(a) With the help of P-θ (pressure-crank angle) diagram, compare the knock in SI and CI engines. Explain that "the factors which…

(a)

In a Spark Ignition (SI) engine, knock is the auto-ignition of the end-gas charge before the flame front reaches it. On a P-θ diagram, this manifests as sharp, erratic pressure oscillations occurring near the end of combustion, after Top Dead Center (TDC). Conversely, in a Compression Ignition (CI) engine, knock results from a long ignition delay; a large mass of fuel accumulates and burns simultaneously, causing a sudden pressure rise after TDC.

The "reversal" of factors is a fundamental thermodynamic distinction:

1. Compression Ratio (CR): High CR increases end-gas temperature and pressure in SI engines, promoting knock. In CI engines, high CR is essential for auto-ignition; a low CR would cause the knock.
2. Fuel Reactivity: SI engines require high octane (low reactivity) to resist knock. CI engines require high cetane (high reactivity) to ensure prompt ignition and prevent the "diesel knock" associated with long delays.
3. Temperature and pressure: High inlet temperature and pressure (for example supercharging) promote knock in SI engines but shorten the ignition delay and suppress knock in CI engines.
4. Engine speed: Higher speed reduces SI knock (more turbulence, faster flame, less time for end-gas reactions) but, because the delay is roughly fixed in time, more fuel is injected during the delay in a CI engine, which promotes knock.
5. Cylinder size: A large bore lengthens the flame path and promotes SI knock, whereas large CI cylinders run hotter and have shorter delays, reducing knock.
6. Timing: Retarding the spark reduces SI knock; in a CI engine, injection timing is set to minimise the delay, and too early injection into cooler air promotes knock. In short, SI knock is prevented by delaying auto-ignition of the end gas, while CI knock is prevented by hastening auto-ignition of the first fuel injected.

(b)

Blowby losses refer to the leakage of high-pressure combustion gases from the cylinder past the piston rings into the crankcase. This phenomenon occurs because the pressure in the cylinder during the power stroke exceeds the pressure in the crankcase.

Factors increasing blowby:

• Ring Wear: Loss of ring elasticity and increased clearance.
• High Peak Pressure: Aggressive engine tuning or high-load conditions.
• High Temperature: Thermal expansion of the piston and rings.
• Ring End-Gap: If the gap is too large, gas escapes easily.
• Worn or oval cylinder bore: poor ring-to-liner sealing.
• Sticking or broken rings: carbon deposits in the ring grooves prevent the rings from sealing.
• Low engine speed: more time per cycle for leakage.
• Ring flutter at high speed: the ring lifts off its groove face and loses sealing.

Effects on performance:

• Power Loss: Loss of working fluid reduces the Indicated Mean Effective Pressure (IMEP).
• Oil Dilution: Combustion products dilute the crankcase oil, reducing its lubricating properties.
• Emissions: Hydrocarbons and unburnt fuel escape into the atmosphere via the breather or Positive Crankcase Ventilation (PCV) system.
• Lower compression pressure: harder starting and loss of efficiency.
• Deposits and oil consumption: blowby gases carry soot, water and acids into the crankcase, forming sludge and increasing oil consumption.
• Higher crankcase pressure: pushes oil past seals and gaskets, causing leaks.

(c)

Supercritical boilers operate at pressures exceeding the critical point of water (22.1 MPa).

Advantages:

• Higher Efficiency: Heat is added at a higher mean temperature, so the cycle efficiency rises by about 2–3 percentage points over subcritical units (roughly 38–42% against 35–37%), which also lowers coal use and CO₂ per kWh.
• Compactness: The absence of a steam drum (once-through design) reduces the plant's physical footprint.
• Lower Water Usage: No drum means no continuous blowdown of water.
• Faster load response: the once-through design without a thick drum allows faster start-up and sliding-pressure operation.

Disadvantages:

• Material Constraints: Requires expensive, high-grade alloys to withstand extreme stress.
• Complex Control: Managing the transition from liquid to supercritical fluid without a drum is technically difficult.
• High Cost: Significant capital expenditure for construction and maintenance.
• Very high feedwater purity: without a drum there is no blowdown, so dissolved solids deposit in the tubes unless full-flow condensate polishing is used.
• Higher feed-pump power: pumping to above 22.1 MPa consumes more auxiliary power.

T-s Diagram: The Rankine cycle for a supercritical boiler with single-stage reheat is plotted on a T-s diagram. The cycle starts at a low-pressure liquid state, is compressed to supercritical pressure, and heated in the boiler. The path moves into the superheated region, bypassing the saturation dome. The steam expands in the high-pressure turbine, is reheated at constant pressure to the maximum temperature, and expands again in the low-pressure turbine before condensation.

(d)

To derive the expression for draught h in mm of water column, we consider the pressure difference between the outside air and the flue gas inside the chimney.

1. Pressure Difference: The draught is the difference in the hydrostatic pressure of the outside air column and the hot gas column of height H, Δp = (ρₐ − ρ_g)gH Pa. Since 1 mm of water column = 1 kg/m² × g = 9.81 Pa, the draught in mm of water is numerically (with ρ in kg/m³ and H in m): h = (ρₐ - ρ_g)H
2. Density (Ideal Gas Law): For outside air, using ρ = P/RT with P ≈ atmospheric pressure: ρₐ = Pₐ / (Rₐ Tₐ)
3. Mass Balance and gas density: For 1 kg of fuel, the mass of air is mₐ and the mass of flue gas is (mₐ + 1). Assuming the volume of the flue gas equals the volume of air supplied at the same temperature (the fuel volume is negligible and the gas constant of flue gas ≈ that of air), mₐ kg of air at T_g would occupy mₐRₐT_g/Pₐ, and the (mₐ + 1) kg of gas occupy the same volume. Hence ρ_g = (mₐ + 1)Pₐ/(mₐRₐT_g).
4. Substitution: h = H · Pₐ · [1/(Rₐ Tₐ) - (mₐ + 1)/(mₐ Rₐ T_g)] h = (H · Pₐ / Rₐ) · [1/Tₐ - ((mₐ + 1)/mₐ)(1/T_g)]
5. Final Constant: Using Pₐ = 1.013 × 10⁵ Pa and Rₐ = 287 J/kgK, the constant Pₐ/Rₐ ≈ 353. h = 353H [1/Tₐ - ((mₐ + 1)/mₐ)(1/T_g)]

(e)

The chemical name of R134a is 1,1,1,2-tetrafluoroethane. It is a Hydrofluorocarbon (HFC) commonly used as a replacement for R-12.

Eco-friendliness: R134a is not fully eco-friendly. While it has an Ozone Depletion Potential (ODP) of 0, meaning it does not damage the ozone layer, it has a high Global Warming Potential (GWP) of 1430 (IPCC AR4). This high GWP means it is a potent greenhouse gas that contributes significantly to global warming.

Clarification: Because of its high GWP, R134a is being phased down under the Kigali Amendment to the Montreal Protocol. It is considered a "transitional" refrigerant; while it is safer for the ozone than CFCs, it is not a long-term solution for climate protection, leading to the development of lower-GWP alternatives like R-1234yf.