Patent Description:
A wide variety of automotive and industrial internal combustion engines routinely rely upon a synchronous belt drive system to transmit torque from a driver sprocket on a crankshaft to a driven sprocket such as on a camshaft. Camshafts produce vibrations which can be detrimental to the operating life of the engine and synchronous belt drive system. In particular, actuation of the intake and exhaust valves through contact with the camshaft lobes causes a cyclic fluctuating torque load to be transmitted through the synchronous belt drive system.

Prior art attempts to attenuate the fluctuating torque loads include use of camshaft dampers as well as damped belt tensioners.

Teachings disclose the use of non-circular (oval) sprockets to control strongly fluctuating torques, for example, VDI Progress Reports No. <NUM>, "Non-Uniform Transmission Belt Drives" by Dipl. Egbert Frenke.

Other attempts include use of a rotor having an oval non-circular profile having at least two protruding alternating with receding portions. The rotary load assembly presents a periodic fluctuating belt tension when driven in rotation in which the angular positions of the protruding and receding portions of the non-circular profile relative to the angular position of the second rotor, and their magnitude, substantially cancels the alternating belt tensions caused by fluctuating load torque of the rotating assembly.

Representative of the art is <CIT> which discloses a synchronous belt drive system comprising an obround sprocket having a toothed surface and at least one linear portion disposed between two arcuate portions, the arcuate portions having a constant radius, the linear portion having a predetermined length, a second sprocket having a toothed surface, the second sprocket engaged to the obround sprocket by an endless toothed member, the second sprocket connected to a rotary load, the rotary load having cyclic torque fluctuations, and a radius of the obround sprocket oriented at a belt entry point which coincides with a maximum amplitude of a cyclic torque fluctuation such that a span length of the endless toothed member is made to vary in a manner that substantially cancels the cyclic tension fluctuations.

<CIT> discloses a timing gear which can reduce the vibration of an engine. The periphery of the timing gear of the engine is provided with teeth and consists of a semicircle and a minor arc. According to a specific vibrating position, the timing gear is designed so that a contact position between a timing belt and the timing gear just corresponds to the minor arc part of the timing gear in the vibrating process when the timing belt is tightened; as the minor arc part is close to the position of axle center of the timing gear, the minor arc needs a shorter belt than the semicircle part and can offset the tightening part of the timing belt without needing a tensioner to adjust, thereby reducing vibration; and when the timing belt is relaxed, the contact position between the timing belt and the timing gear just corresponds to the semicircle part of the timing gear and the relaxing part of the timing belt can be offset.

<CIT> discloses an alternative control belt and timing device for an IC engine.

What is needed is a belt drive sprocket system comprising an obround sprocket having a magnitude and a phase such that an angular displacement timing error between a sprocket and the obround sprocket is less than <NUM>. <NUM> degree peak to peak. The instant invention meets this need.

The primary aspect of the invention is to provide a belt drive sprocket system comprising an obround sprocket having a magnitude and a phase such that an angular displacement timing error between a sprocket and the obround sprocket is less than <NUM> degree peak to peak.

Other aspects of the invention will be pointed out or made obvious by the following description of the invention and the accompanying drawings.

The invention provides an engine as recited in claim <NUM>. Optional features are recited in the dependent claims.

The accompanying drawings, which are incorporated in and form a part of the specification, illustrate preferred embodiments of the present invention, and together with a description, serve to explain the principles of the invention.

<FIG> is a side view of an obround sprocket. The inventive sprocket <NUM> comprises a toothed surface <NUM>. Toothed surface <NUM> engages a toothed belt. Toothed surface <NUM> comprises land areas <NUM> and adjacent grooves <NUM>. The grooves <NUM> have a shape that is compatible with the corresponding design of a tooth form of a toothed belt. Toothed belts are also referred to as synchronous belts since they are used to synchronize the rotation of a driver and driven sprocket.

Sprocket <NUM> comprises portion <NUM> and portion <NUM>. Portion <NUM> has an arcuate toothed surface 11a which comprises a constant radius R2. Portion <NUM> has an arcuate toothed surface 11b which comprises a constant radius R1. Portions <NUM> and <NUM> are segments of a circle since the radii R1 and R2 are equal and constant. Use of circular segments in this manner reduces the complexity of the design and manufacturing process for the inventive sprocket.

