Water Injection?

[Snick]

Water injection even helps above and beyond cooled EGR. Countless benefits, really.

 

[Hotscoots]

Typically , a water Injection system (ADI) only benefits a boosted engine . With the introduction of the Turbo Intercooler , ADI has become obsolete .

 

[Snick]

It helps in improving emissions (NOx, soot) which allows for more powerful tuning, in both spark and compression ignited fuels.

It also keeps engine internals clean--helpful for direct fuel injection implementation, and more.

 

[Dooltd]

Thanks Wheaters for the detailed explanation of water injection used in the last of the piston aero engines. I was aware of this system but had no idea how elaborate it was. The complexity of those multi-banked radial engines boggles the mind, turbines came along just in time.

 

[pistonboy]

Water injection is not really needed in EM/IAV engine because it would be redundant as IAV is using a modern EGR system for the same purposes. As pistonboy correctly stated water injection essentially serves the same purpose as an EGR system however pistonboy does not appear to be aware of just how far EGR systems have come as they are no longer the power robbing problems they once were. EM/IAV have already done an excellent job of explaining why they are using a modern EGR system in Tech Talk V39:




The use of this modern ECU controlled cooled EGR system along with taking advantage of modern computer modeling power to precisely model and plan the air/fuel swirling movement inside the combustion chamber all contributes to the combustion mixture remaining mixed and cooled which results in controlled high compression burn without pre-ignition or detonation. This also results in more complete combustion which reduces harmful emissions. This is how IAV can plan on safely achieving compression ratios of 12:1 on regular unleaded gasoline.

I believe we are talking about apples and oranges. I am referring to what would be the same as a physics experiment where one and only one variable is changed and observing the result. I am not talking about the elio engine which is a computer controlled system where many variables are being changed all the time.

 

[pistonboy]

The last generation of high output aero engines, such as those built for larger aircraft after WW2, successfully used water injection to help prevent detonation at high power settings, e.g. on takeoff.

If the charge temperature is lower, a higher compression ratio can be used without detonation taking place. This makes the engine more efficient. A disadvantage is that adding water to the incoming charge mix displaces some air (i.e. oxygen), so less fuel can be burnt, which reduces the power, defeating the object somewhat.

So, as an added refinement, methanol was mixed with the water. The methanol did two things. First, it prevented freezing of the water at the extremely cold outside air temperatures at high altitude. Secondly, methanol needs very little oxygen to burn, compared to gasoline (it self generates some oxygen in its chemical burning process) so it replaces that "lost" by adding water. The problem with this system was that operators of these aircraft needed to ensure that water/methanol mix was available as well as AVGAS, or the aircraft had to operate at reduced power output, or not at all.

The technology became mainly redundant with the change to gas turbine engines. One exception was the Harrier "jump jet" which used water injection for very similar reasons (to reduce turbine outlet temperature, so the engine could be safely spun to higher rpm without melting the turbine blades).

I have often thought that water/meth injection might be a better way of using sustainable alcohol fuels without the problems caused by adding it directly to the main fuel system.

Thanks for the info Wheaters. Do you know anything about the type of water used. Do if you know if they used distilled water or, did the water injection systems had any time of demineralization system on them?

I love the idea of water injection but am wondering about mineral content doing damage to the engine over time.

 

[outsydthebox]

I don't understand why cooling the air in the cylinder helps. Sure, I know colder air has more oxygen BUT, the air is already in the cylinder. Whatever temperature it was coming in determines the oxygen concentration. Cooling it once in there doesn't add oxygen. I must just be missing something.

Cooler air is "denser". IIRC (from a few years ago) The average (normally aspirated) engine has an "intake charge" efficiency around 70%. This is measured at WOT so the throttle blades aren't a restriction. Since an engine is really a glorified "air pump", It's efficiency is reliant on how well it can "suck" (vacuum) air into the cylinders. The physical act of piston "sucking" the air "stretches it" making it less dense. So, Cooling the incoming air charge ( "before" it enters the cylinder) will make it denser. It would be necessary to introduce the H2o before the key sensors (map/maf/iat) so the computer could take advantage of the denser & cooler charge.
On the other hand...it might end up being more hassle that it's worth. So, let someone else try it first.

