DK177572B1 - An internal combustion engine with a combined fuel oil and fuel gas operation mode - Google Patents

Abstract

An internal combustion engine (1), such as a two-stroke crosshead Diesel engine or a four-stroke Diesel engine, comprising cylinders (11) with combustion chambers and a fuel oil supply system (23) and a fuel gas supply system (19) providing a fuel oil control system (20) and a fuel gas control system (30) with fuel oil and fuel gas to be combusted in the combustion chambers of the cylinders (11), and the internal combustion engine comprises a fuel oil operating mode, wherein engine regulation of fuel provided into the combustion chambers of the cylinders (11) is with the fuel oil control system (20), and a combined fuel oil and fuel gas operating mode, wherein the combined fuel oil and fuel gas operating mode, the engine regulation of fuel provided into the combustion chambers of the cylinders (11) is automatically alternated in time between the fuel oil control system (20) and the fuel gas control system (30) so that the combustion in the combustion chambers of the cylinders (11) is based either on fuel oil or fuel gas in consecutive time intervals.

Description

DK 177572 B1

The present invention relates to an internal combustion engine, such as a two-stroke crosshead Diesel engine or a four-stroke Diesel engine, comprising cylinders with combustion chambers and a fuel oil supply system and a fuel gas supply system providing a fuel oil control system and a fuel gas 5 control system with fuel oil and fuel gas to be combusted in the combustion chambers of the cylinders, and the internal combustion engine comprises a fuel oil operating mode, wherein engine regulation of fuel provided into the combustion chambers of the cylinders is with the fuel oil control system, and a combined fuel oil and fuel gas operating mode.

10 There is an increasing interest in lowering carbon dioxide, nitric oxide and sulphur emissions from the operation of internal combustions engines and hence alternative fuel alternatives to the conventional fuel oil has been investigated. Operation of large two-stroke diesel engines such as a MAN Diesel 12K80MC-GI-S has shown that operation with fuel gas as primary fuel 15 may be both safe, reliable and environmentally desirable as to emissions compared to conventional fuel oil. As regards large two-stroke diesel engines for the maritime market, dual fuel engines with both a fuel oil and a fuel gas supply system are becoming increasingly interesting, especially for liquid natural gas carriers (LNG carriers), wherein boil-off gas from the gas tanks is 20 considered as a natural loss during transportation that must be handled.

From JP2009133256 is known a method for controlling a dual fuel internal combustion engine for preventing formation of black smoke, i.e. for reducing emission during operation of the engine when the heating value of the fuel gas is low, wherein a mix of both fuel oil and fuel gas is adjusted and in-25 jected into the combustion chamber at the same time in a predefined operating condition.

W02009/046713 discloses an internal combustion engine wherein the fuel supply to the cylinders of an internal combustion engine with combustion sensors comprises a mixing unit optimizing usage of a mix of different 30 fuel oils having different fuel oil properties.

Before internal combustion engines are taken into use they are tested not only for all load conditions representative for their later use, but DK 177572 B1 2 also at all fuel supply mixes. Testing is carried out to ensure structural thermal integrity of the engine, that the emission levels are within the required limits and that the fuel consumption is as wanted. In a conventional marine engine using only fuel oil this implies testing for a representative set of loads along 5 the propeller curve. In a dual fuel engine using both fuel oil and fuel gas testing is required for a representative set of loads along the propeller curve also covering a representative set of possible mixes of fuel oil and fuel gas. Hereby the dimensionality of the test matrix increases from one to two dimensions.

10 US-A-2003/0187565 describes a diesel engine, which may be oper ated in two modes, the two modes being diesel only mode or dual fuel mode, i.e. gaseous fuel with pilot oil injection. US-A-2003/0187565 furthermore describes automatic switching between these two modes based on a determined ability of the engine to effectively operate in dual fuel mode under pre-15 vailing engine operational characteristics.

US-A-2009/0076712 discloses an internal combustion engine for e.g. a gas tanker. The engine, which is not necessarily a diesel engine, is capable of utilizing both liquid and gaseous fuel.

Based on this it is an object of the present invention to provide an in-20 ternal combustion engine, which takes advantage of both fuel oil and fuel gas combustion and reduces the extent of required engine testing before the engine is taken into operation.

