Keywords

1 Introduction

Human behavior, perception and productivity are very affected by the environmental characteristics in which an individual is inserted. Some can be controlled and adjusted but others are inherent to the local. When a certain physical environment answer the user’s functional (physical and cognitive) and spatial (psychologic) needs it most certainly will have a positive impact on task fulfillment.

On public transportation vehicles, such as urban busses the vehicle heating, ventilation and air conditioning (HVAC) systems are the most flexible tools to control users comfort level and so ensuring the best environment possible to both driver and passengers inside the vehicle. According to Riachi and Clodic [1] the higher we find the occupation rate to be, more efficient air control and exchange systems are required. Thermal comfort plays an important role as per the driver under optimal conditions won’t be distracted and users would be more likely to choose the bus as their way of transportation.

Urban busses environment differs from those found inside buildings, which have fixed geographical orientation and low relative occupation rates, making thermal control through ventilation openings management more effective.

The project for thermal comfort in automotive vehicles is one of the most important features at the conception phase and must consider variants as air flux, humidity, human and environmental heat exchanges among others [2,3,4,5]. Without this system, thermal comfort is acquired by adjusting windows and hatches for optimal airflow and therefore heat exchanges. In Brazil, notoriously famed by its tropical climate, HVAC technologies aren’t systematically applied to the urban bus fleet, reason why it’s so important to verify the possibility of thermal comfort status acquisition through passive solutions on heat exchanges. The regulation “ABNT NBR15570:2011 – Transport—Technical specification for vehicles of urban characteristics for public transport of passengers manufacturing” [6] oversees the urban bus characteristics and specifies the number and area of windows and other ventilation devices, allowing projects with window openings in the top, bottom or both halves.

Thermal comfort sense evaluation, considering the human body and environment interactions is a serious topic to ensure the best comfort state for both passengers and driver inside the vehicle. Several studies focus on thermal well-being on humans and a considerable amount of information is available in the literature, with most studies considering stable and uniform thermal conditions amongst the various vehicle occupants [7, 8].

In a general manner, thermal comfort can be defined a condition when the body temperature doesn’t change abruptly, skin moisture is low and the physical effort to temperature regulation is at its minimum [9]. As this concept changes, accordingly to the local climate and user adaptation to the bioclimatic features, as well as their psychological and physiological factors, it is possible to define through a bioclimatic diagram – in which relations between temperature and air relative humidity are related using a local climatologic series – the hygrothermal comfort values throughout the year.

To be possible to assume a certain individual to be in a thermal comfort situation, Frota and Schiffer [10] says that some premises must be evaluated such as the subject’s thermic neutrality (which consists on dissipating all the body heat to the environment throughout conductive, radiative and evaporative heat exchanges), the sweating rate and skin temperature must be under acceptable limits according to the activity in play, considering the energy needed to perform the given task and the fact that the individual must be free from stress caused by temperature differences due distinct object’s heat absorption rates or wind gusts. Lamberts [11], in the other hand, uses the information provided by the ISO 7730 [12] regulation to suggest that a person dressed with regular clothes (insulation = 0.6clo) is in a comfort situation between 23 °C and 27 °C.

To Lin et al. [13] studies on urban transportation vehicles can be classified on two types: the first approach consists on tests under controlled laboratory circumstances and does allow large scale production vehicles to be studied under a variety of scenarios such as the influence of infrared radiation on windows with and without tinted films. Many regulations rely on this approach, like the ISO 14505-2:2006 [14]. In the second approach, which consists on study the thermal comfort at the field, the premise that user’s behavior cannot be reproduced on laboratory controlled environments and any attempt implies on uncertainties about real-life conditions which can affect the obtained results is made. Field experiments, on the other hand, could detect and analyze physiological, psychological and behavioral variables and, therefore, can determine how certain local groups relate with the thermal environment in ways that cannot be completely covered by mathematical models of thermal balance.

The authors also found differences in the preferred way to reach a comfort status between short-haul and long-haul bus lines users. Short-haul trips have an average duration of 30 min and average speed of 40KPH and passengers usually adjust airflow to obtain thermal comfort, as opposed of long-haul trips, which last an average of 60 min at an average speed of 100KPH, where the users prefer to adjust solar incidence by opening or closing the curtains.

