You said it yourself…more rice, more power.
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How does the process inside the cylinder differ from what’s happening in the turbo?
Inside the cylinder heat causes the pressure to increase, hot gases push the piston down and the gases cool.
When the exhaust valve opens pressure drops and volume increases and the temp drops.
Is this just semantics or is there some basis to call one a heat engine and the other not? Or am I missing something here?
Is it a cause and effect thing? What is causing the transfer of energy and which is the result? ie the drop in temp in the exhaust gases in the turbo?
How can you misquote me and take me out of context at the same time? Clever. Very clever.
If you sort out the difference between heat and temperature, it becomes more manageable.
So lets assume a 4 stroke engine with a conventional turbo.
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Suck: the piston drops, cylinder volume increases, charge air is drawn in because the manifold has more pressure (red line) than the cylinder. Exhaust valve closes, and now we consider the cylinder gas to be air. The air has a temperature and a mass and so contains some quantity of heat. As the pressure in the cylinder approaches the pressure in the manifold, we don’t have to worry about the change in volume at this stage, it washes out. The intake valve closes. Now we have a set volume, set temperature, set mass, set pressure, and set heat. Just a closed jar of air.
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Squeeze: The piston rises, the volume falls, the pressure rises, the temperature rises, but the heat and the mass stay the same. Fuel is injected and combustion starts.
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Bang: Now we have different gasses, different number of molecules, more heat, a little more mass, more pressure, volume more-or-less a wash at first, more temperature. Then the piston is pushed down by the pressure, so more volume, less temperature, same mass, same heat, less pressure. The exhaust valve opens.
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Blow: Cylinder pressure is more than exhaust manifold pressure, so mass flow is outwards. Lets stick with the gas’s point of view. Volume increases a little, Temperature drops a little, mass stays the same, pressure drops a little, heat stays the same. Now approaching turbine.
- Turbine: within the turbine, volume increases (3-4), temperature falls, mass stays the same, pressure falls, heat stays the same. ah… something about heat rejection…? Take it away, @Steamer!
It seems to me the turbo is one step removed. If a turbo was run on compressed air the temperature would drop just the same as it does using exhaust gases. Of course at some point ΔQ is involved in the process of compressing air.
I would think a mathematical model of a turbo would be simpler if the pressure difference was analyzed rather then starting with ΔQ.
That is to say given the difference in temp between inlet and outlet the pressure difference could be found but if the pressure difference was known that step could be skipped.
Kinda… I think of a heat engine as the complete system, having a means of adding heat to your working fluid (the cold sink), a means of rejecting heat (the heat sink), a means of compressing the fluid and a means of expanding the fluid while removing mechanical energy. In the case of a turbocharger, the cold sink is the engine itself, so it’s only a part of a heat engine. Likewise, a steam turbine is only a part of a heat engine (the steam plant). I’m sure someone with impeccable sources thinks otherwise, so yeah… semantics.
I was reacting to the notion that a turbocharger is motivated by heat rather than the kinetic energy of the exhaust gas. It is most definitely motivated by the kinetic energy of the gas, which in turn is motivated by the heat added in the combustion chamber. For the purpose of understanding exhaust gas power turbines, I find it a useful distinction.
Heat is added through compression. If the mass stays the same and the temperature rises, what must happen to the heat?
Again, heat is removed through expansion, and some of it converted to mechanical energy.
The turbine itself only represents the expansion phase of the cycle (3-4 in your pv diagram), where heat is removed as mechanical energy. Heat rejection (maintaining low pressure on the low pressure side) is by dumping gas to the atmosphere in the case of a gas turbine, or the condenser in the case of a steam plant.
Is that what you say when unlearned peasants such as myself try to speak thermodynamics? 
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Oops - didn’t mean to reply to Klaveness
A Hadley cell can be (is) considered a heat engine. Wind turbines are using pressure differentials in the atmosphere.
You did a good job with #5, don’t need me.
The best example I might add is that of a steam turbine. The mass of the operating fluid doesn’t change but its temperature, heat content, and volume change dramatically as energy is extracted. Consider that the mass of stays the same between the throttle and condenser while the temperature drops from more than 900 degrees to 100 degrees or less.
Exactly. With no temperature change there is no energy exchange. When temperature changes those calories have to go somewhere. In a turbine they make motion. A “jet engine” is another example. One can spray burning kerosene into the air and it is just a smelly fire but introduce that heat into a restricted space and things will move or melt 
Super critical steam is the best example I can think of for heat being a generator of mechanical energy. Though super critical steam requires high pressure to produce that is simply because of the boiling point of water, which to this day remains an inexpensive resource for transferring energy, There are other fluids that boil and can produce high heat at lower pressure but they are very expensive.
That is how an aircraft air conditioning system “air cycle machine” operates. Hot bleed air from an engine compressor stage is expanded by a turbine to create cold air.
Yet the pressure in the burner can does not increase, it actually drops slightly while temperature rises dramatically. Both pressure and temperature then drop rapidly as the hot gas expands across the power turbine.
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True ! So heat movement makes the motion. You know that of course
Temperature is not energy and its units are not calories. This is why people are confused.
1 kg of gas, at 0 deg C, at 1 atm will have a certain volume and a certain amount of heat. at half an atm it will be minus something deg C, larger, and (under adiabatic conditions) have the same heat. at 2 atm, it will be above freezing, smaller, and still have the same heat.
Except for geeks and thermodynamicists, temperature is the conversational term used to describe heat. Don’t make a big deal of it. Most of those who understand the concept don’t have a problem and those who don’t understand it don’t know any better and still live normal lives.
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Ya, but we are trying to answer a question about why turbos are not pinwheels.
If you have a fireplace in your house féel the draft with the pinwheel before you build a fire. The natural draft created by the height of the chimney is constant.Your pinwheel held in front of the hearth may turn slowly. Build a fire, hold that pinwheel close to the hearth and it will turn a little faster.
What is the difference between these two cases? The air starting turbine and the turbocharger turbine?
In the case of the starting air turbine a valve is open and compressed air is released into the turbine where it is converted to mechanical motion.
In the case with the turbocharger it’s the same but the compressed gasses are released when the exhaust valve is open (or uncovered).
I stand corrected on the P&W vs Wright. Not too familiar with either since my personal experience has been with R-985’s , PT-6 and Garrett’s. Just saw cut-aways in museums of the corncob engines.
What is DD16V92?
Direction of flow (axial vs radial), pressure drop across the turbine, energy extraction efficiency, impulse vs reaction …