6.) You have a number of heat reservoirs available, the hottest being at $T_2$ and the coolest being at $T_1$. Which engine (a-d) will you choose for maximum efficiency?

a.) Diesel	$\eta=\displaystyle 1-{1\over\gamma V_1^{\gamma-1}}\left(\frac {V_3^\gamma-V_2^\gamma}{V_3-V_2}\right)$
b.) Joule ideal gas	$\eta=\displaystyle 1- \left({P_1\over P_2}\right)^\frac{\gamma-1}{\gamma}$
c.) Carnot	$\eta=\displaystyle 1-\frac{T_1}{T_2}$
d.) Otto	$\eta=\displaystyle 1-\left({V_2\over V_1}\right)^{\gamma-1}$

The Carnot engine is the most efficient. It draws all its heat from the hottest reservoir, and discards all the waste heat to the coolest reservoir.

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