(SEM V) THEORY EXAMINATION 2021-22 IC ENGINE, FUEL AND LUBRICATION

IC ENGINE, FUEL AND LUBRICATION (KME-054)

B.Tech (Sem V) – Exam Notes & Solved Guide

SECTION A – Short Answer Type (2 Marks Each)

a. Heat Engine
A heat engine is a device that converts thermal energy (heat) into mechanical work by operating in a cycle.

b. Relative Efficiency
Relative efficiency is the ratio of actual thermal efficiency of an engine to the thermal efficiency of the ideal (air-standard) cycle.

c. Ignition Delay in CI Engine
Ignition delay is the time interval between the start of fuel injection and the start of combustion in a compression ignition engine.

d. Flame Speed in SI Engine
Flame speed is the rate at which the flame front travels through the air-fuel mixture in the combustion chamber.

e. Injection Timing
Injection timing refers to the crank angle at which fuel injection starts in a CI engine. Correct timing improves efficiency and reduces emissions.

f. Lean and Rich Mixture
A lean mixture has excess air compared to fuel, while a rich mixture has excess fuel compared to air.

g. NOx Emission
NOx emissions are nitrogen oxides formed at high combustion temperatures. They cause air pollution and smog.

h. Cetane Number and Diesel Index
Cetane number indicates ignition quality of diesel fuel. Diesel index is an approximate measure calculated from fuel density and aniline point.

i. Working of Radiator
A radiator removes heat from engine coolant by transferring it to surrounding air through fins and tubes.

j. Stratified Charge Engine
A stratified charge engine has layers of rich and lean air-fuel mixture inside the cylinder to improve efficiency and reduce emissions.

SECTION B – Descriptive Answers (10 Marks Each)

a. Classification of IC Engines

IC engines are classified based on cycle (Otto, Diesel), ignition method (SI, CI), fuel used (petrol, diesel, gas), cooling method (air, water), number of strokes (two-stroke, four-stroke), and number of cylinders. This classification helps in selecting engines for specific applications.

b. Combustion Stages in SI Engine

Combustion in an SI engine occurs in three stages:
first is ignition delay and flame kernel development, second is rapid flame propagation, and third is after-burning where remaining fuel burns slowly. Proper combustion ensures smooth engine operation.

c. MPFI System and Its Types

Multi-Point Fuel Injection supplies fuel separately to each cylinder near the intake valve. Types include sequential MPFI, simultaneous MPFI, and batch injection. MPFI improves fuel economy, reduces emissions, and enhances engine performance.

d. Particulate Emissions and Control Methods

Particulate emissions consist of soot and unburnt carbon particles, mainly from diesel engines. Control methods include diesel particulate filters, better fuel quality, optimized combustion, and exhaust after-treatment systems.

e. Cooling System and Comparison

A good cooling system maintains engine temperature, prevents overheating, and improves engine life.
Air cooling is simple and lightweight, while liquid cooling offers better temperature control and uniform cooling.

SECTION C – Long Answer / Numerical Type

3(a). Thermal Efficiency of Otto Cycle

Thermal efficiency of air-standard Otto cycle is:
η = 1 − (1 / r^(γ−1))
where r is compression ratio and γ is ratio of specific heats. Higher compression ratio increases efficiency.

3(b). Diesel Cycle Numerical (Outline)

Given bore, stroke, clearance volume, and cut-off, the compression ratio and cut-off ratio are calculated. Using Diesel cycle efficiency formula, thermal efficiency is determined. Increasing cut-off ratio reduces efficiency, hence delaying injection lowers efficiency.

4(a). Combustion Process in CI Engine

CI engine combustion occurs in four phases: ignition delay, rapid uncontrolled combustion, controlled combustion, and after-burning. A neat pressure-crank angle diagram explains these stages clearly.

4(b). Knocking in SI Engine

Knocking is abnormal combustion caused by auto-ignition of end-gas. It reduces efficiency and damages engine parts. It can be reduced by high octane fuel, proper ignition timing, and improved cooling.

5(a). Air-Fuel Ratio Derivation

Exact air-fuel ratio is derived using chemical equation of fuel combustion considering air as incompressible. It helps in proper mixture formation.

5(b). Fuel Injection Quantity & Velocity (Numerical)

Fuel per cycle is derived using BSFC, brake power, and rpm. Injection velocity is calculated using pressure difference, fuel density, and coefficient of discharge.

6(a). EGR System & Catalytic Converter

EGR (Exhaust Gas Recirculation) reduces NOx by lowering combustion temperature.
Catalytic converter converts harmful gases like CO, HC, and NOx into harmless gases using catalysts.

6(b). Alternative Fuels & SI Engine Fuel Rating

Alternative fuels include CNG, LPG, ethanol, biodiesel, and hydrogen.
SI engine fuel rating is based on octane number, which measures anti-knock quality.

7(a). Lubrication in IC Engines

Lubricants reduce friction, wear, cooling, sealing, and corrosion.
Wet lubrication systems include splash system, pressure system, and combined system.

7(b). Battery Ignition System

Battery ignition system uses battery, coil, distributor, and spark plug to produce spark.
Electronic ignition systems improve reliability and timing accuracy.