Thermal Comfort in Automobiles

Evaluation of Thermal Comfort in Vehicles

Major Breakthrough

LumaSense Technologies provides quality gas measurement for thermal comfort evaluation in vehicles. Two suppliers of passenger cars in Korea, namely Hyundai Motor Company and Kia Motors have invested in LumaSense's INNOVA thermal comfort solutions. The evaluation of the Thermal Environment in cars is a very complex task. These contracts are considered as a major break through in the Korean market. LumaSense has a solid reputation in this market segment with numerous orders from car manufactures in the USA, Europe and Japan.

A more thorough description of the equipment and system is given below. Read more about evaluation of Thermal Comfort in Vehicles in this application note.

High demand for thermal comfort precision

The thermal environment in a vehicle cabin is very complex and thus difficult to evaluate. These difficulties are due to the influence of convective, radiative and conductive heat exchange created by external thermal loads and the internal heating and ventilation system. Last, but not least, neither driver nor passengers are able to make up for the asymmetric climate conditions that set high demands for the Instrument and Transducers.

Thermal Comfort Measuring Methods

The usual method of evaluating the efficiency of the air conditioning system in vehicles is to apply sensors to measure the air temperature at feet and at head level. The main purpose being to investigate how quickly the system will raise or lower the temperature in a warm or cold vehicle and to study the difference between the temperature at feet and head level. However, using this method only one of the three needed parameters (air temperature, mean radiant temperature and air velocity) that concerns the thermal comfort sensation is measured. This fact is especially unfortunate in vehicles as the mean radiant temperature differs more from the air temperature that in buildings and because the air conditioning system can create high local air velocities.

Theory

Man's thermal comfort is a result of the combined effect of six parameters: activity level, clothing, air temperature, air velocity, air humidity and mean radiant temperature which are all described in ISO 7730. This method can be used for whole body evaluation only, however, based on measurement with several transducers at different positions simulating various body parts. The applicable transducer is the MM0057, which in shape simulates the body shape of a person, is heated and thus influenced by the air temperature, mean radiant temperature and air velocity. The activity level, clothing and air humidity are set as constant parameters. The activity level in a car will be sedentary which means that the level will be between 1.2 and 1.6 MET. The clothing in a car can vary from 0.3 to 1.5 Clo in relation to the time of the year. The humidity will depend on the outside climate, the air conditioning system and the number of persons in the car. The dependent parameter for sensation of warm or cool is the water vapor pressure. Normally this may vary from 0.8 to 1.8 kPa.

The Equivalent Temperature

The combined effect of: the air temperature, mean radiant temperature and air velocity can be expressed as the Equivalent Temperature that is related to the dry heat loss from the body. A number of standards are under preparation for the evaluation of the thermal environment in vehicles. ISO/NP 14505 defines the Equivalent Temperature as an integrated physical measurement of the thermal climate in the vehicle and presents methods for its determination.

Summary:

A new standard, ISO/DIS 14505 is under preparation, dealing with the assessment of the thermal environment in vehicles, based on the equivalent temperature. The scope of this paper is to demonstrate the different results obtained when evaluating the thermal environment inside a vehicle, using the three well-known parameters, air temperature, operative temperature and equivalent temperature.

The new ISO/DIS 14505 standard recommends the use of the equivalent temperature as the parameter used for evaluation of the thermal environment in vehicles. This paper shows that reasonable results can not be obtained by using either the air temperature or the operative temperature as the evaluation parameter. The measured time to each thermal comfort (PMV=0) is underestimated by 20-45% compared to the equivalent temperature.

ISO/DIS 14505 also describes various methods for measurement of the equivalent temperature, where one of the methods is the dry heat loss transducer.  According to K. Zimney et. al. (1), the measurement results obtained by using a number of dry heat loss transducers on the aluminium are of the same quality as those of a thermal manikin.(1) K. Zimney, H. Zenker, S. Doemoek, M. Ellinger; Comparison between measuren and computer simulated Teq. JTI report 270, Assesment of thermal climate in operators cabs, Florence, November 18-19, 1999.

Measured Thermal Comfort Parameters

The usual method for evaluating the performance and efficiency of the air conditioning system in vehicles is to measure the air temperature with small thermocouples at the feet and head level. The main purpose of such a measurement is to see how quickly the temperature will rise inside the cabin and reach the level where thermal comfort can be established. By measuring the air temperature only, any influence of air velocity and radiation (cold or hot), is neglected and the measurements might lead to false conclusions as only one of the three main climatic parameters that influences the degree of thermal comfort is measured.

Using the operative temperature, measured with an unheated ellipsoid shaped sensor, for the performance evaluation, the influence of the air velocity is neglected. As the air conditioning system in vehicles is based on heating/cooling by air flow, very high local air velocities are often seen. The cooling effect of these air velocities will therefore be neglected, and as demonstrated in the measurement this might also lead to false conclusions.

The combined effect of the air velocity, air temperature and mean radiant temperature can be expressed by the equivalent temperature. The difference between the operative temperature and equivalent temperature was carefully studied by Madsen et. al. [3] and was found to be the preferred parameter for the evaluation of thermal comfort, if high air velocities are present.

Measuring each parameter requires a lot of instrumentation and it is difficult to measure all the parameters in the exact same location and then later calculate the combined influence. Using a transducer that measures the combined effect of all climatic parameters, the equivalent temperature, makes the evaluation much easier.