JP6398543B2 - Natural gas engine and method of operating natural gas engine - Google Patents

Description

  The present invention relates to a natural gas engine and a natural gas engine that can burn natural gas with high efficiency by compression ignition of an ignition fuel, which is a fuel different from natural gas, without using a spark ignition system. It relates to the driving method.

In vehicles such as passenger cars and trucks, as a measure against global warming, the CO ₂ emission coefficient is 0 on a high calorific value basis for light oil with a CO ₂ emission coefficient of 0.0187 tC / GJ (gigajoule) on a high calorific value basis. Natural gas engines have been developed that use 0135tC / GJ natural gas (CNG) as fuel. By switching from this light oil to natural gas, which has a CO ₂ emission coefficient of 72% of that of light oil, a simple heat calculation is expected to reduce CO ₂ emissions by about 28%, so the fuel is changed from light oil to natural gas. The effect of global warming prevention by switching is great.

  In this natural gas engine (CNG engine), as shown in FIG. 5, the natural gas C compressed by the piston 63 is ignited by spark ignition by the spark plug 62 provided in the cylinder head 61. It is burning. In order to reduce NOx in the exhaust gas, the three-way catalyst is used to purify the NOx in the exhaust gas, but in order to exert the catalytic action of the three-way catalyst, the intake air amount is limited by the throttle valve. Then, the stoichiometric combustion in which the fuel amount and the air amount become 1.0 in terms of the excess air ratio (λ) is performed to obtain exhaust gas without oxygen.

  For this reason, in the idling operation region and the light load operation region where the required horsepower is small, the amount of intake air is reduced as the fuel decreases in order to perform stoichiometric combustion. However, there is a problem that the flame propagation is interrupted and there are many unburned areas, and the combustion of natural gas becomes unstable or misfires.

  Further, in the case of a large engine having a high output and a large cylinder bore diameter, as shown in FIG. 6, the high-temperature combustion gas G on the exhaust system passage 67 side of the engine 10X and the low-temperature intake air on the intake system passage 65 side are provided. In relation to A, a portion (shown by cross-hatching) H on the exhaust system passage 67 side tends to be high in temperature with respect to the intake system passage 65 side. Therefore, the natural gas fuel C is not ignited by ignition of the spark plug 62 but is ignited by touching the high temperature portion H before entering the combustion stroke, and the combustion spreads from the high temperature portion H side to the entire combustion chamber 64. A phenomenon called detonation (abnormal combustion) occurs. When this phenomenon occurs, the piston 63 hits the cylinder 70 and cannot smoothly reciprocate, causing engine failure. This detonation is also one of the causes of knocking.

  In addition, natural gas engines have poor ignitability of natural gas, and the number of spark plugs that can be placed in the combustion chamber is structurally limited. Normally, there is only one ignition source, so ignition is ensured every cycle. There is a problem that the combustion efficiency is poor. If this combustion efficiency is poor, a large amount of fuel is required to produce the required output, resulting in a deterioration in fuel consumption. In addition, if a large amount of fuel is put into the combustion chamber in order to ensure ignition, the combustion temperature rises, and the temperature of the spark plug, exhaust valve, exhaust manifold, etc. rises, and the exhaust system parts are easily damaged. The problem arises.

Further, at the time of engine start, it is difficult ignition of natural gas, there is a problem that time-consuming compared to diesel fuel with regard start. In particular, when the air temperature in winter is low, there is also a problem that unburned natural gas is discharged due to misfire and odor is generated.

  As countermeasures against these problems, natural gas (CNG) is injected into the intake passage with the CNG injector, light oil is injected into the combustion chamber with the light oil injector, and mixed with light oil with high compression ignitability. A fuel control device for an internal combustion engine has been proposed in which the ratio of CNG and light oil is changed based on the maximum pressure during combustion in the combustion chamber (see, for example, Patent Document 1).

  In the natural gas engine 10Y that uses this light oil fuel together, as shown in FIG. 7, the light oil fuel f is injected from the liquid fuel injector 69 in the compression stroke in which the air-fuel mixture in which the natural gas C and the intake air A are mixed is compressed. When the temperature of the air-fuel mixture rises due to adiabatic compression of the air-fuel mixture while being diffused in the combustion chamber 64 and exceeds the ignition (ignition) temperature of the light oil, the light oil fuel f starts to combust due to the compression ignition. The natural gas C around is also burned.

  Since the light oil fuel f is diffused in the combustion chamber 64 at the start of the combustion, multi-point ignition can be performed and ignition from the high temperature portion of the exhaust system can be prevented, and the entire combustion chamber 64 is combusted. Since a substantially uniform force is applied, the piston 63 reciprocates smoothly. Therefore, in the engine using this light oil fuel f and natural gas C, detonation can be prevented. In addition, since no spark plug is used, the heat damage of the spark plug does not occur.

