Internal combustion engine

A cylinder block for an internal combustion engine which, in use, supports a rotary crankshaft 40, the block having pairs of opposed lock width surfaces 37 formed thereon for forming an interference fit with respective co-operating lock width surfaces 38 on the crankshaft bearing caps 34 which in use are secured to respective bearing cap support surfaces 36 also formed on the block 31. The lock width surfaces 37, 38 are spaced from the respective bearing cap support surfaces 36 so as to improve stress cracking resistance of the block 31.

FIELD OF THE INVENTION

This invention relates to internal combustion engines and in particular to the mounting of the crankshaft for rotation within the cylinder block of an engine

BACKGROUND AND SUMMARY

In a typical internal combustion engine the engine crankshaft is mounted for rotation in bearings mounted on the cylinder block. The bearings are typically split with one half of the bearing supported in the cylinder block and the other half of the bearing supported in a bearing cap mounted to the cylinder block. The bearing caps retain and locate the crankshaft, or bearings for a crankshaft shaft which in turn retain and locate the shaft, relative to a cylinder block. The journal bearings on the crankshaft run against the two half shell bearings which are fitted to the main bearing cap and semi-circular surfaces on internal supporting walls of the block. For vibration free, low friction and quiet running, the roundness of the bore produced by the main bearing cap and the bulkhead is very important. This roundness is achieved by a machining operation called line boring. The main bearing caps are bolted to the internal supporting walls and then all the circular bearing surfaces are machined in a single operation. This ensures the two half rounds formed by the main bearing cap and the bearing block form as near to a perfect circle as possible and that all the circular bearings are coaxial. The bearing surfaces may then be honed to a fine finish to achieve the extremely fine tolerances needed for quiet running and efficient engine performance.

However, to install the crankshaft, it is necessary to remove the main bearing caps from the engine block. After the crankshaft is put in place, it is necessary to reposition the main bearing caps so that they are replaced in the identical position they occupied during the line boring operation. Any deviation from that original position produces an out-of-round condition that, in turn, leads to vibration, noise and possibly stiff, high friction crankshaft operation and rapid wear. There are a number of conventional methods for relocating and attaching the main bearing caps to bulkheads when installing the crankshaft. In one such method the main bearing cap has a very precisely machined dimension across the transverse axis (that is normal to the rotational axis of the crankshaft journal) of the main bearing cap across the foot of the bearing cap adjacent the bearing split line. This dimension is known as a lock width or snap width. A pair of cooperating opposing surfaces are precision machined on pads on the engine block support wall to produce a controlled interference fit with the cap when the main bearing cap is refitted after crankshaft installation.

The location in the fore and aft direction (i.e., in the direction of the rotational axis of the crankshaft journal) may be controlled by the bearing cap bolts or the use of hollow dowels pressed into counter-based holes in the support wall. The dowels then locate in precisely machined counter-bores in the corresponding main bearing cap.

The interference fit between the main bearings caps and pads on the engine block cause some stresses to both the cap and the cylinder block. The machined pad surfaces and the machined clamping surface for the mounting cap on the support wall are substantially normal to each other and meet at a relatively sharp line of intersection. The lock width pads are subject to high compressive forces during the fitting of the main bearing cap and during operation of an engine, loads generated by combustion will also be transferred to the lock width pads.

The present invention provides a cylinder block in which the stresses caused by the lock width for the main bearing caps are reduced increasing the durability of the engine.

According to a first aspect of the present invention there is provided a cylinder block for an internal combustion engine, the cylinder block in use supporting a rotary crankshaft, the block having pairs of opposed lock width surfaces formed on the block for forming an interference fit with respective co-operating lock width surfaces on the crankshaft bearing caps which in use are secured to respective bearing cap support surfaces also formed on the block, wherein the lock width surfaces are spaced apart from the respective bearing cap support surfaces.

The lock width surfaces spaced from each respective bearing cap support surface such that a portion of the lock width surface in use aligns with the apex of the bearing cavity in the cap and preferably the mid height of the lock surface is at a distance from the bearing cap support surface substantially equal to the radius of the crankshaft bearing.

The lock width surfaces on the block may also be displaced relative to the end of the bearing cap mounting surface (that is radial with respect the axis of rotation of the crankshaft).

The cylinder block may be cast from any suitable material, such as cast iron or aluminium, and is particularly advantageous for materials which have a high notch sensitivity such as compacted graphite iron (CGI).

According to a second aspect of the invention there is also provided an internal combustion engine having a cylinder block according to a first aspect of the invention with a crankshaft rotationally mounted in bearings on the cylinder block, at least some of said bearings comprise two half bearings with one half of each bearing being on the block and the other half thereof being on a respective bearing cap mounted to the respective bearing cap support surface, each bearing cap having spaced apart lock-width surfaces thereon which form an interference fit with the respective co-operating lock width surfaces on the block.

