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Water Injection

Water injection (and water mixtures like mixed methanol injection) has been around for decades.  Its sole purpose is to suppress engine knock.  Keeping in mind that most engine knock is caused by uncontrolled and/or unintended combustion of the fuel mixture.  Water with it high specific and latent heat is an excellent tool for bringing charge temperatures and combustion back into control when charge and cylinder temperatures or compression get too high for the fuel being used.  Consider the following table from ERL / Aquamist:

**Source: Aquamist MF2 Manual

This table illustrates very clearly why water or a water/methanol mix should be used for induction charge and in-cylinder cooling rather than using excess gasoline.  Water has over twice the specific heat of gasoline and more importantly to its application of gasoline has over six time the latent heat of evaporation that gasoline does.

To give you an idea of exactly how much work needs to be done, the following charge temperatures measured at the turbo outlet in an Aquamist study are very illustrative:

Effect of water injection in the induction system:

Water is generally introduced to the induction system after the turbo-charger and before the throttle body (unless multi-port water injection is being used, which is more complicated than this paper contemplates).  It may be injected pre- or post-intercooler or both.

The injection of water into the induction charge will immediately absorb heat from the charge thereby increasing its density. Evaporation of the water at this point is unlikely or very minor.  The induction charge can at a maximum reach 100% relative humidity and will stabilize at an equilibrium state that is governed by temperature of the charge exiting the turbo and the relative humidity of the ambient air. At full saturation the induction charge will have a significantly reduced temperature somewhere between the ambient temperature and the temperature of the injected water.  This heat absorption is achieved primarily through the water's specific heat very little vaporization will occur at this point.  The boiling point of water is higher than most charges exiting the turbo and the cooling effect of the water will reduce even the hottest charges without significant vaporization and never beyond the point of saturation and equilibrium.

Look at the density losses from heated induction charges (which directly works against the objectives of forced induction) and the improvement in charge density from various temperature drops again from Aquamist:

The effect of water on the induction charge will be somewhat greater in drier climates where more water can be evaporated into the air.  However this should not be taken as an indication that it is not useful in more humid climates.  Once the induction charge gets heated it will always have room to accept the evaporated water whether the initial relative humidity was high or low - and this evaporation will always absorb heat from the induction charge.

As the water absorbs heat the droplet sizes will decrease and the surface area of the water droplets will increase.  Any additional volume from this reaction will be more than made up for by the reduction of the charge temperature and its resulting increased density.  Water is not displacing the volume or weight of air in the induction charge and actually working to increase the volume and weight of air that reaches the cylinder.  Sizeable droplets of water will reach the cylinder with significant heat absorbing potential intact.

The cooling of the induction charge is the first way that water injection suppresses knock and permits more air-fuel mixture to enter the cylinder.

Effect of water injection while entering and in the cylinder:

During the intake stroke as the induction charge passes the valve the charge picks up heat from the heated surface of the intake valve.  Additionally the effort of sucking the charge past the valve transfers heat to the charge as well.  The presence of water in the induction charge absorbs this heat more readily with a lower increase in the temperature of the charge.  Remember that it takes more heat to increase the temperature of the water present in the charge.  Air and fuel alone in the charge increases in temperature much more readily and rapidly as it absorbs the heat from the valve and effort exerted to bring the charge into the cylinder.

*Thanks to hotrod on NASIOC for pointing out the starting basis for the conclusions reached here.

Once the charge enters the cylinder, water in the induction charge following the exit of the super heated exhaust gases (1500+ degrees)  begins to immediately cool the surfaces within the cylinder including the cylinder walls, piston head, combustion chamber, valves and spark plug tip.  This cooling will have its most dramatic effect on any possible hot spots the coolant system has not adequately addressed.  Surface temperatures will be significantly reduced by the water even more so than the extra fuel that was previously being used to cool the cylinder surfaces.

With twice the specific heat and six times the latent heat of gasoline, 1/6 as much water by weight is necessary to fully replace the heat absorption of the decreased gasoline in the induction charge.  Richer than 12:1 none of that extra fuel is being burned in combustion and the extra fuel robs significant power while serving as a poor coolant.  Since the amount of water being added is only a fraction of the amount of the decrease in gasoline there is actually more air and as a result more oxygen in the induction charge for additional combustion to take place.

