Instructions: In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Input: What is the difference between a Wankel engine and a conventional reciprocating steam engine?, Context: It is possible to use a mechanism based on a pistonless rotary engine such as the Wankel engine in place of the cylinders and valve gear of a conventional reciprocating steam engine. Many such engines have been designed, from the time of James Watt to the present day, but relatively few were actually built and even fewer went into quantity production; see link at bottom of article for more details. The major problem is the difficulty of sealing the rotors to make them steam-tight in the face of wear and thermal expansion; the resulting leakage made them very inefficient. Lack of expansive working, or any means of control of the cutoff is also a serious problem with many such designs.[citation needed]
Output:
a pistonless rotary engine