heat engine

Examples of heat engine in the following topics:

• Carnot Cycles

  • (See our atom on "Heat Engines. ") How efficient can a heat engine be then?
  • Any heat engine employing the Carnot cycle is called a Carnot engine.
  • This increases heat transfer Qc to the environment and reduces the efficiency of the engine.
  • Carnot also determined the efficiency of a perfect heat engine—that is, a Carnot engine.
  • What Carnot found was that for a perfect heat engine, the ratio Qc/Qh equals the ratio of the absolute temperatures of the heat reservoirs.

• Heat Engines

  • In thermodynamics, a heat engine is a system that performs the conversion of heat or thermal energy to mechanical work.
  • Gasoline and diesel engines, jet engines, and steam turbines are all heat engines that do work by using part of the heat transfer from some source.
  • Most heat engines, such as reciprocating piston engines and rotating turbines, use cyclical processes.
  • We define the efficiency of a heat engine (Eff) to be its net work output W divided by heat transfer to the engine Qh:
  • (b) A heat engine, represented here by a circle, uses part of the heat transfer to do work.

• Global Warming Revisited

  • As an engine operates, heat flows from a heat tank of greater temperature to a heat sink of lesser temperature.
  • In between these states, the heat flow is turned into useful energy with the help of heat engines.
  • This brings up two important points: optimized heat sinks are at absolute zero, and the longer engines dump heat into an isolated system the less efficient engines will become.
  • Department of Energy, 70% to 72% of heat produced by burning fuel is heat lost by the engine.
  • The excess heat lost by the engine is then released into the heat sink, which in the case of many modern engines would be the Earth's atmosphere.

• Thermal Pollution

  • As we learned in our Atom on "Heat Engines", all heat engines require heat transfer, achieved by providing (and maintaining) temperature difference between engine's heat source and heat sink.
  • Water, with its high heat capacity, works extremely well as a coolant.
  • Some may assume that by cooling the heated water, we can possibly fix the issue of thermal pollution.
  • However, as we noted in our previous Atom on "Heat Pumps and Refrigerators", work required for the additional cooling leads to more heat exhaust into the environment.

• Heat Pumps and Refrigerators

  • A heat pump is a device that transfers heat energy from a heat source to a heat sink against a temperature gradient.
  • Actually, a heat pump can be used both to heat and cool a space.
  • Since the efficiency of a heat engine is Eff=W/Qh, we see that COPhp=1/Eff.
  • Since the efficiency of any heat engine is less than 1, it means that COPhp is always greater than 1—that is, a heat pump always has more heat transfer Qh than work put into it.
  • The efficiency of a perfect engine (or Carnot engine) is

• The First Law

  • Heat engines are a good example of this—heat transfer into them takes place so that they can do work.
  • It's pitched at undergraduate level and while it is mainly aimed at physics majors, it should be useful to anybody taking a first course in thermodynamics such as engineers etc..
  • Q represents the net heat transfer—it is the sum of all heat transfers into and out of the system.
  • Q is positive for net heat transfer into the system.
  • Explain how the net heat transferred and net work done in a system relate to the first law of thermodynamics

• Heat and Work

  • Heat transfer by convection occurs through a medium.
  • Lastly, heat can also be transferred by radiation; a hot object can convey heat to anything in its surroundings via electromagnetic radiation.
  • Heat and work are related.
  • Work can be completely converted into heat, but the reverse is not true: heat energy cannot be wholly transformed into work energy.
  • Scientists and engineers have been able to exploit the principles of thermochemistry to develop technologies ranging from hot/cold packs to gasoline powered combustion engines.

• Convection

  • Convection is the heat transfer by the macroscopic movement of a fluid, such as a car's engine kept cool by the water in the cooling system.
  • The specific heat of air is a weighted average of the specific heats of nitrogen and oxygen, which is c=cp≅1000 J/kg⋅C (note that the specific heat at constant pressure must be used for this process).
  • Instead heat diffusion in solids is called heat conduction, which we've just reviewed.
  • An example of convection is a car engine kept cool by the flow of water in the cooling system, with the water pump maintaining a flow of cool water to the pistons.
  • If the water vapor condenses in liquid droplets as clouds form, heat is released in the atmosphere (this heat release is latent heat)  .

• What is Entropy?

  • We can see how entropy is defined by recalling our discussion of the Carnot engine.
  • where Q is the heat transfer, which is positive for heat transfer into and negative for heat transfer out of, and T is the absolute temperature at which the reversible process takes place.
  • The definition of ΔS is strictly valid only for reversible processes, such as used in a Carnot engine.
  • Now let us take a look at the change in entropy of a Carnot engine and its heat reservoirs for one full cycle .
  • Also shown is a schematic of a Carnot engine operating between hot and cold reservoirs at temperatures Th and Tc.

• Heat as Energy Transfer

  • Heat is the spontaneous transfer of energy due to a temperature difference.
  • This observation leads to the following definition of heat: Heat is the spontaneous transfer of energy due to a temperature difference .
  • Heat is often confused with temperature.
  • Heat is a form of energy, whereas temperature is not.
  • In some engineering fields, the British Thermal Unit (BTU), equal to about 1.055 kilo-joules, is widely used.