Our Point of View: VVT-i: It's All a Matter of Timing

February 08, 2008

by Toyota Open Road Blog

Just when you begin thinking that engineers have gotten everything out of the internal-combustion that they can get out of it, they come up with something new that gets more out of it.

Here’s a case in point: Variable Valve Timing-intelligent, or VVT-i.

VVT-i is a way to enhance performance, economy and emissions through control of the phasing, or timing, of an engine’s camshafts.

Stick with me, now, I’m going to get technical: The valves that admit air-fuel mixture into the engine’s combustion chambers, and that open to allow the burned gasses to be pushed out as exhaust, are controlled by egg-shaped lobes on these camshafts, as you see on the image to the left.

The shape of those lobes, and their positioning on the shaft, determine a cam’s timing. In most cases, cam timing is fixed, and that means that it’s a compromise that provides optimal performance only at the speed at which the engine is likely to do most of its work.

But engines operate over a wide range of speeds, from an idle speed of about 600 rpm to a peak speed of more than 6,000 rpm. What if cam timing could be optimized over the engine’s entire operating range?

That’s precisely what VVT-i does.

Using a combination of electronics and hydraulics, VVT-i allows the intake cam to change phase, or timing, over a range of about 30 degrees. Changing the cam timing improves engine breathing at all rpm because the combustion chambers get an optimized volume of air/fuel mixture over a broader range of conditions. The result is more power, more torque, reduced emissions and enhanced fuel economy.

But what if we were able to change the phase of not just the intake cam, but also the exhaust cam? That’s exactly what Dual VVT-i does. By varying the phasing of the exhaust cam by about 30 degrees, we can vary the phasing of the intake cam by a wider range – by about 45 degrees. That means there’s a broader range over which we can open and close the intake valves. And that means we can extract still more power, torque, economy and better emissions.

The way this all works is that a computer controls an oil pump, which controls an oil pressure valve. This valve controls oil pressure in two chambers of the cam-sprocket housing. Based on the oil pressure on both sides of a cam vane inside the sprocket housing, seen here in the below image, the cam’s timing will advance or retard. More oil pressure on one side of the vane rotates the cam in one direction, more oil pressure on the other side of the vane rotates it the other way. This of course is independent of its basic operational rotation, which is provided by the cam’s chain drive .

The oil pressure that controls cam position is, in its turn, based on rpm, load and throttle input. It’s all quite nicely and efficiently managed; the system’s operation is seamless, and works as quickly as a computer can think. Applied to any of our engines, dual VVT-i results in 10-12% more horsepower and torque, and in improved efficiency.

The system also performs the function of exhaust-gas recirculation (EGR) without the external hardware usually associated with this function. It works by leaving the intake valve open just a little, early on the piston’s exhaust stroke. This lets the piston blow a little exhaust gas back into the intake tract – which is basically what an external EGR system does. So there’s no need for a separate EGR system.

There’s another benefit, as well. Use of VVT-i basically trumps cylinder deactivation. Cylinder deactivation, of course, is a way to burn less fuel by allowing a V8 engine, for instance, to run on as few as four cylinders. But cylinder deactivation works best on unladen vehicle driven at small throttle openings. If you add a load or if you accelerate, the engine has to work harder, so it picks up cylinders again and becomes a full V8. The benefits of the technology disappear. Indeed, testing has shown that it’s hard to keep these engines in cylinder-deactivation mode. If you accelerate or add a load, they go to full V8 mode, and you lose the benefits of the system.

VVT-i, on the other hand, is active all the time. It provides its benefits all the time, loaded or unloaded, to optimize economy, emissions and horsepower. That’s why we’re so enthusiastic about it.
Originally published on the Toyota Open Road Blog on Feb. 8, 2008.

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