05-02-2015, 10:42 PM
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chinnikrushna
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variable valve timing in ic engine of mechanical engineering ppt
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please allow me to download this.
chinnikrushna
14-03-2017, 02:19 PM
In internal combustion engines, variable valve timing (VVT) is the process of altering the timing of a valve lift event, and is often used to improve performance, fuel economy or emissions. It is increasingly used in combination with variable valve lift systems. There are many ways in which this can be achieved, ranging from mechanical devices to electro-hydraulic and tubeless systems. Increasingly stringent emissions regulations are causing many automakers to use VVT systems. Two-stroke engines use a power valve system to obtain VVT-like results. The main factor that prevents this technology from widely used in production cars is the ability to produce a cost-effective means of controlling the timing of the valve under internal conditions to a motor. A motor running at 3000 rpm will rotate the Cams 25 times per second, so valve timing events have to occur at precise times to deliver performance benefits. The electromagnetic and pneumatic valve actuators offer the most precise control of the precise moment of the valve, but in 2014 they are not profitable for the vehicles of production.
Early and late shutdown of the exhaust valve can also reduce emissions. Traditionally, the exhaust valve is opened and the exhaust gas is pushed out of the cylinder and into the exhaust manifold by the piston as it moves up. By manipulating the exhaust valve timing, engineers can control the amount of exhaust gas remaining in the cylinder. Keeping the exhaust valve open slightly longer, the cylinder empties more and is ready to be filled with a higher air / fuel charge in the intake stroke. By closing the valve slightly early, more exhaust gas remains in the cylinder which increases fuel efficiency. This allows for more efficient operation under all conditions. Manufacturers use many different names to describe their implementation of the various types of variable valve timing systems. These names include: AVCS (Subaru), AVLS (Subaru), CVPS (Nissan, Infiniti), CVVT (Alfa Romeo, Citroën, Geely, Hyundai, Iran Khodro, Kia, Peugeot, Renault and Volvo) (General Motors), DVVT (Daihatsu), MIVEC (Mitsubishi), N-VCT (Nissan), S-VT (Mazda), VANOS (BMW), VarioCam (Porsche), VVC (VW), VVC (VW), VVT (VW), VVT-i (Toyota, Lexus), VTVT (Hyundai, Kia)
The first variable valving systems were used discretely (stepwise adjustment). For example, one synchronization would be used below 3500 rpm and one used above 3500 rpm. The more advanced "variable valve timing" systems offer a continuous (infinite) valve timing adjustment. Therefore, the timing can be optimized to suit all engine speeds and conditions. The simplest form of VVT is the cam phase, where the angle of a camshaft is rotated forward or backward (relative to the crankshaft). Thus, the valves open and close before or after; However, lifting and length of the camshaft can not be altered with a cam phase system. Obtaining a variable duration in a VVT system requires a more complex system, such as multiple cam profiles or oscillating cams.
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