Energy-Efficient Air Conditioning Systems Utilizing Pneumatic Variable Compressors

Transcription

1 Energy-Efficient Air Conditioning Systems Utilizing Pneumatic Variable Compressors Mingyu Wang, Mark J. Zima and Prasad S. Kadle Delphi Corporation Copyright 2009 SAE International ABSTRACT Air Conditioning systems with reheat reduction based for energy efficiency have generally been implemented with either electronic variable compressors through active stroke control or with fixed placement compressors through modifying the cycling set point. The present work demonstrates a unique concept of achieving energy efficiency via cycling a pneumatic variable compressor at elevated set points. The energy efficiency of such a system approaches that of an electronic variable but significantly higher than that of a fixed placement compressor system. The cost of the system, on the other hand, is substantially lower than that of an electronic compressor. Secondary benefits include a softer start than with a fixed compressor and a considerably simpler control scheme than that required by an electronic variable compressor. INTRODUCTION As a capacity control method, cycling of fixed placement compressors has been used since the beginning of mobile air conditioning systems. Under moderate or low thermal load conditions, excess capacity of the air conditioning system designed for soak and cool-down now poses a problem of excess: the air cooled by the evaporator of the air conditioning system is so low in temperature that the condensate from the air stream becomes iced and blocks further airflow to be charged to the passenger compartment. The preferred method for controlling the excess capacity is to turn the compressor momentarily off by using a switch which is installed in the refrigerant line and sensitive to the low side refrigerant pressure of the AC system. When the air temperature is close to freezing and the low side refrigerant pressure decreases to a trigger point, the switch connects the power supply to the compressor clutch to stop the compressor. When the air stream temperature subsequently increases and the refrigerant pressure rises to a second pressure setting, the compressor switch reconnects the clutch to a power supply and the compressor resumes operation. This method of operation continues to be used today due to the simplicity and low cost. With the introduction of the microprocessor into the automotive industry, capacity control for the fixed placement compressor gained a more precise and direct method. The temperature of the exit air from the evaporator is measured with a thermistor directly and communicated to a microprocessor either in the engine control module or in the air conditioning control head. The microprocessor compares the measured temperature with a pre-set control band, similar to a thermostat in the home air conditioning system, and makes the decision whether to turn on or off the compressor. For this compressor cycling scheme, typically there is a temperature set point and a hysteresis band. The temperature set point determines the level of temperature control, which is typically 1 to 2 degrees Celsius above the water freeze point. The hysteresis band typically is 1.5 degrees Celsius, and is designed to limit the frequency of the compressor cycling and the temperature variations at the charge outlets. The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE s peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE. ISSN Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper. SAE Customer Service: Tel: (inside USA and Canada) Tel: (outside USA) Fax: CustomerService@sae.org SAE Web Address: * * Printed in USA

2 The concepts of set point and hysteresis band also exist in the cycling pressure switch in the form of a low pressure set point for compressor engagement and a second, higher pressure set point for compressor reengagement. The difference between the two pressure set points is equivalent to the hysteresis band in the thermistor control scheme. The considerations for designing the hysteresis band include the protection of the compressor clutch and the thermal comfort variation as perceived by the passengers. Too narrow of a hysteresis band causes the compressor to cycle at a higher frequency which may cause the clutch to wear out prematurely. Too wide a hysteresis band can lead to excessive temperature variations at the charge outlets which can be an annoyance to the passengers in the car. Recently, the cycling of the compressor gained a new application beyond the simple capacity control for freeze protection. With rising gasoline price and increased environmental concerns, more energy efficient air conditioning systems are being demanded by the vehicle manufacturers to help increase the overall fuel economy of vehicles. One popular method of increasing the energy efficiency of an AC system is to reduce the reheating of cooled air to a higher temperature to meet the charge temperature requirement in the passenger compartment, either directly commanded by the passenger, which is the case with manually controlled air conditioning systems, or indirectly through the use of an automatic climate system. Air Evaporator Air Out Breath Reheat Discharge Target protection. As a result, there are two fixed air temperature levels available to an air conditioning system: one temperature