Abstract:
A method and apparatus for controlling combustion in a gas engine connected to an electric generator to compose a power generator unit, the unit being installed near a coal mine site, the gas engine being of a pilot ignition type gas engine which can utilize recovered methane gas and ventilation air methane gas taken out from the coal mine as its fuel by adjusting methane concentration to produce lean air-methane gas mixture are provided. 
     With the method and apparatus, gas engine output torque is controlled so that a relation of |ΔTd|−|ΔTs|&gt;0 is maintained between |ΔTd| which is absolute value of change rate of load torque Td required to drive the generator in relation to engine rotation speed and |Δ Ts| which is absolute value of change rate of output torque Ts in relation to engine rotation speed at an intersection of torque curves, and excess air ratio is controlled to be 2 or larger so that lean mixture burning is performed while evading occurrence of misfire and knock by controlling mixing ratio of recovered methane gas with ventilation air methane.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a continuation-in-part of U.S. application Ser. No. 10/803,975, filed Mar. 19, 2004. 
     
    
     1. FIELD OF THE INVENTION 
       [0002]    The present invention relates to a method and apparatus for controlling combustion in a gas engine employed in a gas engine electric power generator to effectively utilize coal mine methane gas which is low in methane concentration and large in variation thereof, thereby serving to smoothly advance the economic development in developing countries by utilizing the profit made by electric power generation and also made by GHG (greenhouse gas) emission trade. 
       2. DESCRIPTION OF THE RELATED ART 
       [0003]    In growing awareness worldwide of environmental problem, country-by-country objectives of reduction of carbon dioxide emission was decided at the 3rd Conference of the Parties to the United Nations Framework Convention on Climate Change held in 1997 in Kyoto. In the meeting, Kyoto mechanism for the reduction of GHG (CO2, CH4, N2O, etc.) emission in accordance with the conditions of countries and for the promotion of the efficiency of reduction was acknowledged. 
         [0004]    Kyoto mechanism is a system to promote worldwide cooperation and emission credit dealing for the reduction of GHG, in which a concept of carbon dioxide emission credit (right to emission a certain amount of carbon dioxide) was introduced and which aims to utilize market principle as supplementary scheme for achieving the reduction objective of each country. When each entity (nations, enterprises, stores, families, etc.) conducted action of directly exhausting GHG (for example, consuming of energy for operating machines, consuming of gasoline for running vehicles, etc.) or when it conducted action of indirectly exhausting GHG (for example, mining of coal, selling of gasoline, etc.), it is under an obligation to pay carbon dioxide emission credit corresponding to the exhausted amount of GHG. 
         [0005]    As for energy, coal industries of coal industrial nations of the world (China, CIS, Europe, and the United State) are prospected to be important energy suppliers even in the middle part of this century. 
         [0006]    However, accompanying mining of coal, methane gas of 10˜40Nm3 (in terms of pure methane) per ton of coal is released to the atmosphere as recovered methane gas (30˜50% concentration, air diluted) and ventilated gas (0.3˜0.7% concentration, air diluted). Therefore, technology and business to effectively utilize the methane gas now being released to the atmosphere is very prospective and will make large social and economic contributions. 
         [0007]    There are two kind of coal mine methane gas as shown in  FIG. 10 , one is recovered methane gas recovered by a vacuum pump from bore holes for degassing for the sake of safety, and the other is methane gas exhausted together with the ventilation air from the mine cavity and coal face. The concentration of methane of these gas is low, that of the former is 30˜50% and that of the latter is extremely low as 0.3˜0.7%. 
         [0008]    To use a boiler or gas turbine as a heat engine to utilize methane gas has been considered. 
         [0009]    However, if recovered methane gas of methane concentration of 30˜50% is to be used for a gas turbine or boiler, as combustion temperature is low and methane concentration varies violently, it is not practical. It is recognized difficult to use the recovered methane gas for a gas turbine. Actually, the usage of recovered methane has been limited, it has used as fuel by the nearby household, in the case of a boiler used only as auxiliary fuel. 
         [0010]    Therefore, as for the utilization of coal mine methane gas, even recovered methane gas is seldom utilized, and almost all of the coal mine methane gas is released to the atmosphere. 
