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