- Thread starter MDFJ
- Start date
The answer is actually a combination of three of them. By that I mean it is basically the density(3), of the hot(2), exhaust gas(1) caused by the combustion process, being pushed from an area of higher pressure (the cylinder), to an area of lower pressure (the atmosphere) that exerts force against the faces of the impeller, thus imparting rotary motion to the turbo.
This motion, in turn, pumps a denser charge into the intake side of the engine, causing a higher rate of discharge gasses than would be present in an normally aspirate engine.
The end result is the more air a turbo pumps, the more exhaust gas it created. The more exhaust gas created, the faster the turbo turns, and the more air it pumps..........and on, and on, and on.
The deciding factor in the engine speed, and ultimately the amount of air pumped in, and exhausted out, is the amount of fuel introduced. Too much and the turbo will overheat as it's operation is self perpetuating based on the fuel amount. Too little and the engine will run extremely lean, which isn't good either.
This motion, in turn, pumps a denser charge into the intake side of the engine, causing a higher rate of discharge gasses than would be present in an normally aspirate engine.
The end result is the more air a turbo pumps, the more exhaust gas it created. The more exhaust gas created, the faster the turbo turns, and the more air it pumps..........and on, and on, and on.
The deciding factor in the engine speed, and ultimately the amount of air pumped in, and exhausted out, is the amount of fuel introduced. Too much and the turbo will overheat as it's operation is self perpetuating based on the fuel amount. Too little and the engine will run extremely lean, which isn't good either.
I replied to the other thread too, pasted it on here.
1 & 2, It is the exhaust gasses that drive the turbine, but heat does play a factor. Kinetic energy of the exhaust gasses spin the turbine and from there a pressure and temp drop occurs across the turbine harnessing the energy (kinetic and heat) from the exhaust gasses. So, if the kinetic energy was not being used then the thermal energy would not be able to turn the turbine, it uses both.
1 & 2, It is the exhaust gasses that drive the turbine, but heat does play a factor. Kinetic energy of the exhaust gasses spin the turbine and from there a pressure and temp drop occurs across the turbine harnessing the energy (kinetic and heat) from the exhaust gasses. So, if the kinetic energy was not being used then the thermal energy would not be able to turn the turbine, it uses both.
I will agree with Thermal energy! The answer is,,,,
The EXHAUST DENSITY !
Try this scenario, If you take an engine at (Idle) And step on the throttle until the Governor takes over. YOU DO NOT HEAR BOOST. You hear the turbo whining, Not the same and no boost.
(Typically on a diesel Or Gas Engine).
Now Put into gear and power brake it, Or under Load like pulling A hill, You will hear the Turbo spin up and hear it start making boost pressure. Is The best way I know how to explain This.
The (Exhaust Density) Is the UN-BURNT FUEL (Black Smoke) That drives the exhaust side of the Turbo. The heaver the Un-Burnt fuel ( Over Fueling, Un-Burnt Hydrocarbons ) The higher the Boost. Until it's over heated and melts down the Turbo or the Waste Gate opens to relive boost pressure.
All I can Say is Yes Exhaust ! But It's The DENSITY OF THE EXHAUST.
Kinda like driving on A Foggy Day. The engine has more power!
Why, The Density Of the AIR that is introduced to the engine.
(HEAVIER)
I Like You all, And I think there were some good answers,
Just remember DENSITY OF UN-BURNT HYDROCARBONS in the EXHAUST is what drives a Turbo.
So Jacking up your fuel rail is not a good idea! Of what I am saying!! Just trying to help, So you can refer to this in the future. Hope it helps and just remember A Plugged Air Cleaner can do the same and melt it down.
Thanks,
MDFJ
Wrong, EXHAUST DENSITY,
Good answer though!!!!!
It will stump most teachers and instructors as I found in my life time.
Since you answered,
I'LL give you anther one, (Good Bar Joke) NO TRICKS PHYSICS!
I Hook a (CHAIN) to pull your vehicle out of ditch to my rig!!
When I pull you out, (I'M I PULLING YOU OR PUSHING YOU OUT).
