Model-Based Predictive Detector of a Fire inside the Road Tunnel for Intelligent Vehicles

The paper proposes a method for detection of a ﬁre inside the road tunnel without direct view on the ﬁre, using on-board vehicle technologies. The system is based on comparing the measured development of temperature and smoke with model scenarios precomputed for a given road tunnel. The ﬁre scenarios are computed by HW/SW tool TuSim regarding the parameters of the real road tunnel and then the results are presented to the vehicles via car-to-infrastructure communication link. The proper detection of the ﬁre allows early evacuation of the vehicle passengers, which will signiﬁcantly increase chance of their survival. The computed scenarios also provide supporting information for the rescue teams.


Introduction
Safety of transported persons and material is an integral part of toady's transport. From the viewpoint of risk and subsequent damage, the worst place is the road tunnel. In case of accident, several dangerous situations occur in the tunnel. One of the most dangerous accidents at all is the fire inside the tunnel. Even when the fire is detected by sensors and cameras installed in the tunnel tube, statistics says that not all passengers in the threatened zone will evacuate from the vehicles in time. We believe that this drawback can be suppressed when not only the operators of the tunnel but also the vehicles themselves are sensing and detecting the indicators of the fire-rising temperature and opacity.
Subsequent recovery after the accident is also challenging and can expose other persons to danger. erefore, we propose a simulation model that is able to estimate situation in the tunnel during an accident event. Furthermore, such information can be utilized by rescue teams. e paper deals with an effect of the technological equipment on safety of humans and property in the tunnel.
To reach a tolerable level of safety, the heterogeneous complementary systems must be installed in the tunnel. e numerical expression of the risk is problematic. erefore, simulation is one of few possibilities of how to safely compare various variants and test the system limits, socalled worst cases, as an alternative to the prescriptive risk analyses. is paper is focused on the simulation of critical scenarios itself, on the creation of trustworthy models and their subsequent verification and validation.
Nowadays, the tunnel simulators may be classified to the following groups: (1) Simulators used to train tunnel operators, dealing mainly with virtual reality or tunnel visualization [1]. (2) Drive simulators used to train drivers driving through the tunnel; they make it possible to monitor and analyse drivers' behaviours, technical parameters of their drives, minds, subjective feelings, etc. [2]. (3) Specialized simulation tools such as IDA RTV software [3].
(4) Simulators based on the PLC (programmable logic controller) that are mostly used to verify control of tunnel technologies under various modes before putting the control system into operation [4,5]. ese simulators may utilize additional specialized tools for simulation of the tunnel technology and physics; an architecture of such a simulator is depicted in Figure 1.
Fire, in general, can be detected by the following way: (1) SOS button (2) Video detection (3) Smoke sensors (4) FibroLaser line detector In principle, video detection can detect a fire in several ways, by detecting a car/vehicle stop, fire, or smoke in the monitored zone. In time, this method is the fastest method; on the other hand, false alarms can occur from the smoke in tunnel through the passage of truck or light reflection. In this case, in Slovakia the tunnel is usually closed only after approval by the operator. e detection of stopped vehicle is almost immediately. ere are two types of smoke sensors: opacity sensors of specialized smoke sensors. Opacity sensors must be installed closed to portal, branching at maximum distance of 1000 meters along tunnel. According the manufacture's materials SIGRIST, the opacity sensors have a measuring range of 0-100 km; accuracy in the range 0-15 km must be ± 2 km. ese sensors can be also used for fire detection; they are usually installed on the wall of the tunnel. Smoke sensors must be installed at the maximum distance of 150 meters along tunnel. e measure range of these smoke sensors is 0-15 km and accuracy is 0, 2 km; they are usually installed on the tunnel ceiling.
Typical values in measuring opacity: (1) Normal traffic <5 km (2) Heavy traffic ∼5 km (3) Traffic jam ∼7 km (4) Tunnel close 12 km (5) Fire >15 km FibroLaser detects an increased heat in fire in the tunnel. It is built up of optical cables and control unit that emits a laser beam into the cable and analyses its reflection. ere is a Raman effect, when the reflected laser beam is divided into Stokes and AntiStokes signal and temperature change in optic cable is evaluated based on the difference in the intensity of these signals. It is possible to implement Fibro-Laser into simulation programs according the data of the manufacturer Siemens.
ere are three rules for how to detect the increase in temperature: (1) Overrun defined maximum value (2) Overrun the maximum difference from the average temperature zone (3) Overrun the maximum increase of temperature in define time

