Final answer:
The point in travel of a piston in a reciprocating machine that results in the least cylinder volume is called top dead center. It is a critical point in the engine's cycle, marking the change from compression to the power stroke. The compression ratio, on the other hand, is the ratio of maximum to minimum cylinder volumes during the piston's travel.
Step-by-step explanation:
In a reciprocating machine, the point in travel of a piston or plunger that results in the least cylinder volume is known as top dead center (c). At top dead center, the piston is at its highest position within the cylinder, leaving the smallest possible volume in the combustion chamber. This is a crucial point in the cycle of internal combustion engines, as it marks the end of the compression stroke and the beginning of the power stroke.
During the compression stroke, the piston moves upwards, reducing the volume inside the cylinder and compressing the air or air-fuel mixture. Right after reaching top dead center, the mixture is typically ignited, leading to the power stroke where the expansion of gases forces the piston back down. The exhaust stroke then expels the combustion gases, and the cycle begins anew with the intake stroke. These processes are parts of what is known as the Otto cycle or Diesel cycle, depending on the type of engine.
The compression ratio is a separate concept which refers to the ratio of the maximum cylinder volume when the piston is at bottom dead center (the point where the cylinder volume is greatest) to the minimum volume at top dead center.