workflows are different things from bpel engines !
their usage is also different !
you use bpel engine  like pxe in integration projects when you want orchastrate services . but workflow  usage is in situation when you want for example implement automation in process  level in a organization that involves human interactions ! ( BPEL does not support  human interaction  )
after all my suggestion is openwfe or jbpm . Don't go with xpdl standard  in this area because as you know wfmc standard is not a complete one ( surely one of the reasons that some open  source projects like jbpm and openwfe use their own way in this area is the lack of good standard ! they say that xpdl is an incomplete one ! ) .