How much does cylinder management REALLY save in real life?

[The Boston Dangler]

Originally posted by: TekViper
When the engine is in V4 mode, it not only shuts down fuel to the cylinders, but the system hydraulically locks the lifters so that even though the camshaft profile hasn't changed, the pushrod is held in a locked position, keeping both intake and exhaust valves open. This eliminates cylinder pressure/pumping and the piston is just along for the ride. The only friction coming from the piston rings/bearings/weight of the assembly which is minor im comparision to an actual compression load.

It is really a pretty impressive system and well engineered, nothing like the early days of cylinder deactivation. It is however more critical then ever to keep the engine oil changed regularely. The physical components that activate/deactivate the cylinders rely on pressurized oil to do the hydraulic work. If the oil becomes sludged from lack of maintance, the system will fail to operate.

what motor is this? i didn't know any modern engines could clear 2 (or 4) open valves, and the piston.

 

[SJP0tato]

Originally posted by: TekViper
When the engine is in V4 mode, it not only shuts down fuel to the cylinders, but the system hydraulically locks the lifters so that even though the camshaft profile hasn't changed, the pushrod is held in a locked position, keeping both intake and exhaust valves open.

The way the article describes it is the opposite:
It goes through the normal intake stroke (intake valves open, fuel injected into the cylinder, exhaust valves closing/closed, piston descending).
The normal compression stroke (intake and exhaust valves closed, piston ascending).
The normal power stroke (spark ignited somewhere near the top of the piston travel)
The exhaust stroke is different, in that the exhaust valves are kept shut, and the intake valves are never opened.

That way the compressed air/exhaust act as a spring during compression strokes for a theoretical 0 loss of energy (ignoring friction losses).

 

[jagec]

Originally posted by: SJP0tato

Originally posted by: TekViper
When the engine is in V4 mode, it not only shuts down fuel to the cylinders, but the system hydraulically locks the lifters so that even though the camshaft profile hasn't changed, the pushrod is held in a locked position, keeping both intake and exhaust valves open.

The way the article describes it is the opposite:
It goes through the normal intake stroke (intake valves open, fuel injected into the cylinder, exhaust valves closing/closed, piston descending).
The normal compression stroke (intake and exhaust valves closed, piston ascending).
The normal power stroke (spark ignited somewhere near the top of the piston travel)
The exhaust stroke is different, in that the exhaust valves are kept shut, and the intake valves are never opened.

That way the compressed air/exhaust act as a spring during compression strokes for a theoretical 0 loss of energy (ignoring friction losses).

Even in theory you lose some due to entropy...but it's much less than if the valves were going through the motions and pumping air.

 

[LTC8K6]

http://www.popularmechanics.co.../new_cars/1266926.html

Pop Mech builds a 5.7L hemi with MDS. Even more detail in there. This is the early version.

Mine does not have the spark plug wires.

MDS equipped vehicles also get a special exhaust system so they don't sound funny in 4 banger mode.

 

[mb]

My Accord has VCM. It only helps if you know how to take advantage of it. How many people do you see that tailgate and are constantly braking and then accelerating? I see it all the time. Driving like that is bad for fuel economy in any car, and doing that in a car with VCM will ensure that you are almost always using all cylinders. Keep a safe driving distance and you won't have to brake every time the tailgater ahead of you does. Instead you can just let off the gas a little (if you even feel the need) and get back on it without losing much speed or using any extra cylinders.

Some may even think that using cruise control with VCM would be the best, but it's not unless you drive on perfectly flat roads. When you go down a hill the cruise control will try to keep you at the set speed, then once it gets at the bottom it isn't quick/smart enough to maintain just enough throttle to keep the speed going back up the next hill and will thus activate more cylinders. The solution is to speed up slightly while going down the hill, just enough to keep some cylinders deactivated, and then when you go up the next hill just let off the throttle a little, but hold it in place and depending on how steep the hill is you should be back at your original cruising speed without using any extra cylinders. You'll test peoples patience when doing this on steep hills because you'll have to lose some speed to stay in cylinder deactivation mode unless you give it some more gas.

