"Fluorocarbons: Balanced Solutions For Society" Vehicle Air Conditioning...A Working Example

12

A Worldwide Perspective

Vehicle air conditioning is an important part of an integrated system which provides cooling, heating, defrosting, demisting, air filtering and humidity control for both passenger comfort and vehicle safety. Its reliability and convenience are often taken for granted, but it is key to keeping the passenger safe and comfortable in over 350 million vehicles worldwide.

Environmental Considerations for Vehicle Air Conditioning

The dramatic vehicle air conditioning environmental improvements during the past decade represent one of the most rapid and important success stories for responsible environmental stewardship. In the early 1990's, chlorofluorocarbons were completely eliminated by global automakers from use in new vehicle air conditioners, replaced by hydrofluorocarbons (HFCs). This eliminated the potential contribution to ozone depletion from new autos and reduced over 80% of the global warming potential. The current generation of vehicle air conditioners is being improved through refinements yielding additional significant reductions in total greenhouse gases - over 40% in some cases.

Vehicle safety is enhanced through comfort cooling and dehumidifying; drivers are more alert and have better visibility when window demisting becomes necessary. At highway speeds, vehicle air conditioning can lower greenhouse gas output compared to open window driving, which increases fuel consumption due to aerodynamic drag. This trade off between air conditioning and additional carbon dioxide generation due to lost fuel efficiency is frequently overlooked.

Life Cycle Climate Performance

Life Cycle Climate Performance (LCCP) is a measurement that includes both direct air conditioning refrigerant emission and the indirect vehicle energy consumed. System leakage and refrigerant loss during installation, commissioning, servicing and decommissioning must be minimized.

In modern vehicle air conditioners with proper refrigerant recycling, approximately 60% of greenhouse gas releases relate to system energy consumption and 10% relate to transporting the system weight. Only about 30% is refrigerant related, based on typical U.S. driving patterns and conditions. Weight or energy efficiency changes significantly affect system greenhouse gas contributions.

HFC potential alternatives include carbon dioxide (used as a refrigerant)

and hydrocarbons, as well as more exotic systems. Much development work remains to make these alternatives viable, and it is currently unclear whether they ultimately will surpass state-of-the-art HFC-134a

- DRAFT

Air Conditioning Correction Factors in MOBILE6

Report Number M6.ACE.002

March 12, 1998

John Koupal

Assessment & Modeling Division
U.S. EPA Office of Mobile Sources

1 ABSTRACT

Revised air conditioning exhaust emission correction factors are being proposed for MOBILE6. The proposed factors are based on testing of 38 vehicles at two locations, using a test procedure meant to simulate air conditioning emission response under extreme “real world” ambient conditions. These factors are meant to predict emissions which would occur during full loading of the air conditioning system, and will be scaled down in MOBILE6 according to ambient conditions input by the user if appropriate. It was concluded that the data used in the development of the proposed factors adequately represents real world conditions, based on the results of a correlation vehicle tested at both test sites and a full environmental chamber. In general, emissions were found to increase significantly with air conditioning operation, but under some conditions HC and CO emissions decreased. For running emissions, speed-based correction factors were developed separately for Light-Duty Vehicles (LDV's) and Light-Duty Trucks (LDT's) for all pollutants; separate HC and CO corrections were also developed for high emitters. Correction factors for start driving were also assessed.

Recent studies conducted primarily as part of the Supplemental Federal Test Procedure (SFTP) rulemaking development process indicate that vehicle fuel consumption and exhaust emissions increase substantially when the air conditioner is in operation. As the traditional method for accounting for the effects of air conditioner load - increasing dynamometer horsepower by 10% is not adequate for characterizing this emission increase, new certification test procedures aimed at reducing emissions when the air conditioner is in operation were implemented as part of the SFTP rule. Air conditioning correction factors are included as an optional element of MOBILES; however, these factors are based on testing performed in the early 1970's and are considered so outdated that the user is discouraged from using them in the MOBILE User's Guide. Given the recent findings on air conditioning emissions, revised air conditioning correction factors are

clearly needed.

