Patent Publication Number: US-2019170077-A1

Title: Combustion control method and combustion control system with variable excess air coefficient for gasoline engine

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the national phase entry of International Application PCT/CN2017/103980, filed on Sep. 28, 2017, which is based upon and claims priority to Chinese Patent Application No. 201610880511.6, filed on Sep. 30, 2016, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to the field of engines, more particularly to a combustion control method and a combustion control system with variable excess air coefficient for gasoline engine. 
     BACKGROUND 
     Homogeneous charging compression ignition (HCCI) control technology, which realizes low temperature lean combustion by controlling intake air temperature and premixed compression ignition combustion method, effectively improves effective thermal efficiency of the engine and meanwhile reduces pollutant emissions. However, it is difficult to precisely control the intake air temperature. Furthermore, as the engine load increases, the intake air temperature has much greater influence on the compression ignition and even a tiny variation of the temperature will lead to big changes during compression ignition, which may finally result in that the combustion phasing and heat release rate of HCCI control technology get out of control. Meanwhile, as the engine load increases, the rise rate of combustion pressure sharply increases, and uncontrollable abnormal combustion process such as knocking may occur. 
     Technical Problem 
     In existing gasoline engine combustion control technology, the fuel and the air usually are mixed in stoichiometric air-fuel ratio prior to combustion. Since the excess air coefficient does not change along with engine operating conditions (engine load conditions), the advantage that lean combustion effectively improves thermal efficiency of the engine cannot be fully realized. 
     Technical Solution for Solving the Problem 
     SUMMARY 
     In order to overcome the shortcomings of the prior art, the disclosure aims to provide a combustion control method and a combustion control system with variable excess air coefficient for gasoline engine, which use different excess air coefficients under different operating conditions of the gasoline engine and achieve comprehensive improvement on effective thermal efficiency, power performance and pollutant emission in entire range of gasoline engine operating conditions. 
     The disclosure provides a combustion control method with variable excess air coefficient for gasoline engine comprising steps as follows: 
     Monitoring an engine operating condition; 
     Determining current engine load condition based on the engine operating condition, wherein the engine load conditions include a part load condition, a high load condition, or a full load condition; 
     Selecting an appropriate excess air coefficient combustion mode in accordance with the current engine load condition; wherein the engine use a lean combustion mode with an excess air coefficient of 1.6˜2.0 when the current engine load condition is the part load condition; the engine use a combustion mode with an excess air coefficient of 1 when the current engine load condition is the high load condition; and the engine use a combustion mode with an excess air coefficient of 0.8˜0.9 when the current engine load condition is the full load condition. 
     Furthermore, the part load condition is a condition in which the engine operates at 0˜50% rated power, the high load condition is a condition in which the engine operates at 50%-90% rated power, and the full load condition is a condition in which the engine operates at 900/˜100% rated power. 
     Furthermore, when the current engine load condition is the part load condition, the engine use the lean combustion mode with the excess air coefficient of 1.6˜2.0, in which combustion mode, precisely controlling the fuel supply amount by means of the fuel injection system and precisely controlling the amount of intake fresh air by means of the variable valve system, so as to form a lean mixture with the excess air coefficient of 1.6˜2.0 inside the cylinder and ignite it. Meanwhile, the engine uses Miller cycle in combination with exhaust gas recirculation to lower the combustion temperature inside the engine cylinder and make the combustion temperature inside the engine cylinder lower than 1900K. 
     Furthermore, controlling the quantity and the temperature of re-circulated exhaust gas after combustion by exhaust gas recirculation in such a manner that the mixture temperature of the lean mixture inside the cylinder with the excess air coefficient of 1.6˜2.0 at the ignition timing equals to quasi self-ignition temperature. 
     Furthermore, igniting the lean mixture with the excess air coefficient of 1.6˜2.0 by ignition energy greater than 400 MJ. 
