December 9, 2014 

Frederick S. Sciance, Manager 

Environment & Climate Policy 

General Motors Global Headquarters 

Environment, Energy & Safety Policy 

MC: 482-C30-B36 

300 Renaissance Center 

Detroit, MI 48265-3000

FE 6349 

 

Mr. Linc Wehrly, Director 

Light Duty Vehicle Center 

Compliance Division 

Office of Transportation and Air Quality 

2000 Traverwood Drive 

Ann Arbor, Michigan 48105 

Mr. Wehrly: 

Request for GHG Credit for Variable Crankcase Suction Valve Technology 

Pursuant to the provisions of 40 CFR § 86.1869-12(d), General Motors requests off-cycle 

greenhouse gas credit of 1.1 grams CO2 per mile for the use of the Denso SAS air conditioner 

compressor with variable crankcase suction valve technology. This improved air conditioner 

compressor was first used by GM on the 2013 model Cadillac ATS. It was then implemented on 

the 2013 Chevrolet Traverse, Buick Enclave and GMC Acadia vans. In addition to these models in 

2014, the SAS compressor was added to all the GM 2014 full-size trucks, including the Chevrolet 

Silverado, Tahoe and Suburban, GMC Sierra, Yukon and Yukon XL, and Cadillac Escalade and 

Escalade ESV. In addition to these models, in 2015 model year the SAS compressor will be used on 

the Cadillac CTS and Cadillac XTS limousine. It will be used even more widely in the future. 

Background 

EPA currently provides Mobile Air Conditioner (MAC) greenhouse gas credits for reduced reheat 

using an externally-controlled variable displacement compressor (EVDC), which provides 

significant efficiency improvements compared to the baseline fixed displacement compressors 

that were the industry norm at the time that EPA created its vehicle greenhouse gas regulatory 

program. Traditional fixed compressors operate at a single displacement all the time that the 

compressor is switched on. The capacity of the system is only controlled by cycling the compressor 

clutch on and off, which creates waste in two ways. Energy is wasted by continuously warming 

and cooling the MAC system components and by accelerating and decelerating the compressor 

rotating mass as the clutch cycles. Further, traditional fixed displacement compressor systems 

require reheating the cooled air under mild ambient conditions when slightly warmer air is 

required to keep passengers comfortable. In contrast, EVDC technology allows the air conditioner 

compressor to moderate cooling capacity and work less under mild ambient conditions to avoid 

cycling the compressor clutch. When slightly warmer air is needed, EVDC compressors can reduce 

capacity further to cool the air to the required amount and reduce the need to reheat the air. 
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The final rule for the 2012-2016 light duty vehicle greenhouse gas program created MAC credits 

as part of the first-ever overarching national framework for vehicle greenhouse gas regulation in 

the U.S. Following initial discussions, on May 19, 2009, ten automobile manufacturers and the 

UAW joined President Obama at the White House to announce the initial proposal for the 

greenhouse gas regulation, including MAC credits, with the final rule subsequently published on 

May 7, 2010. At the time the regulation was developed in 2009 and 2010, EVDC technology was 

evaluated to be the most efficient technology available for air conditioner compressors, and was 

therefore awarded a credit of 1.7 grams of CO2 per mile, which was the largest credit created for 

any air conditioning technology. Much of the technology development and demonstration work 

supporting the MAC credits had been done through the SAE IMAC Cooperative Research Program, 

a partnership between EPA, DOE and 28 corporate sponsors, which published its final report in 

2006 demonstrating the potential for a 30% MAC efficiency improvement using EVDC and other 

MAC technologies. 

Technology Description 

Since that time, additional technological progress has occurred, producing significant additional 

potential efficiency improvements in mobile air conditioner compressors. The Denso SAS 

compressor is a new EVDC design that improves the internal valve system within the compressor 

to reduce the internal refrigerant flow necessary throughout the range of displacements that the 

compressor may use during its operating cycle. This is achieved though the addition of a variable 

crankcase suction valve (variable CS valve). Conventional compressors have a fixed crankcase to 

suction bleed that regulates the flow of refrigerant exiting the crankcase. The sizing of the bleed 

is a compromise among the conditions when either a high rate of flow or a low rate of flow would 

be more ideal. In conditions where maximum air conditioner capacity is not needed, this fixed 

bleed creates an unnecessary reduction of volumetric efficiency for the compressor. In contrast, a 

variable CS valve can provide a larger mass flow under maximum capacity and compressor startup conditions, when high flow is ideal, then reduce to smaller openings with reduced mass flow in 

mid or low capacity conditions. Thus, the refrigerant exiting the crankcase is optimized across the 

range of operating conditions, creating significant benefits for the energy consumption of the air 

conditioning system. 

