本科生畢業(yè)設計（論文）翻譯
A diesel engine works
【Abstract】In a diesel engine cylinder, the piston in the part of the work cycle of compressed gas,and in another part of the work cycle of the combustion gas mixture within the cylinder so the piston top surface expansion high pressure (about 116 ~ 120Kgf/cm2)under high temperature (about 569°C) gas role, and the pressure through the piston pin, connecting rod to the crankshaft. Can be seen that the piston is a long time under high temperature and high pressure in continuous reciprocating motion of the load, its load and working conditions were appalling. During the design process of the piston will be designed to ensure long-term stability of the work piston. The design of the work done by a brief introduction as follows:Diesel Engine Piston 180C reasonable processing technology is important, the role of parts and technology program analysis, preparation of rough form and process manufacturing line, through the analysis, comparison, use of the relative concentration of processing programs, and ultimately more reasonable to determine the mechanical line processing. The development process of rough line the main consideration, finishing arrangements, choice of processing methods, centralized and decentralized processes, such as processing the order requirements. Then determine the allowance, process size, after the analysis of the characteristics of the process, select the appropriate processing equipment and technical equipment. Calculated look-up table to determine the next major piston cutting process and the mapping of processes card, the design of the final fixture. Fixture design, it is necessary to take various aspects into account, the strict requirements of the fixture a direct impact on the surface of the workpiece processing position accuracy. Therefore, the machine tool design fixture design is an important task is the processing of one of the most active. During the graduation project in a specially designed positioning accuracy, simple structure and easy-to-use precision pin hole boring jig.
Keywords: Piston; Technology; processing equipment; cutting; Fixture
Any type of machine that obtains mechanicalenergy directly from the expenditure of the chemical energy of fuel burned in a combustion chamber that is an integral part of the engine. Four principal types of internal-combustion engines are in general use: the Otto-cycle engine, the diesel engine, the rotary engine, and the gas turbine. For the various types of engines employing the principle of jet propulsion, see Jet Propulsion; Rocket. The Otto-cycle engine, named after its inventor, the German technician Nikolaus August Otto, is the familiar gasoline engine used in automobiles and airplanes; the diesel engine, named after the French-born German engineer Rudolf Christian Karl Diesel, operates on a different principle and usually uses oil as a fuel. It is employed in electric-generating and marine-power plants, in trucks and buses, and in some automobiles. Both Otto-cycle and diesel engines are manufactured in two-stroke and four-stroke cycle models.The essential parts of Otto-cycle and diesel engines are the same. The combustion chamber consists of a cylinder, usually fixed, that is closed at one end and in which a close-fitting piston slides. The in-and-out motion of the piston varies the volume of the chamber between the inner face of the piston and the closed end of the cylinder. The outer face of the piston is attached to a crankshaft by a connecting rod. The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multicylindered engines the crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize irregularity in the motion of the shaft. An engine may have from 1 to as many as 28 cylinders.
The fuel supply system of an internal-combustion engine consists of a tank, a fuel pump, and a device for vaporizing or atomizing the liquid fuel. In Otto-cycle engines this device is either a carburetor or, more recently, a fuel-injection system. In most engines with a carburetor, vaporized fuel is conveyed to the cylinders through a branched pipe called the intake manifold and, in many engines, a similar exhaust manifold is provided to carry off the gases produced by combustion. The fuel is admitted to each cylinder and the waste gases exhausted through mechanically operated poppet valves or sleeve valves. The valves are normally held closed by the pressure of springs and are opened at the proper time during the operating cycle by cams on a rotating camshaft that is geared to the crankshaft. By the 1980s more sophisticated fuel-injection systems, also used in diesel engines, had largely replaced this traditional method of supplying the proper mix of air and fuel. In engines with fuel injection, a mechanically or electronically controlled monitoring system injects the appropriate amount of gas directly into the cylinder or inlet valve at the appropriate time. The gas vaporizes as it enters the cylinder. This system is more fuel efficient than the carburetor and produces less pollution.