is provided by the refrigeration system which is maintained at slightly above the water freeze point and the second temperature is provided by the engine coolant. By taking a portion of the cooled air from the evaporator of the refrigeration system and feeding it through the heater supplied with engine coolant, the air temperature can be increased and subsequently remixed with the remaining stream of air from the evaporator to achieve any temperature bracketed by the water freeze temperature and the engine coolant temperature. The proportioning of air through the heater is accomplished by a device in the HVAC module known as the mix door. This method of temperature control through the mixing of cooled air and reheated air provides a system that is precise and responsive. The inefficiency of such a system in energy use is only highlighted recently as energy conservation and environmental protection became a new priority. To reduce the reheat and, therefore, compressor energy consumption, the compressor is cycled to an elevated set point above that of the water freeze point. The general principle is to cool the air only as low as needed to meet the charge temperature requirement. If the charge temperature target is 10 degrees Celsius, there is no need to cool the air down to 2 degrees Celsius only to reheat it back up to 10 degrees Celsius. The compressor can be operated to the elevated temperature set point of 10 degrees Celsius with the air directly charged to the passenger compartment without reheating. Of course, there is an upper limit as to how high one can raise the set point. Considerations must be given to humidity comfort in the passenger compartment and fogging risk at the windshield. A fixed level of dehumidification must be maintained to provide a comfortable and safe driving environment in the vehicle. Typically, energy efficiency AC systems limit the elevated temperature set point to around 10 degrees Celsius. Time Set Point for Freeze Protection Fig. 1 Fixed Displacement Compressor Cycling The reheat of air conditioned air to a higher charge temperature (Figure 1) is typical in the automotive industry and has a long history - practically since the introduction of the air conditioning systems into automobiles. The main reason is that advanced compressor control method was not available beyond the simple pressure switch cycling control for freeze An alternative to cycling the fixed placement compressors at an elevated set point for reheat reduction is to use the more sophisticated electronic variable compressors, whose control valve is capable of operating at any set point through varying the command current to the valve. Just as before, the compressor operates at a reduced placement to meet an elevated temperature set point that is determined by the charge temperature target, humidity comfort, and fogging risk at the windshield.

3 Air Breath Compressor Control Set Point Is Equal to Discharge Target Discharge Target Freeze Control Set Point Time Fig. 2 Series Reheat Reduction with Electronic Variable Compressors Figure 2 shows a typical charge temperature history when the reheat reduction is achieved with an electronic variable compressor. As the charge temperature change is achieved through active placement control without cycling the compressor, the charge temperature is tinctly smooth and lack of variation. CYCLING OF PNEUMATIC VARIABLE COMPRESSOR FOR SERIES REHEAT REDUCTION The pneumatic variable compressors (Figure 3) are designed to control the capacity of the compressor through varying the stroke of the compressor pistons [1]. Within a pneumatic variable compressor, the case pressure is varied between the suction pressure and the charge pressure of the compressor by way of a 3-port valve. The case pressure works against the charge pressure to modulate the stroke of the piston, thereby controlling the capacity of the compressor. Through the regulation of the compressor placement, the suction pressure of the compressor is controlled to a near constant under various operating conditions which keeps the condensate on the evaporator from freezing. Fig. 3 Pneumatic Variable Displacement Compressor Air Evaporator Air Out Time Breath Reheat Discharge Target Set Point for Freeze Protection Fig. 4 Discharge by Mixing With Pneumatic Variable Compressor under Freeze Control By design, a pneumatic variable compressor intrinsically controls to prevent evaporator freeze. The air temperature coming out of the evaporator is maintained slightly above the freeze point of water. The charge temperature control is achieved using the same method of series reheating and mix door operation. Figure 4 shows a typical temperature control profile for an air conditioning system with a pneumatic compressor. The pneumatic variable compressor is effective in that it eliminates the needs to cycle the compressor in order to prevent evaporator freeze. Higher energy efficiency than cycling fixed placement compressor is achieved due to the steadier and continuous operation of the compressor [2]. However, under the new context of energy efficiency, the pneumatic compressor s internal control is incapable of regulating to a different control set point to allow series reheat reduction. The additional system efficiency associated with series reheat reduction appears to be out of reach for pneumatic variable compressors.

