water/methanol injection's untapped FE potential
 

TLDR; this might be obvious to folk. It just seems like a well-tuned water/methanol injection system not only allows you to run leaner, advance timing, but also allow hotter air-intake temps, and tolerate a higher coolant temp without damage to the valve-train, before causing knock. Cooler exhaust temps means less waste heat. Higher tolerable operating temperatures means less need to waste heat in the form of radiator cooling. Convention says w/m inj., isn't worthwhile until you're playing with really high compression or power-adders like forced-induction or nitrous - but it seems like a lot of things ecomodders want to do are ultimately limited by engine temp/knock/fouling emissions.

I recently read some journal articles about barriers to improved efficiency in ICE engines. A good case was made that the main reason thermal efficiency is so low, is because the valve-train materials are the weakest link. Otherwise, most other components could be made to operate at higher temperatures.

Another weak link is the risk of pre-ignition/knock due to 'hot spots' somewhere in the combustion chamber. I won't get into the supposed 'it cleans your engine' properties of water/methanol injection, but deposits within the combustion chamber is one cause of such hot spots.

It seems as though water/methanol injection has a lot more potential to improve efficiency that it seems the community has exploited. It's cylinder head temperature that contributes most to triggering the thermostat, causing the cooling system to bleed off thermal energy. Exhaust temp is another source of heat loss. Basically, water/methanol injection helps to limit the temperature of the combustion chamber...where we need cooling most to prevent detonation and valve damage.

With a well-tuned water/methanol injection system, we should be able to tolerate higher operating temperatures. If I recall correctly, GM tried cooling the cylinder heads first in their Gen II small block V8s, but reverted to block-first cooling in the Gen III, because the temperature gradient/rate of change of cooling the heads first was too great. My understanding is that coolant channels in the block have gotten smaller over the decades. And in vehicles where the weight penalty is tolerable, the industry seems to be in no rush to go from cast iron blocks to aluminum. Cost aside, I can't help but think this is at least in part of it's mass and thermal conductivity of cast iron blocks don't present much of limit to cooling. If anything, it's properties of a heat sink help to 'pre-heat' coolant to keep temp fluctuation from stressing the cylinder heads.

I'd love to hear people's thoughts - I'm not planning on water/methanol injection for any of my vehicles. At least not anytime soon (if I did, it would be tinkering for its own sake). I hope any discussion could be more theoretical than practical

Have you ever noticed an increase to fuel efficiency when there was a greater amount of moisture suspended in the air?

Lower exhaust temps doesn't necessarily equate to better efficiency. One reason is you're leaning out the fuel ratio. If you start out with 1 part fuel and 14.5 parts air you'll have hot exhaust because you're burning a lot of fuel in a volume that's only 14.5 times bigger than it. But then if you make it 14.5 parts air, 7.25 parts EGR and 7.25 para water, now it's 1 part fuel to 29 parts everything else. You get cooler exhaust because you're heating up a greater mass with the same amount of fuel. Kind of like turning on two range burners to the same setting and putting a bigger pot of water on one and a smaller pot of water on the other. After a minute has gone by the bigger pot will be cooler than the smaller pot because you were heating a bigger mass.

One problem with water is it's specific heat to expansion ratio is lower, about 1.3 instead of about 1.4 like air. So unless you change things like increase the compression ratio or advance the ignition timing you'll actually get worse fuel mileage.

Also there are efficiency losses the cooler you burn. Theoretically you want the flame as hot as possible for best efficiency. For an example, pure oxygen and pure fuel. But we don't have an affordable way of making an engine like that that's anything near practical. Too much cooling from water injection could hurt efficiency.

Still there are efficiency gains that can be made with water injection. One would be to replace high load enrichment with water injection. That way you could keep running a stoichiometric A/F ratio at wide open throttle. The increased CR and advanced timing possibilities are also important.

The water produced when gasoline is burnt is corrosive enough on it's own. Metal likes oil better.

Don't get me started on rubber hoses.

