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ME2041-Advanced IC Engines

Unit-1 Spark Ignition Engines

1. What are different air-fuel mixtures on which an engine can be operated?Ans: Chemically correct mixture, Rich mixture and Lean mixture.

2. What is stoichiometric mixture?Ans: Stoichiometric mixture is one in which there is just enough air for complete combustion

of the fuel. Chemically correct air fuel ratio for octane fuel is 15:1.

3. Why a rich mixture is required for idling?Ans: During idling, the air supply is restricted by the nearly closed throttle and the suction

pressure is very low. This condition of low pressure gives rise to backflow of exhaust gases

and air leakage from the various parts of the engine intake system.

The problem of dilution by residual gases becomes more pronounced at low loads and idling

because the exhaust temperature reduces with decreasing load .

The above phenomena require that the air-fuel ratios used for idling should be rich for

smooth engine operation.

4. What is a Transient operation.?Ans: Transient operation includes starting, warming up and changing from one speed or load

to another, specially for automotive vehicle engines during acceleration and decelerations,

and also idling.

5. Define combustion. What are the general conditions necessary for combustion?Ans: Combustion may be defined as a relatively rapid chemical combination of hydrogen and

carbon in the fuel with the oxygen in the air resulting in liberation of energy in the form of

heat. The conditions necessary for combustion are:

(i) The presence of a combustible mixture(ii) Some means of initiation combustion, and(iii) Stabilization and propagation of flame in the combustion chamber.

6. What are all the stages in SI engine combustion?Ans: Ignition lag, Propagation of flame and After burning.

7. What is ignition lag?Ans: The growth and development of a semi propagating nucleus of flame called ignition lag

or preparation phase.

8. What are the effects of engine variables on ignition lag?Ans: Fuel, Mixture ratio, Initial temperature and pressure, electrode gap, Turbulence.

9. Why is flame speed important? What is the most important single factor affecting flamespeed?

Ans: Flame speed influences the rate of pressure rise in the cylinder, and has bearing or

certain types of abnormal combustion. The most important factor affecting flame speed is

fuel-air ratio and turbulence.

10.What is normal and abnormal combustion?Ans: Normal combustion: In normal combustion, the flame initiated by the spark travels

across the combustion chamber in a fairly uniform manner.

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Abnormal combustion: Under certain operating conditions the combustion deviates from its

normal course leading to loss of performance and possible damage to the engine. This type

of combustion may be termed as an abnormal combustion.

11.List the important abnormal combustions.Ans: Detonation or Knock, Preignition and run-on.

12.What is detonation or knocking?Ans: A very sudden rise of pressure during combustion accompanied by metallic hammer

like sound is called detonation.

13.What are the effects of detonation in SI engine?Ans: Noise and roughness, Mechanical damage, carbon deposits, Increase in heat transfer,

decrease in power output and efficiency and pre-ignition.

14.What are all the various engine variables affecting detonation or knock?Ans: (i) Temperature factors

Raising the compression ratio, super charging, raising the inlet temperature, raisingthe coolant temperature, increasing the load and Advancing the spark timing.

(ii) Density factors

Compression ratio, Mass of inducted charge, Inlet temperature of the mixture,

Temperature of the combustion chamber walls, retarding the spark timing and power

output of the engine.

(iii)Time factors

Turbulence, Engine speed, Flame travel distance, Engine size, Combustion chamber

shape and location of spark plug.

(iv) Composition factorsFuel-Air ratio, Octane value of the fuel.

15.How knock or detonation controlled in SI engine?Ans: Increasing engine rpm, Retarding spark, reducing pressure in inlet manifold by

throttling, Making the ratio too lean or too rich and water injection.

16.How knock or detonation detected in SI engine?Ans: 1. Detonation may be heard if the engine noise is too high.

2. The temperature of the spark plug gasket can be measured by a thermocouple. High

temperature would indicate detonation.

3. The vibrations set up by detonation can be measured by a knock meter having a vibration

sensitive electric pick-up.

4. Presence of intermittent puffs of gray smoke in exhaust indicates detonation.

17. What is pre-ignition?

Ans: The increase in the rate of heat transfer to the walls cause local overheating, especially

of the spark plug, which may reach a temperature high enough to ignite the charge before the

passage of spark, thus causing pre-ignition.

18. What are the basic requirements of a good combustion chamber?

Ans: 1. High power output with minimum octane requirement.