     Furthermore, when the current engine load condition is the high load condition, the engine use the combustion mode with the excess air coefficient of 1, in which combustion mode, precisely controlling the fuel supply amount by means of the fuel injection system and precisely controlling the amount of intake fresh air by means of the variable valve system, so as to form a mixture with the excess air coefficient of 1 inside the cylinder and ignite it. Meanwhile, the engine uses exhaust gas recirculation to adjust the quantity and the temperature of re-circulated exhaust gas so as to realize appropriate combustion. 
     Furthermore, when the current engine load condition is the full load condition, the engine use the combustion mode with the excess air coefficient of 0.8˜0.9, in which combustion mode, precisely controlling the fuel supply amount by means of the fuel injection system and precisely controlling the amount of intake fresh air by means of the variable valve system, so as to form a rich mixture with the excess air coefficient of 0.8˜0.9 inside the cylinder and ignite it. Meanwhile, the engine can adjust the quantity and the temperature of re-circulated exhaust gas by exhaust gas recirculation, so as to realize appropriate combustion. 
     Furthermore, the combustion control method with variable excess air coefficient for gasoline engine further comprises a step of controlling the quantity of the re-circulated exhaust gas after combustion by the exhaust gas recirculation, wherein the quantities of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the quantity in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the quantity in the combustion mode with the excess air coefficient of 1&gt;the quantity in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     Furthermore, the combustion control method with variable excess air coefficient for gasoline engine further comprises a step of controlling the temperature of the re-circulated exhaust gas after combustion by the exhaust gas recirculation, wherein the temperatures of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the temperature in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the temperature in the combustion mode with the excess air coefficient of 1&gt;the temperature in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     Furthermore, after catalytic conversion of the exhaust gas to be discharged that is produced by the combustion in the engine, discharging the exhaust gas. 
     The disclosure further provides a combustion control system with variable excess air coefficient for gasoline engine, which comprises a cylinder block structure, a variable valve system, a fuel injection system, a high energy ignition system, an exhaust gas recirculation system, an operating condition monitoring system, and an engine electronic control unit, wherein the cylinder block structure comprises a cylinder, the variable valve system is configured to control the amount of the fresh air supplied to the cylinder, the fuel injection system is configured to control the amount of the fuel injected into the cylinder, the high energy ignition system comprises a high energy spark plug and is configured for discharge ignition, and the exhaust gas recirculation system is configured to control the quantity and the temperature of re-circulated exhaust gas, wherein: 
     The engine electronic control unit is electrically connected with the operating condition monitoring system, the variable valve system, the fuel injection system, the high energy ignition system, and the exhaust gas recirculation system, respectively; 
     The operating condition monitoring system is configured to monitor an engine operating condition and transmit the monitored result to the engine electronic control unit; 
     Based on the engine operating condition, a current engine load condition can be determined by means of the engine electronic control unit, wherein the engine load conditions include a part load condition, a high load condition, or a full load condition; 
     By means of the engine electronic control unit, an appropriate excess air coefficient combustion mode can be selected in accordance with the current engine load condition, wherein: 
     When the current engine load condition is the part load condition, the engine is controlled, by means of the engine electronic control unit, to use the lean combustion mode with an excess air coefficient of 1.6˜2.0; 
     When the current engine load condition is the high load condition, the engine is controlled, by means of the engine electronic control unit, to use the combustion mode with an excess air coefficient of 1; 
     When the current engine load condition is the full load condition, the engine is controlled, by means of the engine electronic control unit, to use the combustion mode with an excess air coefficient of 0.8˜0.9. 
     Furthermore, the operating condition monitoring system may comprise an engine speed monitoring sensor and a power monitoring device. 
     Furthermore, the exhaust gas recirculation system may comprise an exhaust gas recirculation line, an exhaust gas recirculation control valve, an exhaust gas recirculation intercooler, an exhaust gas bypass return line and a bypass waste gate, wherein the outlet end of the exhaust gas recirculation line is in communication with the intake pipe, and the inlet end of the exhaust gas recirculation line is in communication with the exhaust pipe, the exhaust gas recirculation control valve and the exhaust gas recirculation intercooler are arranged in the exhaust gas recirculation line in series connection, the exhaust gas bypass return line and the exhaust gas recirculation intercooler are arranged in parallel, and the bypass waste gate is arranged in the exhaust gas bypass return line. 