Disposed between portion <NUM> and portion <NUM> is linear portion <NUM>. Portion <NUM> comprises a rectangular section which has the effect of displacing each portion <NUM> and <NUM> from each other, thus giving the obround shape to the sprocket. The sprocket surface <NUM> is straight, i.e., linear or flat between points <NUM> and <NUM>, and <NUM> and <NUM>.

The flat portion <NUM> has a length which relates to a system torque fluctuation amplitude. In this embodiment portion <NUM> has a dimension (W) of approximately <NUM> between points <NUM> and <NUM>, and <NUM> and <NUM>. Hence, the center of curvature <NUM> of portion <NUM> is displaced by a distance of W/<NUM>, approximately <NUM>, from the center of rotation <NUM> of the sprocket. Also, the center of curvature <NUM> of portion <NUM> is displaced by a distance of W/<NUM>, approximately <NUM>, from the center of rotation <NUM> of the sprocket. The dimensions given are only for the purpose of illustration and are not intended to be limiting. It also follows that a major length (ML) of the sprocket has a dimension: <MAT>.

A major segment (MG) for each portion <NUM>, <NUM> has a dimension: <MAT>.

The length (W) of portion <NUM> is determined by the radius of portions <NUM> and <NUM> and is dependent on the dynamic angular vibration characteristic which is being counteracted which is described elsewhere in this specification. Sprocket <NUM> can be designed using constant surface pitch, constant angular pitch or a combination of the two. "Surface Pitch" is defined as the distance between any two consecutive, corresponding, "pitch points" on the OD of the sprocket, measured around the OD line. Constant Surface Pitch is calculated as follows: <MAT> Where.

"Angular Pitch" is defined as the angular difference between any two consecutive, corresponding "pitch points" on a sprocket and may be measured in degrees or radians. Constant Angular Pitch is defined as follows: <MAT> Where.

The sprocket groove profile can be individually designed to suit the particular dynamics of the engine.

The elastic modulus of the span of the belt, in combination with the tooth modulus and the sprocket offset (W/<NUM>) is optimized to substantially reduce or cancel tension fluctuations at predetermined engine speeds. Consequently, in this application the belt is analyzed and designed as a spring member of the system in addition to being sized to transmit the required tensile loads. The system dynamic response is selected by iterative process to arrive at a combination of belt modulus and obround sprocket radius (R1 and R2) which substantially reduces or cancels all of the tension fluctuations otherwise transmitted through the belt and the belt drive system.

<FIG> is a side view of an alternate embodiment of the sprocket. This embodiment comprises three linear segments disposed between arcuate portions <NUM>, <NUM>, <NUM> as otherwise described in <FIG>. The three linear segments (<NUM> to <NUM>) and (<NUM> to <NUM>) and (<NUM> to <NUM>) are disposed between each arcuate portion <NUM>, <NUM>, <NUM>. Each arcuate portion <NUM>, <NUM>, <NUM> comprises, respectively, constant and equal radii R1, R2, R3. The three linear segments are equally spaced about the circumference of the sprocket at intervals of approximately <NUM>°. <FIG> is representative of the <NUM>th order load characteristic in a system using the sprocket shown in <FIG>.

<FIG>, <FIG> and <FIG> are some typical drive layouts for four cylinder, four stroke internal combustion engines using a toothed belt system to drive the camshaft and auxiliaries. These engines typically exhibit a high <NUM>nd order dynamic. Dependant upon fuel pump specification, some diesel engines may have a <NUM>th order which is dominant. Schematic diagrams showing such dynamics can be seen in <FIG>, <FIG>.

In order to counteract <NUM>nd order dynamics, the inventive sprocket <NUM> is attached to the engine crankshaft Crk. Dependant on the presence of other dominant orders, it may be necessary to apply alternative embodiments of the sprocket. These may be attached to the crankshaft, but may equally be applied elsewhere in the system, for example on the water pump, fuel pump or on the camshaft sprocket (s). The engine crankshaft is the driver for the entire belt drive system. The driven direction of the belt is DoR. Due to the sprocket ratio, the engine crankshaft Crk rotates twice for each rotation of the camshaft CAM1.

