Properties of water injection in the diesel cycle

Not much to tell, it was just something to behold the scale of the engines, even though I have seen pictures before: bore diameter nearly 1 m (3.3 ft); nearly 3 m stroke (10 ft!); total engine 5-storeys tall... It's rather interesting actually how Wärtsilä came to be as a company (actually Wärtsilä bought out Sulzer, another ship engine manufacturer). The interesting part is to keep in mind that Sulzer is based out of Switzerland, a land-locked country...

A big step-wise drop in soot emissions was achieved by going to common-rail injection. Remember, the residual fuels that ships run on can contain something like 5% sulfur (that's 50,000 ppm!!!)

Because of the amount of fuel that ships go through, for cost reasons shipping companies are very sensitive to fuel price and economy. Even more stringent emissions regulations would likely require going to distillate fuels, and that would be a huge cost- and supply impact.

For the same reason biofuels have been investigated but more interest has not been demonstrated (reliable supply of the needed quantities was also cited as a disadvantage). Every bit in fuel consumption reduction can mean big cost savings, so things like turbocompounding and CHP have also been investigated. But being that initial capital costs would skyrocket and maintenance issues also increased, very few customers have hopped on-board. A turbocompounded ship engine currently already in service was described. Miller cycle is widely used.

nicklockard said:

If you use the NIST applet or any good steam tables, you find that water injection in the intake tract can *at best* hold temperatures below the boiling point of 125 celcius (for 2.25 bar, absolute,) which is really not that great. Water/methanol can depress that to around about 85 C (guess.)

Click to expand...

Actually, that is not true. When considering the evaporation of the water, you have to consider the *partial* pressure of the water, not the full atmospheric pressure. It turns out to be possible to bring the temperature down to approach the wet-bulb temperature. In some cases (of very dry intake air and with good conventional intercooling prior to water injection - which, by the way, is one of the rules listed in that brochure!!), it may be possible for the air / water-mist mixture to be brought *below* the ambient temperature, but this will depend on atmospheric conditions.

I know from previous tinkering with water injection on my previous vehicle, on a hot day with approx 30 C ambient temperature, on one particular uphill grade in 5th gear with cruise set at 100 km/h, the post-intercooler temperature was something like 62 C (read from VAG-COM). I had the water spray in the intercooler pipe a foot or so in front of the combined pressure/temperature sensor. The moment the water spray turned on, that temperature dropped to around 42 C - and the temperature drop was almost instantaneous with switching on the system.

Re injecting water spray post-combustion, I think you will find that this will not be a benefit. Water takes an awful lot of heat to vaporize - that's why it works well post-intercooler and pre-compression-stroke, to keep the "cool" side of the thermodynamic cycle "cool". If you use it to cool the hot side, the combined pressure of the cooled gases and vaporized water will be lower than the pressure of the hot gases with the same enthalpy, and that is certainly *not* what you want to do if you want efficiency. You want to keep the cool side of the thermodynamic cycle cool, and you want to keep the hot side hot. Also, the moment of NOx formation is the moment of combustion at the interface of a fuel droplet in the compressed air. If you squirt in water after this, you've missed the boat. Quenching the combustion process is likely to increase soot and HC emissions. If a soot particle wants to burn, the best thing to do is to let it burn, and the best way to do that is to let the hot side of the cycle stay hot.

I still have yet to see a proper thermodynamic cycle analysis of drawing in a mixture of water mist and air during the intake stroke, compressing that, and then completing the cycle with standard diesel combustion and exhaust strokes. This is the way it is with ordinary aftermarket-type water injection systems - and there's no major technological obstacle to implementing those (aside from winter freezing the system up).

nicklockard said:

b. It got me thinking over the weekend about the possible effects of injecting water post-combustion

Okay, going to fundamentals, a heat engine such as our CI engines throws a lot of heat away, in the form of warmed up coolant, basically just to protect the metallurgy from melting down. If you could pre-inject just a smidge of water prior to fuel injection for the previously stated goals, then post inject a butt-load of water (at high throttle applications only!), you could extract a lot more useful heat and turn it into steam pressure, which would drive the turbine exducer harder for a given temperature.

This would allow you to kill several birds with one stone: keep NOx under control in the high-load regime (pair it with EGR for low load NOx control,) keep PCT's under control, control high load heat loadings on engine, extract more useful PV work (at the expense of temporarily lost TE.)

Click to expand...

