Most VVT is measured in early and late valve opening and closing. Closing intakes later, for example, can result in reduced pumping losses in partial load conditions, which reduces nitric oxide NOx emissions while only marginally affecting torque output. By contrast, early intake valve closing has the same effect at higher vacuum, but also improves fuel economy by up to seven percent. Similarly, early intake valve opening reduces most emissions and improves fuel economy due to the improvement in volumetric efficiency the process includes.
When opening early, the valve will send hot exhaust through the intake valve where it is momentarily cooled in the manifold before returning to the cylinder in the next stroke. This process is also called valve overlap.
As engine technologies improve and become less expensive, VVT continues to improve performance and economy. Toyota's VVT-i is the latest-generation of this technology in their engines and combines several facets of valve control. To understand how VVT-i works, we first need to look at its underlying technology.
In a nutshell, VVT alters the timing of valve lifting to improve performance and economy in specific driving situations, usually dictated by RPM ranges. The idea behind VVT has been around for nearly two centuries, being first introduced in crude form in steam engines and later becoming common in aircraft and then automotive. In automotive, several methods of varying the valves' lift and timing have been employed.
Toyota's VVT is a cam phasing system, which is one of the most common in use. It uses a variator, which is hydraulically-controlled, to change the size of the inlet and exhaust valve openings, which also affects the duration of those openings as well.
In dual overhead camshaft engines, this allows the timing of each opening inlet and exhaust to be controlled through simple manipulation of the size of the cam lobe being used against the valve lifters. The cam lobe size in Toyota's system is in pairs, with the shorter one being immediately next to a taller lobe. A dual system of lifter arms is used with one for each lobe.
When he shorter lobe is in use, the larger lobe's lifter is "free" unlocked thus creating no lift as the lobe passes under it.
When activated, this second lifter is hydraulically locked and the larger lobe becomes the dictation for cam lift. Hydraulics are controlled through engine rotation speed, with higher speeds activating the higher lift. This basic technology is combined with what Toyota calls "intelligence" to improve its performance further.
Adding the "intelligent" part to VVT, VVT-i improves the timing control further by not only varying the intake valve opening and closing height and duration through the camshaft and lifters, but by further controlling the duration through change in the camshaft's rotation itself.
On a duel overhead camshaft system DOHC , this allows control of the overlap time between intake and exhaust valve closing and opening. The system works by using a variable-speed head for the camshaft. This head, or camshaft gear, is where the timing system belt, scissor-gear, or chain give the rotational force to the camshaft itself.
The gear head is a hollow structure in which oil pressure can be raised or lowered to allow a two-piece floating system to speed up or slow down the head's rotation in relation to the actuator.
Visualize this as a hollow, enclosed gear inside which two star-shaped gears are placed one inside the other. It not only varies the timing but also lift the valves. It varies the timing of the intake valves by adjusting the relationship between the camshaft drive belt, scissor-gear or chain and intake camshaft. Does not lift the valves. Working Principle In an automobile engine the intake and exhaust valves move on a camshaft.
With this technology The intake camshaft is capable of advancing between 25 and 50 degrees when the engine is running. Phase changes are implemented by a computer controlled, oil driven adjustable cam gear. Phasing is determined by a combination of engine load and rpm, ranging from fully retarded at idle to somewhat advanced at full throttle and low RPM. The effect is further optimization of torque output, especially at low and midrange RPM. Valve lift and duration is still limited to distinct low- and high-RPM profiles.
With this technology The timing of the intake valves varies by adjusting the relationship between the camshaft drive belt, scissor-gear or chain and intake camshaft. Engine oil pressure is applied to an actuator to adjust the camshaft position. Adjustments in the overlap time between the exhaust valve closing and intake valve opening results in improved engine efficiency. Follow Share Cite Authors. Anonymous comments 2 January 14, , pm VVTI es el mejor mejor consumo y potencia en vajas y altas y es muy silencioso — Related Comparisons.
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