Natural gas engine and operation method for natural gas engine

Abstract

A natural gas engine in which natural gas fuel is injected into an intake system passage, diesel fuel serving as an ignition source is injected into a cylinder, and when igniting the natural gas fuel, the natural gas fuel is combusted by compression ignition of the diesel fuel that has been injected into the cylinder without the use of a spark ignition system. The engine has a mechanism that introduces exhaust gas into a cylinder during an intake stroke.

Claims

1. A natural gas engine in which an amount of natural gas fuel is injected into an intake system passage, diesel fuel serving as an ignition source is injected into a cylinder, and when igniting the natural gas fuel, the natural gas fuel is combusted by compression ignition of the diesel fuel that has been injected into the cylinder without a spark ignition system, comprising: an exhaust gas introduction mechanism that introduces exhaust gas into the cylinder during an intake stroke by allowing the exhaust gas to flow back into the cylinder by opening an exhaust valve, wherein an amount of the diesel fuel to be injected into the cylinder is set to a constant amount for idling across an entire operation region of the natural gas engine, and wherein output of the engine is increased and decreased by increasing and decreasing, respectively, the amount of the natural gas fuel.

2. The natural gas engine according to claim 1, further comprising: an intake shutter in the intake system passage, wherein the engine is configured to simultaneously perform intake throttle control using the intake shutter when the exhaust gas introduction mechanism introduces the exhaust gas into the cylinder.

3. The natural gas engine according to claim 1, further comprising: an exhaust shutter in an exhaust system passage, wherein the engine is configured to simultaneously perform exhaust throttle control using the exhaust shutter when the exhaust gas introduction mechanism introduces the exhaust gas into the cylinder.

4. An operation method for a natural gas engine in which an amount of natural gas fuel is injected into an intake system passage, diesel fuel serving as an ignition source is injected into a cylinder, and when igniting the natural gas fuel, the natural gas fuel is combusted by compression ignition of the diesel fuel that has been injected into the cylinder without a spark ignition system, comprising: providing an exhaust gas introduction mechanism that introduces exhaust gas into the cylinder during an intake stroke by allowing the exhaust gas to flow back into the cylinder by opening an exhaust valve; setting an amount of diesel fuel to be injected into the cylinder to a constant amount for idling across an entire operation region of the natural gas engine; and increasing and decreasing output of the engine by increasing and decreasing, respectively, the amount of the natural gas fuel.

5. The operation method for a natural gas engine, according to claim 4, further comprising: performing intake throttle control using an intake shutter provided in the intake system passage and exhaust throttle control using an exhaust shutter provided in an exhaust system passage when the exhaust gas introduction mechanism introduces the exhaust gas into the cylinder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a diagram schematically showing a configuration of a natural gas engine according to an embodiment of the present invention.

(2) FIG. 2 is a diagram for explaining exhaust gas introduction.

(3) FIG. 3 is a diagram showing lifting of an intake valve and an exhaust valve during the exhaust gas introduction.

(4) FIG. 4 is a diagram showing a relationship between diesel fuel and natural gas fuel in an operation method according to the present invention.

(5) FIG. 5 is a diagram for explaining normal ignition and a combustion state of natural gas fuel in a natural gas engine according to a conventional technology.

(6) FIG. 6 is a diagram for explaining detonation (abnormal combustion) of the natural gas fuel in the natural gas engine according to the conventional technology.

(7) FIG. 7 is a diagram for explaining ignition and a combustion state of fuel in a natural gas engine simultaneously using diesel fuel according to the conventional technology.

(8) FIG. 8 is a diagram schematically showing comparison between a compression pressure in the natural gas engine and a compression pressure in a general diesel engine.

DETAILED DESCRIPTION

(9) With reference to the drawings, description is given below of a natural gas engine and an operation method for the natural gas engine according to an embodiment of the present invention. In a natural gas engine 10 according to the embodiment of the present invention shown in FIGS. 1 and 2, an intake passage (intake system passage) 12 is connected to an intake manifold (intake system passage) 11a of an engine main body 11, an exhaust passage (exhaust system passage) 13 is connected to an exhaust manifold (exhaust system passage) 11b thereof, and an EGR passage 14 is provided, which connects the exhaust passage 13 to the intake passage 12.

(10) There is also provided a turbo supercharger (turbocharger) 15. A turbine 15a of the turbo supercharger 15 is provided in the exhaust passage 13, and a compressor 15b thereof is provided in the intake passage 12. Through a shaft 15c connecting the turbine 15a with the compressor 15b, rotation of the turbine 15a rotated by exhaust energy of exhaust gas G is transmitted to the compressor 15b. Thus, the compressor 15b compresses intake air A.

