機械外文翻譯--內燃機

1、<p>　　畢業(yè)設計(論文)外文資料翻譯</p><p>　　系　　部： 機械工程系 </p><p>　　專 業(yè)： 機械工程及自動化 </p><p>　　姓 名： </p>

2、<p>　　學 號： </p><p>　　外文出處：http://bbs.jobmet.com/thread-261802-1</p><p>　　-1.html </p><p>　　附 件： 1.外文資料翻譯譯文；2.外文原文。 &

3、lt;/p><p>　　注：請將該封面與附件裝訂成冊。</p><p>　　附件1：外文資料翻譯譯文</p><p><b>　　內燃機</b></p><p>　　我們知道，無論是汽油發(fā)動機還是柴油發(fā)動機，它們都屬于內燃機，都是燃燒燃料后通過推動氣缸內活塞作往返運動來將燃料中的化學能量轉換成為驅動車輛前進的機械能量，因此兩

4、者的工作原理大體是相同的。</p><p>　　作為日常使用的燃料本身，柴油的能量密度最高，比液化天然氣高出近1倍，比汽油高出10%以上。與汽油相比，柴油不易揮發(fā)，著火點較高，不易因偶然情況被點燃或發(fā)生爆炸。由于兩者揮發(fā)性和燃點的不同，導致使用這兩種燃料的發(fā)動機有不同的點火方式。</p><p>　　汽油發(fā)動機的特點：體積小、重量輕、起動性好。</p><p>　　

5、汽油發(fā)動機中，油氣混合氣進入氣缸后，在壓縮接近終了時由火花塞點燃。因此，汽油發(fā)動機需要一套控制何時讓火花塞工作的點火系統(tǒng)，此系統(tǒng)必須精確控制火花塞放電的時刻和火花能量的大小，才能保證汽油機的工作正常，汽油機的燃料供給系和點火系是汽油機上發(fā)生故障比例較高的部位。此外，由于汽油的燃點較低，汽油機的壓縮比就不能太高，以免油氣自燃，因此其熱效率和經濟性較柴油機為差。</p><p>　　汽油機的優(yōu)點在于其體積小、重量輕、

6、價格便宜；起動性好，最大功率時的轉速高；工作中振動及噪聲小，因此，在載客汽車，特別是轎車中，汽油機得到了廣泛的應用，特別是在我們國家目前生產的絕大多數(shù)轎車，都是采用汽油發(fā)動機作為自己的動力系統(tǒng)。</p><p>　　傳統(tǒng)柴油發(fā)動機的特點：熱效率和經濟性較好。</p><p>　　柴油機采用壓縮空氣的辦法提高空氣溫度，使空氣溫度超過柴油的自燃燃點，這時再噴入柴油、柴油噴霧和空氣混合的同時自己

7、點火燃燒。因此，柴油發(fā)動機無需點火系。同時，柴油機的供油系統(tǒng)也相對簡單，因此柴油發(fā)動機的可靠性要比汽油發(fā)動機的好。</p><p>　　由于不受爆燃的限制以及柴油自燃的需要，柴油機壓縮比很高。熱效率和經濟性都要好于汽油機，同時在相同功率的情況下，柴油機的扭矩大，最大功率時的轉速低，適合于載貨汽車的使用。</p><p>　　但柴油機由于工作壓力大，要求各有關零件具有較高的結構強度和剛度，所

8、以柴油機比較笨重，體積較大；柴油機的噴油泵與噴嘴制造精度要求高，所以成本較高；另外，柴油機工作粗暴，振動噪聲大；柴油不易蒸發(fā)，冬季冷車時起動困難。</p><p>　　由于上述特點，以前柴油發(fā)動機一般用于大、中型載重貨車上。</p><p>　　小型高速柴油發(fā)動機的新發(fā)展：排放已經達到歐洲III號的標準。</p><p>　　傳統(tǒng)上，柴油發(fā)動機由于比較笨重，升功率指

