water injection = Higher Efficiency?

[drmiller100]

Originally Posted by t vago

I


That's because it's undefined. Water has a critical point of 373 C and 22064 kPa.


So how would you go about preventing ignition quenching?

again, I don't care about one gram of exhaust gas.

so I went to a kpa to psi conversion thingy, and it says 22064 kpa is 3200 psi.

that seems like a lot to me.

It seems like if I could put a bit of water in, and limit the temperature to a MAXIMUM of 373, I would not have any Nox, and have some decent pressure to drive the piston down.

It also seems like if the max combustion temp was 373, I might lose less to the piston and cylinder walls (delta T and all that.)

Is it possible combustion temps rise above 373 C?

forgive my ignorance. I'm trying to understand it, and I'm probably a bit dense.

[serialk11r]

Originally Posted by t vago

There are, to my knowledge, only three proven effective methods of reclaiming exhaust heat energy to do something useful.

Er, the examples you gave were of reclaiming blowdown pressure though :O

Blowdown surely represents a loss but with perfect recovery you'd be only getting back maybe like 5-10% more energy. It has its place of course, I'd love a blowdown powered alternator using a free flowing turbine Would reduce noise too.

I was talking more about a secondary heat engine running off the thermal energy itself :O BMW turbosteamer is probably the closest one to production, and I don't see that happening for some time. Thermoelectrics seem to be at the very least several years off from even viable prototypes. There's also SMA engines, whose progress I haven't really bothered to follow, though I understand they have a somewhat low theoretical peak efficiency by design.

[t vago]

Originally Posted by drmiller100

again, I don't care about one gram of exhaust gas.

You should. That exhaust gas provides the necessary medium for carrying the heat energy needed to vaporize water in your spiffy little imaginary engine.

Originally Posted by drmiller100

so I went to a kpa to psi conversion thingy, and it says 22064 kpa is 3200 psi.

that seems like a lot to me.

Well, water is a strange animal. Above 373 C and 22064 kPa, it's not really purely a gas nor purely a liquid.

Originally Posted by drmiller100

It seems like if I could put a bit of water in, and limit the temperature to a MAXIMUM of 373, I would not have any Nox, and have some decent pressure to drive the piston down.

Liquid water does not just magically turn into a gas. If it did, people would not be able to survive near a stovetop that had a boiling pot of water on it. All that boiling water expanding 1700 times in volume or so in an instant, you know.

Originally Posted by drmiller100

It also seems like if the max combustion temp was 373, I might lose less to the piston and cylinder walls (delta T and all that.)

Do you understand that liquid water takes a lot of heat energy out of exhaust gas in order to change from a liquid into a vapor? Do you understand that said process will cool off the gas in question, and will lower its pressure as a result?

Because it sure seems like you keep ignoring these basic facts.

Originally Posted by drmiller100

Is it possible combustion temps rise above 373 C?

Well, seeing as how combustion temperatures can range anywhere between 5500 C and 10000 C, inside a firing cylinder on its combustion stroke, I'd say that was pretty possible.

Here's something else that just came to mind: You do realize that you need a pressure differential to inject water into a vessel that's at 150 psia, right? In other words, how are you going to pressurize water to be above 150 psia, so that it'd squirt into your engine? How much power would you need to do this? How are you going to prevent 1200 to 2500 psia burning charge from leaking into your water injection mechanism?

[t vago]

Originally Posted by serialk11r

Er, the examples you gave were of reclaiming blowdown pressure though :O

Um... What do you think happens to the temperature? Do you have any idea of how a turbine works?

Originally Posted by serialk11r

I was talking more about a secondary heat engine running off the thermal energy itself :O

What do you call a turbine? That's not a good enough heat engine for you? You do realize that a turbine in a properly sized turbocharger will recover at least 20% and as much as 60% of the heat energy in the exhaust, don't you? Seems a bit better than the 15% claimed by the BMW turbosteamie.

Originally Posted by serialk11r

BMW turbosteamer is probably the closest one to production, and I don't see that happening for some time.

That turbosteamie thingy seems like a dead end. For the same gain in fuel economy, one could downsize the engine and attach a turbocharger to it. Oh, wait! They're already doing that extensively in Europe! Amazing what they can do over there... Mid- and full-sized sedans getting 40+ MPG right off the showroom floor.

So... instead of using something that's directly powered off the exhaust gas, you'd rather heat something else up, have that heat be lost through radiation as it travels via heat pipes to its heat engine, and then convert that diminished heat energy into mechanical energy - keeping in mind that there'll still be waste heat to get rid of. And all to get a recovery of 15% of the exhaust heat.

Originally Posted by serialk11r

Thermoelectrics seem to be at the very least several years off from even viable prototypes.

