Development of an Air-Cycle Environmental Control System for Automotive Applications

Abstract

The Reverse-Brayton cycle has been used for aircraft cabin cooling for many decades. However, air-cycle cooling hasn’t been popular in the automotive field yet. This study demonstrates that air-cycle technology can provide sufficient cooling for certain applications. The primary focus is a novel forced induction engine control system, where compressor bleed is used both to provide engine boost control and air-conditioning. The bleed-air drives an air-cycle machine (ACM) consisting of typical automotive components: a turbocharger, heat exchanger, and ducting. The components of an ACM system are lightweight and compact compared to those of a typical vapor compression system; both qualities are critical in high performance applications, where such a system seems to make most sense. The ACM was tested first on a test stand and then directly on an engine, in a bootstrap-cycle configuration. The turbocharged test engine’s intake manifold pressure was controlled by bleeding air from the outlet of the engine’s intercooler and feeding the ACM compressor inlet. Once the compressed air was supplied to the ACM it was further compressed by the ACM, cooled by the secondary intercooler, and expanded through the ACM turbine. The engine’s turbocharger was resized to compensate for the increased air flow during ACM operation. The results show that a dry-air-rated (DAR) coefficient of performance (COP) of 0.73 and a DAR cooling capacity of 1.5 tons are possible on a test stand, and a DAR COP of 0.56 and a DAR cooling capacity of 0.72 tons are possible on-engine. The data available from the on-engine testing was limited to lower ACM pressure ratios due to a bearing failure before full testing was complete; performance would likely increase with higher inlet pressures, as shown by the compressed air test stand results. The test results strongly suggest that continued development and in-vehicle testing will provide adequate air-conditioning and engine performance, using only the most benign and environmentally friendly working fluid: air.