With a view to this the internal combustion engine according to the present invention is characterized in that in that in the combined fuel oil and 25 fuel gas operating mode, the engine regulation of fuel provided into the combustion chambers of the cylinders is automatically alternated in time between the fuel oil control system and the fuel gas control system so that the combustion in the combustion chambers of the cylinders is based either on fuel oil or fuel gas in consecutive time intervals.

30 By alternating the engine regulation of fuel provided into the combus tion chambers of the cylinders between the fuel gas control system and the fuel oil control system in the combined fuel oil and fuel gas operating mode a DK 177572 B1 3 continuous usage of both fuels is obtained. This is desirable seen from a user point of view as the fuel gas provided into the combustion chamber of the cylinders in the combined fuel oil and fuel gas operating mode may be a utilization of boil-off gas from LNG-carrier. Moreover, this ensure a stable operation 5 of the fuel gas supply system that may require a constant release of at least a fraction of boil-off gas in order to keep the system pressure stable. Seen from a testing point of view, the testing of the engine must only be carried out for operation of the engine solely on fuel oil and operation of the engine solely on fuel gas. The time intervals of the combined fuel oil and fuel gas operating 10 mode, wherein the engine regulation of fuel provided into the combustion chambers of the cylinders is either with the fuel oil control system or the fuel gas control system are significantly shorter than the operating period of the engine as such and operating periods in which the engine is operated in the fuel oil operating mode. A desired continuous usage of fuel gas may be 15 achieved with time intervals in the range of seconds and minutes, whereas operating periods in the fuel operating mode is typically in the range of days.

This operation of the engine facilitates testing of the engine which must be carried out at several different loads along the propeller curve to prove the specifications of the engine and compliance with emission legisla-20 tion. Hence the internal combustion engine according to the invention reduces the amount of required tests significantly, because it needs only to be tested at several different loads for each type of fuel, whereas a combined simultaneous e.g. injection of fuel gas and fuel oil would require testing at several different loads within a broad range of possible fuel mixes. The internal com-25 bustion engine according to the invention provides a simple engine regulation of e.g. fuel injection into the cylinders even when several fuel control systems and corresponding fuel supply systems are present, and for a more simple design of the fuel control systems compared to an engine with a combustion based on e.g. injection of a mix of different fuel types into the same combus-30 tion process.

A further advantage of the invention is that an internal combustion engine according to the invention may be achieved by retrofitting a fuel gas DK 177572 B1 4 control system on an existing engine without the need of major modifications of an existing fuel oil control system, because the governor control, i.e. the regulation of critical parameters such as timing of injection and amounts of fuel to be injection is either with the fuel oil control system or with the fuel gas 5 control system.

The fuel gas used in time intervals with combustion based on fuel gas in the combined fuel oil and fuel gas operating mode is preferably gas that naturally will boil-off from the gas tank of a LNG carrier, i.e. the available boil-off gas is not sufficient as the only fuel for the entire operation of an inter-10 nal combustion engine during transport.

In a practically embodiment the internal combustion engine further comprises a fuel gas operating mode. In this operating mode the engine regulation of fuel provided into the combustion chambers of the cylinders is handled by the fuel gas control system. This allows the consumption of fuel oil to 15 be kept at a minimum, which may be desirable as to reduction of particle emissions, but also in periods where large amounts of fuel gas must be released from the fuel gas supply system. This may for instance be the case for large LNG carriers where the amount of gas boil-off in the carrier tanks is highly dependent of the surrounding temperatures. Moreover, all required 20 testing may be carried out by only testing the fuel oil operating mode and the fuel gas operating mode since tests of these modes also suffice as a test for the combined fuel oil and fuel gas operating mode wherein governor control is alternated between the fuel oil control system and the fuel gas control system.

The time intervals of the combined fuel oil and fuel gas operating mode, 25 wherein the engine regulation of fuel provided into the combustions chambers of the cylinders is either with the fuel oil control system or the fuel gas control system are significantly shorter than the operating period as such and operating periods in which the engine is operated in the fuel gas operating mode.

