Automobiles 1 Automobiles 2 Introductory Question Automobiles A car burns gasoline to obtain energy but allows some heat to escape into the air. Could a mechanically perfect car avoid releasing heat altogether? A. Yes B. No Automobiles 3 Observations about Automobiles Automobiles 4 6 Questions about Automobiles They burn gas to obtain their power They are rated by horsepower and volume Their engines contain cylinders They have electrical systems They are propelled by their wheels How can an automobile run on thermal energy? How efficient can an automobile engine be? How is an automobile engine a heat engine? Why do cars sometime knock? How is a diesel engine different? What about the rest of the automobile? Automobiles 5 Question 1 Automobiles 6 Heat Engines How can an automobile run on thermal energy? Doesn t the second law forbid this conversion? Doesn t burning destroy gasoline s order completely? An automobile engine is a heat engine A heat engine diverts some heat as it flows naturally from hot to cold and converts that heat into useful work Natural heat flow increases entropy Converting heat to work decreases entropy Overall, some heat can become work without decreasing entropy 1
Automobiles 7 Heat Pumps Automobiles 8 Introductory Question (revisited) A heat pump transfers some heat from cold to hot, against the natural flow, as it converts useful work into heat Reverse heat flow decreases entropy Converting work to heat increases entropy Overall, some heat can flow from cold to hot without decreasing entropy A car burns gasoline to obtain energy but allows some heat to escape into the air. Could a mechanically perfect car avoid releasing heat altogether? A. Yes B. No Automobiles 9 Question 2 Automobiles 10 Efficiency How efficient can an automobile engine be? What fraction of thermal energy can become work? Heat engines and pumps are limited by 2 nd law They cannot decrease the world s overall entropy Their efficiencies depend on temperature differences As the temperature difference increases, a heat pump becomes less efficient a heat engine becomes more efficient Automobiles 11 Question 3 Automobiles 12 Internal Combustion Engine How is an automobile engine a heat engine? An internal combustion engine burns fuel and air in an enclosed space to produces hot burned gases As it allows heat to flow to cold outside air it converts some heat into useful work uses that work to propel a vehicle 2
Automobiles 13 Four Stroke Engine Automobiles 14 Induction Stroke Induction Stroke: fill cylinder with fuel & air Compression Stroke: squeeze mixture Power Stroke: burn and extract work Exhaust Stroke: empty cylinder of exhaust Engine pulls piston out of cylinder Low pressure inside cylinder Atmospheric pressure pushes fuel and air mixture into cylinder Engine does work on the gases during this stroke Automobiles 15 Compression Stroke Automobiles 16 Power Stroke Engine pushes piston into cylinder Mixture is compressed to high pressure and temperature Engine does work on the gases during this stroke Mixture burns to form hot gases Gases push piston out of cylinder Gases expand to lower pressure and temperature Gases do work on engine during this stroke Automobiles 17 Exhaust Stroke Automobiles 18 Ignition System Engine pushes piston into cylinder High pressure inside cylinder Pressure pushes burned gases out of cylinder Engine does work on the gases during this stroke Car stores energy in an electromagnet Energy is released as a high voltage pulse Electric spark ignites fuel and air mixture Two basic types of ignition Classic: points and spark coil Electronic: transistors and pulse transformer 3
Automobiles 19 Efficiency Limits Automobiles 20 Engine, Step 1 Even ideal engine isn t perfect Not all the thermal energy can become work Some heat must be ejected into atmosphere However, ideal efficiency improves as the burned gases become hotter the outside air becomes colder Real engines never reach ideal efficiency Fuel and air mixture after induction stroke pressure = atmospheric temperature = ambient Automobiles 21 Engine, Step 2 Automobiles 22 Engine, Step 3 Fuel/air mixture after compression stroke pressure = high temperature = hot Burned gases after ignition pressure = very high temperature = very hot Automobiles 23 Engine, Step 4 Automobiles 24 Engine, Step 4a Burned gases after power stroke pressure = moderate temperature = high Burned gases after extra expansion pressure = atmospheric temperature = moderate 4
Automobiles 25 Engine, Step 4b Automobiles 26 Question 4 Burned gases after even more expansion pressure = below atmospheric temperature = ambient Why do cars sometime knock? Automobiles 27 Knocking and Gasolines Automobiles 28 Question 5 Compressing a gas increases its temperature During the compression stroke, the fuel and air mixture becomes extremely hot and that mixture can ignite spontaneously in a process called knocking or preignition To avoid knocking, the car can reduce its compression ratio or increase the ignition resistance of its fuel Higher octane fuels are simply harder to ignite How is a diesel engine different? Automobiles 29 Diesel Engine Automobiles 30 Question 6 Uses compression heating to ignite fuel Squeezes pure air to high pressure/temperature Injects fuel between compression and power strokes Fuel burns upon entry into the superheated air Power stroke extracts work from burned gases Because of its higher compression ratio its fuel burns to a higher final temperature and the diesel engine has a higher potential efficiency What about the rest of the automobile? 5
Automobiles 31 Vehicle Pollution Automobiles 32 Catalytic Converter Incomplete burning leaves carbon monoxide and hydrocarbons in exhaust Accidental oxidization of nitrogen produces nitrogen oxides in exhaust Diesel exhaust includes many carbonized particulates Platinum assists oxidization of carbon monoxide and hydrocarbons to carbon dioxide and water Rhodium assists reduction of nitrogen oxides to nitrogen and oxygen. Catalysts supported on high specific surface structure in exhaust duct: catalytic converter Automobiles 33 Transmissions Automobiles 34 Manual Transmission Changes force/distance (actually torque/rotation rate) relationships between the engine and the wheels Two basic types Manual: clutch and gears Automatic: fluid coupling and gears Clutch uses friction to convey torque from engine to drive shaft Opening clutch decouples engine and shaft Closing clutch allows engine to twist shaft Gears control mechanical advantage Automobiles 35 Automatic Transmission Automobiles 36 Brakes Fluid coupling uses moving fluid to convey torque to drive shaft Engine turns impeller (fan) that pushes fluid Moving fluid spins turbine (fan) and drive shaft Decoupling isn t required Gears control mechanical advantage Use sliding friction to reduce car s energy Two basic types Drum: cylindrical drum and curved pads Disk: disk-shaped rotor and flat pads Brakes are operated hydraulically Pedal squeezes fluid out of master cylinder Fluid entering slave cylinder activates brake 6
Automobiles 37 Summary about Automobiles Cylinders expand hot gas to do work Uses the flow of heat from hot burned gases to cold atmosphere to produce work Energy efficiency is limited by thermodynamics Higher temperatures increase efficiency 7