  However, even in a natural gas engine that uses this light oil fuel together, as shown in FIG. 8, the excess air ratio λ is lower than that of a diesel engine with a light oil fuel operated with an excess air ratio λ of 2 to 8 of the prior art. In a natural gas engine operated by stoichiometric combustion of 1, the intake air amount is remarkably reduced, so that the compression pressure in the cylinder is lowered and the temperature rise of the air-fuel mixture in the cylinder in adiabatic compression is also lowered. There is. In particular, in the light load operation region where the engine output (horsepower) is small, the amount of intake air significantly decreases and the compression pressure decreases greatly as the fuel decreases, and ignition and combustion become unstable. There is a problem and this needs to be resolved.

  In connection with this, the present inventor, in a natural gas engine combined with light oil fuel, reliably and stably uses light oil as a compression ignition source and the light oil in a low load operation state, stoichiometric combustion or rich combustion. Equipped with an exhaust introduction mechanism that introduces exhaust gas into the cylinder during the intake stroke in order to burn natural gas with combustion as an ignition source and perform highly efficient combustion with less fuel and less intake air A natural gas engine has been proposed (see, for example, Patent Document 2).

  In addition, natural gas can be ignited reliably and stably in the entire operation range, high-efficiency combustion is achieved with less fuel and less intake air, and knocking due to detonation is suppressed even when the engine operation state is a high load range. For this purpose, an exhaust introduction mechanism is provided, and the amount of light oil injected into the cylinder is the amount of light oil for idle operation in the entire engine operating range. In a high load region in which the accelerator opening is larger than the first opening that is set in advance, a natural gas engine that performs in-cylinder fuel injection of light oil by multi-injection has been proposed (see, for example, Patent Document 3). ).

JP 2012-57471 A JP 2014-109198 A JP 2014-109199 A

  On the other hand, the present inventor has obtained the following knowledge through many experiments and considerations. In other words, in a natural gas engine that uses light oil fuel together, in order to improve the combustion of natural gas, it is important to evenly disperse the light oil used as the ignition source in the natural gas. Has a high density and a relatively high true calorific value of 36.0 MJ (megajoules) / l (liter), which generates the amount of heat required as an ignition source during light load operation such as when starting the engine or idling. Therefore, the theoretical amount of light oil required from the calorific value is relatively small. However, there is a problem that it is difficult for the current fuel injection nozzle to inject this small amount of liquid injection in a good spray state. This is an event in which if the injection volume is small, the injection period is short and the controllability deteriorates.

  On the other hand, in order to realize good ignition of natural gas, it is necessary to make the injected light oil in a uniform spray state with no bias and to mix well with the whole natural gas flowing into the combustion chamber. Therefore, it is necessary to increase the injection pressure to some extent and also to increase the injection time to some extent, and when using the current liquid injection nozzle, if priority is given to the state of spraying or mixing with natural gas, light oil as an ignition source Therefore, it is necessary to ensure the ignitability by increasing the amount of the amount more than the volume necessary for thermal calculation.

  In other words, light oil with a high cetane number is used to help ignite natural gas, which has a high octane number and is difficult to ignite, but due to problems with the current fuel injection nozzle injection mode, natural gas can be ignited stably. The amount of light oil to be injected needs to exceed the theoretical amount of light oil that generates the calorific value required under low load conditions.

Therefore, despite the fact that natural gas is used to reduce CO ₂ emissions against global warming, the effect on global warming can be achieved by burning more light oil than necessary in terms of heat calculations. It will be suppressed.

  In order to solve this problem, the calorific value per volume is smaller than that of light oil, and even if the calorific value is the same, the volume is large, so the injection period can be extended, spraying is easy, and it is mixed with natural gas. If it is possible to use fuel that is easy to use and has high cetane number indicating ignitability, only the amount of ignition fuel that generates a calorific value close to the calorific value required at engine start and low-load operation Thus, the present inventors have obtained the knowledge that natural gas can be ignited satisfactorily and the above-described problems of a natural gas engine using light oil as an ignition fuel can be solved.

An object of the present invention is to ensure that natural gas can be ignited reliably and stably even when the engine is started or at a low load operation in a natural gas engine using liquid fuel as an ignition source and natural gas as a main fuel. High efficiency combustion with a small amount of ignition fuel, natural gas, and a small amount of intake air enables sufficient combustion even when natural gas is mixed from the start of the engine. Even during load operation, natural gas can be burned with a small amount of ignition fuel, so that most of the heat required for engine output is covered by the combustion of natural gas with a low CO ₂ emission coefficient. Another object of the present invention is to provide a natural gas engine that can greatly reduce CO ₂ emissions and prevent global warming, and a method for operating the natural gas engine.