In a further developed embodiment, a pilot fuel oil is provided into the 30 cylinders when the engine regulation of fuel provided into the combustion chambers of the cylinders is controlled by the fuel gas control system. This is known to provide for a better control of the combustion process of compres- DK 177572 B1 5 sion ignition internal combustion engines, but is also desirable for operation when the combustion is based on fuel gas. The pilot fuel causes the initiation of the ignition in the combustion chamber in a manner known per se, but the actual combustion is based on fuel gas provided into the combustion cham-5 bers of the cylinders. Hence a combustion initiator such as a pilot fuel, a laser, a spark or a hot-bulb initiates the gas combustion.

In an even further development embodiment, boil-off gas from the fuel gas supply system is provided into the combustion chambers of the cylinders when the engine regulation of fuel provided into the combustion cham-10 bers of the cylinders is controlled by the fuel oil control system. This will improve operation of the fuel gas control system as it keeps the fuel gas control system at the cylinder cover clean from sealing oil, because the fuel gas supply is kept above zero. This principle may also be applied in the combined fuel gas and fuel oil operating mode, wherein a small fraction of boil-off gas 15 from the fuel gas supply system is provided into the combustion chambers of the cylinders when the engine regulation of fuel provided into the combustion chambers of the cylinders is controlled by the fuel oil control system.

In a preferred embodiment the provision of fuel into the combustion chambers of the cylinders is intermitted for an engine cycle between the con-20 secutive time intervals. Thereby a correct firing order of the individual cylinders when switching the control of provision of fuel into the combustion chambers of the cylinders between different fuel control systems is ensured, because the fuel control systems does not take over the control of the firing sequence of a firing sequence under execution.

25 In a further embodiment the combined fuel oil and fuel gas operating mode is effected cylinder by cylinder so that the cylinders are operating in different modes. In situations where the preferred operating mode is the fuel oil operating mode and the pressure in the fuel gas supply system require that more fuel gas is released than what can be used in the fuel oil operating 30 mode, the pressure may be reduced in a simple and efficient manner by letting the combustion in or more one of the cylinders to be based on fuel gas, whereas the combustion in the remaining cylinders is based on fuel oil. Evi- DK 177572 B1 6 dently, this is more preferred than simply venting the fuel gas supply system in order to reduce the pressure.

In a preferred embodiment the fuel gas supply system comprises a buffer tank smoothing out a discontinuous demand of fuel gas. If for instance 5 the fuel gas is delivered from LNG tanks of an LNG carrier, there will be continuous boil-off from the gas contained in the tank. As the amount of boil-off gas among others vary with the surrounding temperature, the amount of boil-off gas is most likely not in compliance with the amount that may be utilized in the combustion process in the internal combustion engine according to the 10 invention.

In a further development of this embodiment the time intervals are predetermined. Hence the time intervals may be predetermined to fulfil certain demands by the owner of the ship, e.g. in order to keep emissions at a minimum. The time intervals may also be determined based on the configuration 15 of the buffer tank and/or the rate at which the fuel gas supply system can provide fuel gas to the fuel gas control system. At a given temperature the boil-off rate is relatively constant, but when for instance the temperature changes, the time intervals may be adjusted during operation of the internal combustion engine. Such adjustment may be combined with measurements of the 20 amount of fuel gas in the buffer tanks.

In an even further development of this embodiment the time intervals are determined based on a measurement of the amount of fuel oil and/or fuel gas available in the fuel supply systems. Hence the determination of the time intervals may also be determined by taking into account the total amount of 25 available fuel.

With specific demands as to for instance emission rates it is advantageous that the time intervals are determined in order to operate the engine with a fixed amount of fuel gas as primary fuel. This may also ensure that a certain amount of fuel gas e.g. from an LNG carrier is used during transport of 30 LNG if it is desirable to utilize a larger extent of the content of the LNG tank the inevitable boil-offgas.

As an even further alternative the time intervals are determined in or- DK 177572 B1 7 der to operate the engine with a fixed amount of fuel oil as primary fuel. This may be used to keep the consumption of the fuel oil at a minimum, e.g. a certain level determined by the owner of the ship.

In a further embodiment the internal combustion engine comprises a 5 plurality of fuel control systems. This allows for instance the combustion in the fuel oil operating mode to be based on different fuel oils having different fuel oil properties. The different fuel oils may be mixed prior to e.g. injection by a mixing unit or may e.g. be injected by a further independent fuel control system so that the engine regulation of fuel provided into the combustion cham-10 bers of the cylinders is automatically alternated in time between a plurality of fuel control systems.