 

[Ty]

Cooler air is "denser". IIRC (from a few years ago) The average (normally aspirated) engine has an "intake charge" efficiency around 70%. This is measured at WOT so the throttle blades aren't a restriction. Since an engine is really a glorified "air pump", It's efficiency is reliant on how well it can "suck" (vacuum) air into the cylinders. The physical act of piston "sucking" the air "stretches it" making it less dense. So, Cooling the incoming air charge ( "before" it enters the cylinder) will make it denser. It would be necessary to introduce the H2o before the key sensors (map/maf/iat) so the computer could take advantage of the denser & cooler charge.
On the other hand...it might end up being more hassle that it's worth. So, let someone else try it first.

I know cooler air is denser (has more oxygen to burn) but, when it enters the cylinder, it's temperature was already set... in the intake. The air gets pulled into the cylinder where it immediately begins to heat up and expand. The valves are closed at this point so no more air can get in or out (the air maintains it's amount of density at this point though as it heats up, it creates pressure). Now that I've said it like that, the pressure that is created by the air heating up slows the piston down more than if the air didn't heat up as much. If you kept the cylinder cooler, the air would slow the piston less thus increasing efficiency. I think it has more to do with the pressure created by heating up the air quickly than with the oxygen content at that point. cold air intakes are there to cool the air BEFORE it gets into the combustion chamber thus increasing the amount of oxygen available.

It makes sense that the cooler the air before entering the chamber, the more oxygen it'll have and therefor the more thorough combustion you'll get. After it gets into the chamber, the cooler the chamber, the less the air will heat up and thus the less pressure will build up (pressure and heat go together) thus the less it'll try to slow the piston down.

Now, take a separate power stroke... This one gets water instead of gas. The hot chamber flashes the water to steam, driving the piston back down (1,600:1 is the expansion when water flashes to steam and that's more than you get by burning gas). Flashing to steam, the water pulls heat from the surrounding metal, cooling the cylinder for it's next power stroke which will be a normal gas one.

Using this system, the engine would be self cooling and according to the inventor, you could hold your hand on the cylinder wall. He even removed the cooling fins to see the effect. The engine no longer needed the cooling. Imagine the savings from getting rid of the water pump and radiator. Combine that with the benefit of getting 2 power strokes for the same amount of fuel... Man, it'd be neat.

 

[CrimsonEclipse]

De-mineralised water is definitely required, as you suspect. otherwise the inside of the engine might eventually start to look like a furred up kettle! I'm certain that aircraft just had a simple tank for the water/meth mix, as supplied at the point of sale. Any extra on-board equipment adds unwanted weight; very bad idea on any aircraft.

I've flown turbine aircraft that have had water injection.
As previously mentioned, it absorbs heat, but it also adds mass to the air. So more mass is flowing over the turbine.
It allows more fuel to be added without over temping (and melting the turbine blades) and additional mass flow to the turbine to give even more shaft horse power.
The alcohol really didn't add much more than an anti-freeze property and a neat "After burner" effect. It added about 10% horsepower under certain circumstances.

For pistons, it might make certain compounds that would make for a corrosive environment in the pistons and exhaust.
It would, however, have more mass flow over the turbo charger making it more effective.

Frankly, it was more trouble than it was worth.

 

[Lil4X]

A number of old warbirds used water (or water and alcohol) injection for "War Emergency Power" - hopefully to gain a brief power advantage over someone with bad intentions on your six. The Rolls Merlin engine was particularly suited to these applications, but even then for only brief bursts of power - otherwise the engine would turn itself into a big doorstop with amazing alacrity.

The '65-'66 Corvair's flat-six and Oldsmobile's Jetstar V8 were offered in turbocharged versions that used water and alcohol injection to cool the inlet charge to prevent detonation. It worked, but reliability was questionable thanks to either the design or the owner's penchant for full-throttle operation. Properly cared for and operated responsibly, they seemed to hold up very well.