Fractured accuracy

Friday, July 02, 2010

Tags :  engine-structure

heads-blocksAs can be seen from earlier articles in RET-Monitor on race engine block material, a trend can be seen towards Compacted Graphite Iron as a crankcase material. It has been used occasionally in the past on highly loaded motorsport applications but it now seems to have become more widespread.

The reasons for this are the higher mechanical strength of the material in relation to increasing combustion loads, and a broader availability of CGI combined with the ability to cast thin-wall sections. Examples of the application of CGI are Toyota, where CGI is used for its NASCAR engine blocks, and Hyundai, where in the late 1990s CGI was used as block material in its World Rally Cars.
What cannot directly be found in the available information sources though is the fact that CGI has further advantages over more traditional materials, such as aluminum or grey cast iron. One such advantage is the fact that CGI is perfectly usable as a way to optimise the main bearing cap design.

Look at existing CGI race engine blocks and you'll see that the main bearing caps have a traditionally machined split line. This split-line design has some consequences that are less than ideal. Typically the main bearing caps are made from a different material than the block.

One critical parameter in the area of the main bearings is the crankshaft bore. This needs to be highly accurate regarding roundness and straightness, so that it doesn't suffer from possible bearing damage due to misalignment. With a machined split line, therefore, a positioning feature needs to be foreseen between cap and block, where typically positioning pins or bushes are used.
Although this is a well established method, a slight clearance will remain, on which the bearing shell geometry needs to be adapted. Also, not only does the bolt force of the main bearing bolts need to carry the firing and inertia loads, it needs to provide enough clamping load to achieve the required friction in the split line to prevent micro-movements between bearing cap and block.


heads-blocks-fracture-split

This is where the advantage of CGI its mechanical properties comes in. CGI's elongation properties mean it can very easily be fracture split, resulting in a 'perfect fit' between cap and block. For con rods this has become standard procedure, but not for blocks. There are some examples of fracture-split main bearing caps in the automotive industry, but apparently this has not yet crossed over into the race engine industry.

The form fit in the fracture prevents any movement, which enables lower bolt loading and therefore offers the possibility of reducing bolt size. And the material's strength has already made it possible to reduce general wall thicknesses. So with the additional advantage of a reduced bolt size, it now becomes possible to reduce main bearing cap width. This allows some additional improvement possibilities for the crankshaft web's strength (bore size, and therefore cylinder pitch, will not change).

The fractured-split line makes the use of a positioning feature obsolete, which enables better overall roundness of the main bearing bore without the risk of a possible 'step' at the split-line location. This gives better robustness against bearing wear in this horizontal plane of the engine where, depending on cylinder number and balancing level, typically transverse inertia loading occurs.
I am certain that some design improvements in this area will be seen in upcoming race block designs.

Fig.1 - Fracture split main bearing

Written by Dieter van der Put

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