4 It is in this context that the Cycling Pneumatic Compressor concept was proposed. With this unconventional and counter-intuitive method of operating the pneumatic variable compressors [1, 2], higher energy efficiency AC operation can be achieved in several regards. First of all, for AC operations requiring freeze point control, either due to defogging or defrosting, continuous operation of the pneumatic compressor can be maintained under reduced compressor placement, which delivers higher energy efficiency than the cycling fixed placement compressors. As the charge temperature target increases for reheat reduction, the compressor will still operate continuously for certain ambient conditions due to the control properties of the pneumatic valve. For other ambient conditions, it is possible that a combination of stroke reduction and lower frequency cycling work together to provide the elevated charge temperature. It is commonly understood that cycling reduces the COP of the AC system [2, 3, 4, 5]. Consequently, higher energy efficiency operation is achieved than cycling the fixed placement compressor alone. Not intrinsic to cycling the pneumatic compressor, the cycling characteristics can be managed to decrease the cycling frequency so as not to affect compressor reliability on the one hand, and to achieve higher energy efficiency on the other. CYCLING MANAGEMENT FOR THE PNEUMATIC VARIABLE COMPRESSORS Whereas it is straight forward to cycle the pneumatic variable compressor for reheat reduction and higher energy efficiency, the management of the cycling characteristics to achieve enhanced energy efficiency and compressor reliability is a bit more intricate. Herein we present a method that is based on understanding the human comfort impact of the charge temperature variations and the physical mechanism of the charge temperature variations. Figure 5 shows the working of the HVAC module. The blower draws either fresh air from the outside of the vehicle or recirculated air from the inside of the passenger compartment depending on the position of the air inlet door. Regardless of the source, the blower sends the air through the evaporator of the refrigeration system to be cooled to a lower temperature, typically slightly above the freeze point of water for basic systems, or elevated temperature for energy efficient systems. The air coming out of the evaporator is split into two streams. One part of the airflow goes through a heater where it is warmed up by the engine coolant. The second part bypasses the heater. After the heater, the two streams of air are combined in a mixing chamber ready to charge into the passenger compartment to maintain comfort. OSA AI Door Recirc v Blower Evaporator Fig. 5 Control in HVAC Module HEATER OUTFLOW AIR TEMPERATURE CALCULATION Temp Door Heater Assuming that the coolant temperature is known through the vehicle communication bus, and the evaporator air outflow temperature is measured by a thermistor, the outflow air temperature from the heater can be determined by the heater effectiveness map. The heater effectiveness is the ratio of the air temperature rise through the heater over the maximum temperature difference available to the heater, as given in Equation (1): T = T htr evp ε (1) Tclt Tevp For a particular heater, the effectiveness is a function of the air flow rate and the coolant flow rate. Equation (2) describes this relationship: (&, & ) ε = F m air m (2) clt From Equations (1) and (2), the outflow air temperature from the heater can be determined by way of equation (3), htr ( Tclt Tevp ) Tevp T = ε + (3) BULK DISCHARGE TEMPERATURE The bulk charge temperature from the HVAC module can be determined by mixing the two streams of air, that which goes through the heater and that which bypasses the heater. The percentage of the heater airflow is designated by a fractional flow function f ( ω), where ω is the mixing door position spanning the range of Full Cold (0%) to Full Hot (100%). The formula for