2. High thermal efficiency

3. Smooth engine operation.

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19. What were the disadvantages of the following old combustion chambers: (a) T-head design (b)

Side valve or L-head engine?

Ans: Disadvantages of T-head:-having two cam shafts

-being very prone to detonation, the distance across the combustion was long.

There Was violent detonation even at a compression ratio of 4.

Disadvantages of Side valve or L-head:

-lack of turbulence as the air had to take two right angle turns to enter the cylinder

and in doing so lost much of its initial velocity.

- Extremely prone to detonation due to large flame length and slow combustion

process due to lack of turbulence.

-extremely sensitive to ignition timing due to slow combustion process.20. What objectives did Ricardo achieve in his famous turbulent chamber for L -head engines.

Ans: The main objectives of Ricardo turbulent head design were to obtain fast flame speed

and reduced knock.

21. What are the advantages of overhead valve combustion chamber over side valve combustion

chamber?

o Ans: Lower pumping losses and higher volumetric efficiency from better breathing ofthe engine from larger valves or valve lifts and more direct passageways.

o Less distance for the flame to travel and therefore greater freedom from knock, or inother words, lower octane requirements.

o Lower surface volume ratio and, therefore, less heat and less air pollution.o

Easier to cast and hence lower casting cost.21. What are F-head combustion chambers?

Ans: Combustion chamber in which one valve is in head and other in block are known as F-

head combustion chamber.

22. What type of combustion chambers are suited to very high output engines: racing cars and

super charged engines.

Ans: F-head combustion chamber, Bath tube combustion chamber and Rover combustion

chamber.

Unit-2 Compression Ignition Engines

1. What is the basic difference in the combustion process of SI and CI engines?Ans: In the SI engine a homogeneous carburetted mixture of petrol vapour and air, in nearly

stoichiometric or chemically correct ratio, is compressed in the compression stroke through

a small compression ratio (6:1 to 11:1) and the mixture is ignited at one place before the end

of the compression stroke by means of an electric spark.

In the CI engine, air alone is compressed through a large compression ratio (12:1 to 22:1)

during the compression stroke raising highly its temperature and pressure.

2. What are the stages in CI engine combustion?Ans: First stage: Ignition delay period

Second period: Rapid or uncontrolled combustion

Third stage: Controlled combustion

Fourth stage: After burning

3. What is meant by Delay period or Ignition lag?

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Ans: The first stage of combustion in the CI engine is the delay period. During delay period

some fuel has been admitted but has not yet been ignited. The delay period is a sort of

preparatory phase.

4. What is meant by physical delay and chemical delay in delay period of CI enginecombustion?

Ans: Physical Delay: It is the time between the beginning of injection and the attainment ofchemical reaction conditions. In the physical delay period the fuel is atomized, vaporized,

mixed with air, and raised in temperature.

Chemical Delay: In chemical delay preflame reactions start slowly and then accelerate until

local inflammation or ignition takes place.

5. What is the importance of delay period?Ans: The delay period in CI combustion affects rate of pressure rise and hence, knocking. It

also affects engines startability. Therefore the diesel engine designer aims to keep the delay

period as short as possible, both for smooth running and to maintain control over the

pressure changes. But some delay period is necessary otherwise the droplets would not be

dispersed in the air for complete combustion.

6.

What are the variables affecting the delay period?Ans; The various variables affecting the delay period are:

Fuel, Injection pressure or size of droplet, Injection advance angle, Compression ratio, Intake

temperature, Jacket water temperature, Fuel temperature, Intake pressure or super

charging, speed, Air-fuel ratio, Engine size and Type of combustion chamber.

7. Explain the phenomenon of diesel knock.Ans: If the delay period is long a large amount of fuel will be injected and accumulated in the

chamber. The auto-ignition of this large amount of fuel may cause high rate of pressure rise

and high maximum pressure which may cause knocking in diesel engines.

8. Compare knocking in SI engine and CI engine. Ans: In the SI engine, the detonation occurs near the end of combustion whereas in

the CI engine detonation occurs near the beginning of combustion. The detonation in the SI engine is of a homogeneous charge causing very high rate

of pressure rise and very high maximum pressure. In the CI engine the fuel and air

are imperfectly mixed and hence the rate of pressure rise is normally lower than

that in the detonating part of the charge in the SI engine.

