(a) Can alcohols be used as fuel in IC engine? Explain with advantages and disadvantages. (10 marks) (b) A water-filled reactor…

Alcohols as IC-engine fuels. Methanol and ethanol can be used in spark-ignition engines directly or blended, and in compression-ignition engines with cetane improvers or in dual-fuel mode. Their oxygen content promotes more complete combustion, giving lower CO, hydrocarbons, particulate matter and SOx compared with gasoline or diesel. Ethanol and methanol have high octane numbers, allowing higher compression ratios, leaner operation and reduced knocking; in SI engines this permits advanced timing and higher brake mean effective pressure, while in CI engines the low cetane number must be corrected by additives, pilot diesel or dual-fuel injection. Bioethanol also has lower net carbon intensity. The disadvantages are lower volumetric energy density (ethanol about 24 MJ/L and methanol about 15.5 MJ/L against gasoline about 32 MJ/L), so more fuel is needed for the same power and range falls. Their high latent heat of vaporisation makes cold starting difficult, they are hygroscopic, and they can corrode metals and degrade some seals and elastomers. India’s ethanol-blending programme, moving from E10 toward E20 and including E85 flex-fuel trials, shows that blending is practical, but high blending levels require engine calibration, fuel-system compatibility and supply-chain safeguards.

Ruptured reactor containment. The initial water is compressed liquid at 20 MPa and 360 °C; using the supplied table, v1≈0.0015 m³/kg and u1≈1700 kJ/kg, so m=1/v1≈667 kg. The 1 m³ is the initial water volume; after rupture the same mass occupies the room. The room is insulated and initially evacuated, so the rupture is a free expansion with no heat, work, kinetic or potential energy change; no flow work is done on the surroundings, and hence m u1=m u2 and u2=u1=1700 kJ/kg. For the minimum room volume the final pressure is exactly 200 kPa, a saturated mixture. At 200 kPa, u_f=504.5 kJ/kg and u_fg=2025 kJ/kg, so x=(1700−504.5)/2025=0.590. With v_f=0.001061 m³/kg and v_g=0.8857 m³/kg, v2=v_f+x(v_g−v_f)=0.001061+0.590(0.8857−0.001061)=0.523 m³/kg. Therefore V_min=m v2≈667×0.523≈349 m³.

Regeneration and Carnot limit. The schematic is a Rankine cycle with boiler, turbine, condenser and pumps, plus open feedwater heaters fed by steam bled from the turbine at intermediate pressures. Each open heater is a heat exchanger in which feedwater is heated by condensing bleed steam; the drain from a higher-pressure heater is throttled to the next lower-pressure heater. On the T-s diagram, draw the saturation dome; the condenser is a horizontal line at T_min, the pump is a small vertical rise, the feedwater heating stages are short near-horizontal steps at rising temperatures, and the boiler heat addition is a horizontal line at T_max from saturated liquid to saturated vapour. In the unregenerated cycle the heat-addition line starts at a low feedwater temperature and extends to T_max; regeneration replaces that low-temperature portion by heat recovered from the turbine, leaving only a short high-temperature boiler section. As the number of stages increases, the feedwater enters the boiler closer to T_max, so the heat added in the boiler occurs over a narrower temperature range and tends to isothermal addition at T_max. The mean temperature of heat addition, T_h=Q_in/S_in, therefore approaches T_max. Since a reversible cycle’s efficiency is 1−T_c/T_h, the regenerated Rankine efficiency approaches 1−T_c/T_max, the Carnot efficiency.

p-h diagram effects. On the p-h diagram the basic cycle is evaporation at suction pressure, compression to delivery pressure, condensation, and throttling; state 1 is saturated or slightly superheated vapour at suction pressure, state 2 is the compressed discharge state, state 3 is liquid at delivery pressure, and state 4 is the throttled mixture. The refrigeration effect is the horizontal distance h1−h4, and compressor work is h2−h1. Increasing suction pressure raises evaporator temperature and moves state 1 to the right on the lower pressure line; h1 rises while h4 is nearly unchanged, so the refrigeration effect h1−h4 increases, compression ratio and specific volume fall, and COP generally improves. Increasing delivery pressure raises condenser temperature; h2 and h3 rise, h4 rises after throttling, so h1−h4 decreases while compressor work h2−h1 increases, lowering COP and raising discharge temperature. Subcooling the liquid lowers h3 and hence h4, increasing h1−h4 without changing compressor inlet state, so COP rises. Superheating the vapour moves state 1 further right; if the superheat is useful evaporator load, refrigeration effect increases and COP may improve, but if it is parasitic suction-line heat, the refrigeration effect does not increase while work rises, so COP falls.

Coil leaving state and bypass factor. No separate outdoor-air flow is printed; the determinate interpretation is that 40 m³/min is the total air flow through the coil. If it were only the recirculated room-air flow, the mixed-air condition would be indeterminate because the fresh-air flow and room condition are not given. At 32 °C DBT and 18 °C WBT, W1≈0.00714 kg/kg dry air and h1≈50.4 kJ/kg. The specific volume is v1≈0.878 m³/kg, so m_dot≈40/0.878=45.6 kg/min=0.760 kg/s. The coil gives 12 kW, so h2=50.4−12/0.760=34.6 kJ/kg. On the psychrometric chart, the exit point lies on the straight line joining the inlet point to the saturated 4.5 °C point because the leaving air is a mixture of bypassed inlet air and air cooled to the effective surface temperature. For perfect contact with a 4.5 °C effective surface, the air would leave saturated at 4.5 °C, with h_s≈17.5 kJ/kg. The fraction of air actually cooled is (h1−h2)/(h1−h_s)=15.8/32.9=0.480, so the bypass factor BF=1−0.480=0.520. Hence T2=BF×32+(1−BF)×4.5≈18.8 °C and W2≈BF×0.00714+(1−BF)×0.00518≈0.0062 kg/kg, giving a leaving WBT of about 12.2 °C. Thus the coil leaves air at approximately 18.8 °C DBT, 12.2 °C WBT, with a bypass factor of 0.52. Overall, alcohol fuels are feasible but constrained by energy density and materials, the containment volume follows from internal-energy conservation, regeneration approaches Carnot by raising the mean heat-addition temperature, refrigeration performance follows p-h state shifts, and the coil state is fixed by the energy balance and bypass model.