5 calculating the bulk charge temperature is given in Equation (4), T [ f ( ω )] T evp + f ( ω) T htr = 1 (4) Simplification of Equation (4) gives, T [ f ( ω ) ε ] Tevp + f ( ω) εtclt = 1 (5) DISCHARGE TEMPERATURE VARIATION DUE TO CYCLING Air Discharge Evaporator Air Out δ T evp δ T During cycling of the compressor, the air temperature out of the evaporator varies cyclically according to the cycling of the compressor. With the AC running in freeze control mode, the temperature variation of the outflow air from the evaporator is suppressed by passing through the heater. In the energy efficient mode of operation where the compressor cycles at an elevated set point, the air is often directly charged to the passenger compartment, thus the air temperature variation at the evaporator is directly passed into the passenger compartment which is perceivable by the passengers. The relationship between the air temperature variation at the evaporator outflow and the variation of the bulk charge can be obtained by the differential analysis of Equation (5). Taking the difference of both sides of Equation (5), we derive, δt [ f ( ω) ε ] δtevp = 1 (6) From Equation (6), it can be seen that the bulk charge temperature variation is greatest when the temperature door position is at Full Cold where the heater fractional flow is zero, ( ω) = 0 f (7) At this position, the temperature variation of the bulk charge is directly equal to the temperature variation at the evaporator caused by compressor cycling: δ T = δ (8) T evp This equality of temperature variations is shown in Figure 6. On the other hand, when the entire air stream goes through the heater, the charge temperature variation will be at a minimum, as is shown in Figure 7. Equation (9) represents this minimum analytically. [ ] δtevp δt = 1 ε (9 ) Set Point for Freeze Protection Time Discharge Target Fig. 6 Discharge Variation Is At Maximum When Mix Door Is At Full Cold Air Discharge Evaporator Air Out Discharge Target Reheat Set Point for Freeze Protection Time δ T δ T evp Fig. 7 Discharge Variation Is At Minimum OPTIMIZED CYCLING FOR CONSTANT TEMPERATURE VARIATION AT DISCHARGE OUTLETS Based on the prior analysis of the relationship between the temperature variation of the outflow of the evaporator and of the bulk charge temperature, the first method of cycling optimization attempts to maintain a constant charge temperature variation at the charge outlets through a prescribed schedule of temperature hysteresis band at the evaporator for the cycling of compressors as is given in Equation (10), δt evp hys δt = 1 [ f ( ω) ε ], (10) As is shown in Figure 8, under this hysteresis band schedule, the band is at its narrowest when the air mix door is at the Full Cold position, where the temperature

6 band for the cycling of the compressor is identical to the temperature variation allowed at the charge outlets. When the air mix door is at any other position, the hysteresis band will be allowed to widen according to Equation (10), resulting in more cycle-off time for the compressor with the associated benefit of increased energy efficiency for operating the compressor. By definition, the perceived temperature variation at the charge outlet is constant across the entire operating range of the mix door positions. Evap Air Out Temp Hysteresis Band 0 δ T Mix Door Position Fig. 8 Evaporator Air-Out Hysteresis Band Allowed Discharge Variation δt 0 25 C 100% Discharge Fig. 9 Discharge Variation Allowed To Change OPTIMIZED CYCLING FOR CONSTANT CABIN THERMAL COMFORT The second optimization method allows the bulk charge temperature variation at the outlets to be scheduled according to the charge temperature itself, as is showed in Figure 9. This charge temperature variation schedule is based on the fundamental understanding of human thermal comfort sensitivity to temperature variations. It is well known through human comfort engineering that passengers are sensitive to charge temperature variations when the air is charged at a temperature that is close to 25 degrees Celsius. As the conditioned air is charged at a much higher or much lower temperature, the ability to cern temperature variations decreases. The essence of the present method is the maintenance of a constant human comfort in the presence of charge temperature variations. Equation (11) takes the specification of charge temperature variation from Figure 9 and translates it into a schedule for the evaporator air out temperature hysteresis band for the cycling of the compressor. Figure 10 demonstrates the hysteresis band as a function of the charge temperature and air mix door position. δt Evap Air Out Temp Hysteresis Band evp hys 0 δt( T ) [ f ( ω) ε ] = 1, (11) δ T (T = 2C or 60C) δ T (T =25C) Mix Door Position 100% Fig. 10 Hysteresis Band Defined by Human Thermal Comfort Sensitivity The present method of cycling control allows higher energy efficiency operation by allowing greater temperature variations in the insensitive zone of human thermal comfort to the charge temperatures, such as close to the Full Cold or Full Hot range of the charge temperatures. For charge temperature zones where human thermal comfort is sensitive, the charge temperature variation is restricted properly. SERIES REHEAT REDUCTION ON MANUAL AIR CONDITIONING SYSTEMS WITH CYCLING PNEUMATIC COMPRESSOR To properly define Series Reheat Reduction methodology for the manual air conditioning systems, let us first define the envelope of the upper limit of the Evaporator Air Out (EOAT) at various ambient temperatures that will meet the cooling, humidity and safety requirements.

7 The target EOAT temperature is determined as a function of the current ambient temperature. Figure 11 shows one such definition. The top flat portion of the curve is determined by the humidity comfort requirement of 45% relative humidity for a comfortable cabin at 24 degrees Celsius. The part of the curve to the right is defined by the cooling capacity requirement to maintain the cabin at a comfortable temperature when the ambient temperature is high. The part to the left is determined by windshield fogging when the ambient temperature is low leading to reduced windshield glass temperature. The lowest evaporator air out temperature is obtained when the compressor is operating to freeze control. This is the case either when the air conditioning system is working to provide full cooling capacity at high ambient temperature, or to provide full dehumidification capacity at mid to low ambient temperatures. Evaporator Air Out (Deg. C) T frz Acceptable Elevated Evaporator Out Air EOAT Ambient (Deg. C) EOAT temperature will be slightly warmer by the duct temperature rise. If, on the other hand, the knob is set at the EOAT charge temperature, which varies according to the ambient temperature and is defined in Figure 11, the compressor cycling point will be set to the EOAT temperature and the air passing through the evaporator will be directly charged to the outlets. The air mix door will be positioned at the full cold position inside the HVAC module. For the temperature knob positions between the FC and EOAT, the compressor cycling set point is obtained by interpolating between T frz and the EOAT temperature, with the knob position as the independent variable. In this range of operation, the air mix door will be maintained at the full cold position throughout. Designating the knob position with the variable P, which takes the value of 0~100%, with 0% being FC and 100% being FH, and assuming ideal (straight-line) linearity, the requested charge temperature corresponding to any knob position can be given as, T ( FH FC) = FC + P (12) Ignoring the duct temperature rise for the moment, the knob position requesting a charge temperature equal to the highest allowable Evaporator Out target temperature EOAT given an ambient temperature is given by, Fig. 11 Target EOAT for Series Reheat Reduction P EAOT EOAT FC = 100 (13) FH FC For the following range of the temperature knob positions, P EOAT > P > 0 (14) the compressor cycling set point is given by, FC Fig. 12 Manual Knob with Integrated Series Reheat Reduction FH The manual AC system with Series Reheat Reduction relies on how the compressor operates at two key points of the manual temperature control knob as highlighted in Figure 12. At the FC point, the compressor will operate to freeze control set point, i.e., T frz, and the charge T EOAT FC = P FC (15) P set + EAOT For the temperature knob positions between EOAT and FH, the air mix door position is moved away from the Full Cold position (0%) to get a warmer charge temperature than EOAT. For the same range of temperature knob positions, the compressor cycling set point is maintained at the maximum elevation of EOAT, as depicted in Figure 11. With the air mix door away from the 0% position, a stream of air will be passed through the heater and remixed with the remaining bypassing stream. Depending on the temperature knob position, the air mix door will be correspondingly