The natural gas engine of the present invention for achieving the above object uses natural gas and an ignition fuel different from the natural gas as fuel, and the ignition fuel is compressed by ignition in a cylinder. In a natural gas engine that burns natural gas, the ignition fuel is a true calorific value per unit volume in a range of 32 MJ (megajoule) / l (liter) to 35 MJ / l, and a cetane number of 65 to 65. The liquid fuel in the range of 90 is used, and when starting the engine, both the ignition fuel and the natural gas are supplied to start the engine, and the amount of the ignition fuel injected into the cylinder is determined by the engine. In the entire operation region, the fuel consumption amount is set to a constant amount smaller than the amount of ignition fuel with respect to the calorific value required for idle operation .

  According to this configuration, since the amount of heat generated per unit volume is smaller than that of light oil and liquid fuel having a cetane number higher than that of light oil is used, the amount of fuel injected for ignition as an ignition source is less than that of light oil. More than the case, even with the current fuel injection nozzle, the spray state can be improved as compared with the case of light oil, and since the liquid fuel having a high cetane number is used, compression ignition becomes easy. Furthermore, since the mixing amount with the natural gas is improved by increasing the injection amount of the ignition fuel, the ignition fuel mixed evenly with the natural gas is combusted by the compression ignition so that the burned ignition fuel is obtained. As a result, the entire natural gas can be ignited evenly, so that a good combustion state can be obtained.

  In addition, if the true calorific value per unit volume is less than 32 MJ / l, the ignitability and combustibility will deteriorate, and if it exceeds 35 MJ / l, the volume increase will decrease, and the spraying state in fuel injection and into natural gas will decrease. Since the effect of improving the diffusion state is reduced, the effect of switching from light oil is reduced.

  Further, if the cetane number is less than 65, the ignitability is not sufficiently improved, and the effect of improving the ignitability with a small calorific value is reduced. If the cetane number exceeds 90, the selection range of the liquid fuel becomes too narrow. In addition, the ignitability becomes too high and the handling becomes troublesome.

  Incidentally, light oil has a true calorific value (low calorific value) of about 36 MJ / l, a total calorific value (high calorific value) of about 38 MJ / l, and a conversion coefficient from high calorific value to low calorific value is 0.95. It is. Moreover, the cetane number of light oil is about 54.

  Further, as a liquid fuel having a true calorific value per unit volume in a range of 32 MJ / l to 35 MJ / l and a cetane number in a range of 65 to 90, for example, natural gas called liquid synthetic fuel is used. There is a GTL (Gas To Liquid) fuel that is decomposed into carbon monoxide and hydrogen and then recombined with a molecular structure to form a liquid fuel.

  As a feature of this GTL, the true calorific value per unit weight is almost the same, but since the density is about 5% to 10% lower than that of light oil, the true calorific value per unit volume is lower than that of light oil. Accordingly, since the injection amount increases, the injection pressure can be increased and the injection time can be lengthened compared to the case of injecting the heat-calculated amount of light oil, and the fuel injection control can be performed with high accuracy. It becomes like this. Further, since the cetane number is improved by about 15 compared to light oil, the ignitability is high, and the compression ignition performance can be improved. As a result, the substantial injection amount of the ignition fuel can be made smaller than that of light oil.

Therefore, not only the fuel for ignition is changed to a liquid fuel such as GTL instead of light oil, but also the ignitability can be remarkably improved by the aspect of fuel injection and the aspect of cetane number. Even if gas is mixed, it can be burned sufficiently. In other words, the ignition fuel is selected so that it can be sufficiently burned even if natural gas is mixed from the start of the engine. This selection criterion is a liquid fuel having a true calorific value per unit volume in the range of 32 MJ / l to 35 MJ / l and a cetane number in the range of 65 to 90.

By this selection, natural gas can be burned with a small amount of ignition fuel not only at engine start but also at idling or low load operation, so that most of the calorific value of fuel that contributes to engine output is natural gas. It can be generated by burning. As a result, most of the heat required for engine output can be covered by the combustion of natural gas with a low CO ₂ emission coefficient, so CO ₂ emissions can be greatly reduced and global warming is prevented. There is an effect.

  In addition, when GTL is employed, GTL has less sulfur content and less aroma components to become soot than light oil, so that the PM collection device can be made unnecessary or downsized. In particular, since there is almost no soot generated even if a large amount of EGR is performed to reduce NOx, the diesel engine that uses light oil as the main fuel simultaneously reduces NOx and soot that are in a trade-off relationship with each other. In addition, a catalyst device using a NOx occlusion reduction catalyst or a selective reduction (SCR) catalyst can be eliminated or downsized.

  Note that GTL is currently more expensive than diesel oil, and if GTL is used instead of diesel oil, the operating cost of a natural gas engine will be higher due to the use of GTL. Since natural gas is actively mixed even during operation or during low-load operation, the amount of GTL used can be significantly reduced, and an increase in operating cost can be suppressed.

