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  #1  
Old 02-10-2002, 03:47 PM
turbodiesel300
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physics of turbo question

Why does a turbo spool up alot more when the car is under loads like when accelerating, but hardly at all when reved in neutral?

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  #2  
Old 02-10-2002, 04:31 PM
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Here is my take:

The turbo is driven by exhaust gas, i.e, the hot exhaust gas turns the turbo and then the turbo compresses fresh air.

So if there is a heavier load, the car produces more exhaust gas, and therefore the turbo rotates faster.
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  #3  
Old 02-10-2002, 05:41 PM
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Semi correct. The turbo is driven by heat (and more exhaust gas helps, too).
It makes use of expanding gases to spin it.
Under a load the exhaust gases are MUCH hotter, and filled with more energy.
This allows the turbo to spin faster and produce more boost.
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Last edited by whunter; 10-11-2012 at 11:22 PM. Reason: spelling
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  #4  
Old 02-10-2002, 05:58 PM
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Turbos are just like turbines.
Ultimately it is the speed and volume of the exhaust air that determines the speed of the turbo.
Although speed and volume are related to the temp of the exhaust but that is an indirect factor.

If there is lots of cold and faster moving air, the turbo will turn just as fast.

Last edited by whunter; 10-11-2012 at 11:24 PM. Reason: spelling
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  #5  
Old 02-10-2002, 07:09 PM
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Quote:
Originally posted by loubapache
Turbos are just like turbines.
Ultimately it is the speed and volume of the exhaust air that determines the speed of the turbo.
Although speed and volume are related to the temp of the exhaust but that is an indirect factor.

If there is lots of cold and faster moving air, the turbo will turn just as fast.
Turbos rely on the extremely hot expanding exhaust gases to spin the blades. not high flow.
There is nowhere near enough flow alone to spin turbos at the extreme speeds needed to compress enough air.
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Last edited by whunter; 10-11-2012 at 11:24 PM. Reason: spelling
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Old 02-10-2002, 07:21 PM
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Although it is related to temperature, the most important factor here is the volume of the exhaust gas. Under heavy load, the temp is higher but it is the volume of the gas that affects the speed of the turbo more.

Here are two sites about the turbo:

http://www.howstuffworks.com/turbo2.htm

"The more exhaust that goes through the blades, the faster they spin. "


http://www.turbotechnics.com/turbo/turbowork.htm

"Increased exhaust gas drives the turbine wheel faster"
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Old 02-10-2002, 07:54 PM
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Agreed. It's not the energy in the heat that moves the turbine, it's the kinetic energy of the exhaust gas flowing through the system, kind of like a pinwheel that's spinning in the wind.

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Old 02-10-2002, 08:20 PM
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DieselHead:

Yes, kinetic energy or momentum of the exhaust gas.

Two factors that directly determine the turbo speed, the mass (volume) of the exhaust gas and the speed of the exhaust gas.

The mass of the exhaust gas is directly related to how much fuel and air that is injected into the torture chamber. The more air and fuel is injected, the more exhaust gas is passed through the turbo. Since the volume of the torture chamber is a constant, more air/fuel will result in a higher temperature but that is secondary or by-product in this discussion. Since diesel engines use a lot more fuel (many factors more) under load than in idle, this is a big difference.

The speed of the exhausted gas hitting the turbo has more to do with the temperature. However, unlike the mass or volume, I cannot imagine there would be a huge difference here (many factors more).

Hydroelectric plant uses cold water to drive the turbines. Water has a lot more mass per unit volume than exhaust air so the turbines can also spin fast and generate a lot of electrical energy.

Last edited by whunter; 10-11-2012 at 11:25 PM. Reason: spelling
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Old 02-10-2002, 08:20 PM
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Just today I scored a "chilton's diesel guide" from the local used book store. It was published in 1980 and has a lot of references to old Mercedes models. Here is a quote from the book regarding turbos.

"How fast the turbine turns depends on the heat and velocity of the engines exhaust system. As engine speed and power increase, so do the heat and velocity of its exhaust. The the turbine blades-eleven of them on the Garrett Air Research turbocharger, used on the Mercedes-Benz 300SD- are cleverly designed so that the exhausts hot gases, expanding across the blades, are converted from heat energy to mechanical energy, driving the turbine, which in turn drives the compressor."


