Water Injection: It's Not Just For Windshields Any More
By Mike Kojima
[Put into HTML format by Mike Mager]
Water injection relies on water's naturally high specific heat, which means it requires a lot of heat to change liquid water to water vapor (steam). The high specific heat is one of the reasons why water is an excellent coolant. When injected into an engine, water does several things. When it is first injected, some of the water vaporizes, absorbing heat from the incoming intake stream. This helps cool the hot intake charge the same as nitrous oxide injected into the intake.
Once inside the combustion chamber, the water is fully vaporized into steam. This vaporization absorbs a tremendous amount of heat from the combustion reaction, helping cool the engine internally to prevent glowing hot spots that can initiate detonation. The heat absorbed from the water's phase change also acts to buffer the combustion event, slowing it down. Thus the water helps regular pump gas behave much like controlled-burn, high-octane race gas. The super-heated steam also acts like a powerful steam cleaner inside of the engine, removing carbon deposits from inside the combustion chamber.
As water does absorb power-producing heat, it does not produce quite the same horsepower per pound of boost as race gas does, but it does allow quite a bit of additional boost. The water also goes a long way toward eliminating engine-damaging detonation.
The Aquamist water injection system uses a unique, proprietary atomizer nozzle to introduce the water to the intake air stream. The nozzle produces a range of droplet sizes from a fine fog to a course mist. This helps the water work more efficiently. The fine water fog vaporizes almost immediately, helping reduce the intake air temperature by 20 to 30 degrees. The larger droplets do not vaporize until they reach the combustion chamber where they can perform their internal cooling and combustion buffering duties. All the droplets are small enough where the even distribution of water throughout the manifold plenum is assured. Most modern fuel injected cars have manifolds that are designed to flow dry air only, not a mixture of liquid and air so the maintenance of correct water droplet size is critical for even cylinder-to-cylinder water distribution.
The common, old-school water injection system that is typically available makes no attempt to atomize the water. Unequal distribution and over injection of water result, creating bogging and other driveabilty problems with still poor suppression of detonation. The Aquamist system works so well that almost all turbocharged Group-A rally cars in Europe use this identical system with up to 3 bar (44 psi) of boost. In fact the European-market Ford Sierra Cosworth uses this system as original equipment from the factory! This alone speaks highly of the system's reliability and performance.
The Aquamist system is tunable using different sizes of jets ranging from 0.40mm to 1.0 mm. Since the SR20 engine has a relatively-high (for a turbocharged engine) compression ratio of 9.5:1 and is burning 92-octane unleaded pee water, the large 1.0 mm orifice jet with a flow rate of 335cc per minute was selected. Since the SE-R has a huge (more than one gallon) windshield washer reservoir, it was tapped with a pick-up for the high pressure water pump.
To partially offset the power-reducing internal cooling produced by the water, it is possible to run a leaner fuel-air mixture and more ignition advance. Clark Steppler of JWT added sophisticated electronic controls to integrate the water injection into the Engine Control Computer that he reprogrammed earlier. Using a daughter board interfaced to the ECU to control the water injection pump, Clark programmed the daughter board to turn on the injector pump when the mass air flow meter voltage reached a pre-set point (at an air-flow level that occurs around 8 psi of boost) whenever the water injection master arm switch was activated. When the water pump is activated by the ECU, the ECU automatically switches to a new fuel and spark map after about 120 milliseconds delay. This gives the water injection system time to prime before going to the more aggressive fuel and spark maps. The maps are both leaner, and with more ignition timing advance, optimized for water injection.
The water injection system is almost like having a tank full of race gas all the time except without the $5-per-gallon cost. After all water is cheap -- nearly free. For once in the history of hot rodding, something can be done for nearly nothing. As an interesting side note, water injection also reduces toxic emissions of oxides of nitrogen (NOx) by reducing combustion temperatures. All of this and cleaner air too.
Reprinted with Permission