Tunnel Simulator TuSim
e tunnel simulator (TuSim), developed by the authors of the article, is based on the programmable logic controller (PLC). e TuSim is a complex HW/SW solution based on the industrial personal computer (PC), Bernecker and Rainer (BR) Automation PC acting as a PLC. e TuSim is shown in Figure 2 and consists of (top-down view) the Masterview liquid-crystal-display (LCD) switch of the visualization server, BR Automation PC (in the left bottom), and the backup UPS unit (in the right bottom). e uninterruptible power supply (UPS) unit ensures simulation of the continuous operation.
In the BR Automation PC, the PLC of Siemens S7-400 type is simulated including the technological components as well. e TuSim simulator makes it possible to simulate three types of the tunnel, 1000 m long each: (1) Urban tunnel (MST) (2) Highway one-tube tunnel (D1T) (3) Highway two-tube tunnel (D2T) Visualization of technological equipment is ensured through the human-machine interface/supervisory control and data acquisition (HMI/SCADA) displays.
e whole system has an open software concept for future extensions from the level of software in the PLC up to the design of graphical screens. e simulator is not connected to the data flow of a real tunnel. e HMI server takes care of data collection, archiving, and distribution from PLC clients. Selection of data to be archived in the database at the server is configured by the Database Logger [6], and then data may be shown in all clients. In addition to data display, the clients also make it possible to perform control interventions in displays individually for each technological sub-system. To display and change the screens, the tools CimViewer and CimEdit [7] from the HMI/SCADA CIMPLICITY software are being used.

Simulation Models
In the first version, the TuSim was a drive simulator helping service operators to become familiar with the control system of the tunnel and to simulate emergency events manually via so-called reflexes. Since no mathematical-physical models, needed to simulate functionality of the many tunnel systems, were built-in, they had to be integrated additionally. Figure 3 shows models important for basic functionality and interactions between them. e models in blue fields are implemented directly on the PLC level, others on the HMI/ SCADA level. Journal of Advanced Transportation

Traffic Flow Model.
As it is apparent from Figure 3, the model of air flow processes the piston effect resulting from the movement of vehicles inside the tunnel as one of its inputs. Under the one-way traffic, vehicles put the air to motion. e more significant the contribution of that, the higher the volume of vehicle intensity. Products of combustion in the tunnel are also an important input to control air technology. It is also important to know composition of traffic flow, since trucks and buses have much higher impact on emissions in the tunnel environment. More information about the traffic model of the TuSim is in [10][11][12].

Tunnel's Tube
Model. e tunnel tube may be modelled by multiple ways. One of them is based on analysis of the relationship between its inputs and outputs. is approach has been used in [13]. To describe a linear part of the model, we used the state model. e non-linear part of the model includes saturation and transport delay. e model of the tunnel tube is expected to enable changing of carbon monoxide CO and nitrous oxides NO x sensors positions.
at will make it possible to monitor variances in ventilation control. Concentration of emissions in time and space may be calculated using the equation for the longitudinal ventilation [14]: where t is time of simulation (s) and x is distance (m). If the measured and calculated velocity of the air flow in the tunnel is available, we can immediately use solution of the equation from [14], for example, for the values of CO in a steady state: where (1) C(0) is the concentration of CO at the input portal of the tunnel (μg/m 3 ) (2) v is the velocity of the air flow in the tunnel (m/s)   Journal of Advanced Transportation the longitudinal ventilation for one-way traffic will create a linear function, with growing concentration of CO emissions towards the output portal of the tunnel. In [15], there are a discrete microscopic traffic model and emissions modelled for each section occupied by a vehicle according to the tables of emissions [16]. e advantage is that emissions from vehicles do not depend on shape of the tunnel; thus, the model may be used universally. Figure 4 shows comparison of our traffic model extended for the model of the tunnel tube with the model of steady state conditions. After velocity of the air flow becomes stabilized, the values of CO emissions in the microscopic model are close to the values of emission models of the macroscopic traffic model along the whole length of the tunnel. For the needs of the traffic model, we have adopted the PIARC tables [16] for discrete velocities used in the traffic model -0 (km/h), 30 (km/h), 50 (km/h), maximum velocity in the tunnel 80 (km/h), maximum gradient 4%.