8 positioned to provide the requested charge temperature. For the range of knob positions given by, 100 % >= P > (16) P EAOT the expected air mix door position in the module with perfect linearity can be calculated by the following equation, Ψ = P P EOAT 100 (17) 100 PEOAT where Ψ denotes the air mix door position with a range of 0~100%. It is to be noted that near the FH end on the temperature knob, which can be further delineated with a calibratible variable, the requested charge temperature is very high. For comfort heating purpose, the compressor can be safely turned off to gain further energy efficiency. Care should be taken to turn the compressor back on when Defrost or Defog mode of air charge is requested. RESULTS AND DISCUSSIONS The Cycling Pneumatic Compressor methodology outlined above has been implemented and tested on an Air Conditioning System test stand to evaluate the energy saving potential of such a methodology. The refrigeration system components were set up identically to an existing vehicle AC conditioning system so that operating conditions can be established realistically. On the system stand, the ambient conditions of 10Cx90%, 15.6Cx80%, 21.1Cx70%, 26.7Cx60%, 32.2Cx50%, and 40.6x35% (originally defined in English Units as 50Fx90%, 60Fx80%, 70Fx70%, 80Fx60%, 90Fx50%, 105Fx35%) were simulated by controlling the inlet air conditions to the condenser and the evaporator of the refrigeration system. Other controlled parameters such as the air flow rate through the condenser and the compressor RPM are obtained on the modeled vehicle for each vehicle speed. The condenser airflow rate as a function of the vehicle speed was directly taken from the air conditioning development process of the vehicle. The modeled vehicle has an Automatic Climate Control System which automatically adjusts the HVAC module air flow rate and the charge temperature request. By correlating data from road trips, the steady state blower voltage and the associated evaporator air flow rate were established for each ambient conditions and vehicle speed. Similarly, the compressor RPM and vehicle speed correlation were easily obtained from the road trip data. With the supporting data from the modeled vehicle, complete operating conditions were established for the test stand to simulate the above listed ambient conditions and for the vehicle speeds of Idle, 48.3kph, 80.5kph, and 112.6kph (originally 30mph, 50mph, and 70mph in English Units). Four configurations of compressor and control were evaluated. Starting with the baseline fixed placement compressor in freeze control, the fixed compressor with Series Reheat Reduction, pneumatic variable compressor under freeze control, and pneumatic variable compressor cycled for Series Reheat Reduction, were all tested on the bench with the testing matrix of ambient conditions and vehicle speeds. The fixed placement compressor and the pneumatic variable compressor were selected to be close to each other in refrigeration capacity. The fixed placement compressor has 155cc placement with a slightly better volumetric efficiency, which makes it a close match to the variable placement compressor with a 160cc placement. The tested compressor is mounted on a torque bench whereby an electrical motor is used to drive the compressor. The compressor power is obtained by monitoring the torque and the compressor RPM. Figure 13 summarizes the compressor power consumption of the four configurations. For each ambient condition in the chart, the compressor power is averaged across the tested vehicle speeds: Idle, 48.3kph, 80.5kph, and 112.6kph (Idle, 30mph, 50mph, and 70mph in English Units). Needless to mention, the compressor power of all four configurations increases with increasing ambient thermal load. With minor exception at the low ambient, where absolute compressor power is low and relative variations in testing data increases, there is a clear trend that the fixed placement compressor under freeze control requires the highest amount of compressor power. Under the same freeze control condition, the pneumatic variable compressor provides the same cooling but requires less shaft power. The fixed placement compressor with Series Reheat Reduction ranks third in terms of saving compressor power. The best saving is achieved with the cycling pneumatic variable compressor executing Series Reheat Reduction.

