Abstract:
A method and apparatus for determining the balanced condition of an engine having a balancer is provided. An engine balancer tester includes a sensor coupled to the engine. The engine generates first- and second-order vibrations that have vibration levels and the sensor generates a signal corresponding to the vibrations. A meter coupled to the sensor receives the signal and calculates a ratio between the vibration level of the first-order vibrations and the vibration level of the second-order vibrations. A comparator compares the ratio to a predetermined value to assess the balanced condition of the engine.

Description:
FIELD OF THE INVENTION 
     The invention relates to the inspection of motor vehicle engines, and specifically to measuring the vibration of an engine having balancers. 
     BACKGROUND OF THE INVENTION 
     Internal combustion engines typically have at least one piston and connecting rod driving a crankshaft. As the engine operates, reciprocal motion of the piston is translated to rotational movement of the crankshaft. As a result of the reciprocating motion of the piston and connecting rod, engine operation typically generates vibrations of the engine and engine components. The vibration of greatest concern is normally the first-order vibration at a frequency determined by dividing the engine rpm by sixty. Sufficient vibration over a period of time, particularly at certain frequencies, can be uncomfortable for some riders. 
     Some engine manufacturers have attempted to dampen engine vibration by adding balancers within an engine. If correctly designed and installed, the balancers can counteract a significant portion of the engine vibration, resulting in more comfort to some riders. 
     SUMMARY OF THE INVENTION 
     If the balancers are slightly misaligned, the balancers can still dampen the engine vibrations, but the balancers will not function to the designed effectiveness. Because the balancers are located within the engine housing, it is difficult to determine whether the balancers are properly aligned. A diagnostic tool other than visual confirmation is desirable. 
     Prior art methods have sensed and compared first-order vibrations to determine balancer condition, but these methods cannot be used for engines installed in different motorcycle designs, experiencing different motorcycle conditions (e.g., motorcycle tire pressure), or installed on test stands. In other words, the results of these methods vary depending on the engine supporting structure. 
     The invention provides a method and apparatus for determining the balancer condition of a balanced engine, and is based on the recognition that the ratio of first-order vibration to second-order vibration is relatively independent of the engine supporting structure (e.g., whether the engine is mounted on a test stand or a motorcycle). In the present invention, the ratio of first-order to second-order vibrations of an operating test engine is calculated and compared to the ratio for an engine in which the balancers are properly aligned to determine whether the balancers in the test engine are properly aligned. 
     The invention described herein solves the problem of determining whether the balancers in an engine are properly aligned by providing a simple diagnostic tool that allows that determination to be made easily and quickly. The method can be used for engines installed in different motorcycle designs, experiencing different motorcycle conditions (e.g., motorcycle tire pressure), or installed on test stands. As a result, an engine can be inspected for proper balancing before the engine is sold to and operated by a consumer. 
     Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims, and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a motorcycle engine embodying the invention, shown mounted in a motorcycle. 
     FIG. 2 is a perspective view of the motorcycle engine illustrated in FIG. 1 shown mounted on a test stand. 
     FIG. 3 is a graphical view of the narrow-band spectrum of the vibrational response of the engine illustrated in FIG. 1 when operated at 2000 rpm. 
    
    
     Before one embodiment of the invention is explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or being carried out in various ways. Also, it is understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including” and “comprising” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An apparatus embodying the invention is illustrated in FIG.  1 . The method and apparatus described herein may be used with any type of motor vehicle engine. An application of the invention in a motorcycle is illustrated for exemplary purposes. 
     FIG. 1 illustrates a motorcycle  10  including an engine  20 . The engine  20  is mounted within the motorcycle  10  by standard methods and is designed to provide power to the motorcycle  10 . The engine  20  includes two cylinders  25  connected to a crankcase  30 . The engine  20  includes a balancer (not shown) designed to reduce engine vibration. The balancer includes a chain coupled to the crankshaft by a chain and a gear. If the balancer chain is misaligned by one or more teeth, the balancer can create unwanted vibrations in the engine  20 . The invention described herein is designed to detect this type of balancer misalignment. 
     A sensor  40  is removably attached to the engine  20  in the vicinity of the crankcase  30 . The sensor  40  is a motion sensor or a vibration sensor, and can specifically be an accelerometer or any suitable device that senses movement (e.g., a position sensor). It has been found that the type of accelerometer used for balancer testing is not critical, as long as the accelerometer frequency range is up to 100 Hz or above. The sensor  40  is preferably a B&amp;K 4384 accelerometer, but may be replaced by any suitable accelerometer. The sensor measures up-and-down motion (i.e., the z-direction on FIG.  1 ). 
     Preferably, the accelerometer is placed as close to the engine rotation center as possible to avoid detecting torsional vibration. Normally, the rotation center is the same as the geometric center. In the testing performed in association with this invention, the accelerometer was placed on the top of the crankcase on the primary cover side. This location is easily accessed and is a less harmful environment for the hand. The accuracy of the sensor  40  is relatively insensitive to the method of attachment to the engine  20 . The sensor  40  is preferably hand-held or attached using putty or glue, but may be attached by any suitable method including a mechanical fastener. The sensor  40  includes a wire  45  electrically connected to the sensor  40  at one end of the wire  45 . 