It has been found that NOx formation follows the rate of heat release curve, which slightly leads the rate of pressure rise in cylinder. Peak NOx concentration occurs right around the peak of this RoHR curve, which is right around TDC. Beyond this point there's some NOx reduction with radical interactions, but beyond a certain point the reaction kinetics freeze. So post water injection will likely not lower NOx emissions but could reduce exhaust gas temperatures.

However, don't assume that reducing EGT will reduce the amount of convective heat transfer to the engine through the ports and ultimately to the coolant; when, as you say, you inject a "butt-load" of water, you also increase the heat transfer coefficient. Since the specific heat flux is the product of the heat transfer coefficient and the difference in temperature, if you want to reduce the heat flux, you have to make sure that the reduced temperature difference exceeds the increased heat transfer coefficient.

Besides, if you want to extract the exhaust energy in a downstream device like a turbine, thermodynamically it is most efficient to keep the gases at the maximum temperature, which means not dousing the fire with water so to speak. There's no free-lunch of "explosively expanding" water droplets, etc.

GoFaster said:

I still have yet to see a proper thermodynamic cycle analysis of drawing in a mixture of water mist and air during the intake stroke, compressing that, and then completing the cycle with standard diesel combustion and exhaust strokes. This is the way it is with ordinary aftermarket-type water injection systems - and there's no major technological obstacle to implementing those (aside from winter freezing the system up).

Click to expand...

I did it in 2000! But unfortunately I don't have a copy available to give even if I wanted to...

Do you remember what the general outcome was? Was it an efficiency-killer, or an efficiency-improver?

It was an indicated efficiency improver by 7.9% (in the thesis abstract), primarily because the inlet temperature was reduced and the ratio of specific heats was more favourable over the entire cycle (went down on the compression side -- less compression work; negligible change on the expansion side; overall cycle benefit). However, this reflected a 30% H202 solution, and only enough was injected to bring the originally dry intake air to saturation. Dissociation was not considered, though...

I'm not surprised at that.

TDIMeister said:

Besides, if you want to extract the exhaust energy in a downstream device like a turbine, thermodynamically it is most efficient to keep the gases at the maximum temperature, which means not dousing the fire with water so to speak. There's no free-lunch of "explosively expanding" water droplets, etc.

Click to expand...

Hi Dave, thanks for the links you provided me. I haven't had time to catch up, but after I do my reading I'll try and get back to this subset of the topic and the next one brought up in post #34. But, without detailed analysis, I think you can get a similar amount of pressure-volume work upon the turbine exducer at a lower temperature through use of steam. In no way, shape, nor form do I think there is any free nor additional energy to be gained by using steam. But, I think that the energy can be found at lower temperatures and over changed time scales which can protect the turbo from peak heat loadings possibly and lead to more useful work output, respectively, while reducing NOx. Wartsila claims 50-60% reductions in NOx output by direct water injection! Yes!!

TdiMeister said:

It was an indicated efficiency improver by 7.9% (in the thesis abstract), primarily because the inlet temperature was reduced and the ratio of specific heats was more favourable over the entire cycle (went down on the compression side -- less compression work; negligible change on the expansion side; overall cycle benefit). However, this reflected a 30% H202 solution, and only enough was injected to bring the originally dry intake air to saturation. Dissociation was not considered, though...

Click to expand...

Was this done assuming:

• PCP's and PCT's as found in an OEM application, roughly close to 185 bar and 2250-2350 K, respectively?
• And using post-intercooler intake tract water injection?

I'd love to read that thesis if you ever find it on an old floppy somewhere. If not, no biggie... how do you guess the outcome would change if the specific case requirement of exceeding the supercritical threshold pressure and temperature for steam that I proposed were met? Supercritical steam is a very efficient enthalpy carrier

AFAIK all digital copies have been lost due to a crashed hard drive some years ago. The only printed copy is at the University, and I may try to obtain a copy of that for posterity, but it will be some time before that happens while I'm an ocean apart.

GoFaster said:

Actually, that is not true. When considering the evaporation of the water, you have to consider the *partial* pressure of the water, not the full atmospheric pressure. It turns out to be possible to bring the temperature down to approach the wet-bulb temperature. In some cases (of very dry intake air and with good conventional intercooling prior to water injection - which, by the way, is one of the rules listed in that brochure!!), it may be possible for the air / water-mist mixture to be brought *below* the ambient temperature, but this will depend on atmospheric conditions.

Click to expand...