         [0011]    However, greenhouse effect index of methane gas is 21 times that in the case the methane gas is burnt and released to the atmosphere as CO2. For example, coal mine methane gas release in China is 1.44 billion m3, which is equivalent to more than 10% of total amount of CO2 release in Japan. 
         [0012]    Therefore, if Japan establishes an enterprise to effectively consume the coal mine methane gas in China to change the methane gas to CO2 and release to the atmosphere as CO2, reduction of greenhouse effect index of 20 can be achieved compared to the case the methane gas is released to the atmosphere, for greenhouse effect index of methane is 21, on the other hand, greenhouse effect index of CO2 is 1. This reduction of greenhouse effect index can be traded as emission credit. 
       SUMMARY OF THE INVENTION 
       [0013]    As mentioned above, there are two kind of coal mine methane gas as shown in  FIG. 10 , one is recovered methane gas recovered by a vacuum pump from bore holes for degassing for the sake of safety, and the other is methane gas exhausted together with the ventilation air from the mine cavity and coal face. The concentration of methane of these gas is low, that of the former is 30˜50% and that of the latter is extremely low as 0.3˜0.7%. 
         [0014]    The object of the invention is to provide a method of controlling a generator gas engine capable of effectively utilizing coal mine methane gas low in methane concentration and large in variation of methane concentration as fuel of the gas engine. 
         [0015]    First, construction around the combustion chamber of the gas engine to which the present invention is applied will be explained referring to  FIG. 1 . 
         [0016]    As shown in  FIG. 1 , reference numeral  2  shows a main part  2  around the combustion chamber of the gas engine. A combustion chamber  10  is formed above a piston  4  in a cylinder room  8  of a cylinder  6 . A pilot fuel ignition device  12  having an injection nozzle  12   b  is mounted above the combustion chamber  10  so that the injection nozzle  12   b  faces the combustion chamber. Fuel for ignition such as light fuel oil is injected into the subsidiary chamber  12   c  through a supply pipe  12   a , and combustion gas of the pilot fuel produced in the subsidiary chamber  12   c  is injected into the combustion chamber  10  as flame jets  7  through injection holes of the injection nozzle. Lean air-methane gas mixture in the main combustion chamber  10  is ignited by the flame jets  7 . 
         [0017]    Mixture of recovered methane gas and ventilated methane gas mixed beforehand is introduced to a common inlet pipe  18  and recovered methane gas is further introduced to an inlet passage  16  which is opened and closed by an inlet valve  14 . Thus, air-methane gas mixture is introduced into the cylinder room  8 . Ventilated methane gas of methane concentration of 0.3˜0.7% added with recovered methane gas is compressed by a supercharger  19  (see  FIG. 2 ) and supplied to the common inlet pipe  18 . 
         [0018]    As the gas engine to which the present invention is applied is a pilot ignition engine in which a small amount of fuel (light fuel oil) burnt in the subsidiary chamber is spout out into the combustion chamber to ignite the air-fuel mixture in the combustion chamber, lean air-methane gas mixture of methane gas concentration of 10% or lower, preferably super lean air-methane gas mixture of methane gas concentration of about 3˜5% or 3˜4% can be ignited. Therefore, in the invention, recovery methane gas of 30˜50% methane concentration and ventilation air methane gas of 0.3˜0.7% methane concentration are mixed so that charging air-fuel mixture contains 4˜5% of methane gas through controlling by an engine controller  20 . 
         [0019]    An excess air ratio λ control means  22  in the engine controller  20  controls so that excess air ratio λ is 2 or larger. By this, even in the case of very lean air-methane gas mixture, stable ignition and improvement of engine performance can be achieved. 
         [0020]    Further, an engine output control means  24  is provided in the engine controller  20  in order to achieve stable rotation of the engine without rotational fluctuation even when air-fuel mixture in the combustion chamber  10  is a very lean air-methane mixture. 
         [0021]    The engine output control means  24  controls engine output by controlling methane gas flow so that a relation |ΔTd|−|ΔTs|&gt;0 is valid between |ΔTs| which is absolute value of change rate of output torque Ts in relation to rotation speed n, i.e. |         Ts/         N| in the output torque characteristic curve of the engine and |ΔTd| which is absolute value of change rate of torque Td required to drive the generator in relation to rotation speed n, i.e. |         Td/         N| in the characteristic curve of torque required to drive the generator at the intersection point of the Ts curve with Td curve. 