When I drive to take full advantage of VCM it helps a lot. I was averaging 27mpg that way. Right now I've been driving a lot more aggressively (quicker accelerations), not doing the hill 'trick' (I've got a lot of steep hills on my route so I do lose some speed doing it), and I'm only averaging 23-24mpg. 3-4mpg may not seem like much, but it is. If gas prices keep climbing I'll probably start doing it again.

 

[dirtboy]

I have a 5.7L Hemi with MDS. Jeep's are notorious for bad gas mileage and mine is no exception. I try to get it into MDS mode as much as possible, but when you get 13-14mpg in the city and 19-20mpg on the highway, it does make a 1-2mpg difference.

 

[LTC8K6]

My 5.7L Jeep GC 4X4 is getting ~16 city and ~22 highway pretty consistently.

It did 22mpg on a long trip loaded with people and luggage. I was expecting much worse mileage, but it surprised me. Didn't have to pay for as much fuel as I thought I would. It only had about 1100 miles on it at the time, too.

Above 70, MDS doesn't come on near as much, though. So it really helps if you can stay at 70 or less.

 

[LTC8K6]

The Jeep's CC will downshift on even moderate downhill runs to maintain the speed and save fuel. This keeps the revs up for the uphill sections.

 

[boomerang]

Originally posted by: jaha2000
If i understand the system correctly there is no mechanical modifications done to the engine correct?
They shut fuel and spark down to half the cylinders and i would asume diable some knock sensors on those cylinders. I would not think its all that problamatic, computers are very reliable.

The milage increase is only something like what 10%? You are still swinging that mass around inside the motor, you still have all that friction. Plus you still have air going in and out of the cylinders not getting fuel. It takes energy to compress that air inside of the combustion chamber, even if there is no gas present. That engergy comes from some where. Gas engines are terribly inefficient. Most of the energy from burning the gas ends up as heat anyway.

Dont get me wrong, i am sure it helps, but to me, it is not worth any kind of price premium.

It's not worth it but these are the sacrifices that must be made to meet CAFE requirements. Congress, and plenty of our populous too, think that automakers are sitting on secret technology that will allow for fantastic gas mileage. They're legislators and as such, only know how to deal with situations using legislation. No ME degree, background in physics or the like needed by them. Just write a bill - problem solved.

People have a need for trucks, SUV's and the like. In order to make CAFE, technologies of this type must be utilized.

 

[alphatarget1]

Originally posted by: boomerang

It's not worth it but these are the sacrifices that must be made to meet CAFE requirements. Congress, and plenty of our populous too, think that automakers are sitting on secret technology that will allow for fantastic gas mileage. They're legislators and as such, only know how to deal with situations using legislation. No ME degree, background in physics or the like needed by them. Just write a bill - problem solved.

People have a need for trucks, SUV's and the like. In order to make CAFE, technologies of this type must be utilized.

This is more like P/N related material but I attribute that to our education system, as well as people being SO specialized in what they do that they have no idea about anything else. For commuters driving a car/SUV, we're moving like, maybe 200 pounds of the person + over 3000 pounds of mass. Politicians then try to implement mass transit, which for many parts of the US just isn't feasible because people enjoy the freedom to drive.

As for secret technology, I'm sure they are available to make internal combustion engines more efficient, but not by that much. Things like new materials that allow the engine to run hotter to improve efficiency, etc. The problem is the reliability and cost. Like you said, the general public and the officials they elect are usually pretty ignorant about science.

 

[alphatarget1]

Originally posted by: TekViper
When the engine is in V4 mode, it not only shuts down fuel to the cylinders, but the system hydraulically locks the lifters so that even though the camshaft profile hasn't changed, the pushrod is held in a locked position, keeping both intake and exhaust valves open. This eliminates cylinder pressure/pumping and the piston is just along for the ride. The only friction coming from the piston rings/bearings/weight of the assembly which is minor im comparision to an actual compression load.

It is really a pretty impressive system and well engineered, nothing like the early days of cylinder deactivation. It is however more critical then ever to keep the engine oil changed regularely. The physical components that activate/deactivate the cylinders rely on pressurized oil to do the hydraulic work. If the oil becomes sludged from lack of maintance, the system will fail to operate.

This sounds way more complicated than I thought it was. I thought the timing belt/chain drives the camshaft and the cams push the valves open/close? Or am I thinking of just certain engine designs?