The Google unit of Alphabet Inc is set to buy around 100 plug-in hybrid minivans from Fiat Chrysler Automobiles (FCA) to widen its testing program for its self-driving vehicles, the two companies announced on Tuesday.

This is the most advanced collaboration to date between a carmaker and a Silicon Valley-based company. Once these vehicles are refitted with self-driving technologies, they will not be sold to the public.

This is not like the previous trials made by Google when it bought vehicles from Toyota Motor Corp and retrofitted them with self-autonomous tech. The current trials will have the two companies, FCA and Google working closely together on the test models.

In this deal, FCA will equip the 100 or so Chrysler Pacifica minivan it unveiled in February with the self-driving technology of the search giant. The deal does not prohibit the two companies from working with other companies in pursuit of the project.

Their plan is to test the fleet and have the self-autonomous tech integrated into the minivans, not merely an aftermarket proof of concept. In the future, a Chrysler Pacifica version with self-driving tech will be unveiled.

According to Chrysler, this is the first time Google has incorporated sensors and software into a passenger vehicle for public use in a future car unit, and not just as a test in an existing car model.

The additional minivans will more than double the testing fleet of the internet search giant. Currently, the company is testing its self-autonomous cars in four U.S. cities.

This deal calls for Chrysler to "initially design and engineer around 100 vehicles uniquely built for Google's self-driving technology."

"Google will integrate the suite of sensors and computers that the vehicles will rely on to navigate roads autonomously," said the company statement.

After the tech retrofitting, Google will test the minivans in California streets as part of its testing program in U.S. public roads and then have its consumer reveal afterwards.