9、標不如汽油機(轉速較低)，噪聲、振動較高，炭煙與顆粒(PM)排放比較嚴重，所以一直以來很少受到轎車的青睞。</p><p>　　但隨著近年來柴油機技術的進步，特別是小型高速柴油發(fā)動機的新發(fā)展，一批先進的技術，例如電控直噴、共軌、渦輪增壓、中冷等技術得以在小型柴油發(fā)動機上應用，使原來柴油發(fā)動機存在的缺點得到了較好的解決，而柴油機在節(jié)能與CO2排放方面的優(yōu)勢，則是包括汽油機在內的所有熱力發(fā)動機無法取代的，因此，先進的

10、小型高速柴油發(fā)動機，其排放已經達到歐洲III號的標準，成為“綠色發(fā)動機”，目前已經成為歐美許多新轎車的動力裝置，可以預見，我國將出現(xiàn)越來越多的柴油轎車。</p><p>　　內燃機是一種熱力發(fā)動機。通?？砂慈加玫娜剂蟻矸钟校浩蜋C、煤氣機等幾類。因為汽油機輕便、噪音低以及環(huán)保性，它贏得了公眾較好的口碑，因此其有機在國民經濟和國防工程上得到了較為廣泛的應用。內燃機傳熱在很大程度上主宰著汽油機的經濟性、可靠性以及其他

11、各項重要的技術經濟指標。在汽油機負載日益增強的今天，研究汽油機熱負荷問題已經是非常緊迫的任務了。汽油機向高強化方向的發(fā)展使其零部件的機械負荷和熱負荷不斷增大，而汽油機的的汽缸套是氣體壓縮、燃燒和膨脹的空間，并為活塞起導向作用，它的不正常變形將破壞活塞與缸套間的正常間隙，導致工作過程的惡化。因而，如何降低汽油機的熱負荷，則是提高其整機性能的重要研究課題。 </p><p><b>　　發(fā)動機：</b

12、></p><p>　　發(fā)動機充當動力裝置。內燃機是最常見的：它通過在發(fā)動機汽缸里面燃燒一種液體燃料獲得動力。發(fā)動機有兩種類型:汽油機 (也叫做一個火花- 點火發(fā)動機)和柴油機(也被稱為一個壓縮- 點火發(fā)動機)。這兩種發(fā)動機都叫做熱機；燃料在氣缸內燃燒產生的高溫高壓氣體推動曲軸旋轉而傳遞動力。</p><p><b>　　車身：</b></p>&

13、lt;p>　　汽車車身是由窗子，門，一個發(fā)動機罩和一個行李艙蓋建成的一個金屬板殼。它給發(fā)動機，乘客和貨物提供一種保護。車身的設計應使得乘客安全和舒適。車身的外表使得汽車具有吸引人的，五彩繽紛的和現(xiàn)代的外表。</p><p><b>　　底盤：</b></p><p>　　底盤是汽車上主要操作系統(tǒng)組成的一個總成。底盤包括傳動系，懸掛，轉向，以及剎車系統(tǒng)。</

14、p><p>　　傳動系統(tǒng) - 把動力運送到車輪。主要零部件是離合器，變速器，傳動軸，主減速器，和差速器。</p><p>　　懸掛 - 吸收路面震動。</p><p>　　轉向 - 控制運動的方向。</p><p>　　剎車 – 使車輛減速。</p><p><b>　　電氣設備：</b></p

15、><p>　　電系統(tǒng)為點火，喇叭，燈光，發(fā)熱器和起動器提供電。通過循環(huán)充電來維持電量。這條電路由電池，交流發(fā)電機(或者發(fā)電機)組成。蓄電池儲存電。交流發(fā)電機把發(fā)動機的機械能換成電能并給電池再充電。</p><p><b>　　氣缸體：</b></p><p>　　氣缸體是發(fā)動機的基本框架。發(fā)動機的其他零件都安裝在它里面或者固定在它上。缸體里有氣缸，

16、水套和油道。曲軸也固定在氣缸體底部。除了頂置凸輪(OHC)發(fā)動機以外，凸輪軸都安裝在氣缸體里面。在大多數(shù)汽車里，氣缸體由灰鑄鐵或者一種灰鑄鐵和其他金屬的合金(混合物)做成，例如鎳或鉻。氣缸體是鑄件。 </p><p>　　有些氣缸體，特別是在小汽車里的那些，都是由鋁做成的。這種金屬比鑄鐵輕得多。但是，鑄鐵的耐磨性比鋁好。因此，在大多數(shù)鋁制發(fā)動機的氣缸內鑲有鑄鐵或者鋼的軸套。這些軸套叫做氣缸套。而有些氣缸體完全由

