Abstract:
The period required to start a rope-start, two-cycle engine is reduced by enabling the engine&#39;s firing sequence to be initiated immediately upon determining the absolute rotational position of the engine and before determining the engine&#39;s direction of rotation. The rotational direction of the engine is then determined, and the firing sequence is disabled if the engine is counter-rotating. In this manner, the firing sequence is enabled much sooner in the engine&#39;s cycle than if the engine&#39;s rotational direction were determined before the firing sequence is enabled. The engine therefore starts more quickly. The method is particularly useful in battery-less engines which experience a delay in start-up due to the fact that the engine must rotate through at least part of a revolution before generating enough electrical power to operate the computer controlling operation of the engine. It is also particularly useful in short-pull engines in which manual actuation of a rope or other manually-powered starting mechanism drives the engine to rotate through no more than three-to-five revolutions.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to two-stroke engines and, more particularly, relates to a method and apparatus for starting a rope-start, two-stroke engine. 
   Rope-start, two-stroke engines are used in a variety of applications including outboard marine engines, snowmobiles, personal watercraft, snow blowers, and weed trimmers. These engines are started by manually actuating a starter mechanism that drives the engine to rotate. Engine rotation initiates a firing sequence by enabling the supply of electrical power to the engine&#39;s fuel injection and/or ignition systems at the next appropriate engine rotational position(s). The most common manually actuated starter mechanism includes a rope that is wound around a spool coupled to the engine&#39;s flywheel either directly or via one or more gears. The rope unwinds from the spool when it is pulled by the operator, thereby driving the spool and the flywheel to rotate. 
   Consumers demand that rope-start engines start with as little manual input as necessary. Many original equipment manufacturers demand that the engine must start on the first pull. However, starting an engine with one pull of the rope-start mechanism is hindered by several factors. 
   For instance, the rope-start mechanism imparts only a relatively small number of revolutions to the engine, limiting the number of available revolutions to initiate and successfully implement the engine&#39;s firing sequence. In a so-called “short-pull” engine, manual actuation of the rope-start mechanism drives the engine to rotate through no more than three-to-five revolutions. This small number of revolutions creates only a relatively small window of opportunity to initiate and successfully implement an engine firing sequence. 
   In addition, the engine must undergo at least part of a revolution before a firing sequence can be enabled. This limitation on engine starting stems from the fact that the absolute position of the engine must be determined before its firing sequence can be enabled. The engine&#39;s computer typically determines the engine&#39;s absolute rotational position by detecting spaced markers on a rotating component of the engine. These markers may include a plurality of equally-spaced “indicator” markers and a few additional, unequally-spaced “indexing” markers. The locations of and spacings between the markers are stored in a map or table of the computer&#39;s memory. The computer can determine the angle of rotation from a given point by counting the number of indicator markers from that point. The indexing markers form starting points and ending points for determining the engine&#39;s absolute position and direction of rotation upon engine start-up. At least two indexing markers must be detected to determine absolute engine rotational position. Specifically, upon detecting the first indexing marker, the computer resets its internal counter and begins to count the number of indicator markers between the first indexing marker and the second indexing marker. Then, upon detecting the second indexing marker, the computer can determine the angular spacing between the first and second indexing markers. The computer then compares the determined spacing to the table or map of known spacings. Based on this comparison, the computer can identify the indexing marker that is detected second and accordingly, the rotational position of the engine. 
   Quick engine starting is further hindered in a battery-less engine that relies on electricity generated by rotation of the engine to supply electrical power to the computer and other engine components, such as the engine&#39;s fuel injection system and/or ignition system. The typical engine must undergo at least part of a revolution, and sometimes a complete revolution or more, before generating enough power to operate the computer. This “power-up” requirement delays the computer&#39;s detection of the absolute engine rotational position and, therefore, further delays enablement of the firing sequence. All of these factors conspire to render it difficult to initiate a firing sequence in less than about one full engine revolution. 
