In a very summary way, the piston in a four stroke engine It is responsible for moving the air inside to compress it and facilitate the burning or explosion of the fuel or to push it out of the combustion chamber. That is, it is dedicated to pushing the air up or down.
Now imagine a train arriving in a tunnel at more than 300 km/h. Suddenly, the train goes from being outside to moving the air inside the tunnel. To push it to the bottom. Your movement It would be very similar to that of a piston. The train moves in a straight line and around it the tunnel would behave like a combustion chamber.
That doesn’t seem like a problem.
It doesn’t seem like it if we think that the air is simply pushed to the outlet where it is released without further problem. It’s also not a problem if your high-speed lines run over a bridge more than 100 kilometers long.
But if you are a mountainous country and you have made the railway your star medium to move millions of people hundreds of kilometers an hour. Yes, you have a problem. Because the piston effect is pure physics and solving it to gain speed is not being easy.
When they were the best
In 1964, while Spain began to open up to the world, Abebe Bikila won his second Olympic Marathon in the streets of Tokyo. He did it wearing Puma Osaka shoes.nothing to do with the famous 42,195 meters that he covered barefoot in Rome to win four years before.
We do not know if Bikila took that first Shinkansen that linked the cities Tokyo and, precisely, Osaka. The bullet train had begun to operate in Japan that same year, promoted by the Olympic Games in the Japanese capital. Then, the two cities were linked by a train that reached peaks of 210km/hbecoming the first high-speed line in the world.
More than 60 years later, Japan is no longer the country with the highest number of high-speed kilometers of the world. Today it is China. It makes sense, taking into account that the country is huge, so if this means of transportation were promoted, sooner or later they would surpass their neighbors. Spain, by the way, also surpassed Japan in this area years ago.
But it is very likely that something else has hurt Japan more. China is making the bullet train its flag. Its latest advances with the maglev, which levitates thanks to very powerful magnets to avoid friction with the track, has reached a combined speed of 896 km/h at the intersection of two CR450 trains.
The problem for Japan is that China has a lot of money. And if it is necessary to build eight of the 10 longest bridges in the world to solve geographical accidents, they get to work.
Japan has to deal with a lot of mountains and a more traditional system: tunnels. And that when you want to make a train pass at very high speed is quite a problem.
When a train fully crosses the threshold of a tunnel, what is known as piston effecta problem that prevents increasing the walking speed further. The consequences are as simple as they are serious: loud explosions, breakage of equipment… and the eardrums of passengers.
Upon entering the tunnel, the air is compressed and the movement of the train moves it towards the exit. However, some of that air rebounds and generates pressure changes that can be especially painful for passengers, even affecting their middle ear. When moving outside, a pressure wave is created that moves at the speed of sound and when the train leaves the tunnel, a shock wave and a sound explosion are created that, it is calculated, can be heard 400 meters away. It is known as tunnel boom.
Japan is now experiencing a problem carried over from the past. Their trains are wider than the European ones but their tunnels are narrower. This was to reduce infrastructure costs but also to run less risk of landslides in the event of an earthquake. At first this was not a problem but when the speed of the trains increased they realized that they could not continue moving.
In China, trains also use wide tracks like their neighbors but since they do not preserve inherited structuresthe new tunnels built are wider. This reduces the void effect produced with the entry of the train into the tunnel and, therefore, mitigates the problems for passengers. Furthermore, as less resistance is generated when the train passes, energy expenditure is also reduced.
The solution for the Japanese is not simple. On the Tokaido Shinkansen, the first high-speed line (the one that connects Tokyo with Osaka), 13% of total kilometers They run inside tunnels. But the Sanyo Shinkansen line runs through tunnels half of the time. and he Hokkaido Shinkansen which is under construction (this line is only partially open) contemplates the roofing of 80% of the layout.
The most effective solution that has been found to the problem is to produce trains with a very long and sharp nose. The aerodynamics tries to imitate the beak of the Kingfisher that can dive into the water generating minimal splashes. Following the same concept, the longer and sharper the nose of the train, the less resistance the train encounters at the entrance and the more gradually the pressure wave is generated.
The other solution has been expand the section of the tunnel at its entrance. The “door” is wider and also has side openings that allow part of the air to escape. air moved by the train. This escape route generates a lower pressure wave, allowing the train not to cause unwanted discomfort to passengers and to travel faster. It has even been thought of hermetic trains with controlled pressure.
During its tests, Japan continues to search for trains that can reach a top speed of 400 km/h. However, the structures inherited from its past make it extremely difficult to make a new qualitative leap to keep pace with China.
Photo | Rikku Sama
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