(11) In the intake passage 12, through which the intake air A passes, the compressor 15b, an intercooler 16 and an intake shutter (intake throttle) 17 are provided. The intake air A is compressed by the compressor 15b and cooled by the intercooler 16 to have its air density increased. Then, the intake air A is introduced into a combustion chamber 64 in a cylinder 70 shown in FIG. 2 after a flow rate thereof is controlled by the intake shutter 17.

(12) Also, as shown in FIG. 1, in the exhaust passage 13, through which the exhaust gas G generated by combustion of diesel fuel f and natural gas fuel C passes, the turbine 15a and an exhaust gas purification device (aftertreatment device) 18 including a three-way catalyst 18a, an oxidation catalyst 18b, a DPF (diesel particulate filter) 18c and the like, are provided. Also, an exhaust shutter 42 is provided between the turbine 15a and the exhaust gas purification device 18. Moreover, some of the exhaust gas G is introduced into the EGR passage 14 as EGR gas Ge as needed, and the rest thereof is purified by the exhaust gas purification device 18 and emitted into the atmosphere after driving the turbine 15a.

(13) Furthermore, in the EGR passage 14, through which the EGR gas Ge passes, an EGR cooler 19 that cools the EGR gas Ge and an EGR valve 20 that controls a flow rate of the EGR gas Ge are provided. After diverging from the exhaust passage 13, the EGR gas Ge is cooled by the EGR cooler 19, controlled by the EGR valve 20 for its flow rate, and then recirculated into the intake passage 12.

(14) The natural gas engine 10 described above includes a diesel fuel supply line (not shown) to inject the diesel fuel f into the cylinder 70 in the engine main body 11, as in the case of a diesel engine. Therefore, as in the case of a general diesel engine, the diesel fuel f is injected into the cylinder 70 from a diesel fuel injector (diesel fuel injection device) 69 as shown in FIG. 2.

(15) Moreover, as for the engine main body 11 as well as a fuel injection system, a cooling system, and the like, the present invention includes the configuration of the general diesel engine. In addition to such a configuration, the present invention includes a natural gas supply system 30 including a natural gas tank (CNG tank) 31, an electromagnetic valve 32, a pressure regulating device (regulator) 33, a chamber 34, a CNG jet injector (natural gas fuel injection device) 35 disposed on the downstream side of the intake shutter 17 in the intake passage 12, and a CNG pipe 36 connecting those described above, as shown in FIG. 1.

(16) With the natural gas supply system 30, the natural gas fuel C stored in the natural gas tank 31 passes through the CNG pipe 36 and is regulated by the pressure regulating device (regulator) 33 through the electromagnetic valve 32 for its pressure. Then, while being controlled for its injection amount and injection timing by the CNG jet injector 35, the natural gas fuel C is injected into an intake system passage 65 (the intake passage 12 in FIG. 1).

(17) Furthermore, in the present invention, when igniting the natural gas fuel C, the natural gas fuel C is combusted by compression ignition of the diesel fuel f injected into the combustion chamber 64 in the cylinder 70 without the use of a spark ignition system. Moreover, the present invention includes an exhaust gas introduction mechanism (not shown) that introduces the exhaust gas G into the cylinder 70 during an intake stroke.

(18) As the exhaust gas introduction mechanism, in an exhaust cam 68a that actuates an exhaust valve 68 shown in FIG. 2, an exhaust gas introduction cam profile having a phase angle of approximately 90 is formed in addition to a normal cam profile that opens the exhaust valve 68 during a normal exhaust stroke. The exhaust gas introduction cam profile can be actuated according to an engine operation region. Thus, as shown in FIGS. 2 and 3, by lifting and opening the exhaust valve 68 by about 1 mm to 3 mm during the intake stroke, the inside of the cylinder 70 and an exhaust system passage 67 (the exhaust passage 13 in FIG. 1) are communicated with each other. As a result, the exhaust gas G is introduced during the intake stroke.

(19) Moreover, an exhaust gas introduction valve using an electromagnetic solenoid to actuate an on-off valve is provided separately from the exhaust valve 68, and a drive signal is given to the electromagnetic solenoid according to the engine operation region. Thus, by lifting and opening the exhaust gas introduction valve during the intake stroke at the timing as shown in FIG. 3, the inside of the cylinder 70 and the exhaust system passage 67 (the exhaust passage 13) are communicated with each other. As a result, the exhaust gas G is introduced during the intake stroke.

(20) The operation of the exhaust gas introduction mechanism can increase the temperature of an air-fuel mixture of the diesel fuel f, the natural gas fuel C, the intake air A and the exhaust gas G inside the cylinder 70. Thus, stable compression combustion is performed even with a small amount of diesel fuel f, and the natural gas fuel C can also be combusted in a stable manner. Accordingly, stable combustion can be realized. As a result, combustion efficiency is improved, and the amount of the diesel fuel f required for ignition is reduced. Thus, only a small amount of diesel fuel f is required. Moreover, an amount of heat generated by combustion of the fuels f and C is reduced as a whole. As a result, the amount of heat transmitted to the exhaust passage 13 is reduced, leading to reduction in heat damage and improvement in durability.

