Abstract:
A crank angle detecting device that is independent of fluctuations in engine rotational speed and reliably detects a reference crank angle position. A ring gear fixed to a crankshaft of a single cylinder engine has plural projections (teeth) formed around its outer periphery at equal intervals and one irregular interval portion (toothless portion). A crank angle sensor detects start and end on both sides of each projection, a lateral length of each projection and an interval of two adjacent projections, and calculates a ratio therebetween to distinguish the irregular interval portion from the projections. The crank angle sensor and the irregular interval portion are so positioned that the irregular interval portion is detected when the piston is close to bottom dead center.

Description:
FIELD OF THE INVENTION 
   The invention relates to a crank angle detecting device for an engine, more specifically, a positioning arrangement for a crank angle sensor for detecting a reference position of a rotating crankshaft. 
   TECHNICAL BACKGROUND 
   In order to detect a crank angular position of the engine, a ring gear is fixed to the crankshaft, and has plural projections (teeth) formed at equal intervals as well as one toothless portion provided to have an irregular pitch to determine the reference position of the crankshaft. Determining the projections and recesses while the engine is rotating to identify the toothless portion, that is, a reference position by means of a ratio between the projections and the recesses allows detection of the crank angle. Fuel injection timing and ignition timing are controlled based on the detected crank angle. 
   In this case, when each tooth passes a crank angle sensor located facing an outer periphery of the ring gear due to the engine rotation, the crank angle sensor detects a lateral length of the tooth as well as an interval of the two adjacent teeth to calculate the ratio therebetween. This allows distinguishing of the teeth positioned at regular pitch from the toothless portion, thereby detecting the reference position. 
   However, since each tooth is detected when passing the crank angle sensor with the engine rotation, unstable engine speed may cause lower detection accuracy. A four-cycle engine makes two rotations to complete one cycle consisting of intake, compression, expansion and exhaust. In one cycle described above, the fluctuations in the engine speed repeat the increase in the expansion stroke, and then gradual decrease in the exhaust, intake and compression strokes. In particular, restarting the engine after it had a rest results in greater fluctuation in engine rotational speed. 
   A crank angular position is detected by detecting a projection (tooth&#39;s lateral length) and a recess (interval of the two adjacent teeth) of each tooth provided on the ring gear of the crankshaft, and by distinguishing the toothless portion based on the ratio therebetween. In this case, the ratio between the lateral lengths of tooth&#39;s projection and recess varies depending on the fluctuations in engine rotational speed, which may result in incorrect determination of the toothless portion, that is, a reference crank angle position. This requires adequate time for more reliable determination of the reference crank angle position. 
   An advantage of the present invention made in view of the prior art described above is to provide a crank angle detecting device for an engine independent of the fluctuations in engine rotational speed while performing more reliable detection of the reference crank angle position. 
   SUMMARY OF THE INVENTION 
   In order to achieve the above advantage, the present invention provides a crank angle detecting device for an engine, comprising a ring gear fixed to a crankshaft of a single cylinder engine, the ring gear having plural projections formed around its outer periphery at equal intervals as well as one irregular interval portion, and a crank angle sensor for detecting start and end on both sides of each projection formed on the ring gear, the crank angle sensor detecting a lateral length of each projection as well as an interval of the two adjacent projections, and calculating a ratio therebetween to distinguish the irregular interval portion from the projections, wherein the crank angle sensor and the irregular interval portion are so positioned that the irregular interval portion is detected when the piston in the engine is close to bottom dead center. 
   According to this arrangement, the irregular interval portion among the projections is detected when the piston is close to bottom dead center with the smaller fluctuations in engine rotational speed per one cycle. This provides improved reliability to detect the reference crank angle position. 
   In a preferable example of the arrangement, if a lateral length of the projection and an interval of the two adjacent projections are designated as (a) and (b), respectively, each projection is so provided that b/a (when a&lt;b) or a/b (when a&gt;b) is equal to 2 or greater. 
   According to the arrangement described above, the greater ratio between the projection and recess, the more enhanced accuracy in detecting each projection. In addition, the ratio between the projection and recess of a portion positioned at regular pitch differs significantly from that of the irregular interval portion. This allows improved accuracy in detecting the irregular interval portion even in the case of greater fluctuations in engine rotational speed. The ratio, b/a (when a&lt;b) or a/b (when a&gt;b), lower than 2 may cause lower accuracy in distinguishing between the projection and recess. However, an unnecessarily high ratio, b/a, or a/b may also cause lower projection detection accuracy. The values, (a) and (b), are therefore determined as appropriate depending on the ring gear size as well as the number of projections, and the sensor accuracy. 
   In order to achieve the above advantage, the present invention provides a crank angle detecting device for an engine, comprising a ring gear fixed to a crankshaft of a single cylinder engine, the ring gear having plural teeth as well as one toothless portion formed around its outer periphery at equal intervals, and a crank angle sensor for detecting the teeth formed on the ring gear, the crank angle sensor detecting a lateral length of each tooth as well as an interval of the two adjacent teeth, and calculating a ratio therebetween to distinguish the toothless portion from the teeth, wherein the crank angle sensor is so positioned that the toothless portion is detected when the piston in the engine is close to bottom dead center. 
