Posted by : Sayantan Mandal Mar 21, 2010

Do you know that a BMW car engine or its aircraft engine is actually a heat engine. Before we get into various combustion engine lets get into the basics, the building blocks of an ENGINE. It all starts with the concepts of thermodynamics,

In Thermodynamics, Heat Engine is a device by which a system is made to undergo a cyclic process that results in conversion of of heat or thermal energy to mechanical work..

Lets think the way Carnot thought, which leaded to the the invention of the car engine:

Schematic representation of a heat engine
Lets the circle in the middle be an engine and the box at the right be an external HOT reservoir and at the left be an external COLD reservoir:
  1. It consists of a working substance - the system. For example, a mixture of fuel vapor and air in a gasoline or a diesel engine or steam in steam engine are the working substance.
  2. The working substance goes through a cycle consisting of several processes. In some of these processes, it absorbs a total amount of heat QH from an external reservoir at some external temperature TH.
  3. In some other processes of the cycle, the working substance releases a total amount of heat QC to an external reservoir at some lower temperature TC.
  4. The work done (W) by the system in a cycle is transferred to the environment via some arrangement(e.g. the working substance may be in a cylinder with a moving piston that transfers mechanical energy to the wheels of a vehicle via a shaft).

Actually the engine is taking heat energy from hot reservoir  releasing it to cold reservoir and delivers work W to the surrounding.
therefore work,
Total Work, dW = dQc - ( -dQh )

And its efficiency,
Efficiency, n=W/Qh

The cycle repeats itself again and again to get useful work......

For more info you can always refer to wikipedia, Here's the LINK: Heat Engine

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