How much does cylinder management REALLY save in real life?

[alphatarget1]

Granted, I'm just a civil engineer...

It would seem to me that turning 1 bank of the cylinders off wouldn't actually help fuel economy THAT much. In a 4 cylinder engine you have 4 less cylinders with frictional losses, whereas if you turn off 4 cylinders in a v8 (or hell, 3 cylinders in a v6) you still have all of those frictional losses from the moving parts such as the crankshaft, camshaft, etc. You have the same drag coefficient, and you need to burn a similar amount of fuel to overcome the friction and what not.

What do you think?

 

[Howard]

If you close off the intake valves of half the cylinders, you can allow twice the airflow through the throttle valve which, because the ordinarily low level of opening, provides higher efficiency.

That's my guess. I don't know too much about these systems.

 

[alphatarget1]

Originally posted by: Howard
If you close off the intake valves of half the cylinders, you can allow twice the airflow through the throttle valve which, because the ordinarily low level of opening, provides higher efficiency.

That's my guess. I don't know too much about these systems.

Yes, I understand that it does save a little bit of fuel. But is the added complexity just more things to be broken down the road?

 

[The Boston Dangler]

here's me talking out of my ass:

a. when cruising, a car needs a fraction of it's maximum power, unless it's horribly underpowered
b. shutting off (no fuel injected) half of the cylinders would burn about half of the gas, as opposed to leaning out the air/fuel mixture to an unheard of 26:1
c. intake and exhaust valves still open and close on an "off" cylinder. how else would you deal with the dynamic volume of the combustion chamber?
d. pumping, windage and friction losses should be no more than a couple horsepower in a normal car.
e. aero drag increases exponentially with speed, but then if you're accelerating, you're not in cruise mode.

 

[alphatarget1]

Originally posted by: The Boston Dangler

b. shutting off (no fuel injected) half of the cylinders would burn about half of the gas, as opposed to leaning out the air/fuel mixture to an unheard of 26:1

It wouldn't burn "half of the gas" since a minimum power produced by the engine is still needed to propel the car.

 

[herm0016]

i have been driving an 08 gmc pickup with this tech for abut 2200miles. the cylinder management does save gas when you are running in speed where it can be sustained. running around 55 the v8 runs in 4cyl mode and keeps the truck at a constant speed and gets around 25mpg on flat road. on hills it kicks back to v8 to get up the hill and then back to 4 when it gets over the hill. it seems to work ok. i think more efficiency could be gotten out of it if the computer would run up the rpm more in 4cyl mode than in 8 mode, right now it only rises about 200 rpm when it switches modes. around 65 on a flat road it will run in 4cyl for part of the time and v8 part of the time, over about 70 it runs in v8 all the time except down hill. in town it works well also, it will stay in 4cyl a lot even when under very light acceleration and almost always at sustained speeds. it takes some getting use to to know how much you can push the gas in 4cyl mode to keep it in that mode. It does save gas.

 

[The Boston Dangler]

Originally posted by: alphatarget1

Originally posted by: The Boston Dangler

b. shutting off (no fuel injected) half of the cylinders would burn about half of the gas, as opposed to leaning out the air/fuel mixture to an unheard of 26:1

It wouldn't burn "half of the gas" since a minimum power produced by the engine is still needed to propel the car.

that minimum is about 30 hp at 60 mph.

by disabling half of the engine, the active half is operating within a much narrower set of parameters (rpm, afr, valve and ignition timing, et cetera). the fewer conditions an engine has to deal with, the more it can be optimized to operate within those conditions. it's very possible to get dramatic efficiency gains when we're talking about only 30 hp. of course, all this goes out the window when you want to pass someone.

 

[StageLeft]

Apparently engines create the most work for gas used when running at full capacity; their greatest efficiency, so if you do the same amount of work (car moving) but with half the cylinders, they have to work harder, and thus you get the efficiency gain. I think that the frictional losses from a few more cylinders moving is meaningless, especially since they are still moving; they simply are not receiving the fuel air mixture, that's it.

 

[Eli]

Originally posted by: The Boston Dangler

Originally posted by: alphatarget1

Originally posted by: The Boston Dangler

b. shutting off (no fuel injected) half of the cylinders would burn about half of the gas, as opposed to leaning out the air/fuel mixture to an unheard of 26:1

It wouldn't burn "half of the gas" since a minimum power produced by the engine is still needed to propel the car.

that minimum is about 30 hp at 60 mph.

by disabling half of the engine, the active half is operating within a much narrower set of parameters (rpm, afr, valve and ignition timing, et cetera). the fewer conditions an engine has to deal with, the more it can be optimized to operate within those conditions. it's very possible to get dramatic efficiency gains when we're talking about only 30 hp. of course, all this goes out the window when you want to pass someone.

30HP to go 60MPH? Maybe in a full sized truck.

Try 10, 20HP max. It takes amazingly low amounts of power to cruise. It's accelerating that takes power.

Think about it - the original VW Bug had less than 30HP peak. Yes, it was slow.. but it could cruise at 60 no problem.

