Part Ⅰ Text

Lesson One Prime Mover

The source for power is most often a generator set driven by a combustion engine which is fueled with diesel or heavy fuel oil. Occasionally one can find gas engines , and also gas turbines, steam turbines or combined cycle turbines, especially for higher power levels, in light high-speed vessels, or where gas is a cheap alternative (e.g. waste product in oil production, boil-off in LNG carriers, etc.).

In a diesel-electric propulsion system, the diesel engines are normally medium to high-speed engines, with lower weight and costs than similar rated low speed engines that are used for direct mechanical propulsion. Availability to the power plant is of high concern, and in a diesel electric system with a number of diesel engines in a redundant network; this means high reliability but also sophisticated diagnostics and short repair times.

The combustion engines are continuously being developed for higher efficiency and reduced emissions, and at present, a medium speed diesel engine has a fuel consumption of less than 200g per produced kWh at the optimum operation point as seen in Fig.1-1. Even though this is regarded to be a high utilization factor of fuel, it represents only about 40% of the energy in the fuel, the rest of the energy being removed by the exhaust or heat dissipation.

Moreover, the efficiency drops fast as the load becomes lower than 50% of MCR (Max Continuous Rating). At this working condition, the combustion is inefficient, with high NOx and SOx content, and with a high degree of soothing which increases the need for maintenance. In a diesel electric system with several diesel engines it is hence an aim to keep the diesel engines loaded at their optimum operating conditions by starting and stopping generator sets dependent on the load, with an aim to keep the average loading of each running diesel engine closest possible to its optimum load point.

New Words and Expressions

combustion [kəmˈbʌst∫ən] n. 燃烧，发火，点火

diesel[ˈdiːzəl] n. 柴油机，内燃机

turbine[ˈtəːbin,-bain] n.（涡）轮机，叶轮机，汽轮机，透平机

redundant[riˈdʌndənt] adj. 过多的，冗余的，冗长的（文章等）；累赘的

diagnostics[ˌdaiəgˈnɔstiks] n. 征候，特征

sophisticated[səˈfistikeitid] adj. 复杂的，尖端的，高级的；微妙的

emission[iˈmi∫ən] n. 发出，发射，射出，放射；传播

utilization[ˌjuːtilaiˈzei∫ən] n. 利用；效用

dissipation [ ˌdisiˈpei∫ən] n. 消散，分散

Notes

（1）“…as seen in Fig…” as（表示方式、程度、情况等）如同；像；按照

例如：

1）I work as others do.

我跟别人一样工作。

2）I remember it as it were but yesterday.

我想起这件事就仿佛是昨天发生的一样。

3）Parks are to the city as lungs are to the body.

公园对于都市来说就如同肺部对于身体一样。

“as”在英语里是一个比较常见、用法繁多的词。本课 “as” 的用法比较集中，为方便学习，特把几种基本用法归纳如下:

1）引出同位语，常与 “such” 连用

We call such metals as copper and silver good conductors.

我们称这些金属，例如铜和银，为良导体。

There are different forms of energy, such as heat energy, sound energy and electric energy.

有各种不同的能，例如热能、声能和电能。

2）用做介词

Electronics is regarded as an applied science.

电子学被认为是一门应用科学。

Electricity is especially convenient as a source of mechanical power.

电作为机械的动力源特别方便。

3）用做关系代词或关系副词，引出定语从句

This is the same material as is used in the diode.

这和用在二极管中的材料是一样的。

I want to get such a tool as he is using.

我想得到一把像他正在使用的那种工具。

He repaired the machine in the same way as we did.（这句“in the same way”所代替的是介词短语“as”，而该介词短语在主句中作状语，所以“as”作关系副词。）他采取和我们同样的方法修理机器。

4）用做从属连词，引出时间、原因、方式、比较等状语从句

As the piston moves down, the pressure in the cylinder decreases.（时间状语从句）

当活塞下移时，汽缸里的压力就减小。

As the question is of importance, we should tell the chief engineer at once.（原因状语从句）

因为这个问题很重要，所以我们必须立即告诉轮机长。

Light waves are different in frequency just as sound waves are.（方式状语从句）

正如声波一样，光波也有不同的频率。

This generator is as effective as that one (is).（比较状语从句）

这台发电机和那台发电机效率一样。

（2）“…to its optimum load point” to（表示范围、程度）到，达到；……到，……得。

例如：

1）The room was hot to suffocation.

房间热得闷人。

2）to his name be it said

（虽说是敌人然而）真是名不虚传。