第三章液压系统的能量和功率（Energy and Power in Hydraulic Systems）3.1概述（INTRODUCTION）在液压系统中，油液在大气压下进入油泵，这个压力称为吸油压力。
当油液通过油泵时，油液压力的增加使其势能显著增加。
当油液流过管道、阀和管接头时，因摩擦作用引起这些能量损失。
摩擦能量的损失表现为热能。
在输出装臵（液压执行元件）上剩余的能量传递给负载完成有用的工作。
这实质上是能量传递在液压系统中的循环。
油泵将能量加入系统并传递到系统执行元件用来驱动外负载（In hydraulic systems, fluid enters the pump at below atmospheric pressure, called thesuction pressure. As the fluid passes through the pump, its potential energy increases as evidenced by an increase in fluid pressure. Some of this energy is lost due to friction as the fluid flows through pipes, valves, and fittings. These frictional energy losses show up as heat energy and therefore are accounted for. At the output device（hydraulic actuator）the remaining energy is transferred to the load to perform useful work. This is essentially the cycle of energy transfer in a fluid power system. Energy is added to the system by the pump and removed from the system via the actuator as it drives the output load）。
一个液压系统本身是没有能源的。
这个能源是驱动油泵的原动机（如电机或一种内燃机）。
事实上，一个液压系统仅仅是一个能量传递系统。
为什么不取消液压传动而直接将原动机与机械设备连接起来？回答是在传递功率方面液压系统优点非常强。
这些优点包括调速方便、变向容易、易于过载保护、功率－单位重量比高以及发生故障的情况下危险性小（A hydraulic system is not a source of energy. The energy source is the prime mover（such as an electric motor or an internal combustion engine）which drives the pump. Thus, in reality, a hydraulic system is merely an energy transfer system. Why not, then, eliminate hydraulic and simply couple the mechanical equipment directly to the prime mover. The answer is that a hydraulic system is much more versatile in its ability to transmit power. This versatility includes advantages of variable speed, reversibility, overload protection, high horsepower per weight ratio, and immunity to damage under stalledconditions）。
能量守恒定律表明能量既不产生也不消失。
这就意味着系统中任何部位能量的总和保持恒定。
这就意味着系统中任何部位能量的总和保持恒定。
能量总和包含因高度和压力而表现出的势能与因速度而表现出的动能。
如果所有的能量改变了，那么真正说明液压系统总是能量守衡的。
如果所有的能量改变了，那么真正说明液压系统总是能量守衡的。
这将用伯努利原理来完成，当油液经过液压系统时注意这些变化表现在势能和动能的变化。
由于摩擦产生的能量损失变成热能，由油泵输入机械能，负载执行元件输出机械能（The law of conservation of energy states that energy can neither be created nor destroyed. This means that the total energy at any location of the system remains constant. The total energy includes potential energy due to elevation andpressure and also kinetic energy due to velocity. If all the energy changes are properly accounted for, the hydraulic system will always have an energy balance. This will be accomplished by using Bernoulli’s theorem, which keeps track of the changes which occur to the potential and kinetic energy of the fluid as it passes through the hydraulic system. Also included are energy losses due to friction which transfers into heat, mechanical energy added by the pump, and mechanical energy removed by the load actuators）。
3.2 能量守恒（CONSERV ATING OF ENERGY）能量守恒定律表明了能量既不能产生也不能消失。
其意味着系统的能量总和在任何情况下都是恒定的。
总能量包括因高度和压力而表现出的势能和因速度而表现出的动能。
我们来探讨这三种能量（The conservationof energy law states that energy can neither be created nor destroyed. This means that the total energy of system at any location remains constant. The total energy includes potential energy due to elevation and pressure and also kinetic energy due to velocity. Let’s examine each of the three types of energy）。
1.势能（EPE）（Potential energy due to elevation）：如图所示为一距离基准面高度为Z（m）、重量为W（N）的流体。
相对于基准面这个重量的流体具有相应的势能因为已经对流体作了功使其离开基准面一个距离Z （The figure shows a chunk offluid of weight W at an elevation Zwith respect to a reference plane. The weight haspotential energy （EPE ）relative to the reference plane because work would have to be done on the fluid to lift it through a distance Z ）：（3－1）注意EPE 的单位是N•m 。
2.压力能（PPE ）（Potential energy due topressure ）：如果图中重量为W （N ）的流体具有压力p（N/m 2），它就包含了压力能（If the W of fluid in figure possesses a pressure p , it contains pressure energy as represented by ）：WZ EPE ＝（3－2）其中：γ为流体的重度（where γ is the weight density ）。
PPE 的单位是N•m 。
3.动能（KE ）（Kinetic energy ）：如果图中重量为W （N ）的流体以速度v 运动，它就包含了动能，能够用下式计算（If the W of fluid in figure is moving with a velocity v , it contains kinetic energy, which can be found using ）（3－3）其中：g ＝9.81m /s 2；KE 的单位是N•m 。
重量为W （N ）的流体所具有的能量总和既不会生也不会灭。
能量的代数和E T 是常数（The total energy221v g W KE ＝γp W PPE ＝possessed by the W chunk of fluid can neither be created nor destroyed. Mathematically the total energy E T remains constant ）：（3－4）当然，能量可以从一种形式转变为另一种形式。
例如，流体可以损失高度而减小势能。
但是，将导致压力能或动能的增加（Of course, energy can change from one form to another. For example, the chunk of fluid may lose elevation and thus have less potential energy. This, however, would result in an equal increase in either pressure energy or kinetic energy ）。