DK177476B1

DK177476B1 - An internal combustion engine with variable fuel injection profile

Info

Publication number: DK177476B1
Application number: DKPA201270380A
Authority: DK
Inventors: Anders Andreasen
Original assignee: Man Diesel & Turbo Deutschland
Priority date: 2012-06-29
Filing date: 2012-06-29
Publication date: 2013-07-01
Also published as: KR20140002502A; KR101371403B1; CN103511103A; CN103511103B; JP2014058962A; JP5583825B2

Abstract

An internal combustion engine comprising a fuel gas supply system and cylinders provided with fuel gas injectors for injecting fuel gas directly into combustion chambers in the cylinders in a fuel gas operating mode, and a fuel oil supply system, and an engine control unit controlling the fuel injection into the cylinders by adjusting the duration of the fuel injection, where the fuel injection in the fuel gas operating mode comprises a pilot fuel oil injection period (51) and a main fuel injection period (52) separated by a dwell period (53) without fuel injection. In response to variations in the heating value of the fuel gas provided to the cylinders in the fuel gas operating mode, the engine control unit adjusts the timing of the pilot fuel oil injection period (51) and the dwell period (53), and accordingly adjusts the duration of the main fuel injection period (52) with injection of fuel gas.

Description

DK 177476 B1

The present invention relates to an internal combustion engine, such as a two-stroke crosshead Diesel engine, comprising a fuel gas supply system and cylinders provided with fuel gas injectors for injecting fuel gas directly into combustion chambers in the cylinders in a fuel gas operating mode, and 5 a fuel oil supply system providing fuel oil injectors at the cylinders with fuel oil for injection directly into the combustion chambers in the cylinders in a fuel oil operating mode, and an engine control unit controlling the fuel injection into the cylinders by adjusting the duration of the fuel injection, where the fuel injection in the fuel gas operating mode comprises a pilot fuel oil in-10 jection period and a main fuel injection period separated by a dwell period without fuel injection.

There is an increasing interest in lowering carbon dioxide, nitric oxide and sulphur emissions from the operation of internal combustions engines and hence alternatives to the conventional fuel oil have been investigated.

15 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 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, engines using fuel gas are becoming increasingly interesting, 20 especially for liquid natural gas carriers (LNG carriers), wherein boil-off gas from the gas tanks must be handled during transportation.

Hence it is desirable to use natural gas and/or boil-off gas from the gas tanks of LNG carriers for operation of such engines. However, the calorific value of fuel gas from a gas tank of a LNG carrier may vary over time, e.g.

25 because the boil-off gas also comprises e.g. nitrogen, which reduces the energy density of the fuel gas provided to the cylinders. Likewise, variations of the temperature of the fuel gas provided to the cylinders result in a variation in the energy density of the fuel gas injected into the combustion chambers of the cylinders.

30 When the energy density of the fuel gas injected into the cylinders differ from the energy density of the reference fuel gas used for testing the engine and determining a fuel index of the engine, i.e. the amount of fuel to be injected into the cylinders at a given load to obtain a desired engine speed, it is necessary to adjust the fuel index in order to maintain the engine 35 speed and output of the engine. Hence variations in the energy density of the DK 177476 B1 2 fuel gas require more fuel gas to be injected into the engine, which is achieved by adjusting the fuel gas injection time.

Moreover, there is a need to change the operation mode of the dual fuel supply engines of the above-mention kind between the fuel gas operating 5 mode and the fuel oil operating mode and vice verse, without impairing the overall operation of the engine.

From WO2010/139572 is known a method and a device for operating a diesel engine with a dual fuel supply system, wherein the fuel injection comprises at least one pilot injection of fuel oil and a main injection of fuel 10 oil, and where a part of the main injection is replaced by fuel gas.

US57112709 describes a combustion engine operating on dual fuels and comprising a fuel gas supply system, where the amount of fuel gas is changed by changing the fuel gas injection diration, and where the timing of the injection is changed in order to assure that maximum cylinder pressure 15 will occur with correct timing with respect to the engine's working cycle.

It is a purpose of the present invention to provide an internal combustion engine with dual fuel supply with a flexible fuel injection profile.

