The principle of energy-saving in hydraulic hybrid vehicles was introduced. The characteristics of hybrid vehicles with electric or hydraulic were compared. Because of its higher power density and higher efficiency, hydraulic hybrid vehicle is more advantageous when working under the condition changed frequently such as city bus. Different arrangement forms of the hydraulic hybrid system were analyzed to explore their own characteristics and applications. Based on the batteries with high energy density and hydraulic accumulators with high power density, a kind of combination that include their respective advantage were proposed. Keywords-Hydraulic hybrid vehicle; Energysaving ; power density; arrangement form; Status and development trend I. 2005 EPA Eaton UPS(United Parcel Service) 60 70 CO 40 [1] II.A.

Natural gas is a relatively ideal clean alternative energy, but the higher ignition temperature, the slower flame propagation speed and the smaller coefficient of molecular after the combustion all result in lower thermal efficiency. Miller cycle is based on changing the intake valve closing time to achieve high expansion ratio than compression ratio, therefore it can improve the thermal efficiency to some extent. In this paper, the engine performance of applying the Miller cycle on a nature gas engine is studied using GT-Power, and optimized for different levels of compression ratio and for different engine spend. Simulation results indicated that, after applying the Miller cycle engine full load power and fuel combustion rate get worse at low engine speed but improved at high speeds. Also the compression pressure and temperature are lower, which reduces engine knocking tendency and formation rate of harmful exhaust gas emissions. The best intake valve timing for different engine speed and loads needs careful calibration.

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