You want the physics without the mathematics and the consequential intracranial bleeding? OK - here we go:
Today’s chat is inspired by JB007:
"30 years ago I was told the heat loss through the turbo causes it to turn and I have been trying to get my head around this ever since. ie, heat loss creates a pressure differential that creates flow. It’s still bending my brain. How does this work?"
Combustion is the process that drives everything. Fuel has a high amount of stored energy. Combustion byproducts (CO2 and water) have a low amount of stored energy. The difference in the two stored energy states is liberated and used for motivation (and to drive the alternator, etc.)
It’s the heat that energises the gas in the combustion chamber. And it’s the energetic state of the gas that does the mechanical work on the piston.
Unfortunately, we just throw some of that apparently serviceable energy away, out the exhaust pipe. It’s this wasted energy - at least some of it - that is used to drive the turbocharger.
There’s a lot of energy still in the exhaust gas as it exits the cylinder and enters the exhaust manifold.
The energy inside the gas in those headers exists in the form of heat, pressure and motion. Because it’s pressurised relative to atmosphere it’s on the hunt for a way out, and the only exit sign is hanging over the entry to the exhaust turbine part of the turbocharger. It’s like a pressure release valve.
Lots of processes take place here. The pressure inside the exhaust manifold is caused by heat - because there’s a direct, linear relationship between absolute temperature and pressure. So there’s high pressure on the engine side and relatively low pressure on the exit side.
Therefore, there’s flow. (Because gasses flow from areas of high to low pressure.) Gasses have mass. Exhaust is mainly nitrogen gas, CO2 and steam. They’ve all got mass. If you’ve got mass and flow, you’ve got kinetic energy. So the heat and pressure turns into kinetic energy by virtue of its motion through the turbine.
The gas also has viscosity - which is a fancy way of saying it just doesn’t like being pushed around. Or at least it resists being pushed around. So it hits the turbine blades on its way out out the door and the resistance translates to force acting on the blades, and that spins the turbine, which is connected to the compressor on the other end of the shaft.
The compressor pressurises the air on the way into the engine, and that’s basically the mechanism for transferring energy that would have been wasted in the exhaust to additional energy in the inlet air stream (in the form of pressure).
A couple of points on this: Several people took issue with the fact that I said turbochargers are not driven by heat. And I stand by that statement - a turbocharger is driven by mechanical flow - kinetic energy and viscosity. The gas that flows is energised by heat, certainly, but you cannot drive a turbine by heat - you need flow.
If you want to claim it’s heat also driving your alternator, then yeah - OK. We’re totally on the same page here.
There’s a thing called the first law of thermodynamics, which basically covers the conservation of energy. It essentially says the sum of work and energy in a closed system is constant in the time domain.
So if you put a box around the turbine in a turbocharger, you’ve got energy going in (in the form of highly pressurised, hot gas). You can see it’s hot because it’s heating up the headers to bright yellow while it waits for its turn to go through.
You’ve got energy coming out, in the form of mechanical work done spinning the turbocharger (the rotational kinetic energy acquired by the turbine and the compressor). And also in the form of the energy left in the exhaust gas after it passes through the turbine.
Energy in has to equal energy out, or you violate the first law, and that’s not allowed. So the energy coming out of the turbine equals the energy on the way in minus the work done on the turbine and compressor. So the total energy of the exhaust gasses after the turbo must be less than before the turbo.
It’s definitively a lower temperature on the output side of the turbine (you can see that) and the pressure is also lower (otherwise there wouldn’t be flow in that direction). And these are two of the key indicators of the energy state of any gas, all other things being equal.
So instead of thinking about this like: ‘How does the temperature difference (or heat loss) drive the turbocharger?’ just think about it like this (at the risk of dumbing it down to the point where even a politician would get it):
You’ve got high energy gas going into the turbo. It comes out at a lower energy, and the difference between the energy states, before and after, is the energy that’s been returned to the engine by the turbocharger. The temperature and the pressure differences are just individual pieces in the thermodynamics jigsaw puzzle.
Негізгі бет Автокөліктер мен көлік құралдары The physics of turbochargers (for dummies) | Auto Expert John Cadogan
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