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The Google Pixel 2 XL like its Google Pixel predecessor supports only standard USB Power Delivery for fast charging rather than using any proprietary fast charging schemes. This conveniently allows the Google Pixel 2 XL to work with other USB Power Delivery enabled chargers including the Apple 87W USB-C Power Adapter besides Google's own Pixel 2 18W USB-C Power Adapter.

Here we connect the Google Pixel 2 XL with the Apple 87W USB-C Power Adapter using a USB Type-C to Type-C cable and see how they perform over charging when the phone's battery is 7% full. As we observe through the Granite River Labs GRL-USB-PD-A1 power analyzer,  the Apple charger starts off to repeatedly promote its power capabilities of 12 watts (5 volts @ 2.4 amps) over USB Power Delivery.

After the 12W power display has ceased to continue, the Google Pixel 2 XL finally begins to firstly request this 12 watts (5 volts @ 2.4 amps) charging from the Apple charger and gets accepted.

Following up from several Vendor Defined Messages to adjust voltage and current only then the Apple 87W USB-C Power Adapter  advertises full power profiles of 12 watts (5 volts @ 2.4 amps), 27 watts (9 volts @ 3 amps) and 86 watts (20 volts @ 4.3 amps).

We then see the Google Pixel 2 XL takes advantage of the 27 watts (9 volts @ 3 amps) option which it requests and gets accepted by the Apple charger to speed up charging. Later we also observe the phone and charger exchanging Discover Identity and SVIDs Vendor Defined Messages.

Upon initial plugin at 7% battery charge when the phone is on, power delivery negotiations begin with voltage starting at 5.2 volts and current being pulled at about 0.1 amps. As the voltage quickly rises to reach around 9 volts over power requests and Vendor Defined Messages, the phone pulls mostly around 2 amps current to charge at 18 watts (9 volts @ 2 amps). After 30 seconds, the phone enters sleep mode when it draws around 11.7 watts (9 volts @ 1.3 amps) and further down to about 6.3 watts (9 volts @ 0.7 amps) when the phone is being actively used.  

The Google Pixel 2 XL doesn't use the 27W that was established in the above protocol transactions, but rather only up to around 18W over a brief period during initial charging. 18W charging does make sense since the phone also draws similar power when used with its own Google's supplied charger accessory through USB Power Delivery as seen here.