About this previous year paper
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.
(b) Explain the Function of Lubricants and Different Wet Lubrication Systems with Neat Sketch.
Functions of Lubrication in I.C. Engines:
Reduces friction and wear between moving parts.
Provides cooling by carrying away heat.
Acts as a seal between piston rings and cylinder walls.
Prevents corrosion and cleans the system by suspending impurities.
Types of Lubrication Systems:
Wet Sump System:
Oil stored in the crankcase.
Types:
Splash System: Oil splashed by dippers on crankshaft.
Pressure Feed System: Oil pump sends pressurized oil to bearings and crankshaft.
Combination System: Mix of both.
Dry Sump System:
Oil stored in external tank; scavenging pump returns oil. Used in aircraft and racing cars.
Advantages of Wet Sump System:
Simpler design, lower cost, and easy maintenance.
Provides continuous lubrication during engine operation.