To add to what Whopity says, If you are SEP rated and what to fly a Diesel powered SEP then the engine theory element has to be provided during the differences training by the instructor and they need to check your understanding.
While during your basic PPL training your Authority based written exams may have been limited to piston training, if you had presented in a diesel aircraft for the test the examminer will have given you some oral questions regarding the engine.
At PPL level, type / class specific questions are covered during the GST.