17、鋁做成。</p><p><b>　　氣缸蓋：</b></p><p>　　氣缸蓋固定在氣缸體的頂上，正像屋頂套在一所房子上面一樣。氣缸蓋下面與活塞頂上的空間形成燃燒室。最常見的氣缸蓋類型是半球形，楔形和準半球形。這三種說法都是指燃燒室的形狀。氣缸蓋攜帶閥門，氣門彈簧和在搖臂桿上的搖臂，這部分的氣門傳動機構通過推桿工作。有時，凸輪軸直接安裝在氣缸蓋上并且不用搖臂控制閥

18、門工作。這被叫為頂置凸輪軸裝置。像氣缸體一樣，氣缸蓋是由鑄鐵或者鋁合金制成。</p><p><b>　　襯墊：</b></p><p>　　氣缸蓋與氣缸體用高強度的鋼螺栓縛連結。氣缸體和氣缸蓋之間的連接必須密封以便沒有燃燒的混合氣體泄漏。這通過使用氣缸蓋襯墊實現(xiàn)。這是一個夾層襯墊，即在兩片銅之間放一片石棉，這兩種材料都能禁得住在發(fā)動機內的高溫和高壓。</p&g

19、t;<p><b>　　油底殼：</b></p><p>　　油底殼通常由鋼沖壓形成。油底殼和氣缸體的下半部分一同被叫做曲軸箱；他們把曲軸封閉起來。潤滑系統(tǒng)中的機油泵從油底殼抽取油并把油輸送到發(fā)動機內全部正在工作的部分。機油流出后又流回油底殼。因而在油底殼和發(fā)動機工作零件之間有機油不斷流動循環(huán)。</p><p><b>　　進氣配氣定時：<

20、;/b></p><p>　　如果進口門在進氣行程的上止點打開并且在這次行程的下止點關閉，它將有180°的開度。氣門在180°轉角內完全打開。然而氣門達到全開位置需要一定時間，完全關閉也需要一定時間。因此閥門在上止點(BTDC)之前被打開，在下止點(ABDC)之后關閉。</p><p>　　排氣配氣定時： </p><p>　　如果排氣

21、門在排氣行程的下止點打開并且在這次行程的上止點關閉，它將有180°的持續(xù)。但是像進氣門一樣，排氣門需要時間到達充分打開和關閉的位置。因此排氣門在下止點之前打開，在上止點之后關閉。 </p><p><b>　　氣門重疊：</b></p><p>　　進氣門在上止點前17°打開，排氣門在上止點后17°關閉。 因此，有34°的一

22、段時期，兩個閥門都是開的：(17°+ 17°= 34°)。這時期被稱為氣門重疊。排氣門的關閉和進氣門的開啟重疊。在這個時候，新的混合氣推動燃燒后的廢氣從排氣門排出。在渦輪增壓發(fā)動機上氣門重疊角被保持在一個最小值。這就防止廢氣倒流入進氣管。 </p><p><b>　　配氣機構：</b></p><p>　　那些打開和關閉氣門的氣門傳動

23、是為了協(xié)調四沖程的工作循環(huán)(使他們各自上下移動)。這些閥門運動必須正好在合適的時刻進行。每個閥門的開啟由凸輪軸控制。</p><p>　　頂置凸輪軸氣閥傳動：</p><p>　　凸輪是一在軸上的蛋形的金屬，通過曲軸協(xié)調旋轉。那金屬軸叫凸輪軸，在發(fā)動機里的每個氣門一般有各自的凸輪。當凸輪軸旋轉時，凸輪凸起的或者高點的位置，推動氣門座。這行動強迫閥門向下移動。這過程能使進氣門在進氣行程打開，

24、或者排氣門在排氣行程打開。 </p><p>　　因為凸輪軸繼續(xù)旋轉，凸輪軸上的凸起部分離開氣門裝置。當這發(fā)生時，氣門彈簧緊緊地關閉氣門口，叫做氣門座。</p><p>　　現(xiàn)代汽車發(fā)動機里的閥門位于發(fā)動機頂上的汽缸蓋。這被稱為頂置氣門(OHV)結構。另外，當凸輪軸位于汽缸蓋上面時，這種方式被稱為是頂置凸輪軸(OHC)結構。一些高性能發(fā)動機有兩個單獨的凸輪軸，分別負責開關進氣門和排氣門。