Air Flow Model.
Simulation of the air flow in the tunnel is a complex problem demanding numeric solution of Navier-Stokes non-linear differential equations. eir solution is too time demanding to be used in the real-time in the PLC. As an alternative, one might use Bernoulli equation for one-dimensional liquid flow while we consider that the air is as an incompressible fluid: where (4) h is the height difference between the ends of the tunnel (m) is formula is valid only in the case of an ideal liquid; in the case of real liquid, the equation must be extended for a member representing friction losses. Velocity of the flow in the tunnel is influenced by many factors: difference in temperatures between the tunnel and outside environment, gradient of the tunnel, piston effect, effect of fans, friction, change of the cross section, weather conditions at the portal, etc. In our case, the majority of Slovak tunnels have nonrugged profiles; i.e., their cross section is the same, and so only one equation must be solved. Otherwise, a system of equations should be solved for each section of the tunnel in the case of the profile change, branching-off inside the tunnel, when the ventilation shaft is used (Branisko tunnel)  or combined ventilation systems (Višňové tunnel being under construction) [17].
In order to compare the methods, we have created a model similar to the Bôrik tunnel in the IDA RTV [6], with circumference P � 29.22 m and cross section area A � 57.26 m 2 . To calculate the air flow, we used the substitutionary circular cross section of the tube whose hydraulic diameter D can be calculated [18]: Flow fans put air to motion; pressure change depends on a number of fans, efficiency, and fan area. References [19,20] give multiple versions of the equation for both mobile fans and ceiling flow fans: where (1)  For one-way traffic, the moving vehicles move air in the tunnel on, creating so-called piston effect. e higher the velocity and traffic volume are, the higher this effect is. Due to emergency situations in the tunnel, we are also interested in the situation with stopped vehicles when their velocity is lower than velocity of air flow in the tunnel. Since air flow decelerates, we used the equation with the absolute value of velocity [21]: where (1) A is the area of the tunnel cross section (m 2 ) Under fire conditions, temperature in the tunnel will increase which will cause temperature difference between internal temperature and temperature of surrounding environment. e fire represents barrier to air flow. e local loss of pressure caused by the fire depends on temperature power, shape of the lateral cross section of the tunnel, and other factors. e most accurate way of determining temperature in the tunnel is using the CFD simulation or evaluation of real fire experiments. e average temperature in the whole fire section may simplistically be calculated [17]: where (1) ρ is the air density (kg/m 3 ) (2) A is the area of the tunnel cross section (m 2 ) (3) T 0 is the temperature in front of the place of fire (K) (4) T fire is the temperature at the place of fire (K) (5) α is the coefficient of the heat transfer (W/m 2 K) (6) P is the circumference of the tunnel (m) (7) c p is the specific heat capacity of the air (kJ/(kgK)) (8) x is the distance from the place of fire (m) e final differential equation of the air flow velocity was obtained as sum of all mentioned pressure differences and also others described in [22]: where (1) ΔP is the all elements mentioned above (Pa) Furthermore, other members of the equation, such as the temperature differences and the influence of the wind, can be considered.

