The following is an article I submitted to Audiworld in 2006 shortly after I first began using water-methanol injection with my B5 S4.

Since the article was initially published I’ve learned some finer points about the theory behind water-methanol injection and how well the systems function on the B5 S4, but the general concepts covered by the article are still applicable.

Turning Water into Horsepower

Water injection, amongst the many options for increasing engine performance, this subject is probably one of the least understood. While frequently dismissed as simply a Band-Aid for poor intercooling, the benefits of water injection stretch beyond simply cooling engine intake air, and when properly implemented, can lead to more power than intercooling alone.

Water injection can be traced back to the earliest years of the internal combustion engine. During the early 1930’s one of the foremost engine designers and researchers, Sir Harry Ricardo, whose studies on pre-ignition led to the development of the octane rating system, investigated the problem of pre-ignition and the affect of alcohol-water injection on inhibiting detonation. Later, during World War II, the National Advisory Committee for Aeronautics, in studies aimed at increasing the power from aircraft piston engines, came to the following conclusion: “The data indicated that water was a very effective internal coolant, permitting large increases in engine power as limited by either knock or by cylinder temperatures.”1 More recently, Saab engineers have investigated using water injection on their production vehicles to maintain low emissions at high power levels. Clearly, the practice of injecting water into an engine is not new, and its benefits are well documented.

Why use water injection?

Engine power production, referred to as brake mean effective pressure (BMEP), is measured by taking the average effective pressure of the cylinders as they progress through intake, compression, ignition, and exhaust strokes. Added power comes as a result of greater pressure, but a higher temperature inside the cylinder accompanies greater pressure. These higher temperatures can lead to detonation, referred to as engine knock, or pre-ignition, both of which are cases where the fuel-air mixture burns in an undesirable manner and can be destructive to an engine. To combat knock and pre-ignition as power increases, a richer air-to-fuel ratio is normally required. If the addition of extra fuel doesn’t provide enough knock protection, then a higher-octane fuel, which is more resistant to knock and pre-ignition, may be used. However, once the knock limit of a higher-octane fuel is reached, can anything be done? This is where a water injection system presents an appealing option.

Water vs. Gasoline

When it comes to absorbing heat, water is far more effective than gasoline. For some perspective, consider: the amount of heat energy needed to vaporize one cup of liquid water would be sufficient to vaporize almost half a gallon of gasoline.

Specific heat is used when describing the capacity for heat absorption. Specific heat is the amount of heat energy (joules) per unit mass (grams) required to raise the temperature of a substance by one degree Celsius. The greater the specific heat, the more energy a substance absorbs before it heats up. Gasoline has a specific heat value of 2.02 joule/gram-°C. Water has a specific heat of 4.18 joule/gram-°C. This means, for an equivalent volume, liquid water will absorb about twice as much heat as gasoline while increasing in temperature the same amount. Or put another way, for a given quantity of gasoline, half as much water is as effective when it comes to absorbing heat energy.

Fuel and other liquids also have a property called the latent heat of vaporization. This refers to the heat absorbed when a substance changes phase from liquid to gas. In this respect, water is even more effective at absorbing heat energy, with a latent heat of vaporization about 6-7 times that of gasoline.

Returning to the original question, why might one want to use water injection in an engine? Because at some point, as you increase power (BMEP), the engine reaches a limit where no more fuel can be added to combat knock during combustion. Water can then be introduced to prevent knock, allowing more power to be produced. It is worth pointing out that the extra gasoline that is added to the combustion chamber to combat knock does not get burned because there isn’t enough oxygen present to support burning it. Therefore this additional fuel does not contribute any energy to produce power, it is there only to prevent knock, and is exhausted from the cylinder with the combustion by-products. Because water is much more effective in this role, a small amount of water can replace the larger quantity of excess fuel, allowing the engine to run a more desirable air-fuel ratio, and the water will be more effective at cooling inside the cylinder and preventing knock. The end result is that with the help of water injection the engine can produce more power, while consuming less fuel, than it otherwise would.

When to use it?

Water injection is not always necessary; otherwise we would all be driving cars with a water injection system. However, if you’re trying to get more power from an engine, at some point a water injection system becomes an alternative that is worthy of consideration. What if you were tuning an engine to produce greater power, but didn’t want to use higher-octane gasoline? Could you run a motor on regular pump gas with water injection and get improved performance? This was the situation I found myself in. I wanted to see if I could get more performance from my car while running the engine on ordinary 93-octane pump gas.

