Piston effect

Piston effect refers to the forced-air flow inside a tunnel or shaft caused by moving vehicles.

The effect is also similar to the pressure fluctuations inside drainage pipes as waste water pushes air in front of it.

Non-return dampers are sometimes needed to prevent stalling of ventilation fans caused by this air flow.

[3] The piston effect has to be considered by building designers in relation to smoke movement within an elevator shaft.

At wider stations with multiple tracks, air quality remains the same and can even improve when mechanical ventilation is disabled.

At narrow platforms with a single tunnel, however, air quality worsens when relying on the piston effect alone for ventilation.

Reducing these disturbances is a significant challenge for high-speed lines such as Japan's Shinkansen, France's TGV and Spain's AVE.

Japan has enacted a law limiting noise to 70 dB in residential areas,[8] which include many tunnel exit zones.

A diagram showing the piston effect as a vehicle moves through a tunnel.
The elongated nose of the E5 Series Shinkansen in Japan is designed to counter the piston effect. [ 2 ]
The clearance between train and tunnel is often small. London Underground train exiting a tunnel.
A tunnel in the French high-speed TGV network with an entrance hood to mitigate tunnel boom.