THEORY EXAMINATION (SEM–VI) 2016-17 AUTOMOTIVE FUELS AND LUBRICANTS

AUTOMOTIVE FUELS AND LUBRICANTS (NAU602)

Time: 3 Hours  Max. Marks: 100

SECTION – A (Short Answer Questions)

(10 × 2 = 20 Marks)

(a) Flash point

Flash point is the lowest temperature at which a fuel or lubricating oil gives off vapours that can ignite momentarily when exposed to a flame. It indicates fire safety of fuels.

(b) Difference between drop point and pour point

Drop point: Temperature at which grease becomes fluid enough to drip.

Pour point: Lowest temperature at which oil can still flow under gravity.

(c) Viscosity of lubricating oil

Viscosity is the resistance offered by a lubricating oil to flow. It determines the oil’s ability to form a lubricating film between moving parts.

(d) Classification of specific fuels

Specific fuels are classified as:                       Solid fuels

Liquid fuels                                                    Gaseous fuels

(e) Composition of gases in CNG

Compressed Natural Gas (CNG) mainly consists of:

Methane (CH₄): 85–95%

Small amounts of ethane, propane, nitrogen, and CO₂

(f) Calorific value of a fuel

Calorific value is the amount of heat released when a unit quantity of fuel is completely burnt.

(g) Thermo-chemistry of a fuel

Thermo-chemistry of a fuel deals with the heat energy changes during combustion reactions of fuels.

(h) Cetane number

Cetane number is a measure of ignition quality of diesel fuel. Higher cetane number means shorter ignition delay and smoother combustion.

(i) Physical concept of diesel index

Diesel index indicates ignition quality of diesel fuel and is calculated from aniline point and API gravity. Higher diesel index implies better ignition quality.

(j) Classification of petroleum fuels

Petroleum fuels are classified as:               Petrol

Diesel                                                         Kerosene

LPG                                                             Furnace oil

SECTION – B (Long Answer Questions)

(Attempt any FIVE – 5 × 10 = 50 Marks)

2(a) Thermo-chemistry of fuels with examples

Thermo-chemistry of fuels studies heat release during combustion. When fuel reacts with oxygen, chemical energy is converted into heat.

Example (Petrol combustion):

C8H18+12.5O2→8CO2+9H2O+HeatC_8H_{18} + 12.5O_2 \rightarrow 8CO_2 + 9H_2O + HeatC8​H18​+12.5O2​→8CO2​+9H2​O+Heat

This heat is used to produce power in IC engines.

2(b) Cracking – thermal and catalytic cracking

Cracking is the process of breaking large hydrocarbon molecules into smaller ones.

Thermal cracking: Uses high temperature and pressure. Produces petrol but with lower quality.

Catalytic cracking: Uses catalysts like silica-alumina. Produces high-octane petrol with better efficiency.

Catalytic cracking is more important due to higher yield and better fuel quality.

2(c) Effect of flames in SI and CI engines

SI engine: Flame propagates smoothly from spark plug. Knocking occurs if combustion is abnormal.

CI engine: Combustion starts due to self-ignition. Delay period causes rapid pressure rise.

Correct flame propagation ensures efficient and smooth engine operation.

2(d) Need of alternative fuels and features in India

Need:                                                       Depletion of petroleum reserves

Pollution control                                       Rising fuel cost

Features in India:

Availability of CNG, LPG, ethanol blends    Lower emissions

Government support and policies

2(e) Knocking in SI engines and influencing factors

Knocking is abnormal combustion due to auto-ignition of end gas.

Factors affecting knocking:                    Compression ratio

Fuel octane number                                   Engine speed

Intake temperature                                    Spark timing

2(f) Dry sump and wet sump lubrication systems

Wet sump system:
Oil is stored in the crankcase sump. Simple and widely used.

Dry sump system:
Oil is stored in an external tank. Provides better lubrication at high speeds and racing engines.

2(g) Reid vapour pressure test

Reid Vapour Pressure (RVP) test measures volatility of petrol. Higher RVP indicates easier starting but may cause vapour lock in hot conditions.

2(h) Short notes

(i) Deterioration:
Fuel deterioration occurs due to oxidation, gum formation, and contamination during storage.

(ii) Importance of fuel additives:
Fuel additives improve:

Octane/cetane rating

Combustion efficiency

Engine cleanliness

Fuel stability

SECTION – C (Very Long Answer Questions)

(Attempt any TWO – 2 × 15 = 30 Marks)

3. Suitability of CNG in conventional diesel engine

CNG can be used in diesel engines in dual-fuel mode.

Advantages:                                Lower emissions

Reduced smoke                            Lower fuel cost

Results (comparison):

NOx and particulate matter reduce in CNG mode

Slight power reduction

Better thermal efficiency

Graphs show lower emissions and smoother combustion with CNG compared to diesel.

4. Hydro-dynamic lubricating system

In hydrodynamic lubrication, a thick oil film fully separates moving surfaces.

Working:
Relative motion creates pressure in oil film which supports load.

Advantages:

No metal-to-metal contact           Low wear                                         High efficiency

Disadvantages:                            Not suitable at very low speeds      Requires continuous oil supply

5. Properties of automotive fuels and effect of volatility

Properties:                                   Volatility

Calorific value                                Octane/cetane number

Viscosity                                         Flash point

Effect of volatility on engine operation:

Cold starting                                Acceleration                                    Fuel distribution

Vapour lock                                 Crankcase dilution

Proper volatility ensures smooth operation under all conditions.