So I used to think the heat capacity ratio was important, but air's heat capacity ratio falls with temperature, and EGR has a low heat capacity ratio too, so water isn't really different. You can think of water like ultra-cold EGR.

You're right though, if you lower the combustion temperature and slow down combustion, you could easily lose efficiency instead of gaining efficiency from cooler temps. Running water injection with an ultra high compression ratio is certainly a valid way to get more efficiency (though I don't think most people want to bother with filling a water tank). For people with easy access to E85, running more ethanol in the tank an easier way to accomplish the same thing.

The heat ratio is important. Like with anything, there's pros and cons. With EGR you can open up the throttle more, advance timing, use higher compression ratios, etc., etc., etc. So, just like with water injection, there are efficiency advantages at the same time. But do too much and you hurt efficiency. Or just add something with a worse heat ratio and not adjust things elsewhere and you hurt efficiency.

The heat capacity ratio is an important part of the thermodynamic equation. You can't get better efficiency than the the thermodynamic equation. And if you lower the heat ratio number, guess what, your thermodynamic efficiency also lowers unless you increase the compression ratio at the same time.

Efficiency = 1 - 1/CR^y-1

For an example, a 1.4 heat ratio gives a 10:1 CR a maximum thermodynamic efficiency of about 60%. But 1.3 gives the same 10:1 CR a maximum thermodynamic efficiency of about 50%. And the problem is that the efficiency gains are even less at even higher compression ratios. In reality, with a 1.3 heat ratio you'd need a 21:1CR to match the same efficiency of a 10:1CR with a 1.4 heat ratio working fluid of around 60%.

This is why it makes sense to use air made of 70% nitrogen. Nitrogen has a higher specific heat ratio than CO2 or water.

This is why non-internal combustion engines (external combustion engines) tend to get better efficiencies with fluids with higher heat ratios like hydrogen or helium.

Forget about the methanol systems, the engine has to be built around water methanol to take advantage of it.
Closest thing to an ideal water methanol engine is a diesel engine.

So the heat capacity ratio determines pressure and temperature change under adiabatic compression and expansion. Gasoline engines are knock limited (by charge temperature) and don't have any heat exchange steps, so using a lower heat capacity ratio working gas is okay, you just pair it with a higher compression ratio, and the forces seen by the piston are more or less the same.

Air is a desirable dilutant since it has oxygen to promote more complete combustion, but water can be useful too, e.g. at high load where you need to remove excess heat. Absolute heat capacity rather than heat capacity ratio can make a gas more useful for reducing temperatures and heat conduction losses, that's why cooled EGR is combined with lean burn in experimental high efficiency engines rather than just running even more lean.

Or those newer direct-injection gassers too. As they're often pointed out to get more carbon buildup at the intake tract than their port-injection counterparts, and are plagued with an increase to the NOx emissions, water injection is likely to decrease those side-effects.

That's kind of what I'm saying. IF you don't increase the compression ratio, then the pressure will be lower, because you're taking a fluid that expands less and exerts less force when you inject the same amount of heat energy into it. The way to extract that lost energy is to increase the compression ratio.

This is definitely doable, especially with limited amounts of water injection. Water injection reduces the chances of knock, so therefore you can increase the compression ratio. But if you inject a whole lot of it you would need a very high compression ratio to get the same efficiency. So there's obviously going to be a point of diminishing returns is what I'm saying.

I don't understand how cooled EGR works better than air, but water I can understand since it changes phases and reduces temperatures that way. But that is interesting.

As the recirculated exhaust gases are inert (or at least supposed to be), they're meant to keep the AFR richer without an actual increase to fuel volume at the intake. Eventually a hot EGR flow could also lead to a more complete vaporizing of the fuel, which may become desirable while driving on cold weather. Water injection is more useful on hot weather, or with a somewhat extreme load.

I think it's because EGR has a substantial amount of water vapor which will absorb a lot of combustion heat energy and lower temperatures. Water vapor has a very high thermal capacity, though its heat capacity ratio is low.

