GB2310704A - Forming cylinder bores - Google Patents

Abstract

Cold distortion of an engine cylinder bore 2 is caused by the tightening of bolts 4 which secure the cylinder head and/or the crankcase to the engine block 10. Hot distortion is caused by uneven heating of the bore when the engine heats up to its normal running temperature. A method of forming an engine cylinder bore which is circular 8 in cross section when the engine is running at its normal operating temperature comprises machining the bore at ambient temperature to have a non-circular cross section so that hot and cold distortion cause the bore to become more nearly perfectly circular in cross section.

Description

FORMING CYLINDER BORES The present invention relates to a method of forming a bore in a cylinder of an internal combustion engine.

Engine cylinder bores are typically machined to an exact circular cross section to accommodate circular pistons which reciprocate therein. However in practice it is found that bores which are machined to be perfectly circular when manufactured deviate from circularity in use in a hot engine.

There are two principal reasons for this deviation, namely "cold distortion" and "hot distortion".

Cold distortion of the bores is caused by forces produced by the tightening of bolts that secure the cylinder head to the cylinder block, and to a lesser extent by the tightening of bolts which secure the crankshaft housing.

Hot distortion is caused by the metal walls defining the bore being subjected to different temperatures in different regions, for example because of the effects of cooling jackets. Hot distortion causes the bores to change shape as the engine heats up from cold to normal running temperature.

It is known to make some allowance for cold distortion by bolting a fixture to the top of the cylinder block to induce cold distortion so that when the cylinder head is fitted the bore "distorts" to a true cylinder.

It has been proposed in Soviet patent number 1 807 926 to allow for hot distortion in a reconditioned cylinder liner by reboring and honing the liner at the operating temperature of the engine so that the bore "distorts" to a true cylinder when heated from ambient temperature to normal running temperature.

There are problems with both of the above methods.

Bolting a fixture to the cylinder block adds another step to the production process and will not necessarily produce the same distortion as the bolting of the cylinder head to the cylinder block in practice.

Similarly, having to machine the cylinder bore at the normal running temperature of the engine will add to production costs and slow production down while the cylinder block is heated up. Application of an external heat source to the cylinder block also may not produce the same hot distortion as is produced under normal engine running conditions.

According to a first aspect of the present invention there is provided a method of forming an engine cylinder bore which is circular in cross section when the engine is running at its normal operating temperature, the method comprising machining the bore at ambient temperature to have a non-circular cross section so that hot and cold distortion cause the bore to become more nearly perfectly circular in cross section.

By taking account of the effects of hot and cold distortion and forming a bore which is distorted from perfect circularity at ambient temperature the invention overcomes the problems identified in the prior art.

The cylinder bore as formed will be the "antithesis" of a bore which is circular when formed, but distorted in a running engine. By this is meant that if a reference point on the wall of a conventional bore is at a distance beyond the nominal radius of the bore, a corresponding reference point for a bore formed in accordance with the invention, at ambient temperature, will lie within the nominal radius by the same distance, and vice versa.

Accordingly, a second aspect of the present invention provides a method of forming a cylinder bore in an engine cylinder block, the bore being circular in cross section with a longitudinal central axis and having radius r when a cylinder head is mounted on the cylinder block and the engine is at its normal operating temperature, which method comprises the steps of a for a cylinder bore in a similar cylinder block, which bore is circular and has radius r when not subject to cold or hot distortion, determining the internal profile when a cylinder head is mounted on the cylinder block and the engine is at its normal operating temperature; b for a plurality of reference points around the internal wall of that bore at the operating temperature of the engine, determining the difference between the nominal radius r and the actual radius r'; and c forming a bore at ambient temperature in which each corresponding reference point around the internal wall of the bore is at a distance 2r - r' from the longitudinal central axis.

The internal profile of a conventional cylinder bore in a running engine may be determined either by direct or indirect measurement, or by mathematical modelling techniques such as finite element analysis.

The more precise a determination which can be made of the distorted profile of a conventional bore in a running engine, the better an antithesis bore can be formed, within the tolerance limits of the machine tool which is used.

An experimental determination of bore profile may be made by any suitable technique, for example by the use of a piston which is provided with a piston ring fitted with one or more transducers. Other known experimental techniques may of course be also used.

The invention also provides an engine cylinder block which has a bore of non-round shape at ambient temperatures characterised in that the bore distorts to a round shape under the influence of cold distortion and hot distortion.

When the bore has been formed by machining, it may optionally be modified to change its surface characteristics to improve oil consumption or reduce engine emissions. For example a cross-hatch pattern may be formed on the bore wall by means of known techniques such as the use of a laser or by chemical etching. A suitable laser honing technique is described in European patent application number 0 565 742.

The invention will now be further described, by way of example, with reference to the following drawing in which: Figure 1 shows sections through a conventional engine block; Figure 2 is a section through an engine block having a cylinder bore formed using a conventional method, at the operating temperature of the engine; and Figure 3 is a section through an engine block having a cylinder bore formed in accordance with one aspect of the method of the present invention.

The cylinder block 10 shown in Figure la has four cylinder bores 2 defined by bore walls 3. The bore walls 3 are surrounded by a water jacket 12. Cylinder head bolts 4 are used to secure the cylinder head to the cylinder block 2. The volume of water (and hence the cooling effect) in the water jacket 12 around the wall 3 of each bore 2 differs for different points around the circumference of the bore. This difference causes hot distortion of the cylinder bore.

