Abstract:
According to a method and an apparatus of the present invention for diagnosing a fault of an engine, a cranking rotation state is created by rotating a crank shaft while explosions in each cylinder are stopped; a variation of angular velocity of the crank shaft is detected for each cylinder in the cranking rotation state; a cylinder the compression pressure of which is insufficient is detected based on the variation; and if a cylinder indicated as a misfiring cylinder by a fault code is the same as the cylinder detected as being insufficient in compression pressure in the cranking rotation state, the cylinder is specified.

Description:
TECHNICAL FIELD 
     The present invention relates to a method of and an apparatus for diagnosing an engine for a fault, by identifying a misfiring engine cylinder and storing a diagnostic trouble code representative of the misfiring cylinder. 
     BACKGROUND ART 
     There is known a technology for detecting the occurrence of a misfire in each of cylinders of an engine. See, Japanese Laid-Open Patent Publication No. 2009-280082 (hereinafter referred to as “JP2009-280082A”). According to JP2009-280082A, when a management ECU (117) judges that a misfire has occurred in an internal combustion engine (107), the management ECU turns on a warning lamp (125) (see paragraph [0027]). 
     There also is known a technology for detecting an abnormal compression pressure in each of cylinders of an engine. See, Japanese Laid-Open Patent Publication No. 2004-019465 (hereinafter referred to as “JP2004-019465A”). According to JP2004-019465A, while a fuel system and an ignition system are inactivated, the engine is cranked in order to rotate a crankshaft (1a), and a rotational variation of the engine is detected using a difference between instantaneous rotational speeds at preset crankshaft angles in compression strokes of the cylinders. An abnormal compression pressure is detected based on the rotational variation (see claims 1 through 6). 
     SUMMARY OF INVENTION 
     Based on a warning, such as turning on of a warning lamp as disclosed in JP2009-280082A, the engine is diagnosed as suffering from a fault, and then the engine is inspected and repaired at a service shop. However, since there are many factors responsible for a misfire, a large amount of time and effort is required to identify the trouble spot. 
     For example, if a troubleshooting process, which is carried out at a time that a misfire occurs in a multi-cylinder engine, includes an inspection item for confirming whether or not a compression pressure in an engine is abnormal, then the troubleshooting process may include a process for directly measuring compression pressures in the engine cylinders as the engine is being cranked, with pressure gauges inserted respectively into ignition plug insertion holes from which the ignition plugs have been removed, as well as a process for inspecting all of the valve clearances, i.e., the clearances of intake valves and exhaust valves, in the cylinders. However, such mechanical inspecting processes require a large expenditure of man-hours for disassembling the engine and adjusting and servicing the engine. 
     Causes of misfires are generally classified into electrical causes and mechanical causes. In particular, mechanical causes are problematic in that the process of identifying mechanical causes is highly tedious and time-consuming, since the engine has to be disassembled and serviced. 
     According to the technology for detecting an abnormal compression pressure in each of the cylinders as disclosed in JP2004-019465A, even if the compression pressures in the cylinders are slightly different from each other, misfiring may not actually occur and the engine may operate normally without any trouble. The difference between instantaneous rotational speeds at the time that the engine is cranked tends to be affected by different frictional properties of cylinder components as well as compression pressures. Thus, it may not be easy to judge whether or not a detected compression pressure is abnormal and requires the engine to be repaired, based simply on the relative difference between instantaneous rotational speeds. 
     As described above, the process for detecting an abnormal compression pressure in each of the cylinders as disclosed in JP2004-019465A uses a difference between instantaneous rotational speeds at preset crankshaft angles in compression strokes of the cylinders. Consequently, in order to detect an abnormal compression pressure, there is a need for a new diagnostic arrangement (including software such as judgment logic software) for detecting the difference between instantaneous rotational speeds in compression strokes of the cylinders. As a result, the engine diagnosing system is inevitably complex and costly. 
     The present invention has been made in view of the aforementioned problems. It is an object of the present invention to provide a method of and an apparatus for diagnosing an engine for a fault in a reduced number of diagnostic man-hours, simply by judging whether or not a misfire has occurred due to a mechanical fault, and identifying which of the cylinders is suffering from mechanical trouble. 
     Another object of the present invention is to provide a method of and an apparatus for diagnosing an engine for faults with a simplified arrangement and in a reduced number of diagnostic man-hours, simply by judging whether or not a misfire has occurred due to mechanical trouble, and identifying which of the cylinders is suffering from mechanical trouble, through utilization of an arrangement and judgment logic for judging misfires, based on the recognition that the occurrence of a misfire during operation of the engine is judged based on a detected variation in angular velocity of the crankshaft during power strokes of the engine while in operation. 
     According to the present invention, there is provided a method of diagnosing an engine for a fault, which is monitored by a misfire monitor for judging a misfiring cylinder that suffers from a misfire while an engine having a plurality of cylinders is in operation, and storing a diagnostic trouble code representative of the misfiring cylinder, comprising cranking the engine to rotate a crankshaft while canceling fuel explosion in the cylinders, detecting variations in angular velocity of the crankshaft for each of the cylinders while the engine is being cranked, and determining a compression pressure shortage cylinder, which suffers from a shortage of compression pressure, based on the detected variations, and identifying one of the cylinders, which coincides with the misfiring cylinder represented by the diagnostic trouble code and the compression pressure shortage cylinder that is determined while the engine is being cranked. 
     According to the present invention, a cylinder is identified, which coincides with a misfiring cylinder indicated by a diagnostic trouble code and a compression pressure shortage cylinder that is determined while the engine is being cranked. The cylinder is thus identified as suffering from a shortage of compression pressure, which needs to be repaired. Consequently, it is possible to judge whether or not there is a shortage of compression pressure (a mechanical fault) responsible for a misfire, without the need for disassembling the misfiring cylinder, so that the efficiency with which the engine is diagnosed for a fault can be increased. 
     The method may further comprise detecting variations in angular velocity of the crankshaft in a power stroke of the engine for each of the cylinders while the engine is in normal operation, and determining one of the cylinders, which exhibits small variations in angular velocity, as the misfiring cylinder, and detecting variations in angular velocity of the crankshaft in a power stroke of the engine for each of the cylinders while the engine is being cranked, and determining one of the cylinders, which exhibits small variations in angular velocity, as the compression pressure shortage cylinder. 
     While the engine either is operating normally or is being cranked, variations in angular velocity of the crankshaft in the power stroke are detected, and a cylinder, which exhibits small variations in angular velocity, is judged as a misfiring cylinder or a compression pressure shortage cylinder. Accordingly, the logic of a vehicle ECU for judging misfiring can also be used as a logic for judging a compression pressure shortage cylinder. Consequently, an arrangement (including software such as judgment logic software) for detecting a compression pressure shortage cylinder can be simplified. 
     According to the present invention, there also is provided a method of diagnosing an engine for a fault, which is monitored by a misfire monitor for detecting variations in angular velocity of a crankshaft of an engine having a plurality of cylinders, in a power stroke of the engine for each of the cylinders, and judging a misfiring cylinder that suffers from a misfire based on the detected variations, comprising cranking the engine to rotate the crankshaft while canceling fuel explosion in the cylinders, detecting variations in angular velocity of the crankshaft in the power stroke for each of the cylinders while the engine is being cranked, and determining one of the cylinders, which exhibits variations in angular velocity that are equal to or smaller than a predetermined value, as a compression pressure shortage cylinder that suffers from a shortage of compression pressure. 
     While the engine is being cranked to rotate the crankshaft and while canceling fuel explosion in the cylinders, variations in angular velocity of the crankshaft are detected, thereby making it possible to judge the occurrence of an abnormal compression pressure in the cylinders. Therefore, it can be determined whether or not there is a shortage of compression pressure (mechanical fault) that is responsible for a misfire, without the need for disassembling the cylinders. Accordingly, the efficiency with which faults of the engine are diagnosed can be increased. 
