VCM in 09 vs. 06-08

[youbetcha]

The VCM in the new pilot chooses between 6, 4, or 3 cylinders; the prior version only chose between 6 or 3.

Is the difference just software/chip related, is it something that can be "upgraded" in the prior engine?

[iivtecracerii]

its both. they need new camshafts and VTEC mechanisms to fun 4 cylinder mode, since 4 cylinder cuts one cylinder in the front bank and one cylinder in the rear bank.

[nikonster]

FYI- VCM was available in '08 but only on the 2WD. Mine has it. I like the "ECO" indicator on the dash. It definitely influences my driving.

[btk467]

Originally posted by iivtecracerii
its both. they need new camshafts and VTEC mechanisms to fun 4 cylinder mode, since 4 cylinder cuts one cylinder in the front bank and one cylinder in the rear bank.

Are they the same cylinders? Or they different each time when ECO mode engages?
Thanks.

[kramerd1506]

Could someone explain how VCM works?

After thinking about it, it seems like over time on an '06 model like mine, certain parts would receive more wear. Some cylinders aren't running nearly as much as others. What keeps those cylinder walls, rings, valves, cam shaft etc... from wearing at a different rate than the other 3 cylinders that run all the time?

[youbetcha]

Originally posted by kramerd1506
Could someone explain how VCM works?

After thinking about it, it seems like over time on an '06 model like mine, certain parts would receive more wear. Some cylinders aren't running nearly as much as others. What keeps those cylinder walls, rings, valves, cam shaft etc... from wearing at a different rate than the other 3 cylinders that run all the time?

It isn't the case that some are being used more than others.

All cylinders are going up and down all the time, in either VCM or "normal" mode.

VCM doesn't save on frictional metal-to-metal losses in the engine. It solves a different problem. When you are operating at very low throttle, there is a loss of efficiency in moving air past the throttle. When the VCM comes on, it shuts off the AIR to three cylinders (or 3-4 cyls in the 09 on) by closing off the valves. In order to produce the same power, the remaining 3 cylinders are loaded up with additional air and fuel, which means that the throttle is opened up. Opened throttle means less energy is lost moving the air through the engine. It is good for about 10% efficiency on the highway.

In all other aspects, the shut-off cylinders are moving up and down, are being lubricated, etc.

I also read that Honda tested the underlying VTEC system for 400,000 on-off cycles. I wouldn't be concerned about durability.

[kramerd1506]

Wow! Thanks for that. It explains a lot.

[whizmo]

Originally posted by youbetcha
VCM doesn't save on frictional metal-to-metal losses in the engine. It solves a different problem. When you are operating at very low throttle, there is a loss of efficiency in moving air past the throttle. When the VCM comes on, it shuts off the AIR to three cylinders (or 3-4 cyls in the 09 on) by closing off the valves. In order to produce the same power, the remaining 3 cylinders are loaded up with additional air and fuel, which means that the throttle is opened up. Opened throttle means less energy is lost moving the air through the engine. It is good for about 10% efficiency on the highway.

The concept is right here, but I don't think the throttle position or throttle plate has much to do with it. AFAIK, the Pilot engine has a single throttle plate and basically the same amount of air flows past it for a given power level whether it is in VCM mode or not. To make a given amount of power, it takes a certain amount of intake charge whether you're running on three cylinders are six. The throttle is in the same position whether in VCM mode or not and the pumping losses past the throttle plate are the same.

But you do save losses by eliminating the pumping losses in the shut-down cylinders. (When the valves are closed, the air in the cylinder acts like a big spring and you get nearly all the energy of compression back out on the power stroke.) Pumping losses in the working cylinders are essentially unchanged so you save the losses in the shut-down cylinders.

Basically a few cylinders working harder are more efficient than lots of cylinders working less hard.

There are some new technology engines that eliminate the throttle plate (and its losses) by throttling the engine with changes in valve lift, duration, and timing. But this isn't very common yet.

- Mark

[btk467]

Thanks for the reply. It makes more sense now.

[youbetcha]

Originally posted by whizmo


The concept is right here, but I don't think the throttle position or throttle plate has much to do with it. AFAIK, the Pilot engine has a single throttle plate and basically the same amount of air flows past it for a given power level whether it is in VCM mode or not. To make a given amount of power, it takes a certain amount of intake charge whether you're running on three cylinders are six. The throttle is in the same position whether in VCM mode or not and the pumping losses past the throttle plate are the same.

But you do save losses by eliminating the pumping losses in the shut-down cylinders. (When the valves are closed, the air in the cylinder acts like a big spring and you get nearly all the energy of compression back out on the power stroke.) Pumping losses in the working cylinders are essentially unchanged so you save the losses in the shut-down cylinders.

Basically a few cylinders working harder are more efficient than lots of cylinders working less hard.

There are some new technology engines that eliminate the throttle plate (and its losses) by throttling the engine with changes in valve lift, duration, and timing. But this isn't very common yet.

- Mark

I think you'll agree that when you have three cylinders working instead of 6, you have half the displacement available each revolution. And I agree that you need to fill them up with approximately the same amount of charge (the same work is being done).

So the question is, what is actually different and why is it more efficient? It sure looks like you need the same amount of air and fuel! Well ...

Keep in mind that the amount of time the intake valves are open for 3 cylinders is only about 1/2 the available time that you have with 6 cylinders. The throttle HAS to be open wider to accommodate the increased airflow requirement (about the same total airflow in a full engine revolution but greater bursts occurring in less time). Remember you have half the displacement to fill but the same amount of air.

The reason for doing this is that a "mostly" closed throttle creates much lower cylinder pressure which reduces the efficiency of the charge. Opening the throttle increases airflow when the cylinder is filled and also the cylinder pressure, increasing the efficiency.

Other technologies exist to increase cylinder pressure, like turbocharging, of course, which work in high airflow situations, but the purpose of VCM is to increase cylinder pressure at very low throttle.

The spring action of the air in the closed cylinders is a zero sum, by itself this doesn't contribute nor take away from the operation of the other three cylinders.

[theirishscion]

Originally Posted by youbetcha

I think you'll agree that when you have three cylinders working instead of 6, you have half the displacement available each revolution. And I agree that you need to fill them up with approximately the same amount of charge (the same work is being done).

So the question is, what is actually different and why is it more efficient? It sure looks like you need the same amount of air and fuel! Well ...

Keep in mind that the amount of time the intake valves are open for 3 cylinders is only about 1/2 the available time that you have with 6 cylinders. The throttle HAS to be open wider to accommodate the increased airflow requirement (about the same total airflow in a full engine revolution but greater bursts occurring in less time). Remember you have half the displacement to fill but the same amount of air.

The reason for doing this is that a "mostly" closed throttle creates much lower cylinder pressure which reduces the efficiency of the charge. Opening the throttle increases airflow when the cylinder is filled and also the cylinder pressure, increasing the efficiency.

Other technologies exist to increase cylinder pressure, like turbocharging, of course, which work in high airflow situations, but the purpose of VCM is to increase cylinder pressure at very low throttle.

The spring action of the air in the closed cylinders is a zero sum, by itself this doesn't contribute nor take away from the operation of the other three cylinders.

Wow! And I thought I had a pretty good idea how VCM worked before I read that. Thanks for the excellent information/education.