Abstract:
An engine control apparatus for a multi-cylinder compression ignition internal combustion engine includes a combustion status sensor and a controller. The combustion status sensor is installed in a selected one of cylinders of the engine. The controller works to sample an output from the combustion status sensor to determine a combustion status parameter and determine injection timings so as to bring the combustion status parameter into agreement with a target value. The controller also corrects the injection timing for at least one of the cylinders in which the combustion status sensor is not installed so that a combustion status parameter of the at least one of the cylinders lies on an advanced side of that of the one of the cylinders in which the combustion status sensor is installed, thereby avoiding the deterioration of quality of combustion of the fuel arising from a shift of the time of the ignition in the retarded direction from a desired time point.

Description:
CROSS REFERENCE TO RELATED DOCUMENT 
   The present application claims the benefit of Japanese Patent Application No. 2005-343777 filed on Nov. 29, 2005, the disclosure of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1 Technical Field of the Invention 
   The present invention relates generally to an engine control apparatus for multi-cylinder compression ignition internal combustion engine which is designed to correct injection timings based on the time of ignition of fuel in the engine. 
   2 Background Art 
   Typical multi-cylinder compression ignition internal combustion engines such as diesel engines have installed therein fuel injectors to inject fuel directly into cylinders thereof to produce combustible air-fuel mixture. Engine control systems for such a type of engines are usually designed to correct times of injection of the fuel into the cylinders (i.e. injection timings) based on the status of combustion of the engine in order to ensure desired levels of output of and quality of emissions from the engine. Specifically, the engine control systems work to sample an output from a combustion pressure sensor installed in the engine to determine an actual time of ignition of the fuel in the engine and correct the injection timings so as to bring the actual time of ignition into agreement with a target value. 
   Use of combustion pressure sensors in all the cylinders of the engine to correct the injection timings independently on a cylinder basis result in disadvantage in terms of cost. In order to alleviate this problem, there may be proposed systems which have a combustion pressure sensor installed only in one of cylinders of the engine to sample an actual time of ignition of fuel therein, determine the injection timing for the one of the cylinders so as to compensate for an ignition lag that is a difference between the actual time of ignition, as sampled, and a target time, and correct injection timings for the other cylinders based on the ignition lag under assumption that the other cylinders usually undergo the same ignition lag. 
   When the time of ignition of the fuel is retarded from some time point, it usually results in an undesirable drop in output from the engine. This is because the ignition of the fuel upon a drop in pressure in the cylinder results in a lack in utilizing combustion energy or a misfire in the course of combustion of the air-fuel mixture without adequate expansion thereof, leading to a reduction in the combustion energy. 
   In order to avoid the above problem, Japanese Patent First Publication No. 2000-130224 teaches an engine control system for internal combustion engines equipped with an EGR (Exhaust Gas Recirculation) system working to return or recirculate a portion of exhaust gas of the engine (i.e., EGR gas) into an intake pipe. The engine control system has a combustion pressure sensor installed in only one of cylinders of the engine which is the greatest in ratio of the amount of the EGR gas to the amount of intake air in the engine and works to determine and correct all target times of injection of fuel (i.e., injection timings) into the cylinders based on an actual time of ignition of the fuel, as determined using an output of the combustion pressure sensor. The engine control system is designed based on the fact that the one of the cylinders that is the greatest in the EGR gas-to-intake air ratio is apt to experience a misfire due to a drop in concentration of oxygen in the intake air arising form addition of the EGR gas thereto to correct the injection timings based on the actual time of the ignition in the one of the cylinders in which the combustion pressure sensor is installed to avoid the misfire. 
   However, fuel injectors have usually individual variability in injecting fuel into cylinders of the engine. A crank angle sensor has also individual variability in measuring the angular position of a crankshaft of the engine arising from manufacturing tolerances thereof. This will result in a variation in time of ignition of the fuel among the cylinders of the engine, which may lead to an undesirable shift of the times of ignition of the fuel in the cylinders in the regarded direction from a time point at which the quality of combustion of the fuel is deteriorated, resulting in a drop in torque, as produced by the engine. 
   SUMMARY OF THE INVENTION 
   It is therefore a principal object of the invention to avoid the disadvantages of the prior art. 