         [0022]    By this, the engine can operate stably at a rotation speed at which a curve of engine output torque having inclination |ΔTs| coincide with a curve of torque required for driving the generator having inclination of |ΔTd| in the torque versus rotation speed characteristic graph, because rotation speed converges to that at the intersection point of both curves. Therefore, rotational fluctuation can be suppressed even in the case of burning very lean mixture. 
         [0023]    This is explained as follows. In  FIG. 8(   a ), engine output torque characteristic is represented by a curve Ts and load torque characteristic is represented by a curve Td. In this case, operation of the engine is stable as ΔTd&gt;ΔTs. The engine operates stably at a rotation speed at the intersection point of both torque curves of Ts and Td. 
         [0024]    In this case, when rotation speed increases from that at the intersecting point to that at a point A 1  by any cause, Td&gt;ΔTs in the case of  FIG. 8(   a ), load torque Td is larger than output torque Ts at rotation speed at point A 1 , rotation speed of the engine tends to reduce to that at a point A 2  in order to operate at a reduced load torque. As engine output torque at rotation speed at the point A 2  is larger than load torque at the point A 2  as shown by a point A 3  on the Ts curve, rotation speed of the engine tends to increase. Ultimately, rotation speed converges to that at the intersection point of both curves and rotation speed is stabilized there. 
         [0025]    In a case torque characteristics of Ts and Td are as shown in  FIG. 8(   b ) which is another example of stable operation, when rotation speed increases from that at the intersection point of both curves of Ts and Td to that at a point B 1  by any cause, as load torque Td is larger than output torque Ts at rotation speed at point B 1 , rotation speed of the engine tends to reduce to that at a point B 2  in order to operate at a reduced load torque. 
         [0000]    As engine output torque at rotation speed at the point B 2  is smaller than load torque at the point B 2  as shown by a point B 3  on the Ts curve, rotation speed of the engine further tends to decrease. Ultimately, rotation speed converges to that at the intersection point of both curves and rotation speed is stabilized there. 
         [0026]    In a case torque characteristics of Ts and Td are as shown in  FIG. 8(   c ) in which Td&lt;ΔTs, which an example of unstable operation. In  FIG. 8(   c ), when rotation speed increases from that at the intersection point of both curves of Ts and Td to that at point C 1  by any cause, as load torque Td is smaller than engine output torque Ts at rotation speed at point C 1 , rotation speed of the engine tends to reduces to that at a point C 2 . But in this case, engine output torque Ts decreases as rotation speed decreases and engine torque Ts at rotation speed at the point C 2  is smaller than load torque at the point C 2  as shown by a point C 3  on the Ts on the Ts curve, so rotation speed increases, but as there is no point of load torque that is the same as the engine output torque at the point C 3  on the Td curve near the intersection point, rotation speed diverges and does not converge. So rotation speed fluctuates and does not stabilize. 
         [0027]    In a case of  FIG. 8(   d ), although ΔTd&gt;ΔTs, |ΔTd|         |ΔTs|. In this case, also rotation speed does not converge. When rotation speed increases from that at the intersecting point of both curves of Ts and Td to that at a point D 1  by any cause, as load torque Td is larger than engine output torque Ts at the point D 1 , rotation speed tends to reduce to that at a point D 2 . As engine output torque at rotation speed at the point D 2  is larger than load torque at the point D 2  as shown by a point D 3  on the Ts curve, rotation speed tends to increase to a point D 4 , but engine torque at rotation speed at the point D 4  is smaller load torque at the point D 4 , rotation speed must again decrease. Thus, rotation speed diverged and does not converge, it fluctuates and does not stabilize. 
         [0028]    This method of judging convergence or divergence is well known as return map method, and the present invention adopted this method to apply to output torque characteristic of the gas engine and drive torque characteristic of the generator driven by the gas engine to achieve the stabilization of engine operation. 
         [0029]    As a result, by controlling the gas engine so that the relation |ΔTd|−|ΔTs|&gt;0 is maintained at the intersection point of the Ts curve with Td curve, rotation speed can be stabilized at that at intersection point of the controlled engine output torque with the generator drive torque, and stable operation of the gas engine at very lean mixture burning is made possible. 
