How Toyota's Valvematic Works

[jeff88]

Been researching Toyota engines recently and my research has found that Toyota has developed a technology called Valvematic, which changes the lift amount based on load.

Because it controls lift, it does not require a throttle plate (which helps reduce pumping losses), but I am of two minds here. Yes the removal of the throttle plate reduces pumping losses, but wouldn't the intake valves just act like the throttle plate, in that they stop/slow air flow, making the pumping losses the same?

Can anybody give me insight into how the system works? What actually makes it open more or less?

Are there any other makes that employ similar technology?

[t vago]

"Pumping Losses" is shorthand for describing how much work is needed to generate and maintain a vacuum in the intake manifold. If there is no throttle plate, there is no intake manifold vacuum - hence, no pumping losses.

[cRiPpLe_rOoStEr]

Originally Posted by jeff88

Are there any other makes that employ similar technology?

BMW has the Valvetronic, Fiat has the MultiAir/TwinAir, and right now I don't remember about other manufacturers...

[serialk11r]

There's 2 parts to it. The Valvematic (and VVEL/Valvetronic) essentially adds an adjustable rocker between the camshaft and the valve actuator so you can change essentially the point at which the cam is "engaged". This means lift and duration are decreased together, which is why the peak torque on the Valvematic engines can be a little bit lower (though the powerband is likely going to be better overall).

If you were to have say an adjustable rocker arm that only changed lift but not duration, that would give you independent throttle bodies right at the valve. This is better than a throttle body because once the valve is closed no air leaks through, so you don't have to draw as deep of a vacuum as say an independent throttle body setup because while the intake is closed the air is still leaking in. It's also better because 100% of the vacuum is contained in the cylinder, meaning your losses are exactly the increase in entropy from the expansion of the air into the cylinder, no more (draw some PV diagrams to see this). With a throttle, those losses look smaller at first, but then since air is constantly leaking through the throttle you end up with more losses.

Being able to adjust duration though means you get some air metering capability that does not cost you pumping power, which is where most of the gain comes from.

Another benefit is that with less lift you get more favorable flow velocity (aka better fuel vaporization) at low speeds.

[jeff88]

Originally Posted by serialk11r

It's also better because 100% of the vacuum is contained in the cylinder, meaning your losses are exactly the increase in entropy from the expansion of the air into the cylinder, no more (draw some PV diagrams to see this).

Thanks serialk11r! After rereading this I was wondering what a 100% vacuum in the cylinder means for PCV (i.e. does it even need one, or would it actually increase crankcase pressure?)


Is this what you are talking about with the PV diagrams? (I've never actually seen this before.) Are you saying that with Valvematic, the PV "rectangle" will be flat rather than slightly curved?

[mr220v]

Looks like it employs some kind of intermediate rocker shaft. Looks like a good system. Not overly complicated.

Vtec also manipulates lift, but it is not variable, and only has 2 settings.

[some_other_dave]

VTEC basically gives you two completely independent cam grinds to choose from on the fly. Porsche has a "Vario-Cam Plus" system that pretty much does the same thing, lets you pick one of two cam grinds on the fly. Porsche's system lets you also adjust the cam timing (or at least phasing, the timing relative to the other camshaft) while I'm not sure if VTEC lets you vary that or not.

Most manufacturers have variable-valve-timing mechanisms available on at least some of their engines. At least some have mechanisms that also can vary the lift, either by choosing a specific cam grind, or in an arbitrary manner.

-soD

[Occasionally6]

Originally Posted by jeff88

Thanks serialk11r! After rereading this I was wondering what a 100% vacuum in the cylinder means for PCV (i.e. does it even need one, or would it actually increase crankcase pressure?)


Is this what you are talking about with the PV diagrams? (I've never actually seen this before.) Are you saying that with Valvematic, the PV "rectangle" will be flat rather than slightly curved?

Yes, that's a PV diagram but it's only for the working strokes; compression and power strokes. The area described by those lines is the work done by those strokes. (Check: the units of P x V are those of work.)

What's missing is the pumping strokes; inlet and exhaust. They form (ideally) a rectangle that is described by the inlet and exhaust manifold pressures and the volumes at the top (same as 1,2) and bottom (same as 3,4) the strokes.

That is also work but work the engine has to do. It has to be subtracted from the working stroke work (!) to get the net work out.

Lower inlet manifold pressures i.e. light engine loads and throttling increase the area of the pumping work. That also moves the working loop down (all the pressures are reduced) but doesn't alter its shape.

[PaleMelanesian]

Honda has a system on many of their newer engines, that holds the throttle valve open and uses valve timing instead. At low load, it holds the intake valve open past bottom center, so some of the cylinder's contents are pushed back to the intake. A smaller fuel/air charge is ignited in the cylinder, producing lower power while still having a more open throttle.