   It is another object of the invention to provide an engine control apparatus for a compression ignition internal combustion engine which is designed to control the status of combustion of fuel in all cylinders of the engine correctly based on a sampled combustion parameter indicating the status of combustion in a selected one of the cylinders. 
   According to one aspect of the invention, there is provided an engine control apparatus for a multi-cylinder compression ignition internal combustion engine such as a diesel engine which is designed to ensure the stability of combustion of fuel in all cylinder of the engine. The engine control system comprises: a combustion status sensor and a controller. The combustion status sensor is installed in a selected one of the cylinders of the engine equipped with injectors each of which injects fuel into a corresponding one of the cylinders. The combustion status sensor works to output a signal as indicating the state of combustion in fuel in the one of the cylinders. The controller works to sample the output from the combustion status sensor to determine a combustion status parameter representing one of a time of ignition of the fuel and a time of a preselected status of combustion of the fuel. The controller determines injection timings at which the injectors commence injection of the fuel into the cylinders so as to bring the combustion status parameter into agreement with a target value. The controller also corrects the injection timing for at least one of the cylinders that is other than the cylinder in which the combustion status sensor is installed so that a combustion status parameter representing one of a time of ignition of fuel in the at least one of the cylinders lies on an advanced side of that of the one of the cylinders in which the combustion status sensor is installed. The controller controls the injectors to initiate injection of the fuel into the cylinders at the injection timings. This avoids the deterioration of quality of combustion of the fuel arising from a shift of the time of the ignition in the retarded direction from a desired time point due to the factors, as described in the introductory part of this application. 
   In the preferred mode of the invention, the one of the cylinders in which the combustion status sensor is installed may be the latest in time of ignition of the fuel in the engine. 
   The controller corrects the injection timing for the at least one of the cylinders based on a predetermined variation in the combustion status parameter among the cylinders. 
   The controller may correct the injection timing for the at least one of the cylinders so that it is advanced from that of the one of the cylinders in which the combustion status sensor is installed by the predetermined variation in the combustion status parameter among the cylinders. 
   The controller may be designed to correct the ignition timings for ones of the cylinders in which the combustion status sensor is not installed so that combustion status parameters representing times of ignition of fuel in the ones of the cylinders lie on the advanced side of that of the one of the cylinders in which the combustion status sensor is installed. The degrees to which the ignition timings are corrected may be selected to be constant, respectively, on a cylinder basis. 
   The controller may change a degree to which the injection timing for the at least one of the cylinders is corrected as a function of an operating condition of the engine. 
   Only when a given condition of combustion in the engine is met, the controller may correct the injection timing for at least one of the cylinders. 
   The controller may work to control combustion of the fuel in the engine selectively in a first combustion mode where the fuel is burned at a lower concentration of oxygen in the cylinders and a second combustion mode where the fuel is burned at a higher concentration of oxygen in the cylinders. The given condition is when the engine is in the second combustion mode. 
   The one of the cylinders of the engine in which the combustion status sensor may be installed is the greatest in ratio of an amount of a portion of exhaust gas, as recirculated by an exhaust gas recirculation device into the cylinders, to an amount of air charged into the cylinders in the engine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
       FIG. 1  is a view which shows an engine control system for compression ignition internal combustion engines according to the invention; 
       FIG. 2  is a graph which demonstrates a relation between engine torque and ignition timing; 
       FIG. 3(   a ) is a graph which demonstrates a relation between the amount of NOx contained in exhaust emissions from an engine and the ignition timing of fuel in the engine; 
       FIG. 3(   b ) is a graph which demonstrates a relation between PM (particulate matter) contained in exhaust emissions from an engine and the ignition timing of fuel in the engine; 
       FIGS. 4(   a ),  4 ( b ),  4 ( c ), and  4 ( d ) are views which show relations between injection timings and rates of heat release in first to fourth cylinders of an engine, respectively; 
       FIG. 5  is a flowchart of a program to determine and correct injection timings in an engine; and 
       FIGS. 6(   a ),  6 ( b ),  6 ( c ), and  6 ( d ) are views which show relations between injection timings, as determined and corrected by the program of  FIG. 5 , and rates of heat release in first to fourth cylinders of an engine, respectively. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly to  FIG. 1 , there is shown an engine control system according to the invention which is constructed by an electronic control unit (ECU)  60  to control the ignition of fuel into an internal combustion engine  10 . The internal combustion engine  10 , as referred to herein, is, for example, a four-cylinder diesel engine. The engine control system is used with a common rail fuel injection system. 
