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The Chrysler Hemi engine, known by the trademark Hemi, is a series of V6 and V8 engines built by Chrysler with a hemispherical combustion chamber. Three different types of Hemi engines have been built by Chrysler for automobiles: the first (known as the Chrysler FirePower engine) from 1951 to 1958, the second from 1964 to 1971, and the third beginning in 2003. Although Chrysler is most identified with the use of "Hemi" as marketing term, many other auto manufacturers have incorporated similar designs. During the 1970s and 1980s, Chrysler also used the ''Hemi'' name for their Australian-made Hemi-6 Engine and applied it to the 4-cylinder Mitsubishi 2.6L engine installed in various North American market vehicles. ==Hemispherical engines== (詳細はcylinder head ("hemi-head") gives an efficient combustion chamber with minimal heat loss to the head, and allows for two large valves. However, a hemi-head allows no more than two valves per cylinder, and these large valves are necessarily heavier than in a multi-valve engine. The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design. Since the combustion chamber is virtually a hemisphere, a flat-topped piston would yield too low a compression ratio unless a very long stroke is used, so to attain desired compression ratio the piston crown is domed to protrude into the head at top dead center, resulting in a combustion chamber in the shape of the thick peel of half an orange. The hemi-head design places the spark plug at the center of the chamber to promote a strong flame front. However, if (as is typical) the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly. Thus, hemi-heads, because of their lack of squish, are more sensitive to fuel octane rating; a given compression ratio will require a higher octane rating to avoid detonation in a hemi engine than in a conventional engine. The hemi head usually has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and overhead cam engines. This adds to the overall width of the engine, limiting the vehicles in which it can be installed. Significant challenges in the commercialization of engines utilizing hemispherical chambers revolved around the design of the valve actuation, and how to make it effective, efficient, and reliable at an acceptable cost. This complexity was referenced early in Chrysler's development of their 1950s hemi engine: the head was referred to in company advertising as the ''Double Rocker'' head. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Chrysler Hemi engine」の詳細全文を読む スポンサード リンク
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