water/methanol injection's untapped FE potential

[livewire516]

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

[cRiPpLe_rOoStEr]

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

[Isaac Zachary]

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.

[freebeard]

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.

[serialk11r]

Originally Posted by Isaac Zachary

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.

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.

[Isaac Zachary]

Originally Posted by serialk11r

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.

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.

[oil pan 4]

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.

[serialk11r]

Originally Posted by Isaac Zachary

The heat ratio is important.

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.

[cRiPpLe_rOoStEr]

Originally Posted by oil pan 4

Closest thing to an ideal water methanol engine is a diesel engine.

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.

[Isaac Zachary]

Originally Posted by serialk11r

... you just pair it with a higher compression ratio, and the forces seen by the piston are more or less the same.

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.

Originally Posted by serialk11r

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

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.