   The engine  10  connects with an intake pipe  11  in which a throttle valve  12  and a throttle position sensor  13  are installed. The throttle valve  12  is moved in a valve position thereof by an actuator such as a dc motor. The throttle position sensor  13  works to measure the valve position (i.e., an open position) of the throttle valve  12  and output a signal indicative thereof to the ECU  60 . The intake pipe  11  extends to an intake manifold downstream of the throttle valve and leads to each cylinder of the engine  10 . 
   The engine  10  has installed therein injectors  15  each for one of the cylinders thereof. The injectors  15  connect with a common rail  16 . The common rail  16  connects with a high-pressure pump  17 . When actuated, the high-pressure pump  17  pumps fuel out of a fuel tank (not shown) and feeds it to the common rail  16  in which the fuel is accumulated at a selected high pressure at all times. A common rail pressure sensor  18  is installed in the common rail  16  and works to measure the pressure of the fuel within the common rail  16  to output a signal indicative thereof to the ECU  60 . 
   The engine  10  also has an intake valve  21  and an exhaust valve  22  which are installed in an intake port and an exhaust port of each of the cylinders thereof. When the intake valve  21  is opened, air is charged into a combustion chamber  23  and mixed with the fuel sprayed by the injector  15  so that it is burned at a controlled timing. After the mixture is burned, the exhaust gas is emitted to an exhaust pipe  31  upon opening of the exhaust valve  22 . A diesel particulate filter (DPF)  32  is installed in a downstream portion of the exhaust pipe  31  to trap particulate matter contained in the exhaust gas. 
   The engine  10  also has connected thereto an EGR (Exhaust Gas Recirculation) device working to recirculate a portion of the exhaust gas (will also be referred to as EGR gas below) into the intake pipe  11 . The EGR device is made up of an EGR pipe  33 , an EGR cooler  34 , and an EGR valve  35 . The EGR pipe  33  extends to connect between a portion of the intake pipe  11  located downstream of the throttle valve  12  and the exhaust pipe  31 . The EGR cooler  34  is installed in the EGR pipe  33  to cool the EGR gas flowing through the EGR pipe  31 . The EGR valve  35  is installed in a joint of the EGR pipe  33  and the intake pipe  11 . The EGR  35  is actuated by the ECU  60  to control the amount of the EGR gas to be recirculated to the intake pipe  11 . The EGR gas is mixed with the air in the intake pipe  11  to decrease the combustion temperature in each cylinder of the engine  10  to reduce the amount of NOx to be contained in the exhaust gas. 
   Disposed between the intake pipe  11  and the exhaust pipe  31  is a turbocharger  40  which is made up of a compressor impeller  41  exposed inside the intake pipe  11 , a turbine wheel  42  exposed inside the exhaust pipe  31 , and a rotary shaft  43  connecting the compressor impeller  41  and the turbine wheel  42  together. The turbine wheel  42  is rotated by the flow of exhaust gas within the exhaust pipe  31 , which is then transmitted to the compressor impeller  41  through the rotary shaft  43 , so that the compressor impeller  41  rotates to compress the air flowing through the intake pipe  11 . The compressed air is cooled by the inter-cooler  45  and the charged into the cylinders of the engine  10 . Use of the turbocharger  40  results in an enhanced efficiency of charging the air into the engine  10 . 
   The engine  10 , as described above, has the four cylinders which will be expressed by # 1 , # 2 , # 3 , and # 4  below. A combustion pressure sensor  51  is installed in the first cylinder # 1  of the engine  10 . The first cylinder # 1  is the greatest in ratio of the amount of the EGR gas to the amount of the intake air in the engine  10  depending upon the configuration of the intake pipe  11 . The engine control system also includes an air-fuel ratio sensor  52 , a crank angle sensor  53 , and an accelerator position sensor  54 . The air-fuel ratio sensor  52  works to measure the concentration of oxygen (O 2 ) contained in the exhaust gas from the engine  10  and output a signal indicative thereof to the ECU  60 . The crank angle sensor  53  works to output a rectangular crank angle signal at given angular intervals (e.g., 30° CA) of a crank shaft of the engine  10  to the ECU  60 . The accelerator position sensor  54  works to measure a driver&#39;s effort on or position of an accelerator pedal (not shown) and output a signal indicative thereof to the ECU  60 . 
