Abstract:
There is provided a method of controlling an internal combustion engine system. The method comprises supplying a first amount of electric energy to heat to an upstream sensor located in an exhaust gas passage from the internal combustion engine and upstream of an exhaust gas after-treatment device and adjusting an air-fuel mixture supplied to the internal combustion engine based on an output of the upstream sensor during a first engine operating condition, and supplying a second amount of electric energy, which is smaller than the first amount, to heat the upstream sensor and adjusting the air-fuel mixture based on an output of a downstream sensor located in the exhaust gas passage and downstream of the exhaust gas after-treatment device during a second engine operating condition. By adjusting the air fuel ratio based on an output of the downstream sensor in the second engine operating condition, the air-fuel mixture can be adjusted under less influence from the condensed water, because the downstream sensor is located downstream of the exhaust gas after-treatment device. Therefore, during the second engine operating condition, the air fuel ratio can be more precisely adjusted.

Description:
BACKGROUND 
       [0001]    The present description relates to a method of controlling an internal combustion engine, and more particularly relates to a method of feedback controlling an air fuel ratio of air fuel mixture supplied to an internal combustion engine using a heated exhaust gas oxygen sensor. 
         [0002]    There is known and presented, for example in U.S. Pat. No. 6,848,439, an exhaust gas oxygen sensor arranged in an exhaust passage between an internal combustion engine and a catalytic converter. The sensor is capable of outputting a signal that corresponds linearly to the oxygen concentration in the exhaust gases. The &#39;439 patent also shows a method of using the sensor output for feedback controlling an air-fuel mixture to an internal combustion engine. The exhaust gas oxygen sensor outputs a linear signal when its temperature is within a higher operative temperature range, between 700-800° C. for example. On the other hand, the sensor outputs a non-linear signal around the stoichiometric air-fuel ratio at a lower operative temperature range, between 300-400° C. for example. The exhaust gas oxygen sensor is provided with a heater, which may be used to heat the sensor temperature to the operative range. 
         [0003]    When the exhaust gas oxygen sensor is cooled down after an engine stop and an internal combustion engine is started again, a water content of the exhaust gas or combusted gas may be partly condensed by contacting the sensor surface. If the heater is then used to heat the exhaust gas oxygen sensor, the condensed water may cause the sensor output to degrade. The &#39;439 patent describes a method to avoid such degradation by choosing the lower temperature range as its target temperature. The &#39;439 patent also describes using the sensor at lower operating temperatures to provide feedback control of engine air fuel mixtures around the stoichiometric air fuel ratio within a predetermined time period after an engine start. 
         [0004]    However, it is possible under certain circumstances to increase the amount of water in the exhaust gas that condenses on the sensor surface. For example, when hydrogen is used as a fuel instead of fossil fuels, such as gasoline, combustion of hydrogen may create more water in the exhaust gas because hydrogen readily combines with air to produce water. As the amount of the condensed water increases, it may make it difficult to heat the sensor after an engine start, even to the lower target temperature. Further, since combusted hydrogen exhibits lower exhaust gas temperatures, the time period that condensation occurs in the exhaust system can be increased when compared to combusted fossil fuels. The condensation may make it difficult to precisely feedback control the engine air-fuel ratio based on feedback from the sensor output. Consequently, engine emissions and fuel economy may be degraded when exhaust gases condensate into water in the exhaust system. 
         [0005]    Therefore, there is a need to improve the prior art method of feedback controlling an air-fuel ratio using a heated exhaust gas oxygen sensor. 
       SUMMARY 
       [0006]    Accordingly, there is provided, in one aspect of the present description, a method of controlling an internal combustion engine system. The method comprises supplying a first amount of electric energy to heat to an upstream sensor located in an exhaust gas passage from the internal combustion engine and upstream of an exhaust gas after-treatment device and adjusting an air fuel mixture supplied to the internal combustion engine based on an output of the upstream sensor during a first engine operating condition. The method further comprises supplying a second amount of electric energy, which is smaller than the first amount, to heat the upstream sensor and adjusting the air-fuel mixture based on an output of a downstream sensor located in the exhaust gas passage and downstream of the exhaust gas after-treatment device during a second engine operating condition. 
         [0007]    By adjusting the air fuel-ratio based on an output of the downstream sensor during the second engine operating condition, the air fuel-ratio can be adjusted under less influence from the condensed water. Since the downstream sensor is located downstream of the exhaust gas after-treatment device, much of the water vapor in the exhaust gas may be condensed before the exhaust gases reach the downstream sensor. This allows the downstream sensor to operate with less influence from the condensing water vapor. Therefore, during the second engine operating condition, the engine air fuel ratio can be more precisely adjusted so that engine exhaust emissions and fuel economy may be improved. 