         [0030]    Concretively, methane gas flow for achieving the relation |ΔTd|−|ΔTs|&gt;0 at the intersection point of the Ts curve with Td curve is determined for the combination of gas engine and generator driven by the engine beforehand by an experiment. That is, methane gas flow is determined for engine rotation speeds in a map, and the engine controller controls methane gas flow according to the map. 
         [0031]    Next, as to controlling excess air ratio λ to be 2 or larger, it is preferable that ventilated methane gas of methane concentration of 0.3˜0.7% is introduced to the compressor of the supercharger, recovered methane gas is added to individual inlet passage branching from a common inlet passage and connecting to each inlet port of each cylinder, and charging pressure is controlled to control excess air ratio by controlling compression pressure of a supercharger provided in the upstream side from the common inlet passage. 
         [0032]    For example, as shown in  FIG. 6 , when it is required to shift excess air ratio λ 1  at point X to ratio λ≧2 at point Y in a lean burn zone, charging pressure is increased to increase charged air amount introduced into the cylinder room. 
         [0033]    By increasing charging pressure, it may be feared that knock occurs. However, as increased charged air amount effects to lower combustion temperature, lean burning atmosphere of excess air ratio λ of 2 or larger can be produced in the combustion chamber without the fear of occurrence of knock. 
         [0034]    As has been described in the foregoing, by controlling output torque of the gas engine so that the relation |ΔTd|−|ΔTs|&gt;0 is maintained at the intersection point of the Ts curve with Td curve, and further controlling charging pressure by controlling the super charger so that excess air ratio λ becomes 2 or larger without occurrence of misfire and knock in combustion in the combustion chamber of the gas engine, stable lean mixture burning of methane gas can be achieved. 
         [0035]    In the apparatus of the invention, it is preferable that a common inlet passage for supplying ventilation air methane to cylinders of the engine, a supercharger having a compressor connecting to the common inlet passage, and a supercharger control means to control excess air ratio of charged air-methane gas mixture to be 2 or larger by controlling charging pressure produced by the supercharger provided upstream the common inlet passage, are provided. 
         [0036]    Further, it is preferable that a mixer for mixing a part of the recovered methane gas with the ventilation air methane gas before the ventilation air methane gas is introduced to the compressor of the supercharger is provided. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a schematic cross-sectional view of the structure of the part around the combustion chamber of the gas engine to which the present invention is applied. 
           [0038]      FIG. 2  is a schematic representation of total construction of the gas engine of  FIG. 1 . 
           [0039]      FIG. 3  is a flowchart of combustion control of the gas engine. 
           [0040]      FIG. 4  is a graph showing a relation between torque requirement Td of the generator and output torque Ts of the gas engine versus engine rotation speed n. 
           [0041]      FIG. 5  is a graph showing apportionment proportion of gas flow Vg 1  via the first electromagnetic valve and gas flow Vg 2  via the second electromagnetic valve versus output torque Ts of the gas engine. 
           [0042]      FIG. 6  is a graph showing a misfire zone and knock zone on a plane represented by mean effective pressure Pme and excess air ration λ. 
           [0043]      FIG. 7  is a graph showing an opening of the exhaust bypass valve. 
           [0044]      FIG. 8  are graphs showing convergence of rotation speed of the engine output torque characteristic curve Ts and the load torque characteristic Td.  FIGS. 8(   a ) and ( b ) shows the example of when the operation of the engine is stable, and  FIGS. 8(   c ) and ( d ) shows when the operation of the engine is unstable. 
           [0045]      FIG. 9  is a schematic representation of total construction of the gas engine of another embodiment. 
           [0046]      FIG. 10  is a schematic representation showing recovering of coal mine methane gas at a coal mining site. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0047]    Preferred embodiments of the present invention will now be detailed with reference to the accompanying drawings. It is intended, however, that unless particularly specified, dimensions, materials, relative positions and so forth of the constituent parts in the embodiments shall be interpreted as illustrative only not as limitative of the scope of the present invention. 