In <FIG>, sprocket <NUM> is connected to the camshaft CAM1 and sprocket <NUM> is connected to a second camshaft CAM2. Idlers Idr1 and Idr2 known in the art are used to maintain proper belt routing and tension control. Sprocket <NUM> is connected to the water pump WP. Belt <NUM> is trained among the several sprockets. The direction of rotation for belt <NUM> is shown as DoR. The point at which belt <NUM> engages crankshaft sprocket CRK is <NUM>. The camshaft inertia and torque loads are represented by <NUM>.

Toothed belt <NUM> is trained between sprocket <NUM> and cam sprocket <NUM>. The belt entry point <NUM> is that point at which the belt <NUM> engages the sprocket. The belt span length between the crankshaft CRK and the cam sprocket <NUM> is "SL".

Similarly in <FIG> and <FIG>, camshaft sprocket <NUM> is attached to the engine camshaft CAM. In <FIG>, the load characteristic <NUM> includes the torque characteristic of a fuel pump attached to the rear of the camshaft whereas in <FIG>, the fuel pump torque is represented by load characteristic <NUM>. Inertias and torque loads (<NUM>, <NUM>, <NUM>) caused by other components such as water and vacuum pumps may also be present as well, namely, WP (<NUM>) in <FIG> and <FIG>. In <FIG> IDR1 and IDR2 are idlers known in the art to properly guide belt <NUM>. In <FIG>, the belt span length between the crankshaft sprocket <NUM> and the cam sprocket <NUM> is "SL".

For a gasoline engine the dominant cyclic fluctuating torque loads are normally a characteristic of the camshaft. For a diesel engine the dominant order can be produced by the camshaft and/or a fuel injection pump which may be included in the drive system. The torques caused by the water pump and vacuum pump may vary but they are not cyclic, within their own right, on the same period or frequency as the camshafts and are not normally dominant characteristics of the drive dynamics.

<FIG> is a perspective view of another single cam engine embodiment with fuel injection pump included in the drive for a diesel engine. In this embodiment, in addition to the system shown in <FIG> the system further comprises sprocket <NUM> connected to the fuel pump IP. Also shown is sprocket P1 which is engageable with another multi-rubbed belt used to drive various engine accessories (not shown). In <FIG> the cam loads are depicted by <NUM> and the fuel pump load by <NUM>. Sprocket <NUM> is connected to the water pump WP. In <FIG> the torque load caused by a fuel injection pump is represented by <NUM>.

A typical total load characteristic for a four cylinder, four stroke engine is represented by curve "E" in <FIG>. Curves "D" and "C" represent typical <NUM>nd and <NUM>th order characteristics which have been extracted from the total load characteristic. The load characteristic of an in-line four cylinder, four stroke, gasoline driven engine would not normally include a <NUM>th order.

The change in average radius at belt engagement point <NUM> of the inventive sprocket <NUM> as it rotates is curve "C" in <FIG>. The integral of curve "C", which is the effective length change of the belt in <FIG>, is curve "D" on <FIG>. The derivative of the change in average sprocket radius is the acceleration of a given point on the toothed surface, <NUM>, due to the change in sprocket shape.

In order to counteract <NUM>nd order dynamics, the flat portion <NUM> of the obround sprocket <NUM> is arranged in timing relation to the camshaft sprocket <NUM> such that an effective length of the belt <NUM> between sprocket <NUM> and sprocket <NUM> in <FIG> is made to vary in a manner that substantially cancels the alternating belt tensions caused by the cyclic camshaft torque fluctuations. As an example of a design to cancel <NUM>nd order dynamics, this can be achieved by timing the maximum length of the sprocket <NUM> (R1+R2+W) to coincide with the belt entry point <NUM> when the camshaft torque, and therefore belt tension, is at a maximum.

The absolute dimensional characteristic of a drive containing an obround sprocket is dependant on parameters such as the fluctuating torque, the belt span modulus, the inertias of each of the driven accessories in the system, the belt installation tension and the interaction between the belt and sprockets. The interaction between the belt and sprockets is dependant on parameters such as the number of teeth in mesh on the sprocket, the belt tooth modulus, the belt dimensions and the coefficient of friction between the belt and sprocket surfaces.