With a view to this the present invention is characterized in that in response to variations in the heating value of the fuel gas provided to the 20 cylinders in the fuel gas operating mode, the engine control unit adjusts the timing of the pilot fuel oil injection period and the dwell period, and accordingly adjusts the duration of the main fuel injection period with injection of fuel gas.

By adjusting the timing of the pilot fuel oil injection period and the 25 following dwell period, and the duration of the main fuel injection period where fuel gas is injected in the fuel operating mode, the engine is operated at optimal conditions with the maximum combustion pressure regardless of the heating value of the fuel gas.

In the fuel gas operating mode an injection profile comprise a pilot 30 fuel oil injection and a main fuel injection separated by a dwell time without any injection of fuel, and the main fuel injection is injection of fuel gas. The duration of the pilot fuel oil injection is so short that it does not govern maximum combustion pressure achieved in the combustion chamber during combustion, which is controlled by the duration and timing of the main fuel 35 injection where fuel gas is injected into the combustion chamber. Hence it is 3 DK 177476 B1 possible to adjust the timing of the pilot fuel oil injection period and the dwell period used in the fuel gas operating mode, without affecting the main fuel injection period of fuel oil in the fuel oil operating mode. This makes it possible to change between operating modes with an unchanged maximum com-5 bustion pressure even with an abrubt safety shut-down of the fuel gas supply system, because in both the fuel gas operating mode and the fuel oil operating mode the maximum combustion pressure is controlled by the main fuel injection period, i.e. with the same fuel injection profile governing the combustion pressure.

10 The pilot fuel oil injection serve to ignite the fuel gas injected in the main fuel injection period. Due to a high pressure in the combustion chamber the fuel oil injected in pilot fuel oil injection period ignite and fuel gas injected into the combustion chamber in the main fuel injection period is therefore injected into an ongoing combustion process or a sufficiently preheated envi-15 ronment which will ensure ignition of even with poor ignition properties of the fuel gas.

In the fuel oil operating mode the injection profile comprise a pilot fuel oil injection and a main fuel injection separated by a dwell time without any injection of fuel, and the main fuel injection is injection of fuel oil. Since 20 the fuel oil injected into the combustion chamber in the main fuel injection period ignite due to the pressure in the combustion chamber, the pilot fuel oil injection may be dispensed with in the fuel oil operating mode.

In a preferred embodiment, in response to a decrease in the heating value of the fuel gas provided to the cylinders in the fuel gas operating mode, 25 the engine control unit advance the timing of the pilot fuel oil injection period and the dwell period, and advance the timing and the duration of the main fuel injection period with injection of fuel gas. When the heating value of the fuel gas is lower than the heating value of a reference gas used for testing the engine less energy is injected into the combustion chamber per time unit.

30 Therefore by advancing the timing of the pilot fuel oil injection period and the dwell period it is possible to increase the duration of the main fuel injection period so that the desired amount of energy is injected into the combustion chamber. Thereby the fuel gas is injected earlier into the combustion chamber and the combustion pressure is maintained even with a decrease in the 35 heating value of the fuel gas.

4 DK 177476 B1

In a further developed embodiment of the invention, in response to a decrease in the heating value of the fuel gas provided to the cylinders in the fuel gas operating mode, the engine control unit decrease the duration of the dwell period so that the duration of the main fuel injection period with injec-5 tion of fuel gas is increased. It is possible to reduce the duration of the dwell period until the timing of the pilot fuel oil injection and the main fuel injection period with injection of fuel gas is the same. Beyond that point the timing of the main fuel injection period with injection of fuel gas cannot be advanced further because the fuel gas need to be ignited by the pilot fuel oil injection.

10 In an embodiment of the invention the engine control unit change between the fuel oil operating mode and the fuel gas operating mode by progressively reducing the duration of fuel oil injection in a main fuel injection period of the fuel oil operating mode and correspondingly increasing a period with fuel gas injection. When fuel gas operating mode is ordered a fuel gas 15 supply pressure is ordered from the fuel gas supply system by the engine control unit. In order to obtain a stable operation of the fuel gas supply system the amount of fuel gas delivered at the required pressure is gradually increased.

In a further embodiment that provides the same advantages as to a 20 stable operation of the fuel gas supply system, the engine control unit change between the fuel gas operating mode and the fuel oil operating mode by progressively reducing the duration of fuel gas injection in the main fuel injection period of the fuel gas operating mode and correspondingly increasing a period with fuel oil injection.