   According to this arrangement, the toothless portion is detected when the piston is close to bottom dead center with the smaller fluctuations in engine rotational speed per one cycle. This provides improved reliability to detect the reference crank angle position. 
   A preferred example of the arrangement is characterized in that if tooth&#39;s lateral length and the interval of two adjacent teeth excluding the toothless portion are designated as (a) and (b) respectively, each tooth is so provided that b/a is equal to or greater than 2. 
   According to the arrangement described above, the greater ratio between the projection and recess of each tooth, the more enhanced accuracy in detecting each tooth. In addition, the ratio between the projection and recess of each tooth positioned at regular pitch differs significantly from that of the toothless portion. This allows improved accuracy in detecting the toothless portion even in the case of greater fluctuations in engine rotational speed. The ratio, b/a, lower than 2, may cause lower accuracy in distinguishing between the projection and recess of each tooth. However, an unnecessarily high ratio, b/a, may also cause lower teeth detection accuracy. The values, (a) and (b), are therefore determined as appropriate depending on the ring gear size as well as the number of teeth, and the sensor accuracy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an entire control system of a motorcycle according to the present invention. 
       FIG. 2  is a schematic diagram of a crank angle detection apparatus for an engine according to the present invention. 
       FIG. 3  is a graph showing fluctuations in engine rotational speed per cycle. 
       FIG. 4  is an explanatory view of teeth detecting method by the crank angle sensor of the present invention, 
       FIG. 5  is an explanatory view showing positioning at top dead center according to an embodiment of the present invention. 
       FIG. 6  is an explanatory view showing positioning at bottom dead center according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of the present invention will be described below with reference to the accompanying drawings. 
     FIG. 1  is a block diagram of an entire control system of a motorcycle according to the embodiment of the present invention. 
   An engine control unit (ECU)  1  is unitized to be an integral component. A control circuit CPU (not shown) of the ECU  1  receives inputs including an on/off signal from a main switch  2 , a crank pulse signal from a crank angle sensor  3 , an intake air pressure detection signal from an intake air pressure sensor  4 , an intake air temperature detection signal from an intake air temperature sensor  5 , a cooling water temperature detection signal from a water temperature sensor  6 , a voltage signal from an injector voltage sensor  7  for controlling an injector, and a checking input signal from a switch box  8  having a plurality of switches SW 1  to SW 3 . The ECU  1  is also connected to a battery  20 , from which battery power supply is inputted. 
   For outputs from the ECU  1 , the ECU  1  outputs a pump relay output signal to a pump relay  9  for driving a fuel pump, an injector output signal for driving an electromagnetic coil of an injector  10 , an ignition coil output signal for driving an ignition coil  11 , an automatic choke output signal for driving an automatic choke  12  in response to cooling water temperature, a diagnosis warning signal for driving a diagnosis warning lamp  13  in a meter  22  when abnormality is detected, a water temperature warning signal for driving a water temperature warning lamp  14  to indicate a warning when the cooling water temperature exceeds a given temperature, and an immobilizer warning signal for driving an immobilizer warning lamp  15  when an immobilizer  17  of an engine key or the like is abnormally operated. Power supply voltage is outputted for supplying power to each sensor either through a sensor power supply circuit  21  or directly. 
   The ECU  1  is also connected to an external general purpose communication device  18  and capable of inputting/outputting control data or the like through a general purpose communication line. The ECU  1  is further connected to a serial communication device  19  and capable of handling serial communication. 
     FIG. 2  is a system structure diagram of a crank angle detection device according to the embodiment of the present invention. 
   A single-cylinder four-stroke engine  30  is formed with a combustion chamber  32  on top of a piston  31 . An intake pipe  33  and an exhaust pipe  34  are connected to the combustion chamber  32  so as to communicate with the combustion chamber  32 . A throttle valve  35  is provided in the intake pipe  33 , and an intake valve  36  is disposed at an end thereof. An exhaust valve  37  is provided at an end of the exhaust pipe  34 . A reference numeral  38  denotes an ignition plug. A cooling jacket  39  is provided around a cylinder of the engine  30 , to which the water temperature sensor  6  is attached. The piston  31  is connected to a crankshaft  41  via a connecting rod  40 . 
   A ring gear  42  is integrally secured to the crankshaft  41 . The ring gear  42  has plural teeth (projections)  43  formed at equal intervals, among which one toothless portion (irregular interval portion)  44  is provided. The crank angle sensor (crank pulse sensor)  3  is provided for detecting the teeth  43  formed on the ring gear  42 . The crank angle sensor  3  detects each tooth  43  to generate a pulse signal having a pulse width that corresponds to a lateral length on the upper side of the tooth. In this example,  12  portions to be each provided with the tooth  43  include one toothless portion  44  so that the sensor generates  11  (eleven) pulse signals one per 300 of one crank rotation. 