(21) Moreover, an in-cylinder temperature can be quickly increased even at the start-up when the natural gas engine 10 is still cold. Thus, startability is improved. Also, at the start-up of the engine, the engine can be started with a small amount of diesel fuel f. Thus, accidental fires and unburned hydrocarbon emissions during the start-up are also significantly reduced. Furthermore, since the in-cylinder temperature can be quickly increased, smooth acceleration can be achieved.

(22) Furthermore, the use of the exhaust gas introduction mechanism that increases the in-cylinder temperature maintains the in-cylinder temperature at a temperature at which the diesel fuel f is readily ignited, thus enabling stable ignition with a small amount of diesel fuel. Since stable combustion is performed with a small amount of fuels f and C even during a low load operation condition, ride quality (drivability) can be improved while reducing engine vibration. Also, the amount of exhaust gas can be reduced during the low load condition.

(23) Moreover, in the present invention, a (excess air ratio) sensor 41 is provided on the downstream side of the turbine 15a in the exhaust passage 13 to enable measurement of the excess air ratio in the exhaust gas G. Furthermore, the exhaust shutter (exhaust throttle valve) 42 is provided on the upstream side of the turbine 15a in the exhaust passage 13.

(24) Furthermore, a control device 51 called an engine control unit (ECU) is provided to control the diesel fuel injector 69, the CNG jet injector 35, the turbine 15a of the turbo supercharger 15, the intake shutter 17, the EGR valve 20 and the like by receiving signals from various sensors such as an accelerator sensor 52, an engine rotation speed sensor 53 and a coolant temperature sensor (not shown) provided in the engine main body 11, an intake air amount sensor (MAF: not shown) provided in the intake passage 12, the sensor 41, an exhaust gas temperature sensor (not shown), and an NOx sensor (not shown) provided in the exhaust passage.

(25) Note that, in the configuration shown in FIG. 1, a bypass passage 13a is provided as a bypass of the exhaust passage 13. In the bypass passage 13a, a low-capacity compact continuous regeneration DPF 18d is provided, such as a DPF carrying an oxidation catalyst and a combination of an oxidation catalyst device and a DPF. Also, an exhaust channel switching valve 43 for switching the flow of the exhaust gas G between the bypass passage 13a and the exhaust passage 13 is provided in a portion, of the exhaust passage 13, parallel to the bypass passage 13a. The exhaust channel switching valve 43 is controlled by the control device 51. The exhaust gas G passes through the exhaust passage 13 in an opened state of the exhaust channel switching valve 43, and passes through the bypass passage 13a in a closed state thereof, before passing through the compact DPF 18d.

(26) Next, description is given of an operation method for the natural gas engine 10 described above according to the embodiment of the present invention. The operation method for the natural gas engine is an operation method for the natural gas engine 10 in which the natural gas fuel C is injected into the intake passage 12, the diesel fuel f serving as an ignition source is injected into the cylinder 70, and when igniting the natural gas fuel C, the natural gas fuel C is combusted by compression ignition of the diesel fuel f injected into the cylinder 70 without the use of a spark ignition system, the natural gas engine 10 including an exhaust gas introduction mechanism that introduces the exhaust gas G into the cylinder 70 during an intake stroke. The operation method includes the step of performing control to facilitate ignition of the diesel fuel f by introducing, using the exhaust gas introduction mechanism, the exhaust gas G into the cylinder 70 during the intake stroke.

(27) According to the operation method for the natural gas engine, the operation of the exhaust gas introduction mechanism can increase the temperature of an air-fuel mixture of the diesel fuel f, the natural gas fuel C, the intake air A and the exhaust gas G inside the cylinder 70. Thus, stable compression combustion is performed even with a small amount of diesel fuel f, and the natural gas fuel C can also be combusted in a stable manner. Accordingly, stable combustion can be realized. As a result, combustion efficiency is improved, and the amount of the diesel fuel f required for ignition is reduced. Thus, only a small amount of diesel fuel f is required. Moreover, an amount of heat generated by combustion of the fuels f and C is reduced as a whole. As a result, the amount of heat propagated to the exhaust passage 13 is reduced, leading to reduction in heat damage and improvement in durability.

(28) Furthermore, as shown in FIG. 4, control is performed in which an amount mf of the diesel fuel f injected into the cylinder 70 is set to a diesel fuel amount mfa for idling across the whole operation region of the natural gas engine 10, and engine output is increased and decreased by increasing and decreasing the amount of the natural gas fuel C.

