Last post on Jan 27, 2012 at 3:45 PM
You are in the Chevrolet Suburban & Tahoe
What is this discussion about?
Chevrolet Suburban, Fuel System, SUV
#20 of 29 Re: Does Active Fuel Management inprove MPG [kiawah]
Jul 09, 2008 (4:32 pm)
You get the same MPG as me on highway so that AFM does not really save fuel on highway. It could save if it was made to take 4-cyl mode every time power demand from engine allows and perhaps this happens during highway driving but as you say it is kept in 8-cyl mode when the engine is on idle you don't get the savings while driving in city traffic. Idling in 4-cyl mode would probably be the biggest benefit from that mode. Remember a few years ago when police departments in the country changed from using Crown Victoria to Impala just to have a smaller 6-cylinder engine for their idling as that is what police card do a lot.
About the engine curves. Yes, have seen a couple. Power is simply torque multiplied by engine revolution speed. As the engine speed is a linear straight line as the horizontal axis of the graph the torque curve is not. Torque curve is the one which tells how good the engine is and I strongly disagree with your statement that 4-cyl mode provides the engine with another torque curve.
Torque curve is controlled by engine design as how gases flow in and out the engine and how ignition spark and intake / exhaust valve timing is set up. A big thing with this is with valve timing and new high end engines now use variable valve timing for both intake and exhaust valves. This technology certainly has not reached Tahoe engines yet. Also intake and exhaust manifold design has a lot to do with gas flow handling.
Valve timing is done to improve that torque curve to maximize the area for best torque. The problem with torque curve always is that it drops off at certain point and variable valve timing has been found to be an excellent toll to improve this.
It still is the fact that no matter what the power, it is the FORCE, i.e. ENGINE TORQUE that makes the car move. Power just tells how much of that torque can be taken out from the engine in a time unit.
Spraying fuel in 4 cylinders instead of 8 does not change the engine properties for gas flows or valve timing etc. It just basically means that you will be spraying twice as much fuel/cylinder in those 4 cylinders to keep the vehicle going at set speed compared to using all 8 cylinders.
Yes, and the two mode system is one more thing that needs service and repair...
#21 of 29 Re: Does Active Fuel Management inprove MPG [arrie]
Jul 09, 2008 (8:19 pm)
and I strongly disagree with your statement that 4-cyl mode provides the engine with another torque curve.
I never said the engine would have two torque curves....I said it would have two different power (as in horsepower) curves. When running in 4 cylinder mode, you'll have 4 power strokes per 2 engine revolutions. When running in 8 cylinder mode, you'll have 8 power strokes per 2 engine revolutions. You're burning twice as much fuel in 8 cylinder mode.
It just basically means that you will be spraying twice as much fuel/cylinder in those 4 cylinders to keep the vehicle going at set speed compared to using all 8 cylinders.
The engine does not spray twice as much fuel/cylinder. The piston moving up and down is a given volume displacement. You do not vary the mixture in the cylinder to double the hp. If you doubled the amount of gas in the same volume of air, you would run extremely rich, the mixture would not burn completely, and it would clog up the cat converter. If you leaned out the mixture by not providing enough gasoline in that volume of air, you'd burn out the valves.
#22 of 29 Re: Does Active Fuel Management inprove MPG [kiawah]
Jul 10, 2008 (2:02 pm)
You seem to have so good handle of this on your own level I'll let it be.
but as an absolute last comment from me on this...
It just is that the car takes certain amount of force (energy per mile) to push against wind resistance (and rolling resistance) at given speed. As long as that force (and energy) comes from the gasoline in the car's tank the same amount of that gasoline is needed to push that car with that given speed REGARDLESS OF HOW MANY CYLINDERS OF THE ENGINE IS DOING THE WORK!
#23 of 29 Re: Does Active Fuel Management inprove MPG [arrie]
Jul 10, 2008 (2:33 pm)
Lol....yes, correct so far. You are just missing the point that the engine actually produces more HP than the vehicle may require.
It will take the some force and energy to move that vehicle forward cruising at 60mph (let's call it X, or better yet just assume /define it to be 65HP). I think we both can agree to that.