     While the engine either is operating normally or is being cranked, variations in angular velocity of the crankshaft in the power stroke are detected, and a compression pressure shortage cylinder is determined based on the variations. Accordingly, the logic of a vehicle ECU, which is employed to judge misfiring, can also be used as a logic for judging the existence of a compression pressure shortage cylinder. Consequently, an arrangement (including software such as judgment logic software) for detecting the presence of a compression pressure shortage cylinder can be simplified. 
     The present invention can be used not only when there are misfiring cylinders, but also to confirm operation of the engine when the vehicle is checked and serviced after the engine has been assembled. 
     The method may further comprise, while the engine is being cranked, comparing individual average values, which represent average values of the individual variations of the cylinders, with a total average value, which represents an average value of the variations of the cylinders, and determining one of the cylinders, which exhibits a smaller individual average value than the total average value, as the compression pressure shortage cylinder. Therefore, the compression pressure shortage cylinder can be determined by a relative comparison of the cylinders. Even if a change in the voltage of a battery for energizing a starter motor that actuates the crankshaft, or a change in the ambient temperature, etc., affect variations in angular velocity of the crankshaft to a certain extent, such variations are less likely to affect the determination concerning the compression pressure shortage cylinder. 
     The method may further comprise displaying, in a plurality of stages, a degree of difference from the total average value, in connection with individual average values that are smaller than the total average value. If an individual average value is smaller than the total average value, then the degree of the difference of the individual average value from the total average value is representative of the magnitude of the shortage of compression pressure. The magnitude of the shortage of compression pressure depends on the cause thereof (e.g., a compression pressure leakage from the cylinders, a clearance of the intake valve or the exhaust valve). Therefore, the degree of the difference of an individual average value from the total average value can be used as a means for estimating the cause of the shortage of compression pressure. Thus, if the degree of the difference is displayed, it is possible for the operator to estimate the cause of the shortage of compression pressure. If the cause of the shortage of compression pressure is displayed as well as the degree of the difference, then diagnostic efficiency can be increased. 
     The variations in angular velocity of the crankshaft while the engine is being cranked may start being detected upon elapse of a predetermined time from the start of a motor that actuates the crankshaft. In this manner, since the variations in angular velocity are detected when cranking of the engine becomes stable, it is possible to determine the compression pressure shortage cylinder accurately. 
     The method may further comprise monitoring a voltage of a battery for energizing the motor that actuates the crankshaft, and stopping determination of a compression pressure shortage cylinder if the voltage of the battery drops from a predetermined voltage. Since the judgment is avoided when cranking of the engine becomes unstable due to a voltage drop of the battery, it is possible to avoid errors in determining the compression pressure shortage cylinder. 
     The method may further comprise stopping determination of a compression pressure shortage cylinder if an engine coolant temperature or an engine oil temperature is lower than a predetermined value. If the predetermined value is set to a value that is unlikely to occur in a normal environment during normal usage, then the determination of the compression pressure shortage cylinder in a peculiar environment of usage can be avoided, and thus, it is possible to avoid errors in determining the compression pressure shortage cylinder. 
     According to the present invention, there is provided an apparatus for diagnosing an engine for a fault, which is monitored by a misfire monitor for judging a misfiring cylinder that suffers from a misfire while an engine having a plurality of cylinders is in operation, and storing a diagnostic trouble code representative of the misfiring cylinder, wherein the apparatus cranks the engine to rotate a crankshaft while canceling fuel explosion in the cylinders, detects variations in angular velocity of the crankshaft for each of the cylinders while the engine is being cranked, and determines a compression pressure shortage cylinder, which suffers from a shortage of compression pressure, based on the detected variations, and the apparatus identifies one of the cylinders, which coincides with the misfiring cylinder represented by the diagnostic trouble code and the compression pressure shortage cylinder that is determined while the engine is being cranked. 
     According to the present invention, there is further provided an apparatus for diagnosing an engine for a fault, which is monitored by a misfire monitor for detecting variations in angular velocity of a crankshaft of an engine having a plurality of cylinders, in a power stroke of the engine for each of the cylinders, and judging a misfiring cylinder that suffers from a misfire based on the detected variations, wherein the apparatus cranks the engine to rotate the crankshaft while canceling fuel explosion in the cylinders, detects variations in angular velocity of the crankshaft in the power stroke for each of the cylinders while the engine is being cranked, and determines one of the cylinders, which exhibits variations in angular velocity that are equal to or smaller than a predetermined value, as a compression pressure shortage cylinder that suffers from a shortage of compression pressure. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a general configuration of an engine diagnosing system having an engine trouble diagnosing apparatus (hereinafter referred to as a “diagnosing apparatus”) according to an embodiment of the present invention; 
         FIG. 2  is a view showing a general internal structure of a cylinder; 
         FIG. 3  is a view showing the appearance of a crankshaft angle sensor; 
         FIG. 4  is a diagram showing by way of example an output signal from the crankshaft angle sensor; 
         FIG. 5  is a flowchart of a sequence of an engine ECU for judging whether or not a misfire has occurred in a cylinder when the vehicle travels normally, i.e., when the engine is under normal operation; 
         FIG. 6  is a diagram showing by way of example a relationship between strokes of a piston in each cylinder and the magnitude of a load applied to the crankshaft as the piston operates during normal operation of the engine, when the cylinder operates normally and the cylinder suffers from a misfire; 
         FIG. 7  is a flowchart of a sequence of the engine ECU for judging whether or not a misfire has occurred; 
         FIG. 8  is a diagram showing by way of example a relationship between crankshaft angles and crankshaft angular velocities and strokes (intake, compression, power, and exhaust strokes) of cylinders, when first through fourth cylinders operate normally and the first cylinder suffers from a misfire; 
         FIG. 9  is a diagram showing a relationship between crankshaft angles and variations in angular velocity shown in  FIG. 8  together with power strokes of the cylinders; 
         FIG. 10  is a flowchart of a sequence for judging whether or not a compression pressure failure is occurring in each cylinder after the engine ECU has warned of the occurrence of a misfire; 
         FIG. 11  is a diagram showing by way of example changes in engine rotational speed in the event that a tappet clearance is normal; 
         FIG. 12  is a diagram showing by way of example changes in engine rotational speed in the event that the tappet clearance is large; 
         FIG. 13  is a diagram showing by way of example changes in engine rotational speed NE in the event of no compression pressure (zero compression pressure); 
         FIG. 14  is a diagram showing by way of example a relationship between strokes of a piston in each cylinder and magnitudes of loads applied to the crankshaft as the piston operates during cranking of the engine, when the cylinder operates normally and when the cylinder suffers from a shortage of compression pressure; 
         FIG. 15  is a first flowchart of a sequence of the diagnosing apparatus for judging whether or not there is a shortage of compression pressure; 
         FIG. 16  is a second flowchart of the sequence of the diagnosing apparatus for judging whether or not there is a shortage of compression pressure; 
         FIG. 17  is a timing chart of events that occur when the sequences of the flowcharts shown in  FIGS. 15 and 16  are carried out; 
         FIG. 18  is a diagram showing by way of example the relationship between crankshaft angles and crankshaft angular velocities and strokes (intake, compression, power, and exhaust strokes) of cylinders, when first through fourth cylinders operate normally and the first cylinder suffers from a misfire while the engine is being cranked; 
         FIG. 19  is a diagram showing a relationship between crankshaft angles and variations in angular velocity shown in  FIG. 18  together with power strokes of the cylinders; 
         FIG. 20  is a diagram showing by way of example individual average values of variations in angular velocity in the case that the tappet clearance of the first cylinder is normal, in the case that deviation of the tappet clearance is small, in the case that deviation of the tappet clearance is large, and in the case that the compression pressure is zero, at a time that the first cylinder is abnormal and the second through fourth cylinders are normal; 
         FIG. 21  is a diagram showing ratios of the individual average values to a total average value, based on the individual average values of the cylinders shown in  FIG. 20 ; 
         FIG. 22  is a diagram showing at an enlarged scale a portion of the ratios shown in  FIG. 21 ; 
         FIG. 23  is a diagram showing by way of example variations in angular velocity of the cylinders, individual average values, ratios of the individual average values to the total average value, and judgments made by the diagnosing apparatus; 
         FIG. 24  is a diagram showing by way of example indications used to display on a display unit mechanical faults judged by the diagnosing apparatus as causes of a misfire in a misfiring cylinder, and an inspection process and a repair process to be carried out subsequently; 
         FIG. 25  is a diagram showing a first example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments; 
         FIG. 26  is a diagram showing a second example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments; 
         FIG. 27  is a diagram showing a third example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments; 
         FIG. 28  is a diagram showing a fourth example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments; 
         FIG. 29  is a diagram showing a fifth example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments; and 
         FIG. 30  is a diagram showing a sixth example of a relationship between diagnostic trouble codes stored in the engine ECU, ratios calculated by the diagnosing apparatus, judgments made by the diagnosing apparatus, and inspection items and confirmation areas of the engine, which are displayed by the diagnosing apparatus based on the judgments. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A. Embodiment 
     1. Arrangement 
     (1) Overall Configuration 
       FIG. 1  is a block diagram showing the general configuration of an engine diagnosing system  10  (hereinafter referred to simply as a “system  10 ”) having an engine trouble diagnosing apparatus  14  (hereinafter referred to as a “diagnosing apparatus  14 ”) according to an embodiment of the present invention. The system  10  includes a vehicle  12 , which incorporates an engine  16  as an object to be diagnosed, and a diagnosing apparatus  14  for diagnosing the engine  16 . 