     Furthermore, the exhaust gas recirculation system is configured to control the quantity of the re-circulated exhaust gas after combustion, wherein the quantities of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the quantity in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the quantity in the combustion mode with the excess air coefficient of 1&gt;the quantity in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     Furthermore, the exhaust gas recirculation system is configured to control the temperature of the re-circulated exhaust gas after combustion, wherein the temperatures of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the temperature in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the temperature in the combustion mode with the excess air coefficient of 1&gt;the temperature in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     Furthermore, the ignition energy of the high energy spark plug can ignite the rich mixture having an excess air coefficient of 0.8˜0.9 and the mixture having an excess air coefficient of 1, and can ignite the lean mixture having an excess air coefficient of 1.6˜2.0. 
     Advantages 
     The above-mentioned combustion control method and combustion control system with variable excess air coefficient for gasoline engine use different excess air coefficient combustion modes in accordance with different operating conditions of the gasoline engine, achieving comprehensive improvement on effective thermal efficiency, power performance and pollutant emission in entire range of gasoline engine operating conditions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural drawing of a combustion control system with variable excess air coefficient for gasoline engine of the disclosure; 
         FIG. 2  is a schematic modular diagram of a combustion control system with variable excess air coefficient for gasoline engine of the disclosure; 
         FIG. 3  is a flowchart illustrating a combustion control method with variable excess air coefficient for gasoline engine of the disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     In order to further explain the technical solutions of the disclosure that achieve the above objectives as well as the advantages, specific embodiments, structures, features and effects of the disclosure are described in detail hereinafter with reference to the accompanying drawings and preferred embodiments. 
       FIG. 1  is a schematic structural drawing of a combustion control system with variable excess air coefficient for gasoline engine of the disclosure, and  FIG. 2  is a schematic modular diagram of a combustion control system with variable excess air coefficient for gasoline engine of the disclosure. Referring to  FIGS. 1 and 2 , a combustion control system  100  with variable excess air coefficient for gasoline engine according to the disclosure comprises a cylinder block structure  10 , a variable valve system  20 , a fuel injection system  30 , a high energy ignition system  40 , an exhaust gas recirculation system  50 , a catalytic converter  60 , an operating condition monitoring system  70 , and an engine electronic control unit  80 . 
     The cylinder block structure  10  comprises a cylinder  11  and a cylinder head  12 . Herein, the cylinder head  12  is arranged above the hollow cylinder  11 , and the cylinder  11  comprises a hollow cylinder body  111 , a piston  112  inserted in the cylinder body  111  and movable along an axis of the cylinder body  111 , and a connecting rod  113  connected with the piston  112  in an articulated manner. The space between the bottom surface of the cylinder head  12 , the top surface of the piston  112  of cylinder  11  and the cylinder body  111  of cylinder  11 , forms a combustion chamber  13 . Since the present embodiment uses Miller cycle technology, the combustion chamber  13  is a Miller cycle combustion chamber. 