9 Energy Efficiency of Cycling Pneumatic Variable Compressor and Fixed Displacement Compressor 2.5 Compressor power is obtained by averaging over Idle, 48.3, 80.5, kph (idle, 30, 50,70 mph) Compressor Power (KW) SP15 AC 6CVC160 AC SP15 EEAC 6CVC160 EEAC CX90% 15.6Cx80% 21.1Cx70% 26.7Cx60% 32.2Cx50% 40.6Cx35% Ambient Conditions Fig. 13 Energy Efficiency of Compressors on the Bench 10.0 Fuel Economy Improvement (%) % Improvement During FTP Highway % Improvement During SC VCe VCpSRHR VCpFrz FixSRHR FixFrz Compressor and Control Fig. 14 Vehicle Fuel Economy Improvement due to Compressors and Controls with Fixed Displacement Compressor in Freeze Control as Baseline

10 After the concept was successfully validated on the test stand, further evaluation was carried out on a compact production car with a small engine. The fuel economy impact of the Cycling Pneumatic Compressor was tested, along with the fixed placement compressors and electronic variable compressors, in a certified Emission Test Chamber. Evaluation of the compressor and controls were carried out using the EPA FTP and SC03 driving cycles with the compressor turned on for both. Figure 14 shows the vehicle fuel economy impact of the compressors and controls under the SC03 driving cycle when the ambient temperature is at 24 degrees Celsius (75 degrees Fahrenheit). With the fixed compressor performing freeze control as the baseline, the vehicle Fuel Economy improvement due to compressor upgrade or control methodology change is summarized. What is clearly indicated by these tests is that the electronic variable compressor executing Series Reheat Reduction is the most effective in helping increasing the vehicle fuel economy. This is achieved, of course, at some cost penalty for the electronic variable compressor. What is to be highlighted here is that immediately next to the electronic variable compressor, the most energy efficient air conditioning system operation is achieved by the pneumatic variable compressor performing Series Reheat Reduction. As expected, the fixed placement compressor under freeze control is ranked the lowest and consumes the highest amount of fuel. A slight surprise is that the fixed placement compressor with Series Reheat Reduction did not outperform the pneumatic variable compressor under freeze control, as is the case in the system stand testing. The crepancy may be explained by the fact the two classes of compressors are used: the bench testing used compressors in the 155~160cc placement range and the vehicle testing here used compressors in the 125~130cc placement range. Additionally, Powertrain Control Module s anticipation routine may bias the tests result slightly. In the same figure, the FTP Highway driving cycle test results for the same vehicle and compressors are shown. The FTP driving cycle tests do reconfirm what has been learned from the bench testing and the SC03 cycle testing, that the cycling pneumatic compressor provides energy efficiency benefits surpassing those possible with the cycling fixed placement compressor, and often very close to those from the electronic variable compressors. Additional validation of the present Cycling Pneumatic Compressor methodology was carried out on a midsized sedan with a V-6 engine. Integration of the control head interface as outlined in earlier sections was implemented. This vehicle implementation allows seamless phasing-in and phasing-out of the Series Reheat Reduction, based on both the ambient temperature and the temperature knob position. A key point of evaluation on the mid-sized sedan is the drivability impact of cycling the pneumatic variable compressor. Road trip tests of the vehicle indicate that the cycling of the pneumatic variable compressor is completely imperceptible to the driver of the vehicle. The low torque impact on the powertrain is attributed to the stroke control characteristics of the pneumatic variable compressors. When declutched, the pneumatic variable compressor s internal mechanisms return the resting stroke position to a default value that can be customdesigned. Presently, the default resting stroke is either at 10% or 40%. When the compressor is re-engaged, the compressor stroke will move from the resting stroke to a stroke dynamically determined by the AC system operation. This re-engagement property of the pneumatic variable compressor