     (2) Vehicle  12   
     (a) Overall Configuration 
     The vehicle  12  includes, in addition to the engine  16 , an engine electronic control unit  18  (hereinafter referred to as an “engine ECU  18 ” or an “ECU  18 ”) for controlling operations of the engine  16 , and an ignition switch  20  (hereinafter referred to by “IGSW  20 ”). 
     (b) Engine  16   
     As shown in  FIG. 1 , the engine  16  comprises a so-called in-line four-cylinder engine having first through fourth cylinders  22   a  through  22   d  (hereinafter referred to collectively as “cylinders  22 ”), a crankshaft  24 , a crankshaft angle sensor  26 , a starter motor  28 , a battery  30 , a voltage sensor  32 , and a temperature sensor  34 . 
       FIG. 2  shows the general internal structure of one of the cylinders  22 . The cylinder  22  has an intake valve  40 , an exhaust valve  42 , a fuel injection valve  44 , an ignition plug  46 , and a piston  48 . The intake valve  40 , the exhaust valve  42 , and the ignition plug  46  are disposed in facing relation to a combustion chamber  50  in the cylinder  22 . 
       FIG. 3  shows the appearance of the crankshaft angle sensor  26 .  FIG. 4  shows by way of example an output signal Sa 1  from the crankshaft angle sensor  26 . The crankshaft angle sensor  26  detects a rotational angle (hereinafter referred to as a “crankshaft angle Ac”) [°] of a pulse rotor  52  mounted on the crankshaft  24 , and outputs the detected crankshaft angle Ac to the engine ECU  18 . More specifically, as shown in  FIG. 4 , the output signal Sa 1  from the crankshaft angle sensor  26  is output as a pulse signal each time that the pulse rotor  52  turns through a predetermined angle (6° in  FIG. 4 ). The ECU  18  receives the output signal Sa 1  from the crankshaft angle sensor  26 , and shapes the waveform of the output signal Sa 1  into a signal Sa 2 . The ECU  18  measures positive-going periods P1 of the signal Sa 2  in order to detect an engine rotational speed NE and an angular velocity (hereinafter referred to as a “crankshaft angular velocity ω” or an “angular velocity ω”) of the crankshaft  24 . 
     The starter motor  28  actuates the crankshaft  24  based on electric power supplied from the battery  30 . The voltage sensor  32  detects an output voltage Vb [V] of the battery  30 , and outputs the detected output voltage Vb to the ECU  18 . 
     The temperature sensor  34  detects the temperature Tw [° C.] of an engine coolant, not shown, and outputs the detected temperature Tw to the ECU  18 . The temperature sensor  34  may also detect the temperature To [° C.] of an engine oil, not shown. 
     (c) Engine ECU  18   
     The engine ECU  18  serves to control operations of the engine  16 . As shown in  FIG. 1 , the engine ECU  18  has an input/output unit  60 , a processor  62 , and a memory  64 . 
     (3) Diagnosing Apparatus  14   
     The diagnosing apparatus  14  serves to diagnose the engine  16  for faults. As shown in  FIG. 1 , the diagnosing apparatus  14  includes a cable  72  that connects to the engine ECU  18  through a data link connector  70  on the vehicle  12  for inputting and outputting intravehicular data, an input/output unit  74  to which the cable  72  is connected, an operating unit  76  in the form of a keyboard, a touch pad, etc., not shown, a processor  78  for controlling various components and judging each of the cylinders  22  for a malfunction, a memory  80  for storing various data and various programs including a control program used by the processor  78  and a trouble diagnosing program, and a display unit  82  for displaying various items of information. 
     The diagnosing apparatus  14  may consist of hardware in the form of a commercially available laptop computer, for example. 
     For diagnosing each of the cylinders  22  for faults using the diagnosing apparatus  14 , the operator (user) connects one end of the cable  72  to the input/output unit  74  and the other end of the cable  72  to the data link connector  70 , which is mounted on an instrument panel, not shown, of the vehicle  12 . Thereafter, the operator operates the operating unit  76  in order to instruct the diagnosing apparatus  14  to diagnose each of the cylinders  22  for faults. The diagnosing apparatus  14  causes the engine ECU  18  to operate the engine  16 . Details of a process carried out by the diagnosing apparatus  14  to diagnose each of the cylinders  22  for faults will be described later. 
     2. Diagnosis of Cylinders  22  for Faults 
     (1) Outline of Fault Diagnosis 
     According to the present embodiment, while the vehicle  12  is traveling normally, i.e., while the engine  16  is operating normally, the engine ECU  18  judges whether or not a misfire has occurred in the cylinders  22   a  through  22   d . If the engine ECU  18  detects the occurrence of a misfire, then the engine ECU  18  stores a diagnostic trouble code indicative of which one of the cylinders  22   a  through  22   d  is suffering from a misfire, and displays the diagnostic trouble code via a warning lamp, not shown, on the instrument panel. In the event that the engine ECU  18  judges that a misfire has occurred, the operator connects the diagnosing apparatus  14  to the ECU  18  and operates the diagnosing apparatus  14  in order to perform a trouble diagnosis, whereupon the diagnosing apparatus  14  judges whether or not there is a shortage of compression pressure in the cylinders  22   a  through  22   d . Based on the judgment made by the diagnosing apparatus  14 , the operator carries out subsequent inspection and repair processes. 
     (2) Judgment of a Misfire 
     (a) Outline of Judgment of a Misfire 
       FIG. 5  is a flowchart of a sequence of the engine ECU  18  for judging whether or not a misfire has occurred in the cylinders  22   a  through  22   d  when the vehicle  12  is traveling normally, i.e., when the engine  16  is operating normally. 
     In step S 1 , the ECU  18  judges whether or not a misfire has occurred in the cylinders  22   a  through  22   d . If no misfire has occurred in any one of the cylinders  22   a  through  22   d  (S 2 : NO), then control returns to step S 1 . If a misfire has occurred in any one of the cylinders  22   a  through  22   d  (S 2 : YES), then the ECU  18  stores a diagnostic trouble code (DTC), which indicates the occurrence of the misfire and the cylinder  22  that has misfired, in the memory  64 . In step S 4 , the ECU  18  issues a warning by turning on the warning lamp, not shown, thereby indicating to the user that the engine  16  has suffered from a fault. In response to the warning, the operator or user takes the vehicle  12  to a repair shop or the like. 