I Know Don't Laugh, Look at a chain and it's links. You'll see the hook on your bumper is PUSHING YOUR BUMPER FORWARD,
And The CHAIN LINKS ARE PUSHING EACH OTHER FORWARD,
ALL THE WAY TO THE HOOK THAT IS Attached to my Rig.
Look at the what the links are doing. Pull it between your hands and tell me what EACH LINK IS DOING To THE NEXT?
THINK About it IT's True and OLD. I Know you CAN"T PUSH CHAIN! Look at the Physics of a chain. YOU'LL GET A LAUGH!!
MarkB_MI
Well-known Member
- Location
- Motown USA
It's very simple to prove whether or not "exhaust density" powers a turbocharger. Just plug your exhaust pipe and see how much boost you get. After all, back pressure increases the density of the exhaust gasses.
The correct answer, which isn't on your list, is "exhaust energy". Specifically, it's the enthalpy of the exhaust gas leaving the turbo subtracted from the enthalpy of the exhaust gas entering the turbo. Combustion of unburned fuel COULD contribute to the exhaust energy, but 1) modern gas and diesel engines don't have a lot of unburned fuel leaving the combustion chamber, and 2) the fuel would have to burn prior to entering the turbo in order to contribute to the exhaust enthalpy.
The correct answer, which isn't on your list, is "exhaust energy". Specifically, it's the enthalpy of the exhaust gas leaving the turbo subtracted from the enthalpy of the exhaust gas entering the turbo. Combustion of unburned fuel COULD contribute to the exhaust energy, but 1) modern gas and diesel engines don't have a lot of unburned fuel leaving the combustion chamber, and 2) the fuel would have to burn prior to entering the turbo in order to contribute to the exhaust enthalpy.
Wayne is the closest. Think this way, What turns the compressor of a jet engine? The air, cold or hot, that enters the front of the engine. What turns the compressor of a turbo? The air/gas entering the turbo. The exhaust gas leaving the cylinder enters the turbo and spins the compressor to continue to get out thus compressing oxygen to increase the charge in the cylinder. If there was another source high velocity air/gas available you would not need to use exhaust so heat is not necessary and not fuel is burned. If you plug the exhaust pipe no exhaust can flow and therefore not spinning of the turbo.
It's the exhaust gas velocity.
It's the exhaust gas velocity.
What turn a aircraft compressed is the expansion and fuel the air fuel mixture that is ingnited after the air is compressed and before it passes over the power turbine. So its heat heat causes the expansion and has to take the easiest way out which if you have ever seen a compressed going back through those vanes is not it. There is actually so much air compressed that some is rerouted around the HOT section to help keep ng temps down.
Traditional Farmer
Well-known Member
- Location
- Virginia
You're right because when the mechanical power of the engine is used do exactly the same thing its called a Super Charger.
cannonball
Member
The pistons...
Ken Macfarlane
Well-known Member
Turbos are powered by the expansion of the exhaust gases
following combustion.
If you want to get really technical, the turbo is extracting waste
energy from the exhaust stream but it takes a nice diagram to
show that and you have to talk about the intake side of the
cycle.
following combustion.
If you want to get really technical, the turbo is extracting waste
energy from the exhaust stream but it takes a nice diagram to
show that and you have to talk about the intake side of the
cycle.
farmer boy
Well-known Member
If were going to try to get it down to a single factor, you're wrong. As a single factor,
it's hot, higher pressure gasses trying to get to a cool, lower pressure area. That's it.
exhaust pressure(heat) and exhaust density make a turbo spin.
it's hot, higher pressure gasses trying to get to a cool, lower pressure area. That's it.
exhaust pressure(heat) and exhaust density make a turbo spin.
notjustair
Well-known Member
That's what I tell kids who say , "this sux"! In science, there is no such thing as a suck or pull, only
pushes.
pushes.