 

[nakedfrog]

Originally posted by: Eli

Originally posted by: The Boston Dangler

Originally posted by: alphatarget1

Originally posted by: The Boston Dangler

b. shutting off (no fuel injected) half of the cylinders would burn about half of the gas, as opposed to leaning out the air/fuel mixture to an unheard of 26:1

It wouldn't burn "half of the gas" since a minimum power produced by the engine is still needed to propel the car.

that minimum is about 30 hp at 60 mph.

by disabling half of the engine, the active half is operating within a much narrower set of parameters (rpm, afr, valve and ignition timing, et cetera). the fewer conditions an engine has to deal with, the more it can be optimized to operate within those conditions. it's very possible to get dramatic efficiency gains when we're talking about only 30 hp. of course, all this goes out the window when you want to pass someone.

30HP to go 60MPH? Maybe in a full sized truck.

Try 10, 20HP max. It takes amazingly low amounts of power to cruise. It's accelerating that takes power.

Think about it - the original VW Bug had less than 30HP peak. Yes, it was slow.. but it could cruise at 60 no problem.

My 40HP Beetle cruised at 60MPH on three cylinders :Q

 

[RU482]

Originally posted by: alphatarget1
Yes, I understand that it does save a little bit of fuel. But is the added complexity just more things to be broken down the road?

yeah, those computers are pretty problematic

 

[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.

 

[LTC8K6]

http://www.wkjeeps.com/wk_engines.htm#MDS

A thorough explanation of Chrysler's Hemi MDS system and it's components.

 

[SearchMaster]

The EPA rating difference for my wife's Odyssey EX-L (which has the cylinder deactivation) versus the EX (without the technology) is 1 MPG.

I think that's probably pretty accurate. Maybe 2 MPG on a good day.

 

[LTC8K6]

The Honda can go to four or three cylinders, and probably does better than you think, imo.

What you have to do is try to get the indicator to stay on while you are driving.

You soon start accelerating less aggressively and holding a steadier throttle.

My 5.7L Hemi impressed me on a run from NC to PA and back.

 

[SJP0tato]

From what I've read it's a minor increase in efficiency. Maybe 5-8%.

It seems more likely to me it's good for marketing purposes. "Mr Jones, our truck has this super advanced tech that lets it run on HALF the cylinders while cruising for BETTER gas mileage!"

 

[jaha2000]

Originally posted by: LTC8K6
http://www.wkjeeps.com/wk_engines.htm#MDS

A thorough explanation of Chrysler's Hemi MDS system and it's components.

BTW... I am a mechanical engineer, and parts of it confuse me 🙂

Alright then... That sounds VERY complicated... Controlling lifters with oil pressure? What happens if one of those relays fails?

At any rate. Like i said it helps, but its a gimmic.. How much harder did they make the motor to work on? Maintaining that system much be quite a chore. What about people that run over on oil changes? Metal shavings in the oil? Just seems like LOTS of room for failure in a system like that.

 

[SJP0tato]

Originally posted by: jaha2000
At any rate. Like i said it helps, but its a gimmic.. How much harder did they make the motor to work on? Maintaining that system much be quite a chore. What about people that run over on oil changes? Metal shavings in the oil? Just seems like LOTS of room for failure in a system like that.

I'm curious about the additional wear on the cylinders that are active under 4-cyl operation vs those that are sitting with the exhaust pressure in them. Maybe it turns out to not be that big a factor under the testing they've done.

 

[jaha2000]

Originally posted by: SJP0tato

Originally posted by: jaha2000
At any rate. Like i said it helps, but its a gimmic.. How much harder did they make the motor to work on? Maintaining that system much be quite a chore. What about people that run over on oil changes? Metal shavings in the oil? Just seems like LOTS of room for failure in a system like that.

I'm curious about the additional wear on the cylinders that are active under 4-cyl operation vs those that are sitting with the exhaust pressure in them. Maybe it turns out to not be that big a factor under the testing they've done.

Yeah thats true... Wonder what the typical compression pressure is on that motor...

 

[Dman877]

We've had this discussion here before and IIRC, the valves for the unused cylinders stay closed. Then you have two cylinders boomeranging off of each other as one compresses air while the other decompresses.

 

[dainthomas]

A friend of ours has a Tahoe or something with this feature, and she said the mileage is pretty much just as bad as a normal Tahoe. She might have kind of a lead foot though.

 

[StageLeft]

Originally posted by: SJP0tato

Originally posted by: jaha2000
At any rate. Like i said it helps, but its a gimmic.. How much harder did they make the motor to work on? Maintaining that system much be quite a chore. What about people that run over on oil changes? Metal shavings in the oil? Just seems like LOTS of room for failure in a system like that.

I'm curious about the additional wear on the cylinders that are active under 4-cyl operation vs those that are sitting with the exhaust pressure in them. Maybe it turns out to not be that big a factor under the testing they've done.

I wonder, too. I also don't think that these cars typically swap it up; so an 8 cylinder will always use the same 4 to operate, which would put extra stress on them to some extent, though it's on the highway where stress is low anyhow.

 

[LTC8K6]

The 5.7L Hemi with MDS has been in service for more than 4 years now. It doesn't seem to be causing any reliability or maintenance problems.

The newest 5.7L Hemi with Variable Cam Timing does even better on fuel.

 

[Bignate603]

It's not designed to run as efficiently as an engine with half the cylinders, it's just supposed to be a bit more efficient than another vehicle with a large engine. It's so you can have a truck that runs a bit more efficiently when you're not hauling and you've got the power you need when you' are pulling something.

 

[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.