Thermal Engineering: Thermodynamics, IC Engines and Boilers
Thermal Engineering: Thermodynamics, IC Engines and Boilers tests the laws that govern heat and work, the four-stroke cycle, and the hardware of steam generation - mountings versus accessories is a favourite distinction.
Core rules
- First law for a cycle: ΣQ = ΣW; for a non-flow process Q = ΔU + W. For an ideal gas PV = mRT, cp − cv = R and γ = cp/cv (1.4 for air).
- Processes: isothermal (T constant, PV = C), adiabatic (PV^γ = C, Q = 0), isobaric, isochoric.
- Four-stroke engine: suction, compression, power, exhaust - two crankshaft revolutions per cycle. SI engines use a spark plug and petrol; CI engines inject diesel and ignite by compression heat (ratio about 16–20).
- Compression ratio r = (swept volume + clearance volume) ÷ clearance volume.
- Otto air-standard efficiency = 1 − 1/r^(γ−1).
- Boiler mountings are safety and control fittings on the boiler itself (safety valve, water level indicator, pressure gauge, fusible plug, blow-down cock). Accessories improve efficiency and sit in the gas path (economiser, superheater, air preheater).
Worked example
An engine has a clearance volume of 50 cm³ and a swept volume of 450 cm³. r = (450 + 50)/50 = 10. Otto efficiency = 1 − 1/10^0.4 = 1 − 0.398 = 60.2 %.
Traps that cost marks
Calling the economiser a mounting; saying a CI engine has a spark plug; using four revolutions per cycle for a four-stroke; applying 1 − 1/r^(γ−1) to a Diesel cycle; mixing gauge and absolute pressure in PV work.