In all engines some means of igniting the fuel in the cylinder must be provided. For example, the ignition system of Otto-cycle engines described below consists of a source of low-voltage, direct-current electricity that is connected to the primary of a transformer called an ignition coil. The current is interrupted many times a second by an automatic switch called the timer. The pulsations of the current in the primary induce a pulsating, high-voltage current in the secondary. The high-voltage current is led to each cylinder in turn by a rotary switch called the distributor. The actual ignition device is the spark plug, an insulated conductor set in the wall or top of each cylinder. At the inner end of the spark plug is a small gap between two wires. The high-voltage current arcs across this gap, yielding the spark that ignites the fuel mixture in the cylinder.
Because of the heat of combustion, all engines must be equipped with some type of cooling system. Some aircraft and automobile engines, small stationary engines, and outboard motors for boats are cooled by air. In this system the outside surfaces of the cylinder are shaped in a series of radiating fins with a large area of metal to radiate heat from the cylinder. Other engines are water-cooled and have their cylinders enclosed in an external water jacket. In automobiles, water is circulated through the jacket by means of a water pump and cooled by passing through the finned coils of a radiator. Some automobile engines are also air-cooled, and in marine engines sea water is used for cooling.
Unlike steam engines and turbines, internal-combustion engines develop no torque when starting, and therefore provision must be made for turning the crankshaft so that the cycle of operation can begin. Automobile engines are normally started by means of an electric motor or starter that is geared to the crankshaft with a clutch that automatically disengages the motor after the engine has started. Small engines are sometimes started manually by turning the crankshaft with a crank or by pulling a rope wound several times around the flywheel. Methods of starting large engines include the inertia starter, which consists of a flywheel that is rotated by hand or by means of an electric motor until its kinetic energy is sufficient to turn the crankshaft, and the explosive starter, which employs the explosion of a blank cartridge to drive a turbine wheel that is coupled to the engine. The inertia and explosive starters are chiefly used to start airplane engines.
The ordinary Otto-cycle engine is a four-stroke engine; that is, in a complete power cycle, its pistons make four strokes, two toward the head (closed head) of the cylinder and two away from the head. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke, the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.
The efficiency of a modern Otto-cycle engine is limited by a number of factors, including losses by cooling and by friction. In general, the efficiency of such engines is determined by the compression ratio of the engine. The compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber) is usually about 8 to 1 or 10 to 1 in most modern Otto-cycle engines. Higher compression ratios, up to about 15 to 1, with a resulting increase of efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent—in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy
Theoretically, the diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most diesels are also four-stroke engines but they operate differently than the four-stroke Otto-cycle engines. The first, or suction, stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression, stroke the air is compressed to a small fraction of its former volume and is heated to approximately 440°C (approximately 820°F) by this compression. At the end of the compression stroke, vaporized fuel is injected into the combustion chamber and burns instantly because of the high temperature of the air in the chamber. Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts and until it warms up. This combustion drives the piston back on the third, or power, stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.
The efficiency of the diesel engine, which is in general governed by the same factors that control the efficiency of Otto-cycle engines, is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesels are, in general, slow-speed engines with crankshaft speeds of 100 to 750 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Some types of diesel, however, have speeds up to 2000 rpm. Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built than Otto-cycle engines, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.
By suitable design it is possible to operate an Otto-cycle or diesel as a two-stroke or two-cycle engine with a power stroke every other stroke of the piston instead of once every four strokes. The power of a two-stroke engine is usually double that of a four-stroke engine of comparable size.
The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke. In the simplest type of two-stroke engine, the poppet valves are replaced by sleeve valves or ports (openings in the cylinderwall that are uncovered by the piston at the end of its outward travel). In the two-stroke cycle, the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows, and the charge is ignited when the piston reaches the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape from the combustion chamber.
In the 1950s the German engineer Felix Wankel developed an internal-combustion engine of a radically new design, in which the piston and cylinder were replaced by a three-cornered rotor turning in a roughly oval chamber. The fuel-air mixture is drawn in through an intake port and trapped between one face of the turning rotor and the wall of the oval chamber. The turning of the rotor compresses the mixture, which is ignited by a spark plug. The exhaust gases are then expelled through an exhaust port through the action of the turning rotor. The cycle takes place alternately at each face of the rotor, giving three power strokes for each turn of the rotor. Because of the Wankel engine's compact size and consequent lesser weight as compared with the piston engine, it appeared to be an important option for automobiles. In addition, its mechanical simplicity provided low manufacturing costs, its cooling requirements were low, and its low center of gravity made it safer to drive. A line of Wankel-engine cars was produced in Japan in the early 1970s, and several United States automobile manufacturers researched the idea as well. However, production of the Wankel engine was discontinued as a result of its poor fuel economy and its high pollutant emissions. Mazda, a Japanese car manufacturer, has continued to design and innovate the rotary engine, improving performance and fuel efficiency.