   Another complicating factor that hinders quick-start and that is unique to two-stroke engines is the need to prevent engine counter-rotation. Counter-rotation occurs when the engine runs in reverse so that the crankshaft rotates in a direction opposite the intended direction. Because counter-rotation risks damage to the engine and possibly components powered by it, counter-rotation must be detected to prevent firing of the counter-rotating engine. In a system in which the engine&#39;s position is determined by detecting and identifying two indexing markers on a rotating component of the engine, counter-rotation is detected by detecting and identifying a third indexing marker disposed at an angle β from the second indexing marker that is different from the angle α separating the first and second indexing markers. The engine&#39;s rotational direction can then be determined by determining the sequence in which the second and third indexing markers are detected. 
   Unfortunately, the need to detect and identify a third indexing marker additionally delays enablement of an engine&#39;s firing sequence and further hinders quick-start. In a short-pull engine, this additional delay in firing sequence enablement may mean the difference between a successful first pull start and an unsuccessful first pull start. 
   The need therefore has arisen to provide a method for quick starting a rope-start, two-cycle engine that does not require the direction of rotation of the engine to be sensed before enabling a firing sequence. 
   SUMMARY OF THE INVENTION 
   Pursuant to the invention, the period required to start a rope-start, two-cycle engine is reduced by enabling the engine&#39;s firing sequence immediately upon determining the absolute rotational position of the engine and before determining the engine&#39;s direction of rotation. The rotational direction of the engine is then determined, and the firing sequence is disabled if the engine is counter-rotating. In this manner, the firing sequence is enabled much sooner in the engine&#39;s operational cycle than if the engine&#39;s rotational direction were determined before the firing sequence were enabled. The engine therefore starts more quickly. Absolute engine rotational position and engine rotational direction may be sensed by detecting and identifying indexing markers on a rotational component of the engine and determining the sequence in which the indexing markers are detected. The indexing markers may, for instance, comprise magnetic markers (i.e., teeth or other markers made of a magnetically conductive material such as steel) that are located on the engine&#39;s flywheel or crankshaft and that are capable of being detected by a magnetic pick-up device, in which case the detector preferably comprises a magnetic pick-up device located adjacent the rotating component. 
   The method is particularly useful in battery-less engines, which experience a delay in start-up due to the fact that the engine must rotate through at least part of a revolution before generating enough electrical power to operate the computer controlling engine operation. It is also particularly useful in short-pull engines in which manual actuation of a rope or other manually-powered starting mechanism drives the engine to rotate through no more than three-to-five revolutions. 
   In accordance with another aspect of the invention, a two-stroke engine is provided with improved quick start capability. The engine includes a manually-powered starter, a monitor, an electrically powered device which controls at least one aspect of an engine&#39;s firing operation, and a computer. The starter typically comprises a pull-rope coupled to the engine&#39;s flywheel. The monitor comprises a pick-up device or other detector that detects magnetic teeth or other markers on a rotational component of the engine such as a flywheel or a crankshaft. The powered device may comprise the engine&#39;s fuel injection system and/or its ignition system or components of those systems. The computer is operable, in conjunction with the monitor, to determine an absolute rotational position of the monitored component (and hence the engine as a whole) and to enable the supply of energizing current to the powered device. Then, after enabling the supply of energizing current to the powered component, the computer determines the rotational direction of the monitored component and disables the supply of energizing current to the powered device if it determines that the monitored component is counter-rotating. 
   Preferably, the monitored component bears first and second angularly-spaced indexing markers, and the monitor includes a detector that is configured to detect passage of the first and second indexing markers. The computer is configured to determine an angular spacing between the first and second indexing markers and to identify the second detected indexing marker and, hence, determine the absolute rotational position of the engine based upon this determination. In order to permit the rotational direction of the engine to be determined, the monitored component preferably bears a third indexing marker that is angularly-spaced from both the first indexing marker and the second indexing marker. The computer is configured to identify the third detected indexing marker and determine the sequence of passage of the second and third detected indexing markers based upon this determination. 
