In development engines not available in "Does Engine Fit" systems.

In development engines not available in for reference while designing other components

in the "does engine fit" where you can select an engine while designing a chassis, you cannot select a in-development engine. Same while designing a gearbox. This hasn't always been the case, and isn't the case chassis or gearboxes (meaning while designing an engine you can select a chassis or gearbox that is in development)