         [0048]    Referring to  FIGS. 1 and 2 , reference numeral  2  shows a main part  2  around the combustion chamber of the gas engine. A combustion chamber  10  is formed above a piston  4  in a cylinder room  8  of a cylinder  6 . A pilot fuel ignition device  12  having an injection nozzle  12   b  is mounted above the combustion chamber  10  so that the injection nozzle  12   b  faces the combustion chamber. Fuel for ignition such as light fuel oil is injected into the subsidiary chamber  12   c  through a supply pipe  12   a , and combustion gas of the pilot fuel produced in the subsidiary chamber  12   c  is injected into the combustion chamber  10  as flame jets  7  through injection holes of the injection nozzle. Lean mixture of methane gas in the main combustion chamber  10  is ignited by the flame jets  7 . Mixture of recovered methane gas and ventilated methane gas mixed beforehand is introduced to a common inlet pipe  18  and recovered methane gas is further introduced to an inlet passage  16  which is opened and closed by an inlet valve  14 . Thus, inlet air added with those methane gases is introduced into the cylinder room  8 . Inlet air added with said mixture of recovered methane gas and ventilated methane gas is compressed by a supercharger  19  and supplied to the common inlet pipe  18 , as already explained referring to  FIG. 1 . 
         [0049]    As shown in  FIG. 2  showing total construction of the gas engine, an electric generator  32  is connected to a crankshaft  30 , a rotation speed sensor  34  for detecting engine rotation speed is attached to the crankshaft  30 , a cylinder pressure sensor  36  for detecting pressure of the combustion chamber is provided, and a manifold pressure sensor  38  and temperature sensor  40  are attached to the common inlet pipe  18 . Detected speed, pressure, and temperature are inputted as detected signals to an engine controller  20 . 
         [0050]    A supercharger  19  having a gas turbine  41  driven by exhaust gas of the engine and a compressor  42  is connected to the common inlet pipe  18 . An exhaust bypass valve  44  for bypassing a part of exhaust gas entering the turbine is provided at the exhaust gas entrance of the turbine. The exhaust bypass valve  44  is driven by a high speed electric actuator and opening of the exhaust bypass valve is controlled by a signal from the engine controller  20 . 
         [0051]    An inlet air pipe  46  is connected to the compressor  42  of the supercharger  19 , and ventilated methane gas (ventilation air methane (VAM)) added with recovered methane gas through a mixer  48  compressed by the compressor  42  is introduced to the common inlet pipe  18  via an air cleaner  43 . The inlet air pipe  46  is provided with the mixer  48  and a filter  50 . Ventilated methane gas (VAM) is introduced from the upstream side of the filter  58 . Methane concentration of the ventilation air methane is very low, usually it is 0.3˜0.7%. The mixer  48  is to add recovered methane gas (CMM, coal mine methane) to the ventilation air methane (VAM) flowing in the inlet air pipe  46  to be mixed with the VAM. 
         [0052]    The coal mine gas (CMM) is methane gas of methane concentration of 30˜50% recovered from a bore for degassing by a vacuum pump. 
         [0053]    The recovered methane gas reserved in a buffer tank  54  is introduced to a filter  56 , from where a part thereof is introduced to the mixer  48  through a recovered methane gas passage  52  via a first electromagnetic valve  58  and the remnant is introduced to a compressor  58  to be compressed there. The compressed gas is introduced to each of the inlet passages  16  via each of second electromagnetic valves  60 . Thus, recovered methane gas (CMM) is further added to the mixture of VAM and CMM flowing in the inlet passages  16  of each cylinder through each of the second electromagnetic valves  60 . 
         [0054]    Opening period of the first electromagnetic valve  58  and second electromagnetic valves  60  are controlled by the engine controller  20 . 
         [0055]    In the gas engine composed as mentioned above, pressure of the recovered methane gas in the buffer tank  54  is adjusted by a pressure adjusting means (not shown) and recovered methane gas is added to the mixture of VAM and CMM flowing in the inlet air pipe  46  through the first electromagnetic valve  58 , and further recovered methane gas is added to the mixture of VAM and CMM flowing in the inlet passages  16  through the second electromagnetic valves  60 . 
         [0056]    The recovered methane gas introduced through the first electromagnetic valve  58  is mixed with the ventilation air methane (VAM) flowing in the inlet air pipe  46 . This mixture of VAM and CMM is compressed by the compressor  42  of the super charger  19  and flows into the common inlet pipe  18 , from where it flows into the cylinder room  8  of each cylinder passing through the inlet passage  16  with recovered methane gas further added in the inlet passage  16 . This air-fuel mixture is introduced into the cylinder room in the suction stroke and compressed in the compression stroke, then the flame jets  7  is injected from the injection nozzle of the pilot fuel ignition device  12 , and the air-fuel mixture is burned in the combustion chamber  10 . 
