15m3船用LNG储罐设计与分析外文翻译资料

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7 Pressure charging

7.增压

A naturally aspirated engine draws air of the same density as the ambient atmosphere. Since this air density determines the maximum weight of fuel that can be effectively burned per working stroke in the cylinder, it also determines the maximum power that can be developed by the engine. Increasing the density of the charge air by applying a suitable compressor between the air intake and the cylinder increases the weight of air induced per working stroke, thereby allowing a greater weight of fuel to be burned with a consequent rise in specific power output.

自然吸气发动机吸入的空气密度与周围大气的密度相同。由于空气密度决定了气缸内每个工作冲程能有效燃烧的最大燃油重量，它也决定了发动机可以产生的最大功率。通过在进气口和气缸之间应用合适的压缩机来增加增压空气的密度，从而增加每个工作冲程引入的空气重量，因此使更大重量的燃料能够燃烧，所以比功率输出提高了。

This boost in charge air density is accomplished in most modern diesel engine types by exhaust gas turbocharging, in which a turbine wheel driven by exhaust gases from the engine is rigidly coupled to a centrifugal type air compressor. This is a self-governing process which does not require an external governor.

在大多数现代柴油机类型中，增压空气密度的增加是通过废气涡轮增压来实现的。其中由发动机废气驱动的涡轮与离心式空气压缩机牢固耦合。这是一个自动过程，不需要外部操作。

The power expended in driving the compressor has an important influence on the operating efficiency of the engine. It is relatively uneconomical to drive the compressor direct from the engine by chain or gear drive because some of the additional power is thereby absorbed and there is an increase in specific fuel consumption for the extra power obtained. About 35 per cent of the total heat energy in the fuel is wasted to the exhaust gases, so by using the energy in these gases to drive the compressor an increase in power is obtained in proportion to the increase in the charge air density.

驱动压缩机所消耗的功率对发动机的工作效率有重要影响。通过链条或齿轮传动从发动机直接驱动压缩机是相对不划算的，因为一些额外的动力由此被吸收，并且对于所获得的额外动力而言，特定的燃料消耗会增加。燃料中大约35%的总热能被浪费到废气中，因此通过利用这些气体中的能量来驱动压缩机，可以获得与增压空气密度的增加成比例的功率增加。

The turbocharger comprises a gas turbine driven by the engine exhaust gases mounted on the same spindle as a blower, with the power generated in the turbine equal to that required by the compressor

涡轮增压器与鼓风机安装在同一主轴上，它由发动机废气驱动的燃气轮机所驱动，涡轮中产生的功率与压缩机所需的功率相等。

There are a number of advantages of pressure charging by means of an exhaust gas turboblower system:

·a substantial increase in engine power output for any stated size and piston speed, or conversely a substantial reduction in engine dimensions and weight for any stated horsepower.

·an appreciable reduction in the specific fuel consumption rate at all engine loads.

·a reduction in initial engine cost.

·increased reliability and reduced maintenance costs, resulting from less exacting conditions in the cylinders.

·cleaner emissions (see section below).

·enhanced engine operating flexibility. Larger two-stroke engines may be equipped with up to four turbochargers, each serving between three and five cylinders.

通过废气涡轮增压器系统进行增压有许多优点：

·它可以使任意尺寸或活塞转速的发动机功率输出都大幅增加，或者使规定马力的发动机的尺寸和重量都大幅下降。

·在发动机工作的情况下，单位燃油消耗率显著降低。

·降低了初始发动机成本。

·它可以使气缸的严苛的工作条件有所改善，使其可靠性提高，维护成本降低。

·排放物更清洁（请参阅以下部分）。

·增强了发动机的操作灵活性。

较大的二冲程发动机最多可配备四个涡轮增压器，每个涡轮增压器为三到五个气缸工作。

FOUR-STROKE ENGINES

四冲程发动机

Exhaust gas turbocharged single-acting four-stroke marine engines can deliver three times or more power than naturally aspirated engines of the same speed and dimensions. Even higher power output ratios are achieved on some engine types employing two-stage turbocharging where turbochargers are arranged in series. At one time almost all four-stroke engines operated on the pulse system, though constant pressure turbocharging has since become more common as it provides greater fuel economy while considerably simplifying the arrangement of exhaust piping.

相同速度和尺寸的废气涡轮增压单作用于四冲程船用发动机的功率是自然吸气发动机的功率的三倍或更多。在某些采用两级涡轮增压的发动机类型中，涡轮增压器串联布置，可以获得更高的功率输出比。过去几乎所有的四冲程发动机都采用脉冲系统，但由于恒压涡轮增压提供了更大的燃油经济性，同时大大简化了排气管的布置，因此它变得更加普遍。

In matching the turboblower to the engine, a free air quantity in excess of the swept volume is required to allow for the increased density of the charge air and to provide sufficient air for throughscavenging of the cylinders after combustion. For example, an engine with a full load bmep of 10.4 bar would need about 100 per cent of excess free air, about 60 per cent of which is retained in the cylinders, with the remaining 40 per cent being used for scavenging. Modern engines carry bmeps up to 28 bar in some cases, requiring greater proportions of excess air which is made possible by the latest designs of turbocharger with pressure ratios as high as 5:1.

在匹配涡轮增压器与发动机时，自由风量要大于扫气量，以增加增压空气的密度，并提供足够的空气用于燃烧后气缸的扫气。例如，满载空燃比为10.4 bar的发动机将需要约100％的过量空气，其中约60％的空气保留在气缸中，其余的40％用于扫气。由于最新设计的涡轮增压器的压力比可以高达5:1，导致现代发动机在某些情况下的空燃比最高可达28bar，这就需要更大比例的过量空气，

Toensure adequate scavenging and cooling of the cylinders, a valve overlap of approximately 140ris normal with, in a typical case, the air inlet valve opening at 80r before top dead centre and closing at 40r after bottom dead centre. The exhaust value opens at 50r before bottom dead centre and closes at 60rafter top dead centre.

为了确保对气缸进行适当的扫气和冷却，气门重叠约140°是正常的，通常情况下，进气门在上止点之前的80°处打开，而在下止点之后的40°处关闭。 排气则在下止点之前的50°处打开，并在上止点之后的60°处关闭。

For a low-rated engine, with a bmep of 10.4 bar compared with 5.5 bar for a naturally aspirated engine, a boost pressure of some 0.5 bar is required, corresponding to a compressor pressure ratio of 1.5:1. As average pressure ratios today are between 2.5 and 4, considerable boost pre

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