A diesel-methanol dual-fuel system for medium- and high-speed marine diesel engines and a control method thereof

Abstract

The present invention relates to a diesel-methanol dual-fuel system and a control method thereof for a medium- and high-speed marine diesel engine. The diesel-methanol dual-fuel system comprises: the original marine diesel engine and the original fuel system, and a diesel ECU; and further comprises: a methanol supply system, an alcohol injector assembly, an intake air temperature control system, a methanol electronic control system, and a DOC catalytic oxidizer directly installed on the basis of the original engine; the present invention applies the diesel-methanol combined combustion technology on the marine diesel engine to effectively achieve efficient combustion and greatly improve the thermal efficiency of the engine under medium and large load conditions.

Description

Technical field:

[0001] The invention relates to a diesel engine, and further to a diesel-methanol dual-fuel system applied to a medium- and high-speed ship diesel engine and a control method thereof. Background technology:

[0002] With its advantages of large transport capacity, low transport cost and long transport distance, water transport plays an irreplaceable role in the field of transportation. Diesel engines have an absolute dominant position in ship power plants due to their strong power, good economy, high thermal efficiency and high stability. They not only occupy the field of ordinary ship power plants, but also occupy the fields of VLCC, large bulk carriers and container ships, which are traditionally considered to belong to steam power plants. Another major application of diesel engines on ships is as the prime mover of generators. From the actual use of ships, the vast majority of ocean-going, offshore and inland ships use diesel engines as the prime movers of generators, and diesel engines also occupy an absolute dominant position as the prime movers of generators.

[0003] But at the same time, the exhaust emissions of marine diesel engines cause much more pollution to the environment than automotive engines, especially the extremely high emissions of nitrogen oxides, sulfur oxides and particulate matter. In order to solve the energy and environmental problems brought by marine diesel engines, clean alternative fuels have become one of the effective technical measures. Among them, natural gas, liquefied petroleum gas and methanol have attracted the interest of shipowners and engine manufacturers as clean marine fuels. But in comparison, methanol has its unique advantages. First, methanol is liquid under standard atmospheric conditions and can be stored in ordinary non-pressure containers, which makes its installation cost on board very low, easy to transport, and the construction of related infrastructure is relatively simple. Second, although the cost of methanol is higher than that of heavy oil, it does not contain sulfur and will play an important role in areas with strict emission controls such as sulfur emission control areas, river traffic control areas and inland waters. In addition, methanol also has the advantages of diverse production sources and smokeless emissions. In addition, the agreement adopted by the International Maritime Organization in September 2018 that low-flashpoint fuels including methanol can be used as ship fuels also paved the way for methanol to be used as ship fuel.

[0004] Chinese patent CN1470752A discloses an intake alcohol injection device and injection control method for a diesel engine, that is, a methanol nozzle is installed in the intake duct of the diesel engine, and the injected methanol forms a homogeneous mixed gas with air and enters the cylinder, and then the mixed gas is ignited by diesel directly injected into the cylinder. This method has little modification to the original engine and is flexible in application. However, this patent is based on a vehicle diesel engine, and the operating conditions are very different, and the environmental conditions are also essentially different. For example, the air cooler through which the intake air of a ship diesel engine passes is directly cooled by seawater or river water sucked in by a water pump, and the temperature is too low, which is extremely unfavorable for the evaporation of methanol. In particular, when the engine is running at small and medium loads, it will also cause the unevenness of each cylinder to increase. Therefore, in order to effectively apply the intake alcohol injection device and injection control method to ship engines, this patent has made some changes to the previous patent. Summary of the invention:

[0005] The present invention uses diesel / methanol combined combustion technology, only makes minor changes to the original marine diesel engine, installs an alcohol injector assembly on the intake manifold, and combines the matching methanol supply system, intake temperature control system, and methanol electronic control system to realize the application of methanol fuel in medium and high-speed marine diesel engines for the first time, effectively reducing diesel consumption. The technical solution of the present invention is as follows:

[0006] A diesel-methanol dual-fuel system for a medium- and high-speed marine diesel engine includes a marine diesel engine, an existing fuel system, and a diesel ECU; and also includes:

[0007] The methanol supply system, methanol injector assembly, intake temperature control system, methanol electronic control system, and DOC catalytic oxidizer are directly installed on the original engine;