This report presents the "full-usage" air conditioning exhaust correction factors proposed for MOBILE6. Full-usage correction factors are meant to represent the emission increase when the A/C system is inducing full system load on the vehicle, as would occur under extreme ambient (temperature, humidity and solar load) conditions. Since it not appropriate to apply these factors to all ambient conditions, MOBILE6 will scale these factors down based on the ambient conditions under which the model is being run (the development of appropriate scaling factors is discussed in Report Number M6.ACE.001, "Air Conditioning Activity Effects in MOBILE6"). Discussion in this report includes the testing used to generate A/C emission data, correlation between the two test sites and with expected real-world results, and the development of the fullusage correction factors. It should be noted that the correction factors presented in this report apply to vehicles which do not comply with the SFTP requirement. The treatment of air conditioning correction factors for vehicles complying with the SFTP requirement will be addressed in a separate report.

The data used for this analysis was generated through testing performed at EPA's National Vehicle and Fuel Emissions Laboratory and through an EPA contractor, Automotive Testing Laboratories (ATL), in East Liberty, Ohio. 26 vehicles were tested at EPA and 12 were tested at ATL, including one vehicle tested at both locations for correlation purposes (treated as two separate vehicles for the purpose of this analysis). A list of the vehicles tested is contained in Table 1. The sample consisted of 1990 and later vehicles categorized as follows: 24 cars / 14 trucks, 32 Ported Fuel Injection (PFI) / 6 Throttle-Body Injection (TBI), and 28 Tier 0 / 10 Tier 1. Each vehicle was designated either as a "normal" emitter or "high" emitter using the following emission cutpoints over the Running LA4' : 0.8 g/mi HC, 15.0 g/mi CO and 2.0 g/mi NOx (the cutpoints were applied independently for each pollutant, so that a vehicle could be a high emitter for HC and a normal emitter for NOx). These cutpoints yielded five high emitters for HC, three for CO and two for NOX.

EPA's new air conditioning test procedure is based on use of a full environmental chamber at 95° F, 40% Relative Humidity and full solar load (850 Watts/Meter'). This type of facility was not available to EPA at the time of testing, so use of a procedure which simulated these conditions was required. A/C-on tests were conducted in a standard emission test cell at 95° F and 50

1

"Running LA4" emissions were derived from the combination of emissions from Bag 2 and a 505 cycle run warmed-up (ie. without a soak). More detail on this calculation can be found in MOBILE6 Report No. M6.STE.002, "The Determination of Hot Running Emissions from FTP Bag Emissions"

grains/pound of humidity with standard cooling and the driver window down. The A/C system was set according to the SFTP requirements; maximum A/C and blower setting with recirculation mode if so equipped. Rather than attempting to represent a condition that would actually occur in-use, this simulation is meant solely to induce the level of A/C system load on the vehicle which would occur in the real world under extreme ambient conditions. Operating with the driver window down and with standard cooling is meant to compensate for the lower humidity level and lack of solar load inherent in the standard cell. This simulation method showed adequate correlation with SFTP environmental cell conditions during the development of the SFTP rulemaking2, and is a straightforward way to approximate real-world air conditioning emissions using a standard cell setup. A/C-off tests were run in standard FTP ambient conditions (75° F, 50 grains/pound humidity).

The vehicles were run in a warmed-up condition over EPA's facility-specific inventory cycles3, ARB's Unified Cycle (the LA92), and the New York City Cycle one time each with the A/C on and A/C off. A cold start ST01* cycle was also run in both conditions for the purpose of assessing start A/C factors (information on all driving cycles used in this test program is shown in Table 2). The EPA tests were run on a 48-inch electric dynamometer, while the ATL testing used a twin 20-inch electric dynamometer; all tests were run without the 10% A/C load adjustment factor typical to standard emission tests. Both bag and modal data were collected.