     (b) Principles of Judgment of a Misfire 
       FIG. 6  shows a model representation of the relationship between strokes of a piston  48  in each of the cylinders  22   a  through  22   d  and the magnitude of a load L 1  applied to the crankshaft  24  as the piston  48  operates during normal operation of the engine  16 , at times when the cylinder  22  operates normally and when the cylinder  22  suffers from a misfire. The load L 1  causes a reduction in the engine rotational speed NE [rpm], i.e., a reduction in the angular velocity ω of the crankshaft  24 . 
     In the example shown in  FIG. 6 , the load L 1  remains essentially unchanged when the cylinder  22  is operating normally as well as when the cylinder  22  suffers from a misfire, as long as the cylinder  22  is in the intake stroke, the compression stroke, and the exhaust stroke. However, when the cylinder  22  is operating normally at the time that the cylinder  22  is in the power stroke, an explosion in the cylinder  22  produces a torque, which increases the engine rotational speed NE, thereby reducing the load L 1 . 
     Consequently, it is possible to judge that a misfire has occurred based on the fact that the angular velocity ω in the power stroke is made lower (a variation thereof is made lower) as a result of the misfire than when the cylinder  22  is operating normally. 
     (c) Details of Judgment of a Misfire 
       FIG. 7  is a flowchart of a sequence (details of step S 1  of  FIG. 5 ) of the ECU  18  for judging whether or not a misfire has occurred. In step S 11 , the ECU  18  acquires a crankshaft angle Ac from the crankshaft angle sensor  26 . In step S 12 , the ECU  18  calculates a crankshaft angular velocity ω based on the acquired crankshaft angle Ac. 
       FIG. 8  shows by way of example a relationship between crankshaft angles Ac and crankshaft angular velocities ω together with strokes (intake, compression, power, and exhaust strokes) of the cylinders  22   a  through  22   d  at times that the cylinders  22   a  through  22   d  are operating normally and when the first cylinder  22   a  is suffering from a misfire. In  FIG. 8 , the solid-line curve  90  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at times that the cylinders  22   a  through  22   d  are operating normally, whereas the broken-line curve  92  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at a time when the first cylinder  22   a  is misfiring. 
     In the example shown in  FIG. 8 , the angular velocity ω sharply drops during the power stroke of the first cylinder  22   a . Therefore, the first cylinder  22   a  can be judged as misfiring. 
     In step S 13  of  FIG. 7 , using a non-illustrated high-pass filter, the ECU  18  removes variations in the engine rotational speed NE, which are caused when the vehicle  12  is accelerated and decelerated. 
     In step S 14 , the ECU  18  carries out a process of distinguishing strokes of each of the cylinders  22  (stroke distinguishing process). More specifically, certain crankshaft angles Ac are determined as corresponding to power strokes of the cylinders  22   a  through  22   d . In the present embodiment, since the engine  16  is a four-cylinder engine, the strokes, i.e., the intake, compression, power, and exhaust strokes, of the cylinders  22   a  through  22   d  occur successively in two revolutions (720°) of the crankshaft  24 . Consequently, crankshaft angles Ac, which are incremented by 180° (=720°/4), are assigned respectively to the power strokes of the cylinders  22   a  through  22   d.    
     In step S 15 , the ECU  18  calculates an angular velocity variation Δω during the power strokes of each of the cylinders  22 . For example, the ECU  18  may calculate the angular velocity variation Δω as a difference between an angular velocity ω at the start of the power stroke and an angular velocity ω at the end of the power stroke of each of the cylinders  22 . Alternatively, the ECU  18  may calculate the angular velocity variation Δω as a difference between greatest and smallest values of the angular velocity ω during the power stroke of each of the cylinders  22 . 
       FIG. 9  shows a relationship between crankshaft angles Ac and angular velocity variations Δω, which correspond to the data shown in  FIG. 8  and the power strokes of the cylinders  22   a  through  22   d . In  FIG. 9 , the solid-line curve  100  represents a relationship between crankshaft angles Ac and angular velocity variations Δω during a time that the cylinders  22   a  through  22   d  are operating normally, whereas the broken-line curve  102  represents a relationship between crankshaft angles Ac and angular velocity variations Δω during a time that the first cylinder  22   a  is misfiring. The broken-line curve  102  indicates negative angular velocity variations Δω during the power strokes of the misfiring cylinder. 
     In step S 16  of  FIG. 7 , the ECU  18  judges whether or not a misfire is occurring in the cylinders  22   a  through  22   d , based on the angular velocity variations Δω during the power strokes of the cylinders  22   a  through  22   d . More specifically, the ECU  18  judges that a misfire has occurred if the angular velocity variation Δω drops to a negative value, and further determines that the cylinder, which is in the power stroke corresponding to the negative angular velocity variation Δω, is a misfiring cylinder. 
     (d) Diagnostic Trouble Code 
     According to the present embodiment, as described above, a diagnostic trouble code is indicative of the occurrence of a misfire and any one of the cylinders  22   a  through  22   d  that has suffered from a misfire. For example, if a misfire has occurred in the first cylinder  22   a , then a diagnostic trouble code “P0301” is stored in the ECU  18 . If a misfire has occurred in the second cylinder  22   b , then a diagnostic trouble code “P0302” is stored in the ECU  18 . If a misfire has occurred in the third cylinder  22   c , then a diagnostic trouble code “P0303” is stored in the ECU  18 . If a misfire has occurred in the fourth cylinder  22   d , then a diagnostic trouble code “P0304” is stored in the ECU  18 . 
     (4) Judgment of Shortage of Compression Pressure 
     (a) Outline of Judgment of Shortage of Compression Pressure 
       FIG. 10  is a flowchart of a sequence for judging whether or not a compression pressure failure is occurring in each of the cylinders  22   a  through  22   d  after the engine ECU  18  has warned of the occurrence of a misfire. 
     In step S 21 , the operator connects the diagnosing apparatus  14  to the ECU  18  through the cable  72  and the data link connector  70 . In step S 22 , the operator operates the operating unit  76  in order to instruct the diagnosing apparatus  14  to read a diagnostic trouble code (DTC) from the ECU  18 . 
     In step S 23 , the operator judges whether or not the read diagnostic trouble code indicates the occurrence of a misfire. If the diagnostic trouble code does not indicate a misfire (S 23 : NO), then in step S 24 , the operator performs a diagnostic process depending on the diagnostic trouble code. 
     If the diagnostic trouble code indicates that a misfire has occurred (S 23 : YES), then in step S 25 , the diagnosing apparatus  14  judges whether or not there is a shortage of compression pressure in the cylinder  22  that the diagnostic trouble code indicates is misfiring (misfiring cylinder). When the diagnosing apparatus  14  judges a shortage of compression pressure, as will be described in detail later, the diagnosing apparatus  14  cranks the engine  16  in order to rotate the crankshaft  24 , while at the same time stopping supply of fuel and igniting the fuel in the cylinders  22   a  through  22   d  in order to prevent further fuel explosions therein. 
     If a shortage of compression pressure exists in the cylinder  22  (S 26 : YES), then in step S 27 , the diagnosing apparatus  14  judges that the misfire occurring in the misfiring cylinder is caused by a mechanical fault, and displays the mechanical fault responsible for the misfire together with subsequent inspection and repair processes on the display unit  82 . The operator then carries out the inspection and repair processes according to the displayed information. 
