Abstract
Human thermal comfort is the result of the combined effect of several parameters. Not just air temperature, but also mean radiant temperature, air velocity and air humidity. More precisely, people are in thermal comfort when the heat produced by their body, depending on their activity level, can be dissipated through the clothes or at the skin to the surrounding environment as much uniformly as possible.
From this point of view, car cabin is a highly non homogeneous environm ent with very peculiar features. For example surfaces which may have rather different temperature (like glazing or the dashboard) are very close to passengers; moreover, passengers are also very close to air outlets.
It is clear that just car cabin air temperature cannot be the correct way to estimate comfort conditions. There is therefore the need for an equivalent temperature which can be defined as the uniform temperature of a environment where air velocity is equal to zero, and where air temperature is equal to the walls one so that a person exchanges the same overall thermal power that one would exchange in the real environment. Once this concept is established, the next step is to define a sensor set and a measurement procedure able to evaluate human thermal comfort conditions in cars, with the final goal to have a reliable feedback for a climate control system based on equivalent temperature control.
Fiat Research Center has established a methodology to measure thermal comfort by means of equivalent temperature estimators. An equivalent temperature estimator is a mathematical expression that, combining signals from the different sensors installed on board, reproduces thermal comfort measured on board. The methodology makes use of comfort thermal manikins (developed by Fiat Research Center) to assess comfort conditions in chosen car cabin with its A/C system. Latest Fiat Car models already employ equivalent temperature estimators as feed-back for climate control employing sensors in order to measure the inside air temperature, the external temperature, solar sunshine and the dealt air temperature in air conditioning group.
The present paper describes an alternative approach which makes use of a mean radiant temperature sensor. A methodology has also been developed to find the best sensor position inside the car. It will be shown that the use of this sensor may allow a reduction in the number of sensor which are necessary to correctly reproduce equivalent temperature, thus allowing cost reduction. Moreover, it allows to better take into account the impact on thermal comfort of irradiation from dashboard and from glazing with respect to what can be done by means of a traditional sun irradiation sensor placed on dashboard.
A prototype car has been set up to perform on road and subjective tests.