Principles of Similitude Applied to Engine Design

Principles of Similitude Applied to Engine Design

Similitude Applied to Engine Design 

  • For a new design the number, and, therefore, the size, of cylinders to be used for a given power output is an extremely important decision
  • In practice, the greatest differences in cylinder design are caused by the differing requirements of various types of service.
  • Within each service category, however, a surprising degree of similitude in cylinder design is found
  • Thus the principles governing similar designs can be used to obtain qualitative comparisons of engine design and performance within a given category.
  • The use of reduced-scale models to simulate the behavior of full-scale prototypes is necessary when physical testing constraints limit the feasibility of full scale prototype testing
  • Engine performance is predicted based on cylinder diameter, stroke length, fuel supply, combustion methods, and speed and compression ratio.
  • Designing of a new engine is based on experience about the existing engine similarities and performance.
  • Cylinder-size effects
  • Diesel engines are built in a wide range of cylinder sizes, and for a given type of service the cylinder designs used are quite similar
  • The mean values of bmep, piston speed, and specific output all tend to fall as bore increases – the trends are explained as
  • Small cylinders are generally used in applications in which high output in proportion to size and weight is very important
  • In general, large cylinders are used only in services in which there is great emphasis on reliability, durability, and fuel economy – encourages low ratings in terms of bmep and piston speed.
  • Stresses due to temperature gradients increase with increasing cylinder size unless gas temperatures are reduced – leads toward lower F/A ratios, hence lower rated bmep as the bore increases
  • Extensive development work, and especially destructive testing, become less practicable as cylinder size increases  – thus, with little development testing, both bmep and piston-speed ratings must be low in order to insure proper reliability.
  • With increasing cylinder size, it becomes necessary to build up such elements as crankcases, crankshafts, and cylinders out of many parts fastened together, whereas, with small cylinder sizes, one-piece construction is generally used.
  • The foregoing  factors appear sufficient to account for the fact that average rated bmep and piston speed grow smaller as cylinder size increases
  • In spite of this fact, rated power is much more nearly proportional to piston area than to piston displacement
  • Similar engines will, of course, have weights proportional to their piston displacements
  • If power is proportional to piston area, for similar cylinders weight per horsepower increases in direct proportion to the bore. 

Please read :
Combustion in CI Engine .
Comparison of ignition systems 


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