         [0008]    In another aspect, the method comprises adjusting an air-fuel mixture supplied to the internal combustion engine by more heavily weighting an output of the downstream sensor than an output of the upstream sensor during a first predetermined period, and adjusting the air-fuel mixture by more heavily weighting the output of the upstream sensor than the output of the downstream sensor after the first predetermined period. By adjusting the air-fuel mixture by more heavily weighting the output of the downstream sensor prior to more heavily weighting the output of the upstream sensor, the air-fuel mixture can be adjusted under less influence of the water in the exhaust gas because the water is less likely to condense as the period goes by and the engine system temperature increases. Therefore, the engine air fuel ratio can be more precisely adjusted over time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    The advantages described herein will be more fully understood by reading an example of embodiments in which the above aspects are used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein: 
           [0010]      FIG. 1  shows a schematic view of an engine system in accordance with an embodiment of the present description; 
           [0011]      FIG. 2  is a circuit diagram showing an upstream sensor which detects an oxygen concentration in the exhaust gas and has an electric heater in accordance with the embodiment; 
           [0012]      FIG. 3  is a circuit diagram the electric heater of the upstream sensor in accordance with the embodiment; 
           [0013]      FIG. 4  is a map which defines engine operating regions on an engine speed and desired engine torque; 
           [0014]      FIG. 5  is a flowchart showing a routine to control the engine system in accordance with the embodiment of the present description; 
           [0015]      FIG. 6  shows time charts of temperatures of exhaust gas oxygen sensors, heater control, and fuel control; and 
           [0016]      FIG. 7  shows a graph of NOx emission versus air fuel ratios. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    An embodiment of the present description will now be described with reference to the drawings, starting with  FIG. 1 , which shows a schematic view of an engine system including an internal combustion engine  1  fueled with gaseous hydrogen. The engine system is mounted on a vehicle, such as an automotive vehicle, and its output is transmitted to vehicle driving wheels through a power transmission mechanism as is well known in the art. 
         [0018]    The engine system comprises an intake air passage  2  for inducting fresh air to the engine  1 , an exhaust gas passage  3  for expelling an exhaust gas from the engine  1 , and an exhaust gas recirculation (EGR) passage  4  for circulating a part of the exhaust gas back to the intake air passage  2 . 
         [0019]    The engine  1  is a rotary piston engine having two substantially triangular shaped rotors  11  and  21 . The rotary piston engine  1  has two rotor housings  10  and  20 , which are arranged at both sides of an intermediate housing not shown and between front and rear housings also not shown. The rotors  11  and  21  are housed respectively within the rotor housings  10  and  20 . The inner periphery of the rotor housing  10 , the outer periphery of the rotor  11 , and the intermediate and front housings collectively define three combustion chambers, while the inner periphery of the rotor housing  20  and the others define three combustion chambers as well. The rotors  11  and  21  are arranged rotatably around eccentric shafts  12  and  22 , which have a common rotational axis also common with an output shaft of the engine  1 . When the output shaft makes one rotation, each rotor makes three rotations and causes the operational chambers to change the volumes and make an engine cycle (Otto cycle). 
         [0020]    An intake port  2   a  is arranged in one of the rotor, intermediate and front or rear housings so as to communicate to a combustion chamber in an intake stroke. Also, an exhaust port  3   a  is arranged in one of the housings so as to communicate to a combustion chamber in an exhaust stroke. 
         [0021]    Pairs of spark plugs  13  and  23  are arranged in ones of the housings so as to face a combustion chamber in compression and expansion strokes. The spark plug is coupled to an ignition circuit not shown. The ignition circuit is controlled by an engine controller  100  so that the spark plug can spark at desired timing determined by the engine controller  100 . 
         [0022]    Direct fuel injectors  14  and  24  are also arranged in one of the housings respectively so as to face an operational chamber in intake and compression strokes. The direct fuel injectors  14  and  24  are supplied with gaseous hydrogen fuel from a hydrogen storage tank, such as a metal hydrate tank, through a fuel supply system not shown. The direct fuel injector has a solenoid valve inside. The solenoid valve is actuated by a driver circuit not shown which is controlled by the engine controller  100 . Therefore, the direct fuel injector can directly inject gaseous hydrogen directly into a combustion chamber in a compression stroke or an intake stroke at desired timing determined by the engine controller  100 . As is known in the art, when the fuel is injected in a compression stroke, the air-fuel mixture can be combusted even if an overall air fuel ratio of the charged mixture is substantially leaner than the stoichiometry. At that time, the air-fuel mixture is stratified. On the other hand, when the fuel is injected in an intake stroke, the air-fuel mixture will be homogeneous. 
