HEAT ENGINE: Converts Thermal Energy Into Mechanical Energy

Since, we know that thermodynamics is concerned with the conversion of heat into work and work into heat. That's why we need a machine that converts heat into work and work into heat. 
"A heat engine is a machine that converts heat into work whereas a heat pump or refrigerator is a machine that converts work into heat". 

In this article, we will learn about heat engine, its types, its working principle & efficiency of heat engine in detail. Here, we use both the concepts of 1st law & 2nd law of thermodynamics to understand the working operation of a heat engine. The first law of thermodynamics tells us about the conservation of energy while the second law of thermodynamics tells us about the direction of energy transfer.


Introduction To Heat Engine
!!!!Converts Thermal/chemical Energy Into Mechanical Energy!!!!!
You might have seen such engines. It works with the help of fuel. The fuel burns inside the engine and produces heat. This heat is used to do some useful work.


I’ll give you the short and simple definition of heat engine.
......"A heat engine is a Machine that operates between two thermal reservoirs in which one acts as a heat source while the other acts as a heat sink. Its function is to convert heat into work by absorbing heat from the heat source and transfer the remainder to the heat sink after producing work from heat".........
These reservoirs are capable of absorbing or rejecting an unlimited amount of heat without a change in its temperature. It is a work producing device.
.........."Another thing to remember is that heat cannot be completely converted into work. Thus, a heat engine does not have 100% efficiency"............
For Example
Steam power plant, Gasoline and Diesel engines, Carnot engine, etc.

!!!! A "Carnot Engine" Is A Special Type Of Heat Engine Whose Efficiency Is The Maximum Possible Efficiency Relative To The Efficiency Of Any Other Heat Engine!!!!!

Take Example Of Your Bike- 
The engine produces heat from combustion of fuel. This heat creates high temperature and pressure inside the cylinder which pushes the piston outwards producing required work. Whole cylinder and exhaust gases dissipate heat in the sink(Atmosphere).


Types Of Heat Engine
There are mainly two types of heat engine based on the combustion of fuel in and out side of the engine chamber and these are;
(1) External Combustion Engine 
(In which the fuel is burnt outside the engine chamber where the force and motion are produced. A steam engine where the coal is burning outside and some distance from the cylinder and piston, is a good example of External Combustion Engine) 
(2) Internal Combustion Engine 
(In which the fuel is burnt inside the engine chamber. For example: A typical car engine)
☛ Internal combustion engines are generally far more efficient than external combustion engines because no energy is wasted transmitting heat from a fire and boiler to the cylinder; everything happens in one place.


Heat Engine Working
Any heat engine is comprised of mainly three parts namely;
  1. A Heat Source
  2. A Working Substance
  3. A Heat Sink
Heat Engine is a device which converts low grade energy (Heat) into high grade energy (work) and delivers some amount of heat into sink(Atmosphere). We will now examine how a heat engine works;
  1. Working fluid absorbs heat from high temperature reservoir.
  2. It converts a part of this absorbed heat into work such as moving a piston, lifting weights, etc.
  3. Finally, the remaining absorbed heat is released into the lower-temperature reservoir.
  4. Returns to its initial state.
It is generally assumed that the heat engine operates in a cycle so that after each cycle, the engine returns to its original state without affecting its internal energy (∆U = 0). That is why any loss in heat is only due to the conversion of heat into work and not any other way.
            
Since, we know that heat cannot be completely converted into work. Therefore, that part of the heat which cannot be converted into work, we discharge it into the heat sink. The working of heat engine can be understood by diagram;
Here,
        Q₁ = Heat transferred from the hot body
        Q₂ = Heat received by the cold body.

This diagram shows that heat engine takes heat 'Q₁' from high temperature reservoir and discharges 'Q₂' amount of heat to a low temperature reservoir and the lost amount of heat 'Q₁-Q₂' is converted into work, 'W'. 

Since, there is no change in internal energy (∆U=0). Therefore, according to the first law of thermodynamics (energy is conserved and only transferred from one form to other);
                 ➩ [ W ＝ (Q₁-Q₂) ]
While the second law of thermodynamics tells the direction of heat flow from higher temperature to a lower temperature reservoir. A Heat engine is the best example to understand the Kelvin-Planck Statement of second law clearly.


Heat Engine Efficiency 
The efficiency of an engine (real or theoretical) depends on the maximum and minimum temperatures between which it operates. It is an important factor that describes how much heat is converted into work. 
"Heat engine efficiency is defined as the ratio of work output to the heat input or heat supplied". It is denoted by a symbol, 'η'. This efficiency of a heat engine can never be 100%.
Since, 
     From the above figure work output is nothing but the difference between the heat absorbed from the high temperature reservoir (Q₁) and heat rejected to low temperature reservoir (Q₂). Whereas heat input or heat supplied is 'Q₁'.
             ➩ Work output, W ＝ (Q₁ - Q₂)
Thus;
If Q₂ becomes zero then the efficiency of the heat engine will be 100% but in reality this is not possible.
       
Making the temperature of the fluid inside the cylinder higher at the start of the cycle makes it more efficient; making the temperature lower at the end of the cycle also makes it more efficient. In other words, a really efficient heat engine operates between the greatest possible temperature difference. That's why real engines—in cars, trucks, jet planes, and space rockets—work at such enormously high temperatures. 

!!!!The Performance Of A Heat Engine Is Expressed In Terms Of Energy Conversion Efficiency Whereas The Performance Of A Heat Pump Or Refrigerator Is Expressed As A Coefficient Of Performance Rather Than Thermal Efficiency!!!!
 
  
Example: 
(based on Kelvin Planck’s statement)
A heat engine absorbs 360 J of energy and performs 25 J of work in each cycle. Find the following;
(a) the efficiency of the engine
(b) the energy expelled to the cold reservoir in each cycle.
Solution    Here, 
            Q₁ = 360 J, W = 25 J
(a) Efficiency of heat engine
                η = W/Q₁
                   = 25 / 360
                   = 0.07 or 7%
(b) The heat energy expelled to the cold reservoir in each cycle
                Q₂ = Q₁ – W
                      = 360 – 25
                      = 335 J



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