A modification of the conventional spark-ignition piston engine, the stratified charge engine is designed to reduce emissions without the need for an exhaust-gas recirculation system or catalytic converter. Its key feature is a dual combustion chamber for each cylinder, with a prechamber that receives a rich fuel-air mixture while the main chamber is charged with a very lean mixture. The spark ignites the rich mixture that in turn ignites the lean main mixture. The resulting peak temperature is low enough to inhibit the formation of nitrogen oxides, and the mean temperature is sufficiently high to limit emissions of carbon monoxide and hydrocarbon.
柴油機的工作原理
【摘要】在柴油機氣缸內，活塞在一部分工作循環(huán)壓縮氣體，而在另一部分工作循環(huán)氣缸內混合氣體燃燒膨脹使活塞頂面承受高溫（約569°C）高壓（約116～120Kgf/cm2）氣體的作用，并把壓力通過活塞銷、連桿傳給曲軸?？梢?，活塞是在高溫高壓下作長時間連續(xù)變負荷的往復運動，它的負荷和工作環(huán)境很惡劣。在本設計中將對活塞的加工工藝進行設計，以保證活塞長久穩(wěn)定工作?，F(xiàn)將設計中所做的工作簡要介紹如下：柴油機活塞加工工藝合理性是很重要的，通過對零件的作用及工藝方案分析，擬定毛坯的制造形式及工藝路線，通過分析、比較，采用了相對集中加工工藝方案，最終確定比較合理的機械加工工藝路線。制定工藝路線時主要考慮粗、精加工安排、加工方法選擇、工序集中與分散、加工順序等方面的要求。接著確定加工余量、工序尺寸，經過對工序特點的分析，恰當選擇相應加工設備和工藝裝備。接下來經過計算查表確定活塞各主要工序的切削用量并繪制工序卡片，最后設計夾具。設計夾具時，要多方面考慮，嚴格要求，機床夾具的好壞直接影響工件加工表面的位置精度。所以，機床夾具設計是裝備設計中的一項重要的工作，是加工過程中最活躍的因素之一。在本畢業(yè)設計中特別設計了定位準確、結構簡單和使用方便的精鏜銷孔夾具。
關鍵字：活塞；工藝路線；加工設備；切削用量；夾具
任何通過燃料在氣缸中燃燒，使燃油的化學能轉化為機械能，從而獲得動力的引擎都成為內燃機。最常見的內燃機有四種：奧托循環(huán)式發(fā)動機，柴油機，轉子發(fā)動機和煤氣機。根據這四種發(fā)動機的優(yōu)點，把它們應用于不同的工況。奧托循環(huán)式發(fā)動機，是根據其發(fā)明者，德國機械師尼古拉斯.奧格事特.奧托的名字來命名的。是飛機上很常見的一種發(fā)動機；而柴油機是由法籍德國工程師Rudolf Christian Karl Diesel命名的。它是一種用柴油作為燃料的先進的發(fā)動機。普遍用在電子控機械、戰(zhàn)斗機、公共汽車、貨車以及一些小車上。奧托式發(fā)動機和柴油機的工作方式都是二沖程或者四沖程。
