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What is Water Injection?

By Lori Kilchermann
Updated May 23, 2024
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Water injection is the act of injecting a water solution, typically a 50/50 mixture of water and alcohol, into the fuel system or the combustion chamber of an internal combustion engine. The process of water injection allows for much higher compression ratios within a high-performance engine. This equals higher horsepower output by raising the octane rating of the fuel and reducing the detonation that accompanies higher-compression racing engines. By introducing water into the fuel stream as it flows into the combustion system, the water drops the intake temperature enough to produce a denser fuel charge and release more power from the compressed fuel charge.

Heat is the enemy of horsepower in an internal combustion engine. Heat causes the fuel to be ignited prematurely due to hot spots within the combustion chamber. The premature ignition, also known as detonation, can actually destroy an engine by damaging pistons, valves and head gaskets. By using a water injection system, the incoming spray of water cools the combustion chamber and allows the fuel mixture to be ignited at the proper time. The resulting steam coming from the introduction of water into the hot chamber through water injection produces pressure that, in turn, creates more compression and more horsepower production.

The practice of water injection on gasoline engines has been used worldwide since the first systems were used on World War II fighter aircraft. When a fighter pilot required more speed to avoid becoming a casualty, he would push the throttle forward and activate the water injection system. This allowed the airplane engine to produce maximum power without damaging the engine. The practice of introducing water into auto racing engines followed at places such as Indianapolis, Indiana in the United States and Le Mans, France. In the 1950s, the introduction of nitro-methane in American drag racing was soon followed by water injection to produce even more power.

Gasoline engines are not the only recipients of the benefits of water injection; diesel engines, with their inherently high compression ratios, also produce more power when coupled with the injection of water into the combustion chambers. Engines operating with propane fuel systems can also benefit from a water injection system since the octane rating of propane is typically as high or higher than most brands of racing gasoline, depending on the manufacturer. In any form of internal combustion, piston-driven engine application, the power-producing capabilities are enhanced by the introduction of water into the fuel system. Some jet engine designs also use this type of power enhancer in the search for ultimate power production capabilities.

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