Quote:
Originally Posted by dcb
I have to say to this point that I've been assuming a simple bomb calorimeter type measurement of the BTU of gasoline. I am struggling to understand how using an intermediate value makes any sense though. If the engine (or powerplant) releases hot gasses (or even liquid that is slightly above room temperature) it is releasing energy that it might otherwise be using.
One doesn't get to apply a handicap because the discharge happens to be in gaseous state, I don't get that at all.
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I think Ernie isn't explaining it quite right (I understand because I already know what he's talking about, but I can see it being very confusing)... I am happy, however, that low heat is being brought in
Firstly,
In the microscopic sense of thermo efficiency - low heat isn't a handicap. On the macroscopic level, it is because low heat ignores the inherent problem with the process itself.
High heat versus low heat comes down to the energy in water. Simply put, high heat assumes water after a combustion event is in liquid form whereas low heat takes water to be in steam. While this seems kinda crazy, water's latent heat of vaporization is the culprit for the difference.
This is why Ernie keeps mentioning "gasses."
I normally don't do this because sometimes the book is difficult to understand but...
Fundamentals of Engineering Thermodynamics by M. Moran, H. Shapiro
Quote:
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The heating value of a fuel is a positive number equal to the magnitude of the enthalpy of combustion. Two heating values are recognized by name: the higher heating value(HHV) and the lower heating value (LHV). The higher heating value is obtained when all the water formed by combustion is a liquid; the lower heating value is obtained when all the water formed by combustion is a vapor. The higher heating value exceeds the lower heating value by the energy that would be required to vaporize the liquid formed. Values for the HHV and LHV also depend on weather the fuel is a liquid or a gas. Heating value data for several hydrocarbons are provided in tables A-25
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It then goes on to show how to calculate the enthalpy of combustion, which is not exactly trivial. One finds low heat by taking high heat and subtracting the latent heat energy required to vaporize water (something EV components need not worry about
).
Here's the rub (and where your point comes in, dcb) - high heat is the gross, low heat is net. We can't stop water from vaporizing, it
will take that energy, so the only available thermal energy to extract mechanical work is the energy available in low heat.
Ignoring the high heat value of combustible fuels is like ignoring the charger efficiency of an electric vehicle. Yes, the motor might be 90% efficiency - but that doesn't mean 90% of the energy put in gets converted to useful work
It's not very truthful to ignore everything except the motor...