Now lets talk about what energy the engine 'actually' produces. At 60mph, in high gear, let's for the sake of discussion say the gearing puts the engine RPM at 3000rpm. Using the prior 2GR Toyota chart, at 3000 rpm the engine is producing 140 HP. (Yes I know the Tahoe doesn't turn at 3K at 60mph, but we're using the Toyota charts since I already posted them, and the gearing for both are higher). Even though the vehicle only needs 65HP to move thru the air and down the highway, the engine is producing MORE horsepower than is needed. That extra energy (heat) is wasted and expelled (through the radiator and exhaust).
Now what APM can do, is shut down 4 of the cylinders, and not fill them with fuel mixture. The engine's power curve now, since it's burning half as much fuel for any given RPM, would be approximately half. So now the engine at 3000 RPM is producing 70HP. 70HP is still more than the 65HP needed to move the vehicle thru the air at 60mph, so the speed is maintained. It doesn't slow down, or downshift. There is now a closer balance between the HP needed to move the vehicle, and the HP actually produced, so there is less wasted energy.
Why then build engines that produce 140HP? Because when you are starting a vehicle from a stop sign, or carrying a heavy load, or driving up an incline......you need much more than the 65HP that the vehicle might need cruising on the flat interstate.
#24 of 29 Re: Does Active Fuel Management inprove MPG [kiawah]
Jul 11, 2008 (3:12 pm)
I am afraid I have to take back my promise of the last post as I feel I need to post this after seeing how lost you are with the torque (power) curve.
First I need to correct my statement that the 4-cyl mode does not provide a new torque curve. Of course it does as the torque roughly is half of the 8-cyl mode at given rpm but what I mean is it does not provide a useful additional torque curve and I stay behind that. The only way 4-cyl mode could be helpful is that if it would take the 4-cyl mode every time the power requirement from the engine allows to do so. From what I have learned about it this is not how it works.
Now about the torque curve. The engine does not produce torque (and power) by the curve when you normally drive the car. It only produces it when you pull the maximum torque from the engine, i.e. THE TORQUE CURVE ILLUSTRATES MAXIMUM TORQUE OUTPUT THAT THE ENGINE CAN PRODUCE WITH CORRESPONDING ENGINE REVOLUTION IN THE CHART!
When you normally drive say that 60 mph speed the engine provides exactly that amount of torque (and power) to keep the car going that speed. If it would provide more than is required the car's speed would increase unless you step on brake pedal or other vice cause more "friction" for the engine to work against. The engine does not provide the torque (power) what the torque chart shows for that engine rpm unless the engine's performance is on the max limit for that engine rpm.
Seems like you were a perfect customer to buy one of these AFM hoax vehicles.
This was my last post on this topic with you.
#25 of 29 Re: Does Active Fuel Management inprove MPG [arrie]
Jul 11, 2008 (5:14 pm)
I found this reference for you if you'd like to read up on the theory of how Automobile Engines work. It's a little long and detailed for easy casual reading, but it covers the physics at work in an automobile internal combustion engine.
Physics in an Automobile Engine
#27 of 29 2007 suburban active fuel problem
Dec 27, 2010 (4:16 pm)
My suburban recently started a dead miss on #1 cyl. I tried all the normal fixes i.e. Replace plug, wire,a and coil pack. No change. I took it to the local dealer and told them what I found. They told me that #1 had no compression and that I needed to replace the engine. (it has 120k) I took it home and started tearing it apart. Cylinder wall looked perfect as did the head. When I pulled the lifters I discovered that one appeared to be collapsed. I replaced the lifters and reassembled the engine. Same dead miss. I removed the valve cover this morning and the valves are not working. I can't find anyone to tell me what to do now. Any help would be greatly appreciated.
#28 of 29 Re: 2007 suburban active fuel problem [davec1124]
Jan 27, 2012 (8:47 am)
Have you checked the cams? (assuming you still own the vehicle). Now if you look up how the AFM works, there is a small pin that goes into the lifters to lock them and deactivate them. I'm pretty sure that is what the problem is, it doesn't seem to be re enabling the lifters. My email is corvettezo7splive.com, I would be happy to help you if you need it. It sounds like the engine should be fine mechanically.... except that. Excuse my spelling as i was never really good at it. But send me an email and ill try and help you to the best of my abilities!
#29 of 29 Re: 2007 suburban active fuel problem [corvettezo7sp]
by Stever@Edmunds HOST
Jan 27, 2012 (3:45 pm)
Please don't offer to provide "personal" help via email. We all benefit when a problem is discussed on the open forum. Plus you'll get spammed. Thanks.