【正文】 ke valve early helps overe the static inertia of the airfuel mixture at the beginning of the intake stroke , while leaving the intake valve open after BDC takes advantage of the kentia of the moving airfuel mixture . This increase volumetric efficiency .As the piston moves down on the power stroke past the 90 ATDC position , pressure in the cylinder has dropped , and the leverage to the crankshaft has decreased due to connecting rod angle and crankshaft position . This ends the effective length of the power stroke , and the exhaust valve can now be opened to begin expelling the burned gases . The exhaust valve remains open until the piston has moved up past the TDC position . This helps to remove as much of the burned gases as is possible and increase volumetric efficiency . Cam Design and Control Dynamics The function of the cam is to open and close the valves as far as possible , as fast as possible and as smoothly as possible . The closing force for the valves is applied by the valve spring which also maintain contact between the cam and the valves . Dynamic force impose limits on cam and valve lift . The entire valvetrain assembly can be view as a spring \mass system in which the conversion from stored to free energy causes force vibration . Valvetrain assemblies with overhead camshafts can be represented with sufficient accuracy by a 1mass system ( consisting of the moving mass , the valvetrain assembly stiffness and corresponding damping ) .For system with valve bottommounted camshaft and push rods , a 2mass system is being increasingly used . The maximum permissible contact stress , usually regarded as the parameter which limits camlobe radius and the rate of opening on the flank , currently lies between 600700Mpa depending upon the material parings used . Camshaft Drive MechanismEach cam must revolve once during the fourstroke cycle to open a valve. A cycle, remember, corresponds with two revolutions of the crankshaft . Therefore, the camshaft must revolve at exactly half the speed of the crankshaft . This is acplished with a 2:1 gear ratio .A gear connected to the camshaft has twice the number of teeth as a gear connected to the crankshaft. The gears are linked in one of three ways: DriveA cogtype belt can be used .Such belts are made of synthetic rubber and reinforced with internal 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 overheadcam valve trains. DriveOn some engines, a metal chain is used to connect the crankshaft and camshaft gears. Most pushrod engines and some OHC engines have chains. DriveThe camshaft and crankshaft gears can be connected directly, or meshed. This type of operating linkage monly is used on older sixcylinder, inline engines. A camshaft driven by a chain or belt 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 typical timing belt is made of neoprene (synthetic rubber) reinforced with fiberglass. Electronic Valve Control SystemAn electronic value control (EVC) system replaces the mechanical camshaft, controlling each value with actuators for independent value timing. The EVC system controls the opening and closing time and lift amount of each intake and exhaust valve with independent actuators on each value. Changing from a mechanical camshaft driven value into independently controlled actuator valves provides a huge amount of flexibility in engine control strategy. Vehicles utilizing EVC can realize several benefits including:1) increases engine power and fuel economy,2) allows centralized and distributed EVC systems to perform at their full potential,3) adapts to engines of varied cylinder counts.With all of the improved efficiencies and consumer benefits, auto manufacturers are eager to get their first EVC systems on the road. The EVC system is targeted to operate in temperatures up to 125, while the actuator is targeted to run up to 6000 r/min. The actuator can be controlled in a centralized system with a highspeed multiplex bus (up to 10Mbps) or in a distributed system with a nominal speed bus.EVC systems must be pact in size, specifically the actuators that must be small enough to fit in the engine space. A vehicle that uses a 42V system is ideal for EVC because it requires high voltage to control the value actuators, and EVC is targeted for V8 and V12 engines. The EVC system is also highly flexible, allowing adaptability for a number of cylinder engines. New Wordscoordinate 協(xié)調(diào) valve train 氣閥傳動(dòng)respectively 分別的，各自的 overhead camshaft 頂置凸微輪軸guide 導(dǎo)管 tappet 挺桿valve insert 氣門座 cotter 鎖銷，鎖片opening 口 lobe 凸起spot 點(diǎn)，位置 stem 桿dual 雙的 cam follower 凸輪挺桿seesaw 蹺蹺板，杠桿 value clearance 氣門間歇gastight seal 氣封 liable to 容易likelihood 　 　可能 tapping 輕敲valve timing 配氣正時(shí) intake valve 進(jìn)氣閥 exhaust valve 排氣閥 static 靜態(tài)的，靜力的kinetic （運(yùn)）動(dòng)的，動(dòng)力（學(xué)）的 volumetric 測(cè)定體積的leverage 杠桿作用 offset 偏移量dynamics 動(dòng)力學(xué) valve lift 氣門挺桿valve…as… 把…..看成… parameter 參數(shù)，參量radius 半徑，范圍 flank 側(cè)面 pairing 配對(duì)，成對(duì) correspond with 相當(dāng)于 gear ratio 傳動(dòng)比 cogtype belt 齒型帶synthetic rubbe