It's a bit too annoying to pipe in and extract things to activate the furnace, and quite unreasonable to do this too. Can a furnace engine just check if there are proper fuel in bound furnace, consume it and lit furnace? It could simplify tons of redundant designs in a system that needs high energy input, making the furnace engine much practical to use in the late game.

The furnace engine is supposed to make you think of a design to keep a furnace running for as long as possible. Added convenience would defeat the point of it's high reward.
Future updates will however add more engine types that may suit your taste more.