  The above-described natural gas engine includes an exhaust introduction mechanism that introduces exhaust gas into a cylinder during the intake stroke.

  As an exhaust introduction mechanism, for example, in an exhaust cam that operates an exhaust valve, which has already become a well-known technology, a normal cam feel is added with a phase angle of approximately 90 ° according to the operating state of the engine. By making the formed exhaust introduction cam profile operable, the exhaust valve is introduced during the intake stroke by connecting the inside of the cylinder and the exhaust system passage with a lift of about 1 mm to 3 mm of the exhaust valve during the intake stroke. Depending on the operating state of the engine, an exhaust introduction valve that operates an on-off valve with an electromagnetic solenoid different from the exhaust valve is provided, and the exhaust introduction valve is lifted and opened during the intake stroke by this electromagnetic solenoid. Thus, it is possible to adopt a configuration in which exhaust gas is introduced during the intake stroke by communicating the inside of the cylinder and the exhaust system passage.

  According to this configuration, by operating the exhaust introduction mechanism, the hot exhaust gas in the exhaust system passage flows back into the cylinder as needed during the intake stroke, and immediately after this combustion, the hot exhaust gas is used for ignition in the cylinder. The temperature of the mixture of fuel, natural gas, intake air and exhaust gas can be raised. In this exhaust gas introduction, high-temperature exhaust gas is introduced immediately after combustion, so that the effect of increasing the in-cylinder temperature is remarkably increased as compared with the introduction of EGR gas whose temperature is lowered via an EGR passage equipped with an EGR cooler.

  As a result, compression ignition and combustion can be stably performed even with a smaller amount of ignition fuel, and natural gas combustion can also be stably performed using the ignition fuel combustion as an ignition source. . Therefore, stable combustion with good combustion efficiency can be realized, and the amount of ignition fuel can be further reduced, so that the amount of consumption of ignition fuel can be reduced, and the amount of heat generated by combustion of the fuel as a whole is reduced. Therefore, the amount of heat flowing to the exhaust system passage is reduced, and the heat damage caused by this amount of heat is reduced, so that the durability of the exhaust system parts of the engine is improved.

  Further, the operation of the exhaust gas introduction mechanism improves the startability because the temperature in the cylinder can be quickly raised even when the engine is at a low temperature. In addition, since the engine can be started with a small amount of ignition fuel even at the time of starting, misfire and soot are not generated by the starting. Further, since the temperature in the cylinder can be quickly raised, smooth acceleration can be achieved.

  Also, by using an exhaust introduction mechanism that raises the temperature in the cylinder, the temperature in the cylinder is maintained at a temperature at which the ignition fuel is easy to ignite even in a light load operation state, and stable with a small amount of ignition fuel. Since ignition can be obtained and combustion can be stabilized with a small amount of ignition fuel, engine vibration can be reduced and riding comfort (drivability) can be improved. In addition, the amount of exhaust gas during light load operation can be reduced.

  In the above-described natural gas engine, the amount of ignition fuel injected into the cylinder is set to a constant amount that is smaller than the amount of ignition fuel for the calorific value required for idle operation in the entire operation region of the engine. If the engine output is increased or decreased by increasing or decreasing the amount of natural gas, in this case, the ignition fuel can be reliably compressed and ignited with very simple control of a constant amount of ignition fuel. Regardless of the amount, natural gas can be combusted efficiently with the minimum intake amount. In this case, it is preferable to control the opening degree of the intake throttle valve by measuring the air-fuel ratio, the excess air ratio λ, and the oxygen concentration of the exhaust gas, and performing a reciprocal ratio determination so as to achieve stoichiometric combustion.

In the natural gas engine, in the operating state in which the operation of the exhaust introduction mechanism, intake throttle control by intake air shutter disposed in an intake system passage, or one exhaust throttle control by the exhaust gas shutter disposed in the exhaust system path Or, when configured to use both, the intake air amount (fresh air amount) can be burned stoichiometrically by operating the exhaust shutter and operating the intake shutter in the valve closing direction to throttle the intake air. In addition, the pressure on the intake system passage side can be reduced, so that the exhaust gas can flow back into the cylinder more efficiently, the temperature in the cylinder can be raised more, and the combustion efficiency can be further increased. be able to.

  Furthermore, if the exhaust shutter is operated in the valve closing direction to throttle the exhaust, the pressure on the exhaust system passage side increases, the exhaust gas on the exhaust system passage side tends to flow backward into the cylinder, and the reverse flow rate increases. Therefore, the temperature rise effect in the cylinder can be further enhanced.