   The ECU  60  is implemented by a typical microcomputer consisting essentially of a CPU, a ROM, a RAM, etc. and works to execute engine control programs, as stored in the ROM, to perform a fuel injection control task, etc., based on instant operating conditions of the engine  10  which are monitored by the throttle position sensor  13 , the common rail pressure sensor  18 , the combustion pressure sensor  51 , the air-fuel ratio sensor  52 , the crank position sensor  53 , and the accelerator position sensor  54 . 
   The ECU  60  samples an output of the combustion pressure sensor  51  to determine the ignition timing in the first cylinder # 1  of the engine  10 . Specifically, the ECU  60  analyzes the output of the combustion pressure sensor  51  to calculate the volume of the combustion chamber  23  of the first cylinder # 1  which is changed by vertical movement of the piston of the first cylinder # 1  and determine the rate of heat release in the first cylinder # 1  based on the calculated volume and the pressure in the first cylinder # 1 , as measured by the combustion pressure sensor  51 . When the rate of heat release exceeds a given reference value, the ECU  60  determines the instant as the ignition timing for the first cylinder # 1 . 
   The engine  10  operates in two combustion modes. In the first combustion mode is a normal combustion mode in which the injectors  15  spray the fuel into the combustion chambers  23  of the cylinders when highly compressed, so that the fuel is ignited in the cylinders in sequence. The second combustion mode is a pre-mixed combustion mode in which each of the injectors  15  sprays the fuel at an early stage of the intake stroke or compression stroke of the piston where the pressure in the combustion chamber  23  is relatively low, so that the fuel is mixed with the charged air without being ignited until the combustion chamber  23  is highly compressed. The pre-mixed combustion mode may alternatively be a mode in which each of the injectors  15  sprays the fuel near the TDC (Top Dead Center), and a large amount of the EGR gas is circulated to the intake pipe  11  to retard the ignition of the mixture to prevent the ignition from occurring during the injection of the fuel into the cylinder. The ECU  60  is designed to switch between the first and second combustion mode upon a change in engine operating range defined by the speed of and load on the engine  10 . Specifically, the ECU  60  selects the second combustion mode (i.e., the pre-mixed combustion mode) in a low-speed range or a low-load range of the engine  10  and the first combustion mode (i.e., the normal combustion mode) in other engine operating ranges. 
     FIG. 2  illustrates a relation between the ignition timing and torque, as produced by the engine  10 . The graph shows that when the ignition timing is on the advance side of some time point, the engine  10  produces substantially a constant torque either in the normal combustion mode or the pre-mixed combustion mode, but the torque drops as the ignition timing is advanced from that time point. This arises from the fact that the ignition of the mixture at the time when the cylinder pressure (i.e., the pressure in the combustion chamber  23 ) drops will result in a lack in utilizing combustion energy or a misfire in the course of combustion of the combustion gas mixture without adequate expansion thereof, leading to reduced combustion energy. The normal combustion mode and the pre-mixed combustion mode are different in time point when the engine torque drops. Specifically, the time when engine torque is produced in the pre-mixed combustion mode lies on the advanced side of that in the normal combustion mode. Within a range where the engine torque is dropping, the amount of drop of the engine torque per the degree to which the ignition timing is retarded in the pre-mixed combustion mode is greater than that in the normal combustion mode. 
     FIGS. 3(   a ) and  3 ( b ) demonstrate relations between the amount of NOx emissions contained in the exhaust gas and the ignition timing in the normal combustion mode and between the amount of PM (Particulate Matter) contained in the exhaust gas and the ignition timing in the normal combustion mode, respectively. The graphs show that the amounts of NOx and PM emissions decrease as the ignition timing is retarded. 
   The graphs of  FIGS. 2 ,  3 ( a ), and  3 ( b ) show that in the pre-mixed combustion mode, the injection timing (i.e., the start of injection of the fuel into the engine  10 ) is preferably so determined that the mixture may start to ignite just before the time point at which the engine torque starts to drop in order to ensure proper engine torque and quality of exhaust emissions. 