     If a shortage of compression pressure does not exist in the cylinder  22  (S 26 : NO), then in step S 28 , the diagnosing apparatus  14  judges that the misfire occurring in the misfiring cylinder is caused not by a mechanical fault, but by an electrical fault, for example, and displays the electrical fault responsible for the misfire together with subsequent inspection and repair processes on the display unit  82 . The operator then carries out the inspection and repair processes according to the displayed information. 
     (b) Principles of Judgment of Shortage of Compression Pressure 
     As described above, when the diagnosing apparatus according to the present embodiment makes a judgment concerning a shortage of compression pressure, the engine  16  is cranked in order to rotate the crankshaft  24 , while at the same time the diagnosing apparatus  14  cancels explosion of fuel in the cylinders  22   a  through  22   d , as described above. While the crankshaft  24  is being rotated while explosion of fuel is canceled in the cylinders  22   a  through  22   d , if the compression pressure in either one of the cylinders  22   a  through  22   d  is lowered due to a change in the tappet clearance or the like, then upon engine cranking, the engine rotational speed NE or the crankshaft angular velocity ω tends to vary greatly. 
       FIG. 11  is a diagram showing by way of example changes in the engine rotational speed NE in the event that a tappet clearance TC is normal.  FIG. 12  is a diagram showing by way of example changes in the engine rotational speed NE in the event that the tappet clearance TC exhibits a large deviation (e.g., TC=0.2 mm).  FIG. 13  is a diagram showing by way of example changes in the engine rotational speed NE in the event of no compression pressure (zero compression pressure). Generally, the tappet clearance represents a gap between the shaft of an intake value or an exhaust valve and a camshaft or a rocker arm. The tappet clearance affects the opening timing of the valve, which indicates an opening/closing point of the valve, as well as the operation timing of the valve. 
       FIG. 14  is a diagram showing a model representation of a relationship between strokes of the piston  48  in each of the cylinders  22   a  through  22   d , and the magnitude of a load L 1  applied to the crankshaft  24  as the piston  48  operates during cranking of the engine, at a time that the cylinder  22  is operating normally and at a time that the cylinder  22  is suffering from a shortage of compression pressure. The load L 1  causes a reduction in the engine rotational speed NE [rpm], i.e., a reduction in the angular velocity ω of the crankshaft  24 . Since fuel explosion is canceled in the cylinders  22   a  through  22   d  when the engine is cranked, no actual fuel explosion occurs during the power stroke shown in  FIG. 14 . Stated otherwise, the power stroke shown in  FIG. 14  represents a stroke having the same range of the crankshaft angle Ac as the power stroke during normal operation of the engine. 
     In the example shown in  FIG. 14 , the load L 1  applied when the cylinder  22  is operating normally and the load L 1  applied when the cylinder  22  is suffering from a shortage of compression pressure are compared with each other. The difference between the compared loads L 1  is significantly larger in the compression stroke than in the intake stroke, the power stroke, and the exhaust stroke. This is because the compressive load is small when a gas leakage exists somewhere in the cylinder  22 . 
     With an engine  16  having plural cylinders  22   a  through  22   d , the strokes of the cylinders  22   a  through  22   d  are kept out of phase with each other, so as to produce regular angular velocity variations Δω while allowing the engine  16  to be cranked stably during normal operation. However, when a compression failure occurs in any one of the cylinders  22   a  through  22   d , the compressive load is not applied as required, thereby causing a disturbance in the angular velocity variations Δω. 
     According to the present invention, which is based on the above observations, the difference between the loads L 1  in the compression stroke is not used directly, but rather, angular velocity variations Δω in the power stroke are used to judge a shortage of compression pressure, similar to the case of judging the presence of a misfire (see  FIG. 9 , etc.). More specifically, such a judgment is based on the fact that, when the engine  16  having the cylinders  22   a  through  22   d  including one cylinder that suffers from a shortage of compression pressure is cranked, the crankshaft angular velocity ω increases in the compression stroke of the cylinder that suffers from a shortage of compression pressure, but in reaction thereto, decreases in the next stroke, i.e., the power stroke, of the same cylinder. Thus, it is possible to judge whether or not there is a shortage of compression pressure based on a reduction (variation) in the angular velocity ω in the power stroke. Accordingly, a similar logic to that used for judging the presence of a misfire can be used as the logic for judging a cylinder that suffers from a shortage of compression pressure. 
     (c) Details of Judgment of Shortage of Compression Pressure 
       FIG. 15  is a first flowchart of a sequence of the diagnosing apparatus  14  for judging whether or not there is a shortage of compression pressure.  FIG. 16  is a second flowchart of the sequence of the diagnosing apparatus  14  for judging whether or not there is a shortage of compression pressure.  FIG. 17  is a timing chart of events that occur when the sequences of the flowcharts shown in  FIGS. 15 and 16  are carried out. 
     In step S 31  of  FIG. 15 , the diagnosing apparatus  14  displays a request to warm up the engine on the display unit  82 . Upon observing the displayed message, the operator turns on the IGSW  20  in order to start warming up the engine (time t1). The operator warms up the engine by increasing the engine rotational speed NE up to a predetermined warm-up speed, e.g., 3000 rpm. 
     In step S 32 , the diagnosing apparatus  14  judges whether or not the engine has been warmed up. More specifically, through the ECU  18 , the diagnosing apparatus  14  acquires a temperature Tw from the temperature sensor  34 , and judges whether or not the acquired temperature Tw is equal to or greater than a threshold value THw, which is indicative of the engine being in a warmed up condition. If the engine is not warmed up (S 32 : NO), then the diagnosing apparatus  14  repeats step S 32 . 
     If the engine is warmed up (S 32 : YES), then in step S 33 , the diagnosing apparatus  14  controls the display unit  82  in order to display a request for ending warming-up of the engine. The request includes a request to turn off the IGSW  20 , and thereafter, to turn on the IGSW  20  again. After observing the displayed request, the operator turns off the IGSW  20  (time t2), and then turns on the IGSW  20  again in order to initiate a measurement process (time t3). 
     When the operator turns on the IGSW  20  again (S 34 : YES), the diagnosing apparatus  14  acquires a voltage Vb of the battery  30  from the voltage sensor  32  via the ECU  18 , and in step S 35 , judges whether or not the acquired voltage Vb is equal to or greater than a threshold value concerning the voltage Vb (battery voltage threshold value TH_Vb). The battery voltage threshold value TH_Vb is a threshold value by which it is judged whether or not cranking of the engine, which is performed by the starter motor  28 , has become stable. 
     If the voltage Vb is lower than the battery voltage threshold value TH_Vb (S 35 : NO), then the sequence for judging whether or not there is a shortage of compression pressure is ended. If the voltage Vb is equal to or greater than the battery voltage threshold value TH_Vb (S 35 : YES), then in step S 36  of  FIG. 16 , the diagnosing apparatus  14  sends a request to the ECU  18  to supply angular velocity variations Δω that occur during the power strokes of the cylinders  22   a  through  22   d  (time t4). The request includes a request for inhibiting fuel explosion in the cylinders  22   a  through  22   d  by stopping supply of fuel and keeping the ignition signals off. 
     In step S 37 , the diagnosing apparatus  14  sends a request to the ECU  18  to transmit a diagnostic trouble code (time t5). In response to the request, the ECU  18  transmits a diagnostic trouble code to the diagnosing apparatus  14 . 
     In step S 38 , the diagnosing apparatus  14  controls the display unit  82  in order to display a request, which asks the operator to crank the engine. Upon observing the request, the operator energizes the starter motor in order to crank the engine (time t6). 