Off Cycle Condition 

EPA created the off cycle credit program to incentivize the implementation of technologies which 

reduce greenhouse gas emissions in the real world by a greater amount than would be captured in 

the two cycle test program. The EPA’s two cycle test program for city and high way drive 

schedules is conducted without the air conditioner switched on, so air conditioner efficiency 

technologies clearly address an “off cycle” condition, since none of the benefits of MAC technology 

improvements are captured by the two cycle tests. Correspondingly, credits for thermal control 

technologies have been included in EPA’s list of pre-defined and pre-approved off cycle credits 

based on the greenhouse gas reductions expected from reduced air conditioner usage on vehicles 

equipped with these thermal control technologies, such as solar reflective glass and paint, 

ventilated seats, active cabin ventilation and passive cabin ventilation. 

Alternative Methodology Requirement 

EPA’s five cycle test program would not adequately measure the real-world greenhouse gas 

reduction benefits of either EVDC or the variable CS valve. Only one of the five tests is conducted 

with the air conditioner switched on. This is the SC03 test, which is conducted at a very high 
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ambient temperature of 95oF, high solar load of 850 W/m2, and high relative humidity of 40%. 

The relatively short 10-minute duration of the SC03 test and the extremely demanding climatic 

conditions result in the vehicle air conditioner systems being operated at maximum capacity 

essentially throughout the test. Indeed, the SC03 procedure specifies that manual controls be set 

to a mode of maximum cooling and a temperature of full cool for the entire test. Technologies 

such as EVDC and the variable CS valve provide their benefits under milder ambient conditions 

when the air conditioner is not forced to operate at maximum capacity. Therefore, no substantial 

portion of the real world benefits of these technologies can be measured on the SC03 test. As a 

result, the five cycle test program cannot be used to evaluate these off cycle technologies, and an 

alternative methodology must be used instead. 

Bench Tests 

The Denso SAS compressor used on the 2013 and 2014 Cadillac ATS was evaluated by Denso using 

the methodologies that were developed and used during the SAE IMAC Cooperative Research 

Program for its evaluations of U.S. average system efficiency. These methodologies have been 

formally adopted as publicly available SAE standards. The SAE J2765 standard specifies a series of 

40 bench tests at various compressor speeds to measure the system coefficient of performance 

(COP). Twenty five of these bench test conditions are then selected as inputs to the GREEN MAC 

Lifecycle Climate Change Performance (LCCP) model jointly developed for comparative evaluations 

by General Motors, EPA, the Japanese Automobile Manufacturers Association, and SAE (GREEN is 

an acronym for Global Refrigerants Energy and Environmental). These twenty five data points 

replicate a broad range of operating conditions for various ambient climate conditions and air 

conditioner system modes. This LCCP model has been adopted as the SAE J2766 standard and can 

be accessed at the following link: 

http://www.epa.gov/cpd/mac/compare.htm

The GREEN MAC LCCP model includes U.S. climate data for numerous U.S. cities as well as vehicle 

on-road operation parameters. Data sources are thoroughly documented within the model, but 

some of the key parameters are temperature and humidity data from a U.S. Department of Energy 

(DOE) database that contains U.S. National Climatic Data Center data, annual driving distances for 

each city from the U.S. Energy Information Agency (EIA), and percentage of drive time at different 

ambient conditions based on research by the National Renewable Energy Laboratory (NREL). 

Combining the J2765 bench test COP data with the NREL, EIA and DOE climate and vehicle on-road 

data provides a simulation of annual U.S. average greenhouse gas emissions for an air 

conditioning system. 

This full analysis was performed for the Denso SAS compressor, resulting in a U.S. average LCCP of 

17.6 grams CO2 per mile per vehicle. The same analysis was then performed for the Denso SBU 

compressor, which is a modern EVDC design that does not have the variable CS valve technology. 

The SBU compressor is currently used on a variety of vehicles, such as the Chevrolet Impala Classic, 

Holden Commodore, Chevrolet Equinox, and GMC Terrain. The SBU compressor qualifies for the 

1.7 grams CO2 per mile MAC credit under the EPA regulation, and, as such, constitutes a valid 

comparative baseline to determine if the SAS compressor with variable CS valve technology 

deserves an additional off cycle credit for emission reductions beyond those already achieved by 

compressors that qualify for the EPA EVDC MAC credit. (Both compressors also feature integrated 

oil separators, and both qualify for the MAC credit of 0.6 grams CO2 per mile.) The SBU 
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compressor achieved an LCCP result of 18.7 grams CO2 per mile, which is 1.1 grams CO2 per mile 

higher than the SAS compressor. Based on this analysis, vehicles equipped with the SAS 

compressor featuring variable SC valve technology should receive an off cycle credit of 1.1 grams 

CO2 per mile. 

