A service yes, but not specifically in a sar. Like you don't have a sar when running unittests from e.g. Eclipse. It's a 'convenience' to be able to deploy the engine once in JBoss and use it from different applications.
As I dive deeper into the code (and learn more and more about jBPM). This is my conclusion so for (I don't know if this is right so please correct me if I'm wrong). Timers etc are stored as jobs and can be retrieved via the managementService.createJobQuery()...
So far so good, but am I responsible for handling all the jobs myself? So in case of timers: do have to write my own implementation for handling them at the right moment (so and keep constant track of them and fire them at the right moment)? If so is there some 'Best practice' available to do this.
No, there is a job executor. Look at the testcases on how to start a jobscheduler from java. You can do that in any appserver with e.g. a startup servlet or whatever.