That was true decades ago, and it'll continue to be true for the foreseeable future. Until technology is perfected to the point where thermoelectric junctions can be efficiently manufactured on a microscopic scale, then it'll always be years away from becoming viable.

Originally Posted by serialk11r

There's also SMA engines, whose progress I haven't really bothered to follow, though I understand they have a somewhat low theoretical peak efficiency by design.

Motors made out of metals that are designed to deform and reform countless times... I dunno... sounds like a recipe for metal fatigue to me.

[toc]

Probably should start it's own thread, but since it's under discussion here..

... A turbo charger improves fuel economy? (By recovering energy from heat and feeding it back into the engine)..

Sounds wrong, so where's it wrong?

[drmiller100]

Originally Posted by toc

Probably should start it's own thread, but since it's under discussion here..

... A turbo charger improves fuel economy? (By recovering energy from heat and feeding it back into the engine)..

Sounds wrong, so where's it wrong?

all else being similar, smaller engines are more efficient then big ones.

for a given vehicle, there is a design spec where the end user expects a certain amount of power. If you turbocharge a small engine, you can get the desired peak horsepower, yet still retain the small engine for economy.

all by itself, a turbocharger slighly reduces economy at lower power outputs, but the difference is pretty small.

[drmiller100]

Originally Posted by t vago

Y

Liquid water does not just magically turn into a gas. If it did, people would not be able to survive near a stovetop that had a boiling pot of water on it. All that boiling water expanding 1700 times in volume or so in an instant, you know.



Do you understand that liquid water takes a lot of heat energy out of exhaust gas in order to change from a liquid into a vapor? Do you understand that said process will cool off the gas in question, and will lower its pressure as a result?


Here's something else that just came to mind: You do realize that you need a pressure differential to inject water into a vessel that's at 150 psia, right? In other words, how are you going to pressurize water to be above 150 psia, so that it'd squirt into your engine? How much power would you need to do this? How are you going to prevent 1200 to 2500 psia burning charge from leaking into your water injection mechanism?

it takes a LOT of energy to turn water to steam. It will get that energy from the temperature of the exhaust gases, which will end up with a lower pressure due to pv and nrt.

But when water turns to steam, it expands a LOT. It expands at a ratio of 1700:1.

Discussing this with you has helped me understand the basics of the entire concept.

And, the cool thing is, like you said earlier, the engine is DESIGNED to handle the water and the water is already in the exhaust.

You don't need to direct inject the water. Inject it in the intake as small droplets (just like gasoline is) and it will do its thing.

It is ok to admit you can't figure out how to do the math on the water expansion - I can't figure it out either.

That is why we try experiments!!!!!

[jtbo]

I found it interesting that guys that took 200-300hp from Volvo redblock motor by slapping turbo on them, reported improved fuel economy over original bit over 100hp engine, but it probably is that working area of engine is much better after modification.

I think that with water you could get away with leaner mixture, but can't remember that much anymore from such old stuff. In marine applications they use water injection too, I have been told, no first hand experience though.

[t vago]

Originally Posted by drmiller100

It is ok to admit you can't figure out how to do the math on the water expansion - I can't figure it out either.

Experiment away. I'd love to see you achieve what nobody else in this entire world has yet been able to do. You seem to think that that the holy 1700:1 ratio will provide you with fuel economy salvation, and that water vapor mysteriously has a much higher pressure at a low temperature than any other gas at that temperature. Those are two tenets of a faith which refuses to be swayed by reality. Go ahead and worship at the holy altar of water injection. Maybe the water injection god will smile on you and grant your prayers. Tell me when you've managed to get 15% fuel economy savings merely from injecting water and by only injecting water.

Just remember that when you fail, I'm going to be right here to laugh at you and to tell you "I told you so."

[t vago]

Originally Posted by jtbo

I think that with water you could get away with leaner mixture, but can't remember that much anymore from such old stuff. In marine applications they use water injection too, I have been told, no first hand experience though.

Yes, water injection will allow people to safely run with a leaner-than-normal charge mixture. Ordinarily, leaner mixtures tend to have slower burn rates, which would be handled in olden times by advancing the ignition timing. This would allow all of the usable heat energy to be turned into mechanical work. However, with today's cars, ignition timing cannot be adjusted. As a result, some of that otherwise available heat energy is left in the exhaust gas. This heats up the engine block more than normal, and if left unchecked, will create conditions that will cause engine destroying detonation and pre-ignition.

Water injection will serve to cool off the charge mixture to be burned in the engine. This will lower peak combustion temperature, and will move the engine away from detonation/pre-ignition conditions. For this reason, water injection is sometimes considered to be an anti-knock additive (another term for detonation is spark knock).