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This paper addresses R1234yf A/C system performance impacted by condenser airflow passage blockages of nonhotspot and hotspot objects. With the modern vehicle design trend, more and more chances exist in blocking condenser airflow passages by objects such as TOC (transmission oil cooler) or fine grills etc. These objects create hotspots and narrowed airflow passages to the condenser and result in A/C performance degradation. It is important to understand the specific area of the condenser which is most impacted by a blockage so this area can be avoided in the design/packaging of front end components. In addition, it is important to understand the magnitude of performance loss associated with the specific areas of blockage. As a result of this understanding, optimal design locations for these blockages (including hotspots and grilles) can be proposed in order to mitigate the impact on A/C cooling performance.

This paper presents the way to optimize vehicle AC system TXV to meet the various AC system requirements. It discusses vehicle AC system TXV sizing and selection process. In today's automotive industry, sizing and selecting the TXV is more complicated than before as various new components are introduced such as external control compressor, internal control compressors and internal heat exchanger etc. These components complicated the system interaction among the components. Thus it requires mapping TXV characteristic to meet the system demand. Sizing TXV capacity, it must start with the vehicle heat load requirement. The type of TXV (i.e. cross charge or parallel charge head) is determined by the system configuration such as compressor, evaporator, and condenser type and with or without internal heat exchanger, etc. To optimize TXV in the system involves in evaluating TXV characteristic and cooling capacity in the various AC operating conditions.

This paper addresses various ways to determine vehicle dual AC system charge level. Traditionally, either checking charge level plateau and/or using the certain condenser outlet subcooling magnitude are adopted to determine AC system charge level. It is challenging to determine refrigerant charge level in the following scenarios: (1) Some AC systems do not exhibit the flatted charge plateau. (2) The condenser outlet subcooling continues to rise. (3) The system has the requirements to run both front and aux evaporators, front evaporator only and aux evaporator only. It was found that compressor compression ratio of absolute discharge pressure to absolute suction pressure always presents the bath tub curve for all AC systems. When the system reaches the optimal charge level, the evaporator air outlet temperatures show the stable trend. In addition to the traditional condenser subcooling method, few approaches are presented in the paper.

The majority of vehicle AC system warranty costs are a result of compressor replacement caused by excessive wear and seizure-related failures. In today's environment, compressor manufacturers can control manufacturing process well and maintain a stable product quality. Thus, compressor durability heavily relies on a durable AC system design. Both vehicle compressor suppliers have a variety of procedures and test methods to evaluate AC system and compressor durability. Often times, we still see very different compressor warranty return rates (one higher, the other lower) for the same compressor from the same production line in similar vehicle AC systems. In many cases, both AC systems passed vehicle and component durability tests. In addition, compressor manufacturing process quality was controlled well. The question remains why is there such different compressor warranty return rates?

The paper addresses compressor body temperature (crankcase) importance to the vehicle AC system long-term durability. Majority of OEM vehicle test evaluation is to see if AC system can pass compressor discharge temperature and discharge pressure targets. Most OEMs adopt 130°C max compressor discharge temperature and 2350 kpag head pressure as the target. From the field, although some of the compressor failure results from a high compression ratio, and compressor discharge temperature that are caused by the poor front end airflow, etc., high percentage compressor failed systems exhibit not too high compression ratio and compressor discharge temperature, but having the trace of high temperature in the shaft area, gasket area, etc. With introducing more and more variable swash plate compressor applications, OEMs start to see more and more compressor failures that are not related to a high compressor discharge temperature but the trace of high compressor body temperature.

The first challenge to properly size a vehicle A/C system is to define the vehicle air conditioning heat load requirement. Within automotive industry, a model to accurately define vehicle heat load is still under development. In this study, a simple method to calculate vehicle heat load is developed. The cooling load temperature differential (CLTD) method[1] is used to calculate the heat gain of a sunlit roof and wall (door). This is done in one step by using ASHRAE data. The calculation presented here takes into account the geometrical configuration of the vehicle compartment including glazing surfaces (shading), windshield and roof angle, and vehicle orientation, Special attention is given to the calculation of direct and diffuse incidence solar radiation through the windshield and skylight glass. The vertical glass' solar heat gain is evaluated by using ASHRAE[1] data. The U value method is used to calculate heat transfer between the outside and inside cabin.

Oil in circulation (OIC) in a vehicle AC system is an index to demonstrate the system having a proper oil charge. A well lubricated compressor is key for AC system, as it relates to compressor durability. The challenge that climate control development engineering faces is how to correlate OIC from test to test (test stand to vehicle level etc.). In this study, a method to correlate vehicle AC system OIC from test-to-test, stand to vehicle is developed. The study found that oil charge amount ratio, refrigerant mass flow ratio, and vehicle engine speed ratio are the key factors to correlate the test results such as stand versus vehicle, vehicle test A versus test B etc. To further reflect compressor oil lubrication conditions, compressor compression index (polytropic number) is computed for the different testing, and the ratio of compression index is also introduced to the correlation. The method accuracy was validated by multiple vehicle/stand tests.