Suppressing Knock with Water Injection
You have a turbocharged DSM, and you want to tune for more timing, more boost, and lean her out a bit for more power. You get a huge front mount intercooler, big injectors and electronics to control them, an overkill pump so that you don’t have to drop your gasoline for another upgrade, and a beefy turbo that can handle the boost and horsepower level you’re craving. In your tuning you quickly come to a fork in the road. Should I just run race gas all the time? Or, should I dial her back to where it is safe and feed her pump gas until track day?
Water and water/alky injection is another option. Ultimately, you can keep your “race” tune even on the street w/ premium pump gas. How is this? Before we delve into the why, we need to understand what restricts us from running the highest boost the turbo can throw, the most timing possible before no gains are felt, and running lean enough to make peak power. Detonation and pre-ignition.
Detonation and Pre-ignition
- What is Detonation?
- Detonation or knock is when a portion of the gas, usually at a corner of :the combustion chamber furthest away from the spark plug, self ignites but AFTER the spark :plug has fired. Detonation is significantly more serious than pre-ignition, as it is harder to :cure and potentially more damaging to the engine. It is many times (but my no means :always) accompanied by a rattle-like noise much like marbles being shaken in a tin can. The :pinging noise that is heard, is the gas ignited by self-ignition, colliding with the ignited gas :from the spark plug.
- So then, what is pre-ignition?
- Pre-ignition is when the gas and air is ignited BEFORE the spark :plug has fired. This causes :a collision of the burning gasses to occur before the piston has :reached the top of the bore :and so not at maximum cylinder pressure, thus less damage is l:ikely.
Detonation and pre-ignition can have similar causes.Some are:
- Poor octane levels
- A lean condition
- Compression that is too high
- Aircharge that is too hot
- Poor combustion chamber design
- Overheated engine head
An easy way to raise the level of performance of a turbocharged engine is to raise the maximum boost the turbo delivers. However, as you would guess, there are tradeoffs:
- Raising the boost raises the airflow of the system. If you do not have enough fuel flow to match the airflow, then you are encouraging an overly lean condition. A lean mix likes to ignite on its own either before or after the spark plug fires.
- As well, this raises the dynamic compression of the setup, as more air is being compressed than at stock boost. Static compression is the total volume of the cylinder divided by the total volume of the combustion chamber alone. Dynamic compression is the actual compression ratio seen by the combustion chamber at TDC. This fluctuates as volumetric efficiency fluctuates. And, as boost rises, so does volumetric efficiency. Volumetric efficiency is the percent volume of air that actually enters the cylinder versus the actual cylinder volume. VE rises and peaks at around 5500-6000 rpms in a stock 4G63 longblock. A highly compressed air/fuel mix to ignite on it’s own either before or after the spark plug fires.
- Further, raising boost compresses the aircharge. When air is compressed, it heats up. This is called the adiabatic process. A very hot air/fuel mix likes to ignite on it’s own either before or after the spark plug fires.
- At some point, you reach the octane limit of whatever gas you’re running. Premium gas becomes prone to knock and so you’re looking for race gas, or you just accept a boost level just below the knock threshold as fate.
So here we have 3 potential allies to detonation and pre-ignition in an attempt to yield more horsepower. We add the proper fuel upgrades, lower the compression ratio, and add an intercooler to keep the adiabatic process at bay.
How Does Water Injection Work
But what if you want more? Enter water injection. Water injection is the action of spraying a fine mist of water into the intake track. This lowers intake aircharge temperatures, and raises the effective octane in the combustion chamber so that you can run leaner, run more timing, and more importantly higher boost with 93-octane gas.
Though injecting a mist of water into your intake stream may be considered a radical concept, by no means is it new, unexplored, or unproven. The U.S. Air Force experimented extensively with water injection in the early 1940s and fitted combat aircraft with water injection apparatus for the enhanced performance necessary during evasive maneuvers. There are two reports,  formally written by the NACA(National Advisory Committee for Aeronautics): The Induction of Water to the Inlet Air as a Means of Internal Cooling in Aircraft Engine Cylinders, presented August 15 1942; and Knock-Limited Performance of Several Internal Coolants, presented February 1 1944. It was concluded by both government reports that water properly injected into the aircharge allowed MUCH higher boost levels AND peak cylinder pressures (more timing and leaner a/f mix) to be achieved.