So right you are. Big oversight on my part! Yes, the cooling can bring temps down below B.P.

MethylEster, I just wanted to mention that the acid comment didn't escape me...I'm ignoring that sub-discussion for now because it is far more complicated than it first appears.

Dave,

Can I post any of the .pdf papers you sent me? These are excellent. I'm printing and reading some on lunch.

Thank you very much!

http://homepages.cae.wisc.edu/~rutland/research.dir/NOx_water/2000-01-2938.pdf

This gives me chills 'cuz it's so cool

I'm still digesting it all, but this is a great read.

Edit: if anyone can find related papers, please post


"The reason for the improved fuel economy for the 44% load cases can be seen from figure 9.The preak pressure only decreased 10% and the pressure is higher than the baseline about 30 degrees ATDC, resulting in increased work output and a corresponding 1% improvement in SFC." (--Second from last paragraph on page 7.)

And this is injecting water under unchanged timing conditions. If they mapped water\fuel injection timing (and ratios) to run more advanced injection timing at increased water ratio's...wow.

There are some really good observations here, some completely anti-intuitive like the above quote.

I think if hybrid water/fuel injectors like the experimental Bosch injectors used in this study were used with deionized water (plus addition of small amounts of non-ionic surfactant) were used (refer to diagram in link above, page 2,) WITH ECU-mapped more agressive timing (timings mapped to injected water volumes,) efficiency gains could be had for a given amount of engine out NOx emissions. I think the water/fuel ratio also should be ECU-mapped.

Read page 9 for an overview. Summary comments: "Advancing the injection timing allows significant decreases in SFC, PM, and NOx for 44% loads, and large decreases in PM and NOx but with a small increase in fuel consumption for 86% loads."

The question is, is the increase in fuel consumption at 86% loads less than the amount of fuel wasted in the present thinking where engineers are throwing raw fuel down the exhaust pipe for Selective Catlytic Reduction schemes?

Wow, is SCR ever a wrong turn down a dead end alley!

http://www.ilot.edu.pl/Journal%20of%20KONES%202002%20No%201_2/01/str275.pdf

Good read about NOx emissions.. I also had another very good pdf about PCP, flame temperaturs concerning the NOx process but strange enough I can't find it anymore, it seems to be disappeared..

Refound it

http://www.ilot.edu.pl/Journal%20of%20KONES%202001/JOK2001%20NO%201-2/R5.pdf

Thanks for the links!

The more I read of water injection, the more stoked I get.

Been busy, but I want to go over the recent papers and fully digest.

Have a great weekend.

Good discussions forum:

Interesting topic: http://www.aquamist.co.uk/phpBB2/viewtopic.php?t=1081&postdays=0&postorder=asc&start=15

Nick, Hugh MacInnes exactly described a water injection system that pressurized a water tank from boost pressure to squirt water pre-compressor after a certain threshold is reached.

Although an amazingly simple setup, the huge disadvantage is that water being sprayed at ambient temperature and pressure will remain liquid and impinge on the compressor blades. Most people dismiss this entirely as being a non-issue, but they do it at the peril of their compressors... Even if the quantity of injected water is calibrated to never exceed the dew point of the ambient air pre-compressor (which would be VERY LITTLE water), and if you set up the spray nozzle sufficiently far upstream to assumptively make sure all the water evaporates before entering the compressor, this is thermodynamically disadvantageous. Try this exercise on this link. Change nothing except the order between TC, IC and WI in "Water Injection (2)". The setup that places the WI AFTER both the TC and IC is the most advantageous by a healthy margin, while placing the WI before the others is the least advantageous.

Another problem of injecting water pre-compressor is that you cannot control when- or if the water recondenses. At the elevated boost pressure post-compressor, it takes a higher temperature to reach the condensation point of the water, and liquid water could pool up somewhere in the inlet where you don't want it do (the intercooler would be the most likely spot).

From the standpoint of maximizing the charge density and maximizing the cooling effect for NOx reduction, it is most advantageous to have the water injected as close to the intake valves as possible, and in fact there is an optimal timing during which water is injected directly into the cylinder which gives the globally best NOx reduction (at the possible expense of CO, HC and soot emissions).

Okay Dave. I must confess: I'm still trying to get my head around effects of water injection in different schemes.

Let's ignore for the moment the possible blade damage. Let us also ignore water pooling in the IC for now. I think both have very simple solutions.

Doesn't injecting a fine fog of water pre-compressor decrease the work of compression?