奧托式發(fā)動機和柴油機的基本構造都是一樣的。壓縮燃燒室是由一個一段由缸蓋另一端由活塞之間的空間所形成?；钊纳舷逻\動使得氣缸與活塞間的空間發(fā)生大小變化，從而改變壓縮空間的大小?；钊c曲軸之間通過連桿相互連接。曲軸將活塞的運動轉化成旋轉式的運動。多氣缸式發(fā)動機的曲軸，在每一個氣缸處都會多一個稱為曲拐的結構部分。這樣每個氣缸的動力才能很好的傳遞給曲軸，是曲軸的轉動平穩(wěn)。曲軸上接有飛輪并有平衡坑。這樣能夠使曲軸運動的慣性最小化，達到平衡的目的。不同的發(fā)動機會有一個到二十四個等的氣缸。
內燃機的燃料供給系統(tǒng)又油箱、油泵、和分油管以及使液體燃料霧化的機構組成。在奧托式發(fā)動機上，并不是靠化油器來進行燃油霧化的，而是利用燃油的直接噴入，一直到現(xiàn)在都是如此。在大多數(shù)發(fā)動機上，燃料都是通過化油器霧化后通過壓氣機進入進氣管道。在部分發(fā)動機的排氣系統(tǒng)中，也會用到類似的裝置來通過利用廢氣的能量對進氣充量進行壓縮。燃料平均分配給各個汽缸，而廢氣則通過排氣門排出。進排氣門的開閉都是通過凸輪軸的轉動從而牽動氣門彈簧作用到挺桿，在正確的時間是氣門開閉。在上世紀80年代，缸內直噴技術開始用于內燃機領域，從很大程度上代替了傳統(tǒng)的燃油與空氣相混合的技術。在有直噴裝置的發(fā)動機上，燃料會通過噴射系統(tǒng)在正確的時刻噴入汽缸或者進氣管。這樣燃料就會在汽缸里混合，這比化油器混合更充分，污染更小。
所有的發(fā)動機上，火花塞的位置都必須適宜。比如奧托式發(fā)動機的點火系統(tǒng)包括低壓電源，即具有變壓性質的初級線圈，從而導出直流電。電流會被一個機械式的定時調節(jié)器在一秒鐘內方向發(fā)生多次變化。初級線圈中電流的擾動會產生脈沖，從而會在次級線圈中產生高壓電流。這個高壓電流會被分電器分配到各個汽缸，件叫做火花，一個安裝在汽缸頂部被叫做火花塞的零件。在火花塞末端的兩極間有一個間隙，高壓電流會擊穿這個點火間隙，從而點燃汽缸中的混合氣體。由于燃燒室的溫度太高，所有的發(fā)動機都必須有相應的冷卻系統(tǒng)。一些飛機、汽車、和船只上的舷外發(fā)動機采用風冷。這些采用風冷的發(fā)動機都必須有很多散熱片，一邊有較大的散熱面積，從而很好的帶走汽缸的熱量。除此之外的還有水冷系統(tǒng)，它是在發(fā)動機的汽缸中設有水套來達到冷卻的目的。在汽車上，冷卻液借助水泵的壓力在水套中流動，帶走熱量。還有一些汽車是利用風冷，海上船只則是用海水作為冷卻的介質。與蒸汽機和渦輪機不同，內燃機在發(fā)動時并不會產生轉矩，并且扭矩的輸出必須要靠曲軸的轉動才行。汽車發(fā)動機的啟動要靠一個與曲軸箱嚙合的摩擦片，通過摩擦片的分離才能向外輸出力矩。小型的發(fā)動機有時需要手動的進行多次使離合器的松脫才能發(fā)動。有時候在大型發(fā)動機上，會有慣性啟動裝置，或者是借助手工輸入力矩知道驅動能量能使曲軸轉動。一邊帶動增壓器工作，來增加發(fā)動機的功率。一般，慣性啟動裝置和爆炸性質的裝置都是在飛機上采用的。普通的奧托式發(fā)動機都是四沖程，也就是說，每一個工作循環(huán)中，活塞會有四個行程，兩個離缸蓋最近，另外兩個離缸蓋距離最遠。在第一個行程時，活塞遠離缸蓋，同時進氣門打開。活塞在這個過程中的運動，使得燃料和空氣進入燃燒室混合。接著的行程，就是將混合后的氣體壓縮到燃燒室里。當活塞上行到最高點時，燃燒室的體積達到最小，火花塞就會點燃混合氣體，燃燒產生的膨脹壓力會作用在活塞上，使活塞遠離缸蓋，這就是第三個行程。在最后一個行程中，排氣門打開，活塞的上行會對燃燒后的氣體進行擠壓，是廢氣排出燃燒室，做好下一循環(huán)的準備。發(fā)動機的效率會受到很多因素的限制，例如冷卻損失以及摩擦損失。通常，發(fā)動機的效率是由其壓縮比決定的?