The Science Behind Why Water-Methanol Injection Works So Well
“Water injection doesn’t work. That’s like pouring water on a fire. Why would I want to do that?”
That’s a typical reaction to the concept of water or water/methanol injection. While that quote might seem like a logical statement, there are several underlying concepts that if traced through their proven physical properties will help to explain why water/methanol injection is actually a very good idea for supercharged or turbocharged engines. It can even be employed for aggressive normally aspirated engines with high static compression ratios.
This story will take an in-depth look into how water/methanol injection works and what happens when water and methanol is sprayed into the discharge change from a liquid to a gaseous state and how this transformation is genuinely worthwhile.
Instead of thinking about water/methanol injection as dousing flames with water, think of this instead as chemical intercooling. Water/methanol injection is really just an easy way to significantly reduce the inlet air temperature for a boosted or a naturally aspirated engine. Reducing inlet air temperature adds a small amount of power by increasing air density but more importantly reduces an engine’s sensitivity to detonation.
Is all this worth the effort? Our friends at Westech Performance Group dyno tested a twin-turbo LS engine without intercooling or water injection to generate baseline of 830 horsepower. Adding a 50/50 mix and water and methanol also allowed them the luxury of adding a couple more degrees of timing. That was worth an additional 30 horsepower. Then with a mix of 75 percent methanol and 25 percent water the power jumped to 890 for a gain of roughly 60 horsepower. So a 60-hp increase is nothing to scoff at. Now that we have an idea of what water/methanol injection is worth, let’s get into why all this happens. There’s more to this gig than you might think.
Let’s start with the basics of what happens when air is compressed. We could dive into a scholastic adiabatic process explanation but this isn’t a physics class. Instead, we all know that if you put your hand on the tank of an air compressor while it’s busy squeezing air that the tank will be warm to the touch. That’s because as air is compressed, the molecules are pushed closer together which creates heat. Plus, there is also a natural transfer of heat into the air from the mechanical device whether it’s an air compressor or a supercharger. You can think of this as energy being put into the air.
These factors combine to create a significant increase in air temperature when creating boost. Typically, with a decent efficiency rating on a supercharger you will still see high discharge temperatures even at conservative boost levels. To put this in an easy-to-remember perspective Kenne-Bell Superchargers contends that a typical supercharger will increase the air temperature roughly 9 degrees for every 1 psi of boost. This means that if we start with an under-hood air temperature of 90 °F and add 10 psi of boost, the temperature entering the engine after exiting the supercharger will be 180 °F (90 + (9x10) = 180 degrees).
Heated air is not conducive to making horsepower because hot air is less dense with fewer molecules of air per cubic foot than air at the same pressure but at a lower temperature. Common sense dictates that 130-degree air at 10 psi offers greater density (more oxygen) than 180 °F air at 10 psi. So it is beneficial to making horsepower to reduce the inlet air temperature. This is certainly not a new revelation as racers, hot-rodders, and production engine designers have employed intercoolers since the early 1900’s. But intercoolers are expensive to build and install and are also a restriction to flow. A far simpler approach would be to use chemical intercooling.
When introduced into a hot inlet air stream, water and methanol have the ability to reduce inlet air temperature as these liquids transform into a gas. This falls under the category of something called latent heat of vaporization. In this situation, if the inlet air temperature is higher than the temperature of the liquids, then the conversion from liquid to a gas will absorb the heat from its surroundings. This process will lower the inlet air temperature as the liquid (water or methanol) changes state. The energy absorbed can be expressed in British thermal units (Btu) per pound. As our chart shows, it requires 970 BTU’s of energy to convert one pound of water to a gaseous state.
By injecting water using a high-pressure pump, we can create thousands of small droplets that will be much easier to transform from a liquid into a gas. By doing so, this will reduce the inlet air temperature. By studying the Latent Heat of Vaporization chart, it would appear that water does twice the job of methanol. If that’s the case, then why add methanol?
To answer that question, we have to look at the boiling points of water and methanol. Everyone knows that water boils at sea level at 212 °F. However, methanol boils at a significantly lower temperature of 148.5 °F which is more than 60 degrees lower. This means that a majority of the methanol introduced into a 180 °F airstream will boil as opposed to only a portion of the water. The combination of the two liquids injected at high pressure will, however, reduce the inlet air temperature typically by anywhere from 25 to 40 °F.
This may not seem like a big change but there are multiple reasons why water/methanol injection is definitely worth the effort. Many years ago, the OE’s did a study that determined that for every reduction of 25 °F of inlet air temperature reduced the octane requirement of a normally aspirated engine by one full octane number. So if an engine is on the edge of detonation with 91 octane fuel at 90 °F inlet air temperature, a decrease in inlet air temperature of 25 °F would lower the minimum octane to 90. This reveals inlet air temperature to be a critical factor in octane requirements. If you’ve ever driven an over-heated engine then you have probably experienced how the engine detonates because the higher operating temperature superheats the incoming air temperature.