The cooling of the cylinder area surfaces permits additional air/fuel mixture to enter the cylinder thereby increasing volumetric efficiency.  The combination of decreased liquids in the induction charge combined with the increased VE from in-cylinder cooling disproves the myth that water injection will displace air in the induction charge or reduce the amount of fuel that will be burned in combustion.

During this process the continued absorption of heat will decrease the water droplets further increasing their surface area and volume but full vaporization accompanied by the resulting 1700 times increase in the volume of the water will not have occurred at this point.  The water will still reach equilibrium prior to this point, albeit in a hotter state.

The absorption of the heat gain while actually entering the cylinder and the cooling of the cylinder area and volume is the second way that water suppresses knock - the cylinder and the charge is much better prepared for a controlled burn during combustion.

Effect of water injection during compression and combustion:

By this time water has lodged itself between the air/fuel mixture although the percent of water to the mixture is only around 1%.  Though the ratio is small the water further suppresses any remaining tendency of the fuel to pre-ignite by lodging itself between the air and fuel during compression.  Before actual combustion occurs the water works against auto-ignition by continuing to absorb the heat generated during compression.

Water is a byproduct of combustion it is the chemical reaction of oxygen with the hydrogen freed from the hydrocarbon chains during combustion.  How could the injection of additional present water before combustion contribute further to the combustion process?  This is where it gets a bit complex and I will try to do the best I can.  This pieces together various pieces of my understanding from different sources.

During early combustion when the fastest reactions occur the effect of the water in the mixture is to cause a more controlled and stable flame front. The freeing of hydrogen and carbon to combine with oxygen has to work around the present water to form OH radicals and CO.  By slowing this early combustion process there is further suppression of the potential for the mixture to burn too fast and contribute to knock.

Later in the combustion process slower and more complex reactions occur.  The formation of OH radicals is very fast and interferes with the completion of the combustion process to form CO2 from the first step creation of CO.  It is during this phase of combustion where present water helps to complete the slower reaction to complete the formation of CO2 since water is about the only way to complete the oxidation of CO.  The additional present  water actually speeds this reaction which also happens to be when as much of two thirds of the energy from carbon combustion is released.

* Sources: Bob Harris note on DIY_EFI thread and the reference within to Combustion, Third Edition by Glassman (reference provided by Jon [in CT] on NASIOC).

As additional evidence of why using fuel to suppress knock is a horribly inefficient exercise consider the following:

"...thus one can conclude - correctly - that hydrocarbons inhibit the oxidation of CO.

It is apparent that in any hydrocarbon oxidation process CO is the primary product and forms in substantial amounts. However, substantial experimental evidence indicates the oxidation of CO to CO2 comes late in the reaction scheme. The conversion to CO2 is retarded until all the original fuel and intermediate hydrocarbon fragments have been consumed."

* Source: Combustion, Third Edition, Glassman, p. 76

Performance tuning that involves fuel dumping necessitates overly advanced timing resulting in timing induced knock (and as has been experienced by others engine destruction) because the significant power release from combustion is delayed to the point where MBT is very late while waiting for the excess fuel is broken down during the extended initial stages of combustion.  In upgraded turbo applications this becomes extremely apparent as the increased air flow necessitates ever higher fuel flows to maintain these supposedly safe AFRs of 11:1 and richer.

More simply consider the following - what power enhancing purpose would excess fuel serve once all the oxygen in the induction charge has been utilized in combustion?  It can not and will not burn, Glassman has shown that it even interferes with the complete combustion of the fuel that is burnt. It is only useful as a coolant and most fuels make poor coolants.

As illustrated below water injection can actually help reduce maximum cylinder pressures while increasing BMEP.  This is because a properly mixed AFR of 12.5:1 accompanied by water injection increases the rate of the combustion reaction exactly when it is desirable to do so during the oxidation of CO => CO2.

*Source: The High-Speed Internal Combustion Engine by Sir Harry Ricardo from here http://www.carrollsupercharging.com/oldsite/gaseous/GI-01.pdf (reference provided by Jon [in CT]).

Using a mixture of 50/50 water/methanol for injection will have lower specific heat and latent heat than straight water and besides having a very high 113 octane level itself, it also contributes additional fuel to the combustion process.  Contrary to what their heat values would suggest some studies have even reported such a mixture can have greater knock suppression than straight water. Additionally climates that have cold winters benefit from preventing the freezing of the injection mixture.