Even though vehicular HVAC technology has been improving rather fast over the last few years, the studied short-haul bus scenario doesn’t seem to follow the trend on most cities leaving both passenger and driver under poor environmental conditions.

2 Review of Current Standards

The assessment of thermal environment inside a vehicle cabin correlates several variables and can be carried out trough different approaches. In response to these different possible evaluation forms the standards present several methodologies and procedures to obtain the most reliable results when considering the environmental and physiological variables that can be applied in predictive mathematical models which can end up in objective results or personal and psychological results that addresses the individual’s perception of the environment and can vary individually [15].

The Brazilian standards for thermal comfort assessment are predominantly focused to be applied on buildings [16,17,18]. The ABNT NBR-15570 [6], which brings the technical specifications to produce urban vehicles with characteristics suited for public passenger transport, briefly presents design requirements and partial evaluation – as in the case of motor insulation – to achieve thermal comfort without, however, presenting a methodology for evaluating the entire environment. In the case of vehicles equipped with HVAC systems, it is possible to apply NBR 16401-2 [17] as an alternative.

When talking about international standardization, there’s a significant range of standards dealing with indoor thermal environment analysis. The most used are the ISO 14505 [14, 19, 20], which deals directly with the matter applied to vehicles and the ASHRAE 55 [21], although not referring to vehicle cabin application, defines the conditions for human thermal comfort using factors such as the air relative humidity, considered as an influential factor if less than 30% or greater than 70%, and effective temperature, if less than 20 °C and greater than 24 °C [22].

Medeiros [9] indicates ISO 14505: 2007 [14, 19, 20] – Ergonomics of the thermal environment - Evaluation of thermal environments in vehicles as the most relevant in the context of the evaluation of the thermal environment in vehicles, since it is the most used worldwide by vehicle producers in the development and conformity verification of their products, reason why we will emphasize their study. The ISO 14505 [14, 19, 20] is segmented into three parts: the first one introduces the principles and methods to determine the thermal stress, the second indicates how the equivalent temperature must be determined and the third part defines how to evaluate the thermal environment through analysis of individual’s subjective response.

On its first part the ISO 14505-1:2007 [19] – Principles and methods for assessment of thermal stress, it is determined that the thermal environment inside vehicles is influenced by both the climatic conditions of the external environment and the capacity of the HVAC system to act in the internal environment reducing the unfavorable effects on the influence of the external environment. The standard directs the evaluation of the internal thermal environment according to the effects verified in the conditions of heat stress, cold stress or thermal discomfort. After determining the cause of the thermal problem verified by this previous assessment it is suggested the application of subsidiary standards focused on each condition as per specific characteristics of the situation found.

On its second part, the ISO 14505-2: 2006 [14] – Determination of equivalent temperature, guidelines are presented for the evaluation of the thermal conditions inside the vehicle cabins and could be applied in the assessment of other confined spaces with asymmetric climatic conditions, focusing on the evaluation of thermal conditions in situations which deviation from the thermal neutrality is relatively small. This part is based on the determination of the equivalent temperature to verify its applicability – since it is possible to determine the relation between temperature measured in real and ideal conditions (in which, theoretically, there would be neutrality among the body and the interior environment by the sensible heat exchange between them in the same proportion) through the determination of the equivalent temperature that receives different approaches according to the objective of the evaluation. Another valid method is the application of thermal sensors in either manikins that simulate individuals or on human beings using real life clothing, dividing the body into zones that allow correct measurements in both cases. The reliability of the method should be treated with caution when skin evaporation, body transpiration and other human factors are involved.

On its third part, the ISO 14505-3: 2007 [20] – Evaluation of thermal comfort using human subjects, guidelines are given and a standard method of testing for the human perception of thermal comfort in vehicles is presented. The proposed method for evaluation is not restricted to a single vehicle style and makes possible the verification through general principles that can be used to measure and determine the performance of the thermal environment under a given condition of interest, causing it to be applied in both development and verification of product reliability. For those modals without cabins in which the user is directly exposed to the external environment, the speed of movement and the meteorological conditions may interfere excessively in the evaluations. However, the principles of this standard remain applicable and its application is also possible to passengers and operators simultaneously as it does not interfere with the safe operation of the vehicle.

