Applications of Second Law of Thermodynamics

Categories : Thermodynamics
Tags : Mechanical engineering

Introduction

The Second Law of Thermodynamics is considered to be the most fundamental law of science. CP Snow in his Two Cultures wrote: “Not knowing the Second Law of Thermodynamics is like never having read a work of Shakespeare”. This law explains how you heat the water and make hot coffee, why the hot coffee in your mug gets cooled etc. But much more than this, the second law of thermodynamics was found to explain the working of steam engines. In fact in modern times the second law of thermodynamics is being used to even explain popular theories like big bang, expansion of the cosmos and how the time is fast tickling away towards the “heat death” of the universe.

In the simple words the second law of thermodynamics says, “The natural tendency of the heat is to flow from the high temperature reservoir to the low temperature reservoir.” This means that the heat flows from body at high temperature to the body at low temperature. The first of thermodynamics was meant to explain the working of the steam engine or heat engine.

Applications in Automobile Engines

The foundation of second law of thermodynamics was laid by the inventions made by Sadi Carnot, a young French scientist considered to be the father of thermodynamics.. Before the findings of Carnot it was considered that the efficiency of heat engine was dependent on the type of working fluid used in the engine. In his book “Reflections on the Motive Power of Fire” published in the year 1824, Carnot said that the efficiency of the heat engine was independent on the type of fluid used in the engine. As per him efficiency of the heat engine is dependent just on two temperatures: the temperature of the source (hot body) from where the engine absorbs heat and the temperature of the sink (atmosphere) where the engine gives up the exhaust heat.

It was a remarkable discovery, but people found it absurd that the efficiency of the engine was dependent only on the high and low temperatures of the source and sink respectively. Sadi Carnot’s findings were ignored and many of his papers went unpublished. Fortunately his book did get published. The future scientists like Kelvin studied Carnot’s findings in more details and made further advancements.

The concept of the second law of thermodynamics applied to heat engine is equally applicable on the internal combustion engines used in our cars, motorcycles, ships, airplanes, etc. In the internal combustion engines the heat is generated by combustion of fuel inside the engine. The combustion of fuel takes place due to generation of the spark (Spark Ignition or SI engines) as in case of the gasoline engines or due to compression of the fuel (Compression Ignition or CI engines) as in case of the diesel engines.

Some part of the heat generated inside the engine is used to perform the work moving piston inside the engine cylinder. The piston is connected to the crankshaft via connecting rod. The reciprocating motion of the piston is converted into the rotary motion of the crankshaft, which is converted to the rotary motion of the wheels via gear box. The remaining part of the heat generated inside the engine is released to the atmosphere as the exhaust gases or tailpipe emissions. In this case the engine where heat is generated is called as source, whereas the atmosphere where heat is released is called as sink. As per the second law of thermodynamics, higher the temperature of the source and lower the temperature of the sink, higher is the efficiency of the engine.

The second law of thermodynamics is applicable to all the engine cycles including Otto, Diesel etc no matter what type of they are and what type of fluid they are working on. It is discovery of this law that has lead to the progress of mankind to the present day vehicles. There is hardly any other law which as extensive applications as the second law of thermodynamics.

Next: Part-2 of this article on refrigerators