   The engine may, for instance, comprise a battery-less engine which generates electricity to run itself from engine rotation. In this type of engine, the power-up requirement for the computer and other electrically powered components of the engine shortens the window of opportunity to start the engine after the computer powers up. Enabling the firing sequence immediately upon detecting engine absolute position therefore becomes more important in a battery-less engine than in a battery-powered engine. 
   These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
       FIG. 1  is a partially schematic elevation view of a snowmobile incorporating an engine constructed in accordance with a preferred embodiment of the present invention; 
       FIG. 2  is a schematic diagram of a control system for the engine of the snowmobile of  FIG. 1 ; 
       FIG. 3  is a schematic representation of a signal generating apparatus of the engine; 
       FIG. 4  is a diagram illustrating the relationship between a marker pattern and a detector of the signal generating apparatus of  FIG. 3 ; and 
       FIG. 5  is a flowchart of a firing sequence enablement/disablement control scheme performable with the engine of  FIGS. 2-4 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The invention is applicable to virtually any so-called “rope-start,” two-stroke engine. “Rope-start,” as used herein, means any engine in which the power required to start the engine is supplied manually, such as by pulling a rope coupled to a spool on the engine&#39;s flywheel. Rope-start, two-stroke engines to which the invention is applicable are usable in a wide variety of applications. These applications include outboard marine engines, snowmobile engines, snow blower engines, personal watercraft engines, and weed trimmer engines. 
   Referring to  FIG. 1 , a snowmobile  30  is illustrated that employs an engine  32  constructed in accordance with the present invention. As is conventional, the snowmobile  30  includes a seat  34 , a pair of skis  36 , a track  38 , a cowling  40 , and a steering handlebar  42 . The engine  32  is mounted under the cowling  40  and supplies motive power to the track  38  via a drive belt and pulley arrangement  44 . The engine  32  comprises a rope-start, two-stroke engine, and preferably is electronically fuel injected and electronically spark ignited. It is also battery-less. The electrical power required to operate it therefore is generated by engine rotation via an alternator (not shown). A suitable rope-start, battery-less engine is disclosed in U.S. Pat. No. 5,816,221 to Krueger, the disclosure of which is hereby incorporated by reference in its entirety. 
   Still referring to  FIG. 1 , the engine  32  is started by way of a rope-start mechanism  46  operated by an operator straddling the snowmobile. The rope-start mechanism  46  includes a spool  48  and a rope  50  wound around the spool  48 . The spool  48  is mounted on, or otherwise coupled to, the engine&#39;s flywheel  58 . The operator starts the engine  32  by pulling the rope  50  to unwind it from the spool  48 , thereby driving the spool  48  and the flywheel  58  to rotate to initiate a firing sequence. 
   Referring now to  FIG. 2 , the engine  32  is controlled by a control system that includes a computer or ECU  52  and a crank position monitor  54 . The computer  52  receives signals from the crank position monitor  54  and possibly other sensors  60  and transmits control signals to an electronic fuel injection system  62  and an electronic ignition system  64 . The crank position monitor  54  includes (1) a plurality of markers  1 - 24  and A-C ( FIGS. 3 and 4 ) that are mounted circumferentially around a rotating component of the engine in an angularly-spaced apart relationship and (2) a detector  56  that detects movement of the markers past the detector  56 . The rotating component bearing the markers  1 - 24  and A-C may comprise the crankshaft (not shown), the flywheel  58 , or any other rotating engine component whose position is reflected directly or indirectly by the rotational position of the engine  32 . In the illustrated embodiment, the rotational component is the engine&#39;s flywheel  58 . 