         [0023]    Also, port fuel injectors  15  and  25  are arranged in the intake ports  2   a . The port fuel injector is also supplied with gaseous hydrogen fuel from the hydrogen storage tank through the fuel supply system, and has a solenoid valve that is actuated by a driver circuit which is controlled by the engine controller  100 . The port fuel injector can inject gaseous hydrogen into the intake port  2   a  at desired timing determined by the engine controller  100 . Therefore, the port fuel injector can inject gaseous hydrogen fuel into the intake port  2   a  at desired timing determined by the engine controller  100 . When the injected fuel and air are inducted from the intake port  2   a  into a combustion chamber, the air and fuel mixture is substantially homogeneous. 
         [0024]    In the intake passage  2 , an airflow meter  30  and a throttle valve  31  are arranged in that order from the upstream side. The airflow meter  30  detects airflow through the intake passage  2  and outputs a corresponding signal to the engine controller  100 . A throttle valve actuator  42  actuates the throttle valve  31  and adjusts its opening in accordance with a signal from the engine controller  100 . 
         [0025]    In the EGR passage  4 , an EGR valve  35  is arranged, and actuated by an EGR actuator  43  which adjusts an opening of the EGR valve  35  in accordance with a signal from the engine controller  100 . 
         [0026]    In the exhaust passage  3 , a three-way catalyst converter  32  is arranged. The three-way catalyst converter  32  has a conventional structure comprising a casing and a catalyst brick sustained in the casing. The catalyst brick comprises a honeycomb shaped carrier, and a catalyst layer coated on the carrier. The honeycomb shaped carrier may be made of porous material such as cordierite. Upstream of the catalyst converter  32  in the exhaust passage  3 , an upstream oxygen sensor  33  is arranged, which detects a concentration of oxygen in the exhaust gas and outputs electric current in proportion to the detected oxygen concentration as described in greater detail below. It may be called a linear sensor because of linearity of its output. 
         [0027]    Downstream of the catalyst converter  32  in the exhaust passage  3 , a downstream oxygen sensor  34  is arranged, which also detects a concentration of oxygen in the exhaust gas, but outputs electric current that abruptly changes around the stoichiometric air fuel ratio. Therefore, it may be called a lambda sensor since the stoichiometric air fuel ratio corresponding to an excessive air ratio λ (lambda)=1. The downstream sensor may be arranged on the casing of the catalyst converter for a simpler assembly process of an entire exhaust system. It is preferably arranged downstream of the catalyst brick, while it can be arranged between the bricks if there are a plurality of bricks. 
         [0028]      FIG. 2  shows a detailed structure of the upstream sensor  33 . It comprises a sensor element portion  33   a , a heater  33   b  that is basically comprised of an electric resistor and arranged in the proximity of the sensor element part  33   a  and can heat it by transmitting electrically generated heat, a sensor circuit  33   c , and a heater circuit  33   d  that can keep the sensor element portion  33   a  at a predetermined temperature. 
         [0029]    The sensor element portion  33   a  has an oxygen cell element  33   e  and an oxygen pump element  33   f  made of oxygen ion conductive solid electrolyte material such as zirconia. The oxygen cell element  33   e  generates electricity at its both sides in dependence on a ratio of oxygen concentrations between at its both sides, while the oxygen pump element  33   f  pumps oxygen from its one side to the other in dependence on electricity applied to its both sides. Electrode layers  33   e ′ are formed on the both sides of the oxygen cell element  33   e , and electrode layers  33   f ′ are formed on the both sides of the oxygen pump element  33   f.    
         [0030]    A dispersion chamber  33   h  is defined by the pair of oxygen pump elements  33   e  and  33   f , a part of a casing of the sensor element  33   a , and a dispersion layer  33   g . The dispersion chamber  33   h  communicates with the exhaust passage  3  through the dispersion layer  33   g  so that the exhaust gas flows between the exhaust passage  3  and the dispersion layer  33   g  at constant dispersion rate. A relative oxygen concentration chamber  33   i  is formed at one side of the oxygen cell element  33   e , and an oxygen concentration therein is maintained constant, for example, by communicating to the atmosphere. 
         [0031]    The sensor circuit  33   c  is connected to the sensor element portion  33   a , and comprises an operational amplifier  33   j , a resister  33   k , and output terminals  33   m.    
