Proposal for a Vehicle-Level Test Procedure to Measure Air Conditioning Fuel Use

PHEV Charge-Depletion electric range over the UDDS and USO6 drive cycles with and without A/C. Source: Rugh, 2010. Click to enlarge.

At the upcoming SAE 2010 World Congress in Detroit, John Rugh from the National Renewable Energy Laboratory (NREL) will present a paper proposing a vehicle-level test procedure to measure the fuel used by mobile air conditioning units.

Air-conditioning systems can have a significant impact on the fuel economy of a conventional vehicles and on the fuel use/range of plug-in hybrid electric vehicles. A National Renewable Energy Laboratory (NREL) vehicle performance analysis shows the operation of the air conditioner reduces the charge depletion range of a 40-mile range PHEV (midsized parallel hybrid sedan with an 81.9 kW engine and a 51.8 kW, 18.5 kWh Li-ion battery pack) from 18% to 30% in a worst case hot environment.

In fuel efficient vehicles, the impact of the A/C system is more apparent because the fuel used for A/C is a larger percentage of the overall vehicle fuel use. In vehicles with electric powertrains such as PHEVs and EVs, total energy management is essential. A/C energy use will increase the size and cost of the energy storage system (ESS) in order to meet the target performance parameters.

—Rugh, 2010

Designing for air conditioning electrical loads impacts PHEV and electric vehicle (EV) energy storage system size and cost. But while automobile manufacturers have climate control procedures to assess A/C performance, and the US EPA has the SCO3 drive cycle to measure indirect A/C emissions, there is no automotive industry consensus on a vehicle level A/C fuel use test procedure to assess accurately the impact of climate control loads.

There are three primary types of tests to measure A/C fuel use at the vehicle level, Rugh notes in his paper: equivalent capacity, equivalent temperature, and real world.

• Equivalent capacity. The goal here is to measure the fuel required to provide the same thermal performance at the A/C system level. Examples include tests to determine the impact of a new refrigerant or the impact of an improved efficiency technology such as an internal heat exchanger or evaporator. Although refrigerants can be compared at the bench level, testing on a vehicle accounts for varying engine speed, uneven air flow through the condenser, and under-hood conditions.

• Equivalent temperature. This approach requires the same air temperature at the driver (or another location in the passenger compartment) from test to test. The equivalent temperature test can be viewed as drawing a control volume around the passenger compartment and A/C system with equivalent air enthalpy in and out, Rugh says. Within the same vehicle, this type of test could be used to determine the fuel use impact of solar reflective glazings, improved control strategies, increased recirculation air, reheat reduction, a variable displacement compressor, or any thermal load reduction technology