In recent years, researchers and industrialists have sought to find alternative fuels to fossil fuels due to various reasons, such as reducing air pollution or limiting fossil fuel resources, etc. The results of some studies show that adding some substance or nanoparticles to the fuel can improve the performance of the engine or reduce the amount of air pollution (results obtained from the analysis of exhaust fumes). However, the primary basis of these fuels has also been fossil fuels, and it does not help much in view of limited sources of this fuel, although old and experienced automotive manufacturers have also been interested in using gasoline and diesel engines. Unfortunately, this fuel has far less latent energy compared to petrol. In addition, the noise of such engines during operation is more than that of diesel engines. Countries that have natural gas resources have tried to use this potential in their transportation industry. Thus, they have used Compressed Natural Gas (CNG) as a suitable substitute for fossil fuels in cars. After that, electric and hybrid cars have also entered the market. Recently, there has been a lot of research on using hydrogen as a fuel for vehicles. In comparison, all these fuels have disadvantages, limitations, and advantages over each other, which are beyond the scope of this paper. In fact, for the storage of any gas fuel such as CNG or H2, a safe tank is needed because it is exposed to many risks such as bursting, and this issue is directly related to the lives of the driver and passengers. Therefore, the focus of this paper is on gas fuel tanks in cars and their strength.
At first, CNG fuel tanks were made entirely of metal. The first limitation was their large size, which forced them to be placed in the trunk of the car. The second problem was their heavy weight, which led to a solid weight being added as an extra load to the car’s rear suspension system. Since this load was not seen during the initial design of the car and capsules were sometimes manually put on some cars after leaving the factory, many problems were observed for the comfort of the rear passengers of the car, such as in the maneuvers of the car in turns, sometimes the initial acceleration of the car, etc. Apart from these, there were many reports of deaths and financial injuries that were published due to the bursting and explosion of these tanks, which led to people’s distrust in this system. Therefore, new generations of CNG fuel tanks were presented by researchers. In this regard, all-metal tanks are known as the first generation, and all-composite tanks including a polymer liner and an external body made of composite are known as the last generation, i.e., the fourth generation. The biggest advantage of type-IV CNG fuel tanks is their use of the property of gas leakage. In other words, at the time of failure, the gas passes through the various layers of the composite and the gas is sensed (before the tank fully opens and bursts). This is despite the fact that in metal tanks, as soon as the initial crack is created, the crack grows and the gas escapes, an explosion occurs, and the tank breaks into pieces and is thrown around like thousands of shrapnel; each of these pieces can cause the death of a person. In the following, a brief overview of researchers’ achievements in the field of evaluating the strength of composite tanks and their optimization will be discussed.
Gehandler and Lönnermark experimentally studied the behavior of composite CNG tanks exposed to fire [ 1 ]. Moreover, Ou et al. conducted a laboratory study of the behavior of tanks exposed to local and extensive fire in an aluminum liner composite tank filled with hydrogen [ 2 ]. They reported that both internal gas pressure and temperature parameters do not change much when the tank is in the vicinity of local fire. Moreover, filling medium and tank pressure have weak impact on the activation time of TPRD, but they have a remarkable influence on the activation time of pressure-activated PRD. Hence, it is vital to pay attention to various tank-monitoring techniques and ensure the tank’s health during operation. In this regard, Acoustic Emission Energy (AEE)-based Signal Mapping Methods (SMMs) have been employed to identify the source location of damages on the composite CNG tank caused by external shock [ 3 ]. Glisic and Inaudi used long-gauge fiber optic sensors to monitor the health of full composite CNG tanks [ 4 ]. To this end, after the tank was fully manufactured, the sensors were wrapped around it. They analyzed monitoring results at several levels, and damage was detected using algorithms that combine overall deformation and changes in tank stiffness. One of the factors that can increase the temperature of the internal gas or the temperature of the tank body is filling it quickly. Saferna et al. discussed the results of thermodynamic analysis of the CNG composite tank due to the fast filling process [ 5 ]. They also claimed that the composition of natural gas affects the rapid filling time of the tank and compressor performance due to the filling process. Consequently, this process should be as short as possible, and during this time, the tank should be filled with as much gas as possible. Kim and Choi utilized computer modeling and fractography methods to perform risk analysis of composite tanks for compressed natural gas fuel [ 6 ]. They reported that the main cause of failure in CNG composite tanks was the interference