So peng jin is not aiki, but can be applied as aiki? Please explain in detail. And if you can use Japanese terms, because I don't practice taiji.
Short answer: "Peng jin" or peng energy is the core manifestation of internal power (IP). Aiki is manipulation of peng within the body to produce any number of interesting outward results.
I have never heard the Japanese term for "peng jin" although it is inherent in Daito-ryu aiki. IME, this may be due to the lack of a teaching vocabulary in the internal Japanese martial arts. True to old Asian tradition, aiki has been taught through hands-on transmission with very little verbal description.
Go back to the "6 Directions" thread and read the descriptions of what is being done to the body in that process: the use of intent to work the body in 6 (actual 360-degree) directions by simultaneously pushing and pulling the body outward and inward in all those directions. You are pulling your frame and body tissues upward and downward, forward and backward, left and right, diagonally from right shoulder to left foot, left shoulder to right foot, etc. At the same time, you are drawing in your tandan and the soft areas of your chest, and stretching up-and-down your meimon.
This establishes a powerful structure and creates a "feeling" of being spherical, of being at the center of a sphere of force. That "spherical feel" of force is a manifestation of what the Chinese internal martial arts call "peng."
You can use peng jin to repulse an incoming attack back outward. If you make small adjustments in the amount of drawing in the tandan and stretching of meimon, you can direct that force upward or downward, and to some degree in other directions, as though rolling that sphere. That part is the aiki -- the manipulation within yourself... the harmonizing of In and Yo (opposing/complementary forces) that allows you to play with peng (IP). In Japanese internal martial arts, aiki-age and aiki-sage represent aspects of aiki that utilize, respectively, the upward and downward direction of "peng."
This probably should be in the "6 Directions" thread or elsewhere, so I apologize for the thread hijack.