Is the fuel tank better made of plastic or iron?

Out of the need for light weight, more and more auto parts are changed from metal parts to plastic parts, and even some chassis parts have begun to appear plastic.

Because this topic is quite controversial, we will continue with this today and talk about the plasticization of other parts.
Our topic today is also "large parts". As far as passenger cars are concerned, with a volume of about 50L, it naturally can't escape the fate of being lightweight. Yes, it is the plastic fuel tank.

| Fuel tanks are also catching up with the trend of plastics

The fuel tank was originally welded with cold-rolled steel plates, and the structure was relatively simple. However, as passenger cars are getting lower and lower, the requirements for chassis flatness are getting higher and higher, which poses no small challenges to the layout of the fuel tank.
Therefore, the European fuel tank manufacturer Kautex began to explore the plasticization of fuel tanks in the 1960s. Because of its good stability and high degree of light weight, the high-density polyethylene (HDPE) material they developed is still The main material of plastic fuel tanks.

In 1964, Kautex developed the world's first plastic fuel tank for automobiles. In 1973, HDPE material fuel tank was applied to Volkswagen's Passat model, which successfully achieved mass production.
As far as the fuel saving is concerned, the density of HDPE material is only 0.95 g/cm3, while the density of metal steel is 7.85 g/cm3, and the density ratio of the two is 8 times. Even if plastic fuel tanks are thicker than metal fuel tanks to ensure strength, plastic fuel tanks of the same volume are still 50% to 60% lighter than metal fuel tanks.

However, there is always some acceptance process. There is a set of theories earlier: in theory, plastic materials are cheaper, and metal fuel tanks are definitely more expensive than plastic fuel tanks. It can be concluded that manufacturers are "reducing the distribution" in order to save costs.
In fact, the actual situation is just the opposite, because the plastic fuel tank has a complex and irregular shape, which requires high research and development costs in the early stage, and also requires the purchase of expensive molding equipment and testing equipment. Cost madness Lexus, Toyota and other brands have used iron fuel tanks against the trend for a long time. This is a case solved today.

Regarding the development process of fuel tanks, we can roughly draw three stages, from metal fuel tanks to plastic fuel tanks, from single-layer plastic fuel tanks to multilayer plastic fuel tanks. You may ask, why are there multiple fuel tanks? This starts with the shortcomings of plastic fuel tanks.
Plastic fuel tanks are not flawless, for example, they are worse than metal fuel tanks in terms of resistance to fuel penetration. People usually don't pay much attention to fuel permeability resistance. What it says is that due to the constant changes in the internal pressure of the fuel tank during driving, fuel molecules tend to diffuse and penetrate the plastic fuel tank.

The vehicle emission limit of the National VI emission law is 700 mg/day. However, due to the molecular structure of the HDPE material itself, a single-layer plastic fuel tank cannot effectively block the penetration of fuel molecules and cannot meet regulatory requirements. In order to solve the problem of permeability, current plastic fuel tanks adopt a multi-layer shell structure with a barrier layer in the middle.
The barrier layer in the middle of the plastic fuel tank is EVOH (ethylene/vinyl alcohol copolymer), which has very barrier properties to gasoline molecules and can greatly improve the permeability resistance of the plastic fuel tank.

| Safety and product performance of plastic fuel tanks

After understanding the development history of plastic fuel tanks, about its advantages in use, and some concerns of consumers, I will sort out the following for you.

1) Collision safety and fire performance
Compared with plastic, the solidity of metal materials will give people a sense of security. In the event of a rear-end collision or collision, the fuel tank may be squeezed and deformed or even ruptured under impact. Once the oil leak is ignited, the consequences will be disastrous.
Due to the difference in the material properties of metals and plastics, they must be different when they are impacted.
But unlike what we imagined, metal fuel tanks are generally welded, and the weld is the weakest place. The weld is more prone to disconnect and rupture when impacted. More importantly, metal collision is prone to sparks. This is very dangerous. Therefore, the metal fuel tank is usually located between the front and rear axles, reducing the risk of the fuel tank being squeezed and exploded during a rear-end collision or collision.

