**1. Introduction**

Seating comfort is one of the most important indicators of automotive seat performance [1–3]. Around the world, there have been many studies on seating comfort, including car seats, truck seats [4], and bus and train seats [5]. A seat that is comfortable in static conditions may have poor dynamic characteristics that make it uncomfortable on the road. The profile of a bus seat needs to be designed ergonomically for various body sizes of passengers. Most automotive seats are not designed according to anthropometry, neither are the automakers willing to invest resources in designing components appeasing human ergonomics [6].

Nowadays, urban space, especially in large cities, faces several challenges resulting from the permanently increasing number of inhabitants. One of the urgent issues to be solved is a safe, comfortable, and quick way of commuting for the residents of urban agglomerations. However, along with overloaded and rapidly expanding urban space, along with modernization of the existing road infrastructure, the growth of traffic level is also significant [7–11].

A natural way of solving such a challenge is to develop collective transport and encourage the still undecided citizens to use it. Residents choosing the mode of transport in urban agglomerations analyze various factors, among which the largest impact on their

**Citation:** Kernytskyy, I.; Yakovenko, Y.; Horbay, O.; Ryviuk, M.; Humenyuk, R.; Sholudko, Y.; Voichyshyn, Y.; Mazur, Ł.; Osi ´nski, P.; Rusakov, K.; et al. Development of Comfort and Safety Performance of Passenger Seats in Large City Buses. *Energies* **2021**, *14*, 7471. https:// doi.org/10.3390/en14227471

Academic Editors: Guzek Marek, Rafał Jurecki and Wojciech Wach

Received: 16 October 2021 Accepted: 6 November 2021 Published: 9 November 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

decisions are safety and comfort (which affect visual, thermal, acoustic, and vibration aspects) [12]. Research studies have already shown that passenger comfort is a key factor in the choice of the means of transport, and its improvement may attract more users of urban transport [13–17]. Following investments in the safety and comfort of collective transport, the quality of life in contemporary cities may be improved. On the other hand, increased road safety could be achieved in many other ways, too (Figure 1). Passenger protection while travelling is one of them, i.e., the design of safe and comfortable seats along with safety belts. A seat's dimensions must be designed to suit the anthropometry of passengers when designing ergonomic seats. Optimum passenger seat design according to passenger anthropometry can decrease fatigue and discomfort during long periods of sitting [18]. The passengers of the vehicle must be comfortable, as the discomfort can result in fatigue, which can lead to a condition of body imbalance, since the passenger seat used does not usually correspond with the wearer's anthropometry (non-ergonomic). When it comes to the passenger seat design sector, an ergonomic factor and aspect will ensure greater comfort and less fatigue for passengers [19]. According to The Harvard School of Public Health, ergonomics is the science, art, and application of technology that aims to harmonize or balance between all facilities used at work and rest with human abilities and limitations so that the overall quality of life can be increased [20].

**Figure 1.** Examples of ways to improve road safety.

A literature review on the comfort of bus driver seats indicates several medical problems resulting from their long-term use [21–23]. Bus drivers are struggling with such problems as sleepiness, cramps, muscle fatigue, difficult circulation, spine pain, depression, stiffness, pain, numbness in the spine, and other musculoskeletal problems that may become chronic [24–30]. For that reason, to reduce these medical problems resulting from a malformed bus seat, further research should be carried out on the existing bus seat profile. Bearing that that in mind the authors tried to fill the knowledge gap by fully understanding the mechanisms of stress and strength distribution for a specific seat construction design, the present research focuses on specific solid plastic frame construction, which has not been fully studied in the available scientific literature. The study concerns bus seats occupied by an adult (50 years old) and a young (10 years old) passenger, concerning different seat solid plastic frame mounting scenarios.