Of course, boost pressure will also demand an increase in octane. This becomes an additive effect where the engine may require perhaps 96 octane fuel for a 10 psi, supercharged, non-intercooled application. If adding a combination of water and methanol reduces the inlet air temperature by 25 to 40 °F this simultaneously lowers the octane requirement by at least one full octane number. In essence it becomes a safety factor to minimize the chance of detonation.
Plus, methanol offers an anti-knock index (AKI) rating of around 110. While the volume added is a small percentage of the total fuel required, it nevertheless contributes to reducing the engine’s octane requirements while also improving the overall octane number for the main fuel component. All these factors could then combine to pull the anti-knock index (R+M / 2) octane requirement for a 10 psi supercharged engine down to perhaps the mid 90’s depending upon several other factors including ambient air conditions.
Based on the above discussion, it might appear that running straight methanol in a water injection system would be even a better idea. Westech also performed that test as well and found an additional 10 horsepower or so by running straight methanol over a 50/50 mix. The problem is that this involves placing a separate methanol fuel tank under the hood where it is exposed to heat, ignition, and other situations that could easily turn hazardous. The smart move is to run a maximum concentration of 50/50 mix of methanol and de-ionized or filtered water. Plus, the Holley pump is designed for a maximum of a 50/50 mix and could be damaged by continued use at higher concentrations. This minimizes the risk of an under-hood fire compared to straight methanol.
Another consideration in favor of a water/methanol mix is that straight methanol burns with a very faint blue flame that in daylight is nearly invisible. This only adds to the danger of using this fuel in full concentrations. As a point of reference, auto parts windshield washer fluids rated to -20 degrees F generally have concentrations of 30-33 percent methanol. This makes this washer fluid easy to obtain and it does perform well in a performance application. If the higher 50/50 concentration is desired, washer fluid can be treated with pure methanol purchased separately and mixed in the appropriate percentage.
Holley’s water/methanol injection system is available in the 800 horsepower package at 1,000 cc/min. This may appear as a large volume, but with the pulse width modulation control, this allows the tuner to custom set the volume throughout the engine’s entire operating range. The Holley system is not a stand-alone unit but instead must be controlled by a Holley Dominator or HP ECU. With an HP computer, this will also require a separate injector driver module (PN 554-115) that will drive the solenoid with pulse width modulation control.
The Holley water/methanol injection system is sold as a complete unit under the PN VK080030 and includes the pump, reservoir, 1,000 cc/min nozzle, inline filter, and the wiring harness. As mentioned, it is intended to be used in conjunction with the software run by either a Holley Dominator or HP ECU.
The software offers a large graph where the duty cycle can be dictated under both load and rpm. The tuner can also manage output volume with three different nozzle sizes based on horsepower output. Once experience is gained with the system, a tuner can then begin the process of ignition timing experiments can take full advantage of the reduction in discharge temperature.
One additional item to keep in mind is that as the discharge temperature drops, the inlet air will become denser which may affect the air-fuel ratio as will adding this small amount of methanol. It’s a small point, but worth mentioning. Methanol contributes only half the Btu output of gasoline and it can be run very rich without hurting power, so likely the small amount of methanol added to the induction system will not drastically affect the air-fuel ratio, but it is always best to monitor wide-open-throttle air-fuel just to be safe.
Hopefully this short treatise on water/methanol injection will offer some ideas on how this system works and dismiss many of those claims that all this does is “pour water on the flame.” It’s much more sophisticated than that!
Latent Heat of Vaporization
Water Heating Curve Graph
This graph illustrates that as water changes from a solid (ice) to a liquid and then to a gas, it absorbs heat throughout the process. The line between a liquid and a gas reveals that heat is absorbed throughout this process until it reaches its boiling point 100 degrees C (212 degrees F). Heat is absorbed from the high temperature inlet air as the line moves from left to right. The advantage to mixing methanol with water is that methanol offers a much lower boiling point (148.5 degrees F), which places it much closer to a typical supercharger or turbocharger discharge air temperature. If we assume a 160 degree F discharge temperature form a centrifugal supercharger, this means that virtually all of the methanol will be vaporized when injected into the air upstream of the throttle. When mixed with water, all of the methanol will vaporize but only a portion of the water will transform. This explains why a combination of water and methanol works better as a deterrent to detonation compared to straight water.
The boiling point of alcohol increases with its carbon count. Ethanol has a higher carbon count than methanol. This is why ethanol has a higher boiling point than methanol and also explains its higher energy density compared to methanol.
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