that the safety valve (not shown) opens and sprays refrigerant out of the circuit. [0068] When changing from HFO to MDF/LFO the PLC instructs the heat exchange medium pump/s 48 to start based on signal containing measured fuel oil temperature at 24 and viscosity at 26. The PLC controls the cooling capacity of the air cooled heat exchanger 158 by signals containing the measured fuel oil temperature and viscosity. As the air cooled heat exchanger 158 is running, a too early start of chiller unit 60 is avoided, as the return fuel cooling circuit 46, 50 is cooled by air. When the cooling capacity of the return fuel cooling circuit by means of the air cooled heat exchanger 158 is not sufficient based on the information from fuel oil temperature, viscosity and the position of the 3-way temperature control valve 68, the PLC instructs the air cooled heat exchanger to be stopped and the chiller unit 60 to start.

[0069] Figure 6 illustrates a fuel feed system in accordance with a third preferred embodiment of the present invention. This embodiment represents more creative thinking as it has been understood that the fuel may be cooled not only in the return fuel line but anywhere along the fuel flow path. In other words, the most significant difference when compared to the earlier two embodiments can be seen when studying the fuel cooling circuit in more detail. In the earlier embodiments the fuel cooling circuit 46, 50 was arranged to cool down the fuel returning from the engine, whereas in the present embodiment the fuel or fuel mixture travelling from the mixing tank 16 towards the engine 2 is arranged to flow, if needed, via the heat exchanger 144 arranged in the fuel cooling circuit 46, 50. In other words, fuel from the mixing tank 16 is taken by pump 20 to a fuel supply line 136, which introduces the fuel to a three-way valve 138 that directs the fuel, when cooling is needed, to the fuel cooler 144 along line 142 or to the by-pass line 140, when no cooling is needed. Thereafter the fuel is introduced in the fuel supply line 28 including the fuel oil heater 22, the temperature measuring unit 24, and the viscosity measuring unit 26, as in the earlier embodiments. Consequently, the fuel returning from the engine 2 is taken directly along line 18 to the mixing tank 16, or guided by the three-way valve 32 back to the tank 4.

[0070] The need of cooling the fuel oil is determined in accordance with the basic principles discussed already in connection with the earlier embodiments. The chiller unit 60 and the low efficiency heat exchanger 58, 158, which may be either water- or air-cooled one, operate just as described earlier. With regard to the fuel oil heater 22 it should be understood that it may be positioned as shown in Figure 6, i.e. downstream of the fuel cooling, but it may as well be arranged soon after the pump 20 i.e. upstream of the fuel cooling. A further option is to arrange a three-way valve in the fuel oil line after the pump 20 such that the fuel may be taken either to the fuel oil heater or to the cooling circuit.

[0071] It should be understood that the above is only an exemplary description of a novel and inventive fuel feed system for an internal combustion engine. It should be understood that the specification above discusses only a few embodiments of the fuel feed system without any purpose to limit the invention to only the discussed embodiments and their details. Thus the above specification should not be understood as limiting the invention by any means but the entire scope of the invention is defined by the appended claims only. From the above description it should be understood that separate features of the invention may be used in connection with other separate features even if such a combination has not been specifically shown in the description or in the drawings.

Claims

1 . A fuel feed system for an internal combustion engine, the fuel feed system comprising at least two tanks (4, 8) for different fuel oil grades; a fuel changeover valve (10); a feed pump (12, 20); a mixing tank (16); a fuel oil supply line (28) having a fuel oil heater (22), a temperature measuring unit (24), and a viscosity measuring unit (26); and a return fuel line (36, 18) from the engine (2) to the mixing tank (16); the fuel oil supply line (28) and the return fuel line (18, 36) forming a fuel flow path, the fuel flow path being further provided with a fuel cooler (44, 144), characterized in that the fuel cooler (44, 144) is a part of a closed cooling circuit (46, 50) that is provided, in addition to the fuel cooler (44, 144) with at least a pump (48), and cooling means (58, 60, 158) for cooling down the heat exchange medium circulating in the fuel cooling circuit (46, 50).

2. The fuel feed system as recited in claim 1 , characterized in that the cooling means is a chiller unit (60) comprising a compressor (62), a condenser (64), an expansion valve and an evaporator (66).

3. The fuel feed system as recited in claim 1 or 2, characterized in that the cooling means is a water- or air-cooled heat exchanger (58, 158).

4. The fuel feed system as recited in any one of the preceding claims, characterized in that the fuel cooler (44, 144) is arranged in series with at least one of the water- or air-cooled heat exchanger (58, 158) and the evaporator (66) of the chiller unit (60) .

5. The fuel feed system as recited in any one of the preceding claims, characterized in a cooling water inlet valve (52) for directing cooling water either to the water-cooled heat exchanger (58) or to the condenser (64) of the chiller unit (60).

