Heat Engines: Converting Thermal Energy into Mechanical Work, Study notes of Physics

Download Heat Engines: Converting Thermal Energy into Mechanical Work and more Study notes Physics in PDF only on Docsity! 1 PHY 101 Lecture #22 Heat Engines 1 PHY 101 Lecture #22: Heat Engines Prof. Peter R. Saulson saulson@physics.syr.edu http://physics.syr.edu/courses/PHY101/ Office Hrs: Tues 10 –11:30, Physics 263-4, 3-5994 PHY 101 Lecture #22 Heat Engines 2 Outline 1. Why are heat engines important? 2. World’s simplest heat engine 3. Stirling engine PHY 101 Lecture #22 Heat Engines 3 Heat and work Breakthroughs of mid-19th cent. physics: Joule’s “mechanical equivalent of heat” 4184 J of mechanical energy converts into 1 Cal of thermal energy Mayer’s, Helmholtz’s conservation of energy Q - W = ∆E Looked like thermal energy and other forms of energy were freely interconvertible. Same units, symmetrical appearance in 1st Law Is thermal energy just another form of energy? PHY 101 Lecture #22 Heat Engines 4 17th century “energy crisis” Oldest technologies used human or animal muscle as the motive power in manufacturing, transportation, and other “useful work”. Limited, expensive Water power (gravitational potential energy) was first big improvement. Fueled beginning of Industrial Revolution. But limited supply of power, and geographically limited. PHY 101 Lecture #22 Heat Engines 5 Chemical energy resources and the Industrial Revolution Much more energy is available in chemical form than in gravitational form. Coal in 18th, 19th centuries 20th cent. added petroleum and natural gas Only way to release coal’s stored chemical energy is by burning. Directly for heating: OK For work: need to be able to convert thermal energy into mechanical energy. Hence, the need for heat engines. PHY 101 Lecture #22 Heat Engines 6 How best to convert thermal energy into mechanical energy? Technological taming of heat began ca. 1700. Thomas Savery 1698 & Thomas Newcomen 1705 both made practical steam engines Better steam engines due to James Watt, starting in 1769, before the understanding of physics of energy. Physics questions: What is the best way to get mechanical energy from thermal energy? Can 100% of thermal energy be converted to work? 2 PHY 101 Lecture #22 Heat Engines 7 World’s Simplest Engine Gas piston setup can be used to convert thermal energy into mechanical form. 0. Starting position: gas is cool, piston is down. 1. Add mass (from lower shelf) to piston. 2. Warm up the gas. Piston rises. 3. Remove mass onto higher shelf. 4. Cool down the gas. Piston sinks. After this cycle, Ugrav has been added to the outside world. (“Work was done.”) This cycle can be repeated indefinitely. PHY 101 Lecture #22 Heat Engines 8 Pressure-Volume diagram of World’s Simplest Engine Pressure is above atmospheric while lifting the 0.2 kg mass. It stays constant during the expansion of the volume of the cylinder. W=(Pin-Pout)*∆V W is just the area of the red bars. PHY 101 Lecture #22 Heat Engines 9 Why a cycle? Why inefficiency? Engines must be cyclic. Otherwise, we consume the engine. We want to consume only thermal energy. To bring engine back to starting state, we need to send heat into a cool reservoir. That is energy that can’t be converted to mechanical form. Since some energy is sent out as heat, not all of the energy sent in as heat is converted to work. PHY 101 Lecture #22 Heat Engines 10 Stirling engine One of the simplest forms to analyze. Mechanization of steps of heating, expansion, cooling, compression. Four steps in cycle: 1. Expansion while held at high temperature TH. 2. Move gas (at constant volume) from hot reservoir to cool reservoir, lowering temperature to TL . 3. Compression while held at TL. 4. Move gas (at constant volume) from cool reservoir to hot reservoir, raising temp to TH. PHY 101 Lecture #22 Heat Engines 11 Parts of a Stirling engine 1. Hot reservoir (e.g. hot plate or cup of tea), 2. Cool reservoir (bins for cool water or ice), 3. Air space, communicating with hot reservoir, cool reservoir, and power piston, 4. “Displacer piston” shuttles air between hot and cold reservoirs, 5. Power piston to turn crank, 6. Crank, with clever bends to sequence motions of displacer piston and power piston, and 7. Flywheel for rotational inertia (here, propeller.) PHY 101 Lecture #22 Heat Engines 12 What makes Stirling engine go? Engine goes because warm air’s pressure pushes power piston. Warm air does work W = P∆V. But, to make a cycle, you have to compress the air again. Why don’t you need to do as much work to compress the gas as you got out of it during expansion? Compress the gas when it is cool. Pressure is lower. Work P∆V to compress is less.