A kind of heat engine called a pulse tube engine has been recently proposed. The engine consists of only a few parts: differentially heated stacked metal meshes in a cylinder and one piston, coupled to a flywheel through a narrow flow passage called orifice. We built the prototype engine and tested its working mechanism from the standpoint of a thermoacoustic framework. We measured the work flux density distribution over the cross-section of the pulse tube to identify the work source of the engine and elucidate the function of the parts used in the engine. This engine belongs to the standing-wave engine group and the work source resides not in the stacked metal meshes but in the pulse tube. It is demonstrated that the stacked metal meshes are installed to break the thermodynamic symmetry, and that the orifice plays the role of extracting a maximum power from the engine.