兩沖程式發動機的運作---双语

lewisfei

2-STROKE ENGINE OPERATION

        The 2-cycle engine invented by the English engineer Sir Douglas Clerk, dates back in the middle of the 18th century, such engine known also as “Clerk cycle”, has suffered some modifications from its invention.
        To produce power, an internal combustion engine must go through the following cycle: First, fuel must be delivered to the combustion area.  Second, the fuel must be compressed to achieve controlled burning.  This produces greater output than if the fuel is burned in its uncompressed state.  Third, spark must be induced to ignite the fuel.  And finally, the burned mixture must be exhausted from the combustion area.  This readies the engine for a fresh fuel charge, which restarts the repeating series of events.
        Though four-cycle engines are more complex, the process above seems more easily and fully understood in this application.  This is curious because in each cycle necessary to produce a power stroke are: cams, valves, gears, chains pumps, and other devices.
        The two stroke relies on simple basics to accomplish the same task.  Yes there are valves, even though they look different and we call them something else.  There are also pumps, though not always referred to as such. There is also ignition, but in the two stroke, it has to work twice as often and likely, twice as hard.

Two Strokes per Cycle
        For every power cycle there is one up and one down stroke of the piston, as opposed to two up and two down strokes in the four-stroke engine.  This is the main reason that a two-stroke is capable of nearly twice the output of an equal sized four-stroke.  On the other hand, to call a two-stroke a “two-cycle” is incorrect.  Correctly put, it’s two strokes/one cycle (consisting of intake, compression, power and exhaust).  
       
Scavenging Pump Purpose
Each and every  two-cycle engine has a scavenging pump.  It’s the crankcase and it performs a vital function to the operation of the engine.
        On the downward stroke of the piston, sometimes called the power stroke, three things happen.  In the first part of the stroke away from TDC, pressure from combustion is imparted on the piston top, and the force reciprocated to output shaft (crankshaft).  This is the power cycle of the engine.  Somewhere along the same down stroke, the piston travels by an open port (exhaust) and the exhaust cycle begins. The release of spent gases from combustion are helped out the port by pressure build-up from combustion and by the scavenging pump.
        Before the intake charge is transferred to the combustion chamber, it enters the crankcase by way of the inlet valve (port).
        The downstroke created by combustion moves the piston into the crankcase void, creating a higher pressure in the crankcase.  On the way down, the piston uncovers first the exhaust port and then the transfer ports.  The pressure buildup in the crankcase forces the fresh mixture into the combustion chamber via the transfer ports.  This transfer of fuel helps purge the combustion area of burned gases that did not exit upon the initial opening of the exhaust port.  This process is called scavenging, hence the name scavenging pump.
       
        Intake and Compression Cycles
        With the power and exhaust cycles complete, the intake cycle begins as the piston leaves BDC and travels upward in the cylinder.  A negative pressure is created in the crankcase by the action of the departing piston.  When the inlet valve (port) opens, fresh mixture is drawn into the crankcase area.  As the piston continues up, it covers the open transfer and exhaust ports.  The compression cycle now begins.  At a position near TDC, a spark occurs, causing the fuel mixture to burn and the process to begin all over again.  Following this same sequence of events is shown:




1. INTAKE AND COMPRESSION:        During the upward movement of the piston, trapped air over the piston compress.  Meanwhile pressure in the crankcase decreases causing a pressure lower than outside atmospheric pressure.  When the inlet port opens, air is delivered into the engine through the carburetor caused by a much higher atmospheric pressure than that inside the crankcase.  When air passes through the carburetor it mixes with gasoline coming from the carburetor’s holes, filling the crankcase with a new mixture of air-gasoline.

2. IGNITION:  When the piston is about to reach TDC, the air-gasoline mixture is burned by means of a spark induced by the sparkplug.  The expansion of the burned gases forces the piston to move downward.  This is the only event in which the engine is delivering power.