   Referring to  FIGS. 2 and 3 , the markers  1 - 24  and A-C may comprise any devices detectable by an associated detector as the markers rotate past the detector. The illustrated markers comprise magnetic teeth mounted on a rotor  66 . The rotor  66  is, in turn, mounted on the flywheel  58  so as to rotate therewith. The detector  56  may comprise any device capable of detecting the markers  1 - 24  and A-C. In the illustrated embodiment in which the markers comprise magnetic teeth, the detector  56  comprises a magnetic pick-up device such as a ferromagnetic transducer or a Hall effect sensor. With this type of monitor, rotation of the teeth  1 - 24  and A-C past the detector  56  generates magnetic pulses that are detected by the detector  56  to provide an indication of the markers&#39; passage. 
   Still referring to  FIG. 2 , the computer  52  may comprise any programmable device capable of determining the engine&#39;s rotational position and direction of rotation based on signals from the detector  56  and of controlling the engine&#39;s fuel injection and/or ignition systems  62  and/or  64  accordingly. In the illustrated embodiment, the computer  52  comprises a programmable ECU that includes a microcontroller  70 , a signal conditioning circuit  72 , and an input/output device  74 . The signal conditioning unit  72 , which may comprise an analog-to-digital converter, is connected to the detector  56  by a transmission line  76  that converts the analog signals from the crank position monitor  54  to digital signals suitable for use by the computer  52 . The input/output device  74  is coupled to the fuel injection and ignition systems  62  and  64  by respective transmissions lines  78  and  80 . If additional sensors  60  are used to assist in the control of fuel injection and ignition, then the computer  52  may additionally comprise an analog-to-digital converter  81  that is coupled to the additional sensors  60  via a transmission line  82 . The microcontroller  70  includes a pair of memory devices: a RAM  84  and a ROM  86 , a CPU  88 , a timer  92 , and a counter  94 . The CPU  88  is coupled to the A/D converter  81  by a transmission line  90 . The timer  92  and counter  94  are connected to the signal conditioning circuit  72  by a transmission line  96  so as to count pulses generated by the detector  56  and the time between those pulses. 
   The data obtained from the monitor  54  can be compared with information stored in the ROM  86  regarding the spacings between and locations of the markers  1 - 24  and A-C to obtain information regarding the engine&#39;s current operation state, including its absolute rotational position, its speed, and its direction of rotation. Specifically, referring to  FIGS. 3 and 4 , the markers comprise a first plurality ( 24  in the illustrated embodiment) indicator markers  1 - 24  and three additional indexing markers A-C disposed in an angularly spaced-apart relationship on the rotor  66 . The indicator markers  1 - 24  are spaced at equal intervals of 15°. The indexing markers A-C are spaced non-uniformly around the rotor  66  so that the indexing marker B is spaced at an angle α from the indexing marker A and the indexing marker C is spaced at an angle β from the indexing marker B and an angle γ from the indexing marker A. In the illustrated embodiment, α equals 150°, β equals 90°, and γ equals 120°. Other angles may be used so long as α, β, and γ are all non-equal. The timer  92  and counter  94  of the microcontroller  70  are able to count the number of markers detected by the detector  56  and to measure the interval of time between each successive marker&#39;s passage. Because this time interval is constant for adjacent indicator markers but decreases by about half for the additional indexing markers, the computer  52  is able to detect the passage of an indexing marker by noting a decreased interval between pulses when compared to intervals between the indicator markers. The computer  52  can also obtain an indication of the angle between successively detected indexing markers A-C simply by counting the number of pulses between indexing markers A-C. Hence, in the illustrated embodiment, the computer  52  can obtain an indirect measurement of the angle (α) between the indexing markers A and B by counting the number of pulses ( 10 ) between those indexing markers. The counted number is then compared to known numbers stored in the ROM  86  to identify the second detected indexing marker B. 
