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Study Finds That CO2 Emissions in Europe from Mobile AC May Be An Average Six Times Higher Than Predicted by the EPA MOBILE6 Model

A study by researchers at Empa in Switzerland on the impact of mobile air conditioning (MAC) activity on CO2, fuel consumption, and pollutant emissions of diesel cars representing the European fleet confirmed that, as determined in an earlier study of gasoline cars, MACs remain running at ambient temperatures far below the desired temperature of the passenger compartment, unless manually switched off. (This dehumidifies intake air to prevent possible windshield misting, although, the authors note, this is seldom.)

The team developed a simple model to calculate extra emissions and fuel consumption resulting from MAC activity as a function of ambient temperature and humidity: EEMAC, Empa Emission model for Mobile Air Conditioning systems. When applied to the average meteorological year of a central European region and compared with the US EPA MOBILE6 model, the EEMAC model found estimated average annual CO2 output six times higher than that of MOBILE6.

A paper on their work was published online 8 June in the ACS journal Environmental Science & Technology.

...up to two-thirds of these emissions are caused by operating the MAC at temperatures below 18 °C, where mere ventilation is sufficient, and the MAC could thus be switched off without any discomfort to the vehicle occupants. When the MOBILE6 model is applied, the resulting extra emissions are lower by a factor of 3 to 10 and thus greatly underestimate the European situation.

If the actual fleet of a country is modeled, the overall influence of MAC activity on relative emissions is somewhat smaller, as the total fleet is not equipped with a MAC system. However, the discrepancies between the two models remain, and 2% of CO2 and fuel could be saved without any discomfort to the vehicle occupants. This is twice the amount of fuel than is saved annually by renewing the fleet.

—Weilenmann et al.

MOBILE6, developed by the US EPA, is applied all over the world, including Europe. MOBILE6 is based only on measurements above 22 °C (71.6 °F) and assumes that MACs do not run at lower temperatures. The Empa team showed that all systems in European cars, if left switched on, keep running except below 5 °C to prevent freezing. In this range of 5 °C-18 °C, it is not necessary to cool incoming air to maintain a comfortable temperature. However, cooling to about 3 °C dehumidifies the air for the rare case of windscreen misting.

This load on the refrigerant circuit must be taken into account when calculating annual fleet emissions, which EEMAC does. In addition, no data are available for diesel vehicles, where the power for the MACs is delivered with a different efficiency than for gasoline cars.

—Weilenmann et al.

Based on the study, the authors suggest that (a) these findings are used to educate the public and (b) to propose discussions with manufacturers to optimize new MACs such that they turn off completely if no cooling is needed and turn on only when the windscreen mists. Legislation could also support this process, the authors suggest.


  • Martin F. Weilenmann, Robert Alvarez and Mario Keller (2010) Fuel Consumption and CO2/Pollutant Emissions of Mobile Air Conditioning at Fleet Level - New Data and Model Comparison. Environ. Sci. Technol., Article ASAP doi: 10.1021/es903654t



Wouldn't ultra high efficiency electric A/C or Heat pumps be more efficient?


Um these are in CARS so um nope.


They just need to include AC operation at 15 degrees C in the EU's method for calculating CO2 levels, and the manufacturers will sort it out rapidly.

It is probably just a case of programming the AC units differently, or adding a little more intelligence to them (making them all climate control systems, rather than just AC units).

Many Tax rates in Europe are now dependent on CO2 levels, so the manufacturers put a lot of effort into keeping them as low as possible.


Whilst there are times that ambient air is a lower temperature, solar gain caused by srong sun being magnified through glass and dark painted bodywork can still make a car difficult to cool without a/c, at leat initially after start-up, although after 5 minutes or so it is normally possible to run without the a/c after the car is cooled. Unfortunately, a lot of people don't think to switch the a/c unit off after the five munites or so it takes to cool the car down. Opening windows is an alternative if driving at slow speeds.

Regardless, as a/c technicians will advise, its always good practice to run the a/c five minutes or so once a week to keep seals in the a/c system fresh and prevent them from leaking gas, even in colder weather (as long as the a/c can actually work). Too many people forget to do this and then wonder why the a/c loses its effectiveness when they do actually need it.


In sun belts cars with small PV panels and a cooling fan could circulate enough cool air to dissipate the solar heating. The Chevrolet Volt will have a PV roof panel option for this purpose.

The Goracle


Exhaling must be made a crime. You evil CO2 emitters, you.


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