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SUVs Chevrolet Suburban & Tahoe Chevrolet Suburban Active Fuel Management
26 messages, Last post on Jul 11, 2008 at 11:56 PM
You are in the Chevrolet Suburban & Tahoe Forum. Your Hosts are steve_ & tidester
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Replying to: kiawah (Jul 08, 2008 8:26 am) What kind of MPG do you get when you run at speed that the engine stays in 4-cyl mode? As I said in my post I get 20 - 22 MPG if I drive 60 MPH and I have 92 000 miles on the engine. Personally I think this AFM is mostly a hoax to get people buy them trucks as it really can not save gas other than when engine is IDLING or you are coasting down the speed or if the engine is so very poorly designed that it wastes a lot of fuel that is sprayed in the cylinders (which it could very well be too). As I said, for moving the truck at any speed takes the same amount of energy to do, no matter how many cylinders are in use. AFM would make best sense if it would switch to 4-cyl mode when you stop at traffic lights etc. These big engines do not need all 8 cylinders to turn themselves around but perhaps it is about the smoothness of idle and they keep it on 8-cyl mode. So, what is your MPG when you go constant speed of 60 MPH on highway? Arrie
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Replying to: arrie (Jul 08, 2008 1:42 pm) I understand your thought process, but it doesn't necessarily work that way. You are correct for any given speed of the vehicle, it takes a specific amount of energy or HP to make that happen. The problem though, is it would have to be connected to an infinitely variable transmission, so the engine could reduce it's RPM's down to produce just that HP....while maintaining the speed that you wanted. Because of the transmission limitations of the number of gears, when you pick and drive a specific mph...your engine will be turning at a specific resulting RPM as dictated by the gearing, and you would be producing the exact HP or MORE than is necessary. I'll attach an engine HP and torque curve chart, happens to be for a Toyota we have but you'll get the idea. Any given engine's HP produced, is dependant upon the RPM's that it is turning. Internal engine design (like bore, stroke, fuel, etc) dictate the power and torque curves it produces. So when you pick your speed that you are running, and the transmission is in a specific gear, your engine will be turning at a specific RPM. The HP produced will be a given for that engine at that RPM, and most likely will be more HP than is needed. If the HP is more than is needed, you are wasting fuel. The AFM, will effectively give you two power curves for the same engine. A power curve when delivering fuel to all cylinders in 8 cylinder mode, and a power curve when delivering fuel to only 4 cylinders. This gives the engine the ability to drop down to 4 cylinder mode, if the HP of the 4 cylinder curve at that specific RPM is enough to move the car at the speed you have chosen (fighting against the road grade and wind resistance). Engine HP and Torque curves
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Replying to: kiawah (Jul 08, 2008 8:14 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... Arrie
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Replying to: arrie (Jul 09, 2008 3:32 pm) 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.
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Replying to: kiawah (Jul 09, 2008 7:19 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! Arrie
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Replying to: arrie (Jul 10, 2008 1:02 pm) 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.
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Replying to: kiawah (Jul 10, 2008 1:33 pm) 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. ...lol... This was my last post on this topic with you. Arrie
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Replying to: arrie (Jul 11, 2008 2:12 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
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Replying to: kiawah (Jul 11, 2008 4:14 pm) This one's an easy read. It's written by Barry Parker, a physics teacher at one of our local colleges. tidester, host SUVs and Smart Shopper |
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