25 In a practically preferred embodiment the engine control unit change between the fuel oil operating mode and the fuel gas operating mode by progressively reducing the duration of fuel oil injection in the main fuel injection period of the fuel oil operating mode with about 0.1 % of the main fuel injection period per cylinder per engine revolution and correspondingly increasing 30 a period with fuel gas. Depending to the engine speed this allows for a change between the fuel gas operating mode and the fuel oil operating mode and vice verse within 2 min to 10 minutes.

In a preferred embodiment the fuel gas injection pressure is in the range of 150 bar to 300 bar. Hence the fuel gas supply system is a high pres-35 sure fuel gas supply system from which an adjustment of the fuel injection 5 DK 177476 B1 pressure may be ordered by the engine control unit.

In a further embodiment, at 100 % engine load the engine speed is in the range from 45 rpm to 175 rpm.

In a practically preferred embodiment the fuel gas supply system 5 providing fuel gas to the cylinders is connected to a liquid natural gas tank of a liquid natural gas carrier. This allows for utilization of e.g. boil-off gas having a varying heating value. Evidently, the fuel gas supply system may be connected to a dedicated fuel gas tank, i.e. with a separation of the cargo and the fuel gas.

10 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 invention, 15 Fig. 2 is an example of an engine a combined fuel oil and fuel gas operating mode according to a preferred embodiment of the invention,

Fig. 3-7 schematically shows fuel injection profiles according to the invention.

An internal combustion engine 1 according to a preferred embodi-20 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 type is a large engine typically used as a main engine in a ship or as a sta-25 tionary 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 range from 3000 kW to 120.000 kW. The engine speed is typically in the range from 40 rpm to 250 rpm. The compression ignition internal combustion engines according to the present invention are typically capable of using 30 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 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 35 several or all cylinders. In the exhaust gas receiver pressure variations 6 DK 177476 B1 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 from the exhaust gas receiver 8 and deliver compressed air to a scavenge air system comprising at scavenge air receiver 10 which, like the 5 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 rod (not illustrated). A fuel injector injects the fuel into a combustion chamber. When the injected fuel is fuel oil it auto-ignites because of the high tem-10 perature in the air above the piston. The high temperature is present because the piston has compressed the inlet air during the upward compression stroke.

It is preferred that the present invention is implemented in a marine diesel engine with a dual fuel supply system, and in the following the inven-15 tion is described by such example, but of course the invention may be implemented as a single fuel system. The engine is an electronically control engine, which electronic control of both oil and gas injection, ensuring that the fuel injection and combustion is optimized. Further it is based on a high pressure gas injection principle with pilot fuel oil injection for igniting the fuel gas 20 combustion. With this principle the diesel combustion process can be fully utilized and thereby the same high thermal efficiency as for fuel oil combustion can be obtained. 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. As schematically shown in Fig. 2 the internal combustion engine 25 1 comprises a fuel oil supply system 23 and a fuel gas supply system 19 pro viding a fuel oil injection system 20 and fuel gas injection 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 injection system 20 and the fuel gas injection system 30 controls injection of fuel oil and fuel gas into the combus-30 tion chambers of the cylinders 11, respectively. The general principle of the fuel injection 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 depends on the power of 35 the cylinder 11. In a preferred embodiment each cylinder comprises at least a 7 DK 177476 B1 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 5 provided per cylinder 11, there may be one fuel dosing device 15, 16 per injector 13, 14. The engine control unit 12 of the fuel injection 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 10 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 15 increases. 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 injection system 30. At the cylinders a fuel gas dosing device 16 controlled by 20 the cylinder control unit 12 effect the timing and opening of the fuel gas injector 14. The fuel gas is preferably provided to the fuel gas injection system by a double-walled gas supply piping 26 of the common rail design, where a valve of the fuel gas injector 14 is controlled by an auxiliary control oil system. This, in principle, consists of a hydraulic control oil system and an elec-25 tronic gas injection valve, supplying high-pressure control oil to the gas injector 14, thereby controlling the timing and opening of a gas valve of the gas injector 14. The fuel gas supply system is a high pressure fuel gas supply system. An efficient gas injection is obtained when the gas delivery pressure is between 150 to 300 bar depending on the engine load, and the fuel gas 30 has a temperature about 45° Celsius. A buffer tank 22 is used for storage of boil-off gas before provided to the fuel gas injection system 30 by the fuel gas supply system 19. The amount of 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 35 gas may advantageously be used in combination with fuel oil in an internal 8 DK 177476 B1 combustion engine according to the invention. The operation of the fuel gas injection system 30 is insensitive to the gas composition as well as the variation in the gas composition. The dedicated fuel gas tank may then be an additional fuel gas pressure tank on top of the deck, which is fully separated 5 from the cargo. Hence, also liquefied petroleum gas (LPG) that normally consists of higher hydrocarbons like propane and butane may like LNG apply as fuel gas without changing the engine's performance in terms of speed, thermal efficiency and power output, while maintaining the same rating as for fuel oil. The pressure necessary to achieve full atomization of the fuel gas is 10 then about 550 bar and a temperature about 35° Celsius is preferred.