A comparative analysis of existing bus seat profiles and their evaluation may be presented in the following aspects: (i) comfort evaluation and (ii) ergonomic evaluation. In the comfort evaluation part, 48 participants evaluated seven parts (headrest, upperback support, lumbar support, seatback bolster, hip support, thigh support, and seat

pan bolster) [25]. There are 12 components of the bus seat that directly affect passenger comfort, including seatbelts, armrests, recliners, headrests, dorsal support, lumbar support, side back support, seatback overall support, hip support, thigh support, and seat pan overall support. In addition, 17 ergonomic evaluation measures (such as reachability, controllability, tactile sensation, grip sensation, adjustability, size appropriateness, shape appropriateness, cushioning, and overall comfort) were selected to be assessed based on literature reviews [31–33].

In addition to comfort, safety is an essential requirement. The subject of traffic safety has been investigated by many researchers in the context of cars, buses, and coaches [34,35]. Rupp et al. [36] found that the fracture tolerance of the femur is 7.59 kN. In the US, the National Highway Traffic Safety Administration (NHTSA) specified a maximum femur load of 10 kN for a male dummy of the 50th percentile [37]. Leg femurs on both sides experienced 5.2 kN loads. Although within the safe limit, this model predicted a high pelvis load. Due to the dummy posture, the knees are the main point of contact between the seat back and the femurs. The pelvis injury tolerance was determined based on peak pelvis acceleration according to Haffner [38]. Using 130 g of acceleration on the pelvis, he proposed that the occupant's pelvis can suffer serious injury. When the setback impact occurs, the model predicted a peak acceleration of 33 g for the pelvis [39]. There was a similar interval between the peak acceleration of the pelvis and the peak load of the femur. However, it is below the safe threshold of injury for the pelvis acceleration level. Mertz and Patrick [40] conducted a study showing showed that the human chest is capable of bearing a distributed load of 49 g. This value should not exceed 60 g, according to the FMVSS 208 test.

The development of the Bus Safety Standards (BSS) in different countries included seat testing, both in computer simulations and sledge testing (which replicates the collision forces in a repeatable way, but for testing just the seat in isolation and not the entire vehicle). This testing compared traditional low-back seats against medium (taller) back seats and high (for example coach style) back seats. In rear-facing seats, the BSS encourage high back seats. The additional weight of the different seats makes them difficult to implement throughout the entire bus. European Union Transport Politics aims to emphasize the importance of using public transport rather than private transport while focusing on the safety of the passengers and reducing pollution [41,42]. The World Health Organization (WHO) acknowledges this fact in its annual report on global road safety, where they advocate for better public transport that is safe, accessible, and affordable because it is vital to increase safety in urban areas where traffic has become more crowded [43–45].

For the purpose of the present research, the comfort and safety of solid plastic frame seat performance were evaluated based on stress deformation and kinematic behavior. The seats' frame structure and the material used in the modeling and analyses are considered not to be fully investigated in the available literature, making the research performance the biggest motivation.

### **2. Materials and Methods**

Seating comfort analysis can be performed using vibration evaluation, electromyography, electroencephalography, oxygen saturation, posture-image analysis, spinal loading, computer-aided engineering (CAE), pressure, temperature and humidity monitoring, etc. [46–48].

The experimental, purpose-built physical model of the bus that had a front seat and two rows of seats is presented in Figure 2. The model concerned a dummy hybrid representing a 50-year-old man (M50) and a dummy representing a 10-year-old child (P10). The position of the dummies, the seats, and the safety belts were adopted based on the assumed real event scenario. All the characteristic dimensions could be followed in detail in Table 1.

**Figure 2.** Dimensions describing the dummy position concerning the seat and safety belt location: (**a**) front view M50, (**b**) front view P10, (**c**) side view (modified after [49]).


**Table 1.** Distances describing the position of the dummies.

The Mathematical Dynamics Models for Applications (MADYMO) software program uses the method of multibody system dynamics for the formulation of a numerical model. Here, a chain of rigid bodies is linked together using kinematic pairs to copy or model an object. Several physical variables determine the bodies as well as the kinematic joints, which make it possible to solve the equations governing movement. There are three parameters used to describe rigid bodies: mass, moment of inertia, and center of gravity. The kinematic pairs defined specify the bodies that are joined with each kinematic pair. In addition, the coordinate location of those bodies is determined as well.