Since in the CI engine the fuel is injected into the cylinder only at the end of thecompression stroke there is no question of pre-ignition or premature ignition as in

the SI engine.

9. Write the methods of controlling Diesel knock.Ans: Reducing delay period, Reducing delay angle.

10.What are the three methods of generating swirl in CI engine combustion chambers? Ans: By directing the flow of the air during its entry to the cylinder, known asinduction swirl. This method is used in open combustion chambers. By forcing the air through a tangential passage into a separate swirl chamber during

the compression stroke, known as compression swirl. This method is used in swirl

chambers.

By use of the initial pressure rise due to partial combustion to create swirlturbulence, known as induced swirl. This method is used in pre-combustion

chambers and air-cell chambers.

11.What is swirl in CI engine?Ans: An organised air movement, called air swirl, is provided to produce high relative

velocity between the fuel droplets and air.

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12.How CI engine combustion chambers are classified? What type of swirl is used in thesechambers?

Ans: (a) Open Combustion Chamber (induction swirl)

(b) Divided Chamber or Turbulent chamber(i) Swirl Chamber (compression swirl)

(ii) Pre-combustion Chamber (combustion induced swirl)

(iii) Air-cell chamber or lanova (combustion induced swirl)

13.How induction swirl can be obtained in four stroke engine and two stroke engines?Ans: In four stroke engines induction swirl can be obtained by two methods:

(i) By careful formation of the air intake passages, and(ii) By making or shrouding a portion of the circumference of the inlet valve.

The angle of mask is from 90 to 140 of circumference.

In two stroke engine the induction swirl is created by suitable inlet port forms.

14.What is M combustion chamber?Ans: It is a special type of open combustion chamber having combustion chamber in thepiston cavity.

15.What are the important points that should be borne in mind when considering a newcombustion chamber design?

Ans: High thermal efficiency Fuel requirement Ease of starting Capacity of variable speed operation Low exhaust emission. High volumetric efficiency.

16.What are turbochargers?Ans: Turbo chargers are centrifugal compressors driven by the exhaust gas turbines.

17.What are methods of turbo charging? Ans: Constant Pressure turbo charging. Pulse (Buchi)turbo charging Pulse convertor

18.Write the Limitations of turbo charging? Ans: The use of turbo charging requires special exhaust manifolds. Fuel injection has to be modified to inject more fuel per unit time. The efficiency of the turbine blades is very sensitive to gas velocity so that is very

difficult to obtain good efficiency over a wide range of operations.

Unit-3 Engine Exhaust Emission Control

1. What are the main sources of pollutants from petrol engine?o Ans: Evaporative loss (both tank and carburettor)o Crankcase blowbyo Tail pipe exhaust

2. What are the main constituents of exhaust emission from petrol engines?o Ans: Carbon monoxide (CO)o Hydrocarbons (HC)o

Particulate mattero Oxides of Nitrogen (NOx)

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3. What is the cause of formation of NOx in petrol engine exhaust?Ans: High temperatures and availability of oxygen are the two main reasons for the

formation of NOx.

4. What are the problems created by exhaust emissions?o Ans: Global warmingo Acid raino Smogo Odourso Respiratory and other health hazards.

5. What causes the engine emissions?Ans: Non-stoichiometric combustion, Dissociation of nitrogen and impurities in the fuel and

air.

6. What are particulates?Ans: The exhaust of CI engines contains solid carbon soot particles that are generated in the

fuel-rich zones within the cylinder during combustion. These are seen as exhaust smoke andcause an undesirable odorous pollution.

7. What are the other emissions from engines?Ans: Aldehydes, Sulphur, Lead and phosphorous.

8. What are the emission control methods?Ans: Thermal converters, Catalytic converters, Particulate traps and reducing emissions by

chemical methods (Ammonia injection systems, Exhaust gas recirculation)

9. What are catalytic converters?Ans: Catalytic converters are chambers mounted in the flow system through which the

exhaust gases pass through. These chambers contain catalytic material, which promotes the

oxidation of the emissions contained in the exhaust flow.

10.What is smoke and classify the measurement of smoke?Ans: Smoke is due to incomplete combustion. Smoke in diesel engines can be divided into

three categories viz., blue, white and black.

