NCEES Mechanical Engineering PE Exam 1 — Questions and Answers
Question 1: On the NCEES PE Mechanical exam, which thermodynamic cycle best describes the operation of a steam power plant?
- Brayton cycle
- Otto cycle
- Rankine cycle (Correct answer)
- Carnot cycle
Correct answer: Rankine cycle
The Rankine cycle (with water/steam as the working fluid undergoing phase changes) is the standard thermodynamic model for steam power plants.
Question 2: In the NCEES PE Mechanical exam, what does the 'coefficient of performance (COP)' of a refrigeration cycle represent?
- The ratio of heat rejected to work input
- The ratio of useful refrigeration effect to work input (Correct answer)
- The thermal efficiency of the compressor
- The ratio of condenser heat to evaporator heat
Correct answer: The ratio of useful refrigeration effect to work input
COP of a refrigerator equals the heat absorbed at the evaporator (QL) divided by the net work input (Win), measuring how effectively work is converted to cooling.
Question 3: For the NCEES PE Mechanical exam, which equation defines the steady-flow energy equation (SFEE) for an open system?
- Q - W = ΔU
- Q̇ - Ẇ = ṁ[(h2 - h1) + (V2² - V1²)/2 + g(z2 - z1)] (Correct answer)
- PV = nRT
- W = ∫P dV
Correct answer: Q̇ - Ẇ = ṁ[(h2 - h1) + (V2² - V1²)/2 + g(z2 - z1)]
The SFEE for steady open systems accounts for heat transfer, work, and changes in enthalpy, kinetic energy, and potential energy of the flowing fluid.
Question 4: On the NCEES PE Mechanical exam, what is the significance of the 'isentropic efficiency' of a turbine?
- It is the ratio of actual work output to ideal isentropic work output (Correct answer)
- It is the ratio of heat input to mechanical work
- It measures heat loss through the turbine casing
- It is the ratio of inlet to outlet pressure
Correct answer: It is the ratio of actual work output to ideal isentropic work output
Isentropic (turbine) efficiency compares actual work extracted to the theoretical maximum work obtainable in a reversible adiabatic (isentropic) process.
Question 5: In the NCEES PE Mechanical exam, which law states that the total pressure of a gas mixture equals the sum of partial pressures of its components?
- Fourier's law
- Dalton's law of partial pressures (Correct answer)
- Fick's law of diffusion
- Newton's law of cooling
Correct answer: Dalton's law of partial pressures
Dalton's law states that for an ideal gas mixture, the total pressure equals the sum of the pressures each component gas would exert if it alone occupied the volume.
Question 6: For the NCEES PE Mechanical exam, which dimensionless number characterizes the ratio of inertial forces to viscous forces in fluid flow?
- Prandtl number
- Nusselt number
- Reynolds number (Correct answer)
- Mach number
Correct answer: Reynolds number
The Reynolds number (Re = ρVD/μ) compares inertial to viscous forces and determines whether flow is laminar or turbulent.
NCEES Mechanical Engineering PE Exam 1
On the NCEES PE Mechanical exam, which thermodynamic cycle best describes the operation of a steam power plant?