Fire Model.
e course of the fire may be simulated in various tools. For the reason of calculation time, the three-dimensional simulation by the fire dynamics simulator (FDS) was excluded.
ere may be two-dimensional simulation using CFAST [23] taken into account which is primarily designed for simulation of the fire in buildings. CFAST was extended for simulation of air flow in long corridors. In [24], the tunnel consisted of several interconnected corridors for the zone model; the results were compared to FDS. For the empty tunnel and low air flow, the results were comparable. For obstacles in the tunnel and various air flow velocities, we found out it is not possible to use CFAST reliably for fires in the tunnels. erefore, we used the one-dimensional model of the fire, similarly as in the document TP02/2011 [19], or in IDA RTV [6], with pre-determined curves of the fire power for each type of the vehicle. Shape of the curve for the fire power may be mathematically simplified either linearly, exponentially, or quadratically [20]. For simulation, we chose model curves of fire development according to real tests ( Figure 5).
References [25,26] give equation for calculation of temperature at the place of fire: Since we have the one-dimensional model, temperature is considered as an average temperature of the cross section at the place of fire. According to [25], comparison with the three-dimensional computational fluid dynamics (CFD) simulation in situations without backflow of the smoke gives a good coincidence of temperatures (error ca 1%). In the case of backflow of the smoke, an error occurs. Burning efficiency is not 100% so the power in equation must be reduced. We chose the value 90%. Further, we applied only a part of the reduced power of the fire, approximately 70%, in accordance with the references; the residual power is radiated to the wall of the tunnel. For its temperature, we can use the following equation [22]: where (1) ε is the emissivity (− ) (2) δ is the Stefan-Boltzmann constant(W/(m 2 K 4 )) (3) α is the coefficient of the heat transfer (W/(m 2 K)) (4) T wall is the temperature of the wall (K) (5) T fire is the air temperature (K) (6) L FIRE is the length of the tunnel with fire (m) (7) D is the hydraulic diameter of the tunnel (m) e given equation considers radiation to tunnel walls one-dimensionally; there is difference between the ceiling and the pavement.
Generally, the fire may be detected by multiple ways. For our simulations, there were more important autonomous systems: smoke detection and linear detector FibroLaser [27]. For simulations, we used variable detection time.

Smoke Propagation Model.
Smoke propagation depends on velocity of air flow in the tunnel, size of the fire, cross section of the tunnel, and the tunnel gradient. Smoke whose temperature is higher than temperature of air in the tunnel is propagated below the ceiling and depending on velocity of air flow it propagates one way or both ways. Figure 6 shows possibilities of smoke propagation in the tunnel.
In Figure 6(a), velocity of air flow is low, and smoke stratifies evenly. Fire ventilation in the tunnel for both-way operation should follow this case since there are persons along both sides of place of fire. In Figure 6(b), velocity of air flow is lower than critical velocity, and smoke destratification occurs, backflow propagation of the smoke. In Figure 6(c), velocity of air flow is higher than critical velocity, and smoke destratification does not occur. For oneway traffic, knowledge of critical velocity for fires of various vehicle types is key knowledge for safety of persons in the tunnel. Comparison of various ways of how to calculate critical velocity in dependency on fire power is shown in Figure 7 For the simulation, we chose an analytical calculation [27]. e critical expression of critical speed was chosen for its optimality with respect to all commonly used approaches to the calculation of critical speed. e comparison was made based on the authors' analysis method [29] and calculation using software IDK RTV, TP12/2011.
According to the technical conditions of ventilation of road tunnels TP12/2011, this speed for longitudinal ventilation in the direct of traffic in one-way traffic should be greater than the so-called critical speed at which smoke spreads back. If people are only in one direction from the fireplace, fire ventilation has to be controlled, so that flow speed in higher than or equal to the critical speed.
Calculation of critical speed according to TP12/2011 is as follows: where input values are    For 50 MW, we can see that reaction of the control system 5 minutes after stopping traffic is already at the critical value of velocity for the given fire. Similarly, as in the previous comparison, only three fans were sufficient to keep the value of air flow 3 (m/s), so again the requirement of 100% redundancy of fans was fulfilled. Higher velocity of air flow guarantees lower temperature of air in front of the fire place, ensures delivery of fresh air, and keeps smoke in direction from evacuating persons. e maximum value of velocity of air flow in the opposite direction should not be problematic for them.

Evacuation Model.
As we can see from Figure 10, velocity of persons moving within the smoky space decreased even for the value of opacity 0.4 − (1/m). at value also changed based on position of persons in the tunnel, the tunnel did not get smoky immediately along the whole length, and the evacuating model should consider it. e study of the smoke/speed correlation is currently based on two main datasets:

