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Oh, reading the page on the world's biggest engine reminded me of the reason medium and slow speed engines are so tall, the crossheads! According to the article, the crossheads are for isolating torque, which is partially true. Another reason for using the crosshead design is to acheive an "ultra long stroke". The length of a rod in comparison to the radius of a crank's throw is called rod ratio. I have noticed that steam locomotives all have a very long rod ratio, and have been curious why. Anway, rod ratios in IC engines are far shorter, but diesel engines almost always have rod ratios longer than gas engines. I suspect that it is because a long rod ratio is of greater benefit to a constant pressure cycle, and a short rod ratio is better for a constant volume cycle. What's the big deal? Rod ratio affects the acceleration-decelaration curve of a piston, and the curve determining the mechanical advantage based on the angle between a crank and a connecting rod. A con rod swings outside of a crank's radius in the upper 180 deg. of a crank's rotation, and swings on the inside a crank's radius on the lower 180 deg. of rotation. This causes the piston's accelaration-decelaration in the upper 180 deg. of crank rotation to have a much steeper, v-shaped curve, and the same curve for the lower 180 deg. to have a much more circular curve. In other words, a piston moves farther for fewer degrees of crank movement in the upper 180 deg. of crank rotation. Acording to the math involved in determining the shape of these curves for a given rod ratio, as a rod's length approaches infinity, the difference between the curves for upper and lower crank movement approaches zero, and as a rod's length becomes shorter, a maximum in difference is reached. The point of maximum mechanical advantage between a crank and a rod is achieved when there is a ninety deg. angle between the crank and the rod. In most engines, this happens around 67 deg. B/ATDC, but varies with rod length also. I infer by this that the shorter rod ratio is beneficial for a constant volume engine, because in this type of engine pressure falls off quickly after combustion. A shorter rod ratio allowes more piston movement for crank movement in the upper 180 deg of rotation, putting mechanical advantage at the piston. A long rod ratio benefits constant pressure combustion by giving more crank movement for piston movement in the upper 180 deg of crank rotation, taking advantage of the longer duration of cylinder pressure. This also puts mechanical advantage at the crank during the upper 180 deg. of rotation, which can help during compression. While medium and slow speed engines have the longest duration of pressure, most closely following true constant pressure combustion cycle, they benefit the most from an even longer rod ratio.
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