water injection = Higher Efficiency?

[IamIan]

Originally Posted by Old Mechanic

I just wish they would spend as much time and effort at eliminating reciprocation losses.

Have you seen the work being done at LiquidPiston?

[mort]

Originally Posted by drmiller100

I'm seeing 1.2 grams per liter for air, and you look like you are calculating 1.2 liters per gram? Which obviously makes the numbers even worse.

An interesting thing to consider is we are not adding 14 times as much WATER - we are adding 14 times as much STEAM.

Steam is an inert gas, already vaporized, just like exhaust gas in EGR systems is an inert gas.

I don't understand why the extra water/vapor would hurt anything?

Also, FWIW, no one said this has to be a total loss system, and as you pointed out earlier, burning gasoline produces water.

The water tank might not be nearly as big as you are thinking!

Hello drmiller100,
Good thinking, I was looking at air at 100 C not 25 C. 51 g/m of fuel, 14 times more steam.
I wouldn't call steam inert, it is just about the best cleaning solvent. But I know what you mean, it should have little or no effect on ignition. It might increase the breakdown voltage a little. It shouldn't participate in the chemistry at all. It does absorb a lot of heat. I don't know why it seems to lead to misfire at only 20% mixtures. Just solve that and get your boiler ready.

I do most of my driving at cruise. But probably other drivers don't. Cruise conditions would require the most steam. At full power no steam and at idle the rpm should be much lower. So maybe a smaller tank of water for you. After adding a boiler maybe it wouldn't be much more to add a condenser to the exhaust pipe too. You would need to heat the boiler before starting the engine or else use a throttle during warm-up.

Getting back to your earlier post, I don't think there is any benefit in adding steam to the intake. Of course I make lots of mistakes, my math might be off. Thermodynamic efficiency is truly the result of operating at elevated temperatures, the higher the better. If you consider the same model here, an ideal Otto cycle, cruising power at 1500 rpm with about 1/3 of the charge air+fuel and 2/3 steam. Compressing the whole volume by 10:1 compresses the air by 3.3 times adiabatically, and allowing for the addition of compression heat, the pressure is about 0.55 MPa at 330 C. The steam portion compresses by 6.7 times, using a graph of T-S for saturated steam, adiabatic compression from 0.1 MPa at 100 C to 0.67 MPa goes to 330 C, lucky! But also funny, although the cylinder is full the temperature is the same as it would be with no steam added. The new pressure with steam is much higher, 1.2 MPa instead of 0.55 MPa, which is a harder push for the crankshaft.

Heat is added by burning fuel; 51 g = 2244 kJ. The heat added will be divided between the steam and the exhaust (mostly N2, CO2 and H2O) I'm going to add the exhaust water to the steam mass for the calculation, because the specific heat at constant volume of the N2 and CO2 are relatively close, the average is about 0.73. Exhaust mass is about 746 g. But the cV of steam is 1.76 at these conditions. The equilibrium temperature added is 1158. The combustion temperature is 1488 C. If there had been no steam added it would be much higher (but less mass).

Ugh, now shift gears. I'm doing the adiabatic work per mole, sorry. The exhaust contains 585 grams of N2, about 41 moles, and 161 g of CO2, 3.7 moles. There is 801 g of steam, 44 moles. The 89 moles are contained in a volume of 187.5 liter at a temperature of 1488 C (1761 K).
Using 1.33 for the ratio of specific heats (g) on the whole mixture the work of expansion is 2100 kJ. Compressing the mixture took 460 kJ for the air (g=1.4) and about 410 kJ for the steam (g=1.28) (This is a very rough estimate based on expecting the steam fraction to behave as an ideal gas.) The work available is 1230 kJ.

Without steam added the work of compressing the air + fuel (48 moles) is 460 kJ. The combustion temperature is 4400 C, the work of expansion (g=1.38) is about 2700 kJ. The work done to pump the air through a throttle is less than 80 kJ
The work available is 2160 kJ.
The steam added engine would need about 50% more fuel to produce the same work.

Here is a page that calculates some of these things.

-mort

[pete c]

I've spent a few hours today on and off reading this marathon thread. Very interesting, very informative, all though quite a bit of it is beyond my level of understanding.

I do have a few thoughts.

Yes, steam is a hell of a heat sponge as tvago has pointed out pretty well. And if an ICE was anywhere close to being thermally efficient (probably not the right term, so forgive me) then I would buy completely what he says.

But, it ain't.

So, it would seem that if you kept enough heat inside the engine, rather than venting it through the radiator, then you ought to be able to get some sort of benefit from the expansion of the steam.

Tvago, you point out that pressurizing the feed water eats a lot of power. I do recall drmiller saying he'd introduce it as water vapor before everything gets squished.

As for the argument that if it could work, it would have been done by now, i wonder if it's a matter of materials not being up to getting steam cleaned constantly while running and not a simple matter of thermodynamics. If it is a material problem, maybe it still can be conquered.

Of course, I am not an engineer by training, so all this is guessing.

doug, as for your getting all butt hurt by tvago's responses to you, well, you kinda made your own bed there in your build thread where you initiated the smart ass name calling to us "tailgunners". I will admit that as of late, you seem to have gotten past this, but, you know what they say about first impressions.

[chrisoverson]

I'd think on the "it would already have been done by now" point, car manufacturers aren't going to introduce ideas that mean extra work on the part of the driver, i.e. having an extra tank to top up. If they get it wrong, then bad things can happen and they would be held responsible!