The equipment

The first requirement is to have a car that can tap the benefits of water injection. While a normally aspirated engine with high compression could benefit some, it is on forced induction vehicles, turbocharged or supercharged, that water injection can provide significant gains. My turbocharged S4 equipped with an Audi Performance and Racing (APR) Stage III kit was an ideal candidate for using water injection. The APR Stage III kit is built around the larger K04 turbochargers used in the B5 platform Audi RS4. A number of other RS4 parts replace stock S4 components to accommodate the K04 turbochargers, including larger fuel injectors and fuel pump. The ability of the K04 turbochargers to run much higher boost than the S4 with stock K03 turbochargers, up to 21 psi with APR’s software compared to 9 psi on K03 stock software, led me to upgrade some of the cars intake and exhaust components. I replaced my stock downpipes with AWE Tuning’s downpipes and the exhaust with Supersprint’s catback system. The greater pressure developed by the K04 turbo’s would result in hotter air entering the engine, so I replaced the stock S4 intercoolers with AWE Tuning’s larger side mounted intercoolers.

The water injection system I assembled was built around the Aquamist brand 2d system and was installed by Cascade Autosport, a race and rally preparation company located in Redmond Washington.

The water injection system requires no involvement from the driver once it has been configured and powered up, other than monitoring the water flow gauge and blockage indicator to ensure water is flowing to the engine. The primary components of the 2d system are a fuel injection amplifier, adjustable pressure switch, high-speed valve, and 100-psi pump.

The system operates the following way: The fuel injection amplifier continuously monitors the signal going to the fuel injectors and will meter the amount of water to be injected in proportion to the amount of fuel being injected. This is important because too much water can reduce power production.

An adjustable manifold pressure sensor, shown below, is set to activate the system at a desired boost level. Ideally, water should be injected just before the engine encounters peak loads, when maximum torque is being generated.

Once the boost threshold has been met the system activates and a high-speed valve opens allowing the water to be drawn from the windshield washer reservoir and through a filter (1 below) by the 100-psi pump (2). An inline regulator allows the water pressure to be adjusted as desired (3). The water goes through a flow meter (4), the high-speed valve (5), and a 100cc accumulator (not shown), that acts as a reservoir and shock absorber. The accumulator smoothes out the pulses from the pump and enables the system to flow slightly more water than would be possible from the pump alone. An added benefit of the accumulator is to provide additional safety in the event of a pump or high-speed valve failure. Should either of those components fail during operation, the accumulator has enough stored water under pressure to continue injecting water for 5-10 seconds.

After exiting the accumulator the water splits and travels down two hoses that go to two separate jet nozzles placed just after each intercooler (shown in picture below).

These jet nozzles turn the water into a fine mist and discharge it into the intake air stream (6). Placement of the jet nozzles is a subject unto its own, with various options being available. The most common locations for the jet nozzles are before the turbochargers, after the intercoolers, or into the intake manifold. There are various reasons why each location may be chosen, with differing results depending upon location, but the most common location is after the intercoolers, which is what I chose to do.

Two other components complete the system. There is a blockage indicator that illuminates when the pump fails to operate or the high-speed valve does not open. There is also a water flow meter that registers the volume of water passing through the system and displays the amount through a series of LED’s. Both the blockage indicator and water flow display are inside the cabin where the driver can monitor them.

Reduced Intake Air Temperature

The most immediate benefit from water injection is the lowering of intake air temperatures. This can be realized even with a simple setup. The chart below shows how the intake air temperature on my S4 was affected with the addition of water injection.

Once the car begins producing boost, an action that also raises the temperature of the air being compressed, almost immediately the water injection begins to minimize the temperature rise of the intake air beyond what the intercoolers can do alone.

The benefit is most apparent when a series of high boost pulls are made. A situation that will commonly lead to heat soak for a turbocharged car with intercoolers is no problem for an engine with water-injection.

The chart above shows a series of four dynamometer pulls made with minimal cool down between pulls. The first two pulls are made with water injection off. Then the water injection is enabled for the second two pulls. In each instance the intercoolers drop the starting intake air temperature to roughly the same temperature, about 90 degrees Fahrenheit. By the end of each of the first two runs the intake air temperature has risen 50 degrees, to 140 Fahrenheit. Once water injection is activated the temperature rise is greatly reduced, increasing only 20 degrees, and ending at 110 Fahrenheit.

By keeping temperature rise in check the engine is able to produce the same power level over time. The graph above shows the steady decline over time in horsepower produced on my car without the benefit of water injection, and the consistent production of power when water injection is active. Note, the numbers above were produced on a Dynapack dynamometer; therefore it is not wheel horsepower, but hub horsepower.

Making more power

Most engine management systems are designed to run correctly without water injection. If the engine is operating well, water injection may help to maintain power by cooling the intake air, but with no other changes it is unlikely to increase the power produced.

In order to realize performance gains from water injection, it is necessary to have the engine management software in the car tuned to take advantage of the water injection. For this I turned to Audi Performance and Racing, the Alabama engineering firm whose business is improving the performance of Audi automobiles. With a history of tuning the Audi engine, APR had the experience and expertise to tune my car correctly.

