Abstract
Real driving cycles are characterised by a sequence of accelerations, cruises, decelerations and engine idlings, with the frequency of accelerations and decelerations increasing dramatically due to the increasing traffic congestion. Even unrealistic certification driving schedules as the New European Driving Cycle (NEDC), made up of four city segments and one extra urban segment everything but normal driving on European roads, have a braking energy component of 20% the propulsive energy. Recovering this braking energy through a kinetic energy recovery system (KERS) is the most effective way to reduce the propulsive energy supply by the thermal engine. The fuel energy saving may be much larger than the propulsive energy saving, because the thermal engine energy supply may be cut where this engine operates less efficiently, and because the thermal engine can be made smaller thanks to the extra boost by the KERS when needed and therefore more efficient for larger efficiency mostly operating larger brake mean effective pressures and faster warm up. The KERS may be designed as an electrical, mechanical, electro-mechanical, pneumatic and hydraulic device, greatly differing for complexity, weight, packaging, environmental and economic cost, round-trip efficiencies, charging and discharging rates, amount of energy stored and storage efficiency. The present paper discusses the state of the art of pneumatic and hydraulic kinetic energy recovery systems developed in the past for heavy duty commercial and military vehicle applications which are becoming popular also for passenger cars and light duty vehicle applications.
KEYWORDS – Kinetic energy recovery systems, driving cycles, hydraulic systems, pneumatic systems