The first steam engines were vacuum engines, fill a cylinder full of steam and as it condenses it sucks the piston down, Watt's improvement and patent was on a separated condenser unit so you did not have to reheat the whole cylinder each time. This doubled the efficiency, that is, reduced the amount of coal you had to burn by half. But Watt would not sell manufacturing rights to his patent he sold operating licenses, If you had an engine that utilized Watts patent idea you owed him half the price of the coal you did not have to burn by using it. Consequently there were a lot of "pirate" engines, the most common method of working around Watts patent was to claim that your condenser was integrated into(as opposed to separated from) the cylinder some how.