Yu, Yingzhong (2008) Generic approach of refrigerant HFC-134a emission modes from mobile air conditioning (MAC) systems. PhD thesis Energétique, CEP- Centre Energétique et Procédés, ENSMP p.190.

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Abstract

According to the EU regulation 706/2007, a Mobile Air Conditioning (MAC) system using HFC-134a must guarantee its initial leak tightness and the leak flow rate of the MAC system must be tested based on a qualified method. The leakage behavior of MAC systems using HFC-134a is still not fully understood. This research work aims at establishing a test method of leak flow rate measurement for MAC systems and components, and furthermore to give a generic approach of refrigerant emissions in MAC systems. In this thesis, main types of emissions from the MAC system during the life cycle of the vehicle are classified. Regular leakages due to different MAC components are evaluated by explaining their design and sealing principles. A laboratory test method, based on measurement of concentration in an accumulation volume, is introduced for determining the leak flow rate of MAC systems and components. Measurement accuracy is justified. Measurements in standstill mode and at several controlled temperatures are performed to compare the overall system leak flow rate and the sum of leak flow rates of all components. Simulation of temperature variation allows predicting the annual climate condition impacts for any climates. Running mode tests are also discussed to study the contribution of the system running time to the annual MAC system emissions. Based on results of laboratory and fleet tests, a correlation factor is established for MAC systems between real life emissions and laboratory tests. Permeation process of refrigerant through MAC hoses is studied. Both temperature and pressure influences on permeability are analyzed based on experimental data in order to establish the relationship between permeability, temperature, and pressure. Prediction models have been developed to estimate the leak flow rate of hoses taking into consideration the T-P double effects. Typical O-ring seals are discussed and two leakage modes: permeation through sealing materials and gas flow through micro channels are distinguished. Sealing performance of a radial O-ring is studied in detail. With the help of Finite Elements Method, non-linear stress-strain behavior of polymers is taken into account. Main factors such as stress, maximum contact pressure, and contact width are analyzed based on numerical simulation results. Establishing the leakage behavior combining two leakage modes: permeation and leak through micro channels allows understanding the key points of emission dependence and improving the sealing performances. In short, the leak flow rates of a MAC system is the sum of leak flow rates of all leak sources. Those sources are of two types: permeation through polymers and flows through micro channels existing between seals and metallic parts of any fittings or seals. For each emission mode, behavior laws have been established and prediction models validated allowing forecasting emission rates with a limiter number of measurements.

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