The fuel gas supply system may receive fuel gas directly from the cargo of a ship or from a dedicated fuel tank which may be a fuel tank on the deck of a ship. And the fuel gas supply system may receive fuel gas from a boil-off gas reliquefaction system e.g. from the manufacturer Hamworthy or 15 directly from the cargo or a dedicated fuel gas tank. The fuel gas supply system may also be from the manufacturer Hamworthy and generally comprises a booster pump, a high-pressure pump and a heater unit. After pumping the fuel gas to the required pressure by the booster pump to the required pressure, the fuel gas is heated to the required temperature and then provided to 20 the engine cylinders via the double-walled gas supply piping 26. An additional booster pump may be provided after heating of the fuel gas to fuel adjust the fuel gas pressure to the desired level.

In the fuel oil injection 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 de-25 liver 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 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 30 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 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 35 oil injectors 13 via high-pressure fuel oil conduits. A return conduit leads from 9 DK 177476 B1 the fuel oil injectors to a fuel oil return line (not shown). The fuel oil provided to the cylinders 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 sys-5 tern 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 strategy 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, 10 respectively. Likewise, both the fuel oil control system 20 and the fuel gas control system 30 comprise independent engine control units 17a, 17b. Evidently, 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 15 internal combustion engine according to the invention, the engine regulation of fuel injection is either with the fuel oil injection system 20 or the fuel gas injection system 30. Generally, the engine control unit 17 receives an engine speed signal and other engine operating parameters from a sensor arrangement 40, and controls the amount and rate of fuel to be provided into 20 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 bridge of a ship. Switching between 25 operating modes may be performed by an engine switch unit 25 connected to the fuel oil injection system 20 and the fuel gas control system 30, and preferably the engine switch unit is a part of 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 the combustion 30 process in the internal combustion engine is with the fuel oil injection system 20 or the fuel gas injection 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 operating 35 mode and the fuel gas operating mode. Switching between the operating 10 DK 177476 B1 modes may be ordered from the bridge or for safety reasons in case of a gas leak, which immediately will switch the engine to the fuel oil operating mode.

In the fuel gas operating mode the fuel gas injection and the timing of the combustion process is controlled by the fuel gas injection system 30.

5 This is done according to a predetermined engine load index specifying a fuel injection length and a fuel injection pressure required to operate the engine optimally at a given engine load. Before taken into operation the engine is tested at different loads along the propeller curve to determine the optimum fuel injection length and fuel injection pressure at different loads. Hence 10 when the engine control unit makes a look-up in the engine maps for finding e.g. the correct timing of fuel gas injection, timing of opening and closing of the exhaust valve and the hydraulic pressure, it is dependent on both an ordered fuel injection length and an ordered fuel injection pressure required to operate the engine at a desired engine load.

