Understanding Toyota's VVT-i System
By now, you have probably heard a mechanic or automotive enthusiast tell you that an engine is essentially a large air pump. Basically, the more air an engine can suck in to combine with fuel, the more power it can create through combustion. It also follows that the more efficiently an engine can remove exhaust gases from the cylinders, the better it can manage that power. Air flow from one end to the other is the key to a healthy, strong engine.
Air flow is affected by many different components in the motor, but the valves in the cylinder head are what directly control the amount of air entering a cylinder, and the volume of exhaust gases leaving it. The intake valves open up just prior to combustion in order to allow air to flow in and mix with fuel, and the exhaust valves open after the ignition of this mixture in order to suck out the resulting gases. The timing of the valves is controlled by a rotating shaft called the camshaft. The camshaft has lobes which push up on the valves in order to open them and drop them back closed again.
How long these valves remain open, and at what point in the combustion cycle, can have a big impact on the drivability and power generated by an engine. For instance, if you want to have a really fast car, like a race car, you'll want the engine to produce a lot of power at high RPMs. You can adjust the camshaft to perform well at higher RPMs. This will result in poor performance at low RPMs, but that's OK with a race car. Conversely, if you want a lot of low-end torque - which is great for towing - you need to adjust the camshaft to perform well at low RPMs. This, of course, will hurt high RPM performance.
Unfortunately, street vehicles are a compromise between reliability, fuel efficiency and power. While race vehicles have engines with camshaft designs that generate large amounts of power while being used only at specific, high revolutions, your daily driver sees a wide range of RPMs that make a broader power band necessary. While it is ok for a race car to have a lumpy idle that barely runs below 1000 rpm, it would do you no good if your street car stalled out at every stoplight. Regular vehicles usually have to make do with a camshaft that provides a good amount of power in the most often used range of engine RPMs, but runs out of steam at high speeds.
These types of camshafts obviously aren't too efficient. Since they're trying to do everything adequately, they don't really do any one of them superiorly. Your engine needs to be able to perform just as well accelerating from a stop as it does speeding down the highway, which means that much of the time, it's burning too much fuel and also underperforming.
Automakers know about this problem, and have created something called "variable valve timing" (VVT) in response. The Toyota Tundra's i-Force 5.7L V8, Toyota's newest VVT-i engine, can use engine oil pressure to move the camshaft slightly, so that the timing of the valves can be adjusted in relation to engine speed. This way, more aggressive lobe designs can be used when the engine is working at a higher RPM. The VVT system allows the i-Force V8 to run a camshaft profile that gives good fuel efficiency in regular driving, but that can also crank up the power when it's called for.
The dual VVT-i in the Tundra takes things a step further by allowing the exhaust and intake valves to open at the same time at very high RPMs in order to scavenge the airflow as much as possible. This all adds up to a V8 engine that produces 381 horsepower at 5600 rpm while still generating 401 lb-ft of torque at as low as 3600 rpm. Not only that, but in the 2 wheel drive models, the Tundra gets a respectable 20 miles per gallon on the highway. Perhaps most importantly, Toyota's variable valve timing system lets you have killer horsepower without getting killed at the gas pump.
Article Source: http://www.search-raven.com
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The author Jason Lancaster operates TundraHeadquarters.com, a website with info, news, and reviews of Toyota Tundra accessories and Tundra parts.
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by: JasonLancaster
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