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Now onto the fun part- let’s calculate how much water injection will cool down the intake charge. We’ll complete our calculations in Metric units and convert to Fahrenheit at the end. The latent heat of vaporization for water is 40.7 kj per mole. A mole is the standard reaction unit of measure for mass used in chemistry. KJ stands for kilojoules, a standard unit of energy used in chemistry and physics. The mass of water is about 18 grams per mole. Multiply 40.7 Kj/mole by 18 g/mole and you get 2261.11 joules/gram to evaporate 1 gram of water. The Aquamist water injection jets flow from about 150cc/min for the 0.4mm jets up to 335cc/min for the 1.0mm jets. Multiply the 2261.11 joules/gram by the water flow and divide by 1000 to get the number of kilojoules of energy to vaporize the water flow. Let’s see how different jet sizes affect this calculation:
0.4mm = (150cc/min = 150 * 2261.11)/1000 = 339 kj
0.5mm = (190 * 2261.11)/1000 = 430 kj
0.6mm = (225 * 2261.11)/1000 = 509 kj
0.7mm = (265 * 2261.11)/1000 = 599 kj
0.8mm = (295 * 2261.11)/1000 = 667 kj
0.9mm = (310 * 2261.11)/1000 = 701 kj
1.0mm = (335 * 2261.11)/1000 = 757 kj
Next we need to know how much energy it takes to remove heat from air. The specific heat of air, or the amount of energy it takes to change air’s temperature, is about 1.005 Kj per degree Celsius. We will use the corrected air mass flow of the B18C engine at 8000 rpm, or 30.36 lbs/minute, which is 13.77 kg/min.
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| The Aquamist nozzle is compact but requires an external check valve. The Aquamist system is tuned by using many nozzle sizes but the pump volume cannot be adjusted. |
Assuming we used the 0.4mm jets which each flow 150 cc/min, and that the whole mass of water is evaporated, we get 339kj divided by 13.77 kg/min for a temperature drop of 25 degrees Celsius, or 76 degrees Fahrenheit. A 1.0mm jet will drop the temperature 55 degrees Celsius, or 131 degrees Fahrenheit! The temperature drop is subject to the water/air reaching a saturation point. Beyond this, no further evaporation or temperature drop is expected. It is also conditional on the humidity and charge temperature exiting a turbocharger or supercharger. The water injection system will reduce the temperature about the same as an intercooler will!
Aquamist has three systems of water injection. The System 1s is the basic system and is triggered by boost pressure, monitored by an adjustable pressure switch mounted in the manifold that can detect between 3-30 psi. System 2s uses an electronic controller that is mappable between 2000-9000 rpm for 1000 rpm intervals. System 2c is meant for users with their own programmed ECU. It is used to cool the inlet charge as well as keep down cylinder temperatures and detonation to allow the boost to be increased. System 2d uses a fixed water/fuel ratio to maintain a 3D water injection map without programming. Some water injection systems also include a water fault output. If there is a blocked jet or a cut hose that isn’t providing the correct supply of water injection, it can cut the boost, switch the map (with a custom ECU), or switch in a resistor to fool the engine into dumping fuel.
The AEM system has the potential to flow more water than the Aquamist system, with three different nozzles up to 550cc/min. The cool thing about the AEM system is that it is easily tunable and the onset boost pressure can be tuned from 0.5 to 11 psi and a full boost setting from 6 psi to 38 psi so the top of the injection volume curve can be easily tuned by the user. Where the Aquamist system is an continuous fixed volume of water, the AEM system is variable with more water delivered as the boost pressure and RPM climb.
There are a few other things you can do to keep the heat down, such as using Redline Water wetter or Neo’s radiator additive, running a higher pressure radiator cap, or even replacing the stock radiator with a higher capacity unit.
The figure above plots cylinder pressure against crank angle for four different operating conditions. The green line represents the normal combustion cycle. Advancing the ignition by three degrees produces a slight knock (yellow line). Increasing the timing even further produces severe knock (red line). This level of knock is loud enough to hear and if it continues for a period of time, permanent engine damage can occur.
The blue line is placed approximately between the red line and the yellow line. The ignition timing is around 6 degrees advanced of the normal green line, but this time water is being injected into the engine. Before TDC, the blue line followed a predictable path but soon after TDC, the pressure begins to flatten. The injected water begins to evaporate in the combustion chamber and absorbs much of the heat, keeping the pressure and temperature from reaching the point of detonation. The line now follows the green line until the next cycle. The area under the blue line is much larger than the green line indicating an increase in torque. These gains would be even higher with a turbocharged engine.
Got a Tech Question? Email Sarah at asksarah@motoiq.com