Eric Z

Last edited by whunter; 10-11-2012 at 11:26 PM. Reason: spelling
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Old 02-11-2002, 01:57 PM
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the other thing to consider is the length of time you are accelerating. If in neutral, you are probably on the floor for only a second or two, but if under load you would be on the throttle for a much longer period of time.
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Old 02-15-2002, 02:54 AM
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my .02cents. On gases. both loubabache and car 54 are right. the temperature directly dictates the volume of a gas. the higher the temperature, the more the volume(only under certain conditions). so when the car is under load it needs more fuel to burn, to extract more energy, to move a heavier load. more in more out. but now in the same amount of time the car is doing more work(power output work/ time) so it is going to produce more heat. so ultimately the engine produces, BOTH more heat AND more Exhaust gas.

Last edited by whunter; 10-11-2012 at 11:27 PM. Reason: spelling
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Old 02-15-2002, 01:31 PM
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As a summary; the turbocharger's turbine rate of speed, relative to it's acceleration, is directly dependent on the amount of combustion taking place in the engine, not the engine's speed. Therefore, at load the engine requires more fuel to maintain the same engine speed as if it was in neutral. This means that the engine is combusting more at load and therefore creating a greater pressure differential from before the turbine, relative to the atmospheric pressure after it. More combustion, means more hot (though they don't have to be hot in turbine theory) gases wanting to exit the engine. Thus, these gassers build up and the pressure increases. As the high pressure exhaust gases (which just happen to be hot) approach the turbine, the pressure decreases and the gassers expand (volume rises) over the blades of the turbine. Therefore the turbine's rotors can accelerate to reach the speed of the expanding exhaust gases; it approaches this asymptotically.

So in effect, you were all right. Turbine theory is a combination of multiple variables, many of which has an effect on the turbine's speed. Increasing the pressure alone before the turbo (relative to STP) would cause the turbine to accelerate, combustion does this. Increasing the volume of air moving out of the engine alone, actually causes a pressure increase (at a constant volume), which in effect, will also cause the turbine to increase. Increasing the temperature alone, would have no effect on the acceleration of the turbine, if raised to a constant. But in combustion, the air/fuel is burnt at an extremely high rate of speed, starting at a low pressure and temperature. As combustion takes-place, it is the resultant change in pressure (delta p) that controls this reaction, because of the change in temperature (delta t). The heating, causes the gases to want to expand, but since the engine's exhaust manifold has a constant volume, there is a resultant pressure increase in those gases. It is the same force that pushes your piston down, in the power-stroke, except in a turbine, there is no direct barrier. So we can see, that is is actually the interplay between all these factors (temp, pressure, and volume) that causes the turbocharger to work, and in our engines, not ever one alone.

I didn't mean this explanation to be as long as it was, but being an engineer, I thought I'd have to address this issue to the point where there is no ambiguity. If you have any other thermodynamics questions left, please ask me, as I will be happy to answer them privately or publicly, if I can.

Sincerely,
Dr. Adam Ted Luke Delecki
ATLD

Last edited by whunter; 10-11-2012 at 11:35 PM. Reason: spelling
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Old 02-15-2002, 01:51 PM
TurboTurbine
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OK professor now how do we get off the island!
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Old 02-15-2002, 04:06 PM
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TurboTurbine,

By using the good/old 300D, of course!

ATLD
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Old 02-15-2002, 08:08 PM
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Perhaps a simpler explanation (for diesels, gas engines are different!) is that as more fuel is burned, both the temperature, volume, and mass of the gases in the exhaust increase -- more fuel gives more heat and more combustion.

The turbo is a "simple" heat (energy) engine -- more heat (energy) in the expanding gases means more heat (energy) transferred to the other end of the turbo shaft, the compressor, and more pressure in the intake.

As the pressure comes up in the intake, more air goes into the combustion chamber, and more hot gas (mass) goes out the exhaust.

This effect is somewhat limited, unless, as in our MB diesels, the fuel injection metering system also increases the fuel delivered as the pressure in the intake goes up, hence making more energy available.

The whole key is that more fuel makes more energy (mass, temperature, and speed) available, and the turbo neatly converts this to more air stuffed down the intake -- when even more fuel is added, the effect compounds. If you don't add more fuel, nothing much happens.

Peter

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Last edited by whunter; 10-11-2012 at 11:33 PM. Reason: spelling
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