Bench test results using the J2765 procedure for both the SAS and the SBU compressor are 

contained in Appendix A. Denso reviewed this data with the EPA technical specialists for mobile 

air conditioners in April 2013 (see Attachment B for this Denso presentation). The feedback was 

positive, but Denso was instructed to work with an OEM on any greenhouse gas credit request. 

Denso subsequently approached GM to jointly develop this credit application, and the subsequent 

testing and credit application preparation has been coordinated between Denso and GM technical 

experts. 

Vehicle Tests 

While this bench test procedure provides an accurate measurement of the expected benefits for 

U.S. national average climate conditions and driving patterns, consultation on potential off cycle 

credit with EPA’s off cycle technical specialist revealed a desire to see the benefits documented 

through actual vehicle testing. As discussed previously, the SCO3 test would not be useful for this 

purpose, since the extreme air conditioning loads throughout that test would not allow operation 

in the moderate load conditions where the SAS compressor shows its benefits. The AC17 test was 

therefore selected to measure benefits under the more moderate ambient conditions that are 

more representative of average U.S. air conditioner operating conditions. 

Conducting vehicle tests turned into a major endeavor, considering the difficulties of obtaining 

suitable baseline vehicles (without the variable CS valve technology) and comparable production 

vehicles with the technology, obtaining time in suitable SC03 test cells (that are also used for the 

AC17 test), and generating usable test results. The production vehicle was the 2013 Cadillac ATS 

with rear-wheel-drive and a 2.0 liter turbo L4 engine. The Denso SAS compressor with variable 

crankcase suction valve technology has been standard on this vehicle since the vehicle’s 

introduction in 2013. However, no version of this vehicle had ever been built without the SAS 

compressor. 

By coincidence, the Denso SBU compressor that was used in Australia on the Holden Commodore 

had similar mounting points and controls, which allowed this SBU unit, without the variable 

crankcase suction valve technology, to be installed in the Cadillac ATS to gather baseline data. It 

should be noted that it was very fortunate in this instance that a good baseline hardware set could 

be assembled through the use of the Holden Commodore compressor, and it is expected to be 

uncommon to be able gather such comparable data for the “with off cycle technology” and 

“without off cycle technology” cases. At a minimum, use of SBU compressors from other 

applications would have required fabrication of elaborate special mounting brackets, since no 

other version matched the ATS mounting points. This type of hardware obstacle is anticipated to 

be potentially prohibitive in many future situations for performing vehicle tests to evaluate off 

cycle technologies, and in this instance prevents ready testing of the benefits of the SAS 

compressor on other GM models on which it has been implemented. 
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For example, the GM full-size trucks which now use the SAS compressor used fixed compressors in 

prior years. Therefore, there is no good hardware set available to directly compare the benefits of 

the SAS compressor (with variable crankcase suction valve technology) on the current trucks with 

an externally controlled variable compressor without the variable crankcase suction valve 

technology, since these trucks never used that type of compressor. Due to these obstacles and the 

potentially high testing burden, this credit request applies the benefits measured on the Cadillac 

ATS to all GM vehicles using the Denso compressors with variable CS valve technology. 

In addition to the test burden considerations, a relatively small vehicle such as the Cadillac ATS is 

relatively easy to cool, and it uses the smallest displacement of the SAS compressors. Air 

conditioner fuel consumption on the ATS vehicle would be expected to be relatively low compared 

to the larger vehicles with larger compressors comprehended in this credit petition (Chevrolet 

Traverse, Buick Enclave, GMC Acadia, Chevrolet Silverado, Chevrolet Tahoe, Chevrolet Suburban, 

GMC Sierra, GMC Yukon, GMC Yukon XL, Cadillac Escalade, Cadillac Escalade ESV, Cadillac CTS and 

Cadillac XTS limousine). The fuel consumption savings in grams CO2 per mile would also be 

expected to low compared to the same assessment on a larger vehicle with a larger compressor. 

Therefore, the savings in grams CO2 per mile measured for the Cadillac ATS represents a “worst 

case” measurement, since general vehicle parameters and air conditioner size would indicate 

larger savings and a larger credit if assessed on these larger vehicles. 

Vehicle Test Results 

The AC17 procedure is a complicated test, and this particular test series was conducted under 

especially difficult circumstances. The SC03 test chamber had been shut down for a prolonged 

period for upgrades to meet future, more rigorous test requirements. The chamber was reopened 

with a backlog of required tests, and construction was actually continuing nearby while tests were 

restarted to eliminate this backlog. This is worth noting since construction dust was entering the 

SC03 chamber, and test data for two AC17 tests with the SAS compressor were voided where it 

was believed the dust may have had an effect on the dyno rollers, causing tire slip and high CO2 

emissions. Once this was observed, a practice was adopted to sponge off the dust and then dry 

the rollers between tests. 