   The injector  10  is attached to the intake pipe  33 . Fuel pumped from a fuel tank  45  through a filter  47  using a fuel pump  46  is delivered to the injector  10  under a constant fuel pressure maintained by a regulator  48 . The ignition coil  11  controlled by the ECU  1  ( FIG. 1 ) is connected to the ignition plug  38 . The intake air pressure sensor  4  and the intake air temperature sensor  5  are attached to the intake pipe  33 , which are separately connected to the ECU  1 . 
   A secondary air introducing pipe  49  for cleaning exhaust gas is connected to the exhaust pipe  34 . An air cut valve  50  is provided on the secondary air introducing pipe  49 . The air cut valve  50  opens at high engine speed with the throttle opened during normal driving or acceleration to introduce secondary air, while closing at low engine speed with the throttle closed during deceleration to cut off the secondary air. 
     FIG. 3  is a graph showing fluctuations in engine rotational speed per cycle. 
   The greatest fluctuation in engine rotational speed is found in the vicinity of compression top dead center in the course of shifting from a compression stroke to an expansion stroke. In the present invention, as shown by crank pulse signals in the drawing, the toothless portion (pulseless portion) is provided to the position not in the vicinity of top dead center, but the vicinity of bottom dead center with smaller fluctuations in engine rotational speed. As described above, the crank angle sensor and the toothless portion formed on the ring gear are so positioned that the toothless portion can be detected in the vicinity of bottom dead center. This reduces the possibility of incorrect detection caused by the fluctuations in engine rotational speed. 
     FIG. 4  is an explanatory view of teeth detecting method by the crank angle sensor, in which  FIG. 4(A)  shows a pulse signal for a tooth positioned at regular pitch, and  FIG. 4(B)  shows a pulse signal for one toothless portion. In the graph described above, the ring gear has the projection (tooth&#39;s lateral length) of 10° and the recess (intervals of the two adjacent teeth) of 20°. 
   As shown in  FIG. 4(A) , the ratio of the crank pulse between tooth&#39;s lateral length (a) and the interval (b) of the two adjacent teeth, b/a, becomes 2. Also as shown in  FIG. 4(B) , the toothless portion has the ratio, b/a, of 5 resulting from a greater interval (b) of the two adjacent teeth. Thus, presetting a threshold of the ratio, b/a, calculated in the ECU, at its mean value between 2 and 5 allows reliable identification of the toothless portion. 
     FIG. 5  is an explanatory view showing positioning of the ring gear and the crank angle sensor at top dead center. 
   The ring gear  42  has the 11 (eleven) teeth  43  as well as the toothless portion  44  formed at equal intervals of 30°. The tooth in the highest position when the piston is at top dead center is designated as a top dead center tooth  43   a . The top dead center tooth  43   a  is the third closest tooth from the crank angle sensor  3 . The toothless portion  44  corresponds to the fifth closest tooth therefrom. The ring gear  42  rotating in an arrow direction A provides crank pulse signals as shown in  FIG. 3  described above. In this case, as shown in  FIG. 3 , a fifth signal in the first rotation and a 17th signal in the second rotation are missing as they correspond to the toothless portion, respectively. 
     FIG. 6  is an explanatory view showing positioning of the ring gear and the crank angle sensor at bottom dead center. 
   As shown in the drawing, the top dead center tooth  43   a  passes a lowest position when the piston is at bottom dead center. At this point in time, the toothless portion  44  is positioned close to the crank angle sensor  3  (the example in the drawing shows a position of the toothless portion  44  that has just passed the crank angle sensor  3 ). As described above, in the present invention, the crank angle sensor  3  is so positioned that it can detect the toothless portion  44  when the piston is close to bottom dead center. 
   In the above embodiment, the descriptions have been made of a lateral length of the tooth (projection) shorter than the interval of the two adjacent teeth. However, the present invention may also be applicable to a lateral length of the tooth longer than the interval of the two adjacent teeth. In this case, if a lateral length of the projection and an interval of the two adjacent projections are designated as (a) and (b), respectively, each projection is so provided that a/b (when a&gt;b) is equal to 2 or greater. 
   INDUSTRIAL USABILITY 
   As described above, in the present invention, the toothless portion is detected when the piston is close to bottom dead center with the smaller fluctuations in engine rotational speed per one cycle. This provides improved reliability to detect the reference crank angle position. In such a case, if tooth&#39;s lateral length and the interval of the two adjacent teeth excluding the toothless portion are designated as (a) and (b) respectively, each tooth is so provided that b/a is equal to or greater than 2. According to the arrangement described above, the greater ratio between the projection and recess of each tooth, the more enhanced accuracy in detecting each tooth. In addition, the ratio between the projection and recess of each tooth positioned at regular pitch differs significantly from that of the toothless portion. This allows improved accuracy in detecting the toothless portion even in the case of greater fluctuations in engine rotational speed.