Auto A/C expansion valves move to electronic control

  • 25-Apr-2016 04:40 EDT

One comparison of TXV and EXV oscillations in transient conditions on an IHX system. Notice how EXV superheat reading at compressor quickly steadies, vs. TXV. 

Automotive air conditioning accounts for just 5% of motoring fuel use, but its overall impact on engineering the motor vehicle package has made it a continuing target for improvement. The electronic expansion valve (EXV) could be the newest addition. 

For many years the flooded evaporator/simple orifice tube system, highly reliable and low in cost, dominated OE usage for metering liquid refrigerant flow into the under-dash evaporator, particularly in the U.S. The move back to thermostatic expansion valves (TXV) to meter liquid refrigerant flow was a step that produced a small boost in efficiency. But a maker of both TXVs and EXVs, including EXVs for non-automotive applications, has demonstrated there seemingly is a significant reduction in A/C compressor workload in going electronic.

Sanhua Group, a manufacturer of household and automotive air conditioning components, began auto A/C production of EXVs in mid-2015. The company is supplying them to an unnamed U.S. maker of electric vehicles and for incorporation into R-744 systems (carbon dioxide A/C) being engineered for production by a German manufacturer (see: Chinese car maker BYD uses one in its hybrid model.

Effect of IHX introduction

In a conventional TXV system, the TXV senses pressure/temperature at the evaporator outlet, and uses the amount of superheat (heat added to the refrigerant after it has vaporized) to determine the correct refrigerant flow at the evaporator inlet. The valve then opens or closes accordingly. With a simple low-pressure line from the evaporator outlet to the compressor inlet, that provides a reasonably accurate superheat reading downstream at the compressor.

However, the automotive A/C system underwent a significant change with the introduction of the IHX (Internal Heat Exchanger) in the last three years. Although developed to improve efficiency for R-1234yf systems, it also has been incorporated in continuing R-134a systems and was identified in SAE Cooperative Research Group work as a potentially large aid to efficiency for R-744 systems.

An IHX in the refrigeration circuit makes a big difference in compressor inlet superheat, as it's cross-spliced into the liquid line (after the condenser) and the low-pressure line downstream of the evaporator outlet. The IHX transfers heat from the warm refrigerant in the liquid line to the cold vapor in the low-pressure line. The liquid is subcooled and the vapor is further superheated, so the vapor reading is higher than what would be measured at the evaporator outlet.

EXV "fixes" calibration issue

The electronic expansion valve (EXV) "fixes" this problem with a remote pressure/temperature sensor at the compressor inlet, so the superheat-based calibration is downstream of the IHX effect and the opening of the valve therefore is more precise.

How meaningful is this in reducing compressor workload in the real world? Sanhua Group engineers said they had comparison-tested the EXV vs. a TXV in a laboratory per SAE J2765, the Procedure for Measuring System COP (Coefficient of Performance) of a Mobile A/C system on a Test Bench, with the lab fixture including an IHX. The overall result was a greater than 10% increase in COP average of test points along a matrix, a number that should translate into a measurable improvement in fuel economy.

The TXV, as can be expected from a built-in bulb that senses evaporator outlet refrigerant, also is more subject to operating hysterisis vs. an electronic control. And during transient conditions it suffers more oscillation, therefore does not provide as good control, until compressor speed stabilizes.

Because the EXV opening can be controlled even with the A/C off, the EXV also has the potential to provide very slow metering of refrigerant into the evaporator during an idle stop. That can add several seconds to A/C-off cooling—perhaps not enough in itself, but a useful addition in conjunction with systems with thermal storage evaporators, which incorporate Phase Change Material cooling for the idle stop (see:

An EXV can be duty-cycled, which augments control strategy. It also lends itself to more precise splitting strategies for electric vehicles and plug-in hybrids with A/C-based chillers for battery pack cooling.

J2765 is a comprehensive standard and was written to test a system with a single evaporator, plus condenser, compressor and expansion device. It also can be used to cover a secondary cooling loop. And it even can evaluate the front cooling module if air-side pressure drop for the entire system is factored in.

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