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Braking energy recovery can significantly contribute to fuel economy and emission reduction, particularly for commercial vehicles driving in urban environment. By using the compressed air storage, rather than expensive and vulnerable batteries, this paper proposes a pneumatic hybrid system with an integrated compressor/expander unit (CEU) for commercial vehicles, in order to achieve stop/start function and braking energy recovery. During braking, the compressed air is recovered by CEU working in compressor mode and is charged to the air tanks. When the vehicle starts from stop, the CEU works as an expander to crank the engine with compressed air. The compressed air can also be used to supply the air tank of brake boost system, thus reducing its energy consumption. The mathematical models of energy conversion units, including the two modes of CEU and the air brake system, are established and analyzed.

Internal combustion engine is still expected to be the major power unit to propel vehicles for decades from now on. However, for normal driving conditions, more than half of the consumed fuel energy of engine is wasted, in the form of exhaust heat and coolant heat. In order to recover the waste heat generated in the thermodynamic cycle of internal combustion engine, a novel hybrid pneumatic engine concept is proposed. After combustion process, additional compressed air is injected into the cylinder to absorb the heat released by the fuel, and the expansion process of compressed air is optimized. The model of the hybrid pneumatic engine cycle is established and explored in GT-POWER, and it is then used to analyze the influences of the main design parameters on the cycle dynamic and economic performance. The preliminary simulation results show that engine power and economic performance is mainly related to the compressed air supply, the fuel mass and the engine speed.