Noise and vibration considerations are becoming more and more important to the manufacturers of internal combustion engine powered vehicles.

Increases in the specific power output of internal combustion engines and the advent of high speed direct injection diesel engines have increased the potential noise, vibration and harshness (NVH) produced from the engine. In order to improve the overall NVH of the powertrain a number of different technologies have been employed. Two devices which have increased in use over the last few years are the dual mass flywheel and the fitment of balancer shafts.

The dual mass flywheel is a direct replacement for a conventional single mass flywheel. Dual mass- flywheels improve the driveline NVH, in particular gear rattle, by reducing the torsional vibrations transmitted to the gearbox input shaft. This is achieved by allowing the input and output halves of the dual mass flywheel to move relativity to each other about the axis of the input and output shafts.

A set of springs are fitted inside the dual mass flywheel to provide torque transfer from the input to the output. The disadvantage of fitting a dual mass flywheel to an engine is that the rotational inertia of the cranktrain is reduced, and this increases the rotational cyclic velocity fluctuation experienced by the crankshaft.

Balancer shafts are used to reduce the inertia generated forces and couples of the engine. Normally,

either a pair of counter-rotating shafts or a single balancer shaft are used depending upon the configuration of the engine cylinders and crankshaft.

The reduction in inertia generated forces and couples from the engine reduces the forces generated at the engine mounts, which reduces the structure borne noise of the vehicle. Balancer shafts are usually driven from the opposite end of the crankshaft from the flywheel sometimes with a non-unity gear ratio.

Potential drive mechanisms for the balancer shafts include belts, gears and chains. Belt drives may be used for balancer shafts but suffer from the relatively large packaging space required and durability problems with the belts caused by oil contamination and high operating temperatures. Gear driven balancer shafts are used on some current production engines. Gear drives offer a durable solution, but there are issues with the packaging space required, high cost of components and high levels of noise generated by the gear drive.

Chain drives offer the best overall solution to driving balancer shafts. The packaging space required is small due to the compact design of components and the chain drive can be fitted inside the engine as it can withstand high operating temperatures. The cost is lower than a gear driven mechanism and the noise is considerably reduced. However, chain drives are not durable when subjected to high levels of torsional vibration. This can exclude the use of chain driven balancers on internal combustion engines fitted with dual mass flywheels because of the above mentioned problem with large rotational cyclic velocity fluctuations which can be exacerbated by highly rated diesel engines.

Devices exist for the reduction of torsional

vibration for engine front end auxiliary drive components (see, for example US 6,048,284). These drives are usually belt driven and fitted outside the core engine, thus avoiding any problems with oil contamination or high operating temperatures. The devices usually reduce the torsional vibrations in the belt drive using a combination of isolation and damping. This is achieved with the use of springs, rubber, friction and other energy absorbing or storing elements. The large packaging space required and the low temperature limits for this type of device exclude the use of this technology for balancer shaft chain drives fitted within the core of the engine.

The pulley disclosed in US 6,048,284 smooths the load between the input and output members with a combination of isolating means (helically wound springs which store energy upon compression, and release the energy back into the system when the compression load is released) and a damping device (a friction generating device which absorbs kinetic energy caused by relative rotation of the input and output members, and dissipates this as heat). Such an arrangement is entirely counterproductive if used in the context of the sprocket for a chain driven balancer shaft. The problem is that the amplitude of the torsional variations in the drive shaft reaches a peak at a rotational speed which is less than the normal operating speeds of the engine. The provision of the damping device is designed to smooth out this peak hence reducing the maximum torsional vibration transmitted to the output member. However, with a frictional damping device, this is done at the expense of increasing the maximum torsional vibration at higher engine speeds, including speeds within the usual operating range of the engine.

A similar arrangement is shown in EP 0,987,470.

This discloses a sprocket for use on a balancer shaft drive which uses a combination of friction plates and springs to absorb vibrations. As with US 6,048,284, the use of friction plates will be entirely counter productive.

Although there are some specific instances in the prior art, such as JP 10281226, of two-part sprockets where the two parts are isolated only by rubber members and not friction plates, these are used to absorb shock loads. The configuration of such devices is entirely unsuitable to prevent vibration transmission of the torque and frequency that will be experienced by sprockets driving a chain shaft in an internal combustion engine with balancer shafts.

Further, the rubber members cannot withstand the high temperatures that are experienced by a sprocket within the engine.

