Water Injection (engines) - Effects

Effects

In a piston engine, the initial injection of water cools the fuel-air mixture significantly, which increases its density and hence the amount of mixture that enters the cylinder. The water (if in small liquid droplets) may absorb heat (and lower the pressure) as the charge is compressed, thus reducing compression work. An additional effect comes later during combustion when the water absorbs large amounts of heat as it vaporizes, reducing peak temperature and resultant NOx formation, and reducing the amount of heat energy absorbed into the cylinder walls. This also converts part of combustion energy from the form of heat to the form of pressure. As the water droplets vaporize by absorbing heat, it turns to high pressure steam (water vapor or steam mainly resulted from combustion chemical reaction). The alcohol in the mixture burns, but is also much more resistant to detonation than gasoline. The net result is a higher octane charge that will support very high compression ratios or significant forced induction pressures before onset of detonation.

Fuel economy can be improved with water injection. Depending on the engine(*), the effect of water injection, with no other modification, like leaning out the mixture, may be quite significant or rather limited and in some cases negligible.

(*) It should be noted that MOST modern consumer vehicle engines are pre-programmed with specific fuel to air ratios and that introducing water without re-programming the cars computer will not have any benefit, and in most cases will reduce performance and possibly damage them. Most modern fuel systems can not determine that water in any form has been added and cannot "determine" a new compression ratio or take advantage of lower cylinder temperatures. In most cases in cars that are pre-programmed introducing water vapor will cause them to loose power because the water vapor will then take the place of air and fuel that is required to produce the power. Only in vehicles in which the owner can take the car to a "TUNER", a person who can replace the normal car computer with a programmable one will any benefit be seen. But as soon as the water source is depleted the engine will be running rich and power and and fuel economy will be lost again.

Damage can occur to the engine if to much water is injected, or if there is a malfunction of the injector itself. Water is not compressible and if too much water makes it into the cylinder prior to detonation (during the compression cycle) you may exceed the engine blocks head bolts threshold and "blow the heads", Hydrolock and or shatter the pistons connecting rod(s). This damage is fatal to the engine and typically requires a full rebuild.

Overall, Water injections is typically used in Aviation and not intended for use in consumer grade vehicles. And when used in consumer grade vehicles it is for the EXTREMELY high performance arena (1000 hp+) where vehicles are super or turbocharged and produce excessive heat. These vehicles are typically driven only in a event or controlled race and require extremely high levels of knowledge to maintain and only driven for very short distances. A noted Most forms of water injection is mixed with a combustible and not directly from the "tap".

Many product scams exist touting that inserting a tube of misting spray into a intake manifold will increase your millage gains and or horse power. To go back, unless the user re-programs the car ECU/PCM/Computer, power loss and damage will eventually occur.

In some cases water may also reduce CO emissions, this might be attributable to the water-gas shift reaction, in which CO and H₂O shift to form CO₂ and H₂. However, water may also increase hydrocarbon emissions, possibly due to an increased quenching layer thickness.

Some degree of control over the water injection is important. It needs to be injected only when the engine is heavily loaded and the throttle is wide open. Otherwise injecting water cools the combustion process unnecessarily and reduces efficiency.

Direct injection of water is possible and is likely advantageous. In a piston engine, this can be done late in the power stroke or during the exhaust stroke.