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Heat Engines

High Speed Cars High speed cars Racing cars are made with an engine whose power and acceleration is much more than an ordinary car. Race engine components are built to be light and strong, but in most cases, they don‘t have to last much longer than the end of one race. The heat engine of the car has to be cooled from time to time, else it may end up with a blown engine. What type of heat engine is used in these cars? Are these cars efficient enough? Lets discuss it now.

Learning objectives

After completing the topic, the student will be able to:

  • Discuss the expansion and compression of both adiabatic and isothermal cases.
  • Discuss, explore and distinguish the working of a Carnot heat engine from that of a normal heat engine.
  • Explore, identify and compare different types of combustion heat engines based on their working and finally relate them to the everyday science.
  • Discuss, explore and inspect the working of a thermal power plant.
  • Explore and inspect the working of the heat pump by investigating its relevance to everyday science.
  • Explore and investigate the working of an air conditioner.
Vehicles to Convert Heat into Mechanical Work Conversion of heat into mechanical work All motorized vehicles other than purely electric vehicles use heat engines for propulsion.
Heat engines

A heat engine is a device that changes heat energy into mechanical work. It works on the first law of thermodynamics. The basic idea behind a heat engine, whether a steam engine, jet engine or internal combustion engine, is that mechanical work can be obtained only when heat flows from a high temperature to a low temperature. In every heat engine only some of the heat can be transformed into work.

At the heart of every engine is a working substance. In a steam engine the working substance is water, in both its vapor and liquid forms. In an automobile engine the working substance is gasoline–air mixture. If an engine is to do work on a sustained basis, the working substance must operate in a cycle, that is the working substance must pass through a closed series of thermodynamic processes, called strokes, returning again and again to each state in its cycle.

In considering heat engines, we talk about reservoirs. Heat flows out of a high-temperature reservoir and into a low–temperature one.

Heat Engine Heat engine Energy QH is transferred as heat from the high temperature reservoir at temperature TH to the working substance. Energy QL is transferred as heat from the working substance to the low temperature sink at temperature TL. 'W' is the Work done by the engine. (If work is put into a heat engine, the flow of heat may be from the low–temperature sink to the high–temperature reservoir, as in a refrigerator or air conditioner).
Ideal engine

A heat engine:

  • Gains heat from a reservoir of higher temperature, increasing the engine's internal energy
  • Converts some of this energy into mechanical work
  • Expels the remaining energy as heat to some lower–temperature reservoir, usually called a sink.

We can study real engines by analysing the behavior of an ideal engine. In an ideal engine all processes are reversible and no wasteful energy transfers occur due to, say, friction and turbulence. Before scientists understood the second law of thermodynamics, many people thought that a very low friction heat engine could convert nearly all the input heat energy to useful work, but not so.

We shall focus on a particular ideal engine called a Carnot engine after the French scientist and engineer N.L.Sadi Carnot, who first proposed the engine's concept in 1824. This ideal engine turns out to be the best (in principle) at using energy as heat to do useful work. He showed that the greatest fraction of energy input that can be converted to useful work, even under ideal conditions, depends on the temperature difference between the hot reservoir and the cold sink. His equation is

where Thot is the temperature of the hot reservoir and Tcold the temperature of the cold sink. Ideal efficiency depends only on the temperature difference between input and exhaust. Whenever ratios of temperatures are involved, the absolute temperature scale must be used.

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