     A meter  50  is electrically connected to the other end of the wire  45 . The illustrated meter  50  is a meter of standard design and is programmed to interpret the signals from the sensor  40  and to make the calculations described below. The meter  50  is preferably a RION SA-77 FFT analyzer, but may be a Harley-Davidson engine balancer service tool or any other suitable device in alternate embodiments. 
     In operation, the engine  20  is operated in an neutral gear at a pre-selected engine speed, typically 2000±200 RPM, creating certain vibrations inherent in the operation of the engine  20 . These vibrations have frequencies and levels dependent on the structure of the engine  20  and its surroundings. First-order and second-order vibrations of the engine  20  are largely attributable both to the engine  20  itself and to the system in which the engine  20  resides, in this case a motorcycle  10  (FIG.  1 ). 
     Vibrations are detected by the sensor  40 , which generates a signal and transmits the signal over the wire  45  to the meter  50 . The meter  50  detects the signal and processes the signal to determine the level and frequency of the first- and second-order vibrations. A typical frequency response is illustrated in FIG.  3 . 
     The engine  20  may also be tested when mounted on a test stand  55 , as illustrated in FIG. 2, rather than when mounted on a motorcycle  10 . The sensor  40  is permanently mounted underneath the test bed  56 , which is supported by four rubber isolators  57  (see FIG.  2 ). Mounting the sensor  40  on the test bed  56  can save an operator time by not requiring a sensor  40  to be installed on the engine  20 . The sensor  40  is electrically connected by a wire  45  to a production monitoring system  60 . 
     The meter  50  or the production monitoring system  60  calculates the ratio of first-order to second-order vibration accelerations. This ratio is then compared using a comparator within the meter  50  or the production monitoring system  60  to a reference ratio or predetermined value that is representative of a well-balanced engine  20 . Theory and testing have shown the reference ratio to be between about 0.4 and about 0.5, and the preferred reference ratio being approximately 0.45. If the calculated ratio is greater than the reference ratio, then the engine  20  needs to be re-balanced. If the calculated ratio is less than the reference ratio, then the test engine  20  is properly balanced. Testing has shown that the ranges of ratios are typically 0.1-0.3 for a properly aligned balancer, 0.5-1.0 for a balancer misaligned by one tooth, 1.0-1.8 for a balancer misaligned by two teeth, and 2.2-3.7 for a balancer misaligned by four teeth. In an alternate embodiment, a person can calculate the ratio and compare the ratio to a reference ratio. In an alternate embodiment, an order-tracking device may be used to obtain a better prediction by running up the engine, for example, from 1800 to 4200 rpm and then calculating the averaged ratio. The tracking device itself, however, and the set-up time for the device are costly. By using this run-up method, the ratios will converge to 0.20-0.3, 0.7-0.9, 1.4-1.6, and 2.5-2.6 for a balancer misaligned by zero, one, two, and four teeth, respectively. 
     The basis for this ratio method enables the determination of engine balancer condition. For a first-order balancer, the first-order vibration level depends on the shaking force derived from the balancer&#39;s condition and the system&#39;s characteristics (i.e., the mass and stiffness of the motorcycle&#39;s structure). However, other vibration levels such as the second-order vibration are primarily dependent on the system&#39;s characteristics. To eliminate the influence of the system&#39;s characteristics and isolate the balancer&#39;s condition, the relative vibration level, or vibration ratio, γ, is used to determine the balancer&#39;s condition rather than using the absolute level of the first-order vibration. For a balanced engine  20  on a very soft system (one in which the first system resonant frequency is lower than 15 Hz), the ratio, γ, is nearly independent of the variations of the system characteristics and only depends on the balancer&#39;s condition. These system variations could be from different tires, which are a variation of stiffness, different models of motorcycles, which are a variation of mass, or an engine  20  mounted on a test bench  55 , which provides variations of both mass and stiffness. Thus, the same ratio method can be used in many different situations. 
     Using this basis, a method is developed to determine the balancer&#39;s condition in a balanced engine  20 . The method includes measuring the first-order and second-order vibration accelerations at a pre-selected engine speed, preferably 2000±200 RPM. The ratio, γ, is then calculated, where γ equals the level of the first-order vibrations divided by the level of the second-order vibrations. The frequencies of the first- and second-orders for an engine  20  operating at 2,000 RPM are 33 Hz and 66 Hz, respectively. Because the engine speed is not always constant, it is necessary to measure the vibration level by using the 1/1-octave band spectrum rather than using the narrow band spectrum. Two 1/1-octave band filters can allow an accurate measurement of the first- and second-order levels. The center frequencies of these two filters are 31.5 hertz and 63 hertz per ANSI S1.11-1986. A first-order balancer is designed to cancel the first-order vibration only and can not influence the vibration level of any other order. The inverse of the ratio may also be used, i.e., the ratio of the level of the second-order vibrations to the level of the first-order vibrations. In addition, other factors, including multipliers and coefficients, may be included in the ratio calculation within the scope of this invention. 
     Various features of the invention are set forth in the following claims.