6. The fuel feed system as recited in any one of the preceding claims,, characterized in that a three-way temperature control valve (68) is arranged in the fuel cooling circuit (46, 50) for adjusting heat exchange medium flow in the return fuel cooler (44, 144).

7. The fuel feed system as recited in any one of the preceding claims, characterized in that the fuel cooler is a return fuel cooler (44) arranged in the return fuel line (36, 18) returning fuel from the engine (2) to the mixing tank (16).

8. The fuel feed system as recited in any one of the preceding claims, characterized in that the fuel cooler (144) is arranged in a fuel supply line (136) taking fuel from the mixing tank (16) towards the engine (2).

9. The fuel feed system as recited in any one of the preceding claims, characterized in a programmable logic controller (P LC) arranged to collect information from at least the temperature measuring unit (24), and the viscosity measuring unit (26) in the supply line (28) the PLC controlling the operation of the changeover valve (10) based on the collected information.

10. The fuel feed system as recited in claim 9, characterized in that the PLC is connected to the fuel changeover valve (10), the temperature control valve (68), the fuel oil heater (22), the pump (48) in the return fuel cooling circuit (46, 50), the cooling water inlet valve (52), and the chiller unit (60) for controlling their operation.

1 1 . A method of operating a fuel feed system of an internal combustion engine, the fuel feed system comprising at least

• at least two tanks (4,8) for different grades of fuel oil,

• a mixing tank (16),

• a fuel supply line (28), the supply line (28) being provided with a fuel mixture heater (22), a temperature measuring unit (24) and a viscosity measuring unit (26), and

• a return fuel line (36, 18),

• the fuel supply line (28) and the return fuel line (36, 18) forming a fuel flow path,

· the fuel flow path being further provided with a fuel cooler (44, 144), the method comprising the steps of:

• Taking fresh fuel oil from at least one of the tanks (4,8),

• Introducing the fresh fuel oil into the mixing tank (16),

• Mixing the fresh fuel oil with a return fuel recirculated from the engine (2) to the mixing tank (16) to form a fuel mixture, • Taking the fuel mixture to the engine (2) by means of the fuel supply line (28),

• Measuring the temperature of the fuel mixture by the temperature measuring unit (24) and the viscosity by the viscosity measuring unit (26), and

• Returning a part of the fuel mixture from the engine (2) to the mixing tank (16) by means of the return fuel line (36, 18),

• Cooling the fuel oil returning from the engine (2) by means of the fuel cooler (44, 144) arranged in the fuel flow path,

characterized in the steps of, when changing the fuel grade from a high viscosity fuel over to low viscosity fuel:

• circulating in the closed fuel cooling circuit (46, 50) comprising the fuel cooler (44, 144) heat exchange medium for ensu ring a desired temperature gradient of the fuel mixture in the supply line (28), and · cooling the circulating heat exchange medium by means of cooling means (58, 60, 158) arranged in the closed fuel cooling circuit (46, 50).

12. The method as recited in claim 1 1 , characterized in closing the fuel cooling circuit (46, 50) by connecting it to a chiller unit (60) provided with a compressor (62), a condenser (64), an expansion valve, and an evaporator (64), and by arranging the evaporator (64) in series with the fuel cooler (44, 144) in the fuel cooling circuit (46, 50),

13. The method as recited in any one of the preceding claims 1 1 - 12, characterized in by providing the fuel cooling circuit (46, 50) with a low efficiency heat exchanger (58, 158) for cooling the heat exchange medium circulating in the fuel cooling circuit (46, 50).

14. The method as recited in any one of the preceding claims 1 1 - 13, characterized in ensuring the desired temperature gradient of the fuel mixture in the supply line (28) by controlling the capacity of the fuel cooler (44, 144) by adjusting heat exchange medium flow into the fuel cooler (44, 144) by means of a valve (68).

15. The method as recited in claim 14, characterized in controlling, by means of the temperature and/or the viscosity information, the operation of the valve (68).

16. The method as recited in any one of the preceding claims 1 1 - 15, characterized in cooling the return fuel by means of the return fuel cooler (44) arranged in the return fuel line (36, 18).

17. The method as recited in any one of the preceding claims 1 1 - 15, characterized in cooling the fuel mixture by means of the fuel cooler (144) arranged in the fuel supply line (28, 136).

18. The method as recited in any one of the preceding claims 1 1 - 17, characterized in providing the fuel feed system with a programmable logic controller (PLC) for collecting information from at least two of fuel oil temperature and viscosity in the supply line (28), the heat exchange medium temperature (at 160) in the fuel oil cooling circuit (46, 50), the position of the changeover valve (10), the position of the first return fuel valve (32), the position of the second return fuel valve (38) and the position of the temperature control valve (68).

19. The method as recited in claim 18, characterized in controlling the operation of at least one of the changeover valve (10), the fuel oil heater (22), the first return fuel valve (32), the second return fuel valve (38), the temperature control valve (68), the heat exchange pump (48), the inlet cooling water valve (52) and the chiller unit (60) with the PLC and based on the collected information.