Water Injection

Brief history and theory:

Water injection used on internal combustion reciprocating (piston) engines has a long and interesting history. Some WWII military aircraft used water injection to boost horsepower and increase performance. Water injection is still used today on some aircraft and high-performance automotive engines for the same purpose.

It seems counter-intuative for an engine to produce more power when water is sprayed into the intake. The key is that the water doesn't actually increase the engine power directly. It works two ways: delaying detonation onset, and cooling the intake charge.

Detonation is when a fuel/air charge within a cylinder is ignited by something other than the sparkplug. The flame front in the burning charge collides with the flame front initiated by the sparkplug. Where this collision occurs, the pressure rises much higher than would normally occur when the cylinder is firing properly. The added pressure has a runaway effect, causing the fuel/air in that zone to burn so rapidly as to appear to detonate. The pressure spike caused by this can damage pistons, valves, connecting rods, etc.

Detonation can occur due to a number of factors. Lower octane rated gasoline can cause it, as can excessive cylinder compression ratio, and advanced spark timing. Hot intake air and heavy acceleration/engine load can also trigger it. Detonation is also known as "knocking" or "pinging", because of the sound audible when it occurs.

All engine designs are a compromise. Engineers know that the more compression and spark advance they can run, the more energy they can extract from the fuel being burned. The problem is that if they take it too far, the engine will start detonating and if it is severe, the engine will be damaged.

So where does water injection fit into this? Well, water is an excellent anti-detonation agent. Injecting a mist of water into the combustion chamber will help supress detonation, allowing higher compression, more spark advance, or both to be used without damaging the engine. Since more compression and/or more spark advance equate to more power, water injection allows the engine to be retuned to boost power output.

These days water injection seems to be most popular in forced-induction engines. Forced-induction (FI), as opposed to normally aspirated engines use some sort of pump, such as a turbocharger or supercharger, to compress air into the engine. Added air means more fuel can be injected, and more power can be produced. The problem is that detonation is more of a problem on a FI engine, and limits how much boost can be used without causing damage. This is due not only to the added compression caused by the pressurized air, but also the act of compressing the air causes it to heat up. The hotter air makes detonation more likely. Using water injection on a FI car not only takes advantage of waters' anti-detonation properties, but also cools the compressed air by a considerable amount. Cooling the air decreases the tendency to detonate and increases the air density, allowing more air to be packed into the cylinders. Some engines use intercoolers to do this instead of water injection.

Water injection is used to a lesser extent on normally aspirated engines to control detonation in hot weather, under heavy loads (such as towing or drag racing), and when high compression and/or advanced spark timing are used. Most modern fuel-injected engines use ping sensors and continually tune the engine to get peak performance. This is usually done by retarding the spark timing when detonation is sensed, which helps reduce or eliminate the pinging but robs power from the engine. When water injection is used on a modern engine, since the pinging doesn't occur, the ping sensor doesn't signal the computer, and the computer keeps the full spark advance applied, running the engine at peak power. The amount of power increase this causes is highly dependent on a number of factors such as air temperature, engine load, fuel octane, altitude, etc. There is considerable debate as to whether or not the power gains on normally aspirated stock engines are worth the bother and expense of water injection.