water injection = Higher Efficiency?

[drmiller100]

take a closed volume of 40 cc's at 800 degrees F and 150 psi.

add one gram of liquid water.

water has an expansion ratio of about 1700 to 1.

I believe the pressure will rise.

Significantly.

[drmiller100]

Originally Posted by t vago

By the way, what's the pressure of steam at 100 C?


Is this a guess, or is there something you can show? And you do realize that exhaust gas has water vapor in it, right?


Assuming that you mean to flood the intake manifold with steam, then yes, pumping losses do approach zero. However, that's because you're diluting the intake charge with an inert gas (water vapor), thereby forcing the throttle to open up to maintain the same oxygen intake, not because of any magical properties of water becoming steam inside a combustion chamber.

T

In order,

In order, pressure of steam at 100c is 1.0142 bar. which is interesting if the rest of the "atmosphere" is not at one bar.

More interesting is what is the pressure of steam at 400c? I can't find the answer.

it is indeed a guess as a place to start based on an old timer I met who did this in the 70's.

Pumping losses are significant. 10 to 15 percent. So if nothing else, we have magically found 10 to 15 percent possible gains.

[Air-Hybrid]

Originally Posted by serialk11r

At any rate something like 30% of the heat energy is leaving the tailpipe at temperatures around 400-700 (or higher, which is kinda bad but it happens) C. The cooling system runs at 90-100C, and the engine internals are kept well below 200C, and the flame front shouldn't be hitting the piston early when temperatures are highest on a good combustion chamber design. The higher the temperature of the heat source the easier it is to get power out, so you can see why people are turning to the exhaust for waste heat recovery.

Just to clarify, by "engine internals are kept well below 200C", you mean the bottom end (and other moving oil-fed bits)...?
Obviously the inner walls of the cylinder are at considerably higher temps and, going back to the 6-cylinder engines for a moment, the ability to make work (and negating the cooling cct) is dependent on the high temperature (and quantity of heat) of the cylinder walls and piston crown.


This issue about water injection being akin to a steam engine does seem to come up quite often, but as has been said it is completely unfounded. However, the arguments for using W.I. that do make sense are all founded in cooling the charge (whether for power[turbo'd] or efficiency[pumping-loss]), yes?
So why not just use up the water as an external, evaporative cooling agent prior to the charge entering the cylinder? Would the effects be the same or better (providing the charge is not so cool as to deter atomisation)?

[t vago]

Originally Posted by kevman

but then again, your only considering the heat energy available/escaping with the exhaust gasses.

I'm neglecting the available heat energy available due to friction losses, and available due to heat transfer of exhaust heat energy into the engine block. Yes, that's true. Then again, my example has been very much simplified in other respects. You'll find that all of the simplifying assumptions I make are placing the example into the absolute best-case scenario, and that real-world considerations only worsen the case for steam generation.

To a layman's perspective, for instance, nothing is added to the discussion by dragging in steam tables or air tables to more accurately model exhaust gas having water sprayed into it. Treating exhaust gas as an ideal gas is the best-case scenario, and more realistic modelling by using tables would only show worse results.

Nor is the discussion helped by dragging heat transfer equations into the surrounding metal of the combustion chamber. For purposes of the example, no heat transfer is assumed because, again, it's the best-case scenario. That it greatly simplifies the example is a beneficial plus. Besides, most engine heating is due to exhaust heat radiating into the engine block. Think about it - there's an entire stroke devoted just to manipulating exhaust gas. During all the time it takes for that stroke to complete, exhaust gases are touching the sides of the cylinder, the combustion chamber roof, the valves, the piston, and the walls of the exhaust port(s). If we posit that we cool off exhaust gas by spraying water into it, then heat transfer into the engine block will be much less than before, so there is much less heat energy available anyway. This observation is borne out by the fact that the two 6-stroke engines mentioned elsewhere in this thread did not need water cooling at all.

The same goes for the actual spraying of water. In my example, I'm assuming the water is atomized into the exhaust gas such that it's perfectly uniformly mixed with the exhaust gas, so as to uniformly raise its temperature as it sucks heat energy out of the exhaust gas. Again, best-case scenario. In the real world, it's kind of tricky to get truly perfectly uniform atomization of water into a combustion chamber filled with compressed exhaust gas at TDC, and water droplets will fall out out onto the metal surfaces as the exhaust is cooled below its ability to vaporize that water.