   Usually, the injectors  15  have individual variability in injection of the fuel. The crank angle sensor  53  has also individual variability in measuring the angular position of the crankshaft of the engine  10  arising from manufacturing tolerances thereof. Further, the engine  10  undergoes a variation in combustion of the fuel arising from a difference in configuration of the cylinders thereof. This may result in a variation in time when the mixture starts to ignite between the cylinders of the engine  10  even when the ECU  60  commands all the injectors  15  to inject the fuel into the engine  10  at the same timings on the basis of the injection timing determined for a specified one of the cylinders. Therefore, when the ECU  60  determines the injection timing for the first cylinder # 1  in which the combustion pressure sensor  51  is installed and matches the injection timings for all the other cylinders # 2  to # 4  with that of the first cylinder # 1 , it may cause the mixture to start to actually ignite later in a retard direction than a target time point in each of the cylinders # 2  to # 4 , thus resulting in a drop in torque output of the engine  10 . 
     FIGS. 4(   a ) to  4 ( d ) demonstrate a variation in actual ignition timing among the cylinders # 1  to # 4  of the engine  10 . The injection timing for the first cylinder # 1  in which the combustion pressure sensor  51  is installed is so determined that the injection timing thereof may fall within a range where there is no drop in the engine torque. Specifically, the injection timing of the first cylinder # 1  is set to time a 1 . The actual time of ignition in the first cylinder # 1  appears at time c 11 . When the injection timings for the cylinders # 2  to # 4  in which the combustion pressure sensor  51  is not installed are all set to the same time (i.e., time a 1 ) as that for the first cylinder # 1 , it will result in shifts in actual ignition timing to times c 12 , c 13 , and c 14 , respectively. The times c 13  and c 14  of ignition in the third and fourth cylinders # 3  and # 4  are shifted to the advanced side of the time c 11  of ignition in the first cylinder # 1 , while the time c 12  of ignition in the second cylinder # 2  is shifted to the retard side of the time c 11  of ignition in the first cylinder # 1 . This causes the time c 12  of ignition in the second cylinder # 2  to fall in the range where the engine torque drops, which leads to a undesirable change in the rate of heat release in the second cylinder # 2 . A difference between the time c 13  on the most advanced side and the time c 12  on the most retarded side, that is, a maximum variation in ignition timing among the cylinders # 1  to # 4  is an time interval ΔTi which will be referred to an ignition time difference below. 
   In order to alleviate the above problem, the ECU  60  is designed to correct the time (i.e., the injection timing), at which each of the injectors  15  starts to spray the fuel into a corresponding one of the cylinders # 2  to # 4  in which the combustion pressure sensor  51  is not installed, in the advanced direction by the ignition time difference ΔTi which is on the order of 0.5° to 1.0° crank angle (CA) in this embodiment. The ignition time difference a ΔTi may be found experimentally in advance 
     FIG. 5  is a flowchart of a sequence of logical steps or program to be executed in a given cycle by the ECU  60  to control the time of injection of fuel into each of the cylinders # 1  to # 4  of the engine  10  so as to compensate for the ignition time difference a ΔTi. 
   After entering the program, the routine proceeds to step  101  wherein the speed of the engine  10  and the position of the accelerator pedal are sampled as indicating the operating conditions of the engine  10  from outputs of the crank angle sensor  53  and the accelerator position sensor  54 . 
   The routine proceeds to step  102  wherein target injection timings are determined based on the operating conditions of the engine  10 , as derived in step  101 , in a known manner using, for example, a map table. The routine proceeds to step  103  wherein injection timings Ts for the cylinders # 1  to # 4  are determined as injection parameters which compensate for a target-to-actual ignition lag that is a difference between a target time and an actual time of ignition of the mixture in the first cylinder # 1 . The target-to-actual ignition lag is pre-stored in the RAM of the ECU  60 . When this cycle of execution of the program is the second or subsequent cycle, the value of the target-to actual ignition lag which is, as described later, found and stored in the RAM one cycle earlier is used. The rate of injection and injection period are also determined as injection parameters. 