         [0032]    When the oxygen ion conductive solid electrolyte material used for the oxygen cell element  33   e  and the oxygen pump element  33   f  is arranged between two chambers of different oxygen partial pressures (or concentrations), oxygen ions pass through the element depending on a ratio of the oxygen partial pressures of the both chambers until the equilibrium, and generate electromotive force, thereby functioning as an electric cell. On the other hand, when there is a voltage difference between the both sides of the material, it pumps oxygen from one side to the other. 
         [0033]    Then, the operational amplifier  33   j  adjusts current flowing to the oxygen pump element  33   f  in accordance with change of voltage generated at the oxygen cell element  33   e . When oxygen in exhaust gas in the dispersion chamber  33   h  increases, the oxygen pump element  33   f  pumps out the oxygen from the dispersion chamber  33   h  to the outside. When oxygen in exhaust gas in the dispersion chamber  33   h  decreases, the oxygen pump element  33   f  pumps oxygen into the dispersion chamber  33   h  from the outside. The pumping function of the oxygen pump element  33   e  is going to maintain a state corresponding to the stoichiometric air fuel ratio in the dispersion chamber  33   h . But, the exhaust gas flows into the dispersion chamber  33   h  through the dispersion layer  33   g  at the constant rate, and the pumping function does not stop unless an oxygen concentration in the dispersion chamber  33   h  matches to the stoichiometric air fuel ratio. Therefore, an amount of the oxygen pumped out by the oxygen pump element  33   f  is in proportion to a difference between an oxygen concentration in the exhaust passage  3  and an oxygen concentration in the dispersion chamber  33   h  which is supposedly corresponding to the stoichiometric air fuel ratio due to the function of the oxygen cell element  33   e . Then, the current adjusted by the operational amplifier  33   j  for actuating the oxygen pump element  33   f  flows through the resister  33   k . At the terminals  33   m , a voltage in proportion to the current and the oxygen concentration in the exhaust passage  3  can be output. 
         [0034]      FIG. 3  shows the heater  33   b  and the heater circuit  33   d . The heater  33   b  is basically comprised of a resistor, and the heater circuit  33   d  is basically constituted with a bridge circuit including resistors  33   n ,  33   p  and  33   q , a transistor  33   r , and an operational amplifier  33   s . The resistor of the heater  33   b  changes its electric resistance depending on its temperature, as is well known in the art. On the other hand, the resistors  33   n ,  33   p  and  33   q  do not substantially change their resistances. Therefore, a voltage at a point between the heater  33   b  and the resistor  33   p  changes depending on the temperature of the heater  33   b . On the other hand, a voltage at a point between the resistors  33   n  and  33   q  does not substantially change, therefore it can be used as a reference voltage at the operational amplifier  33   s . Output of the operational amplifier  33   s  is input to the transistor  33   r , and it regulates electric current to the heater  33   b  in accordance with the temperature of the heater  33   b . Therefore, it is feedback controlled to be a temperature corresponding to the reference voltage at the operational amplifier  33   s . Although it is not shown, electric supply to the heater circuit  33   d  can be shut down by a switching relay or a power transistor known in the art, which is controlled by the engine controller  100 . 
         [0035]    The downstream lambda sensor  34  basically consists of an oxygen cell element, and does not have an oxygen pump element. Therefore, an output signal of the downstream lambda sensor  34  rapidly changes between below and above a predetermined oxygen concentration. That is, the lambda sensor  34  outputs voltage of about 1 volt at an oxygen concentration of exhaust gas generated when mixture richer than the stoichiometric air fuel ratio is combusted and flows, and outputs voltage of about 0 volt when mixture leaner than the stoichiometric air fuel ratio is combusted and flows. Consequently, it is possible to determine an air fuel ratio of mixture supplied to a combustion chamber is richer or leaner than the stoichiometric air fuel ratio. 
         [0036]    By arranging the linear oxygen sensor  33  and the lambda oxygen sensor  34  upstream and downstream of the three way catalyst converter  32 , degradation of the catalyst  32  can be detected. In particular, when the catalyst  32  functions normally, oxygen in exhaust gas is adsorbed by the catalyst  32  so that an oxygen concentration detected by the downstream lambda sensor  34  is relatively smaller than an oxygen concentration detected by the upstream linear sensor  33 . However, when the catalyst  32  is degraded, oxygen storage capacity is decreased so that detected values by the both sensors  33  and  34  are made similar, and based on this, the degradation of the catalyst  32  can be detected. Also, by providing the two sensors  32  and  33 , a variation caused by an individual difference or aging can be adjusted as well. 
