VTEC Engines
Acura's VTEC Engines
Normal Engines
In normal engines the valves that supply the engine with air and fuel open and close at the same rate, the same distance, and the same length of time at all rpm. The problem is that the car will need air quicker at high rpms. An overlap of exhaust valves closing and intake valves open also helps by providing a suction. This suction brings intake air in faster.
Cars that are designed for speed need to be most efficient at high rpms. At higher speeds more power is require to accelerate 1 mph then at lower speeds. This being said, a sporty car or race car needs more power at the high end rpms to increase its speed quicker. For example, Nascar cars do not run well when idling. The valves open too long and overlap(exhaust is still open when intake opens). The cams are designed so the engine performs its best in the high rpms of the race.
Trucks designed for strength(designed to have large amounts of torque at low rpm to tow large loads) have power at higher RPMs but do not perform well in those high rpms. Regular driving cars lack power at both high and low rpm but work efficiently at mid range rpm.
The Problem
There problem because the valves are designed to open and close at a rate and distance that will work the best for its application. And when that vehicle's rpm is out of that range it loses power and efficiency. This is because the vehicles valve specifications, meaning opening distance and time, do not work well for the RPM.
VTEC Engines
The VTEC(Variable Valve Timing and Lift Electronic Control) systems fixes these problems. It is a system that changes the properties of the valves to fit the RPM. When at a lower RPM it uses 2 smaller cams on the camshaft to push the valves open and closed. This allows for less overlapping of the exhaust and intake valves. They open at a smaller height for a shorter time. When at around 6000 rpm, it switches to one large cam in the middle of the other 2, this single cam pushes both valves. The larger cam pushes the valves farther and for a longer time period, overlapping occurs between exhaust closing and intake opening, which allows more air to enter the cylinder creating more power at a higher RPM. This system makes it possible to get more power at all RPMs. The Vtec system also provides better fuel economy, and lower emissions.
Please go to the link below to find more information and diagrams on the VTEC system.
For More Information
Acura
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