   The routine proceeds to step  104  wherein one of the cylinders # 1  to # 4  of the engine  10  to which the fuel is to be injected in this program cycle is specified. The routine proceeds to step  105  wherein it is determine whether the one of the cylinders # 1  to # 4 , as specified in step  104 , is the first cylinder # 1  in which the combustion pressure sensor  51  is installed or not based on the output from the crank angle sensor  53  indicating the angular position of the crankshaft of the engine  10 . If a YES answer is obtained meaning that the one of the cylinders # 1  to # 4  to which the fuel should now be injected is the first cylinder # 1 , then the routine proceeds to step  106 . Alternatively, if a NO answer is obtained, then the routine proceeds to step  110 . 
   In step  106 , an injection signal achieving the injection parameters: injection timing Ts, the rate of injection, and the injection period, as derived in step  103 , is outputted to a corresponding one of the injectors  15  to commence the injection of fuel into the first cylinder # 1 . 
   The routine proceeds to step  107  wherein the output of the combustion pressure sensor  51  is sampled to determine an actual time of ignition of the fuel into the first cylinder # 1 . The routine proceeds to step  108  wherein the target-to-actual ignition time lag that is a difference between the target ignition timing, as derived in step  102 , and the actual ignition timing, as derived in step  107 , is determined. The routine proceeds to step  109  wherein the target-to-actual ignition lag is stored in the RAM of the ECU  60 . The routine then terminates. 
   If a NO answer is obtained meaning that the one of the cylinders # 1  to # 4 , as specified in step  105 , is not the first cylinder # 1 , then the routine proceeds to step  110  wherein it is determined whether the injection timings Ts for the cylinders # 2  to # 4  in which the combustion pressure sensor  51  is not installed should be corrected or not. Specifically, it is determined whether the engine  10  is now in the pre-mixed combustion mode or not which is to be entered, as described above, when the engine  10  is in the low-speed or low-load range. When the engine  10  is determined to be now placed in the pre-mixed combustion mode, a YES answer is obtained meaning that the injection timings Ts for the cylinders # 2  to # 4  should be corrected, then the routine proceeds to step  111  wherein the injection timings Ts are corrected or advanced by the ignition time difference ΔTi. 
   After step  111  or if a NO answer is obtained in step  110 , then the routine proceeds to step  112  wherein injection signals achieving the injection parameters: injection timing Ts, as corrected in step  111 , the rate of injection, and the injection period, as derived in step  103 , are outputted to corresponding ones of the injectors  15  to commence the injection of fuel into the cylinders # 2  to # 4 , in sequence. The routine then terminates. 
     FIGS. 6(   a ) to  6 ( d ) demonstrate actual times of the ignition of fuel into the first to fourth cylinders # 1  to # 4  of the engine  10  under the injection timing control, as described in step  5 . 
   The injection timing for the first cylinder # 1  in which thee combustion pressure sensor  51  is installed is, as described above, determined to have the actual timing of injection thereof fall within the range where there is no drop in the engine torque. Specifically, the injection timing of the first cylinder # 1  is set to time a 2 . The actual time of ignition in the first cylinder # 1  appears at time c 21 . The injection timings for the cylinders # 2  to # 4  in which the combustion pressure sensor  51  is not installed are shifted in the advanced direction from the time a 2  of the first cylinder # 1  by the ignition time difference ΔTi that is, as described above, the maximum variation in ignition timing among the cylinders # 1  to # 4 . Actual times c 22  to c 24  of the ignition fuel into the second to fourth cylinders # 2  to # 4  are, therefore, advanced from the time c 21  of the ignition of fuel into the first cylinder # 1 . It is found that all the times c 21  to c 24  of the ignition of fuel into the first to fourth cylinders # 1  to # 4  fall within the range where there is no drop in torque, as produced by the engine  10 . 
   As apparent from the above discussion, the engine control system is designed to correct the injection timings Ts for ones of the cylinders of the engine  10  in which the combustion pressure sensor  51  is not installed (i.e., the second to fourth cylinders # 2  to # 4 ) to a time point that is advanced from that for one of the cylinders in which the combustion pressure sensor  51  is installed (i.e., the first cylinder # 1 ), thereby eliminating the undesirable lag of the ignition which leads to the drop in the engine torque. 
   The combustion pressure sensor  51  is, as described above, mounted in the first cylinder # 1  which is the greatest in ratio of the amount of the EGR gas to the amount of the intake air in the engine  10 , thereby enabling the ECU  60  to minimize the delay of ignition of the mixture in the engine  10  which will result in a drop in the engine torque. 