Conclusion on the contributions of water injection:

Throughout the process water injection helps to manage heat, contributing to higher VE and suppressing engine knock. Discounting arguments that water will rob the combustion process of precious oxygen containing air.  Then in the final combustion reactions it actually improves the reaction of CO => CO2 for a faster more effective release of the bulk of the energy during combustion.  Discounting arguments that it slows and inhibits combustion.

Performance tuning and miscellaneous notes on water injection and mixed methanol injection:

Always keep in mind that water injection is being used to suppress engine knock and permit tuning AFR to its maximum power levels.  Water in excess of the amount needed for a safe engine knock suppression margin will hurt net output.  Not leaning AFR enough or leaning it too much can result in decreased power levels as well.  This has been experienced by many tuners who do not have faith in water injection and have left fuel mixtures too rich.

The relationship that you should seek to manage with water injection is the ratio of water to fuel.  Metered to exacting proportions as little as 3% water to fuel can replace the amount of heat absorption that fuel previously provided when leaning from 10:1 to 12:1 AFR.  There is no system that can meter water in that exacting a relationship with fuel that does not utilize a full fuel injector driver, port installed nozzle jets, high flow pump(s) and a rising rate pressure regulator.  At this point you are talking about stand alone engine management like a Motec and a duplication of the fuel system but for water.  That being the case the best solution is to use a system that will get you as close as you can to mirroring the fuel injector flow and run at least 10% water to fuel for margin.

Some water injection users have used significantly higher water to fuel ratios (25%+).  The technique usually involves increasing water flow until the engine bogs down or comes close to misfiring then pulling back a little bit, before leaning the AFR significantly and finding the MBT advance needed.  These techniques take significant amounts of experience and are less compatible with most street setups.

Average street uses should try and target 10%-15% water to fuel.  Aggressive street and moderate track use can pretty readily utilize as much as 25% water to fuel especially when using a mixed methanol injection system.

In general water injection and mixed methanol injection not only permits advancing timing, it is required due to the slower initial flame front (though late stage reactions are sped up) during combustion.  This is contrary to many nitrous techniques so great care and skill must be employed to find the appropriate balance when trying to use both water and nitrous to make significant power gains.

When adding 5%-7% water to fuel without leaning rich fueling levels - a degree or two of advance will produce almost the same power result as without water injection (leaning slightly will increase the power output).  This is good for people who have engines tuned for high octane or race fuel but on a daily basis want to use lower octane grades.  This is purely an economic use of water injection - power use requires fuel and timing tuning.

The more boost you are running and the more fuel you are replacing with water injection the more dangerous it becomes to run a constant pressure / static flow water injection system.  In order to prevent bogging down the engine at lower boost loads the flow of water is less and less likely to be sufficient for your full boost application in these static setups.

The state of your air/fuel mixture and timing after performance tuning for water injection or mixed methanol injection will be more than sufficient to destroy your motor if the water injection fails while under load.  The responsible user will take measures to ensure adequate water is in supply and flowing during high power usage.  Ideally along with a fail safe that will cut boost if a fault in the system occurs.  At a minimum warning lights for water level, water pressure and water flow should be employed.  Ideally triggers that close all boost solenoids in automatic response to faults would be employed by high end systems.

The entire body of information in this paper was developed relatively informally by me over the last 15 odd years through research, reading barely comprehensible scientific papers and through verbal and written discussions with others about our mutual experiences with water injection.  And some of the most important additions to my knowledge have been contributed by the helpful response of people who have read and reviewed this work online.  I hope it has proved a beneficial addition to your research on the subject.

I will make every attempt to update and edit this paper as I receive additional feedback from readers.

Also visit our water injection forum at http://waterinjection.info/phpBB2/ for discussion with other experienced water injection users.

Ed.

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Copyright © 2003  J. Edison Haney II - all rights reserved, no portion of this paper may be copied or otherwise captured without its full contents and credit given to the author and his referenced sources.

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Last modified: 02/05/2004 18:50 -0600

All contents are Copyright © 2003-2004  J. Edison Haney II or their respective authors as otherwise noted. All rights reserved.
Last modified: 02/03/04