Abstract With the continuous improvement of the road condition, commercial vehicles get to be faster and more overloaded than before, which puts higher pressure on the vehicle braking system. Conventional friction braking has been difficult to meet the needs of high-power commercial vehicle. The auxiliary braking equipment will become the future trend for commercial vehicle. Hydraulic retarder is superior to secondary braking equipment. Previously hydraulic retarder research mainly focus on flow field analysis, the braking torque calculation, cascade system optimization and control methods for hydraulic retarder. The gas-liquid two-phase flow in working chamber is less researched. Based on this, this article discusses on the hydraulic retarder from two aspects. Firstly, this paper presents a block modeling method for hydraulic retarder system. Systemic model includes a rotor and a stator, the working chamber, the oil inlet, heat exchanger, fuel tank, the float member and the intake valve mechanism. Using the interface technology connects all the parts together for the subsequent numerical calculations of the flow field. Secondly, Based on the judgment of hydraulic retarder two-phase pattern using Tatid flow theory, choosing VOF two-phase flow numerical model and applying transient sliding mesh technique to deal with the strong interaction between rotor and stator, this paper analyzes the gas-liquid flow pattern and volume changes during the oil-filling process. The results show that, in the process of filling liquid, gas-liquid two-phase flow pattern can be divided into three patterns: smooth stratified flow, wavy flow and dispersed bubble flow according to the working chamber oil accounts. Introduction With the continuous improvement of the road condition, commercial vehicles get to be faster and more overloaded than before, which puts higher pressure on the vehicle braking system. Conventional friction braking has been difficult to meet the needs of high-power commercial vehicle. The auxiliary braking equipment will become the future trend for commercial vehicle. There are mainly two kinds of retarder on the market: the eddy current retarder and the hydraulic retarder. The former is based on Faraday's electromagnetic induction principle and designed to take the heat away by the air, the latter uses oil as the working medium and the heat generated is taken away by the cooling water. Due to the EU ECER13 laws of the long-term braking for the commercial vehicles' travelling downhill, it requires that commercial vehicles be maintained at the constant speed of 30km/h for 6km in length and 7% of the downhill slope. The cooling capacity of the eddy current retarder is so weak that it is difficult to meet the regulatory requirements of long-acting braking, leading to its failure in the market and contributing to the rapid development of the hydraulic retarder whose cooling capacity is much better. For the current situation of foreign retarder research, the more we do is to understand through the related products and patented technology files. Currently, the three companies producing retarder are German VOITH, ZF and US Allision. In 1960, VOITH applied the world's first hydraulic retarder to the diesel trains' brake, and after this, optimized the design continuously and improved the braking performance, shown in Figure1. VOITH products are mainly mounted individually in a vehicle, and connect with the drive shaft directly or through a gear in parallel, shown in Figure2. Figure 1. The variation of the retarderStudy on Transient Oil-Filling Characteristics of Hydraulic Retarder based on Modular Modeling2015-01-1373 Published 04/14/2015 Yulong Lei, Hui Tang, and Xingjun Hu Jilin University ASCL Ge Lin China FAW Group Corpration Bin Song Hangzhou Advance Gearbox Group Co., Ltd CITATION: Lei, Y ., Tang, H., Hu, X., Lin, G. et al., "Study on Transient Oil-Filling Characteristics of Hydraulic Retarder based