all-year simulation, capacity control, bus HVAC, R-744, continuously variable transmission
The reheating of cooled air is the state-of-the-art for controlling the air-side cooling capacity in conventional omnibus HVAC systems. This method is pragmatic, albeit highly energy-inefficient. This paper deals with the structured analysis and improvement of capacity regulation for a conventional system using the example of a coach air conditioning system. Piston compressors are typically used in the air conditioning systems for conventional buses. This compressor type belongs to the group of reciprocating compressors and is a common standard in the constant displacement version for large refrigerant systems in the omnibus sector. The compressor is usually driven directly by the internal combustion engine, which is typically realized by using a V-belt and magnetic clutch. Due to the mechanical connection to the engine and the constant compressor displacement, different control strategies are necessary to realize the required variable cooling capacity. These strategies may influence each other resulting in operating conditions with varying degrees of efficiency depending on the application and refrigerant choice. This leads to the main objectives in this paper: Identifying energy-efficient methods of adapting the capacity of reciprocating piston compressors with a constant displacement volume for air conditioning systems in conventional coaches and identifying energy-saving potentials for different refrigerants and application scenarios. For this purpose, the techniques currently being used in series production for implementing capacity adaption in omnibus air conditioning systems were initially described and the procedures such as cycling-clutch operation and cylinder (bank) shutdown were explained. Subsequently, unestablished and novel methods like speed control by means of pulley transmission and continuously variable transmission (CVT), were presented. Systems were identified using a combination of established and newer methods, and the shift in performance, performance control potentials and the impact on total energy consumption were analyzed. For this purpose, a complete physical vehicle model of a coach was developed and validated. The overall model was split into the following subsystems: driving and environmental conditions as boundary conditions, driving dynamics, bus interior, refrigeration cycle, climate controller, engine cooling and heating cycle and electrical system. Special emphasis was placed on the detailed model of the air conditioning system. Monthly representative simulations for two different refrigerants (R‑134a, R‑744) and three climatically different route and journey scenarios (Germany, Portugal/Spain and India) were analysed and compared with a reference system for a conventional coach. Maximum and average annual fuel saving potentials were identified for various methods of adapting the compressor's transport capacity and identifying the most efficient methods for continuous capacity control in omnibus air conditioning systems. Depending on the application and the refrigerant used, the saving potentials in primary energy were between 2.5% and 6.8% based on one year.