Heat Engines Heat Pumps

1. Heat Engines Heat Pumps Physics Montwood High School R. Casao

2. Heat Engine Cycle • A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. • The first law and second law of thermodynamics constrain the operation of a heat engine. • The first law is the application of conservation of energy to the system, and • the second sets limits on the possible efficiency of the machine and determines the direction of energy flow.

3. First Law of Thermodynamics • The first law of thermodynamics is the application of the conservation of energy principle to heat and thermodynamic processes: the change in internal energy (U) of a system is equal to the heat (Q) added to the system minus the work (W) done by the system. • Mathematically: U = Q - W

4. Internal Energy • Internal energy is defined as the energy associated with the random, disordered motion of molecules. • It is separated in scale from the macroscopic ordered energy associated with moving objects; it refers to the invisible microscopic energy on the atomic and molecular scale. For example, a room temperature glass of water sitting on a table has no apparent energy, either potential or kinetic . But on the microscopic scale it is a seething mass of high speed molecules traveling at hundreds of meters per second.

5. Internal Energy • In the context of physics, the common scenario is one of adding heat to a volume of gas and using the expansion of that gas to do work, as in the pushing down of a piston in an internal combustion engine.

6. Heat engines such as automobile engines operate in a cyclic manner, adding energy in the form of heat in one part of the cycle and using that energy to do useful work in another part of the cycle. First Law of Thermodynamics

7. PV Diagrams • Pressure-Volume (PV) diagrams are a primary visualization tool for the study of heat engines. Since the engines usually involve a gas as a working substance, the ideal gas law relates the PV diagram to the temperature so that the three essential state variables for the gas can be tracked through the engine cycle.

8. For a cyclic heat engine process, the PV diagram will be closed loop. The area inside the loop is a representation of the amount of work done during a cycle. Some idea of the relative efficiency of an engine cycle can be obtained by comparing its PV diagram with that of a Carnot cycle, the most efficient kind of heat engine cycle. PV Diagrams

9. Heat Engines • A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work. Thermodynamics is the study of the relationships between heat and work. • The first law is the application of conservation of energy to the system, and the second sets limits on the possible efficiency of the machine and determines the direction of energy flow.

10. Energy Reservoir Model • One of the general ways to illustrate a heat engine is the energy reservoir model. The engine takes energy from a hot reservoir and uses part of it to do work, but is constrained by the second law of thermodynamics to exhaust part of the energy to a cold reservoir. In the case of the automobile engine, the hot reservoir is the burning fuel and the cold reservoir is the environment to which the combustion products are exhausted.

11. Second Law of Thermodynamics • Second Law of Thermodynamics: It is impossible to extract an amount of heat QH from a hot reservoir and use it all to do work W . Some amount of heat QC must be exhausted to a cold reservoir. • The maximum efficiency which can be achieved is the Carnot efficiency.

12. Second Law of Thermodynamics

13. The most efficient heat engine cycle is the Carnot cycle, consisting of two isothermal processes and two adiabatic processes. The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by physical laws. Carnot Cycle

14. Carnot Cycle • In order to approach the Carnot efficiency, the processes involved in the heat engine cycle must be reversible and involve no change in entropy. This means that the Carnot cycle is an idealization, since no real engine processes are reversible and all real physical processes involve some increase in entropy.

15. The conceptual value of the Carnot cycle is that it establishes the maximum possible efficiency for an engine cycle operating between TH and TC . Carnot Cycle

16. Combustion Engines • Combustion engines: burn fuel to produce the heat input for a thermodynamic cycle. • Burning fuel turns chemical energy into heat energy. • By-products of combustion have a very high temperature and produce a very high pressure. • Results: piston pushed downward and a fraction of the heat energy is converted to mechanical work. • Some heat energy is carried away by the high temperature exhaust gases, and some is lost to the cylinder walls.

17. First law of thermodynamics for combustion engine: • Mathematically: QH = QC + W • QH = heat input due to fuel combustion • QC = heat energy lost • W = work • Net heat absorbed per cycle: QT = QH + QC

18. First law of thermodynamics for combustion engine: • Work output for combustion engine: W = QH - QC • Efficiency for combustion engine:

19. First law of thermodynamics for combustion engine:

20. Gasoline Engine • Five successive processes occur in each cycle within a conventional four-stroke gasoline engine. • During the intake stroke of the piston, air that has been mixed with gasoline vapor in the carburetor is drawn into the cylinder. • During the compression stroke, the intake valve is closed and the air-fuel mixture is compressed approximately adiabatically.

21. Gasoline Engine • At this point, the spark plug ignites the air-fuel mixture, causing a rapid increase in pressure and temperature at nearly constant volume. • The burning gases expand and force the piston back, which produces the power stroke. • During the exhaust stroke, the exhaust valve is opened and the rising piston forces most of the remaining gas out of the cylinder. • The cycle is repeated after the exhaust valve is closed and the intake valve is opened. • How Stuff Works Gasoline Engine Animation • How Stuff Works Gasoline Engine Animation

24. Otto Cycle

25. Otto Cycle

26. Otto Cycle

27. Otto Cycle

28. Otto Cycle

29. Otto Cycle

30. Diesel Engines • The main differences between the gasoline engine and the diesel engine are: • A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously. • A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.

