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.

(1)   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. 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 twice as often as a four-cycle engine.

(2)   Piston action

An engine’s piston action may be classified as (a)single-action, (b)double-action, or(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.

Donble-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 pistons 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.

(3)   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 an down in guides. 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.

(4)   Cylinder arrangement

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°and 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-cylin-der”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.

(5)   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 use on early diesel engines but with the development of solid-injection systems the air-injection engine is rapidly disappearing.

(6) 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 1,200 rpm, but the great majority of other engines run between 350 and 1,200 rpm and are termed medium-speed engines.

参考译文：

发动机可以根据以下几个方法进行分类：（1）工作循环方式，（2）活塞做功方式，（3）活塞连接方式，（4）气缸布置形式，（5）燃油喷射方式，（6）速度。

（1）工作循环方式

根据每一次工作循环活塞运动的行程数，柴油发动机和汽油发动机可以分为两种类型，四冲程或者两冲程。发动机完成一个工作循环，需要四个行程，就称为四冲程循环发动机，或者简称四冲程发动机。如果发动机完成一个工作循环，只需要两个行程，就称为两冲程循环发动机，或者简称两冲程发动机。这样来说，两冲程发动机的点火次数是四冲程发动机的两倍。

（2）活塞做功方式

发动机活塞的做功方式可以分为（a）单向，(b)双向，（c）对置。

单向做功的发动机只利用气缸的一端以及依靠活塞的一个面提供动力。工作区域在远离曲轴的一端，也就是说，在一台垂直的发动机的上端。

双向做功的发动机在上升和下降的冲程时，同时利用气缸的两端以及依靠活塞的两个面提供动力。其结构是相当复杂的，因此双向做功的发动机只适用于大型和相对低速的结构中，比如说，提供内燃机船的动力。

对置式发动机的每个气缸中都用两个活塞沿相反方向运动。燃烧的空间位于气缸中部，在两个活塞之间。有两根曲轴；上面的活塞驱动一根，下面的活塞驱动另一根。要注意的是每个活塞都是单向做功，也就是说，动力只由活塞的一个面来提供。

（3）活塞连接方式

活塞与连杆上端的连接，既可以是直接连接，也可以使间接连接。

在筒状活塞式的发动机中，活塞与连杆上端通过一个水平销子连接。这是最常见的一种结构。

在十字头型发动机中，活塞连接在一根垂直的活塞杆上，活塞杆的下端又连接在一个叫做“十字头”的滑动构件上，十字头在导轨中上下滑动。十字头通过一个十字销与连杆上端连接。这种非常复杂的结构多用于双向做功的发动机。它也用在一些大型的、低速的、单向做功的发动机上。

（4）气缸布置形式

柴油机和汽油机常见的气缸布置形式是：直列式、V型、水平式、径向式。

直列布置形式。这是一种最简单和最普通的布置形式，所有的气缸垂直的排列成一条线。这种结构，发动机最多用到12缸。发动机也制成水平的形成，常见的一缸或者两缸，也有一些事三缸。

如果发动机超过了8缸，就很难保证在曲轴也成直线布置得同时，制造出足够刚性的缸体。而且，发动机变得相当长，占据了相当多的空间。V型排列，由于每个曲柄销上有两根连杆，所以可使发动机的长度几乎缩短一半，这样就使发动机的刚性更大，曲轴也更坚固。这种形式只需要很少的维护工作，安装也方便。这在8缸，12缸，16缸发动机上时很常见的布置形式。气缸在被称作“箱体”的部件里排成一列，不同类型的箱体的角度是根据制造的需要而变化的，从30°到120°，大多数角度是从40°到75°。

水平布置得发动机是一种将箱体的角度增加到180°的形式。这种气缸排列通常用在头部空间很小的机动车上，如货车、公共汽车以及有轨电车上。水平布置发动机也称为“对置式”发动机。

在径向式发动机里，所有的气缸都布置在一个圆圈里，而且都指向圆心。所有的活塞连杆都在同一个曲柄销上工作，绕圆心旋转。这样一来，径向式发动机占据的空间很小。通过把连杆连接到包住曲柄销的一个主盘上，十二个气缸便可在同一个曲柄销上工作。

（5）燃油喷射方法

柴油机分为空气喷射发动机和固体喷射或者机械式喷射发动机。空气喷射发动机利用一股高压气体将燃料吹到气缸内。空气喷射发动机在早期的柴油机上应用，但随着固体喷射系统的发展，空气喷射的发动机已经被淘汰了。

（6）速度

根据速度，柴油发动机和汽油发动机可以分为三个等级：低速、中速和高速发动机。汽车柴油发动机的速度一般高于1200转/分，但是有些发动机转速在350-1200转/分之间，称作中速发动机。