with MDS, are the 4 cy not firing when in mds mode the same each and evry time truck is in MDS, and are the pistons moving? I asume they are just no spark and no flue.

 

Yes, it's the same cylinders all the time. The pistons are still moving (they are attached to the crankshaft and therefore always move as long as it's turning). MDS shuts off the sparkplugs, the fuel injectors, and closes the intake and exhaust valves.

Here's a link to Dodge's animation... Click "Play the video"
http://www.dodge.com/en/2008/ram_150...ility/engines/

 

My 300C got its flu shot. I'm good to go. Yep, it would not be good to stop the crankshaft. All sortsa nasty noises will come out (only once, though ).

 

Maybe I'm missing something here but how does it close the intake and exhaust valves? These are ran off the cam via the lifters and pushrods. Cam is constantly turning so I'm confused on your comment.

 

cuts off the motion at the lifters. I believe it releases the oil pressure from the hydraulic lifters, so the just compress instead of transffering the motion to the push rods.

 

The valves would have to be opening, wouldnt thay? at least partially. If they werent, wouldnt compression build past the point where the compression rings could sustain it? Then you would have compression blowing by the rings into the crank case, and on the exhaust stroke, without the valve open, wouldnt it cause a vacuum, pulling oil out of the crank case, by the oil ringand into the cylinder? I was under the impression that the valves were always slightly off seat when MDS was in operation. I could be wrong, but having the valves always closed seems like a bad idea to me

Like I said, i dont know for sure, I'm just working on my knowledge of engines, so I could verry well not understand the whole MDS enigma.

 

the compression that you create at the top is the same or less than the compression you get at TDC after the compression stroke. you compress the same amount anyway during normal operation, and then light it, so normally your compression is even higher.

you would never create a vacuume because there should always be air in those cylinders. it's the same air getting squeezed and expanded.

Yea, the whole system is a little abstract, but think of it this way, both valves are closed during the CP part o the I-C-P-E cycle anyway right? MDS would also work (i think) by leaving both valves open all the time, but by keeping them closed permenantly, you effectively eliminate the pumping losses of moving air into and out of those cylinders. the air you put energy into to compress realeases that energy in the form of pushing that cylinder back down, and than the whole thing starts over.

 

Compressing a dead hole is bad news in a standard engine. I broke a couple of push rods in my 340 on the same cylinder, and it blew the head gasket. It seems to me that compressing air in the cylinder takes energy, the piston-rod would free wheel if the valves were in a floating attatude, opened just enough to stay off seat. But like I said, im no expert on MDS, I have no use for half an engine, thats why I still have my 04'

 

it's really not bad for the engine. you broke push rods, but the push rods in the dead cylinders during MDS aren't moving or feeling any force since the lifters are sucking it up. I guess maybe the lifters might fail after a while, i don't know.

The pressure is less than the pressure in a cylinder that is firing, so you won't blow a head gasket, not even close.

the energy that is lost during air compression is gained back when that compressed air pushes back down on the piston on the downstroke, with almost 100% efficiency (the only loss being the heat that was rejected through the cylinder wall).

if it freewheeled with the ports open there would be needless pumping of air through the cylinder creating unnecessary parasitic hp loss, and you might even confuse the 02 sensors into thinking there was a lean mixture. it can be made to work with all the valves open, but then what would keep the valves open when the lifter isn't on a cam lobe?

all that being said, i'm going to hold on to my 04 as well.

 

OPERATION
The Multiple Displacement System (MDS) provides cylinder deactivation during steady-speed, low-acceleration and shallow grade climbing conditions to increase fuel economy. Both four- and eight-cylinder configurations have even firing intervals, providing smooth operation. Two cylinders on each bank are active when the engine is in four-cylinder mode - every other cylinder in the firing order. All of the cylinders that are deactivated have unique hydraulic valve lifters that collapse when deactivated to prevent the valves from opening. Engine oil pressure is used to activate and deactivate the valves. It is delivered through special oil passages drilled into the cylinder block. Solenoid valves control the flow. When activated, pressurized oil pushes a latching pin on each valve lifter, which then becomes a “lost motion” link. Its base follows the camshaft, but its top remains stationary, held in place against the pushrod by light spring pressure but unable to move because of the much higher force of the valve spring. Deactivation occurs during the compression stroke of each cylinder, after air and fuel enter the cylinder. Ignition then occurs, but the combustion products remain trapped in the cylinder under high pressure, because the valves no longer open. No air enters or leaves. During subsequent piston strokes, this high-pressure gas is repeatedly compressed and expanded like an air spring, but fuel is not injected.

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