According to a first aspect of the invention there is provided an internal combustion engine comprising at least one piston coupled to a drive shaft which is rotatable about an axis ; at least one balancer shaft ; and a chain drive including a chain coupling the drive shaft and balancer shaft, a sprocket rigidly connected to and rotatable with the drive shaft to transfer the rotary motion of the drive shaft to the chain, wherein the sprocket is positioned in an oil containing part of the engine and is an isolating sprocket comprising an input member connected to and rotatable with the drive shaft about the axis and an output member having teeth on its outer periphery engaging with the chain, the output member being coupled to the input member so as to be rotatable together with the input member about the axis and rotatable with respect to the input member

about the axis through a limited range of angles, and at least one resilient element mounted between the input and output members through which torque is transmitted from the input member to the output member ; wherein fluctuations of torque transmitted from the input member to the output member are absorbed substantially only by deformation of the at least one resilient element.

The approach taken by the invention is to isolate the output member from the input member, rather than damping the output member with respect to the input member. In this respect it is fundamentally different from the prior art. The effect of this is that the peak torque vibrations are transmitted to the output member. However, these only occur for a very brief time during engine start-up and shutdown. More importantly, because this aspect of the invention does not attempt to damp the relative movement between the input and output members, it does not raise the amplitude of torsional vibrations transmitted to the output member in the normal operating range of the engine.

Of course, there will be some damping of the motion between the input and output members as the two are necessarily in contact with one another and mounted so as to rotate relatively to one another, so that friction between the two will inevitably result in some damping. However, the invention aims to reduce the friction between these two elements as much as possible and certainly does not include any device such as that of US 6,048,284 designed to increase friction between the two members. A further benefit of this is that it again simplifies the structure of the sprocket as a friction generation arrangement is not required.

Preferably, the input member comprises two annular plates arranged to be clamped together, and the output member is in the form of an annular plate retained between the annular plates of the input member. This provides an extremely simple construction allowing the sprocket to be more compact than the prior art damping/isolating devices. Indeed, it has been found that the sprocket can readily be fitted in the space provided for a normal non-isolating sprocket. As the output member is retained between the two annular plates of the input member, the resilient element can be enclosed within the input member hence creating a fail safe structure.

The two plates forming the input member may be provided with a clamping element to hold them together. However, preferably, the two plates are arranged to be held together, in use, by a conventional crank nose bolt fitted to the end of the drive shaft to retain all elements in place. This avoids the need for any clamping arrangement on the sprocket itself, again greatly simplifying its construction.

Preferably, the output member has a circumferential bearing surface which is a close fit with a complementary circumferential bearing surface on the input member. This provides a simple bearing for radial loads acting on the output member through the input member which is rigidly fixed to the drive shaft.

The resilient elements may be any type of resilient element that can resist the thermal and mechanical loading to which it will be subjected in use. Preferably, however, the or each resilient element is a helically wound metal spring as these

provide a relatively constant stiffness at various temperatures, and over the operating lifetime of the engine.

In order to limit relative rotational movement of the input and output members, particularly at high loads, at least one snubber spring is provided between the input and output members, the snubber spring having a higher stiffness than that of the resilient element, and being mounted such that it only begins to be compressed after a certain amount of relative rotation has occurred between the input and output members against the action of the resilient element.

This ensures that the snubber springs do not come into operation during the normal running of the engine, but only when there has been a relatively large relative movement of the input and output members, at which point they provide a step wise increase in resistence to relative motion of the input and output members.

This can be useful as the input and output members reach the limit of their relative rotational movement to limit the force of collision between the two members as they approach a stop mechanism.

An example of an engine constructed in accordance with the present invention incorporating an isolating sprocket will now be described with reference to the accompanying drawings, in which: Fig. 1 is plan view of the isolating sprocket with the second annular plate moved for clarity; Fig. 2 is a section through line II-II in Fig.

1 with the second annular plate in place.

Fig. 3 is a diametric section through the first annular plate;

Fig. 4 is a plan view of first annular plate; Figs. 5 and 6 are views corresponding to Figs. 3 and 4 respectively showing the second annular plate; Figs. 7 and 8 are views corresponding to Figs. 3 and 4 respectively showing the chain sprocket; and Fig. 9 is a schematic view showing the engine layout including the relationship between the sprocket, the chain and the balancer shafts.

The isolating sprocket essentially comprises a first annular plate 1, second annular plate 2 and chain sprocket 3.

The first annular plate 1 is best shown in Figs.

3 and 4. The plate is circular and has a central circular opening 11 with a key slot 12. Surrounding the opening 11 is an inwardly projecting annular shoulder 13 (the term inwardly is used to imply an orientation towards the inside of the sprocket, while the term outwardly implies the opposite orientation).

The shoulder 13 is surrounded by an inwardly facing annular recess 14. An annular flange 15 projects radially outwardly beyond the recess 14. In the flange 15 are a first pair 16 of inwardly facing diametrically opposed circumferentially extending blind slots. A second pair 17 of inwardly facing diametrically opposed circumferentially extended blind slots are arranged so that the two pairs of slots 16, 17 are evenly spaced around the flange 15. A pair of diametrically opposed bosses 18 project inwardly from the flange 15.