<FIG> is a schematic of a twin cam, four cylinder, four stroke gasoline engine. The illustrative system comprises cams CM1, CM2 and belt B trained there between. It further comprises tensioner TEN, water pump WP and crankshaft sprocket CRK. The direction of rotation of belt B is DoR. The span lengths of interest are between sprocket CRK and sprocket IDR, sprocket IDR and sprocket WP and sprocket CRK and sprocket WP. In <FIG>, the belt span length between the crankshaft sprocket CRK and the cam sprocket CM1 is "SL". For calculation purposes since there is no major load impact between CM1 and CRK in DoR, these may be treated as one span "SL". Approximate typical values for the variables for the system described in <FIG> are as follows:.

Typical values for the coefficient of friction for the sprocket surface <NUM> are in a range from <NUM> and <NUM>, typically <NUM>.

Typical belt installation tension values can be in the range of 75N up to 900N depending upon system requirements.

The belt span modulus is dependant on the tensile member construction, the number of strands of the tensile member within the belt and the belt width. An example of belt span modulus for a <NUM> wide belt having <NUM> tensile members would be in the region of approximately 240Mpa.

<FIG> is representative of a typical total load characteristic for the driven sprocket of a four cylinder, four stroke diesel engine, including extracted curves for the <NUM>th (curve "C") and <NUM>nd (curve "D") orders. The load characteristic of an in-line four cylinder, four stroke, gasoline driven engine would not normally include a <NUM>th order. The "Offset" refers to W/<NUM>. The "Total Load" refers to <FIG>, line "E".

In <FIG>, Line "A" is zero torque. Line "B" depicts the average torque in the belt drive system. Curve "C" is the <NUM>th order torque characteristic extracted from the total load curve "E". Curve "D" is the <NUM>nd order torque characteristic extracted from the total load curve "E". Curve "E" is the total torque characteristic of the engine measured at the crankshaft CRK. The area under curve "E" represents the work done to turn the engine at a particular speed.

<FIG> is representative of the <NUM>nd order load characteristic (curve "B") for a driver sprocket of a four cylinder, four stroke engine including the change in radius (curve "C") for an obround sprocket and the resultant belt span length change (curve "D").

In <FIG>, line "A" is zero torque. Curve "B" is the <NUM>nd order torque characteristic extracted from the total load. Curve "C" is the variation in effective crankshaft pulley radius as it rotates through <NUM> degrees caused by segment <NUM> in <FIG>. Curve "D" is the integral of curve "C" and is the effective change in belt drive span length caused by the sprocket described in <FIG>.

<FIG> is representative of the <NUM>th order load characteristic "B" for a driver sprocket of a four cylinder, four stroke diesel engine with a three piston fuel pump, (or other driven device that will induce a <NUM>th order), including the change in sprocket radius length (curve "C") for an alternative three lobe embodiment of the obround sprocket (<FIG>) and the resultant belt span length change (curve "D"). Belt span length is the distance between the cam sprocket CAM and crankshaft sprocket CRK on <FIG> for example.

In <FIG>, line "A" is zero torque. Curve "B" is the <NUM>th order torque characteristic extracted from the total load. Curve "C" is the variation in effective crankshaft pulley radius as it rotates through <NUM> degrees. Curve "D" is the integral of curve "C" and is the effective change in drive length caused by the alternate embodiment of the sprocket described in <FIG>.

The elastic modulus of a tensile member of a variety of belts used in the inventive system is shown in <FIG>. The curves SS1 thru SS6 are known as stress-strain curves for a variety of belts <NUM>. Each curve represents a modulus using a different material for the tensile cord in the belt. The elastomeric HNBR belt body is illustrative and not limiting. In addition to HNBR, other belt body materials can include EPDM, CR (chloroprene) and polyurethane, or a combination of two or more of the foregoing. The materials comprise for:.

