cylinder shut down tech, how does a V8 run on 3 cyls?

[IN 05 NT]

all this talk on how new big bore v6's, V8's cars have the option to shut down cylinders for better fuel efficiency,

but how does it work? ie how can a V8 shut down 5cyl's and still run smoothly?

cant find much info on the net....that goes into detail.

[Draven]

5?

I thought most of the 8s shut down 4 cyls..

I assume they have 2 cranks. the principle is easy... getting the cylinders to start firing at the right time as the 2nd load of pistons come on would be the challenging part.

[IN 05 NT]

the chrysler 300c add says 3 cyls i think?

[big_zop]

5?

I thought most of the 8s shut down 4 cyls..

I assume they have 2 cranks. the principle is easy... getting the cylinders to start firing at the right time as the 2nd load of pistons come on would be the challenging part.

Do you mean two V4s connected end on end (with some sort of clutch system to connect the two) as separate cranks would not make a 'vee' 8?

My understanding is that it is even numbers, although the Accord V6 (with the drumming add) goes 6-4-3 so that puts that out the window. I know it would have to do with balance so im not too sure what would be the best in each case.

What i would like to see is the proof that cylinder deactivation actually saves fuel because to travel at a constant speed would require a certain amount of power to overcome all the drags present (powertrain, wheels, wind) and that power is constant and it doesnt matter how many cylinders is producing it, it should consume the same amount of fuel.

(for a V8) The only pros i can see is if its running on 4 cyl that it would be more efficient at converting a certain amount of fuel to power (remembering that the overall combustion process is very inefficient) than it running on 8 cyl. I know that the closer you run to full throttle the more efficient your engine is, so 4cyls would need more throttle percentage to deliver the same power as 8 cyl. But then you would have to deal with the drag caused by having to turn the camshaft/s, and move the valves/pistons/conrods that arent contributing to this power generation.

I just wonder if its more a marketing gimmick than a real world benefit - especially when you bury the throttle and cylinder deactivation is no longer required. Besides this, the biggest impact to fuel economy is weight and cars are getting heavier these days due to safety requirements and equipment and to an extend manufacturing costs - we cant all have exotic materials.

[MR22ZZ]

the Honda V6 in the Accord runs on 6, 4 or 3 cylinders, that may be the one you are confusing it with

EDIT: Big Zop beat me

[CrUZida]

The Chrysler Hemi V8 drops onto 4 cyls I'm pretty sure, and saves either 1 or 2 L per 100km.

Not a huge saving.

No reason why you couldn't do it on a 1UZ.
All you need is a switch that disconnects one ignitor, and at the same time disconnects the corresponding injectors.

[legend]

fuel is cut to the cylinder and valves are held open i heard

[Duk]

From what I've read, they stop oil flow to the hydraulic lifters to prevent the valves from opening. This keeps the pumping loss's down. And obviously keep the injectors closed for the deactivated cylinders.
Don't know how the go about dealing with differences in block and head temperatures between active and non active cylinders. Maybe they cycle the used and unused

[Cuzzo]

Either way they do it it cant be that hard with everyone jumping on the bandwagon.

[MR22ZZ]

it's all a marketing gimmick

[big_zop]

Its all good to speculate (me included) about the theories but the actual execution is dependant upon manufacturer, budget, etc.

Would make a good engineering thesis though...

[Duk]

http://www.autospeed.com/cms/A_2623/article.html

[Talasas]

What i would like to see is the proof that cylinder deactivation actually saves fuel because to travel at a constant speed would require a certain amount of power to overcome all the drags present (powertrain, wheels, wind) and that power is constant and it doesnt matter how many cylinders is producing it, it should consume the same amount of fuel.

Yes but in the case of all cylinders running you have to take into account the forces needed for all the cylinders to draw air through a partially opened throttle (i.e cause of vacuum) which is quite large. There's also the counter argument that cylinders which are shut off have to overcome the force of compression but my assumption is that it's not as much as the vacuum needed from each cylinder in order for it to really be as efficient as they say it is.

Oh and this is the stupidest part of the whole idea:

Interestingly, Chrysler (like Mercedes) teamed with Eberspaecher to design an exhaust system to maintain a V8-like rumble even when 4 cylinders are deactivated.

What the hell for? Wouldn't it be more desirable to hear that the cylinders are actually being shut off? That's right on par with how they actually have to program "shift jerk" into CVTs because people complain "it doesn't feel right"

[big_zop]

Yes but in the case of all cylinders running you have to take into account the forces needed for all the cylinders to draw air through a partially opened throttle (i.e cause of vacuum) which is quite large. There's also the counter argument that cylinders which are shut off have to overcome the force of compression but my assumption is that it's not as much as the vacuum needed from each cylinder in order for it to really be as efficient as they say it is.

Like i said earlier, an engine running at full throttle is more efficient than one running at part throttle. Now i was basing this off the effective compression ratio that was taught to me during my engineering degree (i know it may not be entirely true but it does help illustrate the differences - basice 9:1 compression at full throttle would lead to 4.5:1 compression at half throttle).

And if you assume that the ecu closes the valves so that no air gets in and that they are all connected to the same crank, it would cause the un-used cylinders to create massive vacuums which in turn would create more losses! lol

This can go round and round in circles but the truth is that it seems like everyone is making big claims with out backing it up with proof, so i remain sceptical.

[Talasas]

Good point and well made. I agree, until we see some tested proof, it's anyone's guess how good it really is. 20% improvement in fuel consumption is a big claim.