Then, the operation method of the natural gas engine of the present invention for achieving the above object uses natural gas and an ignition fuel different from the natural gas as the fuel, and this ignition fuel in the cylinder In the method of operating a natural gas engine in which the natural gas is combusted by compression ignition in the above, a true calorific value per unit volume as the ignition fuel is within a range of 32 MJ (megajoule) / l (liter) to 35 MJ / l. In addition, the liquid fuel having a cetane number in the range of 65 to 90 is used, and at the time of starting the engine, both the ignition fuel and the natural gas are supplied and the ignition is performed and injected into the cylinder. how the amount of use the fuel, in the entire operating range of the engine, characterized by a certain amount less than the amount of the ignition fuel to the heating value required by the idling A.

  According to this method, since the ignition fuel having a smaller calorific value per unit volume than that of light oil and having a lower true calorific value than that of light oil is used, the volume injection amount for using the ignition fuel as an ignition source is reduced. More than the case of light oil, the current fuel injection nozzle can improve the spray state more than the case of light oil and uses an ignition fuel having a high cetane number, so that it is easy to perform compression ignition. Furthermore, since the volume of the ignition fuel is increased, the miscibility with natural gas is improved. Therefore, the ignition fuel mixed evenly with natural gas is ignited and burned, so that the entire natural gas is evenly distributed. Can be ignited, so that a good combustion state can be obtained.

  According to the natural gas engine and the operation method of the natural gas engine according to the present invention, in the natural gas engine using the ignition fuel as the ignition source and the natural gas as the main fuel, at the time of engine start and low load operation However, natural gas can be ignited reliably and stably, high efficiency combustion is performed with a small amount of ignition fuel and a small amount of intake air, and even if natural gas is mixed from the start of the engine, it can be combusted sufficiently In addition, natural gas can be burned with a small amount of ignition fuel even during idling or low load operation.

As a result, most of the heat required for engine output can be covered by the combustion of natural gas with a low CO ₂ emission coefficient, and CO ₂ emission can be greatly reduced to prevent global warming. it can.

It is a figure which shows typically the structure of the natural gas engine of embodiment of this invention. It is a figure for explanation of exhaust introduction. It is a figure which shows the lift of the intake valve and exhaust valve in exhaust introduction. It is a figure which shows the relationship between the fuel for ignition and the natural gas in the operating method of the natural gas engine of embodiment of this invention. It is a figure for demonstrating the normal ignition and combustion state of the natural gas in the natural gas engine of a prior art. It is a figure for demonstrating the detonation (abnormal combustion) of the natural gas in the natural gas engine of a prior art. It is a figure for demonstrating the ignition state of the fuel for ignition of the natural gas engine which uses a fuel for ignition together, and the combustion state of natural gas. It is a figure which shows typically the comparison of the compression pressure in a natural gas engine, and the compression pressure in a normal diesel engine.

  Hereinafter, a natural gas engine according to an embodiment of the present invention and a method for operating the natural gas engine will be described with reference to the drawings. The natural gas engine 10 according to the embodiment of the present invention shown in FIG. 1 is provided with an intake manifold 11a and an intake passage 12 as intake system passages of an engine body 11, and an exhaust manifold 11b and an exhaust passage 13 as exhaust system passages, respectively. In addition, an EGR passage 14 that connects the exhaust passage 13 and the intake passage 12 is provided.

  A turbocharger (turbocharger) 15 is provided. A turbine 15a of the turbocharger 15 is provided in the exhaust passage 13 and a compressor 15b is provided in the intake passage 12. The turbine 15a is rotated by the exhaust energy of the exhaust gas G, and the rotation is transmitted through the shaft 15c. The intake air A is compressed by 15b.

  The intake passage 12 through which the intake air A passes is provided with a compressor 15b, an intercooler 16, and an intake shutter (intake throttle) 17, and the intake air A is compressed by the compressor 15b and cooled by the intercooler 16 to increase the air density. Then, the flow rate is adjusted by the intake shutter 17 and introduced into the combustion chamber 64 in the cylinder 70 shown in FIG.

Further, as shown in FIG. 1, a turbine 15a is provided in the exhaust passage 13 through which the exhaust gas G generated by burning the ignition fuel F and the natural gas C passes. Further, an exhaust shutter 42 is provided between the turbine 15a and an exhaust gas purification device (not shown) . A part of the exhaust gas G is introduced into the EGR passage 14 as EGR gas Ge as needed, and the rest is driven by the exhaust gas purifying device as needed after driving the turbine 15a to enter the atmosphere. Released.

  The EGR passage 14 through which the EGR gas Ge passes is provided with an EGR cooler 19 that cools the EGR gas Ge and an EGR valve 20 that adjusts the flow rate of the EGR gas Ge. The EGR gas Ge is branched from the exhaust passage 13. After that, it is cooled by the EGR cooler 19, the flow rate is adjusted by the EGR valve 20, and it is recirculated to the intake passage 12.