     In step S 39 , the diagnosing apparatus  14  judges whether or not the engine rotational speed NE (cranking rotational speed) acquired through the ECU  18  is equal to or greater than a threshold value TH_NE. The threshold value TH_NE is a threshold value by which it can be judged stably that the engine rotational speed NE is high enough to enable determination of a shortage of compression pressure. For example, the threshold value TH_NE is 50 rpm. If the engine rotational speed NE is not equal to or greater than the threshold value TH_NE (S 39 : NO), then the diagnosing apparatus  14  repeats step S 39 . If the engine rotational speed NE has not become equal to or greater than the threshold value TH_NE after elapse of a predetermined time (e.g., 30 seconds), then the diagnosing apparatus  14  cancels the cranking request and brings the diagnostic process to an end. If the engine rotational speed NE is equal to or greater than the threshold value TH_NE (S 39 : YES), then in step S 40 , the diagnosing apparatus  14  judges whether or not a predetermined time (e.g., 1 second) has elapsed after the engine rotational speed NE has become equal to or greater than the threshold value TH_NE. If the predetermined time has not elapsed (S 40 : NO), then control returns to step S 39 . 
     If the predetermined time has elapsed (S 40 : YES), then in step S 41 , the diagnosing apparatus  14  acquires angular velocity variations Δω from the ECU  18  (from time t7 to t8). More specifically, the ECU  18  detects angular velocity variations Δω in the same manner as with steps S 11  through S 15  of  FIG. 7 , and the ECU  18  sends the detected angular velocity variations Δω to the diagnosing apparatus  14 . When the ECU  18  finishes detecting and sending the angular velocity variations Δω, the operator stops cranking of the engine in response to a display on the display unit  82  of the diagnosing apparatus  14 . The operator may also stop cranking of the engine after the ECU  18  has detected the angular velocity variations Δω. 
       FIG. 18  shows a model representation of a relationship between crankshaft angles Ac and crankshaft angular velocities ω, and respective strokes (intake, compression, power, and exhaust strokes) of the cylinders  22   a  through  22   d  when the cylinders  22   a  through  22   d  are operating normally, and when the first cylinder  22   a  suffers from a misfire while the engine is being cranked. In  FIG. 18 , the solid-line curve  110  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at a time that the cylinders  22   a  through  22   d  are operating normally, whereas the broken-line curve  112  represents a relationship between crankshaft angles Ac and crankshaft angular velocities ω at a time that the first cylinder  22   a  is misfiring. 
     In the example shown in  FIG. 18 , the angular velocity ω drops sharply due to a rotational disturbance in the power stroke subsequent to the compression stroke of the first cylinder  22   a.    
       FIG. 19  shows a relationship between crankshaft angles Ac and angular velocity variations Δω corresponding to the data shown in  FIG. 18 , along with the power strokes of the cylinders  22 . In  FIG. 19 , the solid-line curve  120  represents a relationship between crankshaft angles Ac and angular velocity variations Δω, at a time that the cylinders  22  are operating normally, whereas the broken-line curve  122  represents a relationship between crankshaft angles Ac and angular velocity variations Δω, at a time that the first cylinder  22   a  is misfiring. 
     The example shown in  FIG. 18  illustrates compression leakage (zero compression pressure) during compression strokes of the first cylinder  22   a , in order to clearly show a compression pressure failure. In the example shown in  FIG. 19 , the angular velocity variations Δω are reduced during the power stroke of the first cylinder  22   a . This is because a compressive load is not applied, while a corresponding increase occurs in the angular velocity variation Δω during the compression stroke of the first cylinder  22   a , and then in reaction thereto, the angular velocity variation Δω decreases in the power stroke of the first cylinder  22   a . It is thus possible to judge whether or not a shortage of compression pressure has occurred in the first cylinder  22   a , based on a comparison of angular velocity variations Δω in each of the power strokes. 
     In step S 42  of  FIG. 16 , based on the acquired angular velocity variations Δω (from time t8 to time t9 in  FIG. 17 ), the diagnosing apparatus  14  calculates individual average values AVEr, a total average value AVEt, and ratios R 1 . The individual average values AVEr represent average values of angular velocity variations Δω during power strokes of the respective cylinders  22 . The total average value AVEt is an average value of the individual average values AVEr of all of the cylinders  22 . The ratios R 1  (AVEr/AVEt) are calculated by dividing the respective individual average values AVEr by the total average value AVEt. 
     In step S 43 , the diagnosing apparatus  14  judges whether or not there is a mechanical fault in any of the cylinders  22 , based on the diagnostic trouble code acquired in step S 37  and the ratios R 1  calculated in step S 42 , and displays the judgment result on the display unit  82  (from time t10 to time t11). 
     In particular, the diagnosing apparatus  14  judges whether or not there is a shortage of compression pressure in a misfiring cylinder, based on the ratios R 1  with respect to the cylinders  22   a  through  22   d . More specifically, if the ratio R 1  with respect to the misfiring cylinder is smaller than a threshold value by which it is judged whether or not there is a shortage of compression pressure (compression force shortage judging threshold value TH 2 ), then the diagnosing apparatus  14  determines that there is a shortage of compression pressure in the misfiring cylinder. According to the present embodiment, the threshold value TH 2  is 100%. 
       FIG. 20  shows by way of example individual average values AVEr in the case that the tappet clearance TC of the first cylinder  22   a  is normal (e.g., TC=0.23 mm), in the case that the deviation of the tappet clearance TC is small (e.g., TC=0.13 mm), in the case that the deviation of the tappet clearance TC is large (e.g., TC=0.05 mm), and in the case that the compression pressure is zero, at a time when the first cylinder  22   a  is abnormal and the second through fourth cylinders  22   b  through  22   d  are normal. 
     The solid-line curve  130  represents individual average values AVEr in the case that the tappet clearance TC of the first cylinder  22   a  is normal (e.g., TC=0.23 mm). The broken-line curve  132  represents individual average values AVEr in the case that the deviation of the tappet clearance TC is small (e.g., TC=0.13 mm). The dot-and-dash-line curve  134  represents individual average values AVEr in the case that the deviation of the tappet clearance TC is large (e.g., TC=0.05 mm). The two-dot-and-dash-line curve  136  represents individual average values AVEr in the case that the compression pressure is zero. 
       FIG. 21  shows ratios R 1  (=AVEr/AVEt) of the individual average values AVEr to the total average value AVEt, based on the individual average values AVEr of the cylinders  22   a  through  22   d  shown in  FIG. 20 .  FIG. 22  is a diagram, which shows at an enlarged scale a portion of the ratios shown in  FIG. 21 . In  FIGS. 21 and 22 , the solid-line curve  140  corresponds to the first cylinder  22   a , the broken-line curve  142  corresponds to the second cylinder  22   b , the dot-and-dash-line curve  144  corresponds to the third cylinder  22   c , and the two-dot-and-dash-line curve  146  corresponds to the fourth cylinder  22   d.    
       FIG. 23  is a diagram showing by way of example angular velocity variations Δω of the cylinders  22 , individual average values AVEr, ratios R 1  (=AVEr/AVEt), and judgments made by the diagnosing apparatus  14 . In the example shown in  FIG. 23 , the individual average value AVEr of the first cylinder  22   a  is 44.4 [rad/s], the individual average value AVEr of the second cylinder  22   b  is 54.0, the individual average value AVEr of the third cylinder  22   c  is 53.9, and the individual average value AVEr of the fourth cylinder  22   d  is 55.8. Therefore, the total average value AVEt is 52.03 [rad/s]. 
     The ratio R 1  with respect to the first cylinder  22   a  is 85% (=44.4/52.03), the ratio R 1  with respect to the second cylinder  22   b  is 104% (=54.0/52.03), the ratio R 1  with respect to the third cylinder  22   c  is 104% (53.9/52.03), and the ratio R 1  with respect to the fourth cylinder  22   d  is 107% (55.8/52.03). 