In a practical embodiment the combustion in operating modes comprising gas combustion is that of a lean burn gas engine employing premixed combustion, i.e. fuel gas is provided into the combustion chambers via scav-15 enge air or by injection early in the compression stage.

Examples of the present invention and embodiments thereof are in the following described in more detail with reference to the highly schematic drawing, in which

Fig. 1 is a general view of an engine according to the present inven- 20 tion,

Fig. 2 is an example of an engine a combined fuel oil and fuel gas operating mode according to the invention,

Figs. 3a, 3b and 3c illustrate different operating modes according to the invention.

25 An internal combustion engine 1 according to a preferred embodi ment of the present invention may be a two-stroke crosshead diesel engine as illustrated in Fig. 1. Such an engine 1 can e.g. be of the make MAN Diesel and the type MC or ME, or of the make Wårtsilå of the type Sulzer RT-flex or Sulzer RTA, or of the make Mitsubishi Heavy Industries. An engine of this 30 type is a large engine typically used as a main engine in a ship or as a stationary engine in a power plant. The cylinders can e.g. have a bore in the range from 25 cm to 120 cm, and the engine can e.g. have a power in the DK 177572 B1 8 range from 3000 kW to 120.000 kW. The engine speed is typically in the range from 40 rpm to 250 rpm. The engine according to the invention can alternatively be a four-stroke diesel engine with an engine speed e.g. in the range from 300 rpm to 1400 rpm, and an engine power e.g. in the range from 5 1300 kW to 30.000 kW. The compression ignition internal combustion en gines according to the present invention are typically capable of using heavy fuel oil as primary fuel.

The engine 1 of Fig. 1 has a plurality of cylinders with a cylinder liner 2 mounted in a cylinder section 3 of an engine frame 4. An exhaust valve 10 housing 5 is mounted in a cylinder cover 6 and an exhaust gas duct 7 extends from the individual cylinder to an exhaust gas receiver 8 common to several or all cylinders. In the exhaust gas receiver pressure variations caused by the exhaust gas pulses emitted from the exhaust gas ducts are equalized to a more even pressure, and one or more turbochargers 9 receive exhaust gas 15 from the exhaust gas receiver 8 and deliver compressed air to a scavenge air system comprising at scavenge air receiver 10 which, like the exhaust gas receiver, is an elongated pressure vessel.

In the individual cylinder a piston is mounted on a piston rod that is connected with a crank pin on a crankshaft via a crosshead and a connecting 20 rod (not illustrated). The piston is connected directly to the connecting rod if the engine is a four-stroke engine. A fuel injector injects the fuel into a combustion chamber where it auto-ignites because of the high temperature in the air above the piston. The high temperature is present because the piston has compressed the inlet air during the upward compression stroke. However, the 25 invention is generally applicable for internal combustion engines based on premixed combustion, i.e. with a homogenous distribution of fuel in the combustion chamber.

In Fig. 2 cylinder section 3 is illustrated with only a single cylinder 11, but the engine has a plurality of cylinders, such as from 4 to 15 cylinders, 30 when it is a two-stroke internal combustion engine, and such as from 4 to 20 cylinders when it is a four-stroke engine. As schematically shown in Fig. 2 the internal combustion engine 1 comprises a fuel oil supply system 23 and a fuel DK 177572 B1 9 gas supply system 19 providing a fuel oil control system 20 and fuel gas control system 30 with fuel oil and fuel gas to be provided into the combustion chambers of the cylinders 11. In the present example the fuel oil control system 20 and the fuel gas control system 30 controls injection of fuel oil and fuel 5 gas into the combustion chambers of the cylinders 11, respectively. The general principle of the fuel control systems 20, 30 is that each cylinder 11 is associated with a cylinder control unit 12 controlling one or more fuel dosing devices 15, 16 such as fuel pumps or valves connected to fuel injectors 13, 14 in the cylinder cover 6. The number of injectors 13, 14 per cylinder de-10 pends on the power of the cylinder 11. Each cylinder comprises at least a fuel oil injector 13 and a fuel gas injector 14. In smaller engines a single injector per fuel type may be sufficient to inject the amount of fuel required for one combustion process, whereas in larger, more powerful engines two or three injectors for each fuel type may be required. When several injectors are pro-15 vided per cylinder 11, there may be one fuel dosing device 15,16 per injector 13, 14. The engine control unit 12 of the fuel control system 20, 30 are in turn controlled by an engine control unit 17 in communication with the bridge of a vessel.