The total interference fit is preferably in the order of 10–200 microns per bearing cap.

Each bearing cap comprises a body having limbs passing on each side of a semi-circular half bearing cavity and has mounting surfaces on the ends of each limb that sit against the support surface on the block, the outer end portions of each limb (that is outwardly from the crankshaft) adjacent the mounting surfaces having clearance with respect to the block.

Preferably each bearing cap is secured on the support surface bolts vertically (that is normal) to the support surface, and by cross-bolts (that is bolts parallel to the support surface) that engage with opposite outer sides of the bearing cap.

Conveniently, each cross bolt engages with a raised pad on the outer side of each limb.

Preferably the lock width surfaces are formed on the two cross bolt pads.

In the preferred arrangement, the crankshaft runs against the two half shell bearings which are fitted to the main bearing cap and semi-circular surfaces on internal supporting walls of the block.

The invention also comprises a motor vehicle having an engine according to the second aspect of the present invention.

DETAILED DESCRIPTION

FIGS. 1 and 2show the prior art in which a cylinder block11supports a rotatable crankshaft (not shown) on two half bearing shells12and13supported on a main bearing cap14and a wall15of the block11. The wall15has a main bearing cap support surface16formed thereon and the cylinder block15has a pair of precisely machined opposing lock width surfaces17formed thereon adjacent the end of the support surface16.

The bearing cap14has a pair of lock width surfaces18precisely machined thereon that in use co-operate with the surfaces17on the block to precisely locate the main bearing cap14. The bearing cap14is secured in position by vertical bolts21and cross bolts (not shown) which screw into a raised pad22on the side of the bearing cap. The cross bolt pads22are machined to always be in clearance with machined location surfaces23on the block so that the lock width always provides the interference between the bearing cap14and the block11.

The line of intersection19between each lock surface17and the support surface16acts as a stress raiser due to the compressive forces resulting from the interference fit of the cap14between the two surfaces17.

With reference toFIGS. 3 and 4, there is shown a cylinder block31having walls35which in use support the journals40of a crankshaft mounted for rotation relative to the block using bearing caps34. In the present example the block is for a V 6 configuration internal combustion engine but the invention is applicable to other engine types. The cylinder block is cast from CGI and machined in the conventional manner.

The invention will be described with reference only to a single wall and bearing cap as shown inFIG. 3. The crankshaft rotates in a pair of half bearing shells32,33mounted in the cylinder block wall35and bearing cap34. The bearing cap34comprises a body50having limbs51passing on each side of a semi-circular half bearing cavity52in which the half bearing shell33is accommodated. The limbs51have mounting surfaces on the ends of each limb that sit against a support surface36machined on the block31. The outer end portions of each limb51(that is outwardly from the crankshaft) adjacent the mounting surfaces have a clearance C with respect to the block.

The bearing cap34is secured to the support surface36by vertical bolts41, preferably four bolts, and preferably two opposing cross-bolts42. The vertical bolts resist the high compressive forces generated by combustion in the cylinders and the cross bolts42prevent the cap34from twisting axially with the crankshaft (axially means along the axis of rotation of the crankshaft).

CGI, from which the cylinder is cast, has increased strength and stiffness as compared with conventional cast iron and better withstands the increased combustion pressures required by modern diesel engines in particular. However, CGI is sensitive to stress cracking and to reduce the stress concentrations, the lock width surfaces37on the cylinder block are moved away from the support surface36, which is known as the split line. The two lock width surfaces37on the block31align with co-operating lock width surfaces38on the bearing cap34which are machined on raised pads43on the outer side of the limbs51of the bearing cap. The lock width surfaces37on the block do not align with and are radially spaced from the end of the machined support surface36by a gap S. The lock width interference is between 10–200 microns and preferably 20 to 100 microns.

In the preferred arrangement, the lock widths37,38are spaced from the split line36such that portions of the lock width surfaces are in alignment with the apex of the semi-circular cavity52and more preferably such that the mid-height of the lock width surfaces37,38is spaced from the split line by a distance which approximates to the radius of the crank journal. In the present example for a 35 mm crank shaft, the lock width area extends vertically for about 13 to 15 mm and the mid height is about 30 mm from the split line. This mid-height will alter from between 25–45 mm depending on the diameter of the crankshaft to be supported by the bearing cap so that a portion of the lock width area overlaps with the apex of the cavity.

It is convenient if the cross-bolts42engage in threaded bores39which are formed in the lock width surfaces38in the centre of the lock width area.

The location of the lock width surfaces37&38away from the split line decreases the stress concentrations at the split line by a factor of two.