The D21 steam engine is a power machine featuring a double-acting, stationary brass cylinder. The technical specifications match those of the D20, but the distinctive color scheme stands out, giving this machine a particularly elegant finish: all metal parts are painted in black and red. The flywheel is also painted red. This special machine is complemented by high-gloss polished brass components. A technical highlight is the built-in feed water pump, which is exclusively found in the D21 steam engine within the Wilesco range.

This water pump allows for refilling the boiler with water while the steam engine is in operation. The condensate is directed into a condensate tray beneath the chimney, causing the chimney to emit steam. The steam outlet can be manually controlled on the D21 using a steam shut-off valve. The power machine is powered by the dry fuel Witabs, with one load providing approximately 10-15 minutes of steam enjoyment. From the first ignition of the steam engine, it takes about 5 minutes for sufficient steam pressure to build up before the machine can be set in motion. The D21 then has enough power to drive several models, allowing for a clear demonstration of the conversion of thermal energy into mechanical energy. It is also particularly enjoyable to generate electricity and light by connecting the separately available parts Dynamo M66 or Dynamo with Lantern M66/2 to the D21.