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

IGNITION SYSTEM IN PETROL ENGINE

In a petrol-engine , electric spark produced between the points of a spark plug due to this spark the combustible mixture is ignited. 

There are two systems of electric ignition:-

1. Battery ignition ( or coil ignition )

2. Magneto ignition 

1.Battery or coil ignition system -:

It relates to four cylinder petrol engine it has two circuits, the primary circuit and the secondary circuit. The function of ignition coil is to set-up 6 or 2 volts from the battery to the high tension voltage of about 24,000 volts . Closing the switch,  the current from the battery passes through the ammeter , the primary of the induction coil and contact breaker point , the contact pint being closed. The secondary of the induction coil is connected to the distributor arm. The primary winding has a small number of turns of coarse-wire ( between 100 to 200 )whereas a secondary winding has a large number of turns of very fine wire ( of the order of 10,000).

                       When spark is required in a cylinder,  the contact breaker points are opened by the cam and this induces high voltage( the order of 2400 volts ) in secondary. At the same time the distributor arm comes opposite to the point from from where the wire leads to the saprk plug of these cylinder. The high voltage induced in secondary give spark at the spark-plug .

        

         Fig. Battery ignition system 

2. Magnet Ignition System-: 

 In magnet type ignition system , is not required a battery for voltage . It generates its own voltage for the primary. Here an armature having wound upon it in the primary coil of a few hundred turn of enamelled copper wire and secondary coil of several thousand turns of fine insulated wire, rotates in a permanent magnetic field. 

                  In the mordern trend , the armature is to keep stationary and magnets is to keep rotating. By this action,  the primary voltage is generated in accordance with the law of electric generator , which is then transformed into a very high secondary voltage exactly on the lines described in the battery ignition system. Generally magneto system is used in high speed engines like scooter,  racing cars aeroplanes etc .

Valve timing Diagrams

Valve timing Diagram -: The opening and closing of intake and exhaust valves of the engine is graphically represented by a diagram , is known as " valve timing Diagram ". The opening and closing of the valves of the engine depend upon the movement of the piston from TDC to BDC , this relation between piston and valves is controlled by setting a graphical representation between these two , which is known as valve timing Diagram .

                  Theoretically the valve should open and close at the end or start of the stroke. But in actual practice it is not desirable to do so in order to obtain better results. The valves are generally opened earlier and closed later . 

                     Valve timing Diagram comprises of a 360 degrees figure which represent the movement of the piston from Top dead centre to bottom dead centre . In all the stroke of the engine cycle , which is measured in degrees and the opening and closing of the valves is controlled according to these degrees. 

Starting from the suction stroke , the intake valve is opened about 10 ° to 30 ° begore the TDC . Exhaust valve is also open at this movement . By opening the intake valve earlier,  more chances to fresh charge coming in are made . As fresh chagre move in , it helps in expelling the exhaust gases also from the chamber. 

                                                                                   Fig. (a) Inlet valve open, inlet valve close       

  

The inlet valve does not close at B.D.C. , but is closed at after it . It generally closes 30° to 40 ° after B.D.C. by doing so , a large amount of fresh chagre is ensured,  as the charge during suction stroke was get going in and compression by piston has not yet been able to push out the charge. 

During compression , the spark is ignited before the T.D.C. in case of otto . This is about 10 ° before T.D.C. or tge fuel is injected about  5 to 10 ° before T.D.C. . This is done so , because the whole of the fuel can't burn instantaneously.  Fuel also has some delay  period , ( i.e. period between spark and actual ignition ). Also time is required for the spark to travel and spread throughout the cylinder. 

        

        In expansion stroke , the exhaust valve opens about 30 to 40 ° before B.D.C. and closes about 10 ° after T.D.C. . Thus the period of exhaust is increased and complete  and effective scavenging is ensured. If this is not done , then residual  combustible mixture results in the dilution of fresh chagre. 

                                                                                               Fig . ( c ) expansion stroke