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Originally Posted by CarAndDriver
Most modern variable valve-timing (VVT) systems use a cam phaser that rotates the position of each cam shaft relative to the timing chain. Think of making a record turntable go faster or slower by spinning it with your hands. The cam phaser has two basic com ponents: an outer sprocket connected to the timing chain and an inner rotor (connected to the camshaft) that varies the valve timing by adjusting the rotation angle of the cam.
This inner rotor consists of a set of lobes, and oil fills the space between the outer housing and the lobes. Left alone, the rotor will simply spin at the same rate as the outer housing. If you add oil to one side of the lobe and remove it from the other, the rotor moves, and—voilŕ!—there’s your variable valve timing.
The majority of these VVT systems use oil pressure to push the rotor back and forth, but BorgWarner thinks its cam-torque-actuated (CTA) system marks an important step forward. Oil-pressure-actuated (OPA) systems require an upsize oil pump to pro duce the extra pressure that’s required to work the cam phasers, which saps some of the fuel-economy gains of VVT. With a mechanical oil pump, OPA systems don’t work well at low engine speeds because the pump doesn’t build pressure and volume until the revs get higher.
The CTA system avoids those pitfalls by using Newton’s Third Law of Motion—for every action there is an equal and opposite reaction—to move the oil in the cam phas ers. When a cam lobe pushes a valve open, the valve spring resists that force and pushes back. Similarly, when the valve spring pushes a valve closed, it also pushes on the cam lobe in the opposite direction from the valve opening. When multiplied over an entire camshaft, there is enough energy from these back-and-forth forces to make cam phasing work.
Another trick to BorgWarner’s system is the way it moves oil. A center spool valve, controlled by a solenoid inside the cam-phasing rotor, directs the flow. With the valve open in one direction, oil flows into only one side of the oil pockets and can’t leave. By sliding the valve back and forth, the system can mete out the precise amount of oil flow on either side of the rotor lobes.
The key advantages of the CTA system are that it responds quickly even at idle and can operate using a standard engine’s oil pump. But there are downsides. As engine speeds increase, the CTA system becomes less effective. This happens because the valve events occur more frequently, reduc ing the time available to move the oil. Con versely, OPA systems work better as oil pres sure increases and are better at high rpm. So there’s not much of a peak power gain from a CTA system; it improves performance and efficiency in other areas of the rev range. Also, CTA cam phasing is at the mercy of the natural oscillations of those forces on the camshaft. Valve openings and closings in an inline-six are spaced too closely for the sys tem to work well. But a V-6 (or inline-three) is perfectly suited because there isn’t as much overlap between each valve event. The system also works on V-8 engines.
CTA variable valve timing debuted on Ford’s 3.0-liter Duratec V-6, beginning with the 2009 Escape and the 2010 Fusion. The 3.7-liter V-6 in the Mustang uses BorgWar ner’s system, too, as do the 2011 Edge and Lincoln MKX. You can also find it on the Mustang’s 5.0-liter V-8 as well as the V-8 engines used in Jaguar and Land Rover vehi cles. These engines’ efficiencies show the vir tues of the CTA system.
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