31. Diesel Engines • Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder. • How Stuff Work Diesel Animation

32. Diesel Engines • Note that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine.

33. Dodge Hemi • Hemi: (HEM -e) adj. Mopar in type, V8, hot tempered, native to the United States, carnivorous, eats primarily Mustangs, Camaros, and Corvettes. Also enjoys smoking a good import now and then to relax. • The hemispherically shaped combustion chamber is designed to accommodate large valves and put the spark plugs close to the center of the combustion chamber.

34. In a HEMI engine, the top of the combustion chamber is hemi-spherical, as seen in the image. The combustion area in the head is shaped like half of a sphere. An engine like this is said to have "hemi-spherical heads." In a HEMI head, the spark plug is normally located at the top of the combustion chamber, and the valves open on opposite sides of the combustion chamber.

35. Advantage: Horsepower • The engine produces 345 horsepower, and compares very favorably with other gasoline engines in its class. For example • Dodge 5.7 liter V-8 - 345 hp @ 5400 rpm • Ford 5.4 liter V-8 - 260 hp @ 4500 rpm • GMC 6.0 liter V-8 - 300 hp @ 4400 rpm • GMC 8.1 liter V-8 - 340 hp @ 4200 rpm • Dodge 8.0 liter V-10 - 305 hp @4000 rpm • Ford 6.8 liter V-10 - 310 hp @ 4250 rpm • The HEMI Magnum engine has two valves per cylinder as well as two spark plugs per cylinder. The two spark plugs help to solve the emission problems that plagued Chrysler's earlier HEMI engines. The two plugs initiate two flame fronts and guarantee complete combustion.

36. Disadvantage: • If HEMI engines have all these advantages, why aren't all engines using hemispherical heads? It's because there are even better configurations available today. • One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breath easier than two large valves. Modern engines use a pentroof design to accommodate four valves.

37. Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well. Disadvantage:

38. Gas Turbine Engines • In a gas turbine, a pressurized gas spins a turbine. • In all modern gas turbine engines, the engine produces its own pressurized gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. • The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.

39. Gas Turbine Engines • Two big advantages of the turbine over the diesel: • Gas turbine engines have a great power-to-weight ratio compared to gasoline or diesel engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good. • Gas turbine engines are smaller than their reciprocating counterparts of the same power.

40. Gas Turbine Engines • The main disadvantage of gas turbines is that, compared to gasoline and diesel engines of the same size, they are expensive. • Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. • Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load. That makes gas turbines great for things like transcontinental jet aircraft and power plants, but explains why you don't have one under the hood of your car.

41. Gas Turbine Engines • Three parts of the gas turbine engine: • Compressor - Compresses the incoming air to high pressure • Combustion area - Burns the fuel and produces high-pressure, high-velocity gas • Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber. • Gas Turbine Operation Animation

42. Heat Pumps • Heat pumps: a mechanical device that moves energy from a region at a lower temperature to a region at higher temperature. • Heat pump can be described by a thermodynamic cycle just like that of an engine. System absorbs heat at a low temperature and rejects it at a higher temperature.

43. Heat Pumps • Heat pumps have long been used to cool homes and buildings, and are now becoming increasingly popular for heating them as well. • Heat pump contains two sets of metal coils that can exchange energy by heat with the surroundings: one set is on the outside of the building in contact with the air or the ground; and the other set in the interior of the building.

44. Heat Pumps

45. Heat Pumps • In the heating mode, a circulating fluid flowing through the coils absorbsenergy from the outside and releases it to the interior of the building fromthe interior coils. • The fluid is cold and at low pressure when it is in the external coils, where it absorbs energy by heat from either the air or the ground. • The resulting warm fluid is then compressed and enters the interior coils as a hot, high-pressure fluid, where it releases its stored energy to the interior air.

47. Heat Pumps • First law of thermodynamics for heat pump: QH = QC + Win • QC = heat removed from low temperature reservoir • QH = heat pumped into high temperature reservoir • Win = work input

48. Coefficient of Performance • Effectiveness of a heat pump is described in terms of a ratio called the coefficient of performance (COP). In the heating mode, the COP is defined as the ratio of the heat QH moved to a higher temperature region divided by the work input required to transfer that energy. • COP (heating mode) =

49. Coefficient of Performance • The COP is similar to the thermal efficiency for a heat pump in that it is a ratio of what you get (energy delivered to the interior of the building) to what you give (work input). • Because QH is generally greater than Win, typical values for the COP are greater than 1. • It is desirable for the COP to be as high as possible. • Example: if the COP for a heat pump is 4, the amount of energy transferred to the building is 4 times greater than the work done by the motor in the heat pump.

50. Coefficient of Performance • Maximum possible COP is called the Carnot COP and is never achieved by a real heat pump and depends on the high and low temperature between which the pump operates. • Carnot COP (heating mode) =