Heat is a form of energy that can be easily transferred from one body to another. Heat Engine is a kind of heat exchanger that converts heat into thermal and chemical energy. This energy is used to perform mechanical work. It can be done by bringing a working object from a higher state temperature to a lower state temperature. A heat source produced thermal energy that brings the working object to a high-temperature state. During the process, some of the thermal energy is converted into work by utilizing the properties of the working object.
Heat energy is composed of three parts:
- Working object
- Source of heat at high temperature
- Sink of heat at lower temperature
The efficiency of a heat engine depends on the ratio of the work obtained to the heat energy in the high temperature i.e. e = W/Q high. The maximum possible efficiency e max of an engine is
emax =Wmax/Qhigh = (1 – Tlow / Thigh) = (Thigh – Tlow)/Thigh
Example – If a gasoline engine has an efficiency of 30% then what would be the hot reservoir temperature for a Carnot engine having that Engine efficiency, If it runs with a cold reservoir temperature of 200 ºC ?
Solution – Maximum efficiency of a Carnot engine e max = (Thigh – Tlow)/Thigh
If emax = 0.3, then 0.3 = 1 – (473 K)/Thigh
Thigh = 473/0.7 = 675.7 K = 402.7 oC
Heat Engine can be classified into two main types
- Internal Combustion Engines – This process includes the combustion of a fuel that takes place within the system. These types of the engine take place where the fuel is burnt in the engine or where the fossil fuel combustion occurs. Pistons are mostly used in the internal combustion type of heat engines. These pistons move up and down within the cylinders that are present in the heat engines. When a single motion of a piston move in the upward or downward direction inside the cylinder is known as stroke. For Example – Mostly Cars have four-stroke internal combustion heat engines that consist of an Intake stroke, power stroke, combustion stroke and Exhaust stroke.
- External combustion Engines – These type of heat engines takes where the fuel is burnt outside the engine or where the fuel combustion occurs outside the engine. It is a heat engine where a working fluid is included internally and heated by combustion in an external source through the engine wall. This fluid then produced motion and usable work by expanding and acting on the mechanism of the engine.