25、這些發(fā)動機被稱為雙頂置凸輪軸(DOHC)發(fā)動機。</p><p><b>　　推桿氣閥傳動：</b></p><p>　　凸輪軸也裝在發(fā)動機底部的氣缸體內。為了將凸輪的運動傳給氣門需要一些附屬裝置。</p><p>　　在這種布置中，凸輪凸角推動凸輪挺桿。當凸輪的凸角在凸輪挺桿下出現(xiàn)時，它推動凸輪挺桿向上運動(離開凸輪軸)。凸輪挺桿推動控制搖臂

26、的推桿。搖臂以通過它的中心為軸而旋轉。當搖臂的一側上升，其另一側下降，正如一塊蹺蹺板一樣。搖臂向下移動的那一邊推動氣門桿以打開氣門。 </p><p>　　因為推桿氣閥傳動有另外的部分，所以很難以高速運轉。推桿發(fā)動機一般在低速運轉，從而產生比相同大小的頂置凸輪軸較少功率。(記住，功率反映了工作能力。) </p><p><b>　　氣門間隙：</b><

27、/p><p>　　當發(fā)動機處于壓縮行程和做功行程時，閥門必須緊緊地關閉以產生一個不透氣的氣封，以防止氣體逃離燃燒室。如果閥門不完全關閉，發(fā)動機將不能發(fā)揮全部動力。此外氣門頭易于被通過的熱氣體燃燒，這有可能使活塞頻繁沖擊打開的氣門，使發(fā)動機嚴重損壞。 </p><p>　　所以閥門能完全關閉，氣門間隙在操作機構內是必須的。這意味著操作機構必須離閥門足夠遠以允許閥門通過氣門彈簧使其完全關閉

28、。但是，如果間隙太大，將引起金屬輕敲的噪音。</p><p><b>　　凸輪軸驅動機構：</b></p><p>　　在四行程循環(huán)時，每凸輪必須旋轉打開一閥門。記住，一個循環(huán)相當于曲軸旋轉兩次。因此，凸輪軸必須以曲軸正好一半的速度旋轉。這用2：1的傳動比完成。齒輪連接到凸輪軸的齒數(shù)是齒輪連接到曲軸的兩倍。齒輪連結有三種方式：</p><p>

29、<b>　?。?）皮帶傳動</b></p><p>　　齒型帶能被使用。這樣的帶是由合成橡膠做成并且用內部的鋼或者玻璃纖維絞合加強。皮帶上有齒，或者槽以嚙合并且驅動傳動齒輪上的齒。皮帶一般與頂置凸輪閥門傳動一起被用在發(fā)動機上。</p><p><b>　　（2）鏈傳動</b></p><p>　　在一些發(fā)動機上，金屬鏈被用

30、來連結曲軸和凸輪軸齒輪。大多數(shù)推桿發(fā)動機和一些頂置凸輪軸發(fā)動機都有鏈。</p><p><b>　?。?）齒輪傳動</b></p><p>　　凸輪軸和曲軸齒輪可能被直接連結，或者相嚙合。這類操作聯(lián)動通常被用在更老的六氣缸，直列發(fā)動機上。</p><p>　　凸輪軸被鏈或者帶驅動，使其朝著曲軸相同的方向轉動。但是凸輪軸被曲軸齒輪直接驅動，其將在

31、相反方向上轉動。正時皮帶被使用，因為他們花費少于鏈子，而且噪音少。一條典型的正時皮帶由用玻璃纖維加強的氯丁橡膠(合成橡膠)做成的。</p><p><b>　　附件2：外文原文</b></p><p>　　belong engine</p><p>　　We know that both gasoline engines or diesel e

32、ngines, they all belong engine. all from the burning of fuel through the cylinder piston made from the campaign to promote the fuel conversion of chemical energy into mechanical energy driving the vehicle forward. So the

33、 working principle is generally the same.</p><p>　　Daily use as fuel itself, the highest energy density of diesel, liquefied natural gas nearly doubled over, more than 10% higher than gasoline. Compared with

