Mass reduction is a major goal in engine design. Therefore it is known to substitute aluminum for iron in certain engine components. As a common example, automotive engine blocks may be constructed with all aluminum or other light alloys. One drawback in some applications is that commonly employed aluminum alloys have a much higher coefficient of thermal expansion (CTE) than iron. For example, cast iron has a CTE of about 12.times.10.sup.-6 /K, whereas 380 aluminum has a CTE of about 21.times.10.sup.-6 /K, a factor of almost two. Therefore in the case where aluminum bearing caps support a ferrous crankshaft, the aluminum bearing caps thermally grow at a greater rate than the crankshaft as the engine operating temperature increases. This results in an increased bearing clearance and potentially unacceptable bearing life and noise generation. Greater bore clearances require larger capacity lubrication systems to compensate for oil leakage past the main bearings and to maintain adequate oil film thickness on the bearings.
A second drawback in certain applications is that commonly-employed aluminum alloys have a lower elastic modulus and strength relative to ferrous materials. This reduction may cause durability challenges, especially with high bearing loads that are typical with high output, supercharged, and diesel applications.
One solution to the thermal expansion issue, as described in U.S. Pat. No. 5,203,854, is to produce an aluminum bearing support cast with an iron core adjacent to the crankshaft bore to provide comparable coefficients of thermal expansion between the crankshaft and the bearing cap. It is proposed that the bearing clearance does not significantly vary over the range of operating temperatures and therefore noise generation is reduced. The disadvantage is that substituting aluminum with a ferrous insert involves a mass penalty for the engine.
The purpose of the present invention is to provide a main bearing support which has a CTE comparable to the ferrous crankshaft it supports, is more mass efficient than aluminum, and has a greater factor of safety than a ferrous inserted, aluminum support due to lower strains from a higher elastic modulus.