speed up
With or without an object
Go faster, or make something go faster
The train sped up once it left the station.
With or without an object
It can be used with no object, and it can also take one. With an object it is usually separable: turn the volume up, turn it up.
The particle: up
This verb belongs to the group ‘getting bigger, louder or faster’.
Practise it
Add it to your daily practice or see every verb with up.
Further reading: speed up on Wiktionary (CC BY-SA).