"Multiple Displacement"

pwd72s · 01-05-2005, 09:39 AM

Definitely not Porsche related, but we're thinking of buying Cindy a Dodge Magnum wagon. Hopefully a year away, to give MOPAR time to work out any new model bugs. Evidently, it's optional V-8 comes with a "multiple displacement" feature. Thus cuts off 4 cyclinders when in cruise mode, supposedly giving a 20% boost in fuel economy. Okay, so you kill the fuel intake and spark to 4 cylinders? Wouldn't this move make for more wear in those cylinders?
How does this work? Any engine guys care to give me their thoughts on this????

VaSteve · 01-05-2005, 09:46 AM

Hasn't Cadillac's Northstar system done this for years?

Good point about the wear in half the cylenders. Maybe it rotates them around. At least at rebuild time, it will only cost half as much

Jared at Pelican Parts · 01-05-2005, 09:48 AM

Sounds like the old Cadillac system, which was a damn nightmare....

BlueSkyJaunte · 01-05-2005, 09:49 AM

Well.....

Wouldn't the wear increase only if you cut oiling to them as well? I doubt they control individual oil squirters!

notfarnow · 01-05-2005, 09:56 AM

Wasn't it called the Cadillac 8-6-4? If i remember correctly, it would open both valves on the "inactive" cylinders.

When overheated, the Northstar would switch from one bank of 4 cylinders to the other, I think.

cstreit · cstreit's Garage

Actually the way it works is it not only shuts off fuel and spark, but closes the valves as well.

There would be no appreciable additional wear because the cylinder is still being oiled and moving, in fact I would think it would wear less because of the lack of combustion byproducts.

BlueSkyJaunte · 01-05-2005, 10:18 AM

Quote:

Originally posted by cstreit
Actually the way it works is it not only shuts off fuel and spark, but closes the valves as well.

So the motor has to fight the compression of 1/2 the cylinders the whole time?

The Cadillac 16 had this feature. They called it "Displacement On Demand".

http://www.cadillac.com/cadillacjsp/models/gallery.jsp?model=sixteen&df=y

pwd72s · 10:36 AM

The Dodge website only mentions that 4 cyclinders are "deactivated". Wish they'd given more info. Has anybody here bought one? Impressions? (edit) Here's a link to their website. It's the "RT" model that runs the hemi V-8 with the system.
http://www.dodge.com/magnum/

djmcmath · 01-05-2005, 12:36 PM

"Multiple Displacement?" I'm thinking large dual-hulled vessels, which reminds me somehow of the local women -- renowned for their deep draft and large displacement. I'll have to remember this discussion for the next time I'm at the bar with the guys...

Dan

EdT82SC · 01-05-2005, 12:54 PM

The Mercedes has has this on their some of the 8 cylinder cars for years. Very popular feature among the greenies in Europe. I'm sure Dodge gets the technology from them.

arcsine · 01-05-2005, 02:04 PM

I liked the variable compression Volvo was playing with. Basically put a cam and rubber skirt between the block and the crankcase. Mixed it with variable cam timing and lever it up for low compression torque around town and down for high compression horsepower.

cstreit · cstreit's Garage

The valves are closed. Seems weird, but it's not fighting compression because there is no intake.

928ram · 01-05-2005, 02:40 PM

I've been looking into these models as well although I think the new Charger model due out in a couple months will be the likley choice (same car, Dodge version).
The cylinder deactivation is said to be seamless and I have yet to read or hear of anyone stating that they can feel it happening. From what I understand, it's done by deactivating the lifters. Two questions I haven't found an answer for: How they overcome the drag of running the piston in a sealed cylinder, surely the valves are left partialy open; Does the deactivation alternate between (sets of) cylinders, it would almost have to or the 4 that run always would surely wear more. The system was desinged into the engine from the start, so it's not an add-on.
The Magnum is supposed to be a tick quicker than the 300, probably slightly lighter; and has a more sport tuned suspension as opposed to the 300's soft-ride tune.
The Rear suspension is directly from the mid-sized M-B models but mfg. with steel rather thal aluminum components. Don't know if the front susp. is also taken from the same model or not.

pwd72s · 01-05-2005, 04:39 PM

Thanks guys...frankly, this portion of the car had me worried about it being a potential trouble area. Knowing that Mercedes has used the system for years helps. Back in the stone age, we had a station wagon, and we both missed it utilitarian aspects. It's nice to see wagons making a comeback. Not everybody who wants to haul 4 people and "stuff" needs an SUV.

cstreit · cstreit's Garage

Quote:

Originally posted by 928ram
Two questions I haven't found an answer for: How they overcome the drag of running the piston in a sealed cylinder, surely the valves are left partialy open; Does the deactivation alternate between (sets of) cylinders, it would almost have to or the 4 that run always would surely wear more.

Drag? All you're doing is eliminating combustion. The cylinder is still lubed. The piston works against vacuum on the downstroke, but is aided by it on the upstroke. The net net is zero energy loss. (and before the physics guys have heart attacks, I KNOW some energy is lost to friction and air escaping past the rings, but this is negligible overall).

The deactivation is always on the same 4. THere isn't "additional wear". The active cylinders wear as if all were running. The inactive ones would wear minutely less because of the lack of combustion stress.

928ram · 01-06-2005, 03:18 AM

Quote:

Drag? All you're doing is eliminating combustion. The cylinder is still lubed. The piston works against vacuum on the downstroke, but is aided by it on the upstroke.

That's what I meant, "drag" probably wasn't the best word. It would seem that allowing the in. valve open just a crack on intake, and the ex. open during compression stroke would eliminate some excess work but probably to little or no gain?

pwd72s · 01-06-2005, 09:03 AM

Wayne must not visit off topic. I was hoping he'd weigh in here. After all, an MIT degree in automotive engineering must be worth something.

Chris, thanks for your input. Now I think it's just waiting for the first model year bugs are worked out. Unless another sport wagon in the same price range catches our eye.

cstreit · cstreit's Garage

I agree that leaving the valves open just slightly would also work, perhaps even better because you wouldn't be working the rings with pressure so much...

The nature of the way they work is that the valves are activated and de-activated by ****ting off oil pressure to them. They just stop moving at all.

project935 · 01-06-2005, 12:35 PM

A guy here at work has a Magnum RT (Hemi). He gets crappy mileage around town - 14-16 MPG. On his highway trips he's gotten 26-28 MPG. He says accelerating/passing at highway speeds is adequate and doesn't really notice the cylinder shut-down/start-up.

He also put Eibach lowing springs on it ... looks MUCH better.

Any US OEM doesn't really care if these engines wear out too terribly fast - as long as they meet the powertrain warranty time/mileage requirement they'll be happy to SELL you another one when it is shot. Unless you get a poor-quality engine (parts made wrong) they will be fine at their recommended service intervals and 'normal' driving.

The rear suspension links of the 300/Magnum are aluminum castings ... not steel stampings. Close, but not exactly, M-B geometry. The front suspension is unique to the 300/Magnum - take a peek at one and see how close the upper ball joint is to the top of the tire ... kinda scares me.

BlueSkyJaunte · 01-06-2005, 12:35 PM

Still dreaming of the Caddy 16....gorgeous....