A study on compression braking as a means for brake energy recovery for pneumatic hybrid powertrains

Abstract

Hybrid powertrains have become a very attractive technology over the last ten years due to their potential for lowering fuel consumption and emissions. A large portion of all manufacturers worldwide are now focusing on hybrid electric powertrains, which are the most common hybrid powertrains of today. However, in recent years a new technology has emerged, namely the pneumatic hybrid powertrain. Although the concept is still only under research, it has been tested in various laboratories with promising results. The basic idea with pneumatic hybridisation is to take advantage of the otherwise lost energy when braking and convert it to potential energy in the form of compressed air by using the engine as a two-stroke compressor. The compressed air is stored in a pressure tank and when needed it is used to propel the vehicle through expansion in the engine cylinders. Previous studies have shown that more than 40% of the braking energy can be regenerated and used as useful work, leading to lower fuel consumption when compared to a conventional vehicle. The present paper deals with an in-depth investigation of the parameters influencing the performance of the compressor braking mode. Also, a control strategy has been adopted, that controls the pneumatic hybrid powertrain load during compressor mode (CM) at different tank pressures. The results show that the influence of parameters such as valve head diameter, tank valve opening and closing and inlet valve opening (IVO) have a considerable effect on the performance of the CM. Further, the results show that the chosen controller strategy performs well, with an ability to control the CM load almost on a cycle-to-cycle basis. The results shown in the present study are an important step in the development of the pneumatic hybrid powertrain.