It is preferred that the fuel gas supply system 19 is connected to a 20 liquid natural gas (LNG) tank 18 of a LNG carrier vessel operated at sea. The LNG tank of an LNG carrier vessel is kept at low temperatures, but is inevitably heated as external heat from the seawater and atmosphere is transferred through the insulation of tanks. By the intrusion of the external heat, a portion of the LNG is gasified, i.e. boiled off, and the tank pressure gradually increas-25 es. In order to keep the tank pressure at an acceptable level a reliquefaction system (not shown) may be used to reliquefy boil-off gas. Alternatively, or in combination with the reliquefaction system, a boil-off gas compressor may provide high-pressure boil-off gas when such is ordered by the fuel gas control system 30. At the cylinders a fuel gas dosing device 16 controlled by the 30 cylinder control unit 12 effect the timing and opening of the fuel gas injector 14. A buffer tank 22 is used for storage of boil-off gas before provided to the fuel gas control system 30 by the fuel gas supply system 19. The amount of DK 177572 B1 10 inevitable boil-off gas in an LNG tank of a LNG carrier is normally not sufficient as the only fuel for the operation of the internal combustion engine of the LNG carrier, but the amount of boil-off gas may advantageously be used in combination with fuel oil in an internal combustion engine according to the 5 invention.

In the fuel oil control system 20 the fuel oil dosing device 15 may be a fuel pump, and in that case the fuel oil supply system 23 needs only deliver fuel oil from a fuel oil tank 21 to the fuel dosing device at a relatively low feeding pressure in a fuel feeding pipe 24, such as a pressure in the range from 2 10 bar to 15 bar. Alternatively, the fuel oil dosing device 15 may be a valve or a valve in connection with a metering device, and the fuel feeding pipe is then a high-pressure pipe in which the fuel is at a pressure higher than the injection pressure, such as a feeding pressure in the range of 500 bar to 1500 bar. Such a fuel oil supply system 23 is called a common-rail system. In either 15 case, the fuel oil dosing device 15 is connected to fuel feeding pipe 24 by a branch conduit with a valve that is maintained in open position during normal engine operation. Fuel oil dosing device 15 is connected to fuel oil injectors 13 via high-pressure fuel oil conduits. A return conduit leads from the fuel oil injectors to a fuel oil return line (not shown). The fuel oil provided to the cylin-20 ders is typically heavy fuel oil or marine diesel oil.

An internal combustion engine 1 according to the invention may be provided by installing a fuel gas supply system 19 and a fuel gas control system 30 on an existing engine with a fuel oil supply system as described above. In order to simplify installation and generally to make the control strat-25 egy of the combustion process least complex, it is generally preferred that independent cylinder control units 12a, 12b control the amount of fuel oil and the fuel gas provided into the combustion chambers of the cylinders 11, respectively. Likewise, both the fuel oil control system 20 and the fuel gas control system 30 comprise independent engine control units 17a, 17b. Evidently, 30 as known in the art of marine diesel engine at least the fuel oil control system 20 is preferably redundant, viz. a further engine control unit with redundant cabling to the cylinder control units is present. When operating an internal DK 177572 B1 11 combustion engine according to the invention, the engine regulation of fuel control is either with the fuel oil control system 20 or the fuel gas control system 30. Generally, the engine control unit 17 receives an engine speed signal and other engine operating parameters from sensors, and controls the 5 amount and rate of fuel to be provided into the combustion chambers of the cylinders 11, which is also known as the engine’s governor control. The internal combustion engine according to the invention may be operated in a fuel oil operating mode, fuel gas operating mode and a combined fuel oil and fuel gas operating mode. The operating modes may be ordered from the 10 bridge of a ship. Switching between operating modes is performed by an engine switch unit connected to the fuel oil control system and the fuel gas control system, preferably the engine switch unit is connected to the fuel gas supply system 19. In the combined fuel oil and fuel gas operating mode the engine switch control unit 25 determines if the critical timing and regulation of 15 the combustion process in the internal combustion engine is with the fuel oil control system 20 or the fuel gas control system 30, i.e. the engine switch unit 25 switches the governor control between the fuel oil control system 20 and the fuel gas control system 30. Hence in the combined fuel oil and fuel gas operating mode, operation is automatically alternated between the fuel oil op-20 erating mode and the fuel gas operating mode.