         [0057]    Next, combustion control of the gas engine will be explained. 
         [0058]    Combustion control is performed by the engine controller  20  provided with a control means  24  for controlling engine output and a control means for controlling excess air ratio μ. 
         [0059]    The engine output control means  24  will be explained referring to  FIG. 3  showing a control flowchart. 
         [0060]    Control is started at step S 1 . At step S 2 , engine rotation speed is detected by the signal from the rotation speed sensor  34 . Then at step S 3 , methane gas requirement Vg to maintain a relation |ΔTd|−|ΔTs|&gt;0 for the detected rotation speed is calculated from a prescribed map MI. In the map M 1  is determined methane gas requirement versus engine rotation speed obtained by an experiment for a combination of gas engine and generator to be driven by the engine. 
         [0061]    |ΔTs| is absolute value of change rate of output torque Ts in relation to rotation speed n, i.e. |         Ts/         N| in the output torque characteristic curve of the engine at the intersection point of the Ts curve with Td curve, and |ΔTd| is absolute value of change rate of torque Td required to drive the generator  32  in relation to rotation speed n, i.e. |         Td/         N| in the characteristic curve of torque required to drive the generator at the intersection point of the Ts curve with Td curve. 
         [0062]    In  FIG. 4 , load torque Td required to drive the generator decreases as rotation speed n increases, whereas output torque Ts of the engine increases as rotation speed increases, and absolute value of inclination of engine output torque Ts is gentler than that of generator drive torque Td. 
         [0063]    By controlling engine output torque as shown in  FIG. 4 , if rotation speed increases from that at the intersection point of the Ts curve with Td curve, at which point the engine operating stably, by any cause to rotation speed at a point E 1 , rotation speed tends to decrease to that at a point E 2 , and ultimately rotation speed converges to that at the intersection point of both curves Ts and Td, as explained referring to  FIG. 8(   a ), and the engine operates stably at rotation speed at the intersection point. 
         [0064]    At step S 4 , the methane gas requirement Vg obtained from the map M 1  is apportioned between the first electromagnetic valve  58  and each of the second electromagnetic valves  60  so that gas flow of Vg 1  is introduced through the first electromagnetic valve  58  and gas flow of Vg 2  is introduced through each of the second electromagnetic valves  60 . The apportionment is done based on apportionment proportion characteristic shown in  FIG. 5 . 
         [0065]    At step S 5 , the opening period T 1  of the first electromagnetic valves  58  is controlled to allow gas flow of Vg 1 . At step S 6 , the opening period T 2  of the second electromagnetic valves  58  is controlled to allow gas flow of Vg 1 . Thus, the methane gas requirement Vg is into the cylinder room via the first and second electromagnetic valves. 
         [0066]    The engine output control means  24  consists of the steps of S 2  to S 6 . 
         [0067]    Next, the excess air ratio control means  22  will be explained. 
         [0068]    At step S 7 , density γs of air-methane gas mixture flowing in the common inlet pipe  18  is calculated based on pressure and temperature detected by the pressure sensor  38  and temperature sensor  40 . 
         [0069]    At step S 8 , mass flow Gm of the mixture gas flowing in the common inlet pipe  18  is calculated using the calculated density γs, and approximate excess air ratio λ′ is calculated using the mixture flow Gm and the methane gas requirement Vg calculated at the step S 3 . As the mixture gas flowing in the common inlet pipe  18  consists of ventilation air methane and a part of recovered methane gas introduced through the first electromagnetic valve  58 , said λ′ is not accurate calculation value but an approximate value calculated by assuming the mixture in the common inlet pipe  18  consists of only air, for methane concentration of ventilation air methane is usually very small as 0.3˜0.7% and methane concentration in the common inlet pipe  18  is very small. 
         [0070]    Next, at step S 9 , cylinder pressure, i.e. combustion pressure P 0  is detected. At step S 10 , mean effective pressure Pm is calculated by using the detected combustion pressure P 0  and the calculated excess air ratio λ′ (or excess air ratio λ obtained by correcting the value of λ′), and whether the combustion has occurred in a zone between a misfire zone C and knock zone E and further whether occurred in a zone of excess air ratio λ of 2 or larger based on the map as shown in  FIG. 6  is judged. 