34、 conventional gasoline, diesel not volatile, higher ignition point, the situation was not due to accidental fire or explosion. Because of the volatile and, if different, resulting in the use of two different fuels the en

35、gine ignition.</p><p>　　Gasoline Engine features : small size, light weight, good start.</p><p>　　Gasoline engines, the gas mixture into the cylinder, close to the end of the compression ignited

36、 by the spark plug. Therefore, the need for a gasoline engine when to control the spark plug ignition system. This system requires an accurate control of spark plugs and spark discharge energy of the moment, be able to e

37、nsure the normal gasoline engine. gasoline engine fuel supply system and ignition system is a high proportion of the gasoline engine fault location. In addition, the gasoline ignited l</p><p>　　The advantage

38、 lies in its small gasoline engine, light weight, low price; Start good, the speed at maximum power; Small vibration and noise work, therefore, passenger motor vehicles, particularly cars, has been widely used in gasolin

39、e engines. especially in the vast majority of our country's current production cars, as their gasoline engines are used on the power system.</p><p>　　The conventional diesel engine thermal efficiency and

40、 economy better characteristics :</p><p>　　Diesel use of compressed air and improve air temperature, air temperature over the spontaneous ignition of diesel. Then again injected into diesel, hybrid diesel sp

41、ray at the same time their ignition and combustion air. Therefore, no diesel engine ignition system. Meanwhile, the engine oil system is relatively simple and therefore the reliability of diesel engines than gasoline eng

42、ines as good.</p><p>　　From injuries caused by spontaneous combustion due to the restrictions and the need for diesel engine with a high compression ratio. Thermal efficiency and economy must be better than

43、the gasoline engine, while the power in the same circumstances, the engine torque, maximum power at low speed, suitable for use in the truck.</p><p>　　However, due to work pressure Diesel called on the struc

44、tural components with high strength and stiffness. So Diesel relatively bulky size; Diesel fuel pump with the nozzle to create high-precision, high costs; In addition, the engine crude, large vibration and noise; Diesel

45、easy evaporation in winter when cold starting problems.</p><p>　　Because of these features, previously used in diesel engines, medium radial truck.</p><p>　　The new development : small high-spee

46、d diesel engine emission standards has been achieved in Euc22 rope III.</p><p>　　Traditionally, due to relatively heavy diesel engine or gasoline engine power as indicators (low speed), noise, vibration high

47、er. Smoke and carbon particles (PM) emissions is fairly serious, very little has been favored by car.</p><p>　　But with the advances in technology in recent years Diesel, especially small high-speed diesel e

48、ngine is a new development of a number of advanced technologies, For example, electronic control DI, common rail, turbo-charged, in the cold light of technology, such as the use of diesel engines. The shortcomings of the

49、 original diesel engine to be a better solution. and the diesel engine's advantages in energy and CO2 emissions. including the gasoline engine is no way to replace all heat engines, ad</p><p>　　ICE is a

50、heat engine. Usually can burn the fuel used to be in : gasoline engine, gas and other types of aircraft. Because the gasoline engine light, low noise and environmental protection, it won the public good reputation, There

51、fore organic in its national economy and national defense projects has been more widely used. Engine Heat Transfer to a large extent the key to the economic gasoline engine, reliability and other important technical and

52、economic indicators. The gasoline engine load incre</p><p><b>　　Engine</b></p><p>　　The engine acts as the power unit. The internal combustion engine is most common: this obtains its

53、 power by burning a liquid fuel inside the engine cylinder. There are two types of engine :gasoline(also called a spark-ignition engine) and diesel(also called a compression-ignition engine).Both engines are called heat

54、engines; the burning fuel generates heat which causes the gas inside the cylinder to increase its pressure and supply power to rotate a shaft connected to the transmission.</p><p><b>　　Body</b>&l

55、t;/p><p>　　An automobile body is a sheet metal shell with windows, doors, a hood, and a trunk deck built into it. It provides a protective covering for the engine, passengers, and cargo. The body is designed to

56、 keep passengers safe and comfortable. The body styling provides an attractive, colorful, modern appearance for the vehicle.</p><p><b>　　Chassis</b></p><p>　　The chassis is an assemb

57、ly of those systems that are the major operating part of a vehicle. The chassis includes the transmission, suspension, steering, and brake systems.</p><p>　　Transmission systems ― conveys the drive to the wh

58、eels. The main components are clutch, gearbox, driveshaft, final drive, and differential.</p><p>　　Suspension― absorbs the road shocks.</p><p>　　Steering― controls the direction of the movement.

