MEPX
Chapter 8 of 12All chapters
  1. 1What engineers do
  2. 2The design process
  3. 3Forces and loads
  4. 4Stress, strain and materials
  5. 5Structures
  6. 6Mechanisms
  7. 7Electrical basics
  8. 8Energy in machines
  9. 9Manufacturing and tolerances
  10. 10Safety factors and failure
  11. 11Testing and standards
  12. 12Responsibility

Chapter 8 of 12

Energy in machines

Efficiency and where the losses go.

Nothing is free

Every machine turns some input into useful output and the rest into heat, noise and vibration. Efficiency is the ratio, and it can approach but never reach one.

  • An electric motor may reach 90 percent; a petrol engine sits nearer 30.
  • Heat engines are limited by thermodynamics, not by engineering effort.

Sizing

Rating a component means knowing the worst continuous case, not the average. Systems fail on the hot day with the full load, which is the case worth designing for.