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Old 05-01-2021, 09:38 AM   #14 (permalink)
Isaac Zachary
High Altitude Hybrid
Join Date: Dec 2020
Location: Gunnison, CO
Posts: 443

Avalon - '13 Toyota Avalon HV
90 day: 39.66 mpg (US)

Prius - '06 Toyota Prius
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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.

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.

Originally Posted by Ecky View Post
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.

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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