Liquid water offers extra cooling that EGR doesn't since EGR is gaseous, hence its popularity in turbocharged applications.

Does this take into account the phase change, or is it just for water itself?

Generally speaking, anything that slows combustion is bad for efficiency. Fast combustion is the holy grail in highly efficient combustion. And, high temperature is only useful so far as it increases expansion or pressure. If you can get the same expansion with lower temperature, you have less energy loss.

In the case of my particular engine, which was fairly high compression (10.5:1) for an engine of its type in its era, knock is only really an issue above 80% load and below 2500rpm. If I run 93 octane fuel, I can advance timing to MBT and there would be no advantage to cooling combustion, but for any less than 93 I need to pull timing. So, water/meth could allow me to run cheaper fuels.

As far as practical limitations, I live in an area where temperatures can be below freezing as much as half of the year. Water just didn't inject well in those conditions. ;)

Ya, that's how I always understood it. Less oxygen, less NOx for emissions purposes.

But if you were to just build an ultra lean engine compared to mixing a lot of cooled exhaust gases into the intake of the engine the effect would be about the same at bringing combustion temps down because if the oxygen has nothing to burn with it becomes inert like recirclated exhaust gases. The difference is that the ultra lean engine would be better able to burn up more of the fuel and would have better thermodynamics as the CO2 and H2O would expand less than nitrogen and oxygen do.

Ok, that kind of makes sense. I wonder if CO2 also has a higher thermal capacity since it has about the same specific heat ratio as water vapor of around 1.3.

I haven't had much luck in seeing how the phase change effect efficiency. All I know is that both water and water vapor have a lower specific heat ratio. I'm guessing that means even with the phase change there's a lower specific heat ratio. In other words, unless you increase the compression ratio the energy extracted from the resulting steam will be lower than the energy that can be extracted from heated nitrogen.

In other words you add a specific amount of fuel and oxygen, you burn it, that creates a specific amount of heat energy. If that heat energy heats a working fluid in an enclosed cylinder the working fluid expands causing pressure. But if the working fluid is water the resulting pressure will be lower than with nitrogen resulting in less energy that can be extracted. In theory, the way you'd extract more energy from the water would be to have a higher compression ratio.

That's the thing with water injection. It's pros vs cons. It can help in some situations, but isn't the be-all end-all path to efficiency.

Cooler combustion with H2O =

• slower combustion = less efficiency.
• less knock = higher compression ratios/timing advance = more efficiency.
• lower specific heat ratio = less efficiency.
• less heat loss = more efficiency.

And then you have complexity and reliability. Another bad scenario is if you run out of water or if water injection fails and now you have a high compression, lean burning engine ready to destroy itself.

Back on a positive note, in diesel engines water mixed with the fuel and injected can lower CO, PM and NOx emissions. It's one of the fuel ways of lowering both PM and NOx at the same time since most (if not all) other methods lower one but increase the other.

Surplus oxygen ends up reacting with the nitrogen, which by the way is at a higher volume than oxygen on the atmosphere (and at the intake tract of an engine).


Remember CO² is already being used for refrigeration purposes, and I'm not considering only the so-called dry ice. Even some newer HVAC setups resort to CO² instead of those synthetic gases.


The increased moisture content absorbs a higher amount of thermal energy from the intake air stream and the aerodynamic heating inherent to the compression stroke, releasing that heat at a much more convenient and homogeneous way through the entire combustion chamber, leading to a more complete combustion.


Water and fuel are injected always separately on a Diesel engine.

Not if it's emulsified fuel.

Maybe the easiest way to make some emulsified fuel would involve not only water and methanol, but also biodiesel as it would blend more easily with both the Diesel fuel and methanol.

How are you going to keep it emulsified AND run it through all those things that remove the water or not clog up the system? Mold is a diesel fuel tank is not fun.

That's another good point to consider against injecting water and Diesel fuel through the same lines.

Obviously, like with anything, there are pros and cons. And there's a huge difference between what a regular DIY'er can do, what can be done in labs and what might make it as a potentially commercial product.