How does water injection lower intake aircharge? Have you ever noticed that the outside air temperature goes down right after a rain on a summer day? The water droplets in the air draw heat right out of the atmosphere. This is because the water is slowly evaporating either in the atmosphere or on the ground, or once it rests on a surface. A change of state from liquid to vapor at constant temperature requires the input of energy, called the latent heat of vaporization. This implies that while a liquid undergoes a change to the vapor state at the normal boiling point, the temperature of the liquid will not rise beyond the temperature of the boiling point.
The latent heat of evaporation is the energy required to overcome the molecular forces of attraction between the particles of a liquid, and bring them to the vapor state, where such attractions are minimal. Water has a high latent heat of vaporization. To bring water to the boiling point at 100 ºC, the latent specific latent heat of vaporization is 2260 kJ/kg. This means that to convert 1 kg of water at 100 ºC to 1 kg of steam at 100 ºC, the water must absorb 2260 kJ of heat. This is quite a high figure relative to other liquids. Conversely, when 1 kg of steam at 100 ºC condenses to give 1 kg of water at 100 ºC, 2260 kJ of heat will be released to the surroundings.
Latent heats of vaporization vary widely. Here are some others:
- Ammonia: 2466 kJ/kg; Oxygen content 0% (causes pre-ignition)
- Water: 2259.197 kJ/kg; Oxygen content 0% (ignoring dissolved oxygen)
- Methanol: 1099.45 kJ/kg; Oxygen content 49.9% by weight
- Ethanol: 854.62 kJ/kg; Oxygen content 34.7% by weight
- Isopropyl Alcohol: 666.7 kJ/kg; Oxygen content 26.66% by weight
- Gasoline: 586-628kJ/kg
- Toluene: 351 kJ/kg
You can see why most like to use “winter mix” windshield washer fluid. Basic “winter mix” washer fluid contains about 49% methanol, 49% water, and 2% blue dye.
However a bit more goes into cooling the aircharge than just the mist's ability to absorb large amounts of heat when transferring to a gas. One reason an intercooler is so effective in cooling the aircharge is the large total area provided by the many fins. This same principle applies to water injection but on a much grander scale. As the water mists when injected, it separates to a near molecular level. Each micro-droplet has a specific surface area. The finer the droplets are the larger the total surface area. This allows more air molecules and fluid molecules to contact giving the opportunity for heat transfer. Alcohols naturally break into a finer mist than water. Though water can absorb over twice the energy than even the best of the alcohols, methanol; it takes a LONGER time to transfer the heat. This absorption amount over unit time is slow enough to absorb much less total heat energy than methanol would by the time the aircharge reaches the combustion chamber. Throwing methanol in the mix greatly lowers intake aircharge to below ambient (outside) temperatures (more so than water), which the largest and most efficient air-to-air intercooler in the world could never accomplish. Therefore you have a dense oxygen rich aircharge. More aircharge equals more power, just like raising the boost.
Why not use 100% methanol? Many do in fact use pure methanol. It actually burns very well releasing a very large amount of useful energy in the combustion chamber. It has a very high octane level (reportedly over 114). Mixing 93-octane pump gas w/ methanol in the combustion chamber yields a higher fuel octane to around 97-98. In the amount of time available, it does the best in lowering intake charges. BUT, where pure water shines is actually in the combustion chamber. The combustion chamber is hot enough to turn the water to vapor in enough time to be supremely effective. Remember, when water turns to a vapor, it absorbs over twice as much heat as the best alcohol.
Absorbing all that heat intensely cools the combustion chamber: primarily the piston top and any hotspots that may cause the fuel to pre-ignite. However, a large quantity of the water is not completely vaporized. Once the air/fuel mix is compressed and ignited by the spark plug, high heat is released and absorbed by the semi-vaporous water mist. This turns the rest of the water to vapor and in the process slows the actual burn rate of the air/fuel mix to a rate very similar to over 110-octane race fuel. Therefore, it is applicable to say that, if the proper amount of water is delivered BEFORE vaporization, you can perform the same tasks such as high boost, 30+ degree timing advance, 13-14:1 air/fuel ratios. Remember, you’ve supplied pump gas and a few CCs of water!
The benefits do not stop there. The intake track receives a steam cleaning by the partially vaporized water. This cleans the PCV and EGR deposits off of the intake manifold, intake runner, and intake valve. This maintains the higher flow you get from a “fresh” head. Otherwise, carbon build up around the valve occurs inhibiting flow and a good polish job can be nullified by months of carbon build up. As well, carbon deposits are removed and prevented from forming in the combustion chamber. Such carbon deposits can develop hotspots and slightly higher compression that may lend to detonation or pre-ignition. In stead, these carbon deposits flow at out of the exhaust along with the steam leaving a shiny head and combustion chamber.