[0008] The alcohol injector assembly includes: a methanol rail, a methanol nozzle and a methanol pressure plate, and the position is selected at the bend of the intake manifold; the alcohol rail is connected to the methanol pipeline and the inlet end of the methanol nozzle to feed methanol into the methanol nozzle, and the outlet end of the nozzle cooperates with the methanol pressure plate and is fixed at the bend of the intake manifold. The fixing method can be bolted or welded, and the installation angle must ensure that the methanol injection direction coincides with the intake air flow streamline;

[0009] The methanol electronic control system includes a methanol ECU, a control harness, a host computer, a fault display light, an alcohol injection indicator light, a methanol system switch, a methanol level meter and a fixed methanol concentration detector; the methanol ECU directly reads the speed signal, throttle signal, coolant temperature signal and the level signal of the methanol level meter and the flow signal of the flow meter of the original diesel ECU through the CAN mode to judge the engine working condition, the remaining methanol amount and the unobstructed condition of the methanol pipeline to determine the methanol injection; the methanol ECU also controls the intake air temperature control system, reads the temperature signal of the intake air temperature sensor, controls the opening of the electronically controlled exhaust valve, and then realizes the adjustment of the intake air temperature by controlling the exhaust gas introduction amount; the host computer is an integration of the methanol injection control interface, the fault display light, the methanol level meter display interface, the methanol flow meter display interface, the methanol system switch, the alcohol injection indicator light and the fixed methanol concentration detector display interface, which can be The system can simultaneously display the engine speed, coolant temperature, throttle opening, methanol tank level, methanol inlet flow, methanol concentration near the engine and methanol supply system, methanol system operation status and other signals measured from the original diesel ECU; the fault display light is used to display the signal status of each sensor and simple fault indication; the alcohol injection indicator light is used to display whether the ship is in dual-fuel working mode and the amount of alcohol injection; the methanol system switch is used to manually switch the diesel engine working state between the diesel-methanol dual-fuel mode and the original pure diesel mode; the methanol level gauge is used to measure the methanol tank level; there are two fixed methanol concentration detectors, which are respectively installed in the methanol pipeline and beside the engine to detect the methanol concentration in the air atmosphere to ensure that there is no methanol leakage. The methanol electronic control system directly reads the original diesel engine speed, coolant temperature and throttle signal from the diesel ECU through the CAN mode;

[0010] The intake air temperature control system includes: an exhaust gas bypass pipeline, an electronically controlled exhaust gas valve and an intake air temperature sensor; the exhaust gas bypass pipeline leads the exhaust gas treated by the catalytic oxidizer DOC to the intake end of the compressor; the electronically controlled exhaust gas valve is controlled by the methanol ECU and is used to adjust the amount of exhaust gas introduced, and its opening decreases as the engine load increases, and a high-temperature resistant valve is selected; the intake air temperature sensor is used to read the temperature at the position before the alcohol injector and after the air cooler, providing a basis for the opening of the electronically controlled exhaust gas valve.

[0011] The control method of the diesel-methanol dual-fuel system comprises the following steps:

[0012] Step 1: Use the pure diesel mode when starting. After starting, turn on the methanol ECU switch. The methanol ECU will make a judgment based on the engine's operating status. The pure diesel mode is still used when the load is small. When the engine speed, throttle opening, coolant temperature and methanol level meet the methanol injection conditions, the engine will automatically switch to the diesel / methanol dual-fuel working mode and determine the amount of methanol injection according to the calibrated methanol MAP. If the judgment conditions are not met, the methanol system will not work;

[0013] Step 2: As the engine speed decreases, when the engine operating parameters are out of the working range of the methanol system, the methanol ECU will automatically control the methanol nozzle to stop spraying methanol; after the engine is shut down, turn off the methanol system switch to shut down the methanol system; the methanol system stops working, but the engine can still run in pure diesel mode, meeting the classification society's requirements for dual-fuel engines;

[0014] Step 3: When the ship diesel engine is running at small or medium load, the methanol ECU reads the parameters of the intake temperature sensor, controls the opening of the electronically controlled exhaust valve, changes the amount of exhaust gas introduced, and then adjusts the intake temperature to the optimal evaporation temperature of methanol to improve thermal efficiency.

[0015] Preferred solution 1: The static flow rate of the methanol nozzle can reach 700 mL / min at a pressure of 0.4 MPa, and the number of nozzles installed is selected according to the fuel demand.

[0016] In preferred solution 2, the methanol supply system includes a methanol tank, a methanol filter, a methanol flowmeter, a methanol pump and a methanol pressure limiting valve, and the methanol tank is sequentially connected to the methanol filter, the methanol flowmeter, the methanol pump, the methanol pressure limiting valve and the methanol sprayer assembly through a methanol pipeline. Furthermore, a reflux alcohol pipe is provided in front of the methanol pressure limiting valve, and excess methanol returns to the methanol tank through the reflux alcohol pipe.