The elastic modulus of each tensile member is the slope of each curve SS1 thru SS6, as is known in the art. Typically this measurement and calculation is taken on the substantially linear portion of the curve. In addition to fiberglass, carbon fiber and Aramid™, another tensile member material may include fine filament stainless steel wire.

The belt span modulus is dependant on the tensile member construction, the number of strands of the tensile member within the belt and the belt width. An example of belt span modulus for curve SS1, for a <NUM> wide belt with <NUM> strands of fiberglass tensile member, would be approximately 242Mpa.

<FIG> is a series of curves showing the effects of the phasing/misphasing of an obround sprocket major length on the engine dynamic for the system in <FIG>. Curve "D" is the optimum timing arrangement between the position of the sprocket major length to belt entry point <NUM> and torque pulse. Curves A, B, and C are mistimed clockwise from the curve "A" position by +<NUM> +<NUM> and +<NUM> teeth respectively. Curve "E" is mistimed by <NUM> teeth in an anticlockwise direction. Phasing of maximum belt span length to peak torque and inertial load may vary dependent on the dominant orders of the drive and those which are to be diminished by the system. The belt entry point <NUM> is that point at which the belt engages the sprocket. In <FIG> the span length is "SL".

Regarding angular interval or phasing the allowable angular tolerance is calculated using the following: <MAT>.

The belt drive span length is at a maximum when the torque is at a maximum.

<FIG> is a chart showing the effect of a correctly phased obround sprocket on a twin cam, four cylinder, four stroke engine as depicted in <FIG>. Curves "A" and "B" represent measured values for angular vibration at the inlet and exhaust camshaft sprockets respectively for a prior art design using round sprockets.

By way of comparison, curves "C" and "D" represent measured values for angular vibration at the inlet and exhaust camshaft sprockets respectively with an inventive sprocket used on the crankshaft. The resultant reduction in angular vibration is approximately <NUM>%.

Similarly, <FIG> is a chart showing the effect of a correctly phased obround sprocket as described in <FIG> on a twin cam, four cylinder, four stroke engine as depicted in <FIG>. Curves "A", "B" and "C" represent measured values for maximum, average and minimum dynamic tight side tensions respectively over a range of engine speeds for a prior art drive design. In this example, this tension was measured at position IDR in <FIG>. For extended belt lifetime the belt tight side tension should be minimized. Curves "D", "E" and "F" represent measured values for maximum, average and minimum belt tight side tensions with the inventive sprocket in use. The resultant reduction in installation tight side tension is in the range of <NUM>-<NUM>% in the resonant speed range of the engine (approximately 4000rpm to approximately 4800rpm). The decrease in belt tight side tension gives potential for significant improvement in the belt operating lifespan.

The inventive system is useful reduce timing error in IC engines. Timing error is the positional discrepancy between a driver and a driven shaft caused by random factors such as vibration, component inaccuracy and elastic deformation. In this case, it is the rotational inaccuracy of the camshafts (driven) of an IC engine in comparison to the crankshaft (driver) of the engine. It is normally reported in degrees pk to pk. For example, referring to <FIG>, sprocket <NUM> and sprocket <NUM> are each obround. Use of the obround sprockets significantly reduces timing error which in turn gives improvement in fuel economy, lowers emissions and generally improves engine performance and efficiency. At a component level, reduced timing error and lower system loads lead to better durability and less potential for NVH issues. Reduction of tension reduces NVH levels, and especially meshing order, in the drive. The application of obround sprocket to reduce timing error is not limited only to the camshafts of an engine. The benefit can equally be obtained by inserting the obround sprocket on crank or fuel pump.

<FIG> is a chart of crankshaft rotation speed versus angular vibration. The exemplary angular vibration diminishes as engine speed increases. <FIG> displays data for a motored engine and a firing engine. For a motored engine the crankshaft is driven by an electric motor, there is no fuel combustion in each cylinder. For a firing engine the crankshaft is driven in the normal manner for an internal combustion engine, i.e., with combustion of fuel in each cylinder. The motored engine (MER) reflects less angular vibration than the firing engine (FER) for a given engine rotational speed.