Heat energy due to friction can be ignored since there's actually very little of it available to do anything, compared the the heat energy directly found in the exhaust gas itself. Warm-up time, remember?

These are the major assumptions I made with my example. Remember, they push the example into an impossible-to-attain ideal condition, for which worse results would have been shown had real-world considerations been taken into account, and for which the example itself would have been needlessly complicated.

Originally Posted by kevman

let me ask you this, theoretically speaking, an ideal engine will have no/minimum heat transfer to outside the working medium (pison/valves/cylinder ect) correct? that is why ceramic coated pistons/liners/sleeves ect are benificial as i understand it. which means more heat trapped inside to do work.

That is exactly why ceramic coating is popular. However, it's also pricey to have it done correctly to engine internals, because you have to ensure that the coating is correctly bonded to the surfaces where exhaust gases would touch. Otherwise, the coating would eventually flake off and cause no amount of mischief to the piston rings. You also can only coat the surfaces that themselves do not experience frictional rubbing. Therefore, the cylinder wall (which is most of the surface area that exhaust heat leaks into) is not coated.

[t vago]

Originally Posted by serialk11r

t vago, not all theory heavy people are like him :P

I know. Not all engineering types come across as dicks, either. However, as jerky as my posts come across, they're gentle love pats compared to how the real world would act. I'm not interested in gentle persuasion or cajoling, because the real world doesn't gently persuade or cajole people who engage in wishful thinking.

Originally Posted by serialk11r

you can see why people are turning to the exhaust for waste heat recovery.

There are, to my knowledge, only three proven effective methods of reclaiming exhaust heat energy to do something useful. The first method involves shaping exhaust piping in such a way that the exhaust gases themselves generate a partial vacuum to help evacuate the cylinders so as to provide less dilution to charge air inside the engine. Devices that do this are popularly known as headers.

The second method involves passing hot exhaust gas under pressure through a nozzle to convert thermal energy into mechanical energy, and then recover the mechanical energy by use of a turbine wheel. Turbochargers are the best known device that uses this principle.

The third method is really only useful on engines that require a throttle valve to begin with. Exhaust heat energy is used in this case to heat up the intake air such that the throttle plate is forced more open than what would otherwise be needed, in order to lower pumping losses associated with that throttle valve.

Thermal piles are not considered because it's currently not practical to use them in real-world applications, given the state of the art.

[t vago]

Originally Posted by Air-Hybrid

So why not just use up the water as an external, evaporative cooling agent prior to the charge entering the cylinder? Would the effects be the same or better (providing the charge is not so cool as to deter atomisation)?

Several companies make water injection kits that do exactly this. The idea is to push operating conditions away from where engine-destroying pre-ignition and detonation occur.

[t vago]

Originally Posted by drmiller100

take a closed volume of 40 cc's at 800 degrees F and 150 psi.

add one gram of liquid water.

water has an expansion ratio of about 1700 to 1.

I believe the pressure will rise.

Significantly.

I know you believe. You believe that water will just magically flash into steam. Your belief is akin to a religious belief.

Again, you neglect the latent heat of vaporization.

Using your example of 1034 kPa and 700 K (or 427 C, if you will), and assuming that the liquid water was near 100 C, we find that:

The mass of your mystery gas in that closed volume is unknown. However, for a simplified exhaust gas containing 80% diatomic nitrogen, 11 percent carbon dioxide, and 9 percent steam, it's 0.2 grams.

It holds about 150 J of thermal energy. Of that, 91 J is available that can be used to vaporize water.

That gram of water needs 2260 J to completely turn into steam.

Roughly 4% of that gram flashed into steam, bringing the temperature of your closed system to that of the water, i.e. 100 C.

The pressure dropped, too. Ooooh, lookie - 101 kPa.

And you still have 0.96 grams of liquid water to deal with.

So, now you have 39 ccs of mystery gas, 1 cc of water, and it's all at 100 C and 101 kPa. Oh, excuse me - 212 F and 14.7 psia.

Originally Posted by drmiller100

In order,

In order, pressure of steam at 100c is 1.0142 bar. which is interesting if the rest of the "atmosphere" is not at one bar.

Given your example above, it's about 1/10th of the pressure you mentioned.

Originally Posted by drmiller100

More interesting is what is the pressure of steam at 400c? I can't find the answer.

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

Originally Posted by drmiller100

it is indeed a guess as a place to start based on an old timer I met who did this in the 70's.

