The term that you are looking for is "wet stacking", and is due to the fact that diesels utilize the heat and pressure in the pistons and cylinder walls to indu

The term that you are looking for is "wet stacking", and is due to the fact that diesels utilize the heat and pressure in the pistons and cylinder walls to induce combustion (there is no spark plug). That being said, one could inject the bare minimum to "idle" the motor and maintain this temperature, losing a bit of efficiency from completely stopping the motor entirely. There are a few diesel hybrid prototypes, if memory serves me there was a two stroke diesel hybrid motorcycle called the eCycle that got something like 160mpg.