   The computer  52  can determine the rotational direction of the engine  32  simply by determining the sequence in which two consecutive indexing markers are detected. For instance, if the counted pulses reflective of the angles β and γ are detected in sequence, the computer  52  can determine that the detector  56  has detected the indexing markers B, C, and A sequentially and that the engine  32  is rotating forwardly. Conversely, if the counted pulses reflective of the angles β and α are detected in sequence, the computer  52  can determine that the detector  56  has detected the indexing markers C, B, and A sequentially and that the engine  32  is counter-rotating. 
   The inventive method could be implemented without detecting three indexing markers. For instance, if the indexing markers are unique in some way and the detector is capable of simultaneously detecting a particular indexing marker and identifying it as that marker, then two indexing markers could be employed. In this case, the absolute rotational position of engine could be determined immediately upon detecting and identifying the first indexing marker, and the rotational direction of the engine  32  could be determined upon detecting and identifying the second marker. 
   Referring to  FIG. 5 , the computer  52  implements the monitoring logic described above as part of a quick start control routine  100  that enables the engine firing sequence to be initiated before the engine&#39;s rotational direction is known. Upon manual operation of the rope-start mechanism  46  and generation of sufficient electrical power to operate the computer  52 , the routine  100  proceeds from START at  102  and then detects the first indexing marker (e.g., marker B in  FIGS. 3 and 4 ) at  104 . The routine  100  then resets the counter  94  and counts the indicator markers at  106  until the second indexing marker (e.g., marker A in  FIGS. 3 and 4 ) is detected at  108 . The routine  100  then compares the counted number to the numbers corresponding to the three known angles (a, A, o). Based on this comparison, the routine  100  identifies the second detected marker as a specific indexing marker (marker A in this example) and determines the absolute rotational position of the engine  32  from the known position of that indexing marker at  110 . In accordance with the invention, the firing sequence of the engine  32  is enabled at this time, despite the fact that the rotational direction of the engine  32  is not yet known. The electronic fuel injection and ignition systems  62  and  64  will then be energized by the computer&#39;s input/output device  74  at the next appropriate times during the engine&#39;s cycle, e.g., at the next 30° BTDC and at TDC, respectively. Because the firing sequence is enabled relatively early in the rotational cycle of the engine  32 , the chances of a successful first-pull start are maximized. 
   As the engine  32  continues to rotate, the routine  100  counts indicator markers at  112  until it receives an indication of the rotation of the third and final indexing marker (marker C in this example) past the detector  56  at  114 . Routine  110  then identifies the third detected indexing marker at  116  by comparing the counted number of pulses to the table of known numbers stored in the ROM  86 . Then, by determining the sequence that the second and third identified markers A and C pass the detector  56 , the routine  100  determines at  118  whether the engine  32  is counter-rotating. In the present example, by determining that the markers A and C rotate past the detector  56  in sequence, the routine  100  determines that the engine  32  is counter-rotating. It therefore disables the firing sequence at  120  and then proceeds to END at  124 . Although one or, at most, a few incidents of fuel injection and/or ignition may occur before the firing sequence is disabled, these few incidents do not have any significant detrimental effect on engine operation or on the environment. If, on the other hand, the routine  100  determines at  118  that the engine  32  is not counter-rotating, then the firing sequence is continued at  122  and the routine  100  proceeds to END at  124 . 
   The action taken by the computer  52  after implementation of the END step  124  varies depending on the operational state of the engine  32  at that time. If END occurs following disabling of the firing sequence at  120  due to engine counter-rotation, then the computer  52  simply shuts down until the next attempt to start the engine  32 . If, on the other hand, END occurs without disabling the firing sequence, then a separate routine is implemented in which the computer  52  and monitor  54  continue to monitor engine rotation and to control the injection and ignition systems  62  and  64  as the engine  32  runs. If the engine  32  counter-rotates at any time (due, for example, to backfiring), then the computer  52  will disable subsequent firing sequences and shut down the engine  32 . 
   Many changes and modifications may be made to the invention without departing from the spirit thereof. Some of these changes are discussed above. The scope of other changes will become apparent from the appended claims.