Switching on auxiliary devices in vehicular fuel efficiency tests can help cut CO2 emissions by millions of tons

Abstract: Enhancing automobile fuel efficiency is crucial to decarbonizing the transport sector. Recent studies have revealed a gap between real-world fuel efficiency and results from laboratory tests, where “off-cycle” auxiliary devices, like air conditioning (AC) systems, are not turned on. AC consumes the most energy among all off-cycle devices; however, the exact contribution of AC to fuel consumption remains unclear.

Here, by analyzing 1 million trip-level fuel efficiency records from China, we show that AC use when the temperature exceeds 25°C increases gasoline consumption annually by 1.3%. The amount differs across car models, with those produced by Chinese manufacturers showing relatively lower AC efficiency. Improving AC efficiency could cost-effectively reduce CO₂ emissions by 1.6–2.4 million tons in China every year.

Therefore, we suggest integrating off-cycle devices into future fuel efficiency regulations, which will reveal the fuel efficiency gap, incentivize car manufacturers to develop high-efficiency devices, and further tap this emissions reduction potential.