Question Set 76 – Mechanical Engineering

# Question Set 76

1.What is the need of high compression ratio?

Today every machine is expected to give higher efficiency. When we talk about petrol engine and diesel engine whose thermal efficiency is directly related to ‘compression ratio’. That means if we want to achieve higher thermal performance in terms of efficiency, we should provide high compression ratio for both the engine.

### 2.Difference between ‘Thermodynamics’ and ‘Heat Transfer’

Thermodynamics and Heat Transfer both are part of thermal engineering and each has its own meaning and importance. Both deals with heat and energy then, one might ask, what is the real difference between the two subjects ? i.e. Thermodynamics and Heat Transfer

“Thermodynamics” deals with the amount of energy in form of heat or work during a process and only considers the end states in equilibrium. It will not give information about how long it will take to reach to the final state in equilibrium. While,

“Heat Transfer” deals with the rate of energy transfer thus, it gives idea of how long a heat transfer will occur? Heat transfer deals with time and non equilibrium phenomena. Heat can only transfer when there is a temperature gradient exists in a body and which is indication of non equilibrium phenomena.

In short, Thermodynamics gives “Why”  a process will occur and Heat Transfer will tell, “How” a process will occur when there is a concern about transfer of heat.

To support all these arguments, Thermodynamics uses following laws,

●        Zeroth Law of Thermodynamics

●        First law of Thermodynamics

●        Second Law of Thermodynamics

●        Third Law of Thermodynamics

While, Heat Transfer uses,

●        Fourier’s Law (Heat conduction)

●        Newtons Law of Cooling (Heat convection)

●        Stephan Boltzman’s Law (Thermal Radiation)

3.What is the need of high compression ratio?

Today every machine is expected to give higher efficiency. When we talk about petrol engine and diesel engine whose thermal efficiency is directly related to ‘compression ratio’. That means if we want to achieve higher thermal performance in terms of efficiency, we should provide high compression ratio for both the engine.

4.What is compression ratio?

The compression ratio is the ratio of total volume of engine cylinder to the clearance volume of the same cylinder.  As the name suggest, it relates compression of the air fuel mixture during compression stroke. When there is compression, there always be heat associated with and it increases the overall temperature of fuel air mixture.

5.Why compression ratio of petrol engine is kept low compared to diesel engine?

In petrol engine, petrol is used as fuel and when it is subjected to high compression ratio, the heat generated may increase the temperature of petrol+air mixture above the auto ignition temperature of petrol ( 280 °C) and the fuel mixture will automatically ignite without getting sparked.

This auto ignition produces very high pressure which may burst piston. This phenomenon is called ‘detonation’. So, to reduce the detonation effect and to allow the combustion in exactly after compression stroke, high compression ratio cannot be used in petrol engine. In case of otto engines, the compression ratio is of the order of 4 to 8.

6.What will be the change in room temperature if door of refrigerator kept open?

Ans: NO, Rather, temperature of the room will increase over the time, not instantly.

### 7.Difference between ‘Thermodynamics and Heat Transfer’

●        Thermodynamics mainly deals with the transfer of energy in form of heat or work betweensystems and surroundings with other systems in form of either heat or work in equilibriumconditions.  When there is change in one state, which was in equilibrium state and starting to change in other equilibrium state, this information helps to determine heat or work interactions due to this process. While

●        Heat transfer deals with non equilibrium heat exchange or transfer due to temperature gradient while in thermodynamics, during thermal equilibrium (No temperature gradient) no heat exchange is possible.  [Read more interview questions on Thermodynamics]. Heat never transfer when there is no temperature gradient.

8.Difference between Pipe and tube

●        There always a debate between differentiation of pipe and tube. Pipe is always designated by its Inside Diameter (ID) because main purpose of pipe is to carry fluid, while the

●        Tube is always designated by its Outside Diameter (OD). Pipe’s thickness is sometimes called as schedule and tube’s thickness is gauge.

●        As per the tolerance basis, tubes requires higher tolerances as to that of pipe and tubes are more costlier in manufacturing compared to pipe production.

9.Difference between Flywheel and Governor

The function of flywheel is to store the energy/power produced during the power stroke of an engine and this stored energy is used during remaining stroke to make piston up and down during working of engine while governor tries control the speed of an engine due to variation in load. It does this work by reducing or increasing the amount of fuel passing to the engine.

10.Difference between tempering and annealing

Annealing is a process in which workpiece is heated up to certain temperature to get desired changes in grain or micro-structure and then allowed to cool slowly in order to relieve the stresses during work treatment. While tempering is a process wherein hardness and elasticity or ductility can be regain by use of heating and cooling cycle till desired properties can be obtained.