         [0037]    The engine controller  100  is a microprocessor based controller well known in the art, and as shown in  FIG. 1 , receives signals from the airflow meter  30 , the upstream linear sensor  33 , the downstream lambda sensor  34 , an engine speed sensor  40  for detecting an engine rotational speed, an accelerator sensor detecting a position of an accelerator pedal which a driver operates, and other sensors. Based on those input signals, the engine controller  100  computes and outputs signals directly or indirectly, for example through a driver circuit, to the fuel injectors  14 ,  15 ,  24 , and  25 , the spark plugs  13  and  23 , the throttle actuator  42 , the EGR actuator  43 , the switching relay of the heater  33   b  of the upstream linear sensor  33 , and other actuators. Although the heater circuit  33   d  adjusts electricity supplied to the heater  33   b  of the upstream linear sensor  33 , the engine controller may digitally perform the same function as the analogue heater circuit  33   d  does. 
         [0038]    The engine controller  100  stores in its memory an operational map  110 , as shown in  FIG. 4 , which defines three operational modes in accordance with an engine speed which is detected by the engine speed sensor  40  and a desired engine torque which predominantly corresponds to the signal output from the accelerator sensor  41 . The operational map defines a λ=1 mode in a lower speed and higher torque region  112 , a lean mode (1&lt;λ≈2) in a lower speed and lower torque region  114 , and a high power lean region (λ=1.4-1.6) in a higher speed region  116 . 
         [0039]    The engine controller  100  computes a target opening of the throttle valve  31  based on the desired torque, the engine speed and the target air fuel ratio, and controls the throttle actuator  42  to meet the target throttle opening. A base fuel injection amount is computed based on the desired torque, the engine speed and the target air fuel ratio, as well. 
         [0040]    When a predetermined time period, for example two minutes, has passed, the engine controller  100  adjusts an air fuel ratio of air and fuel mixture supplied to the engine  1  with reference to the operational map  100 . At this time, the engine controller  100  closed the switching relay or power transistor between the power supply and the heater circuit  33   d  of the upstream linear sensor. Therefore, the heater  33   b  can receive electricity and maintain the upstream sensor  33  at the predetermined temperature, therefore the sensor  33  is fully operative. The fuel injection amount is feedback controlled using based on the base fuel injection amount and the output of the upstream linear oxygen sensor  33  to meet the target air fuel ratio. At this time, the output of the downstream lambda sensor  34  may be used for a correction of the output of the upstream linear sensor  33 . 
         [0041]    In the lower speed and higher torque region  112 , a target air fuel ratio is set the stoichiometric air fuel ratio (corresponding to λ=1). The EGR valve  35  is opened so that the exhaust gas is re-circulated through the EGR passage  4  to the intake passage  2 . The exhaust gas re-circulated into the combustion chamber decreases a combustion temperature, and reduces NOx generation during the combustion. The direct fuel injectors  14  and  24  inject fuel directly to the combustion chambers. 
         [0042]    In the lower speed and lower torque region  114 , the target air fuel ratio is set an air fuel ratio leaner than the stoichiometric air fuel ratio, for example corresponding to λ=2. The EGR valve  35  is closed, and direct fuel injectors  14  and  24  inject fuel directly to the combustion chambers. 
         [0043]    In the higher speed region, the target air fuel ratio is set an air fuel ratio leaner than the stoichiometric air fuel ratio, for example corresponding to λ=1.4-1.6, which is, in the case of using gaseous hydrogen as fuel, the leanest air fuel ratio as far as pre-ignition that is self ignition before spark ignition by the spark plugs  13  or  23  does not occur. The EGR valve  35  is closed. The direct fuel injectors  14  and  24  inject fuel directly to the combustion chambers, and at the same time, the port fuel injectors  15  and  25  inject fuel to the intake ports  2   a , for higher engine output. 
         [0044]    Control of the engine system, particularly the fuel injectors  14 ,  15 ,  24 , and  25 , and the EGR valve  35  will now be described with reference to a flowchart of  FIG. 5  showing a control routine executed by the engine controller  100 . At a step S 1 , the engine controller  100  reads various signals from the airflow meter  30 , the upstream linear sensor  33 , the downstream lambda sensor  34 , the engine speed sensor  40 , and the others. The routine proceeds to a step S 2 , and determines whether or not 20 seconds has passed since an engine start by reading a counter which is integrated into the engine controller  100  as is well known in the art, and has started when the engine  1  is determined to start a self rotation and counts up as time goes by. Alternatively, the counter may count number of rotations of the engine  1  or number of combustion cycles of the engine  1 , from the fuel injection pulse signal sent to the fuel injectors from the engine controller  100 . 
