RT-flex Control ElementsBasic Engine TrainingWorking Principle of 2-StrokeEngineChapter 10-2二冲程柴油机工作原理RT-flex Control Elements Definition定义•A 2-stroke engine is an engine type that makes a working(power) stroke during each revolution of the crankshaft.•This means that each work process needs one revolutionof the crankshaft or two strokes of the piston. This iscontrary to the 4-stroke engine, where two revolutions ofthe crankshaft or four strokes of the piston are needed toaccomplish the process.•二冲程柴油机是发动机类型之一，它在曲轴每旋转一周的期间内可完成一次工作（做功）行程。
•这意味着，每个做功过程需要曲轴转动一周，或者活塞完成两个冲程。
这方面与４冲程柴油机是不同的，后者完成一次做功过程需要曲轴旋转两周，或者活塞要完成四个冲程。
RT-flex Control Elements Disadvantages of the cross flow systems•only a limited stroke is possible. The air has to flow upwardsand downwards during the little time available •an inadequate scavenging of the cylinder. Especial in theupper part of the cylinder rest gasses are remaining•Short cut of air. Part of the supplied air is flowing straight to theexhaust ports despite the inlet ports are directed upwards•Loss of air. After the inlet ports are closed the exhaust ports are still open and air will flow out the cylinder. The effectivecompression stroke will start later. Oscillating or rotating valves in the outlet are sometimes used to prevent this loss•Pistons with a long skirt are needed to keep the ports closedwhen the piston is in top position, to prevent a short cut between inlet and exhaust ports横流扫气的缺点冲程长度受到限制，空气不得不在短时间内先向上，再向下流动••气缸内扫气不够充分，尤其是在气缸上部会有静止的气体存留•空气会发生短路，尽管进气口指向上方，仍会有一部分供入的空气直接流向排气口•有空气损失，进气口关闭后，排气口尚未关闭，因而空气会流出气缸，有效压缩行程始点将推迟。
为避免此种损失，有时会采用装在出气道内的摆动阀或转阀措施•活塞组件需要有长活塞裙，以便在活塞处于顶部位置时保持气口关闭，从而防止进气口和排气口之间发生短路RT-flex Control Elements Advantages of this system•A much longer stroke is possible. The air only has to flow upwards,which is also a more natural flow for air with a higher temperature•Less air needed because of fewer losses •Better cleaning of-and less rest gasses in the cylinder. Only a littlerest gas under the exhaust valve is possible •Lower inlet ports needed (8 % instead of 10 % of the stroke). Becausethe ports are now situated over the whole circumference they can be lower. This makes the effective compression somewhat longer•Improved combustion because the rotating movement still continuesduring the fuel injection. Mixing of air and fuel is faster and more intense•No need for pistons with a long skirt, short cut between ports andvalve is not possible直流扫气系统的优点•能得到的行程要长得多。
空气只需要向上流动，由于空气温度较高，这种流动方向更加自然•由于空气损失较少，需要的空气量也较少•气缸内扫气效果好，残留燃气较少。
仅在排气阀底下可能有少量残留燃气留下•需要的进气口高度较低（只占行程的８％，而不是１０％）。
此时，由于气口能够在整个圆周长度上布置，因而高度可以较低。
这样做可以使有效压缩行程有所加长•燃烧可得到改善，因为燃油喷射过程中仍继续存在（空气）旋转运动，空气和燃油的混合更迅速更加强烈•活塞组不需要有长活塞裙，进气口与排气阀之间不可能发生短路RT-flex Control ElementsScavenging Ratio Scavenging Ratio •R = supplied volume air per process and per cylinder/volume of cylinder •To clean the cylinder as good as possible, it is necessary that the volume air supplied to the cylinder is more than the cylinder volume.•For cross flow: R = 1.5 to 1.6•For uniflow: R = 1.2 to 1.3•For turbocharger: R = 2.5 to 3.5扫气率•扫气率Ｒ＝每个循环供入每个气缸的空气体积与气缸容积之比•为了使气缸扫气效果尽可能良好，供入气缸的空气量（体积）必须大于气缸容积•横流扫气时：Ｒ＝１．５至１．６•直流扫气时：Ｒ＝１．２至１．３•有涡沦增压器时：Ｒ＝２．５至３．５RT-flex Control Elements Characteristics of 2-Stroke•suitable for high outputs, because of a power stroke per each revolution •normally used for low speed. This gives more time for the scavenging process•of the time of the cross head design, fits best to the high output and low speed 二冲程的特点•适用于高功率的机型，因为每转动一周便有一次做功行程•通常用于低速柴油机。
此种机型有较多的时间可用于扫气过程•当今的十字头结构设计，最适合于高功率和低速机RT-flex Control Elements Assessment测评试题1.Describe/explain the 2-stroke process for an engine with cross-flow.描述／解释横流扫气柴油机的２冲程工作过程。
the devices which were used to supply the necessary air tothe cylinders. Why are these not longer needed?举出曾经使用过的向气缸供给必需的空气的各种装置。
为什么这些装置现已不再需要？3.Why does a 2-stroke needs an auxiliary blower while this is notneeded for a 4-stroke engine?为什么２冲程需要辅助鼓风机，而４冲程柴油机却不需要？ the disadvantages and limitations of an engine with cross-flow compared to an engine with uniflow.列举出横流扫气柴油机与直流扫气柴油机相比较的缺点和局限性。
5.Draw on scale an uniflow scavenging system with a s/d= 2.5 Theheight of the ports is 8% of the stroke, while the exhaust valveopens 20% of the stroke BBDC. Measure the corresponding crankangles. Take for the cylinder diameter 50 mm.按比例给出ｓ／ｄ＝２．５的直流扫气系统图。
扫气口的高度为行程的８％，而排气阀在行程下止点前（ＢＢＤＣ）２０％处开放。
测量相应的各个曲柄角度。
取气缸直径为５０ｍｍ绘出。