Turbo timers are usually retro-fitted to cars and are designed to allow the engine to idle for a set period of time after the car's key has been removed (when you arrive at your destination).
This gives the turbo time to cool and keeps it being lubricated properly, thus preventing premature turbo wear and possible failure.
There are several different timers available from several different manufacturers. Our weapon of choice was a Blitz Fatt DC, mainly because it had a remote LCD screen so the installation looked a lot tidier.
Now if you're thinking of installing a timer into you smart our advice is don't as it totally conflicts with the car's immobiliser system.
When you remove the key from the car's ignition the timer activates and keeps the car running. You exit the car, shut the door, press the lock button to find the smart will not allow the doors to lock via remote while the car is running... Great.
The only way around this is to install an aftermarket alarm and either run the timer alongside or buy an alarm with a turbo timer feature built in.