The Model D was the result of a complete redesign in 1922.
The uniflow engine, perceived as the root of the troubles with the Doble Detroit, gave place to a two cylinder compound type, still with Joy’s valve gear, but with piston valves. Another crucial development was a coiled monotube once-through vertically-mounted cylindrical boiler following the thinking behind the later version of the Detroit boiler, the most distinctive feature of which was the placing of the burner at the top of the boiler. This plus drastic insulation was meant to cause the hot gases to reside within the boiler casing for an optimum length of time giving up the maximum amount of heat to the feedwater.
There was a forced draft burner at the top of the boiler and an exhaust flue at the bottom. The venturi was placed horizontally at the top of the vertical boiler barrel and oriented in such a way as to avoid direct contact with the monotube whilst inducing a swirl motion to the gases. It was thus a counterflow design with water entering the lower end of the coiled monotube and progressing upward toward the burner, which meant that the hottest gases gave superheat to the steam at the top of the coil whilst the cooler gases preheated oncoming the feedwater at the bottom. The distinctive hand operated "miniature steering wheel” rotating a throttle control rod that passed down the middle of the steering column can be observed in D2 which still exists at the present time. Photographic evidence shows that D1 retained the foot throttle pedal, so when the wheel throttle control was first applied is not clear. The latter probably gave more precise adjustment.
No more than five of the D model appear to have been built, if that. It is said that the two-cylinder compound engine sometimes gave difficulty in starting.