In this paper we present the architecture and design of an extended BPEL engine that implements the operational semantics of BPEL4SWS. BPEL4SWS is an extension of the BPEL language with support for Semantic Web Service concepts like mediation and semantic descriptions of activity implementations. We describe the basic communication scenarios of processes with services and the interaction between the engine components involved in the execution of BPEL4SWS processes. The presented prototype is based on the open source BPEL engine Apache ODE, features improved configurability and facilitates the definition of additional BPEL extensions with minimal development effort.