Measurement of smoke:

Visible method Comparison method Obscuration method

Continuous Filtering type

Spot Filtering type

11.Write the internationally accepted methods of measuring the following invisible emission.(i) Oxides of nitrogen (ii) Carbon monoxide (iii) Unburned hydrocarbons (iv)

Aldehydes.Ans: Oxides of nitrogen-Chemiluminescence analyser

Carbon monoxide-Non Dispersive Infra-Red analyser (NDIR)

Unburned hydrocarbons Flame Ionization Detector (FID)

Aldehydes Iodine titration technique, Chromotropic acid method, Dinitrophenyl hydrazine

(DNPH) method.

Unit-4 Alternate Fuels

1. What are the reasons for looking for alternative fuels for IC engines?o Ans: Crude oil and petroleum products will become very scarce and costly to find

and produce.

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o Increases in number of automobiles alone dictate that there will be a great demandfor fuel in the near future.

o Emission problems of gasoline engines.2. What are the merits and demerits of alcohols as alternative fuels for IC engines?

Ans: Merits:

It can be obtained from a number of sources, both natural and manufactured. It is a high octane fuel with anti-knock index numbers of over 100. Alcohols have higher flame speed. It produces less overall emissions when compared with gasoline. More power in the expansion stroke. It has high latent heat of vaporization which results in a cooler intake process. This

raises the volumetric efficiency of the engine.

Alcohols have low sulphur content in the fuel.Demerits:

o The calorific value of alcohol fuels is very low. This means that almost twice as muchalcohol as gasoline must be burned to give the same energy input to the engine.

o Combustion of alcohols produces more aldehydes in the exhaust.o Alcohol is much more corrosive than gasoline on copper, brass, aluminium, rubber

and many plastics.

o Alcohols have poor ignition characteristics in general.o Alcohols have almost invisible flames, which are considered dangerous when

handling fuel.

o There is a possibility of vapour lock in the fuel delivery system.o Headache and dizziness have been experienced when refuelling an automobile.

3. What are the techniques can alcohol be used for CI engines?o Ans: Alcohol/diesel fuel solutionso Alcohol diesel emulsionso Alcohol fumigationo Dual fuel injectiono Surface ignition of alcoholso Spark ignition of alcoholso Alcohols containing ignition improving additives.

4. What are the methods can hydrogen as fuel used in SI engines?o Ans: By manifold inductiono By direct introduction of hydrogen into the cylindero By supplementing gasoline.

5. What are the methods can hydrogen as fuel used in CI engines?o Ans: By introducing hydrogen with air and using a spray of diesel oil to ignite the

mixture that is by the dual fuel mode.o By introducing hydrogen directly into the cylinder at the end of compression.

6. What are the advantages and disadvantages of hydrogen as an IC engine fuel?Ans: Advantages:

Low emissions. Essentially no CO or HC in the exhaust as there is no carbon in thefuel.

Fuel availability. There are number of different ways of making hydrogen, includingelectroltsis of water.

Fuel leakage to environment is not a pollutant. High energy content per volume when stored as a liquid.

Disadvantages:

o Requirement of heavy, bulky fuel storage both in vehicle and at the service station.o Difficult to refuel.

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o Poor engine volumetric efficiency.o Fuel cost would be high at present day technology and availability.o High NOx emissions because of high flame temperature.o Can detonate.

7. What is natural gas?Ans: Natural gas is a mixture of components, consisting mainly of methane (60-95%) withsmall amounts of other hydrocarbon fuel components.

8. What are the advantages and disadvantages of using natural gas as alternate fuels?Ans: Advantages:

Octane number is around 120, which makes it a very good SI engine fuel. One reasonfor this high octane number is a fast flame speed.

Low engine emissions. Less aldehyde than with methanol. Fuel is fairly abundant world wide. It can be made from coal but the process of

making is very costly.

Disadvantages:

Low energy density resulting in low engine performance. Low engine volumetric efficiency because it is a gaseous fuel. Need for large pressurized fuel storage tank. Inconsistent fuel properties. Refuelling is a slow process.

9. What are the advantages and disadvantages of using LPG in SI engines?Ans: Advantages:

LPG contains less carbon than petrol. LPG powered vehicle produces 50% lesscarbon monoxide per kilometre.

LPG mixes with air at all temperatures. There is no crankcase dilution, because the fuel is in the form of vapour. LPG has high antiknock characteristics. Running on LPG translates into a cost saving of about 50%.

Disadvantages:

The ignition temperature of LPG is somewhat higher than petrol. Therefore runningon LPG could lead to a five percent reduction in valve life.

