When a piston is going down in an engine, it pushes air into the crankcase. When another piston is going up, it draws air out of the crankcase. Pumping air back and forth can cost horsepower. Cycle World Technical Editor Kevin Cameron explains how manufacturers reduce such losses.
One of the proud claims made by the Japanese motorcycle industry is that much of the horsepower gain achieved since the 1980s has come from the reduction of friction. That’s a good thing because any power recovered from friction can be used for intentional purposes, such as rapid acceleration. As Mr. Honda once put it, “Primarily, essentials of the motorcycle consist in the speed and thrill.” He said that in 1959 in the manual for the Benly 125 twin.
I have before me a CBR600RR crankshaft. This is a flat crankshaft because it has two up and two down; the crankpins are at 180 degrees from each other. That means in each of these “twins” the pistons are alternating. When one piston is going down, it pushes air into the crankcase. When another piston is going up, it draws air out of the crankcase.
If you look at the upper half of the crankcase, you see the four cylinders, which are normally blocked with pistons going up and down in them. When those pistons go up and down, the air that they push down into the crankcase has to go from one cylinder to the other and then back again every 180 degrees. If there is resistance-if it is hard for the air to get from one side to the other-that will resist the motion of the pistons. In fact, it is very easy to lose several horsepower at high rpm pumping air back and forth inside the crankcase.
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4 окт 2024