     The ratio R 1  is smaller than the threshold value TH 2  (100% in the present embodiment) with respect to the first cylinder  22   a . Therefore, the first cylinder  22   a  is judged as suffering from a shortage of compression pressure. If the diagnostic trouble code stored in the ECU  18  represents the occurrence of a misfire in the first cylinder  22   a , then the first cylinder  22   a  is judged as “NO GOOD” and in need of a mechanical fault check. Since the ratios R 1  with respect to the second through fourth cylinders  22   a  through  22   d  are not smaller than the threshold value TH 2 , the second through fourth cylinders  22   a  through  22   d  are judged as “GOOD” and do not require mechanical fault checks, regardless of the content of the diagnostic trouble code. 
     (d) Judgment Results Made by Diagnosing Apparatus 
       FIG. 24  shows by way of example indications that are displayed on the display unit  82  to indicate mechanical troubles judged by the diagnosing apparatus  14  as being responsible for a misfire in a misfiring cylinder, together with an inspection process and a repair process to be carried out subsequently. In  FIG. 24 , the indications are displayed in three stages depending on the magnitudes of the ratios R 1 , i.e., “SMALL TAPPET CLEARANCE DEVIATION”, “LARGE TAPPET CLEARANCE DEVIATION”, and “COMPRESSION FAILURE”. The indication of “COMPRESSION FAILURE” includes damage to the cylinders  22   a  through  22   d , failures of pistons, not shown, etc. 
     If the ratio R 1  is slightly smaller than 100%, then the diagnosing apparatus  14  displays on the display unit  82  an inspection process and a repair process to be carried out for repairing the small deviation of the tappet clearance TC. If the ratio R 1  is considerably smaller than 100%, then the diagnosing apparatus  14  displays on the display unit  82  an inspection process and a repair process to be carried out for repairing the large deviation of the tappet clearance TC. If the ratio R 1  is extremely smaller than 100%, then the diagnosing apparatus  14  displays on the display unit  82  an inspection process and a repair process to be carried out for repairing the compression failure. 
       FIGS. 25 through 30  show first through six examples of relationships between diagnostic trouble codes stored in the ECU  18 , ratios R 1  calculated by the diagnosing apparatus  14 , judgments made by the diagnosing apparatus  14 , and inspection items and confirmation areas of the engine  16 , which are displayed by the diagnosing apparatus  14  based on the judgments. 
     In  FIG. 25 , the diagnostic trouble codes indicate that the first cylinder  22   a  is misfiring. Since the ratio R 1  with respect to the first cylinder  22   a  is smaller than the threshold value TH 2  (100%), the first cylinder  22   a  is judged as suffering from a shortage of compression pressure. Since the first cylinder  22   a  both is misfiring and is a cylinder suffering from a shortage of compression pressure, the diagnosing apparatus  14  diagnoses that the first cylinder  22   a  is “NO GOOD” and is in need of a mechanical fault check. Depending on the ratio R 1  with respect to the first cylinder  22   a , the diagnosing apparatus  14  displays “POOR TAPPET CLEARANCE” and “POOR COMPRESSION” as inspection terms and confirmation areas with respect to the first cylinder  22   a . Inasmuch as the second through fourth cylinders  22   b  through  22   d  are not misfiring and do not suffer from a shortage of compression pressure, the diagnosing apparatus  14  judges the second through fourth cylinders  22   b  through  22   d  as “GOOD” and not in need of a mechanical fault check. 
     In  FIG. 26 , the diagnostic trouble codes indicate that the first cylinder  22   a  and the third cylinder  22   c  are misfiring. Since the ratio R 1  with respect to the first cylinder  22   a  is smaller than the threshold value TH 2  (100%), the first cylinder  22   a  is judged as suffering from a shortage of compression pressure. Since the first cylinder  22   a  both is misfiring and is a cylinder suffering from a shortage of compression pressure, the diagnosing apparatus  14  diagnoses the first cylinder  22   a  as “NO GOOD”. Depending on the ratio R 1  with respect to the first cylinder  22   a , the diagnosing apparatus  14  displays “POOR TAPPET CLEARANCE” and “POOR COMPRESSION” as inspection terms and confirmation areas with respect to the first cylinder  22   a . Inasmuch as the second and fourth cylinders  22   b ,  22   d  are not misfiring and do not suffer from a shortage of compression pressure, whereas the third cylinder  22   c  is misfiring but is not a cylinder suffering from a shortage of compression pressure, the second through fourth cylinders  22   b  through  22   d  are judged as “GOOD”. 
     In  FIG. 27 , although the ratio R 1  with respect to the first cylinder  22   a  is smaller than the threshold value TH 2  (100%), the diagnostic trouble codes indicate that none of the first through fourth cylinders  22   a  through  22   d  are misfiring. Consequently, the diagnosing apparatus  14  judges all of the cylinders  22   a  through  22   d  as “GOOD”. 
     In  FIG. 28 , the diagnostic trouble codes indicate that the first through third cylinders  22   a  through  22   c  are misfiring. Because the ratios R 1  with respect to the first through third cylinders  22   a  through  22   c  are smaller than the threshold value TH 2  (100%), the first through third cylinders  22   a  through  22   c  are judged as being cylinders that suffer from a shortage of compression pressure. Further, since the first through third cylinders  22   a  through  22   c  both are misfiring and are cylinders suffering from a shortage of compression pressure, the diagnosing apparatus  14  diagnoses the first through third cylinders  22   a  through  22   c  as “NO GOOD”. Depending on the ratios R 1 , the diagnosing apparatus  14  displays “POOR COMPRESSION” as an inspection term and a confirmation area with respect to the first cylinder  22   a , displays “LARGE TAPPET CLEARANCE DEVIATION” as an inspection term and a confirmation area with respect to the second cylinder  22   b , and displays “SMALL TAPPET CLEARANCE DEVIATION” as an inspection term and a confirmation area with respect to the third cylinder  22   c . Since the fourth cylinder  22   d  is not misfiring and does not suffer from a shortage of compression pressure, the fourth cylinder  22   d  is judged as “GOOD”. 
     In  FIG. 29 , the diagnostic trouble codes indicate that the first cylinder  22   a  and the third cylinder  22   c  are misfiring. Because the ratios R 1  with respect to the first cylinder  22   a  and the third cylinder  22   c  are smaller than the threshold value TH 2  (100%), the first cylinder  22   a  and the third cylinder  22   c  are judged as being cylinders that suffer from a shortage of compression pressure. Further, since the first cylinder  22   a  and the third cylinder  22   c  both are misfiring and are cylinders suffering a shortage of compression pressure, the diagnosing apparatus  14  diagnoses the first cylinder  22   a  and the third cylinder  22   c  as “NO GOOD”. Depending on the ratios R 1 , the diagnosing apparatus  14  displays “POOR COMPRESSION” as an inspection term and a confirmation area with respect to the first cylinder  22   a , and displays “POOR TAPPET CLEARANCE” as an inspection term and a confirmation area with respect to the third cylinder  22   c . Since the second cylinder  22   b  and the fourth cylinder  22   d  are not misfiring and are not cylinders that suffer from a shortage of compression pressure, the second cylinder  22   b  and the fourth cylinder  22   d  are judged as “GOOD”. 
     In  FIG. 30 , the diagnostic trouble codes indicate that the fourth cylinder  22   d  is misfiring. The first through third cylinders  22   a  through  22   c  are not misfiring, although the first through third cylinders  22   a  through  22   c  are cylinders that suffer from a shortage of compression pressure. The fourth cylinder  22   d  is not a cylinder that suffers from a shortage of compression pressure, although the fourth cylinder  22   d  is a misfiring cylinder. Therefore, the diagnosing apparatus  14  judges that all of the cylinders  22   a  through  22   d  are “GOOD”. 
     According to the present embodiment, as can be seen from the examples shown in  FIGS. 25 through 30 , a mechanical fault is judged to have occurred only when a misfiring cylinder is indicated by a diagnostic trouble code, and a compression pressure shortage cylinder, the ratio R 1  of which is smaller than the threshold value TH 2  (=100%), coincides with the misfiring cylinder, whereupon an inspection item and a confirmation area depending on the ratio R 1  are indicated. 