The vehicle weight is increased due to the use of heavy pressure cylinders for storingLPG.

A special fuel feed system is required for liquid petroleum gas.10.Compare LPG and petrol as fuel for SI engines.

Petrol LPG

Fuel consumption in petrol engine is less

when compared to LPG.

Compared to petrol, running the engine on

LPG results in around a 10% increase in

consumption.

Petrol has odour LPG is odourless

Octane rating of petrol is 81 Octane rating of LPG is 110

11.What all other possible fuels for engine?Ans: Bio gas, Producer gas, Blast furnace gas, Benzol, Acetone and diethyl ether.

Unit-5 Recent Trends

1. What is stratified charge engine?Ans: It is an engine which is mid-way between the homogeneous charge spark-ignition

engine and the heterogeneous charge compression ignition engine.2. What is meant by stratification?

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Ans: Stratification means providing different fuel-air mixture strengths at various places in

the combustion chamber a relatively rich mixture at and in the vicinity of the spark plug

and a leaner mixture in the rest of the combustion chamber.

3. What are the methods of charge stratification?Ans: Stratification by fuel injection and positive ignition.

Stratification using carburetion alone.4. What are the applications of stratified charge engine?

Ans: It is compact, lightweight design, good fuel economy, speed, range, antilock resistance,

multifuel capability and smooth and quiet engine operation.

5. Write the advantages and disadvantages of stratified charge engine.Ans: Advantages

It can tolerate a wide quality of fuels. It has low exhaust emission levels. It can be manufactured by the existing technology.

Disadvantages:

Charge stratification results in reduced power for a given engine size. It has a higher weight than that of a conventional engine. Its manufacture is more complex and hence, its manufacturing cost is higher.

6. What is 4-valve engine?Ans: An engine has valves that let the air-fuel mixture into the combustion chamber to be

burned and then draw out the exhaust gas after the combustion. A conventional engine has

one intake valve to let in the air-fuel mixture and one exhaust valve to let out the exhaust

gases.

But the 4-valve engine has two intake and two exhaust valves. 4-valves give an engine

steadier low speed performance and a better acceleration feeling. So most race engines and

high performance engines have four valves. (e.g.) Yamahas YZR-M! Motor GP race machine

has four valves.

7. What is over head cam engine?Ans: OHC in general means Over Head Cam engine. In that valves are operated almostdirectly by the camshaft, which makes it easy to achieve the perfect timing at high rpm.

8. What is the disadvantage of OHC engine?Ans: The disadvantage is that an OHC engine requires a timing belt or chain with released

components, which is more complex and more expensive design.

9. What is SOHC and DOHC?Ans: SOHC means single overhead camshaft engine. In this the camshaft is installed in the

cylinder head and valves are operated either by the rocker arms or directly through the

lifters.

DOHC means Double over head cam engine, some times it could be called Twin Cam. It is

possible to install multiple valves per cylinder and place intake valves on the opposite sidefrom exhaust valves.

10.What is Lean burn engine?Ans: Lean burn engine use a homogeneous fuel air mixture having 10 to 25% weak mixture.

11.What are the advantages of lean burn engine?Ans: Advantages

They can burn weak mixture, in the range of 10 to 25% weak, in a engine withoutmuch change in design or without the use of costly and complicated accessories.

The mixture produced is more homogeneous. Higher compression ratio comparedto conventional engine, can be used without tendency to knocking. Hence, thermal

efficiency is higher than that of a conventional engine.

Emission of pollutants such as NO, CO and HC are reduced to very low valueswithout the use of any external aid.

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12.What are the advantages of Gasoline Direct Injection? Ans: Lower exhaust pollution Lower fuel consumption Smooth engine operation due to an even power output from each cylinder. Automatic adjustment of the air-fuel ratio to suit operating conditions.

13.What is Multi point injection and Single (mono) point injection?Ans: Multi point injection uses separate injectors for each cylinder, and each injector is

located close to the inlet valve of the engine.

Single point injection uses only one injector, which discharges fuel into the air stream at the

point used by a carburettor.

14.What is common rail diesel injection system?Ans: The common rail diesel injection system delivers a more controlled quantity of

atomised fuel, which leads to better fuel economy; a reduction in exhaust emissions; and a

significant decrease in engine noise during operation.15.What are all the components in common rail diesel injection system?

Ans: Accumulator, rail, high pressure pump, injectors and electronic diesel control (EDC).