<FIG> is a chart of crankshaft speed versus vibration angle for an inlet cam. An obround sprocket is mounted to the inlet valvetrain camshaft. Three conditions are shown. The first is for a standard drive system with no obround sprocket (Curve A). The second is with an obround sprocket and the third is with an obround sprocket and a high modulus belt (Curve B). The phase and magnitude of the obround sprocket is <NUM> pitches from <NUM> o'clock position and <NUM>. The standard belt modulus is <NUM>,000N and the high modulus belt modulus is <NUM>,000N. The modulus is given in Newtons (N) and is defined as the force required to extend a unit length by <NUM>%.

The vibration angle for the third condition (Curve C) is significantly reduced to less than <NUM> deg peak-to-peak when compared to the value for the standard drive system at about <NUM> deg peak-to-peak, both measured at 4000RPM.

<FIG> is a chart of crankshaft speed versus vibration angle for an exhaust cam. An obround sprocket is mounted to the exhaust valvetrain camshaft. Three conditions are shown. The first is for a standard drive system with no obround sprocket (Curve A). The second is with an obround sprocket and the third is with an obround sprocket and a high modulus belt (Curve B). The vibration angle for the third condition is significantly reduced to about <NUM> deg peak-to-peak when compared to the value for the standard drive system at about <NUM> deg peak-to-peak, both measured at 4000RPM (Curve C). However, depending on the engine the improvement can range from under <NUM> deg peak-to-peak to about <NUM> deg, a reduction of just over <NUM>%. The phase and magnitude of the obround sprocket is <NUM> pitches from <NUM> o'clock position and <NUM>. The standard belt modulus is about <NUM>,000N and the high modulus belt modulus is about <NUM>,000N.

<FIG> is a chart of crankshaft speed versus angular displacement for an inlet cam. Angular displacement is also referred to as timing error and is measured relative to crankshaft position. An obround sprocket is mounted to the inlet valvetrain camshaft. Three conditions are shown. The first is for a standard drive system with no obround sprocket (Curve A). The second is with an obround sprocket and the third is with an obround sprocket and a high modulus belt (Curve B). The angular displacement for the third condition is significantly reduced to less than <NUM> deg peak-to-peak when compared to the value for the standard drive system at about <NUM> deg peak-to-peak, both measured at 4000RPM (Curve C). However, depending on the engine the improvement can range from under <NUM> deg peak-to-peak to about <NUM> deg, a reduction of just over <NUM>%. The phase and magnitude of the obround sprocket is <NUM> pitches from the <NUM> o'clock position and <NUM>. The standard belt modulus is about <NUM>,000N and the high modulus belt modulus is about <NUM>,000N.

<FIG> is a chart of crankshaft speed versus angular displacement for an exhaust cam. An obround sprocket is mounted to the exhaust valvetrain camshaft. Three conditions are shown. The first is for a standard drive system with no obround sprocket (Curve A). The second is with an obround sprocket (Curve B) and the third is with an obround sprocket and a high modulus belt (Curve C). The angular displacement for the third condition is significantly reduced to about <NUM> deg peak-to-peak when compared to the value for the standard drive system at about <NUM> deg peak-to-peak, both measured at 4000RPM. However, depending on the engine the improvement can range from under <NUM> deg peak-to-peak to about <NUM> deg, a reduction of just over <NUM>%. The phase and magnitude of the obround sprocket is <NUM> pitches from the <NUM> o'clock position and <NUM>. The standard belt modulus is about <NUM>,000N and the high modulus belt modulus is about <NUM>,000N.

<FIG> is a diagram showing the effect of the obround sprocket phasing relative to each camshaft timing error. The Y axis is angular displacement, or timing error, of each cam sprocket with reference to the crankshaft. It is quoted as a peak to peak value, that is, the numerical difference between min and max. Columns <NUM> and <NUM> of the diagram report a standard drive set up using all round sprockets. Column <NUM> reports use of a <NUM>rd order obround sprocket installed on the inlet and the exhaust camshaft. Each sprocket is positioned such that max offset is in line with the cam shaft lobes. Columns <NUM> to <NUM> report various trials using differing offsets of the obround sprockets. The "<NUM> o' clock" position is the datum for all offsets. The values given are simply the number of pitches, or grooves "g", through which the sprocket datum point was rotated from that position. "Datum point" is the point used as reference for angular measurements. This is set @ the <NUM> o'clock position. "cw" refers to clockwise. For example, "Ex <NUM> cw" refers to the <NUM> o'clock position and the exhaust cam obround sprocket having an offset of <NUM> grooves in the clockwise direction from the <NUM> o'clock position on the engine.