Al Baker(pumpman)
Member
An Answer a common mechanic can understand is how violent the exhaust gas is exiting the combustion chamber. If you have the wind pickup on a grass fire look how much harder and violent it will burn. When you push more air and fuel into the intake the harder it will exit into the turbine. If it was heat, then explain how you can have lets say 30 pounds of boost and 1100 degrees on the pie gauge. Pull back on the fuel and the boost will drop to 0 while the pie will slowly drop from 1100. Throw on a jake brake and the pie gauge will drop to 0 almost as fast as you look at the gauge. Now smash the pedal and you will see the boost go back to 30 and the pie will start to climb, but not instantaneously. Nothing to do with how much heat, but how "mad" the gasses are leaving the heads. Watch a boost gauge on a day it is 90 degrees verses a day that it is -30. The colder and more dense the air the higher your boost. I have seen head bolts pop the heads off in really cold weather on a juiced up engine if a driver didn't watch his gauges. I have seen many tubos grow cherry red while pulling hard on my dyno. From the time the engine was just warn enough to pull to that point the boost never changed. I have seen guys install a big fancy high dollar turbo on an engine and not get hardly any boost. It takes fuel to make a turbo spin. Example is with a dirty fuel filter. An engine that will build 30 pounds boost any time pulled hard will not make hardly any boost because not enough fuel is being injected. With a dirty air cleaner you will have a very high pie temp. , but hardly any boost. Lots of black smoke and low power. Its not the amount of fuel put in, or the amount of air. Its how much fuel and air together and burning violently that will make a turbo spin. When idling a turbo will have 0 boost. You will here it spinning but it will not make boost. It is just spinning from exhaust being forced threw it. Example, start an engine with the air neck from the tubo taken off. The engine will free rev most times just fine, turbo unable to do anything. Now try to apply a load. You will make lots of smoke, low temps on the pie, but no boost. Takes air/fuel burning to build boost. That why multi turboed pullers take time at the start line to get the engine to come alive. They have to slowly increase the fuel so the turbos can come up to speed to burn the extreme over fuel condition. Fuel and air have to have some balance to make a turbo work. More air, more fuel, more power. Until something finds its weak spot. Not enough air cleaner compassity or maxing out the fuel system delivery. Most times it inability to remove heat from the engine. exhaust ,or liquid coolant.
Janicholson
Well-known Member
Not a flame!! The compressor of a jet engine is not powered by the incoming air, it is powered by a turbine in the combustion zone and that is the only reason it turns. In a turbo prop there is an additional power turbine that extracts power for the propeller through reduction gears. Jim
That is an oversimplification.
The continued expansion of the exhaust gasses is what really creates the energy to spin the turbo, otherwise air in and air out would cancel the energy. You can gain additional boost at the expense of torque by retarding the timing and causing unburned fuel to essentially burn/expand in the turbo. We do this on the big NG engines to get the turbo's hot and spinning on startup. They will not develop any torque to speak of but they will not detonate due to a rich mixture either.
It is easier to create boost with cold air because it is more dense and compresses easier.
The continued expansion of the exhaust gasses is what really creates the energy to spin the turbo, otherwise air in and air out would cancel the energy. You can gain additional boost at the expense of torque by retarding the timing and causing unburned fuel to essentially burn/expand in the turbo. We do this on the big NG engines to get the turbo's hot and spinning on startup. They will not develop any torque to speak of but they will not detonate due to a rich mixture either.
It is easier to create boost with cold air because it is more dense and compresses easier.
buickanddeere
Well-known Member
The forces that make the Delta P.
I will agree!! Again you just said it.
UN-BURNT fuel leaving the engine under load into the exhaust system.
Meaning Denser Exhaust Gasses to drive the turbine impeller. The more raw fuel added and not enough oxygen, Un-Burnt Exhaust hydrocarbons are produced. Remember a Diesel engine is controlled by, the amount of Fuel Injected (Not Air) It takes in the same amount of air Per combustion stroke until boost is applied. Gas engines regulate both Air & Fuel.
