We considered an ignition strategy based on energy injection in the final stages of the target implosion, but well before stagnation. The injected energy is used to set on a high adiabat the small portion of the fuel destined to become the ignition spark. Most of the energy needed to form the spark is provided by the work of the imploding target on the spark fuel. This approach is first introduced by a simple analytical model and then illustrated by 2D numerical calculations, in which the ignition of imploding cylinders by heavy ions is simulated. Different implosion designs and burn modes are presented and discussed. When compared with the standard central self-generated spark, or the fast ignition approaches, the ignition by entropy injection is potentially promising of some distinct advantages.