<FIG> is a diagram showing the effect of the obround sprocket phasing relative to each camshaft timing error with a standard and high modulus belt. The Y axis is angular displacement in degrees peak-to-peak, or timing error, of each cam sprocket with reference to the crankshaft. It is quoted as a peak to peak value, that is, the numerical difference between min and max. Columns <NUM> and <NUM> of the diagram report a standard drive set up using all round sprockets. Each column reports use of a <NUM>rd order obround sprocket installed on the inlet and the exhaust camshaft. Each sprocket is positioned such that max offset is in line with the cam shaft lobes. Columns <NUM> and <NUM> to <NUM> report various trials using differing offsets of the obround sprockets. The "<NUM> o' clock" position is the datum for all offsets. The values given are simply the number of pitches, or grooves, through which the sprocket datum point was rotated from that position. "Datum point" is the point used as reference for angular measurements. This is set @ the <NUM> o'clock position. The phase and magnitude of the obround sprocket is <NUM> pitches for the exhaust and <NUM> pitches for the inlet from the <NUM> o'clock position and <NUM> for each. The standard belt modulus is about <NUM>,000N and the high modulus belt modulus is about <NUM>,000N.

<FIG> is a diagram showing the effect of an obround sprocket on timing error by belt width. Column <NUM> reports a <NUM> wide belt in a system using round sprockets. Column <NUM> reports a <NUM> wide belt in a system using obround sprockets. Column <NUM> reports a <NUM> wide belt using a high modulus belt in a system using standard sprockets. Column <NUM> reports <NUM> wide belt using a high modulus belt in a system using obround sprockets. Column <NUM> reports an <NUM> wide belt using a standard modulus belt in a system using standard sprockets.

<FIG> is diagram showing the effect of an obround sprocket in timing error by magnitude of eccentricity. Each column reports an obround sprocket used on the inlet and outlet camshafts. The magnitude of eccentricity for each system ranges from <NUM> to <NUM>.

Testing to validate the effectiveness of the obround sprocket to reduce belt drive system dynamics can be carried out on both motored and firing engines. The results for timing error improvement included in the Figures were generated on a motored engine. While in most cases these results transfer to a firing engine, in some cases obround sprockets do not reduce dynamics on certain engines. Testing should be performed on a firing engine to assure the required improvements are achieved and are reliable. The steps necessary to conduct the testing are known in the engine dynamics art. These also include that the vibration sensors need to operate in an oil environment, need to be able to withstand up to 160C, and need to be able to withstand chemical attack from oil and additives. Consistency checks are carried out at beginning and end of each series of test runs. Measurements are taken during a run up from idle to max engine speed over a 60sec ramp. A standard Rotec system is used for data capture and analysis.

Claim 1:
An engine with a synchronous belt drive system comprising:
a camshaft (CAM1) with cam shaft lobes and a first obround sprocket (<NUM>) installed thereon, the first obround sprocket (<NUM>) having a toothed surface (<NUM>) and at least one linear portion (<NUM>) disposed between two arcuate portions (<NUM>,<NUM>), the arcuate portions having a constant radius (R1, R2), the linear portion (<NUM>) having a predetermined length; and
a crankshaft (CRK) with a sprocket (<NUM>) installed thereon, the sprocket (<NUM>) having a toothed surface, the sprocket (<NUM>) engaged to the first obround sprocket (<NUM>) by an endless toothed member (<NUM>);
the first obround sprocket (<NUM>) having a magnitude and a phase such that an angular displacement timing error between the sprocket (<NUM>) and the first obround sprocket (<NUM>) is less than <NUM> degree peak to peak, wherein the phase of the first obround sprocket (<NUM>) is in the range of <NUM> grooves to <NUM> grooves when rotated relative to a datum point at which the maximum offset of the first obround sprocket (<NUM>) is in line with the cam shaft lobes.