Suggested languages for you:

Americas

Europe

Q30PE

Expert-verified
Found in: Page 551

### College Physics (Urone)

Book edition 1st Edition
Author(s) Paul Peter Urone
Pages 1272 pages
ISBN 9781938168000

# (a) What is the hot reservoir temperature of a Carnot engine that has an efficiency of $${\bf{42}}{\bf{.0\% }}$$and a cold reservoir temperature of $${\bf{27}}{\bf{.0}}\;{\bf{^\circ C}}$$? (b) What must the hot reservoir temperature be for a real heat engine that achieves $${\bf{0}}{\bf{.700}}$$ of the maximum efficiency, but still has an efficiency of $${\bf{42}}{\bf{.0\% }}$$ (and a cold reservoir at $${\bf{27}}{\bf{.0}}\;{\bf{^\circ C}}$$)? (c) Does your answer imply practical limits to the efficiency of car gasoline engines?

The temperature of the hot reservoir is $$244.24\;^\circ {\rm{C}}$$ for Carnot engine. The hot reservoir temperature for real heat engine with $$0.70$$ of maximum (Carnot) efficiency is $$477\;^\circ {\rm{C}}$$.Yes, our result imply practical limits as temperature of hot reservoir is very high for practical applications.

See the step by step solution

## Step 1: Introduction

We calculate temperature of hot reservoir by using the formula for efficiency of Carnot engine when efficiency and temperature of cold reservoir is given. We further calculate temperature of hot reservoir of real engine. We find that this temperature is very high for practical purposes.

## Step 2:  Given parameters and formula for efficiency of heat engine

Efficiency of Carnot engine$$= 42\% = 0.42$$

Temperature of cold reservoir $${T_c} = 27\;^\circ {\rm{C}} = 300\;{\rm{K}}$$

Efficiency of Carnot engine $$h = 1 - \frac{{{T_c}}}{{{T_h}}}$$

Here,

$$\eta$$ - efficiency of engine.

$${T_c}$$ - temperature of cold reservoir.

$${T_h}$$- temperature of hot reservoir.

## Step 3:  Calculate temperature of hot reservoir for this Carnot engine

Temperature of hot reservoir is calculated as

\begin{aligned}{}h &= 1 - \frac{{{T_c}}}{{{T_h}}}\\{\rm{0}}{\rm{.42}} &= {\rm{1}} - \frac{{{\rm{300}}\;{\rm{K}}}}{{{T_h}}}\\\frac{{{\rm{300}}\;{\rm{K}}}}{{{T_h}}} &= {\rm{1}} - {\rm{0}}{\rm{.42}}\end{aligned}

\begin{aligned}{}{T_h} &= \frac{{{\rm{300}}\;{\rm{K}}}}{{{\rm{0}}{\rm{.58}}}}\\ &= {\rm{517}}{\rm{.24 K}}\\ &= {\rm{244}}{\rm{.24}}{{\rm{ }}^{\rm{o}}}{\rm{C}}\end{aligned}

## Step 4:  Calculate efficiency of Carnot engine and temperature of hot reservoir for it

Efficiency of real engine $$= 0.7$$ times efficiency of Carnot engine

Efficiency of real engine $$= 42\% = 0.42$$

Efficiency of Carnot engine

\begin{aligned}{}h &= {\rm{0}}{\rm{.42}}/{\rm{0}}{\rm{.7}}\\ &= {\rm{0}}{\rm{.6}}\\ &= {\rm{60\% }}\end{aligned}

Temperature of hot reservoir is calculated as

\begin{aligned}{}h &= 1 - \frac{{{T_c}}}{{{T_h}}}\\0.6 &= 1 - \frac{{300\;{\rm{K}}}}{{{T_h}}}\\\frac{{300\;{\rm{K}}}}{{{T_h}}} &= 1 - 0.6\end{aligned}

\begin{aligned}{}{T_h} &= \frac{{300\;{\rm{K}}}}{{0.4}}\\ &= 750 {\rm{K}}\\ &= 477{\;^{\rm{o}}}{\rm{C}}\end{aligned}

Therefore, the temperature of hot reservoir is $${\rm{244}}{\rm{.24}}{{\rm{ }}^{\rm{o}}}{\rm{C}}$$for Carnot engine. The hot reservoir temperature for real heat engine with 0.7 of maximum (Carnot) efficiency is$$477{\;^{\rm{o}}}{\rm{C}}$$.Yes, our result imply practical limits as temperature of hot reservoir is very high for practical applications as this temperature cannot be sustained in engines.