15 Fig. 3 shows a fuel injection profile 50 according to the invention.

The x-axis represents the position of the crankshaft where zero corresponds to the top dead centre of the piston in the individual cylinders. Therefore, the x-axis also schematically represents the timing of the injection of fuel with zero representing the start of the fuel injection profile. The y-axis represents 20 the rate of injection of energy i.e. energy per time unit. The injection profile 50 consists of a pilot fuel oil injection period 51, which is a pre-injection of fuel oil before a main fuel injection period 52. Typically the pilot fuel oil injection period 51 is about 5 % of the main fuel injection period 52 when the engine is operated at 100 % engine load, and therefore not able to govern the 25 maximum cylinder pressure achieved during combustion. The maximum cylinder pressure achieved during combustion is mainly controlled by the timing and the duration of the main fuel injection period 52. The pilot fuel oil injection period 51 and the main fuel injection period 52 is separated by a dwell period 53. In Fig. 3 the start of injection of the pilot oil injection period 51 is 30 started at tSoi,Pre = 0, i.e. at the top dead centre, and the start of injection of the main fuel injection period is tSoi,mam = 3. The values are selected arbitrarily only with the purpose of explaining the adjustment of the fuel injection profile according to the invention. In the fuel gas operating mode where only fuel gas is injected into the cylinder during the main fuel injection period 52, 35 the pilot fuel oil injection period 51 is required to ignite the injected fuel gas.

11 DK 177476 B1

Hence the fuel gas is injected into an ongoing combustion process.

Fig. 4 illustrates the situation where the engine control unit 17 change from the fuel oil operating mode to the fuel gas operating mode. This is done by progressively replacing a part of the fuel oil injected into the cylin-5 der in the main fuel injection period with fuel gas. Hence the main fuel injection period comprises a fuel oil injection period 52a and a fuel gas injection period 52b. The engine control unit 17 orders fuel gas to be injected at the time tEoi,fuei, which is at the end of the fuel oil injection period 52a, i.e. at the time where the fuel oil injection valve closes. Evidently, there may be a slight 10 overlap between the end of the fuel oil injection and the start of the fuel gas injection. Hence the fuel gas injection period 52b represented with the dotted lines is gradually increased until the main fuel injection period 52 only comprises fuel gas injection as shown in Fig. 5 showing a fuel injection profile 50 in the fuel gas operating mode. The injection timing of the fuel gas injection, 15 i.e. the start of injection of fuel gas, tSoi,gas, is adjustable by engine control unit and is used to achieve the desired maximum cylinder pressure. When changing from the fuel gas operating mode to the fuel oil operating mode the opposite procedure as described above is applied.

Especially in the fuel gas operating mode it is advantageous to adjust 20 the timing of the injection of fuel gas in the main fuel injection period 52, i.e. by adjusting the duration of the main fuel injection period 52. This follows from the fact that often the heating value of the fuel gas provided to the cylinders by the fuel gas supply system is lower than the heating value of a reference fuel gas used for testing the engine and determining the duration of 25 the main fuel injection period at a given engine load. When the heating value of the fuel gas decrease compared to the reference fuel gas, the duration of the main fuel injection period 52 is increased to keep the engine load constant. However, when less energy is injected per time unit, the maximum combustion pressure in the cylinders decreases, which in turn results in lower 30 engine efficiency. By advancing the start of the injection of the main fuel injection period in the fuel gas operating mode, the maximum combustion pressure and the engine efficiency is thereby maintained even with a decrease in the heating value of the fuel gas.

As mentioned, with a lower heating value of the fuel gas it is not 35 possible to achieve the desired maximum cylinder pressure with a constant 12 DK 177476 B1 main fuel injection period 52. In that case the start of the main fuel injection period 52, tSoi,gas, may be advanced so that the duration of the main fuel injection period 52 is increased. This situation is illustrated in Fig. 6 where the start of the main fuel injection period 52 is advanced to the instant tSoi,gas = 5 1, because the amount of energy injected into the cylinders per time unit is only at 80 % of the reference gas. However, in order to avoid an overlap between the pilot fuel oil injection period 51 and the main fuel injection period 52, the start of the pilot fuel oil injection period 51 has been advanced to start at the instant tSoi,Pre = -2. Since the fuel gas cannot self ignite under the 10 pressure in the combustion pressure the start of the main fuel injection period with fuel gas injection may not be advanced to start before the pilot fuel oil injection period 51, which is provided to initiate the fuel gas injected into the cylinder in the main fuel injection period 52 represented by the dotted lines in Fig. 5 and Fig. 6. Hence in situations where the main fuel injection 15 period 52 needs to be increased more than the duration of the dwell period 53 or the duration from tSoi,Pre to tSoi,gas, it is advantageous to advance the start of the pilot fuel oil injection period 51, i.e. the instant of tSoi,Pre· Hence it is possible to both reduce the duration of the dwell period and advance start of the pilot fuel oil injection period in order to increase the main fuel oil injec-20 tion period 52.

