The WofklowEngine is a misterious thing in CVW. Unless you want to implement the CVW API you need not care too much about it.
When you want to create a workflow you have to create an engine that the workflow will belong to and ... that is it.
The workflow engine can be a singleton or you can create one for each new workflow definition. It does not matter functionally. The API does not require more than that it exists.
The simple implementation however uses this engine to maintain the loaded instances of action functions. The action functions (conditions, prefunctions, validators and postfunctions) are singletons in this implementation. This means that whenever a condition is instantiated for a specific Java class it will be reused by the engine.
If for some reason you have to be sure that two different actions defined by the same java class are different objects then they should not share engine.