REFRIGERATION PROCESS

Refrigeration -: Refrigeration is artificial cooling process. The term refrigeration mean cooling a space , substance or system to lower or also maintain it's temperature below the ambient one. In other words , refrigeration is the process of moving heat from one location to other in the control conditions. In this energy in the form of heat is removed from a lower temperature reservoir and transferred to a high temperature reservoir. 

                       Refrigeration has many applications,  including,  but not limited to : industrial freezers,  household refrigerator , air conditioning and cryogenics .

                      A refrigeration system is an combination of mechanical equipment in which a refrigerant is circulated for cooling , or extracting heat from bodies or space. A refrigerant having its boiling temperature less than atmospheric temperature will start boiling immediately if subjected to atmospheric conditions, absorbing heat from its surroundings, thereby producing cooling effect . 

Refrigeration system could be of two types-:

1. Compression 

2. Absorption 

                 Compression system, also sometimes known as mechanical system is the more popular of the two .                                                      

In this figure the compression refrigeration system is shown. This cycle is followed by the compression refrigeration system.

* In this process first , the refrigerant gas is compressed by the compressor and this compressed refrigerant then discharged into the condenser.

* In condenser it is cooled by the atmospheric air or water which is being used for abstracting heat from condenser. Condenser now condensed to liquid. 

* Condensed , and warm high pressure refrigerant then travel through the strainer or drier which prevents plugging of the flow control device by tapping scale,  dirt and moisture. 

* Flow control device controls the flow of refrigerant and refrigerant is trottled.

*  Now in the evaporator , refrigerant is subjected to a much lower pressure due to the combined suction of the compressor and the pressure drop across the flow control device.

             As a result,  refrigerant tends to expand and evaporate thereby cooling the air passing over evaporator. Refrigeration takes place from the absorption of heat during vaporization in the cooling coils. The gas is drawn again into compressor and the cycle is repeated .

Pre-ignition , octane number and cetane number

Pre-ignition -: pre-ignition is caused by the spontaneous combustion of the mixture before the end of the compression stroke in spark ignition engine, i.e. , even before the spark occurs in the spark plug . It may occur generally due to the red hot carbon deposited , or the cylinder walls being too hot , or overheated spark plug , or the pressure wave due to detonation. It may also due to the faulty timings .

                 In S.I. engine the fuel air mixture is drawn into the chamber during the suction stroke and is compressed until the spark plug ignite it. When the engine is running on high loads , there might be a hot carbon deposite or overheated spark plug's tip that might lead to pre ignition. 

                    The low flash point temperature,  of the petrol also adds to this problem . This is completely eliminated by the intervention of fuel injection into petrol 

Octane number -: It is defined as the percentage by the volume of iso-octane in a mixture of iso-octane and n-heptane which show the same tendency to knock as the fuel in question under same conditions . Ignition quality of petrol is expressed by octane number. 

Cetane number-: Cetane number is defined as the percentage by the volume of cetane in a mixture of cetane and α-methyl-napthalane which has the same ignition delay as the fuel in question under same conditions. Ignition quality of diesel oil is expressed by cetane number. 

Scavenging and auto ignition

Scavenging -: In an I.C. engine , scavenging is the process of replacing the exhaust gases in the cylinder with the fresh air or fuel mixture for the next cycle. 

                   In other words,  scavenging is the process of removing the brunt gases from the engine cylinder.  Efficient scavenging is necessary for good combustion of fuel in the engine cylinder. The passage of scavenge air will also assist cooling of the cylinder,  pistpn and the valves. 

There are three types of scavenging process-:

1. Loop scavenging 

2. Uniflow scavenging 

3. Cross scavenging 

1.Loop scavenging -: In loop scavenging process , the incoming air passes over the piston crown then rises toward the cylinder head.  The exhaust gases are forced before the air passing down and out of exhaust ports located above the inlet port .

                      Fig -: loop scavenging 

2.Cross scavenging -: In this type of scavenging process,  the incoming air is directed upwards and pushing the exhaust gases before it from the engine cylinder. The exhaust gases then travel down and out of the exhaust ports. 

                   Fig -: cross scavenging 

3.  Uniflow scavenging -: In the uniflow scavenging process, the incoming air enters at the lower end of the cylinder and leave at the top of the cylinder. During this travel incoming air push the brunt gases outside of the cylinder.  The outlet at the top of the cylinder may be ports or a large valve. 