3. POWER DELIVERY:  As the piston moves downward, power is being delivered through the rod that connects the piston to the shaft, the former connects to the exterior of the engine.  The air-gasoline mixture that is contained in the crankcase starts to compress once the piston closes the inlet port.

4. EXHAUST AND INDUCTION:  Shortly after power delivery and with the piston going downward, the exhaust port is opened, allowing the burned gases to escape.  Shortly after the exhaust port is opened, the transfer ports are opened, allowing the new load of air-gasoline that was compressed in the crankcase to pass to the combustion chamber.  Besides, this process aids to a faster evacuation of the burned gases from the combustion chamber.
BEGINNING OF A NEW CYCLE:        Once the piston reaches BDC, it starts its travel upward again due to the effect of the inertia provided by ignition, starting a new cycle.  Just before the exhaust port gets to close, a considerable amount of air-gasoline mixture goes to the atmosphere.

IMPORTANT CONSIDERATIONS
        To keep a two-stroke engine operating at top efficiency, one of the most important considerations is scavenging pump integrity.  Because of its many functions, different scavenging pump faults will cause different symptoms in the engine.
        The scavenging pump operates the fuel pump, it pressurizes the transfer charge, and it creates a negative pressure causing replacement of fresh fuel from the intake tract.  Since the pump operates by pressures, both positive and negative, it needs to be sealed.  It must build and maintain both pressure and vacuum.  Areas where leaks can occur are: crankcase seals, gaskets, casting cracks, loose fasteners, and piston-to-cylinder clearance.  Any area on the downstream side of the inlet valve is where to search for losses in crankcase pressure.
兩沖程式發動機的運作

        由英國工程師 Douglas Clerk爵士發明的兩沖程發動機, 可回溯至18世紀中期, 這種又被稱為 “Clerk 沖程”的發動機, 由發明之日起, 就作了一些修改.
        為了產生能量, 內燃式發動機必須經過以下過程: 首先, 燃料必須送至燃燒區; 其次, 燃料必須被壓縮,  使燃燒得到控製. 與燃料未被壓縮而燃燒相比, 這樣會產生更大的能量. 第三, 火花必須被引入, 以燃燒燃料.  最後, 燃燒的混合物必須由燃燒區排出. 這使發動機準備好裝滿新的燃料, 重新開始.
        盡管四沖程發動機更為複雜,上述過程似乎顯得更易於在應用中全面理解.在每個為了產生能量所作的沖程當中, 凸輪, 閥門, 齒輪, 鏈式油泵及其它設施一起運作.
        為了完成同樣的功能,兩沖程只需要某些簡單的基本動作.它也同樣需要閥門盡管這些閥門看起來不一樣, 我們也可能用別的名稱來稱呼它們; 它們也同樣需要泵--盡管這些泵並不總被稱為泵;它們也同樣需要燃燒----但是兩沖程式發動機的燃燒, 與四沖程式發動機的相比, 頻率是後者的兩倍.

每個循環中的兩個沖程
        每個能量循環中,活塞均需向上及向下各作一個沖程.與此不同的是,四沖程式發動機則需向上及向下各作兩個沖程.這就是為什麼與同樣大小的四沖程發動機相比,兩沖程發動機能夠產生幾近前者兩倍能量的主要原因. 另一方面, 把兩沖程 稱為“兩循環” 是不正確的. 正確的名稱應為,它是兩個沖程/循環 (包括吸入, 壓縮, 提供能量及排氣).  
       