Janicholson
Well-known Member
In my reality the exhaust turbine is driven by the gasses going past it. The blades and snale shell like shape of the
housing are designed to catch this wind and turn it into mechanical power. The blades are air foils with high science
behind their shape/curvature and they capture energy from the pressure applied to them as the exhaust gasses pass them. It
has nothing to do with anything but the density and speed of those gasses and the above mentioned shapes. compressed air
with the same density and volume will produce the exact same capture of energy. The question as posed has no correct single
answer, and is lacking in some components. Jim
housing are designed to catch this wind and turn it into mechanical power. The blades are air foils with high science
behind their shape/curvature and they capture energy from the pressure applied to them as the exhaust gasses pass them. It
has nothing to do with anything but the density and speed of those gasses and the above mentioned shapes. compressed air
with the same density and volume will produce the exact same capture of energy. The question as posed has no correct single
answer, and is lacking in some components. Jim
OK, you can spin a turbo up enough to produce boost with a cold gas such as OX if the flow is high enough. They intercool some to make denser air before it enters the engine. So prove to me that some of the statements are true and that heat and unburned fuel make a turbo boost. Don't seen many tier 3 engines making much, if any black smoke and I've never seen a gasser that has a turbo that blows black smoke so that one statement I can laugh at.
As far as density is concerned ask the pilots on here, 2 identical aircraft both at sea level. One someplace where the air temp is 0 F and the other where it's 100 F, which one is going to have the ability to take off at a high weight and why.
Rick
As far as density is concerned ask the pilots on here, 2 identical aircraft both at sea level. One someplace where the air temp is 0 F and the other where it's 100 F, which one is going to have the ability to take off at a high weight and why.
Rick
Lyndon - AB
Member
I'm going the throw in a new wrinkle. If it's "unburnt fuel" causing the turbo to build boost, how come when I'm jaking down I'm building boost? There is no fuel being injected when I'm jaking. My Western Star will build over 20 Psi of boost when jaking. No wastegate, no variable geometry turbo. Just a simple turbo.
Pressure and flow out the manifold. Under load, more fuel means more combustion pressure, means more boost. The highest your boost will reach is determined by the fuel setttings, or the maximum flow capabilities of your turbo. Or what your heads/head gaskets can handle. :twisted: Polished intake manifolds and polished/coated exhaust manifolds can increase flow volume and velocity, not necessarily increasing boost, but allowing your turbo to flow more freely, giving you the potential to increase the fuel a little more.
Pressure and flow out the manifold. Under load, more fuel means more combustion pressure, means more boost. The highest your boost will reach is determined by the fuel setttings, or the maximum flow capabilities of your turbo. Or what your heads/head gaskets can handle. :twisted: Polished intake manifolds and polished/coated exhaust manifolds can increase flow volume and velocity, not necessarily increasing boost, but allowing your turbo to flow more freely, giving you the potential to increase the fuel a little more.
Pete in MD
Well-known Member
- Location
- Central Maryland
Then we better call tractor pulls tractor pushes.
CenTex Farmall
Well-known Member
Bingo! A correct answer.
CenTex Farmall
Well-known Member
Another correct answer!
CenTex Farmall
Well-known Member
Yet another correct answer
CenTex Farmall
Well-known Member
(The last sentence, anyway)
CenTex Farmall
Well-known Member
The last machine I was paid to fly had an APU (auxillary power unit). The APU was made by Sundstrand and works basically like a big fat turbocharger. It has a centrifugal compressor.
For the APU to start it is spun up by an electric motor (which reverts to a generator after the start sequence). Once it hits a certain RPM the APU FADEC will start the igniter and dump in the fuel and off it goes. Being spun up by a motor easily gets the compressor section to produce enough airflow (no fuel being burned, no heat involved) to get the whole thing up to a point where it can sustain itself.
The APU compressor is oversized enough to not only run itself but also generate airflow for main engine start and will run both air conditioners (air cycle machines).
So what drives a turbocharger? Pressure gradient across the turbine section which results in flowing gases.
For the APU to start it is spun up by an electric motor (which reverts to a generator after the start sequence). Once it hits a certain RPM the APU FADEC will start the igniter and dump in the fuel and off it goes. Being spun up by a motor easily gets the compressor section to produce enough airflow (no fuel being burned, no heat involved) to get the whole thing up to a point where it can sustain itself.
The APU compressor is oversized enough to not only run itself but also generate airflow for main engine start and will run both air conditioners (air cycle machines).
So what drives a turbocharger? Pressure gradient across the turbine section which results in flowing gases.
