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UNIT FOUR

By Tom Warren,2014-10-24 15:07
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UNIT FOUR

    UNIT FOUR

    Engine Classification

    NEW WORDS

    Complete 完成,结束 considerable 相当大?多?的 single-acting 单动的,单作用的 crankpin 曲柄销,曲轴销 develop 发展,发出,产生 manufacture 制造,加工,生产 crankshaft 曲轴,曲柄轴 bank 一排,组,机组 vertical 垂直的,直立的 rigid 刚性的,刚硬的,不易弯的 upstroke ?活塞?上行程 stiff 刚性的,坚硬的,非弹性的 downstroke ?活塞?下行程 vary 变化,使多样化,改变 complicated 复杂的, 结构复杂的 rotate 旋转,转动,使旋转 motorship 内燃机船,汽船 occupy 占有,占据,拥有 crosshead 十字头,滑块 blast 一阵,一股,气流 horizontal 水平的,卧式的,地平的 medium 中间的,中等的,适中的 encircle 环绕,包围,围绕

    PHRASES AND EXPRESSIONS

    Fuel injection 燃油喷射 by far 最…,…得多 divide into 分成…,分为… gas-burning engine 汽油机,可燃气体发动机 be based on 以…为基础,根据… cylinder-in-line 直列式气缸 piston stroke 活塞行程,冲程 a sufficiently rigid frame 足够刚性的机架 for short 简称,为简略起见 radial engine 径向式发动机,星形发动机 (be) away from 离开…,远离,不在…处 as many as 多达…,达…之多 trunk-piston engine 筒状活塞式发动机 the great majority of 大部分…,大多数…

    The engines can be classified in the following several ways: (1) operating cycle, (2) piston action, (3) piston connection, (4) cylinder arrangement, (5) method of fuel injection, and (6) speed.

     Operating cycle. Diesel and gas-burning engines can be divided into two groups based on the number of piston strokes per cycle, either four or two. An engine which needs four strokes to complete one cycle is a four-stroke cycle engine or, for short, a four-cycle engine (Fig.3). If it needs only two strokes to complete a cycle, it is called a two-stroke cycle engine or, for short, a two-cycle engine. Thus, a two-cycle engine fires twics as often as a four-cycle engine.

     Piston action. An engines piston action may be classified as (a) single-action, (b) double-action, (c)opposed-piston. Single-acting engines use only one end of the cylinder and one face of the piston to develop power. This working space is at the end away from the crankshaft, that is, at the upper end of a vertical engine.

     Double-acting engines use both ends of the cylinder and both faces of the piston to develop power on the upstroke as well as on the downstroke. The construction is complicated; therefore, double-acting engines are built only in large and comparatively low-speed units, generally to power motorships.

     An opposed-piston engine has cylinders in each of which two pistons travel in opposite directions. The combustion space is in the middle of the cylinder between the pistons. There are two crankshafts; the upper piston drive one, the lower pistons the other. Note that each piston is single-acting; that is, it develops power with only one face of the piston.

    Piston connection, the piston may be connected to the upper end of the connecting rod either directly (trunk piston type), or indirectly (crosshead type).

    In trunk-piston engines, a horizontal pin within the piston is encircled by the upper end of the connecting rod. This by far the most common construction.

    In crosshead-type engines, the piston fastens to a vertical piston rod whose lower end is attached to a sliding member called a crosshead, which slides up and down in guide. The crosshead carries a

    crosshead pin which is encircled by the upper end of the connecting rod. This more complicated construction is required in double-acting engines. It is also used in some large, slow-speed, single-acting engines.

    Cylinder arrangement (Fig.4). The four basic cylinder arrangements of a diesel or gas-burning

engine are: (a) cylinder-in-line, (b) v-arrangement, (c)flat, and (d) radial.

    A cylinder-in-line arrangement. This is the simplest and most common arrangement, with all cylinders arranged vertically in line. This construction is used for engines having up to 12 cylinders. Engines are also built with horizontal cylinders, usually one or two, in a few cases with three cylinders.

    If an engine has more than eight cylinders, it becomes difficult to make a sufficiently rigid frame and crankshaft with an in-line arrangement. Also, the engine becomes quite long and takes up considerable space. The v-arrangement, with two connecting rods attached to each crankpin, permits reducing the engine length by almost one-half, thus making it much more rigid, with a stiff crankshaft. It also costs less to manufacture and install. This is a common arrangement for engines with eight, twelve, and sixteen cylinders. Cylinders lying in one line are called a bank, and the angle between the banks

    may vary, in manufacturing practice, from 30?to 120?, the most common angles being between 40?

    to 75?. (A complete circle is 360?).

    A flat engine is a v-engine with the angle between the banks increased to 180?. This

    arrangement is used where there is little headroom, as in trucks, buses, and rail cars. Flat engines are also called opposed-cylinder engines.

    In a radial engine all the cylinders are set in a circle and all point toward the center of the circle. The connecting rods of all the pistons work on a single crankpin, which rotates around the center of the circle. Such a radial engine occupies little floor space. By attaching the connecting rods to a master disk surrounding the crankpin, as many as twelve cylinders have been made to work on a single crankpin.

    Method of fuel injection. Diesel engines are divided into air-injection engines and solid or mechanical injection engines. Air-injection engines use a blast of highly compressed air to blow the fuel into the cylinder. Air injection was commonly used on early diesel engines but with the development of solid-injection systems the air-injection engine is rapidly disappearing.

    Speed. All diesel and gas-burning engines may be divided into three classes according to speed; low-speed, medium-speed, and high-speed engines. Automotive diesel engines often run faster than 1200 rpm, but the great majority of other engines run between 350 and 1200 rpm and are termed medium-speed engines.

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