，F(xiàn)在發(fā)動機的壓縮比一般在8---10之間。更高的壓縮比可以達到15，效率的提高也可以通過采用辛烷值較高的燃料來實現(xiàn)?，F(xiàn)在，好的發(fā)動機的效率在20％--25％，也就是說，只有這部分能量真正用于產生機械能量。理論上，柴油周期相比奧托循環(huán)的區(qū)別在于，它的壓縮過程是等容、等壓的。大多數(shù)柴油機都是采用四沖程，但卻與奧托式四沖程不一樣。首先，在進氣時，活塞向下運動，并通過進氣門將空氣吸進燃燒室。其次，在壓縮時，活塞將空氣壓縮到比先前小很多倍的體積，并在這個過程中使空氣的溫度達到440℃（等同于華氏820℉）。在壓縮結束的時候，蒸發(fā)的燃油被噴入汽缸，由于汽缸中的氣體高溫作用而立即燃燒。一些發(fā)動機上設有電子噴射輔助系統(tǒng)，在發(fā)動機發(fā)動直到加熱完成期間進行燃油噴射。這樣的壓縮過程為活塞進行第三個沖程提供強大的動力。第四個沖程跟奧托式四沖程發(fā)動機一樣，都是排氣過程。柴油機的效率，跟一般的奧托式發(fā)動機是受同樣的因素所影響的，但是稍好于奧托式發(fā)動機。事實上，現(xiàn)在發(fā)動機中，基本的效率都不會超過40％。事實上，柴油機的曲軸轉速的100—750轉每分鐘，這等同于奧托式發(fā)動機的2500—5000轉每分鐘。但是也有一些柴油機的轉速達到了2000轉每分鐘。因為柴油機的壓縮比高達14或者15，這使得它們的體積較奧托式大，這個缺點正體現(xiàn)出柴油機的到效率和高燃油經濟特性。好的設計一般采用奧托式循環(huán)或者二沖程的方式來代替四沖程的方式。因為同樣體積的發(fā)動機，二沖程的效率是四沖程的兩倍。二沖程的有點在于，縮短了燃料壓縮的時間，并且減少了燃料的浪費以及用半個沖程完成了四沖程發(fā)動機的一個壓縮沖程。在最簡單的二沖程發(fā)動機上，排氣門被廢氣管代替了。在二沖程循環(huán)中，燃料和空氣的混合氣體在活塞在汽缸中下行時進入曲軸箱。緊接著，燃料開始壓縮，并在活塞到達上至點是點燃。這是活塞在燃氣壓力的作用下下行，廢棄就會從排氣口由汽缸內向外排出去。上世紀50年代，德國機械師菲利克斯.王科爾開發(fā)了一種新型的發(fā)動機。在這種發(fā)動機上，活塞和汽缸被一個在橢圓形燃燒室里旋轉的三角轉子所代替?；旌先剂贤ㄟ^進氣口進入，然后分流到有轉子表面與端面形成的燃燒室里?；旌蠚怏w通過轉子的旋轉得到壓縮，最后被火花塞點燃。然后，廢棄就會隨著轉子的運動從排氣口排出。循環(huán)過程中，轉子的旋轉一周，會出有三個沖程，而且在轉子的正反兩面產生壓力。正因為轉子發(fā)動機與柴油機相比，結構緊湊、質量輕，因而在汽車發(fā)動機中作用很大。另外，它簡單的結構使得生產成本低，冷卻系統(tǒng)質量輕，另外它的重心低，使得他的安全性得到了增加。在上世紀70年代初期，一條轉子發(fā)動機的生產線在日本落成。很多美國的汽車制造商都很看好這個項目。但是，由于轉子發(fā)動機的低燃料經濟性很高污染性，最后沒能得到繼續(xù)的發(fā)展。日本的汽車制造商—馬自達，繼續(xù)了改善轉子發(fā)動機燃油經濟性的設計和研發(fā)。發(fā)動機采用火花點火的改進方式，進行分層點火稀薄燃燒，幫助沒有使用廢氣再循環(huán)和催化轉換器的發(fā)動機減小排放量。它的特點在于在一個汽缸中有兩個燃燒室，當沖入的混合氣體過多是，備用燃燒室就會將多余的混合氣體儲存起來?；鸹ㄈ麜赛c燃多余部分的混合氣，在將另一部分點燃。這樣最高火焰溫度就會比較合適，從而很好的限制NOx化合物的生成量以及CO和HC的排放量。
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