Three different Auto Smelters/Furnace Machines:

A industrial smelter, optimized for many furnaces.
A simple auto smelter.
A charcoal machine.


About the industrial smelter:

The amout of items in the minecarts traveling out on the rails in the industrial smelter can be customized.
This is done by reading how many items are in the minecarts, and sending them out after a specific amount is reached, rather then relying on a clock to sent the minecarts out in fixed intervals.
If a clock is used, the minecarts might not have enough items to fill all the furnaces, and the smelter can not be expanded very much. (If the clock is not a long clock. Long clocks however take up a lot of space, and could slow the smelter down if few furnaces are used.)

To customize how many items must be in the minecarts, two furnaces in the "body" (The part that is fixed, and doesn't change when expanding the capacity of the smelter.) are included.
In these furnaces there are, as a default a single item, that stacks in 64's. I've used redstone dust. To force the minecarts to bring more items, simply add more items in to the furnaces in the "body".


About the auto smelter:

This machine might seem useless, since you might aswell put your fuel and what you want to cook manually in to a furnace.
However, when smelting large amounts of items, the one inventory spot in the furnace will not be enough.
By building a machine like this, you wont have to constantly refill your furnace with what you want to smelt, simply dump it all in to the chest.
(And the auto smelter also looks good.)
(It tastes good aswell.)

No items get stuck in the machine after smelting.