Research On The Braking Energy Recovery Via Air Compression/Expansion Cycle Based On The Internal Combustion Engine

World vehicles increase continuously and internal combustion engines(ICE) will still be the major power units for decades, causing the massive consumption of fossil fuel and significant impact on the environment. Over half of the total fuel energy in ICE is dissipated through exhaust and coolant. In city driving cycles, considerable part of vehicle energy is consumed by frequent braking processes. It is of great significance to recover the waste energy in the vehicle and ICE.In this thesis, vehicle braking energy recovery via compressed air is studied both theoretically and technically. Based on the ICE, theories of compressed air recovery and compressed air utilization, hybrid system application for energy recycling are worked on. The main work and conclusion are as follows:1. Research of air compression cycle based on the modified ICE. Air compression cycles are discussed and two technical solutions based on the ICE are proposed differing by the recovering route of additional valve and exhaust valve. Both four-stroke cycle and two-stroke cycle of the air compression cycles are modeled on the basis of thermodynamics. Influences of equivalent compression ratio, tank pressure and valve opening angle on the cycle performance are analyzed. Under the utilization of variable valves, the improved air compression cycles and the two-stage compression are discussed.2. Numerical and experimental research of the air compression mode. Two air compression modes are numerically modeled and validated, including four-stroke compression mode recovering through additional valve and two-stroke compression mode recovering through exhaust valve, respectively. Results show that four-stroke compression mode has higher cycle COP than two-stroke compression mode but with lower specific power and lower specific recovered air mass. Two-stroke compression mode could provide more braking torque and is better than four-stroke compression mode.3. Analysis of energy recovering efficiency in the vehicle braking cycle based on the air compression mode. Vehicle braking cycle simulation with air compression mode is conducted, and the influences of braking cycle conditions and tank parameters on the energy recovering efficiency are discussed. Results show that for a typical passenger car, braking cycles with initial vehicle speed above 20 km/h and deceleration above 0.4 m/s2 have better braking energy proportion. In the braking cycles with deceleration between 0.6 m/s2-0.8 m/s2, the braking energy recovering efficiency is best. Tank volume and its initial conditions have significant impact on the energy recovering efficiency.4. Research of compressed air utilization based on the expansion cycle. Firstly in this part, compressed air expansion cycle is proposed and its performances are discussed based on the ideal thermodynamic model. Secondly, air expansion mode model is established and cycle performances with different charge valve parameters and intake chamber volumes are discussed. According to the finite recovered compressed air, feasibility of air assisted engine cranking and vehicle starting are analyzed. Results show that air assisted engine cranking is more feasible.5. Applied research of braking energy recovered compressed air. Combining the air compression cycle with air expansion cycle, pneumatic hybrid system with air assisted engine stop-start is proposed. A city bus is used as the research object, and the fuel consumption improvement is calculated in different driving cycles. Based on the pneumatic hybrid vehicle, compressed air in low pressure is used for air injection to the turbocharged ICE aiming to improve the transient performances. Results show that absorbed braking energy by air compression mode of ICE reaches 20.2%,26.8%,34.5%,29.9% of the total driving energy in NEDC, UDDS, JAPAN10-15, CBDC driving cycles, respectively. The improvement of fuel consumption reaches 5.5%,7.3%,9.4%,13.7%, respectively. Among the total fuel consumption improvement, the idling mode elimination during vehicle stop accounts for 3.2%,2.5%,5.5%, 7.1%, and the braking period fuel reduction accounts for 2.2%,4.8%,3.9%,6.6%. In the applied research of air injection for turbocharged ICE, engine transient performances at constant speed with load step and accelerating cases are both improved significantly.