The present paper studies how an electrically assisted charging system influences the functional parameters of spark ignited engines. These influences are studied through the city cycles performed with a modern turbocharged car. The main objective that triggered researches was to improve transient response of the internal combustion engine, with focus on city driving. The issue of turbo lag is well known from decades, but sadly most of turbocharged engines are engineered for peak power output, rather than drivability. Steady–state torque became a simple number car manufacturers use to maintain competition, though the majority of daily driving schedule involves transient conditions. In fact, only a very small part of a vehicle’s operating pattern is true steady – state, e.g., when cruising on a motorway. The fundamental aspect of transient condition lies in its operating differences compared with steady–state operation. Whereas during steady-state conditions, engine speed and fueling, hence all other engine and turbocharger properties remain practically constant, during transient operation, both engine speed and the amount of injected fuel change continuously. Consequently, the available exhaust gas energy varies, affecting turbine enthalpy, air supply and boost pressure. Due to various dynamic, thermal and fluid delays in the system, air–supply is delayed compared with fueling, eventually affecting torque buildup (drivability) and exhaust emissions. Tests were done on a compact SUV (Sport Utility Vehicle) which has a greater ground clearance, and it is powered by a spark ignited turbocharged engine to which an electrical charger unit was also fitted. The latter was powered by the car’s own battery. The analyses made upon test data showed a considerable advantage for the electrically assisted version with regard to transient response, generated by the increased pressure and mass flow of the air entering into the combustion chamber through the intake manifold when the electric charger was active, which allowed a greater fuel quantity to be injected, delivering torque from early idle speed.

transient; steady state; electric charger

Presentation: oral