Engines of the single-acting type have ONE PISTON per cylinder, with the pressure of combustion gases acting on only one sur-face of the piston. This is a feature of design rather than of principle, because the basic principles of operation apply whether an engine is single-acting or opposed-piston. The pistons in most single-acting diesel engines are of the trunk type (length greater than diameter). The barrel or wall of a piston of this type has one end closed (crown) and one end open (skirt). Only the piston crown serves as part of the combustion space surface. Therefore, the pressure of combustion can act only against the crown. Thus, with respect to the surfaces of a piston, pressure is single acting. All 4-stroke cy-cle engines (fig. 2-8) and most 2-stroke cycle engines (fig. 2-9) are single acting.

Figure 2-9.End cross section of a General Motors series 149 2-stroke cycle, single-acting diesel engine.

Figure 2-10.Cross section of a Fairbanks-Morse 2-stroke cycle, opposed-piston diesel engine.