How A Water Injection System Can Use Exhaust To Never Run Dry How Water Injection Systems Work. Water injection is a great method for increasing the horsepower an engine makes, with additional benefits such as efficiency gains and emissions reductions. Water injection makes this possible through evaporative cooling; when the water changes from liquid to vapor, it cools the intake charge, increasing density and reducing the likelihood of knock. As a result, you can advance timing, increase boost, and run engines with higher compression ratios, ultimately making more power. The problem with water injection, however, is that you need to constantly refill the water tank as it runs dry. Tenneco, however developed a test for a potential solution. What if you simply recovered water from your exhaust, and re-injected that water into your intake to make more power!? Will it work?