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The Variable Valve Timing and Lift Electronic Control (VTEC) System

Thảo luận trong 'Tiếng Anh chuyên ngành' bắt đầu bởi bmt-hui, 5/3/10.

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    We’ve all heard the acronym “VTEC” thrown around constantly in the world of cars. So much so that “VTEC” has become massively misused over and over again so that it is used now with sarcasm. But here I’m going to clear up all the misunderstanding and explain the world of valvetrain technology. For those a bit less invested in the world of auto technology, I’ll explain the valvetrain for you a bit. The valvetrain is what controls what goes in and out of the cylinder (intake and exhust). It consists of the camshaft, the rocker arms, and of course the valves. The camshaft is a long rod rotating along with the engine running along the length of the engine. It has a few protrusions (the cam profile) coming out of it which are intended to push a set of rocker arms which then push open the valves. The valves are the “doors” that let the air and exhaust enter and exit the cylinder during the four-stroke combustion cycle.
    For a small history lesson; Honda brought the VTEC system to the masses in Japan in 1989 with their Intergra and CRX SiR models. The U.S. was introduced to it in 1991 with the arrival of their everyday supercar NSX which produced 290 horses with only 3.0 liters of displacement. Honda then installed their new tech into the Type R versions of their B16 and B18 engines for an easy 100 plus hp per liter. The B16B built specifically for the Civic Type R boasted an impressive 115.8 hp per liter in a relatively affordable car. The VTEC system culminates in the S2000, which is the only car to use their infamous F20c engine. It produces 240 horsepower with “only” 2.0 liters of displacement and no forced induction. Introduced in 1999 to celebrate the new millenium, the S2000’s F20c still has the highest specific output for a naturally aspirated 2.0 liter car under $100,000.
    Let’s get down to the tech side here. V-T-E-C stands for Variable valve Timing and lift Electronic Control. In english, VTEC (and all other variable valve technology) modulates the valves of each cylinder with a computer (just like everything else in this world). VTEC doesn’t modify the valves directly, but rather the camshafts that the valves are connected to. As the engine turns, the camshafts turn in relation and they push the valves open for the cylinder. What VTEC does is change both the duration and lift of the valves as engine speeds change by using different cam profiles. For example, for maximum performance at high RPMs, both valve lift and duration should be relatively longer. But for drivability, comfort, fuel consumption, and emissions, valve duration and lift should be relatively minimal. So for a minivan, larger cam profile is not needed, but for a performance-oriented car that is driven both in traffic on the road and on the track, the best of both worlds are needed. If you want shorter duration and lift, you want your profiles to be smaller, since they won’t push open the valves as far and for as long. If you want longer duration and lift, you obviously want a larger profile. How do we get both smaller AND larger profiles onto one camshaft? For our example, let’s imagine two cam profiles pushing two rocker arms to open two valves. Here’s a simple diagram:
    <img class="alignnone size-full wp-image-1311" alt="" height="280" width="199">
    “3_a1.gif.” Image. Honda Motor Co. “The VTEC Breakthrough: Solving a Century Old Dilemma.” Honda Worldwide. N.p., 2009. Web. 3 Dec. 2009. <Honda Worldwide : Honda Motor Co.,Ltd.
    automobile-technology/VTEC/>.

    What Honda decided to do was to build a camshaft with two cam profiles and the ability to switch between them. When the time comes to switch from low-RPM comfort mode to high-RPM performance mode, the camshaft makes a small change and switches to its longer duration and lift cam profile. And then the car slows back down to lower-RPM driving, the camshaft profile switches back. So how does Honda change the cam profiles on the fly? Quite simple actually.
    What VTEC does is install another rocker arm (let’s call it the VTEC rocker) between the two rocker arms, and a larger cam profile to operate this larger rocker arm. This larger cam profile would obviously push the valves open for a longer duration and lift. The larger cam profile and rocker doesn’t push anything open as there aren’t any valves underneath it, but are just situated between the smaller rockers. Like here:
    [​IMG]


    During low-RPM operation, the valvetrain works normally; ie. the smaller profiles push the smaller rocker arms which then push open the valves. But when the engine enters its higher-RPM range, a hydraulic system pushes a pin that connects the VTEC rocker with the two rockers beside it. When that happens, the larger VTEC cam profile pushes all three rockers even further due to the fact that the VTEC rocker arm is now connected to two non-VTEC rocker arms. When the larger cam profile isn’t needed anymore, the pin disengages and unconnects the three rocker arms, reverting back to the smaller cam profile. An animation for your entertainment:
    [​IMG]


    It’s quite a simple mechanism actually, just a simple trigger (a certain RPM), a hydraulic system, and an extra set of cam profiles and rockers. The problem with VTEC is that it is an “on/off” switch, where the cam profiles are either small or large. The next step is to continuously modulate the cam profile for any and every driving condition. BMW introduced its “Valvetronic” system in 2001, which Toyota followed with its “Valvematic” system and Nissann with its VVEL (Variable Valve Event and Lift) system.
    Well, that’s basically how the VTEC system (and most variable valve systems) work. Leave a comment if you’d like.
    SOUSER BY :takemebeyondthehorizon.wordpress.com
     

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