   The ECU  60  may alternatively be designed to determine the degree to which the injection timing Ts are to be corrected or advanced to ensure a desired level of torque, as produced by the engine  10 , based on a given time point during combustion of the mixture in each of the cylinders of the engine  10  correlating to the time of the ignition, such as the position or time of the peak of the rate of heat release, the combustion center of gravity, or the end of the combustion period instead of the variation in ignition timing among the cylinders of the engine  10 . 
   The ECU  60  works to correct the injection timings Ts when the engine  10  is in the pre-mixed combustion mode, but may also be designed to make such a correction during an HCCI (Homogeneous Charge Compression Ignition) mode or a low-temperature combustion mode which is apt to result in a drop in torque of the engine  10  when the time of ignition is advanced from a desired point. Further, in the case where the engine control system is used with automobiles equipped with a NOx catalyst system, the ECU  60  may correct the injection timings Ts in a rich-burn control mode to purge the exhaust gas of NOx through the NOx catalyst system. 
   Use of fuel which is lower in cetane number in the normal combustion mode may cause the ignition timing to be retarded, thus resulting in a drop in engine torque. The ECU  60  may, therefore, also be designed to determine whether the value of the rate of heat release is correct or not during the normal combustion and, when it is incorrect, correct the injection timings Ts based on the ignition time difference ΔTi. 
   The ECU  60  may also be designed to multiply the ignition time difference ΔTi by a predetermined constant α to correct the ignition timings Ts. For instance, the constant α is selected to be greater than one (1) in order to minimize the possibility that the time of ignition of the mixture in each of the second to fourth cylinders # 2  to # 4  is undesirably regarded from a desired point. The constant α may alternatively be selected to be smaller than one (1) to correct the injection timings Ts to the retard of those without use of the constant α. 
   The ECU  60  may also be designed to change the degree to which the injection timings Ts for the second to fourth cylinders # 2  to # 4  (i.e., the ignition time difference ΔTi) are advanced as a function of parameters representing operating conditions of the engine  10  such as the ratio of the amount of EGR gas to that of air charged into the engine  10  and/or the concentration of oxygen (O 2 ) contained in the exhaust gas of the engine  10 . For instance, the degree of which the injection timings Ts is to be corrected in the advanced direction may be increased with an increase in the EGR gas-to-air ratio. This is because an increase in the ratio of the amount of the EGR gas (i.e., the exhaust gas) to that of the intake air will result in a decrease in amount of oxygen contained in the intake air, thus causing the time of ignition of the mixture in the engine  10  to be retarded. Additionally, the degree of the correction may be increased in the advanced direction with a decrease in the concentration of oxygen in the exhaust gas. This is because usually, a decrease in concentration of oxygen in the exhaust gas causes the time of ignition of the mixture in the engine  10  to be retarded. 
   The combustion pressure sensor  51  is, as described above, installed in the first cylinder # 1  which is the greatest in ratio of the amount of the EGR gas to the amount of the intake air in the engine  10  to correct the injection timings Ts for all the other cylinders # 2  to # 4 , but however, it may be mounted in another cylinder to correct the ignition timing(s) Ts for only one or some of the other cylinders which are relatively greater in the ratio of the EGR gas-to-intake air ratio. It is usually possible to find such a cylinder(s) in advance from the configuration of the intake pipe  11 . Specifically, the ECU  60  may work to correct or advance the injection timing(s) Ts for only one(s) of the cylinders of the engine  10  which is apt to experience misfire due to the addition of the EGR gas to the intake air. 
   In the case where the engine  10  is designed to have two banks, two combustion pressure sensors  51  may be installed one in one of cylinders of each of the banks. The ECU  60  works to correct the injection timings Ts for only one or some of the other cylinders of each of the banks which are relatively greater in the ratio of the EGR gas-to-intake air ratio. 
   Two or more combustion pressure sensors  51  may be installed in the cylinders of the engine  10 . In this case, the ECU  60  works to correct the injection timings Ts based on actual times of ignition of the mixture, as sampled by the combustion pressure sensors  51 . 
   In place of the combustion pressure sensor  51 , the ECU  60  may use a combustion light sensor sensitive to emission of light upon combustion of the mixture to determine an actual time of ignition of the mixture in a specified one of the cylinders. 
   While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.