Abstract
The evolution of the working processes at the injection nozzle is the result of reciprocally interaction between nozzle needle movement and fluid exchange, conditioned on their turn, by the pressure variation, factor which depends by the law of cam lifting. In the paper is setting off the role that can be exerted by the differential characteristic of the injector spring on the working processes stability at the nozzle, for several types of diesel engines, destined for motor vehicles. There are presented some considerations regarding its involvement on the engines functionality, especial at the idle speed and small charges.