     The variable valve system  20  is used for precisely controlling the amount of the air that is required for combustion and supplied to the cylinder  11 . The variable valve system  20  comprises an intake pipe  21 , an exhaust pipe  22 , an intake valve  23 , an exhaust valve  24 , a throttle valve  25 , and a variable valve timing mechanism  26 . Herein, the intake valve  23  and the exhaust valve  24  are arranged on the cylinder head  12 , the intake pipe  21  and the exhaust pipe  22  are arranged outside the cylinder head  12  and are fixedly connected with the cylinder head  12 , the intake pipe  21  is in communication with the cylinder  11  through the intake valve  23 , and the exhaust pipe  22  is in communication with the cylinder  11  through the exhaust valve  24 . Since the present embodiment uses Miller cycle technology, a Miller cycle small lift intake valve is used as the intake valve  23 . The variable valve timing mechanism  26  uses variable valve timing (VVT) technology, and can adjust the amount of intake air (exhaust air) by adjusting the valve opening and closing timing and angle of the intake valve  23  (the intake valve  24 ) according to operation conditions of the engine, so as to optimize the amount of intake air and increase combustion efficiency. The variable valve timing mechanism  26  comprises a variable intake valve actuator  261  arranged at an upper end of the intake valve  23  and configured to control opening and closing of the intake valve  23 , and a variable exhaust valve actuator  262  arranged at an upper end of the exhaust valve  24  and configured to control opening and closing of the exhaust valve  24 . The throttle valve  25  is arranged on the intake pipe  21  and is configured to control the amount of intake air supplied through the intake pipe  21 . The variable intake valve actuator  261 , the variable exhaust valve actuator  262 , and the throttle valve  25  are electronically controlled actuator components, and in particular, the engine electronic control unit  80  is electrically connected with the variable intake valve actuator  261 , the variable exhaust valve actuator  262  and the throttle valve  25 , respectively, and controls operations of these components. 
     The cylinder block structure  10  and the variable valve system  20  work in coordination, and Miller cycle technology is used. Miller cycle realizes a combustion cycle that has an expansion ratio higher than effective compression ratio, improves thermal efficiency of the engine and meanwhile reduces maximum combustion temperature. 
     The fuel injection system  30  is used for controlling the amount of the fuel injected into the cylinder  11 , which system uses gasoline direct injection (GDI) technology. The fuel injection system  30  comprises a fuel tank  31 , a fuel delivery pipe  32 , a low-pressure fuel delivery pump  33 , a high-pressure fuel delivery pump  34 , and a high pressure injector  35 . Herein, the two ends of the fuel delivery pipe  32  are respectively connected with the fuel tank  31  and the high pressure injector  35 , the low-pressure fuel delivery pump  33  is connected with the fuel tank  31 , the high-pressure fuel delivery pump  34  is configured to pressurize and deliver the fuel, the high-pressure fuel delivery pump  34  is arranged on the fuel delivery pipe  32  and is configured to further pressurize the fuel inside of the fuel delivery pipe  32 , the high pressure injector  35  is arranged on the cylinder head  12  and used for directly injecting fuel into the cylinder  11 , the high pressure injector  35  has a fuel nozzle disposed inside the cylinder  11  (the combustion chamber  13 ), and the fuel inside the fuel delivery pipe  32  is injected into the cylinder  11  via the fuel nozzle of the high pressure injector  35 . The high pressure injector  35  is an electronically controlled actuator component, and in particular, the engine electronic control unit  80  is electrically connected with the high pressure injector  35  and controls its injecting operation. 
     The fuel injection system  30  and the variable valve system  20  work in coordination, to realize precise control of the amount of intake air and fuel in the cylinder  11 , thereby achieving a variable control of engine excess air coefficient. 
     The high energy ignition system  40  comprises a high energy spark plug  41  and a high energy discharge power supply (not shown) for providing electrical energy to the high energy spark plug  41 . The high energy spark plug  41  is arranged on the cylinder head  12 , and it has an ignition end disposed inside the combustion chamber  21 . The ignition energy generated by the high energy spark plug  41  may reach up to 400 MJ. Such ignition energy can not only ignite the rich mixture having an excess air coefficient of 0.8˜0.9 and the mixture having an excess air coefficient of 1, but also can ignite the lean mixture having an excess air coefficient of 1.6˜2.0. The high energy spark plug  41  is an electronically controlled actuator component, and in particular, the engine electronic control unit  80  is electrically connected with the high energy spark plug  41  and controls its discharge ignition. 