         [0045]    If it is determined that 20 seconds has not passed since the engine start at the step S 2  (NO), the routine proceeds to a step S 3 , and stops to supply electricity to the heater  33   b  of the upstream linear sensor  33 , for example by the engine controller  100  controlling to open the switching relay. Then, it proceeds to a step S 4 , and sets the target air fuel ratio to correspond to λ=1. After the step S 4 , the routine proceeds to a step S 5 , and computes the base fuel injection amount based on the engine speed, the desired torque and the target air fuel ratio without taking account of the outputs of either of the upstream linear sensor  33  or the downstream lambda sensor  34  (open control). Then, it proceeds to a step S 6 , and the engine controller  100  actuates the direct fuel injectors  14  and  24  to inject the base fuel amount determined at the step S 5  at a desired timing without actuating the port fuel injectors  15  and  25  (direct injection). At the same time, the engine controller  100  controls the throttle actuator  42  to regulate intake airflow to the engine  1 , thereby causing the direct fuel injectors  14  and  24  and the throttle valve  31  to function collectively as an air-fuel regulator to adjust the air-fuel mixture supplied to the engine  1 . Finally, the routine proceeds to a step S 7 , and the engine controller  100  controls the EGR actuator  43  to close the EGR valve  35 . Then, the routine returns. 
         [0046]    During the steps S 3  through S 7 , as shown in a time chart of  FIG. 6 , the upstream linear sensor  33  and the downstream lambda sensor  34  are likely not to reach active temperatures. Then, it may be difficult to make precise feedback control based on the outputs of these sensors  33  and  34 . Therefore, the feedback control is not made, but the open control is made with the target air fuel ratio to be the stoichiometry. At this time, catalytic reaction of the exhaust gas over the three-way catalyst converter  32  occurs and generates heat to increase the gas temperature downstream of the catalyst converter  32 . Therefore, the downstream lambda sensor  34  increases its temperature at a greater rate, as shown in  FIG. 6  by a line A. The time period of 20 seconds for the determination at the step S 2  may be predetermined from an experiment or test, not limited to 20 seconds, but may be a time period sufficient for the lambda sensor  34  to reach the active temperature. 
         [0047]    On the other hand, if at the step S 2 , it is determined that 20 seconds has passed after the engine start (YES), since it is a state where the downstream lambda sensor  34  has reached the active temperature (see the line A in  FIG. 6 ), but the upstream linear sensor  33  has not reached the active temperature (see the line B in  FIG. 6 ). Therefore, the routine proceeds to a step S 8 , and determines whether or not two minutes has passed after the engine start by reading the counter described above. If it is determined that two minutes has not passed after the engine start at the step S 8  (NO), the routine proceeds to a step S 9 , and continues to stop electricity supply to the heater of the linear sensor  33 . Then, it proceeds to a step S 10 , and sets the target air fuel ratio to correspond to λ=1. After the step S 10 , the routine proceeds to a step S 11 , and computes the base fuel injection amount based on the engine speed, the desired torque and the target air fuel ratio, and computes the fuel injection amount based on the base fuel injection amount and an output signal from the downstream lambda sensor  34  (feedback control). Then, it proceeds to a step S 12 , and the engine controller  100  actuates the direct fuel injectors  14  and  24  at a desired timing without actuating the port fuel injectors  15  and  25  (direct injection). Finally, the routine proceeds to a step S 13 , and the engine controller  100  controls the EGR actuator  43  to close the EGR valve  35 . Then, the routine returns. 
         [0048]    During the steps S 9  through S 13 , the downstream lambda sensor  34  has reached the active temperature as shown in  FIG. 6 . Also, even if the exhaust gas temperature is lower and water content of the exhaust gas is likely to condense, the catalyst converter  32 , particularly the honeycomb shaped carrier of the catalyst brick, may block the water content from getting condensed on the downstream lambda sensor. Therefore, the air fuel ratio can be precisely feedback controlled to be the stoichiometry (λ=1). This feedback control is configured that the output signal by the lambda sensor  34  and a signal corresponding to the stoichiometric air fuel ratio (λ=1) are compared, and based on this comparison result, a correction amount to correct the base fuel injection amount is calculated. Alternatively, while still heavily weighting the output of the lambda sensor  34 , the output of the upstream linear sensor  33  may be taken account of to some extent for a purpose of watching the function of the lambda sensor  34 . 