         [0071]    In  FIG. 6 , the abscissa represents effective mean pressure Pm and ordinate excess air ratios λ. In this graph, A, B, and C represent a misfire zone respectively, and E a knock zone. D represents a excessive rich mixture zone. The zones A, B, and C represent misfire zones of gas engines different in combustion type respectively, the zone A is a misfire zone of stoichiometrical combustion gas engine, B is a misfire zone of spark ignition with subsidiary chamber type gas engine, and C is misfire zone of light oil injection with subsidiary chamber type gas engine. 
         [0072]    To achieve stable combustion in lean mixture burning of methane fuel, it is necessary to control so that combustion occurs in a zone not belonging to any of the A, B, C, D, and E zones in  FIG. 6 . 
         [0073]    When it is judged at the step S 10  that combustion is occurring in a zone not belonging to any of the zones A˜E, for example, judged that combustion occurring at the point X, the excess air ratio control means  22  controls so that the point X is shifted to Y to increase excess air ratio by controlling opening of the exhaust bypass valve  44  for bypassing a part of exhaust gas entering in the turbine  41  of the supercharger  19  via actuator. The actuator comprises an electric actuator of high speed response and driven by a signal from the engine controller  20 . The actuator actuates to decrease opening of the exhaust bypass valve  44  when increasing combustion pressure P 0  and excess air ratio λ, and increase opening of the exhaust bypass valve  44  when decreasing combustion pressure P 0  and excess air ratio λ. 
         [0074]    Conventional mechanical valve drive device of diaphragm type is slow in response and boost pressure can not be increased rapidly when the actuator receives signal to increase opening of the exhaust bypass valve  44 . Therefore, there was a tendency that, when intending to shift from the point X to the point Y, the point X shift toward a point Z in the knock zone E because of retarded boost pressure rise. By adopting the high speed electric actuator, control response of the exhaust bypass valve  44  is increased and shifting from the point X to the point Y in  FIG. 6  is performed with certainty. 
         [0075]    In this way, excess air ratio λ can be controlled by controlling charging pressure of the supercharger. 
         [0000]    By increasing charging pressure, there may be a fear that knock occurs, however, as increased charged air amount effects to lower combustion temperature, lean burning atmosphere of excess air ratio λ of 2 or larger can be produced in the combustion chamber without the fear of occurrence of knock. 
         [0076]    Although the exhaust bypass valve  44  was explained here, it is also suitable to control charging pressure providing an inlet air bypass valve to the compressor  42 . By controlling the inlet bypass valve, the same effect can be achieved. 
         [0077]    When it is judged that combustion is occurring in zones not belonging to A˜E zones, operation is continued with the opening of the exhaust bypass valve  44  maintained at step S 12 , and the process ends at step S 13 . 
         [0078]    By controlling combustion of the gas engine as mentioned above, stable lean mixture burning with excess air ratio λ of 2 or larger is made possible without occurrence of knock and misfire, and a gas engine can be obtained which can effectively utilize coal mine methane gas which varies considerably in methane concentration including ventilation air methane which is very low in methane concentration as fuel can be obtained. 
         [0079]    Another embodiment of the gas engine is shown in  FIG. 9 . The gas engine of this embodiment is not provided with the electromagnetic valve  58  and mixer  48  of the first embodiment, and other than that is the same as the first embodiment shown in  FIG. 2 . 
         [0080]    According to the embodiment, recovered methane gas is not introduced to the air inlet pipe  46  through the mixer  48  as is in the first embodiment, and recovered methane gas is introduced into each of the inlet passages  16  only through each of the second electromagnetic valves  60 . 
         [0081]    Therefore, only ventilation air methane which is very low in methane concentration is contained in the charging air-methane mixture in the common inlet pipe  18 . As approximate excess air ratio λ′ is calculated by using the density γs calculated based on the pressure P 1  and temperature T 1  of the air-methane mixture in the common inlet pipe  18 , the calculated value of excess air ratio λ′ is more nearer to actual excess air ratio λ as compared with the case recovered methane gas is introduced through the mixer  48  via the first electromagnetic valve  58  before the mixture enters the common inlet pipe  18  as is in the first embodiment. 
         [0082]    Accordingly, combustion control is performed based on more accurate excess air ratio λ, and construction of the control apparatus is simplified as compared with the first embodiment.