provides the low impact on the powertrain. The other factor that imparts low impact engagement is that the Series Reheat Reduction via cycling of the pneumatic compressor occurs mostly at mid to low ambient temperatures. As such, the compressor torque is low or moderate to begin with, which ensures minimum powertrain interference when cycled. At higher ambient conditions, the application of the Series Reheat Reduction is scaled back to provide sufficient cooling capacity. The control of the compressor is governed by freeze control. As a result, capacity control at higher ambient is returned to internal pneumatic control and there is no cycling occurring with high torque value. The low torque cycling of the compressor is beneficial externally to the powertrain in terms of low torque interference. It is also beneficial to the clutch reliability. It may be expected that the clutches designed to cycle the fixed placement compressor should last beyond its standard design life when used in the context of pneumatic compressor. CONCLUSION The pneumatic variable compressor was designed to eliminate the cycling forced by the application of the fixed placement compressor so as to achieve capacity control to prevent freeze. In the two decades after the pneumatic variable compressor was introduced, it has proven superior in its high energy efficiency operation and low powertrain impact. It is interesting to see that the application of the cycling methodology to a compressor designed to eliminate it yields such a beneficial result in achieving Series Reheat Reduction. Through bench testing, small vehicle testing in Emission Chamber, and finally on a mid-sized sedan on the road,

11 it has been demonstrated that cycling the pneumatic variable compressor provides a reliable method to execute Series Reheat Reduction for enhanced energy efficiency operation of the air conditioning system. It offers energy efficiency better than what is possible with the cycling fixed compressors under Series Reheat Reduction. It provides energy efficiency benefit close to that offered by the electronic variable compressor executing Series Reheat Reduction and does it at a reasonable low cost. It has been observed through on-road vehicle operation that cycling of the pneumatic variable compressor is superior to cycling of the fixed placement compressor because of the low, resting stroke of the pneumatic variable compressor and the associated low starting torque. This is especially important for small cars with low placement engine. It frees the powertrain of engagement torque interference from the compressor and allows for smooth powertrain operation. REFERENCES 1. Skinner, T. J. and Swadner, R. L., V-5 Automotive Variable Displacement Air Conditioning Compressor, SAE Transactions, Section 1, Vol. 94, Rattsa, E. B. and Brown, J. S., Experimental Analysis Of Cycling In An Automotive Air Conditioning System, Applied Thermal Engineering, Volume 20, Issue 11, Pages , August 1, Nadamoto, H. and Kubota, A., Power Saving with the Use of Variable Displacement Compressors, SAE , SAE International Congress and Exposition, Detroit, Michigan, USA, March 1-4, Park, Y. C., McEnaney, R., Boewe, D., Yin, J. M. and Hrnjak, P. S., Steady State and Cycling Performance of a Typical R134a Mobile A/C System, SAE , 1999 SAE International Congress and Exposition, Detroit, Michigan, USA, March 1-4, Rasmussen, B., Uribe, T., Alleyne, A. and Bullard, C., Evaluation of Control Strategies for Compressor Rapid Cycling, SAE , 2004 SAE World Congress, Detroit, Michigan, USA, March 8-11, CONTACT Mingyu Wang is a Staff Research Engineer with 14 years of experience in the area of advanced PTC and HVAC systems development. He holds a Ph.D. in Mechanical Engineering from Duke University and is a member of SAE. He can be reached at (716) or mingyu.wang@delphi.com DEFINITIONS, ACRONYMS, ABBREVIATIONS AC AI COP Econ EOAT FC FH HVAC OSA P RPM T Greek Symbols δ ε ω Ψ Subscripts htr evp clt frz hys set Air Conditioning Air Inlet Coefficient Of Performance Economy Evaporator Out Air, also position on temperature knob Full Cold Full Hot Heating, Ventilation and Air Conditioning Outside Air knob percentage position Revolutions Per Minute Difference Heat Exchanger Effectiveness Mixed door position Mixed door position Heater Evaporator Coolant Discharge freeze Hysteresis Cycling set point