Diesel cycle is called constant pressure cycle because the heat addition process in diesel cycle is done at constant pressure whereas petrol cycle is called constant volume cycle (Otto Cycle) as here the heat addition is done at constant volume. (Heat rejection in both the cycles is done at constant volume).

Petrol engines works on Otto Cycle, petrol engines are also called as S.I. Engines.

The Diesel cycle is a combustion process of a reciprocating internal combustion engine. In it, fuel is ignited by heat generated during the compression of air in the combustion chamber, into which fuel is then injected, hence Diesel engines are called compression ignition engines.
A Diesel cycle consists of one constant pressure, one constant volume and two isentropic processes.