     3. Advantages of the Present Embodiment 
     According to the present embodiment, as described above, if the ratio R 1  with respect to a misfiring cylinder indicated by a diagnostic trouble code is smaller than 100%, or stated otherwise, if a cylinder  22  is identified which coincides with the misfiring cylinder indicated by the diagnostic trouble code, and a compression pressure shortage cylinder is determined to exist while the engine is being cranked, then the cylinder  22  is identified as suffering from a shortage of compression pressure and needs to be repaired. Consequently, it is possible to judge whether or not there is a shortage of compression pressure (mechanical fault) responsible for a misfire, without the need for disassembling the cylinders  22   a  through  22   d . Thus, the efficiency with which the engine  16  is diagnosed for a fault can be increased. 
     According to the present embodiment, while the engine is being cranked in order to rotate the crankshaft  24  while explosion of fuel in the cylinders  22   a  through  22   d  is canceled, angular velocity variations Δω are detected, thereby making it possible to judge the occurrence of an abnormal compression pressure in the cylinders  22   a  through  22   d . Therefore, it is possible to judge whether or not there is a shortage of compression pressure (mechanical fault) responsible for a misfire, without the need for disassembling the cylinders  22   a  through  22   d . Thus, the efficiency with which the engine  16  is diagnosed for a fault can be increased. 
     While the engine  16  is either operating normally or is being cranked, angular velocity variations Δω in the power stroke are detected with respect to the cylinders  22   a  through  22   d , so as to determine the presence of a compression pressure shortage cylinder based on the angular velocity variations Δω. Accordingly, a similar logic of the ECU  18  to that which is used for judging a misfire can be used as the logic for determining the presence of a compression pressure shortage cylinder. Consequently, an arrangement (including software such as judgment logic software) for detecting a compression pressure shortage cylinder can be simplified. 
     The present embodiment can be used not only when there are misfiring cylinders, but also to confirm proper operation of the engine  16  after the engine  16  has been assembled, such as when the vehicle is checked and serviced. 
     According to the present embodiment, while the engine is being cranked, individual average values AVEr and a total average value AVEt are compared with each other, and any one of the cylinders  22   a  through  22   d  having an individual average value AVEr that is smaller than the total average value AVEt is judged to be a compression pressure shortage cylinder. Therefore, the presence of a compression pressure shortage cylinder can be determined by relative comparison of the cylinders  22   a  through  22   d . Even if a change in the voltage Vb of the battery  30 , which is used for energizing the starter motor  28  that actuates the crankshaft  24 , or changes in the ambient temperature, etc., affect the angular velocity variations Δω to a certain extent, the angular velocity variations Δω are less likely to affect the judgment concerning the compression pressure shortage cylinder. 
     According to the present embodiment, individual average values AVEr, which are smaller than the total average value AVEt, have degrees of difference thereof from the total average value AVEt displayed in a plurality of stages ( FIG. 25 ). If an individual average value AVEr is smaller than the total average value AVEt, then the degree of difference of the individual average value AVEr from the total average value AVEt represents the magnitude of a shortage of compression pressure. The magnitude of the shortage of compression pressure depends on the cause thereof (e.g., leakage of compression pressure from the cylinders  22   a  through  22   d , a clearance of the intake valve  40  or the exhaust valve  42 ). Therefore, the degree of difference of an individual average value AVEr from the total average value AVEt can be used as an indication for estimating the cause of the shortage of compression pressure. Therefore, when the degree of difference is displayed, it is possible for the operator to estimate the cause of the shortage of compression pressure. If the cause of the shortage of compression pressure is displayed along with the degree of difference, then diagnostic efficiency can be increased. 
     According to the present embodiment, while the engine is being cranked, angular velocity variations Δω start to be detected upon elapse of a predetermined time from initiation of the starter motor  28  that actuates the crankshaft  24 , i.e., upon elapse of a predetermined time after the engine rotational speed NE exceeds the threshold value TH_NE. Since angular velocity variations are detected after cranking of the engine becomes stable, it is possible to reliably judge the presence of a compression pressure shortage cylinder. 
     According to the present embodiment, the voltage Vb of the battery  30 , which energizes the starter motor  28  that actuates the crankshaft  24 , is monitored. If the voltage Vb becomes lower than the threshold value TH_Vb, the process of judging a compression pressure shortage cylinder is canceled (S 35 : NO in  FIG. 15 ). Since the judgment is avoided when cranking of the engine becomes unstable due to a drop in the voltage Vb of the battery  30 , it is possible to avoid errors in determining the presence of a compression pressure shortage cylinder. 
     According to the present embodiment, if the temperature Tw of the engine coolant becomes lower than the threshold value THw, the process of judging a compression pressure shortage cylinder is canceled (S 32 : NO). If the threshold value THw is set to a value, which is unlikely to occur in a normal environment of usage, then the judgment concerning the compression pressure shortage cylinder is avoided in a peculiar environment of usage. Thus, it is possible to avoid errors in determining the presence of a compression pressure shortage cylinder. 
     B. Modifications 
     The present invention is not limited to the above embodiment, but may employ various additional or alternative arrangements based on the above disclosure of the present invention. For example, the present invention may employ the following arrangements. 
     In the above embodiment, the diagnosing apparatus is used to diagnose the engine  16  of the vehicle  12 . However, the diagnosing apparatus  14  may be used in connection with other systems having engines, for example, mobile objects such as ships or the like. In the above embodiment, the diagnosing apparatus  14  communicates with the engine ECU  18  from an external location outside of the vehicle  12 . However, the diagnosing apparatus  14  may be incorporated in the vehicle  12 . Stated otherwise, the engine ECU  18  may include the functions of the diagnosing apparatus  14 . 
     In the above embodiment, the engine  16  is an in-line four-cylinder engine. However, the layout and number of the cylinders  22   a  through  22   d  are not limited to those that make up parts of an in-line four-cylinder engine. The engine  16  may be a V-shaped six-cylinder engine. If the engine  16  is a V-shaped six-cylinder engine, the strokes, i.e., the intake, compression, power, and exhaust strokes, of the six cylinders occur successively in two revolutions) (720°) of the crankshaft  24 . Consequently, crankshaft angles Ac, which are incremented by 120° (=720°/6), are assigned respectively to the power strokes of the cylinders. 
     In the above embodiment, the judgment of a misfire and the judgment of a compression pressure shortage are combined. However, from the standpoint of using angular velocity variations Δω of the crankshaft  24  that correspond to the power stroke, only one of such judgments, i.e., the judgment of a misfire or the judgment of a compression pressure shortage, may be used. 
     In the above embodiment, while the engine is being cranked, both the fuel supply system (the fuel injection valves  44 , etc.) and the ignition system (the ignition plugs  46 , etc.) are disabled. However, insofar as no fuel explosion occurs in the cylinders  22   a  through  22   d , only the fuel supply system may be disabled. 
     In the above embodiment, the temperature Tw of the engine coolant is used to judge whether or not to cancel the process of determining a compression pressure shortage cylinder. However, instead of or in addition to the temperature Tw, the temperature To of an engine oil (not shown) may also be used. 
     In the above embodiment, angular velocity variations Δω in the power stroke are used to judge both misfiring and the occurrence of a compression pressure shortage. However, from the standpoint of combining the judgment of misfiring and the judgment of a compression pressure shortage, the present invention is not limited to using angular velocity variations Δω in the power stroke. For judging a compression pressure shortage, for example, angular velocity variations Δω in the compression stroke may be used. 
     In the above embodiment, a compression pressure shortage is determined by using individual average values AVEr and the total average value AVEt. However, from the standpoint of judging a compression pressure shortage, the present invention is not limited to using individual average values AVEr and the total average value AVEt. Only the individual average values AVEr may be used in order to judge the occurrence of a compression pressure shortage.