Rapid Start of Hybrid Pneumatic Engines

Downsized and supercharged hybrid pneumatic vehicles may prove to be a viable and cost-efficient alternative to hybrid electric vehicles since they offer an almost as high fuel saving potential. One part of the fuel saving potential for this concept originates from the start/stop capability. This paper shows that the pneumatic motor mode can be used to start the engine rapidly enough to justify the use of a start/stop functionality. The paper presents fundamental considerations that have to be made for various possible hardware structures. A physics based model is introduced that describes the pneumatic start dynamics for any number of cylinders and any kind of valve profiles. Based on the validated model, the pneumatic engine start has been optimized and the optimal actuations have been implemented at the testbench for the engine under consideration. The experiments were carried out on a downsized and supercharged hybrid pneumatic prototype engine with two cylinders. It was shown that the engine's idling speed (1200 rpm) can be reached within 310 ms for a tank pressure of 10 bar.