and floating effect between the clamp bolts and the tank. They recommended that the tank’s installation components, i.e., clamp belts and bolts, be redesigned and, in addition, new assembly processes be implemented. Moreover, it is also necessary to carry out periodic inspections every three years to check the health of other parts such as valves, pipelines, etc. It should be noted that conducting only experimental research is very expensive and time consuming. In addition, in this case, it is not possible to manufacture a lot of tanks and perform various tests on a real scale without achieving the correct design. On the other hand, with the development of simulation software and their very good accuracy in examining the behavior of various materials under different working conditions, it is logical to make a sample and perform various tests after obtaining the most optimal state, or the final design. In this regard, Kim et al. have evaluated the structural safety of CNG composite tanks based on Finite Element (FE) simulation [ 7 ]. To this end, they used the von Mises yield criterion and Tsai-Hill theory. In addition, FE simulation has been used to study the behavior of CNG tanks made of composite considering different twist angles [ 8 ]. The researchers used Kevlar fiber for the outer body and finally stated that considering the twist angle of 35 degrees, the least deformation can be achieved in the tank. However, they only considered the constant thickness for the tank wall, which is different from reality. Nouri et al. studied experimentally and numerically the performance of CNG composite tanks under static load and considering variable wall thickness [ 9 ]. For the first time, they used the Finite Element Model (FEM) based on experimental measurements in this field, and by conducting stress analysis, they identified the critical area prone to failure. After that, they used different static failure criteria for composite materials to obtain fracture strength and calculate safety factors in each of the composite layers in the critical region. Finally, they stated that the most accurate criterion should include 3D normal and shear stress components. Seyedi et al. optimized the type-IV composite tank under internal pressure loading according to conditions beyond the working conditions and testing conditions stated in the standard, in other words, the endurance limit until the tank explosion [ 10 ]. They modeled the composite tank as 23 layers in such a way that the first layer was considered as a liner (7.5 mm), the next 10 layers were thick glass fibers (0.9 mm), and the next 12 layers were thinner than glass fibers (0.75 mm). They benefited from the Design of Experiments (DOE) technique to perform the response surface analysis; the pressure created in the critical zone and the tank deformation were considered as outputs. Eventually, they showed that compared to metal tanks, lightweight composite tanks are more resistant to internal pressures, resulting in a 30% reduction in the weight of composite tanks and a 20% reduction in deformation under working pressure. Nouri et al. attempted to obtain the optimum fiber twist angle in the composite CNG tank in two cases of constant and variable wall thickness under static loading conditions [ 11 ]. They reported that considering the lowest value of von Mises stress as the objective function, the most optimal angle is equal to 23 and 15 degrees for the composite tank with constant and variable thickness, respectively. Debondue investigated the advantages of glass fiber applications in the construction of CNG composite tanks, and the efficiency of such tanks was assessed [ 12 ]. A hybrid FE-RSM method has been presented to optimize fatigue lifetime of a polymer composite CNG tank [ 13 ]. Using the results of the finite element simulation and its coupling to the analysis of response surface method, the researchers were able to find the optimal values for the wall thickness in different parts of the tank, including the main body in the form of a cylinder and the beginning and end of the tank in the shape of a hemisphere, as well as the connection area of the tank to the aluminum flanges. In addition, they also optimized the fiber twist angle. They reported that the fatigue life of the tank manufactured using the optimal values is 2.4 times longer than that of the tank in the initial state. Recently, the accuracy of predicting the service life of the composite CNG tank has been evaluated by employing different fatigue failure criteria [ 14 ]. Despite all the efforts made by various researchers and industrialists in the field of strength assessment of vehicle fuel tanks and their structural optimization, there are still the problems of tank failures. Hence, it seems that in addition to the investigated cases, another parameter influences the service life of the tanks, which has been overlooked by designers. In this regard, the authors believe that the driver’s behavior in refueling can be one of these factors. Therefore, in this article, the authors for the first time discussed the behavior of drivers in refueling the gas tank on the service time of the tanks by numerical simulation. To this end, dynamic analyses were performed using the validated FEM of a composite tank with variable wall thickness. Different loadings were considered based on the capacity of the tank, and finally, the service life of the tank was reported according to the distance traveled.