  In the natural gas engine 10, as in the diesel engine for light oil fuel, as shown in FIG. 2, the liquid fuel for injecting the ignition fuel F, which is liquid fuel, into the cylinder 70 of the engine body 11. A supply line 80 is provided. The liquid fuel supply line 80 includes a fuel tank 81, a solenoid valve 82, a pressure regulator (regulator) 83, a chamber 84, a liquid fuel injector 69, and a fuel pipe 85 connecting them. As a result, the ignition fuel F is injected into the cylinder 70 from the liquid fuel injection injector 69 in the same manner as a normal diesel engine for light oil fuel.

  In the present invention, the engine body 11, the fuel injection system, the cooling system, etc., in addition to the configuration of a normal diesel engine for light oil fuel, as shown in FIG. 1, a natural gas tank (CNG tank) 31, electromagnetic A valve 32, a pressure regulator (regulator) 33, a chamber 34, a CNG injection injector (natural gas injection device) 35 disposed on the downstream side of the intake shutter 17 in the intake passage 12, and a CNG pipe 36 for connecting them. The natural gas supply system 30 is configured.

  The natural gas C stored in the natural gas tank 31 is adjusted by the natural gas supply system 30 through the CNG pipe 36, the pressure is adjusted by the pressure regulator 33 via the electromagnetic valve 32, and then injected by the CNG injection injector 35. Are injected into the intake system passage 65 (intake passage 12 in FIG. 1) while the injection timing is controlled.

  Further, in the present invention, the ignition fuel F is combusted by the compression ignition of the ignition fuel F injected into the combustion chamber 64 in the cylinder 70 without using a spark ignition system when the natural gas C is ignited. The natural gas C is combusted using the ignition fuel F as a fire type.

  As the ignition fuel F, a liquid fuel having a true calorific value per unit volume in the range of 32 MJ (megajoule) / l (liter) to 35 MJ / l and a cetane number of 65 to 90 is used. To do. When the engine is started, both the ignition fuel F and the natural gas C are supplied to start the engine.

  If the true calorific value per unit volume is less than 32 MJ / l, the ignitability and combustibility will deteriorate, and if it exceeds 35 MJ / l, the volume increase will decrease, and the spray state in fuel injection and diffusion into natural gas will decrease. The effect of improving the condition is reduced. Further, if the cetane number is less than 65, the ignitability is not sufficiently improved, and the effect of improving the ignitability with a small calorific value is reduced. If the cetane number exceeds 90, the selection range of the liquid fuel becomes too narrow. In addition, the ignitability becomes too high and the handling becomes troublesome. Incidentally, light oil has a true calorific value (low calorific value) of about 36 MJ / l and a cetane number of about 54.

  Here, GTL (Gas To Liquid) fuel is adopted as the liquid fuel having a true calorific value per unit volume within a range of 32 MJ / l to 35 MJ / l and a cetane number of 65 to 90. . This GTL has a true calorific value per unit weight that is almost the same as that of light oil, but since the density is about 5% to 10% lower than that of light oil, the true calorific value per unit volume is lower than that of light oil. Since the injection amount is increased accordingly, the injection pressure can be increased and the injection time can be extended as compared with the case of injecting the heat-calculated amount of light oil, and the injection control of the ignition fuel F can be performed with high accuracy. It becomes like this. In addition, since the cetane number of GTL is improved by about 15 compared to light oil, GTL has high ignitability and can improve compression ignition performance. As a result, the substantial injection amount of the ignition fuel F can be made smaller than that of light oil.

  Further, the natural gas engine 10 is provided with an exhaust introduction mechanism (not shown) for introducing the exhaust gas G into the cylinder 70 during the intake stroke. As the exhaust introduction mechanism, an exhaust cam for operating the exhaust valve 68 shown in FIG. 2 has a phase angle of approximately 90 ° with respect to a normal cam feel that opens the exhaust valve 68 in a normal exhaust stroke. As shown in FIGS. 2 and 3, the exhaust valve 68 is lifted by about 1 mm to 3 mm during the intake stroke by additionally forming a profile and enabling the exhaust introduction cam profile according to the engine operating state. Thus, the exhaust gas G can be introduced during the intake stroke by allowing the inside of the cylinder 70 and the exhaust system passage 67 (the exhaust passage 13 in FIG. 1) to communicate with each other.

  Further, by providing an exhaust introduction valve that operates an on-off valve with an electromagnetic solenoid different from the exhaust valve 68, and giving a drive signal to this electromagnetic solenoid according to the engine operating state, it is shown in FIG. At such timing, the exhaust introduction valve is lifted and opened during the intake stroke, whereby the inside of the cylinder 70 and the exhaust system passage 67 (exhaust passage 13) are communicated to introduce the exhaust gas G during the intake stroke. A configuration can also be adopted.

  In the present invention, the exhaust introduction mechanism does not have to be limited to the above-described two configurations. Even in other configurations, the exhaust introduction mechanism has a function of introducing exhaust gas into the cylinder during the intake stroke. I just need it.