Even with APR on my side for the tuning, I had work to do to set my car up properly. There are many things to consider when implementing a water injection system. Each system has to be tuned specifically to the car and hardware in use. It is for this reason that a generic tune, one that is applicable to multiple cars, doesn’t exist for water injection.

The first consideration was how much water to use. A common starting point is 10 to 15 percent of the maximum fuel flow capacity of the car’s fuel injectors. My APR Stage III kit utilizes the RS4’s fuel injectors, so I chose water injection nozzles that were capable of flowing approximately fifteen percent of the maximum capacity of all six RS4 injectors combined.

The other decision concerned the mixture being injected. Water injection does not require that just water be injected; a percentage of the total mixture consisting of some methyl alcohol (methanol) can also be used. 100% water has the greatest cooling capacity, but because I was planning to use the windshield washer reservoir to supply the system I still wanted to be able to clean off my windshield and keep the fluid from freezing in cold weather. Methyl alcohol is a common additive to windshield washer fluid, and happens to increase the effective octane level of the mixture very slightly as well as evaporating easily. It cannot absorb as much heat energy as water, or release as much energy as gasoline when it burns, but the right mixture would be a good compromise between functionality and performance for my situation. I decided on a mixture of approximately 80% water to 20% methanol.

The Tune

With all the pieces in place it was time for APR to do their stuff. First they evaluated the condition of my car to be sure that there weren’t any problems with its operation. Next they performed some baseline dynamometer runs to determine the power levels the car was making as well as giving them the chance to collect engine operating data. Once they had a clear picture of what they were working with they began to tweak their 93-octane engine code, the gasoline I use most often, to operate in conjunction with the water injection system.

The first step was to adjust the trigger point at which the system would be activated. With knock most likely to occur around the peak engine load, this led to setting the threshold at 5 psi of boost pressure.

The first change to the software involved leaning the fuel mixture. The mixture was leaned out to the point that the exhaust gas temperatures began to rise, and then backed off slightly to a richer mixture. Boost was next increased slightly in the upper rpm range. Finally, the timing advance was looked at, but the decision was made to leave it alone because the ignition retard values had stayed consistent throughout the previous steps.

The Results

When all was said and done, and APR was finished with their work, the car went back to the dynamometer. The chart below shows back-to-back tests comparing the standard 93-octane software to the water injection software. The base case is the standard 93-octane program and the test case is the 93-octane water injection program. An impressive 22-wheel horsepower, or roughly 30 more crank horsepower, were produced with water injection activated.

So how would this compare to using race gas? Well besides the fact that water is much cheaper than 100-octane fuel, water injection cuts in half the performance gap between 93-octane and 100-octane in my car. My S4 produces about 47 wheel horsepower more on 100-octane fuel than it does on 93-octane. Water injection tuning provides a 22 wheel horsepower improvement over regular 93-octane tuning.

Water Injection Drawbacks

Despite the benefits that water injection offers, there are some valid reasons why water injection isn’t being used in every car on the road. It requires the installation of extra equipment that complicates the vehicle and requires some upkeep, like occasionally cleaning the nozzles and inline filter. Also, safeguards need to be installed to ensure that water is always flowing when needed, that the operator is alerted to a low water condition and to remove heavy loads from the engine if the water flow stops. For if the water flow were to stop while the car was under a heavy load, the engine could be damaged.

Conclusions

When it comes to water injection, the increase in performance realized from the tune depends upon what you and your tuner are comfortable with. APR and I both wanted to finesse some more performance from my S4, but we were in agreement that it needed to be done safely. Water injection, when taken to the extreme, allows you to run high boost, advanced ignition timing, and lean air-to-fuel ratios, all conditions which can quickly lead to a ruined engine if not managed carefully. On the other hand, water injection could act more as a safety buffer to a car that is otherwise well tuned.

APR and I did what should be considered a first round of development. We uncovered some significant gains, but we were time limited to testing with a single nozzle size and only the 80/20 water-methanol mixture. In the next stage of development we hope to find out what affect other mixtures have, looking at 100% water all the way to a 50/50 water-methanol mixture. We would also like to test and observe what affect other size nozzles have on this engines performance potential.

In the end, when it comes to water injection tuning, where you choose to stop depends upon your comfort level. As APR has demonstrated, even with a conservative tune, when the tuning is done properly, water injection has great potential for upping the already impressive performance figures of the Audi S4 Stage III

Rothrock, A.M., Krsek, A. Jr., Jones, A.W., “The induction of water to the inlet air as a means of internal cooling aircraft-engine cylinders,” NACA Report 756 (1943): 1.