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Written by Paul Weissler
Automotiveair conditioning accounts for just 5% of motoring fuel use, but its overallimpact on engineering the motor vehicle package has made it a continuing targetfor improvement. The electronic expansion valve (EXV) could be the newest addition. For many years the flooded evaporator/simpleorifice tube system, highly reliable and low in cost, dominated OE usage formetering liquid refrigerant flow into the under-dash evaporator, particularlyin the U.S. The move back tothermostatic expansion valves (TXV) to meter liquid refrigerant flow was a stepthat produced a small boost in efficiency. But a maker of both TXVs and EXVs, including EXVs for non-automotiveapplications, has demonstrated thereseemingly is a significant reduction in A/C compressor workload in going electronic. SanhuaGroup, 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 andfor incorporation into R-744 systems (carbon dioxide A/C) being engineered for production by a Germanmanufacturer (see: http://articles.sae.org/14766). Chinese car maker BYD uses one in its hybridmodel. Effect of IXH introduction Ina conventional TXV system, the TXV senses pressure/temperature at theevaporator outlet, and uses the amount of superheat (heat added to therefrigerant after it has vaporized) to determine the correct refrigerant flowat the evaporator inlet. The valve then opens or closes accordingly. With asimple low-pressure line from the evaporator outlet to the compressor inlet,that provides a reasonably accurate superheat reading downstream at thecompressor. However,the automotive A/C system underwent a significant change with the introductionof the IHX (Internal Heat Exchanger) in the last three years. Although developed to improve efficiency forR-1234yf systems, it also has been incorporated in continuing R-134a systemsand was identified in SAE Cooperative Research Group work as a potentially large aidto efficiency for R-744 systems. AnIHX in the refrigeration circuit makes a big difference in compressor inletsuperheat, as it"s cross-spliced into the liquid line (after the condenser) andthe low-pressure line downstream of the evaporator outlet. The IHX transfers heat from the warmrefrigerant in the liquid line to the cold vapor in the low-pressure line. The liquid is subcooled and the vapor isfurther superheated, so the vapor reading is higher than what wouldbe measured at the evaporator outlet. EXV "fixes" calibration issue Theelectronic expansion valve (EXV) "fixes" this problem with a remotepressure/temperature sensor at the compressor inlet, so the superheat-basedcalibration is downstream of the IHX effect and the opening of the valve therefore ismore precise. Howmeaningful 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 SAEJ2765, the Procedure for Measuring System COP (Coefficient of Performance) of aMobile A/C system on a Test Bench, with the lab fixture including an IHX. The overall result was a greater than 10% increasein COP average of test points along a matrix, a number that should translateinto a measurable improvement in fuel economy. TheTXV, as can be expected from a built-in bulb that senses evaporator outletrefrigerant, also is more subject to operating hysterisis vs. an electroniccontrol. And during transient conditionsit suffers more oscillation, therefore does not provide as good control, untilcompressor speed stabilizes. Becausethe EXV opening can be controlled even with the A/C off, the EXV also has thepotential to provide very slow metering of refrigerant into the evaporatorduring an idle stop. That can addseveral seconds to A/C-off cooling perhaps not enough in itself, but a usefuladdition in conjunction with systems with thermal storage evaporators, whichincorporate Phase Change Material cooling for the idle stop (see:http://articles.sae.org/11031). AnEXV can be duty-cycled, which augments control strategy. It also lends itself to more precisesplitting strategies for electric vehicles and plug-in hybrids with A/C-basedchillers for battery pack cooling. J2765 is a comprehensive standard and waswritten to test a system with a single evaporator, plus condenser, compressorand expansion device. It also can be used to cover a secondary coolingloop. And it even can evaluate the frontcooling module if air-side pressure drop for the entire system is factored in.
Date written: 25-Apr-2016 04:40 EDT
More of this article on the SAE International Website
ID: 3161
Automotiveair conditioning accounts for just 5% of motoring fuel use, but its overallimpact on engineering the motor vehicle package has made it a continuing targetfor improvement. The electronic expansion valve (EXV) could be the newest addition. For many years the flooded evaporator/simpleorifice tube system, highly reliable and low in cost, dominated OE usage formetering liquid refrigerant flow into the under-dash evaporator, particularlyin the U.S. The move back tothermostatic expansion valves (TXV) to meter liquid refrigerant flow was a stepthat produced a small boost in efficiency. But a maker of both TXVs and EXVs, including EXVs for non-automotiveapplications, has demonstrated thereseemingly is a significant reduction in A/C compressor workload in going electronic. SanhuaGroup, 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 andfor incorporation into R-744 systems (carbon dioxide A/C) being engineered for production by a Germanmanufacturer (see: http://articles.sae.org/14766). Chinese car maker BYD uses one in its hybridmodel. Effect of IXH introduction Ina conventional TXV system, the TXV senses pressure/temperature at theevaporator outlet, and uses the amount of superheat (heat added to therefrigerant after it has vaporized) to determine the correct refrigerant flowat the evaporator inlet. The valve then opens or closes accordingly. With asimple low-pressure line from the evaporator outlet to the compressor inlet,that provides a reasonably accurate superheat reading downstream at thecompressor. However,the automotive A/C system underwent a significant change with the introductionof the IHX (Internal Heat Exchanger) in the last three years. Although developed to improve efficiency forR-1234yf systems, it also has been incorporated in continuing R-134a systemsand was identified in SAE Cooperative Research Group work as a potentially large aidto efficiency for R-744 systems. AnIHX in the refrigeration circuit makes a big difference in compressor inletsuperheat, as it"s cross-spliced into the liquid line (after the condenser) andthe low-pressure line downstream of the evaporator outlet. The IHX transfers heat from the warmrefrigerant in the liquid line to the cold vapor in the low-pressure line. The liquid is subcooled and the vapor isfurther superheated, so the vapor reading is higher than what wouldbe measured at the evaporator outlet. EXV "fixes" calibration issue Theelectronic expansion valve (EXV) "fixes" this problem with a remotepressure/temperature sensor at the compressor inlet, so the superheat-basedcalibration is downstream of the IHX effect and the opening of the valve therefore ismore precise. Howmeaningful 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 SAEJ2765, the Procedure for Measuring System COP (Coefficient of Performance) of aMobile A/C system on a Test Bench, with the lab fixture including an IHX. The overall result was a greater than 10% increasein COP average of test points along a matrix, a number that should translateinto a measurable improvement in fuel economy. TheTXV, as can be expected from a built-in bulb that senses evaporator outletrefrigerant, also is more subject to operating hysterisis vs. an electroniccontrol. And during transient conditionsit suffers more oscillation, therefore does not provide as good control, untilcompressor speed stabilizes. Becausethe EXV opening can be controlled even with the A/C off, the EXV also has thepotential to provide very slow metering of refrigerant into the evaporatorduring an idle stop. That can addseveral seconds to A/C-off cooling perhaps not enough in itself, but a usefuladdition in conjunction with systems with thermal storage evaporators, whichincorporate Phase Change Material cooling for the idle stop (see:http://articles.sae.org/11031). AnEXV can be duty-cycled, which augments control strategy. It also lends itself to more precisesplitting strategies for electric vehicles and plug-in hybrids with A/C-basedchillers for battery pack cooling. J2765 is a comprehensive standard and waswritten to test a system with a single evaporator, plus condenser, compressorand expansion device. It also can be used to cover a secondary coolingloop. And it even can evaluate the frontcooling module if air-side pressure drop for the entire system is factored in.
Date written: 25-Apr-2016 04:40 EDT
More of this article on the SAE International Website
ID: 3161
