engineermike
Well-Known Member
Jackson, my friend, I respect your mechanical prowess deeply but I'm afraid you have this one mistaken. You don't have to believe me on the issue. A couple hundred years of Thermodynamics and every gas turbine cogeneration unit powering our grid proves it to be true that the energy in gases exists almost completely as a function of it's temperature, not its pressure. The first time I was taught this, I doubted it as well but later learned [again] that it is undoubtedly true.
What further proves that heat is the driver is the way that cogeneration works, where the exhaust gas of the turbine is 1000 deg F but near ambient pressure. If pressure did the work, it would be all done at this point because there is no pressure left. However, the hot gas is then used to heat water into steam that goes on to do more work. Furthermore, the steam pressure was added by a boiler feedwater pump that takes very little power to drive, but the steam has much higher potential to do work than its boiler feedwater because it had heat added to it.
You can not determine the power produced by the mass flow of a working gas by looking at the pressures alone, but you can determine it by looking at the temperatures alone.
Please forgive my horrible explanation, as I would make a poor instructor.
Consider the common gas turbines located all across the country used to drive generators. Their operating principal is what's called the Brayton cycle, as opposed to the Otto cycle we know and love. The Brayton cycle compresses air from ambient up to 100-200 psi, then combusts fuel, which adds heat at constant pressure, unlike the Otto cycle. This hot gas then expands through a turbine back down to ambient pressure. The turbine produces 1.5x more power than the compressor absorbs even though the mass flow and pressures across them are virtually identical. How could the turbine produce 1.5x more power than the compressor absorbs if the working pressures pushing the blades are the same? The answer is heat. The compressor only adds about 500 deg F when it compresses the air, but the turbine removes over 1000 deg F.
What further proves that heat is the driver is the way that cogeneration works, where the exhaust gas of the turbine is 1000 deg F but near ambient pressure. If pressure did the work, it would be all done at this point because there is no pressure left. However, the hot gas is then used to heat water into steam that goes on to do more work. Furthermore, the steam pressure was added by a boiler feedwater pump that takes very little power to drive, but the steam has much higher potential to do work than its boiler feedwater because it had heat added to it.
You can not determine the power produced by the mass flow of a working gas by looking at the pressures alone, but you can determine it by looking at the temperatures alone.
Please forgive my horrible explanation, as I would make a poor instructor.
No, it exits the cylinder because of the pressure. It's also not doing work at this time. Please understand that gas doing work is very different from a gas simply flowing.
Yes.
It has everything to do with the combusting fuel because that is the mechanism by which heat is added to the working gases. Heat can be added at a constant volume (Otto cycle) or constant pressure (Diesel and Brayton cycles). There are many more and I find the Atkinson cycle to be interesting because our Mustangs actually use some level of simulated Atkinson cycle at light loads to improve efficiency. This is what I was referring to earlier when discussing how aftermarket SC calibrations sacrifice some of this Atkinson effect.
Technically, on an NA engine, there is typically still 50-100 psi of pressure in the cylinder when IVO occurs. This blows down to near ambient soon after BDC. If your exhaust system is half-decent, not much expansion occurs after the first 20-30 deg after EVO. With a turbine, you basically set up 2 stages of expansion to extract more energy from the heat of the exhaust gas.
I completely agree on both counts, but you're implying that you can defy the laws of thermodynamics if you spend enough money, which of course isn't true. You can achieve less drive pressure than boost if you size the engine, turbocharger, and piping system with that goal (and don't mind terrible lag). The simple fact that drive pressure-less-than-boost is possible and has been demonstrated proves that the heat is doing the work.
The law of conservation of energy means that 100% of the heat of combustion of the fuel must go somewhere. Some is used to spin the crank, some heats the water, and a lot goes out the exhaust pipe. If we are talking about WOT of a turbocharger vs a supercharger, then less heat goes out the tail pipe because it is used to spin the turbine, and more goes to the crankshaft. As a result, for any given amount of fuel, the turbocharged combination can produce more power. I can't help but notice that the turbocharged coyotes typically make the same power as SC but they do it at a few psi less boost, or make up to 100 more hp at the same boost. I've actually converted a prior car from SC to TC myself and gained about 100 hp at the same boost level even at a 2:1 drive pressure:boost ratio.
Are we comparing TC vs NA? If so, it would be silly to think that adding a turbocharger to an NA engine could improve mileage because of the throttle valve. You can actually use a turbocharger to improve mileage on a diesel if it's sized appropriately because the cold side compressed air can reduce pumping losses on the intake stroke.
I still maintain that the calibration is much more influential than the compressor drive mechanism because neither are doing much at light load. I haven't had the opportunity to pick apart a turbo Coyote calibration, to know if they retain more stock-like cam timing, spark timing, IMRC function, and shift points. The supercharged cals look like they spent little to no time worrying about efficiency.
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