59、</p><p>　　Brake― slows down the vehicle.</p><p>　　Electrical Equipment</p><p>　　The electrical system supplies electricity for the ignition, horn, lights, heater, and starter. The e

60、lectricity level is maintained by a charging circuit. This circuit consists of the battery, alternator (or generator). The battery stores electricity. The alternator changes the engine's mechanical energy into electr

61、ical energy and recharges the battery.</p><p>　　Engine Block</p><p>　　The engine block is the basic frame of the engine. All other engine parts either fit inside it or fasten to it. It holds the

62、 cylinders, water jackets, and oil galleries.The engine block also holds the crankshaft, which fastens to the bottom of the block. The camshaft also fits inside the block, except on overhead-cam engines (OHC). In most ca

63、rs, this block is made of gray iron, or an alloy (mixture) of gray iron and other metals, such as nickel or chromium. Engine blocks are castings. </p><p>　　Some engine blocks, especially those in smaller ca

64、rs, are made of cast aluminum. This metal is much lighter than iron. However, iron wears better than aluminum. Therefore, the cylinders in most aluminum engines are lined with iron or steel sleeves. These sleeves are cal

65、led cylinder sleeves. Some engine blocks are made entirely of aluminum.</p><p>　　Cylinder Head</p><p>　　The cylinder head fastens to the top of the block, just as a roof fits over a house. The u

66、nderside forms the combustion chamber with the top of the piston. The most common cylinder head types are the hemi, wedge, and semi-hemi.All three of these terms refer to the shape of the engine's combustion chamber.

67、 The cylinder head carries the valves, valve springs and the rockers on the rocker shaft, this part of the valve gear being worked by the push-rods. Sometimes the camshaft is fitted directly int</p><p><b

68、>　　Gasket</b></p><p>　　The cylinder head is attached to the block with high-tensile steel studs. The joint between the block and the head must be gas-tight so that none of the burning mixture can es

69、cape. This is achieved by using cylinder head gasket. This is a sandwich gasket, i.e. a sheet of asbestos between two sheets of copper, both these materials being able to withstand the high temperature and pressures with

70、in the engine.</p><p>　　Oil Pan or Sump</p><p>　　The oil pan is usually formed of pressed steel. The oil pan and the lower part of the cylinder block together are called the crankcase; they encl

71、ose, or encase, the crankshaft. The oil pump in the lubricating system draws oil from the oil pan and sends it to all working parts in the engine. The oil drains off and runs down into the pan. Thus, there is constant ci

72、rculation of oil between the pan and the working parts of the engine.</p><p>　　Inlet Valve Timing</p><p>　　If the inlet valve opened at TDC of the intake stroke and closed at BDC of that stroke,

73、 it would have a duration of 180°.It would have remained open for half of a complete 360° revolution, or 180°. However, it takes some time for the valve to open to its full position. It also takes time for

74、 it to close tightly. Therefore the valve is opened before TDC (BTDC) and closed after BDC (ABDC).</p><p>　　Exhaust Valve Timing</p><p>　　If the exhaust valve opened at BDC at the beginning of t

75、he exhaust stroke and closed at TDC at the end of the exhaust stroke, it would have a duration of 180°. But like the inlet valve, the exhaust valve needs time to reach the full-open position, It also needs time to r

76、each the full-closed position. So the exhaust valve opens before BDC and closes after TDC.</p><p>　　Valve Overlap</p><p>　　The intake opens at 17° BTDC and the exhaust closes at 17° AT

77、DC. Thus, for a period of 34°, both of the valves are open: (17° + 17° =34°). This period of time is known as valve overlap. The closing of the exhaust valve laps over the opening of the intake valve.