Set of experiments from Jin [1976] Set of experiments from Frantzich and Nilsson [2003]
e first dataset (from Jin) collected data and they were used for providing correlation between the extinction coefficient and walking speeds, visibility levels, and cognitive abilities when exposed to smoke. Jin used two types of different smoke. First was irritant smoke, which was produced by burning wood cribs. Second was non-irritant smoke, which was produced by burning kerosene. is experiment was performed in 20-meter long corridor that was filled with smoke corresponding to an early stage of fire. e experiment involved 17 women and 14 men, ranging from 20 to 51 years in age. e second dataset (from Frantzich and Nilsson) is from more recent studies.
is experiment was performed in tunnel for studying the influence of different visibility conditions on individual walking speeds.
Frantzich and Nilsson used artificial smoke and, for simulation irritation, acetic acid was used. is experiment was performed in 37-meter long tunnel tube. e experiment involved 46 people. Cellular automaton (CA) model of traffic was extended for a number of persons inside vehicles; therefore, the evacuation model accepts an initial distribution of persons in the tunnel. Interconnection of the CA model of traffic and the evacuation model is shown in Figure 11.
To estimate speed of walking in the evacuation model, the following fuzzy inference system (FIS) was designed: Density of persons (at the evacuation path): low (<1.5(person/m 2 ), middle, and high (>4(person/m 2 )) Smokiness: low (<(2/m)), middle, and high (>(6/m)) Illumination (the level of operational lighting): low (<25%), middle, and high (>70%) In the first step, all membership functions were chosen as trapezoidal and linguistic variables were deduced expertly. It turned out it was possible to reach lower deviation from experimental data when tuning the shape of membership function by genetic algorithms. Comparison of various approaches is documented in [31,32].
To compare the evacuation model we decided to use data from [30] where detailed comparison of multiple evacuation tools is available. We repeated selected experiments with our evacuation model; comparison of input data for experiments is given in Table 1.
For all the scenarios, one-way traffic was applied, stopping at the emergency exit without switched-on ventilation. e scenarios A1.1, A1.2, and A2.1-A2.3 were analysed in [30]. e first two simulated a standard course of evacuation with its immediate initiation. en, various walking speeds were tested. e average evacuation times for individual scenarios are given in Table 2 together with standard deviations (in parentheses).

Risk Analysis in TuSim
e TuSim extended for mathematical-physical models is a model suitable for simulation experiments with technological equipment based on scenario analysis. To evaluate scenarios, it is appropriate to compare ASET (Available Safe Egress Time) and RSET (Required Safe Egress Time), or mortality rates for individual scenarios. Meaning of times RSET and ASET is apparent from Figure 12. Figure 13 shows a clearer time-spatial way of visualization of times RSET/ASET [35], where there is no problem to assess the situation at the particular place in the tunnel. On the left side of the picture, there is velocity of air flow indicated; on the right side of the picture, there is smoke propagation shown and the red lines represent escape paths to individual exits. e way used to make simulation in TuSim is depicted in Figure 14. Setting of the simulation parameters is in the top left corner of the picture. In the top right part of the picture, there is the course of simulation for unstopped traffic and in the bottom right part for stopped traffic and evacuated persons.      Figure 12: Meaning of times RSET and ASET [33].
Journal of Advanced Transportation 9