After presenting the standard, it is possible to realize that although the ISO 14505 [14, 19, 20] should be used in the design phase of the vehicles and in the validation of the results obtained by researches that verify the thermal characteristics of the thermal environment inside the vehicle its application is laborious and relies on specific materials, tools and certified technicians. Due these factors, it is proposed the preliminary verification of the thermal environment through this work proposed methodology, enlightening the initial problem understanding through simplified procedures therefore facilitating the decision-making process and reducing efforts to define the nature of the problem.

3 Methodology

Based on the analysis of ISO 14505 [14, 19, 20], ASHRAE 55 [3] and NBR 15570 [6] standards, the main variants to be applied in the method were defined by its ease of measurement with simple tools or could be measured using an instrument capable of multi-variable measurements or mathematic models. The variants were defined as per below: (Table 1).

Table 1. Chosen variants
Full size table

The measurement of Dry Bulb Temperature, Wet Bulb Temperature and Air Relative Humidity (variants 1–3) can be done by a digital thermo-hygrometer with relative ease. In general, it is recommended to switch on the instrument at least 2 h before taking the measures to ensure the time needed for sensors stabilization, although this period may vary for each instrument manufacturer, as the gap between single measurements. The Internal Surface Temperature (variant 4) can be measured by a simple infrared thermometer.

Based on the measurement procedures recommended by the standards, it was decided to carry out the measurements in one point at the center of the vehicle, at a 1.10 m height from the vehicle floor, which is common to sitting or standing passengers, instead of the three heights proposed by the standards. The decision to concentrate the measuring points at the central point is based on the fact that, due to the central doors in the vehicles to apply accessibility criteria, the three standardized measurement points have similar characteristics. In this proposal, vehicle engine is considered as thermally isolated, without contributions to interior cabin heating. To ensure a more reliable result, it is recommended that the measures are made under a real travel, by the period of 30 min, taken every 5 min. These measurements can be after normalized in a mean value that will be used on calculation.

The measurement of the internal surfaces temperatures taken by the infrared thermometer allows verifying if there are significant radiant temperature influences over the other parameters. If the surfaces temperatures are close to Dry Bulb Temperature values, it could be ignored and, if they’re significantly different it is recommended to report this could be an interference factor on the results. This may be used as an additional data once the measurement of the real radiant temperature depends of a Black Body Thermometer, which makes no sense when talking of a simplified methodology.

After the proposed measurements were taken, it is suggested to use the Effective Temperature Index calculated and the Air Relative Humidity value to cross-check the data. The Effective Temperature Index (variant 5) can be calculated by Eq. (1), the Nieuwolt equation [21].

$$ ET = DBT{-}[0. 5 5* ( 1 { }{-}0.0 1RH) * (DBT{-} 1 4. 5)] $$
(1)

In which:

ET:

– Effective Temperature Index

DBT:

– Dry Bulb Temperature

RH:

– Air Relative Humidity

After calculating the Effective Temperature Index the results can be plotted on a graphic that correlates RH and ET. Although its subjective character and wide matter discussion, the proposed comfort zone will remain between Effective Temperature 20 °C to 24 °C and Air Relative Humidity 30% to 70%, once they could be considered as restrictive values. The comfort zone is illustrated on Fig. 1.

Fig. 1.
figure 1

ET – RH comfort zone

Full size image

In purpose of simplifying the methodology, the influence of the activity was deliberately disregarded, considering the users in rest and thermal equilibrium with the environment and not considering the interference of the clothing in the thermoregulation of the organism.

4 Application

The method was tested by taking measurements inside a bus used to transport students inside the Federal University of Juiz de Fora (UFJF) campus. This vehicle shares the same model as the other city busses found in Juiz de Fora city – Brazil, as well as the same seat and window configuration plus have a fixed daily route and schedule. The bus, as all other Juiz de Fora city urban busses does not have a HVAC system, relying only on window management to achieve thermal comfort.

It is important to say that according to ABNT [16] Juiz de Fora city is included under the bioclimatic zone 3, which corresponds to a highland tropical climate, showing slight lower mean temperatures as found in a typical tropical climate region, this is due the altitude difference. Juiz de Fora also is known for high thermal amplitudes during the day. Although occurring throughout the whole year, precipitation is more intense during late winter and mid spring seasons.

Measurements were taken at 08h00m, 12h00m and 17h00m on every day for 65 working days in between September 1st and November 30th 2016. Each trip had an average time of 30 min and measurements were made every 5 min (Fig. 2).

