ALTERNATOR SIGNAL USED AS CRANKSHAFT POSITION SENSOR IN SMALL MOTORCYCLE ENGINES

TECHNICAL FIELD

The present invention relates to internal combustion engines; more particularly, to small engines such as are known to be used for powering relatively small motorcycles; and most particularly, to method and apparatus for sensing crankshaft rotary position in such engines by employing voltage output signals from the engine's alternator (also referred to herein as a magneto).

BACKGROUND OF THE INVENTION

Small motorcycles and mopeds are powered typically by one- cylinder, four-stroke internal combustion engines. Such vehicles and engines, typically in an engine displacement range of 125-150cc, are low in both weight and cost; thus, there is a continuing need in the industry to provide less expensive, lighter, and higher performing engines.

Various engine functions require synchronization for an engine to run properly and to achieve optimum performance. Some of these functions, such as piston motion and open/close timing of combustion valves, typically are synchronized via purely mechanical linkages and gearing between an engine crankshaft and camshaft. However, other functions such as timing of fuel injection and ignition may depend for optimum synchronization on a computerized Engine Control Module (ECM) that monitors the rotary position

of the engine crankshaft and can accurately initiate and terminate those functions during the four-stroke rotary cycle of the engine.

Small engines typically generate their own electric power through use of a magneto comprising a rotor mounted on an end of the crankshaft and having a plurality of permanent magnets and a stator having a plurality of wound poles fixed within the rotor. As the engine rotates, an alternating current is generated in the stator poles having a voltage wave form that is a function of the number of stator poles and phases in the windings. A battery may be provided to permit, for example, electric starting of the engine, although kick starting via a recoil mechanism is also well known.

In the prior art, engine synchronization is achieved typically through use of a rotary encoder (also referred to herein as a crank wheel). A crank wheel, which may be a modified magneto rotor, is mounted on the crankshaft and is provided with typically between one and 24 angularly-spaced magnetic or optical elements (herein referred to collectively as "teeth"). A crank position sensor adjacent the crank wheel sends a stream of signals to the ECM in response to passage of the teeth past the sensor. A mechanical gap in the teeth provides an indexing signal for each engine revolution.

Because a four-stroke engine requires fuel injection and spark firing only every other revolution, the ECM must keep track of the signal pulses to establish the phase status of the camshaft over the full 720° rotational cycle of the engine. Reliable establishment and maintenance of phase status remains an important requirement in the art; obviously, an engine cannot run if the camshaft

(and hence the fueling and firing) is 360° out of phase to the piston and valve timing.

It will be seen that a crank position sensor and associated hardware and wiring represents significant weight, complexity, and expense in the manufacture of an engine.

It is further known that engine rotational speed inherently varies during the rotational cycle. RPM decreases slightly due to negative engine torque as the piston does work during the compression stroke, and then RPM and engine torque increase under combustive pressure during the power stroke. Some such variation occurs even during an unfired rotational cycle, as gas is compressed during the compression stroke and decompressed during the next "power" stroke. US Patent No. 5,562,082 discloses to use such intra-cycle speed variation to determine camshaft phase and thereby establish synchronization of an engine. A drawback of the disclosed system, however, is that a modified crank wheel and sensors are required.

What is needed in the art is an improved means for determining engine rotational position and camshaft phase at all times without requiring additional hardware and/or manufacturing expense.

It is a principal object of the present invention to reduce the weight, complexity, and cost of a small internal combustion engine.

SUMMARY OF THE INVENTION

Briefly described, a method in accordance with the invention for determining engine rotational position and camshaft phase at all times uses the prior art output signal of an engine magneto. There is no need for a conventional independent crank sensor which therefore is deleted from an engine in accordance with the invention, thus reducing the weight, complexity, and cost of an engine. Instantaneous engine speed variation during the compression stroke and the immediately succeeding power (expansion/combustion) stroke causes an uneven period of the magneto output voltage signal, indicated by the period of successive voltage reversals, which period variation is detected by an Engine Control Module and supplied to programmed algorithms to infer the crankshaft rotational position and speed and the camshaft phase at all times. These determinations may be used, among other purposes, to time fuel injection and ignition coil charging and firing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 is an exploded isometric view of a prior art magneto rotor and eight-pole stator, including an additional magnetic crank position sensor;

FIG. 2 is a graph showing in-cylinder pressure during an unfired

720° engine rotational cycle;

FIG. 3 is a graph similar to that shown in FIG. 2, showing cylinder gas torque through a 720° cycle;

FIG. 4 is a graph showing relative engine torque and speed through an unfired 720° cycle;

FIG. 5 is a graph showing an exemplary voltage output signal of a single-phase 8-pole alternator;

FIG. 6 is a graph like that shown in FIG. 5, for a two-phase 12-pole alternator;

FIG. 7 is a graph like that shown in FIG. 5, for a three-phase 18- pole alternator;

FIG. 8 is a graph showing time periods between zero-voltage crossings as a function of rotary position (samples) for an engine alterator with the engine running, also showing acquisition of synchronization;

FIG. 9 is a graph showing output voltage as a function of time after engine starting for an electric-started engine, also showing the acquisition of synchronization and crank sensor signal relative to FIG. 1;

FIG. 10 is a graph like that shown in FIG. 8 for the starting condition shown in FIG. 9;

FIG. 11 is a graph showing output voltage as a function of time after engine starting for a kick-started engine, also showing the acquisition of synchronization and crank sensor signal relative to FIG. 1 ; and

FIG. 12 is a graph like that shown in FIG. 10 for the starting condition shown in FIG. 11.

Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate preferred embodiments of the invention, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a prior art magneto 10 for generating an alternating electric current comprises a rotor 12 and a stator 14. Rotor 12 includes a central bore 16 including a keyway 18 for rotationally fixing the rotor to a shaft such as a crankshaft (not shown) of an engine 20, for example, a small-displacement one-cylinder engine. Rotor 12 is configured for rotation in a clockwise direction 22 and includes a plurality of permanent magnets (not visible in FIG. 1) embedded in the inner wall 23 of the rotor. Stator 14 includes eight wound radial poles 28 disposed symmetrically about magneto axis 30, and a wiring harness 32 for connection to an Engine Control Module (ECM) 34. Upon assembly, stator 14 is non-rotationally mounted within rotor 12 such that during rotation of rotor 12 the embedded permanent magnets pass by each of poles 28 sequentially, creating an alternating voltage and current in harness 32. A magnetic crank

sensor 36 attached to stator 14 is positioned outside of rotor 12 and, in response to proximity passing of trigger bar 24 followed by signal bar 26, delivers a pair of spaced apart electromagnetic signals 38 (see FIGS. 9 and 11) to ECM 34 indicative of the rotational position of the engine crankshaft once during each revolution of engine 20. Because the angular positions of keyway 18 and bars 24,26 are known with respect to top-dead-center (TDC) position of the crankshaft, signals 38 are interpreted by ECM 34 to infer the angular position of TDC at any given time in the rotation cycle of engine 20 between successive sets of signal pairs 38.

A exemplary magneto in accordance with the present invention should be considered as being substantially identical with magneto 10 except that bars 24,26, crank sensor 36, and concomitant signals 38 are omitted. Note that while magneto 10 is shown exemplarily as having a stator with 8 poles wound single- phase (FIG. 5), other magneto configurations are fully comprehended by the invention, for example, 12 poles wound two-phase (FIG. 6) or 18 poles wound three-phase (FIG. 7).

Referring to FIGS. 2 and 3, during a full two-revolution cycle of a four- stroke engine during cranking without firing ("unfired"), the inherent cylinder compression and consequent gas torque varies during the cycle. In these examples, the cylinder pressure is maximum 40 at 0° crank angle (TDC); decreases by a factor of 9.4 (engine compression ratio) during the 180° "power" stroke 42; is constant during the exhaust stroke 44 (exhaust valve opens and closes); is similarly constant during the intake stroke 46 (intake valve opens and closes); and then increases again to maximum pressure 40 during the

compression stroke 48.

Referring to FIG. 4, unfired engine speed 50 and torque 52 are shown in relative terms, normalized, for example, to their values during the intake stroke as shown in FIG. 2. It is seen that unfired torque and speed both decrease during the compression stroke 48. Torque returns to zero at about the minimum engine speed, and then both increase rapidly during the decompression ("power") stroke 42.

Referring to FIGS. 4 and 5, zero-voltage crossings 53 of the engine generator sinusoidal voltage output are readily detected by an ECM and are also plotted as pulses 54. It is seen that because of the changes in engine speed 50 the period 56 between pulses during the passage of the engine through TDC is slightly greater than the other inter-pulse periods 58. Thus, period 56 may be readily detected by the ECM and employed as a phase discriminator in accordance with the invention. Further, from the length of period 56 in relation to engine speed 50, the ECM is able to calculate the exact position of TDC.

Thus, the method of the invention, by utilizing the prior art voltage signal along with algorithms in the ECM, allows the ECM to achieve both precise engine synchronization and phasing without requiring any separate crank position sensor apparatus. Indeed, because the ECM can count zero-voltage crossings 53, the greater the number of poles in the magneto stator, the more closely the ECM can monitor speed variations within each engine revolution. For example, in the 4-pole magneto shown in FIG. 5, crossings 53 occur every

45° (360/8), whereas in the 18 pole magneto shown in FIG. 7, crossings occur

every 10° (360/36). Thus, when using the magneto illustrated in FIG. 7, an ECM can interrogate for engine speed variation every 10° of rotation. This can be very useful in performing precise timing of fuel injection and spark coil charging.

Referring to FIG. 8, in an actual running engine, the varying values of inter-pulse periods show a regular pattern 60 with engine rotation, wherein a synchronizing phase detection signal 62 is determined by the ECM from these engine speed variations.

Referring to FIG. 9, it is seen that in the prior art, when using electric engine starting, achieving synchronization 62 requires a time period equivalent to several zero-voltage crossings (curve 64).

However, referring to FIG. 10, another advantage of the present method is that in almost all instances of engine starting, camshaft synchronization is obtained within the first engine revolution. This is because, when ignition is terminated, an engine coasts to a stop at a rotational resistance point which, as is suggested in FIGS. 2 and 3, typically is compression stroke 48. Thus, an engine typically is stopped by failure to complete an unfired compression stroke, and may be reversed slightly by accumulated partial compression before coming to a dead stop. This means that when an engine is again cranked and the ECM and algorithms are invoked, the first revolution includes the speed variations of the compression stroke and the power stroke, indicating to the ECM that the engine is not on the exhaust/intake revolution.

Referring to FIGS. 11 and 12, it is seen that for a kick-started engine, the

advantages just described for electric starting still pertain.

While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.