Similarly the forces resulting from tightening of the cylinder head bolts 4 are unequal around the bore. This inequality of forces produces cold distortion of the bore.

In the cylinder block 2 shown in Figure lb, each cylinder head bolt 4 is attached to the wall 3 of a bore 2 by a boss 5. This close attachment of the cylinder head bolts causes more extreme cold distortion than for the cylinder block in Figure la. However both hot and cold distortion do occur for both cylinder blocks.

The conventional engine block 10 shown in Figure 2 has a cylinder bore 2 defined by a wall 6. The bore 2 is manufactured with a circular cross sectional shape of radius r as shown by the broken line 8. The bore 2 as manufactured is a good fit for a piston (not shown) which will reciprocate in the bore 2.

When the engine block 10 has a cylinder head secured to it by means of bolts 4 cold distortion occurs as a result of the tightening of the bolts. When the engine is at normal running temperature, thermal stresses cause hot distortion of the wall 6. Hot distortion is uneven around the perimeter of the wall 6 because of the presence of a cooling jacket 12.

The nett effect of the cold and hot distortion forces is to cause the wall 6 of the bore 2 to adopt a configuration which is distorted significantly from its ideal circular shape 8. For ease of illustration the deviations from circularity are shown greatly exaggerated in both of the figures of the drawing.

For a reference point P on the wall 6 of the reference cylinder block 10, a distance r' is determined of the point P from the longitudinal centre axis A of the bore 2.

The distance r' will vary according to where around the perimeter of the bore 2 the distance is determined. If distances r' are determined for a sufficient number of reference points P, the cross-sectional shape of the bore 2 can be known sufficiently well for that information to be used in the design and manufacture of an antithesis (or "negative") bore as illustrated in Figure 3.

The profile of the bore wall 6 could be calculated using finite element analysis and knowledge of the material properties of the engine block 10 and bolts 4, and the temperatures and forces acting on these materials when the engine is running. Additionally or alternatively the profile could be determined experimentally, for example by the use of transducers mounted on a piston ring, or by optical or other measurement techniques.

Referring now to Figure 3, a bore 2a is formed in an engine block 10a by a single point boring operation which is controlled by a numerical computer (NC). The profile of the reference bore 2 is stored in the memory of the NC, together with the radius r which the bore 2a is preferred to adopt in use when the engine is at normal operating temperature.

The NC controls the boring operation so that the bore profile which is produced is the antithesis of the bore profile shown in Figure 2. Thus reference point P in Figure 2 is at distance r' from the central axis A (r' being a shorter distance than r). The bore 2a is formed such that the corresponding reference point P' is at a distance r'' from the central axis A (r'' being a greater distance than r). The distance r'' is equal to 2r - r'.

The bore 2a thus deviates from a circular cross section at ambient temperature in the absence of the cylinder head bolts. However when the cylinder head is attached, and when the engine is running, the forces caused by hot and cold distortion cause the bore 2a to adopt a profile which is more nearly the preferred circular shape shown by the dashed line 8.

The performance of the bore 2a may be evaluated in relation to a parameter such as level of oil consumption, generation of particulates and so forth. The results of the evaluation may be fed back into the NC program and iterative changes made to obtain an optimised set of parameters based on, for example, minimum oil consumption.

The optimised parameters may then be used as the basis for all further production.

Once the reference shape of the bore 2 is programmed in, the NC can control the manufacture of many correctly shaped cylinder bores 2a without the need to bolt a fixture on to simulate cold distortion, and without the need to heat the cylinder block 10a to simulate hot distortion.

The invention therefore provides a simple method for forming cylinder bores which are round in use in a running engine.

Although the invention has for convenience been described with reference to circular bores in which circular pistons are to be located, it is to be understood that the invention is not limited to this embodiment. The techniques described may be used to form bores of any desired shape, for example elliptical bores, by determination of the actual bore profile when the engine is running and using this information to control the boring operation to produce the appropriate antithesis bore shape.

Claims (4)

1. A method of forming an engine cylinder bore which is circular in cross section when the engine is running at its normal operating temperature, the method comprising machining the bore at ambient temperature to have a noncircular cross section so that hot and cold distortion cause the bore to become more nearly perfectly circular in cross section.

2. A method of forming a cylinder bore in an engine cylinder block, the bore being circular in cross section with a longitudinal central axis and having radius r when subject to hot distortion and cold distortion, which method comprises the steps of a for a cylinder bore in a similar cylinder block, which bore is circular and has radius r when not subject to cold or hot distortion, determining the internal profile when the cylinder head is subjected to hot and cold distortion; b for a plurality of reference points around the internal wall of that bore, determining the difference between the nominal radius r and the actual radius r' when subject to cold and hot distortion; and c forming a bore at ambient temperature in which each corresponding reference point around the internal wall of the bore is at a distance 2r - r' from the longitudinal central axis.

3. A method of forming an engine cylinder bore which is circular in cross section when the engine is running at its normal operating temperature, substantially as herein described with reference to the drawings.

4. An engine having an engine block which has a cylinder bore that is circular in cross section when the engine is running at its normal operating temperature, obtainable by the method of any one of Claims 1 to 3.