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Old 05-01-2021, 01:35 AM   #11 (permalink)
It's all about Diesel
 
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Originally Posted by Isaac Zachary View Post
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.
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.

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Old 05-01-2021, 04:39 AM   #12 (permalink)
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Originally Posted by Isaac Zachary View Post
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.
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.
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Old 05-01-2021, 07:44 AM   #13 (permalink)
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Originally Posted by Isaac Zachary View Post
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.
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.
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Old 05-01-2021, 09:38 AM   #14 (permalink)
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Quote:
Originally Posted by cRiPpLe_rOoStEr View Post
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.
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.

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Originally Posted by serialk11r View Post
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.
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.

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

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Originally Posted by Ecky View Post
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.
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.
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Old 05-01-2021, 07:26 PM   #15 (permalink)
It's all about Diesel
 
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Quote:
Originally Posted by Isaac Zachary View Post
if the oxygen has nothing to burn with it becomes inert like recirclated exhaust gases
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).


Quote:
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.
Remember COČ 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 COČ instead of those synthetic gases.


Quote:
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.
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.


Quote:
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.
Water and fuel are injected always separately on a Diesel engine.
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Old 05-01-2021, 07:56 PM   #16 (permalink)
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Water and fuel are injected always separately on a Diesel engine.
Not if it's emulsified fuel.
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Old 05-04-2021, 02:02 AM   #17 (permalink)
It's all about Diesel
 
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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.
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Old 05-04-2021, 10:07 AM   #18 (permalink)
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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.
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Old 05-04-2021, 05:32 PM   #19 (permalink)
It's all about Diesel
 
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That's another good point to consider against injecting water and Diesel fuel through the same lines.
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Old 05-04-2021, 06:07 PM   #20 (permalink)
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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.
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.

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