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As a potential replacement to traditional automotive R134a direct expansion (DX) systems, a secondary-loop system allows for the usage of flammable but low-GWP refrigerants such as propane (R290). However, as the secondary-loop system has an additional layer of thermal resistance, the cycle's transient behavior and cabin thermal comfort during pull-down and various driving cycles may be different from traditional DX systems. This paper presents a Modelica-based model to simulate both steady-state and transient operation of automotive secondary-loop systems. The model includes a lumped cabin component and a secondary-loop automotive air-conditioning system component. The air-conditioning system component consists of a condenser, a compressor, an expansion device, a coolant plate type heat exchanger, a coolant to air heat exchanger and a coolant pump. The developed model was validated against both steady-state and transient experimental data for an R290 secondary-loop system.

In this paper, the application of the separate sensible and latent cooling (SSLC) technology to the mobile air conditioning (MAC) system was investigated. Conventional MAC systems utilize a low evaporating temperature to cool down the cabin air temperature and to remove moisture from humid air. In order to remove the moisture, the supply air temperature has to be below the dew point temperature of the cabin air. Therefore, a reheating process is necessary to increase the air temperature to an appropriate and comfortable level. However, energy is wasted in this reheating process, which results in the reduction of the fuel efficiency. Since the SSLC technology can provide an appropriate solution to these issues of conventional systems, it is proposed to apply the SSLC technology to the MAC system, which can eventually reduce the fuel consumption of the MAC system.

Due to the influence of energy use on electric vehicle range, latent energy storage options could be used to increase thermal comfort and decrease energy consumption during driving. This study focuses on the implications of thermal storage on transient performance of a typical secondary loop system and a combined secondary loop with ice storage system. The use of ice storage in assisting the vapor compression cycle during cabin pull-down and continued cooling, as well as cooling during compressor off periods was experimentally investigated. It was found that the ice storage system was able to decrease energy consumption during pull-down by about 20% and decrease time to comfort by about 15% compared to a regular secondary loop system.

The thermal comfort inside automotive cabin has been extensively studied for decades. Traditional CFD models provide accurate simulation results of the air temperature distributions inside cabins but at a relatively high computation cost. In order to reduce the computational cost while still providing reasonable accuracy in simulating the air temperature profile inside a mid-sized sedan cabin, this paper introduces a new simulation tool that utilizes a proper orthogonal decomposition (POD) method. The POD method, an interpolation technique, requires only one set of multiple CFD simulations to produce a set of “snapshots”. Later, any simulations that require CFD runs to solve algorithm equation sets can be simplified by using interpolation between the snapshots provided that the geometry of the cabin keeps the same. As a result, the computation time can be reduced to only a few minutes.

Complex engineering design optimization often requires multiple executions of computationally expensive simulation tools such as those based on Computational Fluid Dynamics (CFD). Some CFD simulations can take several hours to complete, thus potentially making the design optimization task infeasible. In this paper, a combination of two powerful methodologies is presented that has the potential of reducing the engineering time required for CFD based design by more than 90%. The first methodology, termed as Parallel Parameterized CFD (PPCFD) allows for speeding up multiple CFD runs to explore a given design space very efficiently. The second approach is Approximation Assisted Optimization (AAO). AAO techniques are used to reduce the time and effort involved in conducting optimization with computationally expensive simulations. The PPCFD methodology needs to be tailored or customized for an individual geometry of interest.

Automotive climate control systems are typically equipped with either an orifice tube or a thermostatic expansion valve. The two devices behave differently especially during cycling operation. The variable restriction of the thermostatic expansion valve delays the refrigerant migration when the clutch is disengaged and allows a faster redistribution when the clutch is engaged. The effect of cycling on the performance of two climate control systems, one with a short-tube orifice, and the other with a thermostatic expansion device, was investigated. The cycle period was varied from 10 seconds to 6 minutes. The test results show the change in moisture removal rate, latent capacity, sensible capacity, energy consumption, and coefficient of performance due to cycling. It is shown that the penalty in energy consumption due to cycling depends on the cycle period.

Computer simulation tools for modeling heat exchanger performance can be effective aids in the design, and optimization of heat exchangers. This paper presents a model that has been developed to simulate the performance of flat tube, louvered fin heat exchangers of the type used for automotive applications such as radiators and charge air coolers. Two different types of fin can be modeled-louvered plate fins and louvered corrugated fins. A segment-by-segment modeling approach is employed in which each tube is divided into multiple segments in order to account for heterogeneous fluid flow and to allow for two-dimensional air maldistribution. The energy transfer and the hydraulic equations are solved for each segment. A variety of working fluids such as air, water-glycol, and most refrigerants can be modeled. Multiple correlations are available to model heat transfer coefficients and friction factors for the fluid inside the tubes as well as for the air side.

In measuring the dynamic behavior of automotive climate control systems, the vehicle is placed in a full-scale environmentally controlled wind tunnel. This conventional test method demands considerable effort and cost. To improve the shortcomings of the conventional method, a new technique that requires only the tested climate control system and a simplified facility is proposed. The technique consists of a control program, a numerical cabin model, a small size environmentally controlled closed air loop, its controller, and the climate control system. The cabin model is a transient set of linear ordinary differential equations, which account for the latent and sensible loads and the infiltration and ventilation loads. The solution is obtained numerically and the control program uses the model to predict the conditions of the air inside the cabin of the car and adjusts the set points of the controller of the air loop accordingly.

This paper presents the oil circulation behavior in a CO2 climate control system operating at low evaporating temperature down to -32°C. The increase of oil circulation ratio (OCR) from 0 to 6 wt.% during steady state conditions degrades the coefficient of performance and cooling capacity by 15% and 8%, respectively. The pressure drop across the heat exchangers increases, especially in the gas cooler. In low temperature CO2 systems some fluctuations of oil and refrigerant flow rates were observed during cyclic operations when the system did not equip the oil separator, but was observed only at high oil charge when the system did equip the oil separator. These instabilities lead to a periodic compressor performance fluctuation, which caused system performance degradations. Therefore, the use of an oil separator is recommended for the low temperature operation if an ordinary metering valve is adopted as an expansion device without any special control strategy.

A suction line heat exchanger (SLHX) transfers heat from the condenser outlet to the suction gas. In a TXV (thermostatic expansion valve) system, the performance improvement with a 60 to 80 % effective SLHX is expected to be on the order of 8 to 10 % for capacity, and 5 to 7 % for COP for high outdoor air temperatures of 43ºC. In a FOT (fixed orifice tube) system, the performance improvement was calculated to be about 10 to 15 %. The calculated improvements have been verified experimentally within a few percent.