[0017] The present invention realizes the application of diesel / methanol combined combustion technology to medium- and high-speed marine diesel engines for the first time, filling the technical gap of medium- and high-speed marine diesel engines using diesel and methanol dual fuels. Its advantages are:

[0018] The present invention can enable the ship diesel engine to freely switch between a pure diesel mode and a diesel-methanol dual-fuel mode, and the switching mode can be automatic or manual, meeting various requirements of classification societies for dual-fuel engines.

[0019] The invention effectively solves the problem of poor methanol volatility under small and medium loads caused by the ship diesel engine directly introducing seawater (river water) into the air cooler to cool the intake air, and successfully realizes the application of the diesel engine intake alcohol injection device and injection control method disclosed in Chinese patent CN1470752A in the field of medium and high-speed ship diesel engines.

[0020] The application of diesel-methanol combined combustion technology in marine diesel engines can effectively achieve high-efficiency combustion and significantly improve the thermal efficiency of the engine under medium and heavy load conditions.

[0021] Up to now, in ship diesel engines, the replacement rate of methanol for diesel can reach as high as 60% and as low as 40%. The average replacement rate under normal load is 50%, saving an average of more than 10% of fuel costs.

[0022] The use of a diesel-methanol dual-fuel system and method on a ship diesel engine can effectively reduce exhaust temperature, lower engine room temperature, and improve the working environment of engine room personnel.

[0023] Currently, the emissions from the modified diesel-methanol dual-fuel ship engines meet China's first-stage emission limits.

[0024] The present invention can be applied not only to diesel engines used as the main propulsion power device of ships, but also can be installed on auxiliary engines of ships, and has a very broad application prospect. Description of the drawings:

[0025] Figure 1 1 is a schematic diagram of the system structure of the present invention; in the figure, 1 represents a diesel ECU, 2 represents a host computer, 3 represents a methanol system switch, 4 represents a methanol ECU, 5 represents an electronically controlled high-pressure common rail system, 6 represents a high-pressure oil pump, 7 represents an alcohol injector assembly, 8 represents a methanol pressure limiting valve, 9 represents a diesel injector, 10 represents a fixed methanol concentration detector, 11 represents an intake air temperature sensor, 12 represents a methanol pump, 13 represents an air cooler, 14 represents a diesel filter, 15 represents a methanol flow meter, 16 represents a methanol filter, 17 represents a diesel tank, 18 represents a methanol level gauge, 19 represents a methanol tank, 20 represents a DOC catalytic oxidizer, 21 represents an exhaust gas bypass pipeline, 22 represents an electronically controlled exhaust valve, and 23 represents an exhaust gas turbocharger.

[0026] Figure 2 2 is a schematic diagram of the structure of the methanol injector assembly in the embodiment; in the figure, 24 represents a methanol pressure plate, 25 represents a methanol nozzle, 26 represents a methanol rail, and 27 represents an intake manifold. Specific implementation method:

[0027] Example:

[0028] The present invention is further described in detail below in conjunction with the accompanying drawings and specific embodiments. It is particularly emphasized that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] Figure 1In the illustrated embodiment, a test bench for applying a diesel-methanol dual-fuel system to a medium- and high-speed marine diesel engine is demonstrated. The system includes a marine diesel engine, a fuel system, a diesel ECU, and a methanol system directly installed on the original engine, which is composed of a methanol supply system, an alcohol injector assembly 7, and a methanol electronic control system. A DOC catalytic oxidizer 20 is also installed.

[0030] The original ship diesel engine used for modification is a straight six-cylinder supercharged intercooled four-stroke engine used as a ship main engine, with a cylinder diameter of 123mm, a stroke of 145mm, a rated power of 257Kw, a rated speed of 1800r / min, and a displacement of 10.337L. The engine adopts an electronically controlled high-pressure common rail injection system with a maximum injection pressure of 160MPa, which can achieve multiple injections. The engine is mainly installed on inland ships, coastal ships, river-sea direct ships, strait [ferries] and fishing vessels.

[0031] The fuel system of the test bench includes a diesel tank 17, a diesel filter 14, a high-pressure fuel pump 6, an electronically controlled high-pressure common rail injection system 5, and a diesel fuel injector 9, simulating a real ship. Diesel is pumped out of the diesel tank 17, filtered by the diesel filter 14, pressurized by the high-pressure fuel pump 6, and reaches the electronically controlled high-pressure common rail system 5, and finally enters the cylinder through the fuel injector 9, and the excess diesel flows back to the diesel tank through the return oil pipeline.

