Straight-six Engine - Crankshaft Design

Crankshaft Design

Crankshafts on six-cylinder engines generally have either four or seven main bearings. Larger engines and diesels tend to use seven bearings because of high loadings and to avoid crankshaft flex. Because of the six-cylinder engine's smooth characteristic, there is a tendency for a driver to load the engine at low engine speeds. This can produce crankshaft flex in four main bearing designs where the crank spans the distance of two cylinders between main bearings. This distance is longer than the distance between two adjacent main bearings on a V6 with four mains, because the V6 has cylinder bores on opposite banks which overlap significantly; the overlap may be as high as 100%, minus the width of one connecting rod (1.00" or so). In addition, modern high-compression engines subject the crankshaft to greater bending loads from higher peak gas pressures, requiring the crankthrows to have greater support from adjacent bearings, so it is now customary to design straight-sixes with seven main bearings.

Many of the more sporty high-performance engines use the four bearing design because of better torsional stiffness (e.g., BMW small straight 6, Ford's Zephyr 6). The accumulated length of main bearing journals gives a relatively torsionally flexible crankshaft. The four main bearing design has only six crank throws and four main journals, so is much stiffer in the torsional domain. At high engine speeds, the lack of torsional stiffness can make the seven main bearing design susceptible to torsional flex and potential breakage. Another factor affecting large straight-six engines is the end-mounted timing chain which connects any camshafts to the crankshaft. The camshafts are also quite long and subject to torsional flex as they in turn operate valves alternately near the front of the engine and near the rear. At high engine speeds, camshafts can flex torsionally in addition to the crankshaft, contributing to valve timing for the cylinders furthest from the cam drive becoming inaccurate and erratic, losing power, and in extreme cases resulting in mechanical interference between valve and piston — with catastrophic results. Some designers have experimented with installing the timing chain/gears in the middle of the engine (between cylinders 3 and 4) or adding a second timing chain at the rear of the engine. Either method can solve the problem at the cost of additional complexity.

Another factor reducing the ability of the large six-cylinder engines to achieve high speed is the simple geometric reality of a relatively long stroke (undersquare) design. A straight-six is a long engine, and the designer is usually encouraged to make it as short as possible, while height is not usually a problem. Hence, the tendency to use a longer stroke and smaller bore than in a V engine to achieve a given capacity. By contrast, a long-stroke V engine tends to become too wide, which encourages increasing the bore rather than the stroke to increase displacement. The typically longer stroke of the straight-six increases crank throw and piston speed, and so tends to reduce the rpm rating of the engine.