油泵清除目的
每個兩沖程發動機均有一個清除的油泵.  它就是曲柄箱, 對發動機的運作具有重大意義.
        活塞的向下沖程,有時被稱為動力沖程.在此過程中,會發生三件事:在離開TDC的開始部位,燃燒時的壓力作用於活塞頂部,壓力往複作用於輸出軸(曲柄軸連桿).這就是發動機的動力沖程.同時,活塞經過打開的接口(排氣) , 開始排氣沖程. 燃燒形成的壓力及清除的油泵, 有助於燃燒的廢氣排出接口.
        吸入的氣油混合物, 在進入燃燒室之前, 先由進氣閥(接口)進入曲柄箱.
        燃燒產生的向下沖程, 使活塞進入曲柄箱空間, 在曲柄箱內產生更高的壓力. 在向下的過程中, 活塞先打開排氣接口, 再打開轉移接口. 曲柄箱形成的壓力推動新鮮的氣油混合物通過轉移接口進入燃燒室.  燃料的轉移, 有助於將未能在之前排氣接口打開時排盡的廢氣清除出燃燒區域. 這一過程被叫作清除過程, 所以油泵被命名為清除油泵.
       
        吸入及壓縮循環
        當提供能量及排氣循環完成之後,活塞離開BDC,在汽缸內向上移動,開始吸入循環.活塞離開,在曲柄箱中產生負壓. 打開進入閥門(接口), 新的混合物進入曲柄箱區域. 活塞繼續向上移動, 覆蓋轉換接口及排氣接口. 壓縮循環由此開始. 在TDC附近的位置, 產生火花, 使燃料混合物燃燒, 並且重新開始整個過程. 以下描述了相同的順序:


1. 吸入及壓縮:        在活塞向上運動的過程中, 活塞上部的空氣被壓縮.  同時, 曲柄箱內壓力減少, 使其中的壓力低於外界氣壓. 當入口接口打開時, 由於大氣壓比曲柄箱內的壓力高, 空氣經化油器送至發動機. 當空氣經過化油器的時候, 它與來自化油器軟管的汽油相混合, 使曲柄箱充滿新的氣油混合物.

2. 燃燒:  當活塞即將到達TDC的時候, 火花塞引起的火花燃燒氣油混合物. 燃燒氣體的膨脹, 迫使活塞向下移動. 這是發動機輸送能量的唯一活動.

3. 能量輸送:  在活塞向下移動的過程中, 能量通過聯接活塞與輸出軸的連桿輸送. 輸出軸與發動機的外部相聯. 一旦活塞關閉進入接口, 曲柄箱內的氣油混合物開始壓縮.

4. 排氣及感應:  能量輸送不久以後, 隨著活塞向下移動, 排氣接口被打開, 使被燃燒的氣體得以逃逸. 排氣接口打開不久以後, 轉換接口被打開, 使在曲柄箱中被壓縮的新裝氣油混合物得以傳輸至燃燒室. 此外, 這一過程有助於被燃燒的氣體更快地從離開燃燒室.
新的循環的開始:        一旦活塞到達BDC, 由於燃燒產生的慣性作用, 活塞開始再次向上移動, 開始新的循環. 在排氣接口即將關閉之前, 大量的氣油混合物被排至大氣中.

重要的注意事項
        为了使两冲程发动机保持高效运作, 最需要考虑的事项之一是清除油泵.  由於它具有許多功能,  清除油泵的不同問題, 会導致發動機有不同的表現.
        清除油泵操作燃料油泵, 它對轉移負荷產生壓力, 並產生負壓, 將新鮮的燃料通過吸入管道進行置換. 由於油泵通過正壓及負壓進行操作,它必須密封.它必須產生並維持壓力及真空.可能發生泄露的區域包括:曲柄箱油封, 墊片, 鑄造痕,松的固定螺絲,及活塞至汽缸的清潔程度.在吸入閥門向下的一面,均有曲柄箱壓力的流失.

marjrh

作为一名汽车工程师，对于两冲程发动机运作的理解如下：

两冲程发动机，由英国工程师道格拉斯·克拉克爵士于18世纪中叶发明，也被称为“克拉克循环”。从发明至今，该发动机经历了一些改进。为产生动力，其通过内部燃烧过程实现。此发动机具有独特的工作循环，包括进气、压缩、燃烧和排气四个主要阶段。高效的两冲程发动机设计能确保燃油经济性和良好的性能。现代汽车中广泛应用此技术，证明其成熟度和可靠性。