Another thing I've found about many many mods here is that they are climate dependant. What works for one person in one part of the world may not work so well in another, you'd have to introduce antifreeze and anticorrosion agents and all sorts into the water to make it work on a production vehicle and the costs around such things, the potential headache in getting it working and the fact that not all buyers would be happy with it anyway means the auto manufacturers are not going to do it if it only gives small gains.

We, however, don't care about the hard work, and we don't care if it will break at -20 if we aren't in a cold climate etc. etc. We design things in specific ways to suit ourselves, and we also don't mind too much if the life of the engine is shortened because it doesnt necessarily have to last for hundreds of thousands of miles like this. We also don't care if it only gives us an extra 2 or 3mpg, because we're going for absolute max whereas the big brands only care about matching eachother and EPA targets which they can do by other means. That is why we can take advantage of so many things that the car makers cannot.

[drmiller100]

Dang it. I typed in a big old long response, and hit the delete key.

Ok. So, again Thank You Mort. I think I follow most of this, but am starting to get lost.

So, I want to make sure I have the basic principals correct.

Lets look at three cases - all without fuel.

Case one is wide open throttle, case two is 5 psi absolute, and case three is the same grams of air as case two, plus enough "steam" to make the pressure atmospheric.

Assume all three are the same "temperature".

On the intake stroke, the cylinder takes a gulp, which takes some work in case two. Then there is the compression stroke, and then the powerstroke. My understanding is the all three cases will have the same net work completed for the compression and power stroke - zero.

Now I realize we will lose some heat to the piston and cylinder, but other than the lost heat, no net work. am I correct?

from there. In case two, assume we add the "right" amount of fuel, which is also the right amount of fuel for case three.

Case two will have much higher temperatures then case three. There is more mass in the steam to heat up.

Now maybe this is what you wrote above, and maybe your conclusion was it is half as efficient with the steam than without.

But what about heat loss to the cylinders and heads? lower absolute temps will have less heat loss to pistons and heads.

Is this significant?

Again, thanks for your patience.

[drmiller100]

Originally Posted by pete c

doug, as for your getting all butt hurt by tvago's responses to you, well, you kinda made your own bed there in your build thread where you initiated the smart ass name calling to us "tailgunners". I will admit that as of late, you seem to have gotten past this, but, you know what they say about first impressions.

there is a whole other thread about us wearing our big girl panties.

and in that thread we all agreed if we make claims, we ought to be able to explain them.

several of us are waiting for vago to explain his claims, as they are not obvious to us.

[drmiller100]

Originally Posted by IamIan

Have you seen the work being done at LiquidPiston?

interesting engine.

with ceramics, it seems like a fellow could make a combustion chamber which was fairly well insulated. then use a poppet valve to push the pressurized gas in.

I'm a little puzzled how they are going to "overexpand" down to atmospheric pressures, unless they leave the exhaust valve open on the compression stroke?

For that matter, they could run this thing as a 2 stroke with a separate combustion chamber.

[IamIan]

Originally Posted by drmiller100

interesting engine.

I agree.

But like any 'prototype' ... it will be years before it is available ... if it even makes it at all.

Right now the prototype has decent performance ... ~20HP from ~20 Lbs ~140cc engine ... I wish they would have published with the other SAE stuff the BSFC chart , not just an average efficiency ... I would have liked to see the efficiency distribution over different load and rpm ranges from a BSFC chart.

Although it is their first prototype ... so maybe more details in a year or two , as they make refinements and such.

Originally Posted by drmiller100

I'm a little puzzled how they are going to "overexpand" down to atmospheric pressures, unless they leave the exhaust valve open on the compression stroke?

near atmospheric is different than at atmospheric ... for example what do they mean by 'near'.

In this case the current prototype undergoing testing has a ~1.5 over expansion ratio ... meaning that the expansion ratio is about ~1.5x more than the compression ratio... this is how they get to those lower pressures ... how close they get to atmospheric will depend.

Remember this is a hybrid cycle engine ... in it's operation it combines elements of the Oto cycle , the Diesel cycle , the Atkinson cycle , and the Rankine cycle ... it tries to combine all 4 into one engine cycle that has the best net results... or at least that's the intent.

Originally Posted by drmiller100

For that matter, they could run this thing as a 2 stroke with a separate combustion chamber.

I'm not clear on what you are fore seeing here? Can you elaborate?

[ecomodded]

A Briggs & Stratton 4-stroke lawn mower engine would make a great test subject for water injection.
A mad scientist could rig up his water injected lawnmower and do a-b-a testing on gas usage for a set track consisting of the front & back yard, undoubtedly.

edited to add-

You could run the motor on the bench, running for a set amount of time for a-b-a-b-a testing. Although your neighbors may prefer if you were to cut the lawn..

[bazman]

Does anyone know of an electronic fuel injector that could handle water/meth? I believe that if I ran one of those attached to any custom and programmable ECU then the water could be be injected with almost infinite variability between minimum flow and maximum. This would enable the injection to do something most kits cannot - inject under both vacuum and boost while varying the flow seamlessly.

Most water injection kits require 2 or more jets to vary flow from high to low OR if they have mappable flow control, they can only cope with either boost or vacuum not both.

An old used Link ECU from a raceparts bin, and a fully electronic injector that can atomise water from low flow to high flow would be perfect I'd think.

Wonder if methanol EFI injectors would work/come in acceptable flow rates?