Auto ignition -: It is the phenomenon of fuel catching fire without external source of ignition energy . As the name suggests, the fuel particles auto-ignite. So when the spark is introduced ,the flame front propagates,  this flame boundary further increases the temperature of fuel molecules which is already at a high temperature. 

                  Auto ignition occurs the mixture of the gases or vapour ignites spontaneously with no external ignition source and after reaching a certain temperature , the Auto ignition temperature.  The auto ignition temperature is not an intrinsic property of gases or vapours but is the lowest temperature in the system whene the rate of heat evolved from the gases or vapour increases beyond the rate of heat loss to the surrounding , resulting in ignition .

                 Auto-ignition is accelerated by the normal heptane and resisted by iso-octane. 

SUPERCHARGING

Supercharging -: Supercharging is the process of increasing the mass of air or density of air fuel mixture to the engine cylinder by compressing intake air initially with a view to obtain high power. This is usually done with the help of a compressor or blower.

                          Supercharging is used in I.C. engine to increase the volumetric efficiency. It is most common used in petrol engine. During supercharging the pressure of inlet air is increased above the atmospheric pressure by the supercharger. It can push high volume of air than the volume of air intaked by naturally aspirated engine of same capacity. 

                      Naturally aspirated engine have volumetric efficiency below the 90% but the supercharged engine the volumetric efficiency is above the 100 %.

                           Due to availability of higher oxygen during the combustion of fuel which ensure the better combustion of fuel . The advantage of supercharging is better combustion of fuel into the camber of cylinder. Due to this total power produced by the engine is also increased. There are two units used for increasing volumetric efficiency of engine , supercharger and turbocharger. The main difference in supercharger and turbocharger is that , supercharger is commonly used in petrol engine and turbocharger is commonly used in diesel engine. Turbocharger get the power from the exhaust gases of the engine to run the compressor through the turbine. In petrol engine combustion pressure is low so the energy of exhaust gas is also low as compare to diesel engine. For this reason turbocharger is not used in the petrol engine. 

        Supercharging is used generally at high altitude,  where density of atmospheric air is low . 

Concept of supercharging -: 

  

DETONATION

Detonation -: Detonation is generally caused by fuel with low octane rating - is the tendency for the fuel to pre - ignite or auto ignite in a engine's combustion chamber. The auto-ignition propagates a high pressure wave through fuel air mixture, giving a voilent blow to the walls of cylinder. A fuel having higher self ignition temperature detonates less.

                         The effect of detonation may be anywhere from arbitrary to serve. prolonged and intense knocking can break the piston or the engine , through it is can also endure this slight issue for thousand miles. 

           Detonation is mostly caused by the use of low- grade engine fuel and it is resulting deterioration of your engine's parts. The shape, size and spark location and geometry of the design all help determine where these detonation are likely to occur. An over-heated spark plug tip can also cause pre-ignition 

Detonation in petrol engines can be reduced by the addition of a small amount of lead ethide or ethyl fluid to the fuel. This is the known as doping. 

                           The knocking tendency in a petrol engines can be prevented by ensuring that the charge away from the spark plug has low density , low temperature,  long ignition delay, and rich mixture. It is also decreased by reducing compression ratio, and increasing the engine speed with fixed octane rating fuel. However knocking in the petrol engines is increased by increase in cooling water temperature and supercharging .

                The knocking tendency in diesel engines can be prevented by reducing the delay period, increasing the compression ratio , inlet pressure og air and injection pressure. 

I.C. ENGINE MCQ 02

Q.1 which fuels are taken as a reference for the cetane rating ?

A. Cetane and n- heptane

B. Cetane and α- methyl naphthalene

C. Cetane and isocetane

D. Tetraethyl lead and cetane

Ans-: B

Q.2 In diesel engine, pistons are usually cooled by __ .

A. Water 

B. Fuel oil 

C. Lubricating oil 

D. Air 

Ans-: C

Q.3 Which device is used to reduce exhaust noise ?

A. Exhaust manifold 

B. Filter

C. Tail pipe 

D. Muffler

Ans-: D

Q.4 Cetane number is the measure of _____ .

A. Ignition quality 

B. Viscosity of fuel

C. Additions in fuel

D. Density of fuel

Ans -: A

Q.5 What is the freezing temperature of the petrol engine?

