Steam and IC Engines
Steam and IC Engines
The exam tests steam states and steam-table use, boiler mountings versus accessories, four-stroke and two-stroke operation, and simple power calculations.
Steam
Sensible heat raises water to boiling point; latent heat converts water into dry saturated steam at the same temperature; superheated steam is above saturation temperature. Dryness fraction x = mass of dry steam / total mass, and enthalpy h = h_f + x·h_fg. Boiler mountings are safety and functional fittings: safety valve, water level indicator, pressure gauge, fusible plug. Accessories improve efficiency: economiser, superheater, air preheater, feed pump.
IC engines
Four-stroke SI: suction, compression, ignition/power, exhaust - two crank revolutions per cycle, one power stroke. Two-stroke: one revolution, ports instead of valves, crankcase scavenging, lower efficiency. CI engines use fuel injection and compression ignition. Compression ratio CR = (V_s + V_c)/V_c. Indicated power of one cylinder: IP = p_m·L·A·n/60, with n = N/2 for a four-stroke. Mechanical efficiency = BP/IP.
Worked example
Single-cylinder four-stroke, bore 100 mm, stroke 120 mm, 1500 rpm, IMEP 6 bar. A = π/4 × 0.1² = 7.854 × 10⁻³ m²; p_m = 6 × 10⁵ Pa; n = 1500/2 = 750 per minute. IP = 6×10⁵ × 0.12 × 7.854×10⁻³ × 750 / 60 ≈ 7.07 kW. If BP = 5.7 kW, mechanical efficiency = 5.7/7.07 ≈ 80.6 %.
Traps
- Using N instead of N/2 for a four-stroke engine.
- Calling the economiser a mounting.
- Mixing swept volume and clearance volume in the compression ratio.
- Leaving IMEP in bar instead of converting to N/m².