     The exhaust gas recirculation (EGR) system  50  comprises an exhaust gas recirculation line  51 , an exhaust gas recirculation control valve  52 , an exhaust gas recirculation intercooler  53 , an exhaust gas bypass return line  54 , and a bypass waste gate  55 . Herein, the outlet end of the exhaust gas recirculation line  51  is in communication with the intake pipe  21  downstream of the throttle valve  25 , and the inlet end of the exhaust gas recirculation line  51  is in communication with the exhaust pipe  22 . The exhaust gas recirculation control valve  52  and the exhaust gas recirculation intercooler  53  are arranged in the exhaust gas recirculation line  51  in series connection. The exhaust gas bypass return line  54  and the exhaust gas recirculation intercooler  53  are arranged in parallel. The bypass waste gate  55  is arranged in the exhaust gas bypass return line  54 , and is used for controlling the proportions of the exhaust gas in the exhaust gas recirculation line  51  that passes through and does not pass through the exhaust gas recirculation intercooler  53 , so as to precisely control the temperature of re-circulated exhaust gas. The exhaust gas recirculation control valve  52  and the bypass waste gate  55  are electronically controlled actuator components, and in particular, the engine electronic control unit  80  is electrically connected with the exhaust gas recirculation control valve  52  and the bypass waste gate  55  and controls opening and closing of the valves. 
     In the present embodiment, the exhaust gas recirculation technology is used to control the quantity and the temperature of re-circulated exhaust gas. In particular, a portion of exhaust gas exhausted from the cylinder  11  enters into the exhaust gas recirculation line  51  via the exhaust pipe  22 , the amount of the exhaust gas entered into the exhaust gas recirculation line  51  can be controlled by means of the exhaust gas recirculation control valve  52 . Furthermore, the exhaust gas in the exhaust gas recirculation line  51  is divided into two portions, one of which is cooled by means of the exhaust gas recirculation intercooler  53 , and the other of which bypasses the exhaust gas recirculation intercooler  53 , passes through the exhaust gas bypass return line  54  and enters into the exhaust gas recirculation line  51  to mix with the exhaust gas cooled by means of the exhaust gas recirculation intercooler  53 , and then re-circulates in the intake pipe  21  to mix with fresh air and then enters into the cylinder  11  again. Herein, the proportions of the exhaust gas passing through the exhaust gas recirculation intercooler  53  and the exhaust gas passing through the exhaust gas bypass return line  54  can be adjusted by the opening and closing of the bypass waste gate  55  in the exhaust gas bypass return line  54 , whereby the temperature of the exhaust gas re-circulated to the intake pipe  21  can be precisely controlled. 
     The exhaust gas recirculation system  50  serves for controlling the quantity and the temperature of re-circulated exhaust gas, and realizing control requirements of the quantity and the temperature of re-circulated exhaust gas under different operating conditions. By means of the exhaust gas recirculation system  50 , the temperature and the quantity of re-circulated exhaust gas can be controlled, and thus the temperature of the mixture in the cylinder at the ignition timing can be controlled. After ignition of the mixture, the quantity of the re-circulated exhaust gas directly affects the maximum combustion temperature during combustion. The larger the quantity of the re-circulated exhaust gas, the lower the maximum combustion temperature. 
     The catalytic converter  60  is arranged in the exhaust pipe  22 , and the catalytic converter  60  may be filled with catalysts such as platinum and palladium, so that prior to emission, the exhaust gas, which is exhausted into the exhaust pipe  22  after the combustion inside the cylinder  11  and which contains nitrogen oxides, hydrocarbon, carbon monoxide and so on, can be catalyzed and converted into the gas that meets the environmental protection requirements for emission. Depending on different combustion temperatures and air coefficients, different exhaust gas pollutants may be produced. For example, during low temperature lean combustion, the exhaust gases including hydrocarbon and carbon monoxide may be produced, and during high temperature combustion, the exhaust gases including nitrogen oxides, hydrocarbon and carbon monoxide may be produced. By means of the catalytic converter  60 , the exhaust gas pollutants can be converted into the gas that meets the requirements for emission. 