  By operating the exhaust introduction mechanism, the temperature of the mixture of the ignition fuel F, the natural gas C, the intake air A, and the exhaust gas G in the cylinder 70 can be raised, so that even a small amount of the ignition fuel F is stable. Thus, the combustion of the natural gas C can be performed stably by compression combustion, and stable combustion can be realized.

  In the present invention, a λ (excess air ratio) sensor 41 is disposed downstream of the turbine 15a in the exhaust passage 13 so that the excess air ratio λ in the exhaust gas G can be measured. An exhaust shutter (exhaust throttle valve) 42 is provided upstream of the 13 turbines 15a.

  Further, a control device 51 called an engine control unit (ECU) is provided, and includes an accelerator sensor 52, an engine rotation speed sensor 53 provided in the engine body 11, a cooling water temperature sensor (not shown), and an intake air amount provided in the intake passage 12. Signals from various sensors such as a sensor (MAF: not shown), a λ sensor 41 provided in the exhaust passage, an exhaust gas temperature sensor (not shown), a NOx sensor (not shown), etc. are input, and a liquid fuel injection injector 69, The CNG injection injector 35, the turbine 15a of the turbocharger 15, the intake shutter 17, the EGR valve 20, and the like are configured to be controlled.

  Next, a method of operating the natural gas engine according to the embodiment of the present invention in the natural gas engine 10 will be described. In this natural gas engine operation method, natural gas C and an ignition fuel F different from the natural gas C are used as fuels, and the natural gas C is combusted by compression ignition in a cylinder of the ignition fuel F. This is a method of operating the natural gas engine 10, and as the ignition fuel F, the true calorific value per unit volume is in the range of 32 MJ (megajoule) / l (liter) to 35 MJ / l, and the cetane number is 65 to 65. This is a method of using a liquid fuel in the range of 90 and supplying both ignition fuel F and natural gas C when starting the engine.

  Further, as shown in FIG. 4, the calorific value Fci of the ignition fuel F injected into the cylinder 70 is set as the calorific value Fci of the ignition fuel F at the start in the entire operation region of the natural gas engine 10, The engine output is increased or decreased by controlling the amount of natural gas C.

  According to this operation method, in the natural gas engine 10 that uses the ignition fuel F equipped with this exhaust introduction mechanism, stable ignition and combustion can be maintained in the operation state at the time of starting the engine. By using the combustion of F for the ignition of natural gas C, the amount of fuel for ignition is constant, compared to control in which the ratio of the fuel for ignition F and natural gas C is changed in the engine operating state. With natural control, natural gas C can be burned efficiently with a minimum amount of intake air. Note that the valve opening of the intake shutter 17 in this case is controlled by measuring the air-fuel ratio, the excess air ratio λ, and the oxygen concentration of the exhaust gas G, and performing a reciprocal ratio determination so as to achieve stoichiometric combustion.

  According to the operation method of the natural gas engine 10 and the natural gas engine, a liquid fuel such as GTL having a high cetane number and a relatively large volume with respect to the calorific value is used as the ignition fuel F. As shown in FIG. 4, when starting the natural gas engine 1, the natural gas C can be started with a small amount of ignition fuel F added thereto. That is, since the ignition fuel F is very easy to ignite by compression, the amount of the ignition fuel F necessary for the compression ignition is small.

Then, as shown in FIG. 4, the heat generation amount Fci necessary for maintaining the engine operation at the time of starting or idling can be secured only by the heat generation amount Fci in the amount necessary for the compression ignition of the ignition fuel F. Therefore, the natural gas C is also added and burned at the same time, and the necessary heat generation amount Tci can be secured by using the heat generation amount Cci. By mixing the ignition fuel F and the natural gas C, it is possible to improve the ignitability at the time of start-up, secure the heat generation amount after ignition, and reduce the CO ₂ generation amount.

  By mixing the natural gas C, the consumption amount of the ignition fuel F can be reduced as compared with the method of starting the engine only with the expensive ignition fuel F, so that the operating cost can be reduced and the ignition fuel F can be reduced. The fuel tank can be made smaller.

  Further, by operating the exhaust introduction mechanism, the exhaust gas G is introduced into the cylinder 70 during the intake stroke, and the ignition fuel F, the natural gas C, the intake air A, and the exhaust gas G in the cylinder 70 are introduced. Therefore, even a smaller amount of ignition fuel F can be stably compressed and combusted, and natural gas C can be combusted more stably.

  In other words, the operation of the exhaust introduction mechanism can further improve the ignitability and combustion efficiency, and the amount of the ignition fuel F for ignition can be further reduced, so that a smaller amount of the ignition fuel F is sufficient. In addition, since the amount of heat generated by the combustion of the fuels F and C becomes smaller as a whole, as a result, the amount of heat flowing to the exhaust passage 13 is further reduced, heat damage is reduced, and durability is improved.