In a practical embodiment the engine switch unit 25 is a part of the fuel gas control system 30 and communicates to the engine control units 17a, 17b when the responsibility of the governor control is to be switched between the fuel control systems 20, 30. A switch between the fuel control systems 20, 25 30 may for instance be effected by communication to the respective engine control units 17a, 17b based on inputs from the fuel gas supply system about the level of available boil-off gas in the buffer tank 22. The switch between the fuel control systems 20, 30 may also be effected according to predetermined time intervals, or a leak detected in the fuel gas control system 30. Generally, 30 the fuel gas supply system 19 may trigger the engine switch unit 25 to provide the fuel oil control system 20 with the governor control and abandon the combined fuel oil and fuel gas operating mode or the gas operating mode and DK 177572 B1 12 continue the engine operation in the fuel oil operating mode.

Fig. 3a illustrates the fuel consumption in the fuel oil operating mode wherein the combustion process is based on provision of fuel oil into the cylinders as a main fuel and the governor control is with the fuel oil control sys-5 tern 20. The amount of fuel oil spent is indicated by the area 40 representing the fuel index varying over time, i.e. the amount of fuel oil to be provided into the cylinders and which slowly change over time among others in dependence of the engine load. A portion of the graph is represented as a hatched area 41, which represent a small amount of boil-off gas from the fuel gas 10 supply system 30. In order to keep the pressure of the fuel gas supply system 30 within the specified limits, it is preferred to inject a small amount of the boil-off gas into the cylinders even when the engine is operated in the fuel oil operating mode. However, the governor control is entirely with the fuel oil control system 20. This small amount of fuel gas may also vary over time and 15 it is a very small amount compared to the primary fuel used for combustion in the combustion chambers of the cylinders, and may e.g. be injected into an ongoing combustion process based on the fuel oil.

Fig. 3b illustrates the use of fuel gas in the fuel gas operating mode wherein the combustion process is based on provision of fuel gas into the 20 cylinders as the main fuel and the governor control is with the fuel gas control system 30. The amount of fuel gas spent is indicated by the hatched portion 50 representing the fuel index varying over time, i.e. the amount of fuel gas to be provided into the combustion chambers of the cylinders and which slowly change over time among others in dependence of the engine load. The un-25 hatched portion 51 represents a pilot fuel oil, which also is present when the engine is operated in the fuel oil operating mode of Fig. 3a, and which is preferred to control the combustion process. Generally, the pilot fuel is injected into the combustion chamber before e.g. injection of the primary fuel.

Fig. 3c illustrates the fuel consumption in the combined fuel oil and 30 fuel gas operating mode, wherein the governor control is automatically shifted by the engine control unit 25 between the fuel oil control system 20 and the fuel gas control system 30 in consecutive time intervals. The hatched areas DK 177572 B1 13 50, 41 represent the amount of fuel gas spent in a time interval, and the unhatched areas 51,40 represent the amount of fuel oil spent in a time interval. Again the small portions 41, 51 represent a small fraction boil-offgas and pilot fuel oil, respectively, as explained above. The combustion is based on a sin-5 gle fuel type as primary fuel and a continuous use of the fuel gas over time is achieved by switching the governor control between the fuel oil control system 20 and the fuel gas control system 30 so that the different fuel types are provided into the combustion chambers of the cylinders in consecutive time intervals. Evidently, the fuel oil provided into the combustion chambers of the 10 cylinders 11 may be a mix of different fuel oils, and the fuel gas may be a mix of different fuel gasses present in e.g. natural gas. During a time interval e.g. several fuel injections of the fuel type used as primary fuel are performed and hence the internal combustion engine takes several revolutions during a time interval. The time intervals may be in the range of 1 to 10 minutes and event 15 up to about 1/4 to 1 hour depending on the buffer tank volume and the amount of fuel spent. Evidently, the time intervals will change with the changing rate of boil-off gas in a LNG tank, which also may be increased actively so that a larger amount of the LNG is used for the operation of the internal combustion engine.

