# Heat Engines Heat Pumps

## Presentation on theme: "Heat Engines Heat Pumps"— Presentation transcript:

Heat Engines Heat Pumps
Physics Montwood High School R. Casao

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.

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

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.

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.

First Law of Thermodynamics
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.

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.

PV Diagrams 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.

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.

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.

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.

Second Law of Thermodynamics

Carnot Cycle 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 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.

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

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.

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

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

First law of thermodynamics for combustion engine:

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.

Gasoline Engine How Stuff Works Gasoline Engine Animation
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

Otto Cycle

Otto Cycle

Otto Cycle

Otto Cycle

Otto Cycle

Otto Cycle

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.

Diesel Engines How Stuff Work Diesel Animation
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

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.

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.

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.

The engine produces 345 horsepower, and compares very favorably with other gasoline engines in its class. For example Dodge 5.7 liter V rpm Ford 5.4 liter V rpm GMC 6.0 liter V rpm GMC 8.1 liter V rpm Dodge 8.0 liter V rpm Ford 6.8 liter V 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.

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.

Disadvantage: 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.

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.

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.

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.

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

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.

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.

Heat Pumps

Heat Pumps In the heating mode, a circulating fluid flowing through the coils absorbs energy from the outside and releases it to the interior of the building from the 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.

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

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) =

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.

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) =

Heat Pumps Heat pumps can also operate in the cooling mode. Air conditioners and refrigerators are examples of heat pumps operating in the cooling mode. Energy is absorbed into the circulating fluid in the interior coils; then, after the fluid is compressed, energy leaves the fluid through the external coils.

Heat Pumps The heat pump must have a way to release energy to the outside. Refrigerator as an example: A refrigerator cannot cool the kitchen if the refrigerator door is left open. The among of energy leaving the external coils behind or underneath the refrigerator is greater than the amount of energy removed from the food or from the air in the kitchen if the door is left open. The difference between the energy out and the energy in is the work done by the electricity supplied to the refrigerator. Energy, Win, allows compressor to remove heat from inside the refrigerator and transfer it to the kitchen.

Heat Pumps For a heat pump operating in the cooling mode, “what you get” is energy removed from the cold reservoir. The most effective refrigerator or air conditioner is one that removes the greatest amount of energy from the cold reservoir in exchange for the least amount of work. COP (cooling mode) =

Heat Pumps The greatest possible COP for a heat pump in the cooling mode is that of a heat pump whose working substance is carried through a Carnot cycle in reverse. Carnot COP (cooling mode) =

Second Law: Refrigerator
Second Law of Thermodynamics: It is not possible for heat to flow from a colder body to a warmer body without any work having been done to accomplish this flow. Energy will not flow spontaneously from a low temperature object to a higher temperature object.

Second Law: Refrigerator

Second Law: Entropy Second Law of Thermodynamics: In any cyclic process the entropy will either increase or remain the same. Entropy: a measure of the amount of energy which is unavailable to do work; a measure of the disorder of a system. Entropy DS =