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 + HeatC8H18+12.5O2→8CO2+9H2O+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.