(29) According to the operation method, in the natural gas engine 10 including the exhaust gas introduction mechanism and simultaneously using the diesel fuel f, stable combustion can be maintained in an idling condition. Thus, the use of the combustion of the diesel fuel f for the ignition of the natural gas fuel C enables the natural gas fuel C to be combusted with the minimum amount of intake air and high combustion efficiency through a very simple control in which the diesel fuel amount is constant (mf=mfa) compared with a control in which a ratio of the diesel fuel f to the natural gas fuel C is changed according to the engine operation condition. Accordingly, the exhaust gas temperature can be increased with respect to the ratio of the amount of the fuels f and C. Thus, exhaust gas purification performance of the exhaust gas purification device 18 provided in the exhaust passage 13 can be improved. Note that the valve position of the intake shutter 17 in this case is controlled by measuring the air-fuel ratio, excess air ratio and oxygen concentrations in the exhaust gas G, and determining a stoichiometric ratio for stoichiometric combustion.

(30) Moreover, by simultaneously performing intake throttle control using the intake shutter 17 provided in the intake passage 12 and exhaust throttle control using the exhaust shutter 42 provided in the exhaust passage 13 during the operation of the exhaust gas introduction mechanism, the exhaust gas can be allowed to more efficiently flow back into the cylinder 70. As a result, the in-cylinder temperature can be further increased, and the combustion efficiency can be further improved.

(31) According to the natural gas engine 10 thus configured and the operation method for the natural gas engine, the operation of the exhaust gas introduction mechanism can increase the temperature of the air-fuel mixture of the diesel fuel f, the natural gas fuel C, the intake air A and the exhaust gas G inside the cylinder 70. Thus, stable compression combustion is performed even with a small amount of diesel fuel f, and the natural gas fuel C can also be combusted in a stable manner. Accordingly, stable combustion can be realized. As a result, combustion efficiency is improved, and the amount of the diesel fuel f required for ignition is reduced. Thus, only a small amount of diesel fuel f is required. Moreover, an amount of heat generated by combustion of the fuels f and C is reduced as a whole. As a result, the amount of heat propagated to the exhaust passage 13 is reduced, leading to reduction in heat damage and improvement in durability.

(32) Moreover, the in-cylinder temperature can be quickly increased even at the start-up when the natural gas engine 10 is still cold. Thus, startability is improved. Also, at the start-up of the engine, the engine can be started with a small amount of diesel fuel f. Thus, accidental fires and unburned HC emissions during the start-up are also significantly reduced. Furthermore, since the in-cylinder temperature can be quickly increased, smooth acceleration can be achieved.

(33) Furthermore, the use of the exhaust gas introduction mechanism that increases the in-cylinder temperature maintains the in-cylinder temperature at a temperature at which the diesel fuel f is readily ignited, thus enabling stable ignition with a small diesel fuel amount mf. Since stable combustion is performed with a small amount of diesel fuel f even during a low load operation condition, ride quality (drivability) can be improved while reducing engine vibration. Also, the amount of exhaust gas can be reduced during the low load condition.

(34) When the bypass passage 13a is provided in the exhaust passage 13 to provide the low-capacity DPF 18d for the low load condition, the capacity of the DPF 18d provided in the bypass passage 13a can be reduced to realize a compact size. Therefore, the compact DPF 18d can be disposed in a high-temperature portion closer to the engine main body 11. Thus, the amount of PM generated by diesel combustion in the exhaust gas G emitted into the atmosphere can be reduced. Note that, in a high load operation region a high exhaust temperature, the exhaust gas G is controlled to pass through a high-capacity DPF 18c with a catalyst on the downstream side, which is provided in the exhaust passage 13, rather than through the bypass passage 13a. Thus, exhaust resistance is small, and soot is removed.

(35) According to the natural gas engine and the operation method for the natural gas engine according to the present invention, the operation of the exhaust gas introduction mechanism can increase the temperature of the air-fuel mixture of diesel fuel, natural gas fuel, intake air and exhaust gas inside the cylinder. Thus, stable compression combustion is performed even with a small diesel fuel amount, and the natural gas fuel can also be combusted in a stable manner. Accordingly, stable combustion can be realized. As a result, combustion efficiency is improved, and the amount of diesel fuel required for ignition is reduced. Thus, only a small amount of diesel fuel is required. Moreover, an amount of heat generated by combustion of the fuels is reduced as a whole. As a result, the amount of heat propagated to the exhaust passage is reduced, leading to reduction in heat damage and improvement in durability. Therefore, the natural gas engine and the operation method for the natural gas engine according to the present invention can be used for many vehicle-mounted natural gas engines and an operation method for the natural gas engines.