(a) What do you understand by the term EGR? Explain how EGR reduces NOₓ emission in CI engines. (10 marks) (b) The flue gas…

(a) EGR, or Exhaust Gas Recirculation, is the deliberate recirculation of a small fraction of cooled exhaust gas back into the intake charge of a spark or compression ignition engine. In CI engines the main purpose is NOx control. NOx is formed thermally in the Zeldovich mechanism when local temperatures exceed about 1800 K and oxygen is available. EGR reduces NOx by two linked effects. First, the inert CO2 and N2 in the recirculated gas increase heat capacity and lower peak combustion temperature, often below the 1800 K threshold, so the rate of NO formation falls sharply. Second, dilution lowers the local oxygen concentration and slows the chain reactions that produce NO and NO2. In practice, cooled EGR also reduces charge temperature, improves knock resistance in SI engines, and in CI engines can reduce soot while requiring careful control to avoid smoke and fuel consumption penalties. Under BS VI, light-duty vehicles face an 80 mg/km NOx limit, while heavy-duty engines are regulated by engine dynamometer limits in g/kWh; EGR is therefore a core engine-side strategy in Indian emission control.

(b) Taking 100 kmol of dry flue gas as basis, the Orsat analysis gives CO2 = 8.0, CO = 0.9, O2 = 8.8 and N2 = 82.3 kmol. Since air is taken as 21 percent O2 and 79 percent N2 by volume, the oxygen supplied with the air is 82.3 × 21/79 = 21.88 kmol O2. Carbon balance gives carbon in fuel = 8.0 + 0.9 = 8.9 kmol C. Oxygen balance, written in O2-molecule form, is O2 supplied = CO2 + 0.5 CO + 0.5 H2O + O2 in flue gas. Hence H2O = 2(21.88 - 8.0 - 0.5×0.9 - 8.8) = 9.25 kmol. Hydrogen in fuel is therefore 2 × 9.25 = 18.51 kmol H. On this 100 kmol dry-gas basis the fuel equivalent is C8.9H18.51; normalised to one carbon atom it is CH2.08. The theoretical oxygen required for C8.9H18.51 is 8.9 + 18.51/4 = 13.53 kmol O2. The actual oxygen supplied is 21.88 kmol, so the percentage excess air is (21.88 - 13.53)/13.53 × 100 = 61.7 percent. The total air supplied is 21.88 + 82.3 = 104.18 kmol. If the fuel is written as C8.9H18.51, the molar air-fuel ratio is 104.18 kmol air per kmol fuel; if normalised to CH2.08, the molar air-fuel ratio is 104.18/8.9 = 11.71 kmol air per kmol fuel. The molar mass of C8.9H18.51 is 8.9×12 + 18.51×1 = 125.31 kg/kmol. The mass of air is 104.18 × 28.97 = 3018 kg, and the mass of fuel is 125.31 kg, giving a mass air-fuel ratio of about 24.1 kg air per kg fuel. The same mass ratio is obtained from the CH2.08 basis, confirming the result.

(c) For a steam nozzle, nozzle efficiency is the ratio of the actual kinetic energy obtained at the exit to the kinetic energy that would be obtained in an isentropic expansion from the same inlet state to the same exit pressure. If inlet velocity is small, ηn = (h1 - h2)/(h1 - h2s) = V actual squared / V isentropic squared, where h1 is inlet enthalpy, h2 is actual exit enthalpy, h2s is isentropic exit enthalpy, and V is exit velocity. It accounts for friction, turbulence and other irreversibilities inside the nozzle. The velocity coefficient is the ratio of actual exit velocity to the ideal isentropic exit velocity, Cv = V actual / V isentropic. Since velocities are proportional to the square root of kinetic energy, Cv = square root of ηn. For well-designed nozzles Cv is usually 0.95 to 0.99; lower values indicate greater frictional loss and lower available jet velocity.

(d) In a capillary-tube refrigeration system, the balance point is the operating condition at which the mass flow of refrigerant through the capillary tube equals the mass flow handled by the compressor. On a pressure-enthalpy diagram, for a chosen condensing pressure, the capillary tube flow increases as evaporating pressure falls because the pressure drop increases, while the compressor mass flow generally falls because the pressure ratio rises. Their intersection fixes the stable operation zone, the stable evaporating pressure, condensing pressure, refrigerant mass flow and degree of subcooling. At this point, the liquid supplied to the evaporator matches the vapor removed by the compressor, so refrigerant does not accumulate in the evaporator or condenser. If the evaporating pressure deviates, the mismatch between capillary flow and compressor flow drives the system back toward the intersection; this is why a properly selected capillary tube gives self-regulating operation. In tropical Indian conditions, high ambient temperature raises condensing pressure, shifts the balance point, and can reduce capacity if the capillary tube is too long or too small; hence balance-point selection is important for stable cooling and correct subcooling.

(e) The Equal Friction Method is a duct-sizing procedure in which the frictional pressure drop per unit length of duct is kept constant for all main ducts and branches. The designer first estimates the air volume flow in each duct from room loads, then selects a reasonable velocity, usually 2 to 4 m/s for mains and lower for branches, and reads the corresponding friction loss from a duct friction chart or Moody diagram. A typical design friction rate is 0.5 to 1.0 Pa/m for commercial air-conditioning ducts. The same friction rate is then applied to each branch; the duct diameter is adjusted until the calculated friction loss per metre matches the chosen value. The process is iterative: if the chosen velocity gives a friction rate outside the selected range, the diameter is changed and the velocity recalculated. Fitting losses, grille losses and coil losses are added separately, and the total external static pressure is used to select the fan. If the total pressure is too high, the friction rate or velocities are revised. For a simple layout, a main duct serving three rooms might be sized at 0.8 Pa/m; each branch is then sized so that its straight-run friction loss is also 0.8 Pa/m, after which branch fitting losses are checked. The method is simple, economical and widely used in Indian commercial buildings with relatively uniform loading, such as offices, hospitals and retail spaces, where tropical climate design requires reliable air distribution and low fan energy.