Again, the temperature difference between input and output side of the exhaust turbine is a BYPRODUCT of the reduced pressure after the turbine. Keeping as much heat in the exhaust BEFORE the turbine is a good thing because it keeps the pressure up.

Cart-before-the-horse sort of thing.

The heat is not producing the energy. It's HELPFUL, and loss of heat will HURT the turbo in terms of power, but the heat itself is NOT what's causing the turbo to spin.

 

I am going to step into this again, and try to bring some consensus.
A few basic premises.
1. Energy cannot be created or destroyed. We are not talking about anything nuclear here.
2. About a third to a half of the total chemical energy in a motor vehicle fuel just goes out the exhaust pipe.
3. Gas laws:
"The ideal gas law can be stated as a formula, pV = nRT, where p stands for pressure, V for volume, n for number of moles, and T for temperature. R is known as the universal gas constant, a figure equal to 0.0821 atm · liter/mole · K. (Like most terms in physics, this one is best expressed in metric rather than English units.)"
This was copied from this site: gas laws: Definition from Answers.com

Simply put, you cannot change the temperature of a gas without changing the pressure, if you hold the volume the same. If you allow one thing (pressure, temperature, or volume) to change, at least one other thing has to change.

You can measure the temperature, or the pressure on both the engine side and the outlet side of a turbocharger exhaust side turbine, and both the pressure AND the temperature will be lower, and the gas has expanded in volume.

That pressure and temperature loss is an indication that the gas on the outlet side of the turbine has less energy than it did before it went through the turbine. The turbocharger turbine, by controlling the expansion of the gas has captured some of that energy that is in the exhaust gas. This is energy that is wasted on a non turbocharged engine.

Once this energy is captured from the hot exhaust, it can be used to do something useful, like partially compressing the air on the intake side of the cylinder. If the air is partially compressed when it goes into the cylinder, you can lower the compression ratio in the cylinder, and reduce the amount of energy each piston absorbs from the flywheel to compress the gas in the first place.

This is where a well designed turbocharger system on an internal combustion engine can result in fuel economy gains, or power gains.

 

You last few posts already gave the lie here buddy. This is quite a bit beyond the scope of the elementary classes you have taken so far.




Let's make this simple for you. Engineers measure performance of the turbine and compressor separately using a calibrated test engine. They KNOW all of the inputs and by measuring the compressor performance they know the output. They find that after taking all the losses into account that just using the PRESSURE differential there is not enough energy to account for the output at the compressor side. So genius, explain where the energy is coming from because, as the other poster just so succinctly put it, energy cannot be created or destroyed in this process.


Oh, and this goes FAR beyond the scope of the ideal gas law.

 

You just have a problem admitting when your wrong dont you. lol
Takes a big man dosnt it.

 

Can you provide links to actual research data? I would be very interested in reading it. And by the way, the pressure difference between input and output sides of the exhaust turbine is responsible for the temperature difference, I didn't say it's the only source of mechanical energy.

The only things I can find about this subject specifically state only about 1/3rd of the "heat energy" that is wasted out the exhaust is then turned into mechanical energy.

And by "heat energy" they mean the flow/pressure of exhaust gases as a result of internal combustion, not that the heat itself is utilized as energy.

I still think semantics is the issue here, not that the heat itself is being used to produce mechanical energy (torque) at the turbo shaft but that you're reading more into the process than what really exists.

 

Im a little past elementary class's i think you need to go back to school take a couple class's

mass flow rate is what drives the majority of a turbo, simple as that, what little expansion that happens doesn't effect the turbo nearly as much as the flow rate of gas's going through.

 

Let's start over with something basic.

What's moving the exhaust turbine side of the turbo is individual molecules of exhaust gas imparting kinetic energy to the impeller. Period.

Heat is the random vibration of molecules(atoms) moving in all directions at once (or at least, they all cancel each other out). Otherwise, the desk I'm leaning on would move. There is no vector.

Of course, the TEMPERATURE of the exhaust gases is important to keep the pressure up on the input side of the turbine.

But heat is not doing the work, the flow is doing the work.

 

thats why i said mass flow rate does the work. anyways i think we are just beating our heads against a wall with him.

 

The exhaust gas before the turbocharger has a certain internal energy (enthalpy) at that pressure and temperature. After the exhaust turbine the exhaust gas is now at a different lower pressure and temperature; this corresponds to a different enthalpy.

This reduction in enthalpy (reduction of exhaust gas internal energy) can produce a certain amount of ideal work; the useful work is this maximum ideal amount times the efficiency of the exhaust turbine process.

The larger the drop in temperature and pressure the larger the enthalpy change (reduction) one gets.