One additional source of test variability may have occurred in the SBU compressor tests on Days 3 

and 4. During these AC17 tests, torque and pressure measured at the SBU compressor were 

unexpectedly low. This was never fully unexplained, but could be an example of the types of 

control issues that may be expected to arise when attempting to do this type of baseline 

technology testing for hardware on a vehicle that was never actually designed and optimized to 

use that hardware. Whatever the explanation, these test results were kept, even though the low 

torque and low pressure conditions would be expected to save energy for the SBU air conditioner 

operation on these tests, thereby making the baseline SBU compressor look more efficient. 

Keeping these test results therefore may have diminished the off cycle credit amount by making 

the baseline technology look better than it should. 

The AC17 tests were conducted over four days, consisting of four AC17 tests for the SAS 

compressor (with the variable CS valve technology) and four AC17 tests for the baseline SBU 

compressor (without variable CS valve technology). Detailed test results are contained in 

Attachment C. As mentioned previously, two of the four AC17 tests for the SAS compressor were 
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voided due to tire slip. The following table summarizes the average data supporting an average 

savings of 1.3 grams CO2 per mile for the SAS compressor with variable CS valve technology: 

Grams CO2 per mile SC03 Drive Cycle Highway Cycle Combined

SAS Compressor (2 tests) 38.7 9.1 23.9

SBU Compressor (4 tests) 40.2 10.3 25.2

Differential (credit) 1.3

This vehicle data, measuring a 1.3 gram CO2 per mile benefit on the AC17 test, is consistent with 

the credit value of 1.1 grams CO2 per mile estimated from the bench tests data and U.S. LCCP 

calculations. In view of the test variability on the AC17 test and other vehicle testing difficulties 

and considerations, we are basing our credit request on the bench test data, which can be 

considered to be more precise. However, the vehicle test data provide important confirmation 

that the benefits of the variable crankcase suction valve compressor technology are measurable in 

comparable amounts on actual vehicle tests. 

Durability 

Durability of the SAS compressor has been thoroughly tested to meet General Motors 

specifications. This includes at least 14 separate durability bench test procedures, such as high 

refrigerant charge durability testing, low charge testing, vibration testing, corrosion testing, liquid 

slugging durability testing, clutch durability, resonant speed testing, control valve durability, startstop durability, reverse torque durability, over voltage durability, jump start test, power 

temperature cycling, and humid heat cycling. 

Perhaps most importantly, six compressors were evaluated in the Customer Usage Test, which 

runs the six compressors three times through a stressful 700-hour cycle of various compressors 

speeds. This 2,100 hours of testing is designed to simulate the wear of a normal compressor life. 

The components are then disassembled and examined for excess wear. The SAS compressors 

successfully passed all durability testing. 

The objective of our durability evaluation was to predict any expected in-use emission 

deterioration rate, and to determine the performance level that effectively represents a 

significant majority of SAS compressors in actual use over the full useful life of candidate in-use 

vehicles of each vehicle design. 

Based on these tests, GM is willing to attest that the SAS compressor is expected to meet EPA 

requirements for in-use durability over the complete vehicle lifetime of at least ten years for cars 

and eleven years for trucks, and no reduction has been applied for in-use degradation of the 

benefits of the variable crankcase suction valve technology. Durability test results are included in 

Attachment D. 

Conclusion 

Based on the bench test and vehicle test data presented in this petition, combined with calculation 

procedures in the Lifecycle Climate Change Performance tool, General Motors hereby requests 

that EPA approve an off cycle greenhouse gas credit of 1.1 grams CO2 per mile for all 2013, 2014 

and 2015 model vehicles equipped with the Denso SAS compressor with variable crankcase 
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suction valve technology. This 1.1 gram credit amount has been estimated to be representative of 

the fuel savings that can be expected from this technology in actual real-world usage in U.S. 

national average climate conditions. The relevant models and their expected sales volumes are 

listed in Attachment E. 

Thank you for your consideration of this application for off cycle greenhouse gas credits. 

Fredrick S. Sciance, Manager 

Environment, Energy and Safety Policy 

General Motors Public Policy Center 

 

c: Jessica Brakora 

 Arvon Mitchum 

 

Attachment A - Compressor Bench Test Data 

Attachment B - Denso Presentation to EPA, April 2013 

Attachment C - AC17 Vehicle Test Data 

Attachment D - Durability Tests (Confidential) 

Attachment E - Sales Volumes of Models with SAS Compressor (Confidential)