One thing would be to remove the return line all together and mix water as needed with fresh fuel near the engine. That way you could keep the fuel tank water free as well as have emulsified water in as few lines as possible. You may need some sort of mini fuel cell up near the engine for a return and add fuel from the main tank as needed. That way cleaning out the fuel system from bacterial build up could be made easier. Also, if you can keep the emulsified fuel and water within a pressurized system, like the fuel rail, then the temperature could be increased without causing boiling. The high temperature could help mitigate or stop bacterial and fungal growth.

There are additives that either help absorb water or expel water. If the water is mixed well enough into the fuel then mechanical water separation becomes basically imposible. You can run the emulsified fuel through a water separator all day and not get a drop of water with the right additives, depending on the amount of water of course.

Also, water can be basically homogenized into the fuel by means of ultrasonic mixing without the need of additives. Of course there'd be a greater chance of separation of water and fuel over time without the additives.

Of course, besides bacterial growth, there's the problem of corrosion. Water can cause things like expensive fuel injectors to rust from the inside. So building parts out of materials like stainless steel might be the only way for it to work.

Another problem would be startup would be difficult unless you had a way to switch between plain fuel and emulsified fuel. One way of acheiving this is having a small loop that uses emulsified fuel and switching over to pure fuel for the last part of the trip in order to use up the emulsified fuel in the loop so that during the next startup it can start on pure fuel.

But one theoretical type of emuslified fuel injection system that would solve a lot of the problems would be an injector that basically injects water and fuel at the same time and somehow mixes them together during the injection event.

But in any case, cold weather and freezing would be a problem that would have to be dealt with, as in the case of any water injection system.

The main question is if the benefits outweigh all of these design challenges.

• By injecting the water directly into the cylinder during combustion you eliminate any cylinder wear from water droplets affecting lubrication.
• Also, the emulsified water boils in "microbursts" during combustion which helps with fuel atomization and ultimately burning, which reduces particulate matter.
• And of course it also reduces combustion temperatures which reduce the formation of NOx.
• It also puts the water right where it needs to be in a stratisfied charge, like in a diesel engine, that is to say right where combustion is occurring. This allows for the use of less water which can reduce the unwanted side effects of water injection, like overcooling and lowering the specific heat ratio of the combustion charge, while maintaining or even improving the benefits of water injection.

I think that if it can be pulled off it would be a great alternative to EGR. EGR on a diesel seems to only reduce efficiency. This could not only increase efficiency, but also reduce major diesel pollutants such as NOx and particulate matter at the same time, and do so much more effectively than just spraying water into the intake.

NOx and PM emissions decrease exactly because the water injection is done into the intake. Injecting it along the fuel won't either cool the charge air which leads to lower NOx, or decrease heat irradiation through the engine coolant (thus retaining a higher amount of energy during the compression stroke to be converted into motion at the power stroke). Such heat conservation also increases the speed of the flame spread, thus leading to a more accurate combustion decreasing the PM emissions.

All I know is that according to the papers I've read, emulsified water injection reduces NOx and PM emissions better than intake spraying does.

NOx is reduced because the overall combustion in a diesel is already very cool and only produces NOx at the rich point where the injection is occuring. Lowering the whole cylinder temperature through intake injection won't help lower NOx as much as putting the water where the NOx would normally be reduced.

I don't see how heat conservation increases flame spread if the injection is local and rich. Water droplets there push the flame and fuel out into the "fresh air" of the combustion chamber.

alcohol / water
 

In Vietnam, we mixed methanol-alcohol and demineralized water together, for use in the Douglas A1 Skyraider, to protect the radial engine during high ordnance load takeoff. Once aloft, any remaining solution was jettisoned to conserve weight. I understood that this was it's sole purpose. Protection against destructive pre detonation.

Interesting. I've been aware of the usage of WM-50 to prevent hot starts on gas turbines, while on piston-engined aircraft I've been more aware of its usage at high altitude.


That's the most usual purpose on spark-ignited engines, as the water content allows a leaner AFR.