20 Preferably, the length of the time intervals is predetermined so that the automatic shift of the governor control is optimized for specific operating conditions of the internal combustion engine. The time intervals are therefore preferably determined based on the size of the buffer tank 22, which again is configured with respect to the general fuel gas tank, such as a LNG. When 25 the governor control is with the fuel oil control system 20, boil-off gas from the fuel gas tank is stored in the buffer tank 22, and when the governor control is with the fuel gas control system 30, the boil-off gas stored in the buffer tank 22 is used as fuel gas. Evidently, the specific amount of boil-off gas stored in the buffer tank may be used to determine the time intervals, and even further 30 the time intervals may be determined based on a measurement of the amount of fuel oil and fuel gas available in the fuel supply systems 20, 30. As the boil-off rate of the gas tank 18 feeding the fuel gas supply system 19 may change DK 177572 B1 14 over time, a corresponding adjustment of the time intervals may be performed.

In an embodiment the combined fuel oil and fuel gas operating mode may be effected on a cylinder basis so that the cylinders 11 are operating in 5 different modes. This embodiment is easy to implement because the engine control unit 17b need only to control one cylinder control unit 12b to perform such operation.

With the present invention it has been recognised that a practical simple testing of an internal combustion engine and a controlled usage of dif-10 ferent fuel types may be achieved without mixed fuel types, but by automatically shifting the governor control between different types of fuel control systems.

Claims (13)

An internal combustion engine (1), such as a two-stroke cross-head diesel engine or a four-stroke diesel engine, comprising cylinders (11) with combustion chambers and a fuel oil supply system (23) and a fuel gas supply system (19) which supplies a fuel oil control system (20) a fuel gas control system (30) with fuel oil or fuel gas to be combusted in the pre-combustion chambers of the cylinders (11), wherein the combustion engine comprises a fuel oil operating state in which the engine control of fuel delivered into the combustion chambers of the cylinders (10) 20) and a combined fuel oil and fuel gas mode, characterized in that in the combined fuel oil and fuel mode, the engine regulation of fuel delivered into the combustion chambers of the cylinders (11) is automatically switched between fuel oil control system (11) and fuel the system (30) so that for the firing in the combustion chambers of the cylinders (11) for successive time intervals is either based on fuel oil or fuel gas. An internal combustion engine (1) according to claim 1, wherein the internal combustion engine further comprises a fuel gas operation state. An internal combustion engine (1) according to claim 2, wherein a pilot fuel oil is supplied into the cylinders (11) when the engine control of fuel supplied into the combustion chambers of the cylinders (11) is controlled by the fuel gas control system (30). An internal combustion engine (1) according to claims 1 to 3, wherein boil-off gas is supplied from the fuel gas supply system (19) into the combustion chambers of the cylinders (11), when the engine control of fuel supplied into the combustion chambers of the cylinders (11), controlled by the fuel oil control system (20). An internal combustion engine (1) according to claims 1 to 4, wherein the delivery of fuel into the combustion chambers of the cylinders (11) is interrupted in a motor cycle between successive time intervals. An internal combustion engine (1) according to any preceding claim, wherein the combined fuel oil and fuel gas operating state occurs cylinder by cylinder, so that the cylinders (11) operate in different states. An internal combustion engine (1) according to any preceding claim, wherein the fuel gas supply system (19) comprises a buffer tank (22) for equalizing a discontinuous fuel gas demand. The internal combustion engine (1) according to claims 1 to 7, wherein the time intervals are predetermined. An internal combustion engine (1) according to claims 1 to 7, wherein the time intervals are determined on the basis of a measurement of the amount of fuel oil and / or fuel gas available in the fuel supply systems (19, 23). An internal combustion engine (1) according to claims 1 to 7, wherein the time intervals are intended to operate the engine with a fixed amount of fuel gas as primary fuel. An internal combustion engine (1) according to claims 1 to 7, wherein the time intervals are intended to operate the engine with a fixed amount of fuel oil as primary fuel. An internal combustion engine (1) according to any preceding claim, comprising multiple fuel supply and fuel control systems (23, 20 20; 19.30). An internal combustion engine (1) according to any preceding claim, wherein the combustion in operating conditions comprising gas combustion is as for a lean burn gas engine utilizing premixed combustion. 25