[0032] The diesel ECU receives signals such as engine speed, coolant temperature, throttle opening, etc. measured by the sensors on the original engine, and controls the injection timing and injection pulse width of the diesel injector 9.

[0033] The methanol supply system includes a methanol tank 19, a methanol filter 16, a methanol flow meter 15, a methanol pump 12, a methanol pressure limiting valve 8, and a methanol pipeline for connecting various components. The methanol pipeline and other equipment in contact with methanol are all designed to be alcohol-resistant.

[0034] The alcohol injector assembly 7 includes a methanol rail 26, four methanol nozzles 25 and a methanol pressure plate 24. The position is selected at the bend of the intake manifold 27, and its purpose is to allow methanol and intake air to be mixed more fully. The methanol rail 26 is connected to the methanol pipeline and to the inlet end of the methanol nozzle 25. After voltage stabilization, methanol is sent to the methanol nozzle 25. The nozzle outlet cooperates with the methanol pressure plate 24. Both ends of the methanol nozzle 25 have alcohol-resistant rubber rings to ensure sealing. The alcohol injector assembly 7 is fixed at the bend of the intake manifold 27, and the installation angle must ensure that the methanol injection direction coincides with the intake air flow streamline.

[0035] The intake air temperature control system includes an exhaust gas bypass line 21, an electronically controlled exhaust gas valve 22 and an intake air temperature sensor 11; the exhaust gas bypass line 21 leads the exhaust gas treated by the catalytic oxidizer DOC to the intake end of the compressor; the opening of the electronically controlled exhaust gas valve 22 is controlled by the methanol ECU to adjust the amount of exhaust gas introduced, and a high-temperature resistant valve is selected; the intake air temperature sensor 11 reads the temperature at the position before the alcohol injector 7 and after the air cooler 13, providing a basis for the opening of the electronically controlled exhaust gas valve 22;

[0036] The methanol electronic control system includes: methanol ECU, control harness, host computer 2, fault display lamp, methanol injection indicator lamp, methanol system switch 3, methanol level meter 18 and fixed methanol concentration detector 10. The methanol ECU directly reads the engine speed, coolant temperature, throttle opening and other signals measured by the original diesel ECU through the CAN mode to judge the engine working condition, and reads the signals of the methanol flow meter 15 and the methanol level meter 18 at the same time. When the injection conditions are met, an injection instruction including injection time and pulse width is sent to the methanol nozzle 25. The methanol ECU also controls the intake air temperature control system, reads the temperature signal of the intake air temperature sensor 11, controls the opening of the electronically controlled exhaust valve 22, and then adjusts the intake air temperature by controlling the amount of exhaust gas introduced. The host computer integrates the methanol injection control interface, fault display lamp, methanol level meter display interface, methanol flow meter display interface, methanol system switch, methanol injection indicator light and fixed methanol concentration detector display interface, and can simultaneously display the engine speed, coolant temperature, throttle opening, and the liquid level of the methanol tank 19, methanol inlet flow, methanol concentration near the engine and methanol supply system, methanol system operation status and other signals measured from the original diesel ECU. The control harness includes a signal line connecting the methanol ECU and the original diesel ECU, a signal line connecting the methanol ECU and the methanol level meter 18, a signal line connecting the methanol ECU and the methanol flow meter 15, a signal line connecting the methanol ECU and two fixed methanol concentration detectors 10, a control harness connecting the methanol ECU and the diesel injector 9, and a communication line connecting the methanol ECU and the host computer 2.

[0037] The specific method of using the above system to conduct substitution rate test and emission test on marine diesel engine is as follows:

[0038] according to Figure 1 Build the system as shown and check whether each component can operate normally;

[0039] Perform normal vehicle preparation before starting the engine;

[0040] After the engine starts normally, it first runs in pure diesel mode until it stabilizes at the target operating condition. The methanol ECU reads the engine operating condition signal from the original diesel ECU. When the engine operating boundary conditions meet the methanol injection conditions, the methanol injection amount is controlled on the control interface of the upper computer 2. After methanol is injected, the engine speed regulating mechanism adjusts the diesel circulation fuel supply to make the speed and torque the same as the target operating condition under the pure diesel mode.