There are a few places where it is claimed that 'it makes the turbo act like a bigger turbo (higher mass flow rates at increased PR's) on forums and such, but I can not find any substantiation.

Edits: Okay, I've found this link which suggests that pre-compressor water injection could shift the surge line to the right and reduce safety margin: http://books.google.com/books?id=zxRdacCvjVsC&pg=PA570&lpg=PA570&dq=surge+water+injection&source=web&ots=m-suFABF03&sig=A0EaLCSyv13HDleJYuHryTPVT1o&hl=en#PPA571,M1 (the comments are about fixed power generation axial turbines. Do they apply to automotive turbochargers as well?)

More: http://www.aquamist.co.uk/phpBB2/viewtopic.php?t=1216&postdays=0&postorder=asc&start=0

Member JohnA suggests that the entire turbo map is right-shifted in proportion with increasing water injection--efficiency islands, surge lines, choke lines, and all.

More: http://www.aquamist.co.uk/phpBB2/viewtopic.php?t=267&postdays=0&postorder=asc&start=45

Pic of precompressor water injection holder:

ASME members?

Can anyone download this paper for me?

http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=JETPEZ000128000004000849000001&idtype=cvips&gifs=yes

Abstract: The present paper examines the effect of water injection at the compressor inlet or between stages, on its operation. A wet compression model coupled with an engine performance model is used. The wet compression model produces the compressor performance map when water is present and consists of a one-dimensional stage stacking model, coupled with a droplet evaporation model. The effect of water injection on overall performance and individual stage operation is examined. The map-generation procedure is embedded in an engine performance model and a study of water injection effect on overall engine performance is undertaken. The possibility to evaluate the effect on various parameters such as power, thermal efficiency, surge margin, as well as the progression of droplets through the stages is demonstrated. The results indicate that water injection causes significant stage rematching, leading the compressor toward stall and that the performance enhancement is greater as the injection point moves towards compressor inlet.

The effect of water injection on compressor work is ambiguous without more detailed calculation. The specific compressor work may go in either direction, either up because of the increased specific heat capacity at constant pressure of the water-air mixture, or down because the inlet temperature is reduced due to evaporation of water. The specific heat ratio usually decreases with added water, so this is good for reducing specific compressor work. However, since you've added water and furthermore cooled the inlet charge, the density and mass flow is definitely going to be higher, and your compressor power (mass flow rate multiplied by specific compressor work) will likely be at any rate higher than the baseline without water injection.

As you can see, determining this will depend partly on the amount of water injected relative to the amount of airflow, and how much of it evaporates.

Heywood has a nice chart showing the effect of water in air on properties like R, Cp, Cv and specific heat ratio (gamma). See Fig. 3-2 on p. 67. Of course, your NIST applet should tell you the exact same thing.

Will dig up link, but several sources saying precompressor injection is polytropic compression (more isothermal than adiabatic?)

Polytropic can be anything from 1 < n < gamma (well, it could technically also be > gamma). For most technical analysis using plain air, calculating using gamma suffices, because you would have inlet- and outlet temperatures to determine the adiabatic efficiency and therefore the you can determine the net effective compressor work. Once you have liquid water in there you cannot technically use gamma because you cannot treat the mixture as an ideal gas. In this case an empirically-determined polytropic exponent is used. Compression (or expansion) at very high temperatures and pressures result in dissociation, so again empirical n's are used or real gas relations.

TDIMeister said:

As you can see, determining this will depend partly on the amount of water injected relative to the amount of airflow, and how much of it evaporates.

Click to expand...

Yes, exactly. Also, I think this is best determined empirically/experimentally as it will differ from one turbo/car combination to the next.

Moreover, the water/alcohol/ketone composition will change this. There are some excellent postings in that thread I linked to before: particularly the discussion of water/acetone, water/alcohol, water/alcohol/acetone mixtures and the use of surfactants (non-ionic) to control droplet size distribution. Surfactant use is a well known trick for controlling droplet size in fog injection systems by lowering surface tension. Also read member hotrod's contributions: he offers both theory and practical, as he has done precompressor injection with good success.

More links: http://www.eng-tips.com/viewthread.cfm?qid=72284

Well, the only benefits to the pre-compressor setup is its simplicity, passive fail-safe nature and requiring no pumps. It's very easy for a home enthusiast to rig up such a system if he can find a suitable reservoir (a radiator expansion tank would be perfect but offer limited capacity).