Injection Delivery and Basic Components
The water or water/alky mix does not have to be delivered in precise quantities. But there does need to be some semblance of balance between fuel injected and the water injected. Generally speaking, the amount of pure water should be injected at 15% of the total fuel injected at peak horsepower. In a water/alky that number goes up to 20-25%. So, for those of us who aren’t sure whether pure water is going to be best or if a mix is preferred, using a nozzle that injects the same amount of the fluid as one injector is more than enough for deciding between the two and provides SOME future growth in performance. For those who want to run straight alcohol, i.e. “meth”, you will need a nozzle that flows around 3 times more than on needed for pure water. That is not to say that running straight alcohol is less efficient. It is simply that you will not receive the full individual benefits of what straight alcohol provides in any given setup. The only real down side to running straight methanol is that a different tuning strategy needs to be employed, as methanol is a fuel, but burns at a different rate than gas, likes a different mixture ratio with air, and so forth… Such is a discussion for another article.
It is always good to place the nozzle in such a way that the mist permeates the entire aircharge. Placing it at the throttlebody elbow at the bend so that it partially faces the throttle plate has worked very well. As well, several small nozzles that total the required flow works better than one larger one. This is because smaller nozzles tend to atomize the mist better adding more surface area to the fluid as it is injected into the aircharge.
A turbocharged engine operates in vacuum most of the time (less for some :D ). Also, many choose to activate injection at a particular boost level. This means that the water line attached to the nozzle is subject to vacuum and some boost. In vacuum, fluid is sucked into the engine inhibiting light throttle performance and often bogging the engine. In extreme cases, some have experienced hydro-lock if measures aren’t taken to prevent this. In light boost, the water can be pushed out of the line, all the way to the pump, through the pump and to the fluid reservoir. It would take a relatively long time for the pump to prime and flow the fluid through the lines to the nozzle. This time can be damaging. Without water injection and with an aggressive tune, if one second elapsed at 4500 rpms, the air/fuel mix has detonated 75 times on each cylinder! To prevent these occurrences, a solenoid or check valve needs to be used in the water line to the nozzle(s). A solenoid opens to let the fluid inject and closes to prevent it from being sucked or pushed by the various engine intake conditions. A check valve must flow to the nozzle to prevent boost from pushing the fluid. It also must have a high “crack” pressure, on the order of 4-5 psi. This is the pressure it takes to suck it open. Either should be places as close to the nozzle as possible.
You’ll need a pump that flow enough fluid for your goals (just like a fuel pump). You’ll also need one that can tolerate extended usage of your fluid choice. Some pumps don’t like water and rust up. This includes MOST fuel pumps. Some pumps don’t like alcohol and their rubber parts dry out and let go. Most pumps in a kit that you purchase can handle both alcohol and water for years. And most provide plenty of flow for the highest goals. If you’re purchasing your own pump, go with a good name like SHURflow, look for 100 psi, and get as close to 1.5 gpm or more as you can. Almost all SHURflow pumps have a pressure adjustment screw. This is to fine-tune your injection and allows you to get a larger nozzle set for down the road.
An easy way to control your injection is through a Hobbs switch, which can be purchased at NAPA for $30 or so. This is a pressure switch that closes when a certain pressure is detected. AND it’s adjustable! I’ve had great success with this and it is FAR cheaper than a simple on off boost controller that is sold by any of the kit manufacturers out there.
Fuel injection hose has worked best for me. It is not cheap but you need something that can handle 100+ psi repeatedly. Simple screw or worm clams have worked fine. But, you will have to tighten them up every once in a while. Check them on a regular basis. I check mine when I check my oil.
Mount your pump in a plane below your fluid tank. This insures a primed pump. Many just tap the wiper fluid reservoir. I placed my tank in the hatch. This guarantees cooler fluid. It will draw up even MORE heat to completely vaporize the fluid.
Finally, keep an eye on your fluid levels. It would be sad to hear you've blown your engine on a hard run at 30 psi running, 38 degrees timing on an EVO3 16G with pump gas because you ran out of fluid during the run! Further, use distilled water or keep an eye on your nozzle of you use wiper fluid. Nozzles will retain deposits and clog up making them less and less effective.
--Dsmonster-34728 22:08, 4 December 2007 (EST)