Because plastic fuel tanks use high-performance engineering plastics as raw materials, they have an impact resistance that is not weaker than that of metal materials. More importantly, the plastic fuel tank is made of one-piece blow molding, which is like an egg shell. It can still maintain good elasticity and rigidity at the extreme temperature of -40℃ and +90℃. This is not the case with metal fuel tanks. The advantages.

Of course, the theory has to be verified by actual tests. In order to verify the anti-strike ability of the fuel tank, some impact tests will be carried out during product development, such as impact resistance test, angle hammer impact test, etc., which are used to evaluate the impact of external energy. Can the fuel tank remain intact?

The impact resistance test is to verify whether the fuel tank will rupture after a sudden frontal impact. The plastic fuel tank needs to be fixed on the slide rail, and then the front and back sides of the fuel tank are impact tested by a hydraulic linear actuator. The maximum impact acceleration can reach 100G.
Angle hammer impact is to evaluate the low temperature impact resistance of the plastic fuel tank. The plastic fuel tank will be placed in a -40℃ low temperature environment box for 24 hours, and then an angle hammer impact tester will be used to impact the vulnerable parts of the fuel tank with 30J energy.

Taking into account the extreme conditions of fire from the outside, the plastic fuel tank is also subjected to fire resistance tests. Even if the plastic fuel tank really catches fire and burns, the fuel tank will only gradually begin to melt, and there is not enough time to explode due to heat expansion, so you don’t need to worry about the safety of the plastic fuel tank.

After the fuel tank is preheated, it will be directly exposed to the free-burning flame for 60 seconds, and then a flame-retardant isolation cover is added for indirect combustion. After all the steps are completed, observe the leakage of the fuel tank.

2) Product performance
Although safety is our first consideration, it cannot be at the expense of product quality. The fuel tank has transitioned from metal to plastic. Which properties will be affected?
According to different materials, automobile fuel tanks can be generally divided into single-layer plastic fuel tanks, multilayer plastic fuel tanks and metal fuel tanks. Through the characteristics of different material fuel tanks, it can be found that plastic fuel tanks have excellent corrosion resistance.

Rusting of metal fuel tanks is not new. Although the surface has been nickel-plated and zinc-plated, it is unavoidable because it is located under the car. Rust is very common. In addition, gasoline is often mixed with oil impurities. After a long period of soaking, the metal The steel may be gradually corroded to produce impurities, which may block the oil circuit.
In contrast, plastic fuel tanks are more resistant to acid, alkali, salt, etc. than steel plates, so they are very suitable for use in areas with greater pollution.

Although the material characteristics of metal and plastic are different, the performance of fuel tank products will be different, and the replacement of metal fuel tank by plastic fuel tank will not reduce the performance of fuel tank. Strict performance testing is required before entering the market, such as the tightness of the fuel tank cap, vibration durability test, pressure resistance test and many other tests, in order to ensure that it is sufficiently reliable.
Vibration durability test: Add rated capacity of water to the fuel tank, seal all inlets and outlets, and then perform vibration durability tests in the horizontal and vertical directions. It is necessary to observe whether the fuel tank is cracked or leaked.
Because it is connected to the floor, the vibration and impact of the fuel tank are still very strong. This type of test has a strong practical reference significance.

Pressure resistance test: Add rated capacity of water to the fuel tank, maintain an ambient temperature of 50°C, seal all inlets and outlets, apply a pressure of 30kPa to the fuel tank, maintain the pressure for 5 hours, and check whether the fuel tank is leaking.
Just like the human lungs, everyone will worry about whether the fuel tank will not be able to withstand the pressure and someday will be discouraged. Withstand voltage test is used
To evaluate whether the sealing of the fuel tank is good enough and pressure resistant.

| Summary

The plasticization of automobiles has become a major development trend. The materials used in automobiles are gradually transitioning from metal to plastic. It should be known that plastics are much lighter than metals, which can greatly reduce the weight of the car body and reduce manufacturing costs. .
In the 1970s, the use of plastics accounted for only 2% to 3% of the total mass of automobiles, but it has now doubled several times to 8% to 10%. It can be said that the more plastic used in a car is one of the more advanced automobile manufacturing technology.
From the process of upgrading the fuel tank, we can see the epitome of the development of automobile technology. Although plastic materials seem to give people a cheap feeling, all the engineering plastics used to build cars have undergone strict performance tests, and there is no need to "talk about plastic color changes."

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