  Further, by utilizing the exhaust introduction mechanism for increasing the temperature in the cylinder, the temperature in the cylinder can be maintained at a temperature at which the ignition fuel F is easily ignited, and stable ignition can be obtained with a small amount of fuel. In addition, since the combustion can be stabilized with a small amount of fuels F and C in the entire operation region including the light load operation region, it is possible to reduce engine vibration and improve ride comfort (drivability). Further, it is possible to reduce the exhaust gas amount during operation in the idling operation region and the light load machine.

  In addition, the improvement in ignitability reduces the misfire, and by ensuring good fuel and heat generation, the temperature in the cylinder can be quickly raised even when the engine 10 is at a low temperature. In addition, since the temperature in the cylinder can be quickly raised even after starting, smooth acceleration can be achieved. Furthermore, when GTL is adopted, the GTL does not contain aroma that is a source of soot, so the amount of PM emission is greatly reduced.

  Further, when the exhaust throttle control by the intake shutter 17 provided in the intake passage 12 and the exhaust throttle control by the exhaust shutter 42 provided in the exhaust passage 13 are used in combination during the operation of the exhaust introduction mechanism, the exhaust gas G is more efficiently obtained. Can be caused to flow back into the cylinder 70, the temperature in the cylinder can be further increased, and the combustion efficiency can be further increased.

  According to the natural gas engine and the operation method of the natural gas engine of the present invention, in a natural gas engine that uses liquid fuel as an ignition source and natural gas as a main fuel, at the time of engine start, idling operation, and low load operation Even when the natural gas can be ignited reliably and stably, high efficiency combustion is achieved with a small amount of liquid fuel and a small amount of intake air. In addition, natural gas can be burned with a small amount of ignition fuel even during idling or low-load operation.

As a result, most of the heat required for engine output can be covered by the combustion of natural gas with a low CO ₂ emission coefficient, and CO ₂ emission can be greatly reduced to prevent global warming. it can.

  Therefore, it can be used as a driving method for many natural gas engines and natural gas engines that are mounted on vehicles.

10 Natural Gas Engine 11 Engine Body 12 Intake Passage (Intake System Passage)
13 Exhaust passage (exhaust system passage)
13a Bypass passage 14 EGR passage 15 Turbo supercharger (turbocharger)
16 Intercooler 17 Intake shutter (intake throttle)
19 EGR cooler 20 EGR valve 30 Natural gas supply system 31 Natural gas tank (CNG tank)
35 CNG injector (natural gas injector)
41 λ sensor (excess air ratio sensor)
42 Exhaust shutter (exhaust throttle valve)
43 Exhaust flow path switching valve 51 Control unit (ECU)
52 Acceleration sensor 53 Engine rotation speed sensor 61 Cylinder head 62 Spark plug 65 Intake system passage 67 Exhaust system passage 68 Exhaust valve 69 Liquid fuel injector (liquid fuel injection device)
70 Cylinder A Intake air C Natural gas F Fuel for ignition G Exhaust gas Ge EGR gas λ Excess air ratio

Claims (4)

In a natural gas engine that uses natural gas and an ignition fuel different from the natural gas as fuel, and burns the natural gas by compression ignition in a cylinder of the ignition fuel,
As the ignition fuel, a liquid fuel having a true calorific value per unit volume in the range of 32 MJ (megajoule) / l (liter) to 35 MJ / l and a cetane number in the range of 65 to 90 is used. When starting the engine, both the ignition fuel and the natural gas are supplied to start the engine .
The amount of the fuel for ignition injected into the cylinder is configured to be a constant amount smaller than the amount of fuel for ignition with respect to the calorific value required for idle operation in the entire operation region of the engine. Characteristic natural gas engine.   2. The natural gas engine according to claim 1, further comprising an exhaust introduction mechanism that introduces exhaust gas into a cylinder during an intake stroke. In the operating state of performing the operation of the exhaust introduction mechanism, intake throttle control by the intake shutter provided in the intake system passage, or to a combination of one or both of the exhaust throttle control by the exhaust gas shutter disposed in the exhaust system path The natural gas engine according to claim 2 , wherein the natural gas engine is configured. In the method of operating a natural gas engine, natural gas and an ignition fuel different from the natural gas are used as fuel, and the natural gas is combusted by compression ignition in a cylinder of the ignition fuel.
As the ignition fuel, a liquid fuel having a true calorific value per unit volume in the range of 32 MJ (megajoule) / l (liter) to 35 MJ / l and a cetane number in the range of 65 to 90 is used. When starting the engine, both the ignition fuel and the natural gas are supplied to start the engine .
A natural gas engine characterized in that the amount of the ignition fuel injected into the cylinder is a constant amount smaller than the amount of the ignition fuel with respect to the calorific value required in the idling operation in the entire operation region of the engine. Driving method.

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