Fig. 2.
figure 2

Bus route inside the campus (Color figure online)

Full size image

During the trip, with the route illustrated in red on Fig. 2, variables 1 through 4 (described in Table 1) were simultaneously measured using an ICEL TD-990 infrared thermometer and an Instrutherm HTR-157 digital thermohygrometer. The infrared readings showed the bus internal surface temperatures quite near the dry bulb air temperatures, and therefore allowing the radiant temperature effects to be neglected without highlighting its possible influences within the thermal environment. After the measurements, the obtained values were normalized and organized in a graph which corresponds to the relations between DBT, WBT and RH, as shown below: (Fig. 3).

Fig. 3.
figure 3

08h00m series measurement

Full size image

At 08h00m is possible to verify high RH rates while the temperatures remain relatively low. This situation is due the local climate factor added the fact it’s the beginning of spring in the southern hemisphere and therefore the rain season, that falls mostly during the night keeping the RH high until the middle of the mornings (Fig. 4).

Fig. 4.
figure 4

12h00m series measurement

Full size image

By noon, the high temperature and RH rate variation became more evident, as well as the temperature increase with the summer approach. The verified situation is due the higher solar incidence which increase the temperatures, some sparse rain occurrence increasing RH rates on a few days, while on other days it is possible to perceive a drastic reduction when compared to the 08h00m readings. The increased distance between DBT and WBT readings is mainly due this reduction in RH (Fig. 5).

Fig. 5.
figure 5

17h00m series measurement

Full size image

At 17h00m it’s possible to verify an overall raise in temperatures and a further reduction on RH due to the increase in solar incidence and the higher temperatures. The variations between DBT and WBT are caused by the lower RH measured with a few peaks due sparse rains in the end of the afternoon.

With all the data acquired and in possession of the Nieuwolt equation previously described, the effective temperature index calculation was possible. These calculations were done to all three series of data as presented in the following images: (Figs. 6, 7 and 8).

Fig. 6.
figure 6

08h00m series effective temperatures

Full size image
Fig. 7.
figure 7

12h00m series effective temperatures

Full size image
Fig. 8.
figure 8

17h00m series effective temperatures

Full size image

After comparing each effective temperature for a given day and time with its relative humidity rates, the graph presented on Fig. 1 could be populated for a better analysis of the thermal comfort situation, as follows: (Fig. 9).

Fig. 9.
figure 9

08h00m series thermal comfort graph

Full size image

By 08h00m only one amongst all 65 trips showed satisfactory thermal comfort indexes. On the remaining 64 trips, it’s possible to realize that the low effective temperatures and high relative humidity rates cause discomfort by cold to the passengers (Fig. 10).

Fig. 10.
figure 10

12h00m series thermal comfort graph

Full size image

At noon, only 09 of all 65 trips presented satisfactory values according to the comfort parameters adopted in this simplified methodology. In the remaining trips a cold discomfort can be perceived when high RH and low ET are combined. Heat discomfort is found when high RH and comfortable ET values occur, giving the passengers a fug sensation (Fig. 11).

Fig. 11.
figure 11

17h00m series thermal comfort graph

Full size image

The biggest comfortable situation found in this survey occurred in the end of the afternoon, when 28 out of 65 trips presented ET and RH readings amongst the adopted comfort parameters. In this scenario, cold discomfort is caused by low ET readings and high RH, while heat discomfort is caused by high RH readings at comfortable ET readings and high RH readings paired with high ET values.

5 Conclusions

Analyzing the results presented in the previous section, a table containing the absolute number of comfortable and uncomfortable trips was formulated and presented as follows: (Table 2).

Table 2. Comfort rates by each measurement series
Full size table

It’s evident the high number of uncomfortable rides in any given time series, which shows how inadequate the Brazilian city bus model is for Juiz de Fora City facing the local climate and without a proper hygrothermal regulation system.

The rates shown above can indicate a poor open window dimensioning or even inadequate use by the passengers. These hypothesis, although raised, must be further studied by future researches once the current paper focus on proposing a simplified methodology for thermal comfort assessment, without seeking the causes of discomfort.

Although the problems could be extinguished by adopting an HVAC system, more sustainable alternatives, such as passive shading and ventilation devices, can be studied and implemented reducing energy consumption and operator expenses while maintaining or improving user comfort.