78、 During this time, the first of the new mixture pushes the last of the burned gases out the exhaust valve. Valve overlap is held to a minimum on turbo-charged engines. This prevents the intake charge from being blown out

79、 the exhaust.</p><p>　　Valve Operation</p><p>　　To coordinate the four-stroke cycle, a group of parts called the valve train opens and closes the valves (moves them down and up, respectively). T

80、hese valve movements must take place at exactly the right moments. The opening of each valve is controlled by a camshaft.</p><p>　　Overhead camshaft (OHC) valve train</p><p>　　The cam is an egg-

81、shaped piece of metal on a shaft that rotates in coordination with the crankshaft. The metal shaft, called the camshaft, typically has individual cams for each valve in the engine. As the camshaft rotates, the lobe, or h

82、igh spot of the cam, pushes against parts connected to the stem of the valve. This action forces the valve to move downward. This action could open an inlet valve for an intake stroke, or open an exhaust valve for an exh

83、aust stroke.</p><p>　　As the camshaft continues to rotate, the high spot moves way from the valve mechanism. As this occurs, valve springs pull the valve tightly closed against its opening, called the valve

84、seat.</p><p>　　Valves in modern car engines are located in the cylinder head at the top of the engine. This is known as an overhead valve (OHV) configuration. In addition, when the camshaft is located over t

85、he cylinder head, the arrangement is known as an overhead camshaft (OHC) design. Some high-performance engines have two separate camshafts, one for each set of inlet and exhaust valves. These engines are known as dual ov

86、erhead camshaft (DOHC) engines.</p><p>　　Push-rod valve train</p><p>　　The camshaft also can be located in the lower part of the engine, within the engine block. To transfer the motion of the ca

87、m upward to the valve, additional parts are needed.</p><p>　　In this arrangement, the cam lobes push against round metal cylinders called cam follower. As the lobe of the cam comes up under the cam follower,

88、 it pushes the cam follower upward (away from the camshaft). The cam follower rides against a push rod, which pushes against a rocker arm. The rocker arm pivots on a shaft through its center. As one side of the rocker ar

89、m moves up, the other side moves down, just like a seesaw. The downward-moving side of the rocker arm pushes on the valve stem to open</p><p>　　Because a push-rod valve train has additional parts, it is more

90、 difficult to run at high speeds. Push-rod engines typically run at slower speeds and, consequently, produce less horsepower than overhead-camshaft designs of equal size. (Remember, power is the rate at which work is don

91、e.)</p><p>　　Valve Clearance</p><p>　　When the engine runs in compression stroke and power stroke, the valves must close tightly on their seats to produce a gas-tight seal and thus prevent the g

92、ases escaping from the combustion chamber. If the valves do not close fully the engine will not develop full power. Also the valve heads will be liable to be burnt by the passing hot gases, and there is the likelihood of

93、 the piston crown touching an open valve, which can seriously damage the engine.</p><p>　　So that the valves can close fully some clearance is needed in the operating mechanism. This means that the operating

94、 mechanism must be able to move sufficiently far enough away from the valve to allow the valves to be fully closed against its seat by the valve spring. However, if the clearance is set too great this will cause a light

95、metallic tapping noise.</p><p>　　Camshaft Drive Mechanism</p><p>　　Each cam must revolve once during the four-stroke cycle to open a valve. A cycle, remember, corresponds with two revolutions of

96、 the crankshaft. Therefore, the camshaft must revolve at exactly half the speed of the crankshaft. This is accomplished with a 2:1 great ratio. A gear connected to the camshaft has twice the number of teeth as a gear con

97、nected to the crankshaft. The gears are linked in one of three ways:</p><p>　　(1) Belt drive</p><p>　　A cog-type belt can be used. Such belts are made of synthetic rubber and reinforced with int

98、ernal steel or fiberglass strands. The belts have teeth, or slotted spaces to engage and drive teeth on gear wheels. A belt typically is used on engines with overhead-cam valve trains.</p><p>　?。?）Chain driv

99、e</p><p>　　On some engines, a metal chain is used to connect the crankshaft and camshaft gears. Most push-rod engines and some OHC engines have.</p><p>　　（3）Gearlchains drive</p><p>

100、;　　The camshaft and crankshaft gears can be connected directly, or meshed. This type of operating linkage commonly is used on older six-cylinder, inline engines.</p><p>　　A camshaft driven by a chain or belt

101、 turns in the same direction as the crankshaft. But a camshaft driven directly by the crankshaft gear turns in the opposite direction. Timing belts are used because they cost less than chains and operate more quietly. A