     Referring to  FIG. 2 , the engine electronic control unit  80  is electrically connected with the operating condition monitoring system  70 , the variable valve system  20 , the fuel injection system  30 , the high energy ignition system  40 , and the exhaust gas recirculation system  50 , respectively. The operating condition monitoring system  70  is used for monitoring engine operating conditions such as engine speed and engine power. The operating condition monitoring system may comprise an engine speed monitoring sensor (not shown), a power monitoring device (not shown), and the like, to monitor engine speed, engine power and the like and transmit the monitored results to the engine electronic control unit  80 . Then, based on the monitored results, the engine electronic control unit  80  may control the variable valve system  20 , the fuel injection system  30 , the high energy ignition system  40  and the exhaust gas recirculation system  50  to work. 
       FIG. 3  is a flowchart illustrating a combustion control method with variable excess air coefficient for gasoline engine of the disclosure. Referring to  FIG. 3 , a combustion control method with variable excess air coefficient for gasoline engine of the disclosure comprises steps as follows. 
     S 1 . Monitoring engine operating conditions; 
     S 2 . Determining current engine load condition based on the engine operating conditions; 
     S 3 . Selecting an appropriate excess air coefficient combustion mode in accordance with the current engine load condition; 
     S 4 . After catalytic conversion of the exhaust gas to be discharged that is produced by the combustion in the engine, discharging the exhaust gas. 
     In the step S 1 , engine operating conditions can be monitored, for example engine operating parameters including engine speed and engine power can be monitored, to obtain engine operating conditions. 
     In the step S 2 , current engine load condition can be determined based on the obtained engine operating conditions, wherein the engine load condition may be a part load condition, a high load condition, or a full load condition. In the present embodiment, the part load condition is a condition in which the engine operates at 0˜50% rated power, the high load condition is a condition in which the engine operates at 50%-90% rated power, and the full load condition is a condition in which the engine operates at 90%-100% rated power. 
     In the step S 3 , an appropriate excess air coefficient combustion mode can be selected based on the obtained current engine load condition. In the present embodiment, the engine may use a lean combustion mode with an excess air coefficient of 1.6˜2.0 when the current engine load condition is the part load condition, use a combustion mode with an excess air coefficient of 1 when the current engine load condition is the high load condition, and use a combustion mode with an excess air coefficient of 0.8˜0.9 when the current engine load condition is the full load condition. 
     In the case that the current engine load condition is the part load condition, the engine may use the combustion mode with the excess air coefficient of 1.6˜2.0. In such combustion mode, the amount of intake air is precisely controlled by means of the variable valve system  20 , and the amount of fuel is precisely controlled by means of the fuel injection system  30 , so that a lean mixture with the excess air coefficient of 1.6˜2.0 can be formed inside the cylinder  11 , and the ignition can be achieved with the ignition energy of the high energy ignition system  40 . Meanwhile, the engine uses Miller cycle in combination with exhaust gas recirculation (the exhaust gas recirculation system  50 ) to lower the combustion temperature inside the engine cylinder  11  and make the combustion temperature inside the engine cylinder  11  lower than 1900K. Furthermore, the quantity and the temperature of re-circulated exhaust gas after combustion can be controlled by exhaust gas recirculation in such a manner that the mixture temperature of the lean mixture inside the cylinder with the excess air coefficient of 1.6˜2.0 at the ignition timing equals to quasi self-ignition temperature, thereby realizing controllable combustion phasing and meanwhile increasing isochoric degree of combustion. Furthermore, the lean mixture with the excess air coefficient of 1.6˜2.0 is ignited by the ignition energy greater than 400 MJ, that is, the ignition energy generated by the high energy ignition system  40  can reach over 400 MJ. 
     In the case that the current engine load condition is the high load condition, the engine may use the combustion mode with the excess air coefficient of 1. In such combustion mode, the amount of intake air is precisely controlled by means of the variable valve system  20 , and the amount of fuel is precisely controlled by means of the fuel injection system  30 , so that a mixture with the excess air coefficient of 1 can be formed inside the cylinder  11 . Meanwhile, with the exhaust gas recirculation (the exhaust gas recirculation system  50 ), the engine can adjust the quantity and the temperature of re-circulated exhaust gas to match with the current combustion mode, thereby improving knock suppression. 