[0041] According to the standard of "Emission Limits and Measurement Methods of Exhaust Pollutants from Marine Engines (China Phase I and Phase II)", the test cycle of the marine engine is operated according to the propulsion characteristics, and the operating points of the engine are calculated according to the four-condition cycle test operating condition inspection table (1134r / min539.7N·m, 1440r / min850.0N·m, 1638r / min1120.9N·m, 1800r / min1360.0N·m). In addition, the single-cylinder displacement of the marine engine of this embodiment is 1.723L. According to the requirements of "Emission Limits and Measurement Methods of Exhaust Pollutants from Marine Engines (China Phase I and Phase II)", the emission limits of the first phase are as follows: CO: 5.0g / kW·h, HC+NOX: 7.2g / kW·h, PM: 0.2g / kW·h.

[0042] The test results of this embodiment show that in the automatic operation dual-fuel mode, the highest methanol substitution rate is 53.7%, the substitution rate of the 50% and 25% power points in the four working points exceeds 50%, and the 100% power point is limited by the maximum cylinder pressure, making the substitution rate lower. The average substitution rate of the four working points is 42.3%. The replacement ratio of methanol to diesel at the four working points is less than 2, the highest replacement ratio is 1.88, and the average replacement ratio is 1.82. The power performance of the high-speed ship diesel engine using the diesel-methanol dual-fuel system and method is not lower than that of the original engine, and the economy is greatly improved. From the emission perspective, the specific fuel consumption in automatic operation dual-fuel mode is lower than that in pure diesel mode, and the weighted emission indicators of the four operating points are all lower than those in pure diesel mode. In detail, the weighted emissions of CO are 0.003g / kW·h, the weighted emissions of THC are 0.009g / kW·h, the weighted emissions of NOX are 5.813g / kW·h, and the weighted emissions of soot are 0.067g / kW·h, all of which meet the first stage emission limit requirements of China.

[0043] The ideal replacement rate of methanol under various operating conditions is dynamically calibrated from the test bench to the methanol ECU and then applied to the actual ship.

[0044] The diesel-methanol dual-fuel system and method were also applied on a real ship. The test ship was a fishing boat. The engine was a straight-six-cylinder water-cooled four-stroke medium-speed ship main engine with a cylinder diameter of 160mm, a stroke of 225mm, a rated power of 184kW, a rated speed of 1000r / min, a displacement of 27.14L, and a mechanical straight-line pump system. The original ship engine was converted from ship diesel mode to diesel-methanol dual-fuel mode without any changes to the original fuel system. Only one set of the above-mentioned methanol system was installed, and an alcohol sprayer assembly was installed on the intake manifold, and a methanol supply system, an intake air temperature control system, and a methanol electronic control system were equipped. A DOC catalytic oxidizer was then installed, and the methanol ECU was calibrated on the bench according to the ideal replacement rate of methanol under various working conditions. The installed engine can realize pure diesel mode and dual-fuel mode operation. The chief engineer performed start-stop operations according to the daily operating procedures. The automatic switching of the dual-fuel mode and the one-button shutdown method are simple and easy to master.