The second plate 2 is best shown in Figs. 5 and 6. The structure of this second plate 2 is broadly

the same as the first plate 1 in that the second plate 2 is circular with a central opening 21 having a key slot 22. However, where the first plate 1 has an annular shoulder 13, the second plate 2 has an inwardly facing annular recess 23 which extends radially outwardly as far as the outer periphery of the annular recess 14 in the first plate 1. The second plate 2 has a similar flange 25 as the flange 15 on the first plate with the same layout of a first 26 and second 27 pairs of slots having shapes corresponding to the shapes of the recesses 16,17 in the first plate 1.

The chain sprocket 3 is best shown in Figs. 7 and 8. The chain sprocket 3 also has a circular plate like construction with a central opening 31 which is larger than the openings 11,21 in the first and second plates. The plurality of teeth 32 surround the periphery of the chain sprocket for engagement with a chain. A first pair of diametrically opposed through slots 36 are provided which correspond in size and location to the first pair of slots 16,26 on the first and second plates. A second pair of diametrically opposed through slots 37 correspond in location to the second pair of slots 17,27 on the first and second plates. However, the circumferential length of the through slots 37 is significantly shorter than the circumferential length of the second pairs of slots 17,27. A third pair of diametrically opposed through slots 38 are provided such that a diametric line passing through the centre of the through slots 38 is angularly offset by 45° with respect to a line passing through the centre of either the first pair of through slots 36 or the second pair of through slots 37. The third pair of through slots 38 are positioned so as to correspond to the location of the bosses 18 but have. a greater circumferential

length than the bosses 18.

The complete assembly will now be described with reference to Figs. 1 and 2. Fig. 1 shows the sprocket wheel 3 placed onto the first plate 2. In this condition, and as can be seen in Fig. 2, the chain sprocket 3 fits overs the annular shoulder 13 in the first plate 1 so as to provide a bearing surface for any radial load acting on the chain sprocket. A path for lubricant (not shown) will be provided to the bearing surface.

A helical spring 40 is provided in each of through slots 36 and is held in place at each end of the slot by a spring seat 41. This is essentially a component which has a mushroom-shape cross-section with a head which is convexly curved in two dimensions. This head 42 engages with the end of the through slot 36, while the opposite end 43 projects into the helical spring 40. The seats 41 project from the plane of the chain sprocket 3 into the first 16, 26 pairs of slots as shown in Fig. 2.

A similar spring 44 and seat 45 arrangement is provided in the pair of shorter through slots 37.

This time, the seats 45 project into the second 17,27 pairs of slots.

As shown in Figs. 1 and 2 the bosses 18 fit into the third pair of through slots 38.

With the second plate 2 in place as shown in Fig.

2, the annular shoulder 13 bears against the annular recess 23 in the second plate 2. The components may be glued together in this configuration as a temporary measure allowing easy handling while the sprocket is mounted to the crankshaft of an engine. The key slot

12,22 ensure that the sprocket is fitted in the correct orientation to the crankshaft. When the crank nose bolt is fitted in place, the axial load caused by this bolt is transmitted through first and second plates by virtue of the contact between the shoulder 13 and recess 23. This ensures that the two halves are rigidly clamped together while the chain sprocket 3 is held so as to be rotatable through a limited angle about the crankshaft axis while being prevented from moving radially. The clamped plates are also designed to resist clamping loads required for the fixing of other drive pulleys as may be required e. g. for an alternator, a water pump, power steering, belt drives etc.

During operation when the drive shaft may rotate at up to 6000 rpm, any torsional vibrations in the crankshaft are transmitted by the first 1 and second 2 plates to the springs 40 causing them to be compressed rather than transmitting the vibrations to the chain sprocket.

During start-up, the torsional vibrations acting on the sprocket will pass through a resonant peak. At this time the torque variations will be sufficient to cause a relatively large relative circumferential displacement of the chain sprocket 3 with respect to the first 1 and second 2 plates. This will cause the end of each second through slot 37 to reach the end of the corresponding second slots 17,27, at which point the stiffer springs 44 will begin to be compressed, thereby providing greater resistence to relative rotation of the chain sprocket 3 with respect to the first 1 and second 2 plates. Further relative rotation will cause the bosses 18 to hit the end of the circumferential through slots 38 thereby providing a stop preventing further rotation. The action of the

springs 44 ensures that the force with which the boss 18 collides with the end of the slot 38 is reduced.

Fig. 9 shows the basic engine layout incorporating the chain sprocket 3. The drive shaft 100 of the engine fits into the circular opening 11 within the sprocket and engages in the key slot 12.

Chain drive 101 engages with the teeth 32 on the chain sprocket 3. The chain 101 also engages with a pair of toothed balancer shafts'102 and an idler shaft 103.

11 As described above, torsional vibrations in the drive shaft 100 are isolated from the chain and balancer shafts by the chain sprocket 3.