         [0049]    Then, the combustion at λ=1 raises a temperature of exhaust gas, and may promote heating of the linear sensor  33 . The time period of two minutes for the determination at the step S 8  may be predetermined from an experiment or test, not limited to two minutes, but may be a time period sufficient for the linear sensor  34  to reach the active temperature. Further, it may be number of rotations or combustion events of the engine  1  as described above. 
         [0050]    Also, if it is determined that two minutes has passed after the engine start at the step S 8  (YES), it is a state where both of the linear sensor  33  and the lambda sensor  34  have reached the active temperatures (see the line B of  FIG. 6 ). The routine proceeds to a step S 14 , and supplies electricity the heater  33   b  through the heater circuit  33   d  of the linear sensor  33  by the engine controller  100  closing the switching relay. Then, the heater circuit  33   d , as described above, controls the heater  33   b  to maintain the linear sensor  33  at the predetermined temperature. Then, the routine proceeds to a step S 15 , and determines the engine operating condition is in which of the regions  112 ,  114 , and  116  in the map  110  of  FIG. 4 , based on the desired torque and the engine speed. 
         [0051]    If it is determined at the step S 15  that the engine operating condition is in the lower speed and higher torque region  112 , the routine proceeds to a step S 16 , and sets the target air fuel ratio to be the stoichiometry (λ=1). Then, it proceeds to a step S 17 , and feedback controls the air fuel ratio (λ=1) based on the output of the upstream linear sensor  33  that has reached the active temperature as described above. This feedback control is configured that the output signal of the linear sensor  33  and a value corresponding to the stoichiometric air fuel ratio (λ=1) are compared, and based on this comparison result, a feedback amount for the base fuel injection amount is calculated. 
         [0052]    Further the feedback correction amount to the base fuel injection amount at the step S 17  is corrected by the output of the lambda sensor  34 . That is, the output signal of the lambda air fuel ratio sensor  34  and the stoichiometric air fuel ratio (λ=1) are compared. Based on the comparison result, the feedback amount for the basic fuel injection amount is corrected. In particular, if an air fuel ratio corresponding to the output of the lambda sensor  34  is determined leaner than the stoichiometric air fuel ratio, the feedback amount by the output of the linear sensor  33  is decrementally corrected by a predetermined amount, and if the lambda sensor  34  determines it is richer than the stoichiometry, the feedback correction amount by the output of the linear sensor  33  is incrementally corrected by a predetermined amount. 
         [0053]    After the step S 17 , the routine proceeds to a step S 18 , and the engine controller  100  actuates the direct fuel injectors  14  and  24  at a desired timing without actuating the port fuel injectors  15  and  25  (direct injection). Finally, the routine proceeds to a step S 19 , and the engine controller  100  controls the EGR actuator  43  to open the EGR valve  35  to re-circulate part of exhaust gas to the intake passage  2  through the EGR passage  4 . With this exhaust gas recirculation, the combustion temperature can be decreased to reduce the NOx emission. Then, the routine returns. 
         [0054]    If it is determined at the step S 15  that the engine operating condition is in the lower speed and lower torque region  114 , the routine proceeds to a step S 20 , and sets the target air fuel ratio to be an air fuel ratio leaner than the stoichiometry (λ=2). Then, it proceeds to a step S 21 , and feedback controls the air fuel ratio (λ=2) based on the output of the upstream linear sensor  33  that has reached the active temperature as described above. This feedback control is configured that the output signal of the linear sensor  33  and a value corresponding to the target lean air fuel ratio (λ=2) are compared, and based on this comparison result, a feedback amount for the base fuel injection amount is calculated. 
         [0055]      FIG. 7  shows a relationship between NOx emission and an excess air ratio A. The NOx emission increases from a little more than λ=1 corresponding to the stoichiometric air fuel ratio to λ=1.3. Then, it decreases to approximately zero around λ=1.8-2.0. Accordingly, when the operating condition is in the lower speed and lower torque region  112 , the target air fuel ratio will be set corresponding to λ=2 for the substantially zero NOx emission. 
         [0056]    After the step S 21 , the routine proceeds to a step S 22 , and the engine controller  100  actuates the direct fuel injectors  14  and  24  at a desired timing without actuating the port fuel injectors  15  and  25  (direct injection). Finally, the routine proceeds to a step S 23 , and the engine controller  100  controls the EGR actuator  43  to close the EGR valve  35 . Then, the routine returns. 