[0045] The present invention is not limited to the embodiments described above. The above description of the specific embodiments is intended to describe and illustrate the technical solution of the present invention. The above specific embodiments are only adaptive and not restrictive. Without departing from the scope of the present invention and the scope of protection of the claims, ordinary technicians in this field can also make many forms of specific changes under the guidance of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A diesel-methanol dual-fuel system for a medium- and high-speed marine diesel engine, comprising a marine diesel engine, an existing fuel system, and a diesel ECU (1); characterized in that: Also includes: A methanol supply system, a methanol injector assembly, an intake temperature control system, a methanol electronic control system, and a DOC catalytic oxidizer (20) are directly installed on the original engine; The methanol sprayer assembly comprises: a methanol rail (26), a methanol nozzle (25) and a methanol pressure plate (24), and the position is selected at the bend of the intake manifold (27); the methanol rail is connected to the methanol pipeline and the inlet end of the methanol nozzle to feed methanol into the methanol nozzle, and the outlet end of the nozzle cooperates with the methanol pressure plate and is fixed at the bend of the intake manifold. The fixing method can be bolt fastening or welding, and the installation angle must ensure that the methanol injection direction coincides with the intake air flow streamline; The methanol electronic control system comprises a methanol ECU, a control harness, a host computer (2), a fault display lamp, an alcohol injection indicator lamp, a methanol system switch (3), a methanol level meter and a fixed methanol concentration detector (10); the methanol ECU (4) directly reads the speed signal, throttle signal, coolant temperature signal and the level signal of the methanol level meter and the flow signal of the flow meter of the original diesel ECU through the CAN mode to judge the engine working condition, the methanol remaining amount and the unblocked condition of the methanol pipeline, so as to determine the methanol injection; the methanol ECU also controls the intake air temperature control system, reads the temperature signal of the intake air temperature sensor (11), controls the opening of the electronically controlled exhaust valve, and further realizes the adjustment of the intake air temperature by controlling the exhaust gas introduction amount; the host computer serves as the methanol injection control interface, the fault display lamp, the methanol level meter display interface, the methanol flow meter display interface, the methanol system switch, the alcohol injection indicator lamp and the fixed methanol concentration detector The integration of the display interface can simultaneously display the engine speed, coolant temperature, throttle opening, methanol tank level, methanol inlet flow, methanol concentration near the engine and methanol supply system, methanol system operation status and other signals measured from the original diesel ECU; the fault display light is used to display the signal status of each sensor and simple fault indication; the alcohol injection indicator light is used to display whether the ship is in dual-fuel working mode and the amount of alcohol injection; the methanol system switch is used to manually switch the diesel engine working state between the diesel-methanol dual-fuel mode or the original pure diesel mode; the methanol level gauge is used to measure the methanol tank level; there are two fixed methanol concentration detectors, which are installed in the methanol pipeline and beside the engine respectively to detect the methanol concentration in the air atmosphere to ensure that there is no methanol leakage. The methanol electronic control system directly reads the original diesel engine speed, coolant temperature and throttle signal from the diesel ECU through the CAN mode; The intake air temperature control system comprises: an exhaust gas bypass pipeline (21), an electronically controlled exhaust gas valve (22) and an intake air temperature sensor; the exhaust gas bypass pipeline guides the exhaust gas treated by the catalytic oxidizer DOC to the intake end of the compressor; the electronically controlled exhaust gas valve is controlled by the methanol ECU and is used to adjust the amount of exhaust gas introduced, and its opening decreases as the engine load increases, and a high-temperature resistant valve is selected; the intake air temperature sensor is used to read the temperature at a position before the alcohol injector and after the air cooler (13), providing a basis for the opening of the electronically controlled exhaust gas valve.

2. According to claim 1, a diesel-methanol dual-fuel system for medium- and high-speed marine diesel engines is characterized in that: The static flow rate of the methanol nozzle can reach 700mL / min at a pressure of 0.4MPa, and the number of nozzles installed is selected according to the fuel demand.

3. A diesel-methanol dual-fuel system for medium- and high-speed marine diesel engines according to claim 1 or 2, characterized in that: The methanol supply system comprises a methanol tank, a methanol filter (16), a methanol flow meter (15), a methanol pump (12) and a methanol pressure limiting valve (8); the methanol tank is connected to the methanol filter (16), the methanol flow meter (15), the methanol pump (12), the methanol pressure limiting valve (8) and the methanol sprayer assembly (7) in sequence through a methanol pipeline.

4. According to claim 3, a diesel-methanol dual-fuel system for medium- and high-speed marine diesel engines is characterized in that: A reflux methanol pipe is arranged in front of the methanol pressure limiting valve, and excess methanol returns to the methanol tank through the reflux methanol pipe.

5. The control method of the diesel-methanol dual fuel system according to claim 1, characterized in that: The following steps are involved: Step 1: Use the pure diesel mode when starting. After starting, turn on the methanol ECU switch. The methanol ECU will make a judgment based on the engine's operating status. The pure diesel mode is still used when the load is small. When the engine speed, throttle opening, coolant temperature and methanol level meet the methanol injection conditions, the engine will automatically switch to the diesel / methanol dual-fuel working mode and determine the amount of methanol injection according to the calibrated methanol MAP. If the judgment conditions are not met, the methanol system will not work; Step 2: As the engine speed decreases, when the engine operating parameters are out of the working range of the methanol system, the methanol ECU will automatically control the methanol nozzle to stop spraying methanol; after the engine is shut down, turn off the methanol system switch to shut down the methanol system; the methanol system stops working, but the engine can still run in pure diesel mode, meeting the classification society's requirements for dual-fuel engines; Step 3: When the ship diesel engine is running at small or medium load, the methanol ECU reads the parameters of the intake temperature sensor, controls the opening of the electronically controlled exhaust valve, changes the amount of exhaust gas introduced, and then adjusts the intake temperature to the optimal evaporation temperature of methanol to improve thermal efficiency.

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