A. - 20 to - 10 ° C

B. 50 to - 30 ° C

C. - 10 to - 4 ° C

D. 40 to 100 ° C

Ans -: B

Q.6 Which test is done to determine the mechanical efficiency of multi-cylinder engines ?

A. Air flow test 

B. Fuel consumption test

C. Morse test

D. Power test 

Ans -: C

Q.7 The percentage of CO (carbon monoxide) in exhaust gases in the compression ignition engine is ?

A. 10 to 20 %

B. 5 to 10 %

C. Zero

D. 10 to 12 %

Ans -: C

Q.8 High speed diesel engines need a cetane number of ____ .

A. 100

B. 50 

C. 75

D. 25

Ans -: B

Q.9  If diesel is fed by mistake in the oil tank of petrol engine , then engine will __ .

A. Knock

B. Detonate

C. Not run

D. Give lot of smoke 

Ans -: C

Q.10 Which type of carburettor is preferred .

A. Vertical type 

B. Horizontal type

C. Eccentric type 

D. Concentric type 

Ans -: D

Q.11 The compression ratio of motor car is ____.

A. 7

B. 5

C. 10

D. 15

Ans-: A

Q.12 The knocking tendency in a S.I. engine can be reduced by increasing _______ .

A. Compression ratio of fuel

B. Speed

C. Lubricating oil quality 

D. None of these

Ans -: B

Q.13 What is HUCR ?

A. Highest useful compression ratio

B. Highest useful compression radiation 

C. A lubricating device 

D. None of these 

Ans -: A

Q.14 Most high speed compression engines oprate on _____ .

A. Otto cycle 

B. Dual cycle 

C. Carnot cycle 

D. Four stroke cycle 

Ans -: B

Q.15 Piston rings are usually made of which material ?

A. Carbon steel

B. Aluminium 

C. Cast iron 

D. High speed steel

Ans -: C

I.C. ENGINE MCQ :- 01

Q.1 Under which condition otto cycle is more efficient than diesel cycle?

A. At lower compression ratio

B. For same compression ratio

C. For same maximum pressure 

D. For same maximum temperature 

Ans-: B

Q.2 How many working cycle two stroke engine have ?

A. 1

B. 2

C. 1/2

D. 4

Ans -: A

Q.3 In diesel engine, the fuel is injected by ____.

A. Combustion chamber 

B. Spark plug

C. Injected fuel

D. Heat resulting from compressing air that is supplied for combustion 

Ans -: D

Q.4 At what pressure the fuel is normally injected in diesel engine ?

A. 20 - 50 kg/cm^2

B. 50 - 60 kg/cm^2

C. 90 - 130 kg/cm^2

D. 5 - 10 kg/cm^2

Ans -: C

Q.5 The inlet value of a four stroke cycle I.C. engine remain open for _____.

A. 180°

B. 235°

C. 275 °

D. 360°

Ans -: B

Q.6 which of the following is compressed in a diesel engine cylinder?

A. Air 

B. Fuel

C. Air + fuel

D. Air+fuel+lub oil 

Ans -: A

Q.7 For idling speed of a petrol engine,  air fuel ratio is ______ .

A. 10 : 1

B. 1 : 1

C. 5 : 1

D. 20 : 1

Ans -: A

Q.8 How many valves  diesel engine has ?

A. 1 valve

B. 3 valve

C. 2 valve 

D. No valve 

Ans -: B

Q. 9 For increasing power output _______ is usually done .

A. Detonation 

B. Knocking 

C. Scavenging 

D. Supercharging

Ans -: C

Q.10  Iso-octane has octane number of ___.

A. 100 

B. 200

C. 150

D. 250

Ans -: A

Q.11 The specific gravity of diesel oil is __.

A. 0.7

B. 0.5

C. 0.75

D. 0.85

Ans -: D

Q.12 The efficiency of I.C. engine is of the order of ___ % .

A. 25

B. 95

C. 85

D. 35

Ans -: D

Q.13 The magneto system of ignition is used in ____.

A. Motor cycles 

B. Aeroplanes 

C. Racing cars

D. High speed engines 

E. All of the above 

Ans -: E

Q.14  In which type of engine NOx emission is more ?

A. Petrol engine 

B. Diesel engine 

C. Steam engine 

D. Electric  engine 

Ans -: B

Q.15 The difference of research octane number and motor octane number is konwn as _____.

A. Fuel octane number 

B. Fuel density

C. Fuel sensitivity 

D. Cetane number 

Ans -: C