Since the pilot fuel oil injection does not govern the maximum combustion pressure it is possible to change from the fuel gas operating mode to the fuel oil operating mode and keep the maximum combustion pressure as desired and with-in safe limits despite a significant advancement of the main 25 fuel injection period in the fuel gas operating mode. This is depicted in Fig. 7 illustrating that the start of the main fuel injection period as to injection of fuel oil in the fuel oil operating mode is unchanged.

A decrease in the heating value of the fuel gas may is detected by means of the sensor arrangement 40 measuring the engine cylinder pressure, 30 e.g. the mean indicated pressure. With a known relationship between engine load and mean indicated pressure, it can be detected if a longer injection of gas is required in order to keep a steady load. The actual main fuel injection period with injection of fuel gas compared to a main fuel injection period with injection of a reference gas under a test of the engine, enables an approxi-35 mate estimate of the actual heating value. As an alternative to measured cyl- 13 DK 177476 B1 inder pressure the engine load can be estimated with a torque-meter positioned on the crankshaft and a measurement of the engine speed.

Claims

14 DK 177476 B1

An internal combustion engine (1), such as a two-stroke cross-head diesel engine, comprising a fuel gas supply system (30) and cylinders (11) provided with fuel gas injectors (13, 14) for injecting fuel gas directly into combustion chambers in the cylinders (11) in a fuel gas operating mode, and a fuel oil supply system (20) supplying fuel oil injectors in the cylinders (11) with fuel oil for direct injection into the combustion chambers of the cylinders (11) in a fuel gas operating mode (17), and the fuel injection into the cylinders 10 (11) by adjusting the duration of the fuel injection, wherein the fuel injection in the fuel gas operation mode comprises a pilot fuel injection period (51) and a main fuel injection period (52) separated by a non-fuel period (53j) rest period; the engine control unit (17) being burned in response to variations in the calorific value 15 thereof power gas supplied to the cylinders (11) in the fuel gas operation mode adjusts the timing of the pilot fuel injection period (51) and the rest period (53), and accordingly adjusts the duration of the main fuel injection period (52) with the injection of fuel gas.

Internal combustion engine (1) according to claim 1, characterized in that the engine control unit (17), in response to a decrease in the fuel value of the fuel gas supplied to the cylinders (11) in the fuel gas operating state, advances the timing of the pilot fuel injection period (51) and the rest period (53). ), and advances the timing and duration of the main fuel injection period (52) with fuel gas injection.

Internal combustion engine (1) according to claim 1 or 2, characterized in that the engine control unit (17) in response to a decrease in the fuel value of the fuel gas supplied to the cylinders (11) in the fuel gas operating state reduces the duration of the rest period (53), thus increasing the duration 30 of the main fuel injection period (52) with fuel gas injection.

Internal combustion engine (1) according to claims 1 to 3, characterized in that the engine control unit (17) switches between the fuel oil operating state and the fuel gas operating state by progressively decreasing the duration of the fuel oil injection (52a) in a main fuel injection mode (177) B1 (17) for 15 increases a period (52b) of fuel gas injection.

Internal combustion engine (1) according to claims 1 to 4, characterized in that the engine control unit (17) switches between the fuel gas operating state 5 and the fuel oil operating mode by progressively decreasing the duration of the fuel gas injection (52b) during the main fuel injection period (52) in the fuel gas mode (52). oil injection (52a).

Internal combustion engine (1) according to claim 4, characterized in that the engine control unit (17) switches between the fuel oil operating mode and the fuel gas operating mode by progressively decreasing the duration of the fuel oil injection (52a) during the main fuel injection period (0), with the fuel oil operating mode (52) approximately. the period (52) per cylinder per engine rpm and the like increase a period (52b) with fuel gas.

Combustion engine (1) according to any one of the preceding claims, characterized in that the fuel gas injection pressure is in the range 150 bar to 300 bar.

Internal combustion engine (1) according to any one of the preceding claims, characterized in that the engine speed at 100% engine load is in the range of 45 revolutions per minute to 175 revolutions per minute.

Combustion engine (1) according to any one of the preceding claims, characterized in that the fuel gas supply system (30) for supplying fuel gas to the cylinders (11) is connected to a liquid natural gas tank on a liquid natural gas tank. 30