The European Commission regulatory proposed average CO₂ emissions of the EU fleet of new passenger cars and light commercial vehicles in 2030 will have to be 30% lower than in 2020. Hybrid electric vehicles (HEV) are becoming a more and more important vehicle segment in the European car park. In the case of HEV a more efficient energy management can be achieved compared to the conventional internal combustion engine powered vehicles. Due to this reason, HEV represent a viable solution throughout which a diminishing of the fuel consumption as well as vehicle emissions can be achieved. Conventional vehicles and parallel HEV frequently operate using transient state. Otherwise, the series HEV engines operate at steady state. The main objective of this paper is to emphasis this advantages of steady state engines throughout experimental researches compared to transient state. The transient regimes were obtained successively by the variation of following parameters: engine speed, engine load and both. For comparison, the experimental values obtained in the steady state presented. In the case of steady regimes, the engine's performance and environmental characteristics are higher than for the transient regimes. The increase of engine speed in transient regimes mostly affects the engine's performance and environmental performance. The difference between this values obtained in the stabilized and transient regimes is minimal when the load modified compared to the speed variation.