How can I make my LEGO engine go faster
If you need a fast motor but not having too much torque, yet powerful enough 
 - consider using a PF L-motor. It has roughly half of the XL-motor's torque, but twice as much rpm.

The PF XL-motor has high torque, considering its intended application with LEGO. This is known to cause some serious damage to gears if the assembly isn't rigid enough. It is essential to have your gears braced properly from both sides. Otherwise your gears will be slipping and/or grinding under load. In your picture you have your gears braced from one side only - this will cause you problems. You need to construct rigid bracing from the other side as well.

I find Sariel's Gear Ratio Calculator useful for finding the right gears for your application.