     In the case that the current engine load condition is the full load condition, the engine may use the combustion mode with the excess air coefficient of 0.8˜0.9. In such combustion mode, the amount of intake air is precisely controlled by means of the variable valve system  20 , and the amount of fuel is precisely controlled by means of the fuel injection system  30 , so that a rich mixture with the excess air coefficient of 0.8˜0.9 can be formed inside the cylinder. Meanwhile, with the exhaust gas recirculation (the exhaust gas recirculation system  50 ), the engine can adjust the quantity and the temperature of re-circulated exhaust gas to match with the current combustion mode, thereby ensuring available output power of the engine. 
     In particular, the engine controls the quantity of the re-circulated exhaust gas after combustion by the exhaust gas recirculation, wherein the quantities of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the quantity in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the quantity in the combustion mode with the excess air coefficient of 1&gt;the quantity in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     In particular, the engine controls the temperature of the re-circulated exhaust gas after combustion by the exhaust gas recirculation, wherein the temperatures of the re-circulated exhaust gas under three combustion modes have a relationship as follows: the temperature in lean combustion mode with the excess air coefficient of 1.6˜2.0&gt;the temperature in the combustion mode with the excess air coefficient of 1&gt;the temperature in the combustion mode with the excess air coefficient of 0.8˜0.9. 
     In the step S 4 , after catalytic conversion of the exhaust gas to be discharged that is produced by the combustion in the engine, the exhaust gas is discharged. When the current engine load condition is the part load condition, the engine may use the combustion mode with the excess air coefficient of 1.6˜2.0, the exhaust gases to be discharged which is produced by the combustion in the engine include hydrocarbon and carbon monoxide, which exhaust gases are catalyzed and converted by means of the catalytic converter  60  and then discharged. When the current engine load condition is the high load condition, the engine may use the combustion mode with the excess air coefficient of 1, the exhaust gases to be discharged which is produced by the combustion in the engine include hydrocarbon, carbon monoxide, and nitrogen oxides, which exhaust gases are catalyzed and converted by means of the catalytic converter  60  and then discharged. When the current engine load condition is the full load condition, the engine may use the combustion mode with the excess air coefficient of 0.8˜0.9, the exhaust gases to be discharged which is produced by the combustion in the engine include hydrocarbon, carbon monoxide, and nitrogen oxides, which exhaust gases are catalyzed and converted by means of the catalytic converter  60  and then discharged. 
     The combustion control method with variable excess air coefficient for gasoline engine of the disclosure uses exhaust gas recirculation technology, Miller cycle technology which realizes that in the combustion cycle the expansion ratio is higher than the effective compression ratio, in-cylinder direct injection technology which realizes controllable fuel-injection amount in the cylinder, variable valve system technology which realizes controllable intake air amount in the cylinder, and high energy ignition system technology which realizes controllable combustion phasing, to control the combustion in the engine, and uses the catalytic converter to treat the exhaust gas after combustion. With the exhaust gas recirculation technology, reuse of the exhaust gas can be realized, and the quantity and the temperature of re-circulated exhaust gas can be controlled so as to control the combustion. 
     As mentioned above, the technical solutions according to the embodiments of the disclosure has advantages as follows. The above-mentioned combustion control method and combustion control system with variable excess air coefficient for gasoline engine use different excess air coefficient combustion modes in accordance with different operating conditions of the gasoline engine, achieving comprehensive improvement on effective thermal efficiency, power performance and pollutant emission in entire range of gasoline engine operating conditions. 
     The above are merely preferred embodiments of the disclosure, and are not meant to limit the disclosure in any form. The disclosure is intended to cover all changes, equivalent arrangements and various modifications included within the spirit and principle of the disclosure. 
     INDUSTRIAL APPLICABILITY 
     The combustion control method and the combustion control system with variable excess air coefficient for gasoline engine according to the embodiments of the disclosure use different excess air coefficient combustion modes in accordance with different operating conditions of the gasoline engine, achieving comprehensive improvement on effective thermal efficiency, power performance and pollutant emission in entire range of gasoline engine operating conditions.