         [0057]    If it is determined at the step S 15  that the engine operating condition is in the higher speed region  114 , the routine proceeds to a step S 24 , and sets the target air fuel ratio to be an air fuel ratio leaner than the stoichiometry (λ=1.4-1.6). Then, it proceeds to a step S 25 , and feedback controls the air fuel ratio (λ=1.4-1.6) based on the output of the upstream linear sensor  33  that has reached the active temperature as described above. This feedback control is configured that the output signal of the linear sensor  33  and a value corresponding to the target lean air fuel ratio (λ=1.4-1.6) are compared, and based on this comparison result, a feedback amount for the base fuel injection amount is calculated. Then, the routine proceeds to a step S 26 , and the engine controller  100  actuates the direct fuel injectors  14  and  24  and the port fuel injectors  15  and  25  at respective desired timings (direct injection plus port injection). At the same time, the engine controller  100  controls the throttle actuator  42  to regulate intake airflow to the engine  1 , thereby causing the direct fuel injectors  14  and  24 , the port fuel injectors  15  and  25  and the throttle valve  31  to function collectively as the air-fuel regulator to adjust the air-fuel mixture supplied to the engine  1 . Finally, the routine proceeds to a step S 27 , and the engine controller  100  controls the EGR actuator  43  to close the EGR valve  35 . Then, the routine returns. 
         [0058]    As described above, in an engine start, electricity supplied to the heater  33   b  of the upstream linear sensor is stopped at the step S 3  or S 8  until two minutes after an engine start when it is supposed that the upstream linear sensor  33  has reached the active temperature and there is substantially no condensed water on the surface of the upstream linear sensor. Therefore, the linear sensor  33  may not have any distortion on its surface due to the condensed water and the heater. 
         [0059]    During this two minute period, the downstream lambda sensor  34  arranged has substantially no condensed water on its surface thanks to the catalyst converter  32  arranged upstream of the downstream lambda sensor  34 . Also, the exhaust gas reacted and heated at the catalyst converter  32  may cause the lambda sensor  34  to more quickly reach the active temperature so that the air fuel ratio can be precisely feedback controlled based on the output of the lambda sensor  34 . 
         [0060]    In the embodiment above, the oxygen sensors  33  and  34  are supposed to reach the respective active temperatures by determining a time period since an engine start at the steps S 2  and S 8 . Alternatively, a temperature sensor may be provided and detect a temperature of the linear sensor  33  or the lambda sensor  34 . Then, at the step S 2  or S 8  of the control routine of  FIG. 5 , it may be determined whether or not the detected temperature of the sensor  33  or  34  is higher than a predetermined temperature. Further alternatively, the temperatures of the sensors  33  and  34  may be estimated based on cumulated rotations of the engine  1  since an engine start, cumulated fuel injection amount since an engine start, other parameters including the time period since an engine start, an engine temperature, and an atmospheric temperature, or a combination of any of the above. 
         [0061]    The engine  1  is not limited to the rotary piston engine, but may be any type of internal combustion engines including a spark ignited engine having a reciprocating piston with direct fuel injection or port fuel injection. The fuel supplied to the engine is not limited to the gaseous hydrogen described above, but it may be hydrocarbon based fuels including gasoline, diesel oil and ethanol. 
         [0062]    The upstream sensor  33  is not limited to the specific type of linear sensor described above, but may be a different type of sensor showing a linearity of the output without the oxygen pump element, or a lambda sensor like the downstream sensor  34  if the target air fuel ratio is always set the stoichiometry. 
         [0063]    The heater  33   b  and the heater circuit  33   d  are not limited to the above described. Specifically, instead of turning on and off the electricity to the heater, the engine controller  100  may adjust the electricity by controlling a power transistor such as the transistor  33   r  in  FIG. 3 . In that case, instead of stopping the electricity to the heater  33   b  at the steps S 3  and S 9 , small amount of electricity can be supplied to the heater  33   b . That amount is much smaller than what may be supplied at the step S 14 . Then, the upstream linear sensor can more quickly reach the active temperature without a risk of the excessive surface distortion. 
         [0064]    While in the above embodiment, the downstream sensor  34  does not have any heater, the sensor  34  may have an electric heater. Since the downstream sensor  34  is arranged downstream of at least one brick of the catalyst converter  33  which blocks water from condensing on the sensor  34 , the heater of the downstream sensor  34  may be activated just after an engine start so that an air fuel ratio feedback control can be started further earlier. 
         [0065]    It is needless to say that the invention is not limited to the illustrated embodiments and that various improvements and alternative designs are possible without departing from the substance of the invention as claimed in the attached claims.