How much heat input is required, and (b) How much heat is discharged as waste heat from this engine, per second? Solution: a. Otherwise the work done on the system in one part of the cycle would be equal to the work done by the system in another part, and the net work would be zero.ġ1 Heat Engines The efficiency of the heat engine is the ratio of the work done to the heat input: Using conservation of energy to eliminate W, we find:Īn automobile engine has an efficiency of 20% and produces an average of 23,000 J of mechanical work per second during operation. (a), (b), (d), and (e) are the four strokes of the cycle.ġ0 Heat Engines Why does a heat engine need a temperature difference? Figure Four-stroke-cycle internal combustion engine: (a) the gasoline–air mixture flows into the cylinder as the piston moves down (b) the piston moves upward and compresses the gas (c) the brief instant when firing of the spark plug ignites the highly compressed gasoline–air mixture, raising it to a high temperature (d) the gases, now at high temperature and pressure, expand against the piston in this, the power stroke (e) the burned gases are pushed out to the exhaust pipe when the piston reaches the top, the exhaust valve closes and the intake valve opens, and the whole cycle repeats. Note that all three of these quantities are positive.Ĩ Heat Engines A steam engine is one type of heat engine.ĩ Heat Engines The internal combustion engine is a type of heat engine as well. The high-temperature reservoir transfers an amount of heat QH to the engine, where part of it is transformed into work W and the rest, QL, is exhausted to the lower temperature reservoir. Figure Schematic diagram of energy transfers for a heat engine.ħ Heat Engines We will discuss only engines that run in a repeating cycle the change in internal energy over a cycle is zero, as the system returns to its initial state. There are many ways to state the second law here is one: Heat can flow spontaneously from a hot object to a cold object it will not flow spontaneously from a cold object to a hot object.Ħ Heat Engines It is easy to produce thermal energy using work, but how does one produce work using thermal energy? This is a heat engine mechanical energy can be obtained from thermal energy only when heat can flow from a higher temperature to a lower temperature. The second law of thermodynamics is a statement about which processes occur and which do not. If it were, movies run backwards would look perfectly normal to us! Figure Have you ever observed this process, a broken cup spontaneously reassembling and rising up onto a table? This process could conserve energy and other laws of mechanics.ĥ The Second Law of Thermodynamics-Introduction However, the absence of the process illustrated above indicates that conservation of energy is not the whole story. The first law of thermodynamics tells us that energy is conserved. Heat Engines Reversible and Irreversible Processes the Carnot Engine Refrigerators, Air Conditioners, and Heat Pumps Entropy Entropy and the Second Law of Thermodynamicsģ Order to Disorder Unavailability of Energy Heat Death Statistical Interpretation of Entropy and the Second Law Thermodynamic Temperature Third Law of Thermodynamics Thermal Pollution, Global Warming, and Energy ResourcesĤ The Second Law of Thermodynamics-Introduction We discuss many practical matters including heat engines, heat pumps, and refrigeration.Ģ The Second Law of Thermodynamics-Introduction The second law of thermodynamics tells us that as time moves forward, the disorder in the universe increases. Entropy is not conserved, but instead is constrained always to increase in any real process. This great law is best stated in terms of a quantity called entropy, which is unlike any other. The efficiency of any engine-no matter how carefully engineered-is limited by nature as described in the second law of thermodynamics. Steam engines produce steam which does work: on turbines to generate electricity, and on a piston that moves linkage to turn locomotive wheels. There are many uses for a heat engine, such as old steam trains and modern coal-burning power plants.