(SEM V) THEORY EXAMINATION 2022-23 I C ENGINES FUELS & LUBRICATION

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SECTION A – Short Answer Type Questions (2 Marks each)

(a) Define the Term Heat Engine.

A heat engine is a device that converts heat energy into mechanical work.
It operates on a cyclic process where heat is supplied from a high-temperature source, part of it is converted into useful work, and the rest is rejected to a low-temperature sink.

Examples:

Internal Combustion (IC) Engines – petrol and diesel engines.

External Combustion (EC) Engines – steam engines and turbines.

Efficiency of a Heat Engine:

η=1−Q2Q1\eta = 1 - \frac{Q_2}{Q_1}η=1−Q1Q2

where Q1Q_1Q1 = heat supplied, Q2Q_2Q2 = heat rejected.

(b) Explain the Term Ignition Delay in CI Engines.

Ignition Delay is the time interval between the start of fuel injection and the start of combustion (noticeable by a sharp rise in pressure).

Stages:

Physical delay: Time required for atomization, vaporization, and mixing of fuel with air.

Chemical delay: Time required for chemical reactions leading to self-ignition.

Factors Affecting Ignition Delay:

Fuel properties: Cetane number, volatility, viscosity.

Temperature and pressure: Higher temperature and pressure reduce delay.

Injection timing and swirl: Proper mixing shortens delay.

Shorter ignition delay results in smoother engine operation and reduced knocking.

SECTION B – Long Answer Type Questions (10 Marks each)

(a) Discuss an Expression for Thermal Efficiency of Air Standard Diesel Cycle.

The Diesel cycle consists of four processes:

1–2: Adiabatic compression. 2–3: Constant pressure heat addition.

3–4: Adiabatic expansion. 4–1: Constant volume heat rejection.

Thermal Efficiency (η):

η=1−1rγ−1[ργ−1γ(ρ−1)]\eta = 1 - \frac{1}{r^{γ-1}} \left[\frac{ρ^{γ} - 1}{γ(ρ - 1)}\right]η=1−rγ−11[γ(ρ−1)ργ−1]

where:

rrr = compression ratio ρρρ = cut-off ratio (V₃/V₂)

γγγ = ratio of specific heats

Observation:

Efficiency increases with higher compression ratio and lower cut-off ratio.

Diesel cycle efficiency is lower than Otto cycle for same compression ratio but more practical for heavy-duty applications.

Classification of I.C. Engines:

Based on fuel: Petrol, diesel, gas engines.

Based on ignition: Spark ignition (SI) or compression ignition (CI).

Based on cycles: Otto, Diesel, or Dual cycles.

(b) Explain Stages of Combustion in SI Engine and Combustion Chamber Design.

Stages of Combustion in SI Engine:

Ignition Lag Stage: Time between spark discharge and start of combustion.

Flame Propagation Stage: Rapid burning of air-fuel mixture with a sharp rise in pressure.

After-Burning Stage: Remaining unburnt charge burns slowly as pressure drops.

Flame Speed Factors:

Mixture strength (optimum at slightly rich mixture). Compression ratio.

Turbulence and ignition timing.

Combustion Chamber Design Considerations:

Fast flame travel and complete combustion. Minimum knocking tendency.

Compact design for high thermal efficiency. Good cooling and adequate turbulence.

Examples:

T-Head, L-Head, F-Head, and Hemisphere-head chambers.

SECTION C – Very Long Answer Type Questions (10 Marks each)

(a) Demonstrate Phenomenon of Knock in S.I. Engine and Methods to Reduce Detonation.

Knock Definition:
Knocking in SI engines is the spontaneous ignition of the end-gas ahead of the flame front due to high pressure and temperature, producing shock waves and a metallic noise.

Causes:

High compression ratio. Low-octane fuel.

High inlet temperature. Advanced spark timing.

Effects:

Loss of power and efficiency. Engine overheating and mechanical damage.

Methods to Reduce Knocking: Use of high-octane fuel.

Retard spark timing.

Water injection or EGR (Exhaust Gas Recirculation) to lower temperature.

Efficient combustion chamber design (compact and turbulence-promoting).

Use of anti-knock additives (e.g., tetraethyl lead or MTBE).

Conclusion:
Controlling detonation enhances performance, engine life, and fuel economy.