Aplicación de encuesta y resultados

Thermal comfort research has predominantly been conducted within buildings where conditions are closer to steady-state and do not change significantly during the day. The vehicle environment is, increasingly, the focus of research; however, its environment is very different to those in buildings. Past experiences (Wohlwill 1974) and expectations (McIntyre 1980, Brager and De Dear 2001) can affect the perception of thermal environments. The steady physical environment in the laboratory does not reflect vehicle, particularly public transport environments (Lin et al. 2010).

The environment within a vehicle is best described as a transient, non-uniform environment (Walgama et al. 2006, Cengiz and Babalık 2007, Burch et al. 1991a). Vehicle conditions can change rapidly and are often affected by outside weather conditions more than buildings with radiant exchanges, in both the cold and heat having a greater effect (Parsons and Entwistle 1983). It is difficult for drivers and passengers to adopt more comfortable positions in relation to the heating and ventilating systems (Madsen et al. 1986). Behavioural thermoregulation is, therefore, reduced with the possibility of greater thermal discomfort. Vehicle interiors frequently experience strong vertical and horizontal asymmetries and localised sensations can result in thermal discomfort not reflected in PMV (Martinho et al. 2004). Vehicle passengers can also experience localised chilling, non-uniform radiant heat transfer, localised solar irradiation and non-uniform air velocities (Burch et al. 1991a). All these factors result in an environment that inherently differs from that experienced in buildings and therefore warrants specific research.

It is normally accepted to allow a dissatisfaction percentage of 20% in non-uniform vehicle environments rather than the normal 5%. That is, at least 20% of the occupants will be dissatisfied with the environment resulting from the acceptable comfort criteria/range being widened by 0.5 either side. So the acceptable range changed from +0.5 to -0.5 to a new range of approximately +1 to -1 (Lin et al. 2010).

ISO 14505, part 3 (2006) details how to conduct an assessment of a vehicle environment. Examples are given of relevant subjective scales such as: sensation, comfort, stickiness and preference. It suggests that at least 8 participants should be used in experiments that last a minimum of 30 minutes to determine the subjective experience of the environment. Criteria are listed to determine whether or not the environment is within comfort levels these change according to the number of participants used in the study.

Sakai et al. (1977 in Kogi 1979) questioned bus users regarding improvements to bus design and separated their requirements by topic. In the environmental category, air cooling and heating were the two factors with greatest priority for passenger comfort. Ensuring thermal comfort of the passenger is therefore important in establishing overall comfort throughout the journey.

The 6 basic parameters have been shown to integrate in their effect on thermal comfort in steady-state conditions. In the non-uniform conditions of a vehicle however, their magnitudes may not be the same. Madsen et al. (1986) investigated thermal comfort during the warm-up (in winter conditions) and cool-down (in summer conditions) of vehicles. They determined that there is a large impact of solar loads in the vehicle cabin. This was further investigated by Hodder (2002) who developed the PMVsolar model. Sensation increases were shown to be in

proportion to the amount of solar radiation and the model includes a correction to the normal PMV index to account for the effect of solar radiation in vehicles. In conditions with high solar loads, a variable airflow pattern results in the greatest comfort experienced by passengers (Hagino and Hara 1992). It is common for the greatest comfort to result from a transient experience.

Lin et al. (2010) examined sensations on short and long bus and train journeys. They note that conditions across the carriage or bus are often non-uniform, particularly from air velocities, with some passengers experiencing localised increases or fluctuations in this parameter. None, however, exceeded the maximum 0.8 ms-1 recommended by ASHRAE Standard 55 (ASHRAE

2003) even though discomfort was experienced. Therefore, comfort criteria specified in relevant standards may not be applicable in vehicle environments.

Actual mean thermal sensations (AMVs) on public transport are slightly lower than Predicted Mean Vote (PMV) and the difference is greater for longer journeys (Lin et al. 2010). This is likely to result from the non-uniformity of conditions resulting in sensations that differ from PMV. Sensations were collected throughout the year and no seasonal differences were observed, however, the experiment was conducted in Taiwan which has a tropical climate. Sensations in the UK may fluctuate seasonally due to the greater differences in outside environmental conditions during each season.

Passengers wear clothing appropriate for environmental conditions outside the vehicle with little or no adjustments made to adapt clothing for the vehicle environment (Lin et al. 2010). Longer journeys result in passengers wearing higher clothing insulation values, particularly in the range of air temperatures 24°C-28°C. If environmental conditions on the train are set according to appropriate building requirements then current conditions may be too warm for the clothing insulation of its occupants. This is because current indoor specifications are set according to indoor clothing requirements (i.e. no coats or jackets) whereas train passengers are likely to have higher clothing insulation values.

Tanaka et al. (2003) measured sensations at 22°C and -10°C air temperature before entering a vehicle with an interior temperature of approximately 30°C. Results showed that the cooler ‗pre‘ condition resulted in cooler sensations on entering the vehicle and a smaller overshoot compared with the 22°C. Sensations in the 22°C pre condition also resulted in the longest time to reach steady state, approximately 15 minutes in comparison with the -10°C‘s 5 minutes. Larger temperature differences when moving between environments may, therefore, result in an attenuated response to the environment with shorter times to reach steady-state. The paper does not state how many participants were used in the study or their ages, only stating that they were males ‗in their twenties to forties‘. It is therefore difficult to determine whether differences might occur with other experimental groups.

Models of human responses in vehicles

Ambs (2002) describes a segmented model of thermal comfort within in a vehicle that calculates the subjective sensation of the occupant. This is one of the few models within vehicle research that applies a meaningful descriptor to the model output, a factor that is useful for designers and engineers. The model, VISTEON, utilises work conducted by Brown and Jones (1997) who were furthering the work of Gagge et al. (1971). As the model segments the body into smaller

parts, localised sensations can also be calculated; an important factor in non-uniform environments such as vehicles.

Kaynakli and Kilic (2005b) describe a model of thermal comfort during the transient warm-up of a car that predicts both thermal sensations and skin temperatures. The model was compared against experimental work conducted by Burch et al. (1991a) and is relatively accurate in predicting each parameter. The model does not, however, accurately predict overshoots observed in skin temperatures of the hand, pelvis and chest. In spite of this, the model does seem to accurately estimate most other parameters although no comparison with existing models (such as PMV) is made.

A manikin was developed by Della Rolle and Romitelli (1993) and equipped with 16 sensors across the body to estimate passenger thermal comfort within vehicles. The manikin was able to predict both overall and local sensations; however, it was not validated against experimental data from human subjects. It is, therefore, difficult to determine whether this method is a suitable substitution for humans.

Burch et al. (1991b) compared a model that predicts thermal sensation with the existing Gagge (Gagge et al. 1971) model. The ‗Average Thermal Sensation‘ (ATS) model presented in the paper is a more accurate reflection of sensations during car warm-up period in winter conditions. Conditions did not, however, exceed comfort votes of +1 and the accuracy of the model above this point cannot be determined.