Evaluation
We set the same conditions for scenarios: Stopping of one-way traffic with the volume 1600 (veh/h) in the 300th minute of simulation Response of the control system in the 600th minute of simulation We were interested in the best case (switching-on of three pairs) and the worst case (switching-on of no fan).
To simulate the humans' decisions to evacuate, we used the rule of temperature increased to 45°C or smoke present at the place of human appearance. If none of the rules applied, we started evacuation in the 900 th minute with time penalization according to distance from the place of fire. For each scenario, we showed time-spatial way of time visualization RSET/ASET, where red hatching indicates the value of air temperature 50 − 55°C. Figure 15 shows that for fire of the truck velocity of air flow shortly decreased below the estimated value of critical velocity and backflow of smoke may occur for a short period of time. Increased temperature will cause earlier initiation of evacuation; temperature 50°C will reach emergency exit behind the fire place approximately in the 960 th minute. If persons started evacuation in a wrong direction, in addition to smoke they will be endangered by heat as well. Figure 16 shows the course of truck fire with switched-off ventilation where smoke backflow does not reach the emergency exit in front of the place of fire due to increased velocity of air flow to 1 (m/s). e persons complete evacuation by 1404 s. Temperature 50°C will reach the emergency exit behind the place of fire approximately by the 13 th minute, emergency exit in front of the place of fire approximately by the 14 th minute, and the whole evacuation of persons will run in environment with higher temperature. Higher temperature need not immediately cause inability of persons to evacuate, and smoke backflow below the ceiling of the tunnel need not immediately endanger persons. e smoke of the fire will start to mix with bottom layers of the air, even after partial cooling of the gas, and this effect can be very difficult considered in one-dimensional simulation models. Persson [34] utilized the one-dimensional model of the fire and used a fractional effective dose (FED) and fractional incapacitating dose (FID) for mortality calculation. For scenarios of vehicle fire, they got zero mortality, fatalities were caused only by the fire of dangerous goods, or fire with near to immediate increase to maximum power (explosive conflagration). For working ventilation, those findings are following our conclusions.
ree-dimensional simulations in the Horelica tunnel [37]   Journal of Advanced Transportation following start of the ventilation will occur within 5 minutes from the traffic stop. In the case of the correct function of fire detection and ventilation systems, without irrational behavior of the persons (evacuation in a wrong direction), without staying in vehicles, there will be no fatalities for oneway traffic.
In the case of two-way traffic, a part of persons will evacuate in a smoke environment with low visibility. Switching the ventilation off may be problematic in short tunnels due to a change in the airflow. Possible regulation to the given value of airflow, e.g., the particular values 0 or 1 m/ s according to technical conditions, may be reachable with difficulties without the possibility to change the ventilation airflow (frequency inverters, fan blades angle of attack control, etc.).

Detection of Fire by the Vehicle
In the previous sections, we have proposed a method to simulate the physical conditions (opacity, temperature, air speed, carbon oxide concentration, etc.) inside the road tunnel in case of fire. As one could see in Figure 16, if the tunnel ventilation is not working, evacuating persons will be endangered by high temperature and smoke. Figure 17 shows temperature development over space and time inside the tunnel. First detectable rising of the temperature occurs at 600 s, but people will start to evacuate at 700 s. During those 100 s, the temperature will rise by approx. 20°C. e fire can be therefore detected in advance comparing the normal temperature inside the tunnel (computed by the TuSim without the event of fire) and the real measured temperature. If the temperature is above the maximal normal temperature and the vehicle is steady, there is a high possibility of the fire inside the tunnel and the autonomous vehicle has to issue an alert to its passengers prompting them to the nearest egress exit. Similar approach can utilize measurement of COx concentration indicating the presence of the smoke.
e alarm inside the vehicle should be issued when any of these conditions is met: Vehicle is steady, and the temperature is above maximal normal temperature at given distance inside the tunnel Vehicle is steady, and the concentration of COx is above the maximal normal level at given distance inside the tunnel e maximal normal values for temperature and exhaust gasses concentration will be provided by TuSim for given parameters of the traffic and the tunnel. ese values depend on the distance. e distance of the vehicle from the portal of the tunnel can be estimated by odometers inside the vehicle combined with other sensors used in autonomy navigation.

Conclusion
e article proposes a complex simulation method in order to predict the development of the physical properties (e.g., temperature, opacity) in case of fire inside the road tunnel. When evaluating an effect of the technological equipment in case of fire in the road tunnel, we must realize a detailed time-spatial analysis of the given event. e methods based on statistics are not able to consider effect of technologies when an event occurs. e correct functionality of detection and reaction of the tunnel control system is usually assumed. Further, the results from the statistics cannot be scaled linearly with the parameters of the tunnel. e only option is to perform a simulation experiment. An advantageous option for simulation experiments is the use of PLC, as real control systems are built on the same platform. As a starting point of our work, we have used our TuSim tunnel simulator. Once the possible scenarios (fire with different power) are computed, the predicted normal and critical behaviour of the tunnel along with the location of the emergency exits may be presented to the intelligent vehicles. e vehicles may detect the fire early considering changes in temperature and opacity. e proposed method is independent of the external systems in case of emergency (wireless communication inside the tunnel may fail in case of fire). erefore, the passengers will be alerted even when the communication link with the tunnel is lost.

Conflicts of Interest
e authors declare that there are no conflicts of interest regarding the publication of this paper.