Active Fuel Management
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9 ECHO 1
Joined: Aug 2009
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From: Missouri
Active Fuel Management
What happened to this? I remember Chevy had it for a while then it mysteriously disappeared.
For those who are unfamiliar, it is a system that allows a V8 or V6 to "turn off" 2 cylinders.
For those who are unfamiliar, it is a system that allows a V8 or V6 to "turn off" 2 cylinders.
Super Moderator
Joined: Apr 2004
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I think it's still out there, with Honda and MBenz for sure. Search "active cylinder management" or "variable displacement".
Drove a couple Hondas with it. Unless you were looking at the light on the dash, you couldn't tell when it switched back and forth.
Drove a couple Hondas with it. Unless you were looking at the light on the dash, you couldn't tell when it switched back and forth.
Posting Guru
Joined: Feb 2004
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From: Saratoga, NY
Gen IV 5300 have it, and I know the Caddy 6200 has it. Chrysler Hemi at some point got it they call it "Multi Displacement System". For the V-8 it shuts off 4 cylinders. Gm had a 3.9 V-6 that shut off 2. I make my living working on caddies and having the old timers tell me what a POS the V8-6-4 was back in 1981. One guys been there since 78.
Lead Driver
Joined: Sep 2007
Posts: 8,180
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From: Scio, OR
And what, really, would you gain. Shutting off 1/2 the cylinders would just mean that the other half would have to do twice as much work, and they'd use twice as much fuel (per cylinder) doing it. You'd still have all the friction of engine components to overcome.
Doesn't seem to me like you gain much for the complexity.
Doesn't seem to me like you gain much for the complexity.
i ain't rite
Joined: Mar 2005
Posts: 65,565
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From: Marlboro Mental Hospital.
it actually increased fuel mileage by a large amount because when at cruising speed where little power was needed to maintain speed 4 cylinders were "turned off".
the fuel was shut off and the valves were opened making them dead holes.
the fuel was shut off and the valves were opened making them dead holes.
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Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
It's be interesting to see some real life numbers. It takes the same amount of energy to move the vehicle, whether it has a 4 or an 8 cylinder. And you still have all the rotating mass and friction. The only theoretical savings would be in the compression not being done in the 4 dead cylinders, but I don't see that saving much either.
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i ain't rite
Joined: Mar 2005
Posts: 65,565
Likes: 5,601
From: Marlboro Mental Hospital.
the savings come from the fact that the 4 dead cylinders do not have fuel going into them.
i don't know exactly how it works, just that when it was working correctly, it would make a big difference in fuel economy when turned on.
i don't know exactly how it works, just that when it was working correctly, it would make a big difference in fuel economy when turned on.
Lead Driver
Joined: Sep 2007
Posts: 8,180
Likes: 669
From: Scio, OR
I understand that the 4 dead cylinders don't have fuel going to them. I also understand that the 4 live cylinders have to have nearly twice as much fuel going to them in order to generate an equal amount of power.
In order to produce a certain amount of power, it's either 8 cylinders with X amount of fuel, or 4 cylinders with 2 times X amount of fuel, all other conditions being equal.
In order to produce a certain amount of power, it's either 8 cylinders with X amount of fuel, or 4 cylinders with 2 times X amount of fuel, all other conditions being equal.
i ain't rite
Joined: Mar 2005
Posts: 65,565
Likes: 5,601
From: Marlboro Mental Hospital.
negative. the 4 cylinders get the same amount of fuel as if it was still running on 8. if there is a demand for more power, the computer will kick in either 2 or 4 more cylinders.
Lead Driver
Joined: Sep 2007
Posts: 8,180
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From: Scio, OR
That's not possible. If you cut the fuel supply in half, you cut the power produced roughly in half.
Like I said, for any given amount of output power, you're going to use 1x amount of fuel each in 8 cylinders, or 2x amount of fuel each in 4 cylinders. Unless you're going to increase the thermal efficiency of the engine by a factor of 2, or violate the laws of physics.
Like I said, for any given amount of output power, you're going to use 1x amount of fuel each in 8 cylinders, or 2x amount of fuel each in 4 cylinders. Unless you're going to increase the thermal efficiency of the engine by a factor of 2, or violate the laws of physics.
Frmr Ford Trans Engr
Joined: Aug 1999
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From: SE Florida
So let's say the vehicle takes 16 horsepower to keep it moving at a certain speed down the road. You have 8 cylinders running, each producing 2 horsepower. Now you turn off four of them, and you only have 8 horsepower. Will the truck maintain speed at 8 horsepower when 16 are required? No, it won't. The remaining cylinders have to double their production, and that takes more fuel.
Posting Guru
Joined: Feb 2004
Posts: 1,388
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From: Saratoga, NY
What engine are we talking about here the POS V8-6-4 or whats used now cause the unused cylinders are not wide open valves they remain shut after a power stroke the exhaust gas stays in the unused cylinder and acts like a spring. read up
Active Fuel Management - Wikipedia, the free encyclopedia
Active Fuel Management - Wikipedia, the free encyclopedia
i ain't rite
Joined: Mar 2005
Posts: 65,565
Likes: 5,601
From: Marlboro Mental Hospital.
like i said, i don't know how it works, just that it does.
the old 4-6-8 caddy system when working properly would increase fuel mileage on the highway by 5-6 mpg. and that was pretty impressive on a car that would only get 12-13 mpg when running on all 8.
unfortunately, the 4-6-8 system sucked, and broke more than it worked.
the old 4-6-8 caddy system when working properly would increase fuel mileage on the highway by 5-6 mpg. and that was pretty impressive on a car that would only get 12-13 mpg when running on all 8.
unfortunately, the 4-6-8 system sucked, and broke more than it worked.