MarkB_MI
Well-known Member
- Location
- Motown USA
Only one conclusion to be made after reading these posts: A lot of folks managed to get through school without taking a single thermodynamics class.
Actually, a lot of people got really close to the right answer. Your answer isn't any more right than many of the others. It isn't the total energy of the system, as you imply, it is only energy that can be extracted by the turbine. Your answer doesn't account for heat loss in the system.
Indiana Ken
Well-known Member
If one were to blow ambient air into the inlet of the turbo charger exhaust turbine; one would observe the turbine could be made to spin. If one were to measure the air pressure and air temperature on the inlet and outlet side of the exhaust turbine; one would observe the temperature and pressure is lower on the outlet side than the inlet side of the exhaust turbine.
The pressure drop and temperature drop represent energy inputs to the exhaust turbine causing it to spin. Exhaust gases contain both pressure and temperature. Since we are only allowed one answer; exhaust gases, is the correct answer for the quiz.
If pressure and temperature drop after the turbine, it would not necessarily be the result of the interaction with the turbine. How would airflow routed through the turbine reduce temperature or pressure? Those would
more likely be the result of expansion. If volume remains constant, pressure would actually increase after passing the turbine. I don't see how heat energy is transferred to turbine energy at all. But, since MarkB studied
thermodynamics and the rest of us didn't, maybe he can explain. It is the kinetic energy (not heat energy) which is comprised primarily of velocity and density(mass) which is transferred to the turbine. The change in
velocity is the measure of energy imparted. Those are my uneducated thoughts, at least. I think most people got pretty close to getting it right.
more likely be the result of expansion. If volume remains constant, pressure would actually increase after passing the turbine. I don't see how heat energy is transferred to turbine energy at all. But, since MarkB studied
thermodynamics and the rest of us didn't, maybe he can explain. It is the kinetic energy (not heat energy) which is comprised primarily of velocity and density(mass) which is transferred to the turbine. The change in
velocity is the measure of energy imparted. Those are my uneducated thoughts, at least. I think most people got pretty close to getting it right.
jimg.allentown
Well-known Member
Most of you are wrong. It is not heat. the proof is that if you put a turbo in an oven, it will never start to spin no matter how hot you turn up the oven. Exhaust density is irrelevant. It is (in my opinion) the FLOW of exhaust gases.
Exhaust density may have some small contribution, but it is the gas VOLUME that does the real work.
For example, in a diesel, the air is unthrottled yet the turbo does not "run away." What really makes it go is the increased GAS VOLUME that occurs when you add fuel to the mix. More fuel creates more gas volume. Again, the density of that gas is secondary to the rate of flow.
And, the temperature of the turbo or the exhaust gases passing through it is again secondary to the volume passing through the unit.
Exhaust density may have some small contribution, but it is the gas VOLUME that does the real work.
For example, in a diesel, the air is unthrottled yet the turbo does not "run away." What really makes it go is the increased GAS VOLUME that occurs when you add fuel to the mix. More fuel creates more gas volume. Again, the density of that gas is secondary to the rate of flow.
And, the temperature of the turbo or the exhaust gases passing through it is again secondary to the volume passing through the unit.
T in NE
Well-known Member
- Location
- Cambridge, Nebraska
The reduction in temperature is due to the energy taken from the exhaust to turn the turbine.
Energy can't be created or destroyed. Only converted between chemical and mechanical, or transferred from one media to another. In this case from the exhaust gasses to the turbine.
That's why the hydrogen generators don't work. Losses through resistance in the wires, etc, mean that you will never get as much energy out of burning the hydrogen, as you used to make it.
It's also why it's important to know whether the pyrometer probe is pre- or post-turbo when diagnosing high egt. On a 12 valve Cummins I think it's about a 300 degree difference under load.
Energy can't be created or destroyed. Only converted between chemical and mechanical, or transferred from one media to another. In this case from the exhaust gasses to the turbine.
That's why the hydrogen generators don't work. Losses through resistance in the wires, etc, mean that you will never get as much energy out of burning the hydrogen, as you used to make it.
It's also why it's important to know whether the pyrometer probe is pre- or post-turbo when diagnosing high egt. On a 12 valve Cummins I think it's about a 300 degree difference under load.
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