A general equation, Equation (1), is used to describe the relationship between the position of a local coordinate system and the global coordinate system, where *Xi* is the matrix of the coordinates of the position vector, *ri* is the matrix of the coordinates of the vector joining the beginnings of both coordinate systems, *Ai* is the matrix of direction cosines, and *xi* is the matrix of the coordinates of the vector of local displacement of the coordinate system.

$$X\_i = r\_i + A\_i x\_i \tag{1}$$

The numerical model for the solution to the general equation of motion Equation (2) uses a modified single-phase Euler's method with a constant time step *ts*:

$$M\ddot{\mathbf{x}} + \mathbf{C}\dot{\mathbf{x}} + \mathbf{K}\mathbf{x}\_t = P\_t \tag{2}$$

where *M*—mass matrix; *C*—matrix defining the damping of the system; *K*—rigidity matrix; *xt*—displacement; *Pt*—matrix defining external loads applied to the system. This matrix is usually adopted in the form of the so-called proportional damping (depending on *K* and *M* matrices).

There are 3 types of mechanical models, in which fuzzy models and neural networks are not used because of their non-linearity. Unfortunately, due to the nature of the experiment, the ramification cannot be used to achieve the desired damping force in an open control system. In this case, the Bouc–Wen model was used. As described by Spencer et al. [50], the model is commonly used to elaborate the MR damper hysteretic characteristics. In the present experimental study, the controlled MR damper has high vibration restitution when compared with the passive MR damper. A controlled MR damper presents the better performance by 24% of road-holding vehicles when compared with fully active and 22% on an ideal semi-active suspension system [51].

The hysteretic behavior of the MR damper was depicted with the help of the Bouc– Wen model [50]. A scheme of the mechanical diagram of the MR damper is shown in Figure 3. The dampers can be used as comfort and noise protection elements, too [41–54]. Figure 3 presents the general view of the soundproof partition of the bus motor. The crosssection shows the bus septum, where 1—engine compartment, 2—passenger compartment, 3—DVA, 4—internal part of the partition. The partition has an elastic fastening, which is the external part of the sound absorber of reinforcing elements. A damper on the driver's seat is a required feature. However, transport comfort can be increased by fitting dampers to the passenger seats as well.

**Figure 3.** Mechanical model of MR damper (**a**), and general appearance of a bus soundproofing system (**b**).

From the empirical point of view, the mechanical model is governed by the following equations

$$\dot{y} = \frac{1}{c\_0 + c\_1} \left[ \alpha z + k\_0(x - y) + c\_0 \dot{x} \right] \tag{3}$$

$$\dot{z} = -\gamma \left| \dot{\mathbf{x}} - \dot{\mathbf{y}} \right| z \left| \stackrel{n-1}{\quad} z - \mu(\dot{\mathbf{x}} - \dot{\mathbf{y}}) \left| z \right|^{n} + A(\dot{\mathbf{x}} - \dot{\mathbf{y}}) \tag{4}$$

$$f\_{MR} = c\_1 \dot{y} + k\_1 (\mathbf{x} - \mathbf{x}\_0). \tag{5}$$

Based on dimensional differences, 12 bus seats were classified based on their features for the purpose of the study. Accordingly, lumbar supports were classified in reference to dimensional differences between extracted centerline and seat pan lines measured for the measured bus seat profiles. This means that the lumbar was shaped for ≥25 mm (mean) or flattened to achieve the lumbar shape of <25 mm on the center line, and the designed side lumbar support was shaped for ≥45 mm or flattened to achieve the side lumbar support shape <45 mm than the side lumbar support on the seat pan lines (Figure 4) [48].

**Figure 4.** Critical cross-sections and outlines of a bus seat profile.
