Chrysler, Ford, General Motors, the U.S. Environmental
Protection Agency (EPA) and the California Air Resources Board
(CARB) have collaborated over the past two years to develop an
efficiency test for mobile air conditioner (MAC) systems. Because
the effect of efficiency differences between different MAC systems
and different technologies is relatively small compared to overall
vehicle fuel consumption, quantifying these differences has been
challenging. The objective of this program was to develop a single
dynamic test procedure that is capable of discerning small
efficiency differences, and is generally representative of mobile
air conditioner usage in the United States. The test was designed
to be conducted in existing test facilities, using existing
equipment, and within a sufficiently short time to fit standard
test facility scheduling.
Representative ambient climate conditions for the U.S. were
chosen, as well as other test parameters, and a solar load was
included. The procedure was then performed by the OEMs on a wide
range of vehicles to assess repeatability, accuracy, and other
parameters. While these evaluations continue, and the results have
been at times mixed, the test is robust and the overall experience
with the test has been positive.
Repeatability and accuracy have been good, but MAC CO₂ emissions
are small compared to test-to-test variations in CO₂ emissions
associated with other vehicle functions, especially on hybrid
vehicles. Thus, even with normal test repeatability levels, the
benefits of individual MAC technologies cannot always be discerned.
Work continues to understand the performance of the test, including
potential improvements and supplements. The new test is a useful
addition to the tools we can use to assess MAC efficiency in order
to make MAC improvements in the future.