In Wikipedia, a "Workflow Engine" is defined as:
A software application that manages and executes modeled computer processes.
and from the same source, "Business Process Management" is defined as:
A "holistic management" approach to aligning an organization's business processes
with the wants and needs of clients
Hence these nouns should not be used interchangeably; a "Workflow engine" is a generic engine used for modelling any workflow, whereas a "Business Process Management Engine" is a specific implementation of a "Workflow Engine" which models business process management.
So you could make the analogy, "Workflow Engine" is to a block of wood as "Business Process Management Engine" is to a carved wooden walking stick.