This amount of work (reduction in enthalpy is given as BTU/lb mass) times the mass flow (lb mass/sec) gives one the amount of power (BTU/sec, 0.71 BTU/sec = 1 HP) that the exhaust turbine can produce. Likewise this is the amount of work less friction losses turning the turbine shaft that goes into the compressor side which times the compressor efficiency gives the actual work done compressing the air (raising its temperature and pressure from ambient).

If mass flow alone was to provide the turbocharger work then what gives us the mass flow?

When the exhaust valve opens the upstroke of the piston on the exhaust stroke helps to push the exhaust out (providing the mass flow) plus there is a slightly higher pressure in the cylinder from combustion than on the other side the exhaust valve. If there was no change in energy of the exhaust gas then the energy to turn the turbocharger would have to come from the useful work produced by the other cylinders thus robbing the engine of power (other cylinders on their power stroke turn the crankshaft which pushes up the piston for the cylinder on the exhaust stroke).

That is why turbochargers are used instead of superchargers; the turbocharger doesn't have as much parasitic loss (slight increase in exhaust backpressure) versus a supercharger (which is belt driven off of the crankshaft directly robbing useful power). Of course superchargers have their advantages as well.

DDL

p.s. I'm a senior mechanical engineer who has worked on developing industrial natural gas engine air/fuel ratio control for both natural aspirated and turbocharged engines. I hope I laid out my explanation in an easy to follow manner. Sorry for the wordiness!

 

Fords mistake was when they made the 4.6 and 5.4 that they designed it so they couldnt use larger bores. If they had they could go another 20-30 cubes more easily without going to a newer V8 design instead of explanding theexisting one using less new tooling.
But, even if they could use a larger bore on the existing 5.4, they might not have expanded it anyway.

Im late on this conversation, but thats my .02

 

Exhaust heat is important as it helps keep the pressure of the gas up plus it is inidicative of the internal energy of the exhaust gas. This energy is from the work put into the compression stroke and the exothermal (heat releasing) combustion process. The higher the temperature and pressure of the exhaust gas above atmospheric the more energy the gas has available to do work in an expansion down to atmospheric pressure.

If the exhaust gas loses temperature in the exhaust system prior to the turbocharger then its pressure and internal energy will go down leaving less energy available to produce work in the exhaust turbine. Cooling a constant volume of gas reduces its pressure so this is to be avoided if possible.

That is why on a turbocharged engine the exhaust inlet to the turbine is usually insulated in order to retain as much heat (and thus pressure) in the "constant volume" exhaust manifold.

The exhaust turbine is turned by the impulse of the exhaust gas striking the turbine blades and the reaction force of the gas changing direction.

The pressure differential between the cylinder and the outlet of the turbine is what gives the gas velocity; the more the differential the more velocity the gas can achieve and thus impart more energy into the turbine wheel.

The gas leaving the exhaust turbine is hotter than atmospheric and has to be at a higher pressure than atmospheric in order to get through the rest of the exhaust system downstream of the turbine outlet. This is where a lower restriction exhaust system would help so that the turbine could have a larger pressure drop in it (getting closer to atmospheric pressure) and thus produce more power which could be used by the compressor section with less pressure required to exhaust the gas through the remainder of the exhaust system.

The larger the heat drop going through the exhaust turbine (assuming no heat transfer to the turbine) is indicative of more produced work. However the work produced is actually a result of the pressure expansion of the gas in the exhaust turbine which is accompanied by the drop in temperature.

Dropping heat at constant pressure does no useful work unless this heat is transferred to another fluid (or heat engine) which can be used to produce useful work (e.g. exhaust heat boils water into steam which is then used to drive a small steam turbine).

DDL

 

You can come up with all the fancy explanations for the process, but it comes down to this:



The amount of work transferred, the speed of the molecules, the heat involved, drop in pressure, everything you can point out, fine.

However, the actual work being done is mechanical work.

Ever see one of these:

Crookes radiometer - Wikipedia, the free encyclopedia

I mean, turbo impellers aren't painted a different color on one side versus the other, right?

In other words, there is no mechanism where by "heat" is used as a DIRECT energy source.

There is pure mechanical kinetic energy being used to move the turbine. Period.

 

Increase the stroke, increase the CI.

 

I completely agree. It comes from the mass of air being pushed out of the engine. That's what i have been trying to say. The guys earlier where trying to say it was work done by gas expansion from heat after the engine. There is very little work dont from the expansion from heat since the air is cooling as soon as it leaves the engine. For the most part.

The energy is a result of the pressure ratio across the turbo (i.e. the ratio of the pressure in the exhaust manifold/ambient pressure). The mass of air flow is also a factor but basically the same air following into the turbo is flowing out of the turbo

 

Ok, now that I've sifted through a million off topic posts about how turbos work when it doesn't even relate to the 6.2...

The real question is, will I be able to buy a 2010 F350 with a V10, or am I going to be stuck with a torque deficient 6.2?