3.3 Overall Results

As with previous versions of the model, MOBILE6 will contain correction factors which estimate the emission impact of changes in temperature. Emissions at temperatures higher than 75° F will be determined in the model first by applying a base temperature correction, then applying the A/C correction factor appropriate for that temperature. A/C correction factors must be developed separately from the baseline temperature corrections in order to avoid doublecounting temperature impacts. For this analysis, therefore, the A/C-off results were corrected from the temperature the test was conducted (nominally 75°, although minor variability is common) to the A/C-on temperature (nominally 95°) for each paired test. Since MOBILE6 temperature correction factors will not change from the MOBILE5 corrections, MOBILES

2

Results from a correlation program between this simulation and a full environmental chamber over a sample of six Tier 1 vehicles can be found in AAMA/AIAM's comments to EPA on the proposed SFTP rulemaking (EPA Docket No. A-92-64 Item IV-D-10).

"For detail on the development of EPA's facility-specific inventory cycles, see MOBILE6 Report No. M6.SPD.001, "Development of Speed Correction Cycles"

*STO1 is a 1.4 mile cycle developed to specifically characterize driving behavior following startup. The cycle was developed from an in-use driving survey conducted in Baltimore, Spokane and Los Angeles as part of the SFTP

rulemaking process.

temperature corrections were used. The Bag 2 corrections were used for all running tests, and Bag 1 corrections were used for the cold start ST01 test".

Once the temperature correction was applied, the A/C impact was analyzed by taking the ratio of emissions with the A/C on to corrected emission levels with the A/C off results (referred to throughout the report as the "A/C ratio"). This ratio was based not on each individual vehicle, but on the average A/C on and A/C off levels over all vehicles for each driving cycle. Results of this analysis on the running cycles over all vehicles are shown in Figures 1-4 for fuel consumption, HC, CO and NOx; in these figures, the driving cycles are ordered from lowest (NYCC) to highest (FWHS) average speed. Although a more detailed analysis is covered in Section 5, these figures highlight some general trends that shape the development of running correction factors:

Fuel Consumption and NOx: Increases in fuel consumption' and NOx generally result from the added load placed on the engine by the air conditioning system when the A/C compressor (which is propelled by the engine) is engaged. Figures 1 and 2 show a consistent increase over all cycles, with a strong dependency on average speed. In general A/C load is fairly constant over all operation, so the relative additional load placed on the engine depends on the loading condition of the engine itself. Larger relative increases in engine load due to air conditioning occur at lower speeds, while at higher speeds the relative additional load placed on the engine by the air conditioning system is smaller. This results in a decreasing A/C ratio as average cycle speed increases.

HC and CO: Although the changes in relative A/C loading (and hence fuel consumption) mentioned above can also drive increases in HC and CO, more significant increases are usually the result of fuel enrichment. Excess fuel enrichment can result from the added load placed on the engine and/or fuel calibrations that simply add fuel because the air conditioning system is in operation. Although not the case for every vehicle, the effects of this enrichment on emissions (particularly CO) from the vehicles that do experience excess fuel enrichment are so large that average fleet emissions are increased significantly. Figure 3 shows A/C ratio for HC; the ratio is higher at the low and high ends of the speed range, but actually drops below 1.0 in the mid range. One explanation of this is that increased combustion temperatures resulting from higher engine load reduce HC emissions in some situations. Figure 4 shows higher ratios for CO but less dependence on speed. This suggests the stronger role of vehicle calibration in driving the CO

"The temperature corrections will be modified to accommodate the start/running split new to MOBILE6, but the base corrections will not change. The start/running split has not be developed, so for this analysis the MOBILES Bag corrections were applied.

*MOBILES temperature correction factors can be found in "Compilation of Air Pollutant Emission Factors, Volume Il-Mobile Sources" (AP-42), Page H-24

'Fuel consumption correction factors are presented in this report primarily because of the proposed treatment of CO for vehicles complying with the SFTP requirement, to be discussed in a future report.