Why do we not know the name of this old-timer? If he could get this thingy to work, then his name ought to be at least as well known as Otto or Diesel or Brayton or Rankine or Carnot or Atkins or Wankel.

Originally Posted by drmiller100

Pumping losses are significant. 10 to 15 percent. So if nothing else, we have magically found 10 to 15 percent possible gains.

So how would you go about preventing ignition quenching?

[ProDarwin]

Originally Posted by t vago

I know. Not all engineering types come across as dicks, either. However, as jerky as my posts come across, they're gentle love pats compared to how the real world would act. I'm not interested in gentle persuasion or cajoling, because the real world doesn't gently persuade or cajole people who engage in wishful thinking.



There are, to my knowledge, only three proven effective methods of reclaiming exhaust heat energy to do something useful. The first method involves shaping exhaust piping in such a way that the exhaust gases themselves generate a partial vacuum to help evacuate the cylinders so as to provide less dilution to charge air inside the engine. Devices that do this are popularly known as headers.

The second method involves passing hot exhaust gas under pressure through a nozzle to convert thermal energy into mechanical energy, and then recover the mechanical energy by use of a turbine wheel. Turbochargers are the best known device that uses this principle.

The third method is really only useful on engines that require a throttle valve to begin with. Exhaust heat energy is used in this case to heat up the intake air such that the throttle plate is forced more open than what would otherwise be needed, in order to lower pumping losses associated with that throttle valve.

A variation on #2 that is also proven is turbo-compounding.

#3 is home-built used often on this site.


There are many other methods that have also been proven effective, however they have not been proven cost effective:

TEC Modules to recapture heat energy. IIRC one of the big 3 had created a cat covered with them that generated around 1KW average. Of course these things are ridiculously inefficient.

BMW used a steam turbine in the exhaust: BMW Turbosteamer gets hot and goes

<insert heat recovery device here> Sterling engine, etc.

[ConnClark]

I figured I would chime in on this. The boiling point of water is not always 100C. It can be less or more depending on the pressure its under. under ten atmospheres of pressure its boiling point goes up to about 180C. Also at that pressure the steam expansion ratio is less than 175 times its volume as water.

In my research into water injection on a gasoline engine it is most useful as an internal coolant. The next most useful thing it does is act as a knock suppressant allowing more boost or higher compression ratios. Lastly it can be used as a intake charge coolant to cram about 0.5 to 1.5% more air/fuel mixture into the cylinder. None of these things is extremely useful for hypermileing .

On a diesel it is a little different story when used as a charge coolant. Its documented to be useful increasing fuel economy, power output, and reducing NOx emissions. Unfortunately these effects aren't really prevalent unless the diesel engine is operating constantly over 80% of its full output. Even then its only good for an improvement of 0.5 to 2% in fuel consumption and power output.

[t vago]

Originally Posted by ConnClark

I figured I would chime in on this. The boiling point of water is not always 100C. It can be less or more depending on the pressure its under. under ten atmospheres of pressure its boiling point goes up to about 180C. Also at that pressure the steam expansion ratio is less than 175 times its volume as water.

For purposes of illustration, it's enough to show that the water will cool off the exhaust gas due to its sucking out the exhaust gas's heat energy to satisfy its latent heat of vaporization requirement. I'm not about to go into overly complicated details for somebody who does not know what water's critical point is (can't find the pressure of steam at 400 C, indeed).

Originally Posted by ConnClark

In my research into water injection on a gasoline engine it is most useful as an internal coolant. The next most useful thing it does is act as a knock suppressant allowing more boost or higher compression ratios. Lastly it can be used as a intake charge coolant to cram about 0.5 to 1.5% more air/fuel mixture into the cylinder. None of these things is extremely useful for hypermileing .

Agreed. I have done some study into water injection, too. At one point, I was going to do a binary staged water injection system that would deliver approximately the same amount of water per cylinder, regardless of engine speed, for purposes of turbocharging. I even worked out the math using Excel and found out that the water charge would vaporize completely while the cylinder was on its compression stroke, while providing the cooling mentioned above.

BTW, this is why extra gasoline is used at WOT for WOT enrichment, as well as why tuners enrich the charge mixture for forced induction applications. Gasoline also provides cooling by evaporation on the compression stroke, but nowhere near as efficiently as water does. IIRC, gasoline has a latent heat of vaporization of about 2 J/g-K.