Thermodynamically it's not particularly effective and there's the danger of water recondensation in the intake tract. This seems to be the best suited to a turbocharged engine that doesn't have an intercooler, and very small amounts of water injected (which should be the case at any rate!)

If I were designing a system for my own car (or for an industrial engine) I would inject the water as close as possible to the intake valves, and if the objectives warranted the complexity it (e.g. NOx compliance), direct water injection.

I had all the pieces together for a water injection system for my car including a Hobbs switch, mister from McMaster-Carr, SHURflo high-pressure low flowrate pump, and even a larger 8.5L windshield washer fluid reservoir that was a direct fit for my Passat and even had two bungs for fluid pickup (in the original application, one pump was for the windshield washers and the other was for the headlight washers).

But alas, I never got around to installing it...

Apologies for wandering around the sub topics, but I want to understand something better.

In Fix_Until_Broke's thread: Exhaust & Intake Pressure Measurements



Figure 1: FUB's measurements



When EMP > IMP, is this by definition turbo lag? Or, what is going on?

I think it's very relevant to something I've been pondering regarding water injection for a long time, but I want to understand this fully.

nicklockard said:

When EMP > IMP, is this by definition turbo lag? Or, what is going on?

Click to expand...

No, turbo lag is a temporal phenomenon. It is simply the time that elapses from when engine load demand increases to when the desired boost pressure is generated.

The phenomenon of EMP > IMP can be illustrated mathematically by equating compressor power to turbine power (freewheel condition, a.k.a. Büchi's Law).


Pcomp = Pturbine

m_dotair*wisen,comp/(effmech,comp*effisen,comp) = m_dotexh*wisen,turbine*effmech,turbine*effisen,turbine

m_dotair and m_dotexh are mass flow rates w.r.t. time

The specific isentropic work for the compressor and turbine, respectively:

wisen,comp = Cp,air*T1*{(P2/P1)^((kair-1)/kair)- 1}
wisen,turbine = Cp,exh*T5*{1- (P6/P5)^((kexh-1)/kexh)}

The mechanical efficiencies of the compressor and turbine are usually expressed as a total turbocharger mechanical efficiency:

effmech,comp* effmech,turbine = effmech,turbo

Rearranging all these equations, you get the function:

(m_dotexh/m_dotair)* effisen,comp*effisen,turbine*effmech,turbo*(T5/T1) = (Cp,air*{(P2/P1)^((kair-1)/kair) - 1}) / (Cp,exh*{1- (P6/P5)^((kexh-1)/kexh)})

I know the equation looks messy, but it’s not very hard to understand. It can be easily plotted out with multiple lines of constant turbine inlet temperature, and in fact this is done in the Diesel Engine Handbook by Challen and Baranescu (sorry, don’t have the page number on-hand).

(P2/P1) is your compressor pressure ratio; consider this approximately the IMP in absolute units. (P6/P5) is your turbine pressure ratio; consider this approximately the EMP in absolute units.

What becomes clear is that, aside from choking flow, the relationship between the IMP and EMP are mainly functions of the efficiencies of the turbocharger, as well as the temperatures at the compressor inlet and turbine inlet. You can have EMP < IMP, but the requirement for this to happen is a high total turbocharger efficiency and a high turbine inlet temperature. It’s not about the size of the compressor or turbine. As a piece of trivia, the operating condition at which (P2/P1) = (P6/P5), that is, IMP = EMP, is called the Büchi point.

At low loads, the TDI engine due to the low T5 has EMP > IMP. At high-loads and engine speeds, choking causes again EMP > IMP, although in between, there are operating points where it’s possible that IMP < EMP, particularly with the VNT-equipped TDIs.

Thanks. I'm familiar with dot notation for rates. Is a centrifugal turbine or compressor really adiabatic isentropic expansion/compression?

I guess what I'm still trying to see is how can we see evidence of turbo lag in FUB's data?

Next, from what I gather, the formulas above also state that

Cp(ex) * turbine inlet temp T5(?) determines turbine work available and by extension compressor work possible.

So, does having additional water in the cycle increase or decrease the product of Cp*T5?

Circling back around toward my intended question of my last post in asking for help: if we could inject water post exducer (or better yet, post-catalyst), enough to cause the temps to drop below the condensation point of the entrained steam water, it should lower P6 which could increase turbine work. Is this correct, and, will it spool the turbo up faster?

Turbo lag is best displayed by charting requested boost and MAP (obviously).