Water injection: BMEP and Peak Pressure

[obanion]

I'm creating this thread to discuss this one particular effect of water injection. Lots of attention is payed to intake charge cooling, and I understand that very well. But of particular interest to me is the effect water injection has on the relationship between BMEP (brake mean effective pressure, or the average pressure pushing on the piston, and therefore torque), and peak pressure.

The study done in the early 30s by Sir Harry Ricardo, gave us a nice graph, which is on page 134 of the book "Turbochargers" in the WI section. The gist of it is that BMEP kept going up, and at one point, the peak pressures starting coming down, even as BMEP kept going up. Translation: More power with a lower maximum stress on parts, ie, more power before breaking a rod or lifting a head, etc. Unfortunately, the study was only able to go so far. Not as much water as I'd like to try and run was used.

So my first question is....why? How does water (or steam) during combustion somehow reduce peak pressure, and increase BMEP? My guess is it's a combination of combustion effects, slowing combustion so it's more "spead out" over the power stroke, and also the actual thermodynamic behavior of steam vs air, with more steam translating into a better medium for transfering power to the piston.

Am I warm?

Secondly, do the advantages keep going up, the more water you use? Before you yell NO, the example I am considering is where you have lots of intake heat from lots of boost (45-55psi), so you can introduce FAR more water into the cylinder as it will be vaporized into water vapor, rather than liquid. My calculations indicate a air charge of about 140F at maximum humidity, which is about a 10:1 air:water mass ratio. I'd be displacing about 8% of the air with water vapor at that point.

The alternate is to add a bunch of weight and complexity with dual liquid/air intercoolers, and not have all that water in the intake charge. Yes the charge should make more power, not having all the water vapor displacement, but I have the option of running lots more boost anyway (65psi, no problem on compounded turbos), so maximum power at a given boost level isn't my goal. Rather it's the maximum power for the rotating assembly, which is limited by peak cylinder pressues.

So, anyone have any idea what BMEP and peak pressures are like using very high (but vaporizied) water:air ratios?

 

[patprimmer]

The concept sound OK to me in principal.

Problems:-

The upper limit will be as you approach hydraulic lock as there will at one point be to much water still liquid at the top of the compression stroke.

As the water consumption increases, so will the size of the water tank, untill it becomes unworkable. To obtain a car with a reasonable range, you will need to use both intercooler and moderate water injection rates.

A failure in the water delivery system, will result in a seriously damaged engine if you don't have fail safe devices, like dumping boost if there is no water pressure and flow.

Regards
pat

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[obanion]

Solutions:

1. Most water will be vapor from intercooling two stages of turbocharging. Flow will be carefullly monitored, so that the level of water will be such that most is vapor, and perhaps 10% liquid upon induction to cylinder. So, air:water ratio can not be constant. You only need a perhaps 40:1 ratio at 5psi (not much heat), vs a 10:1 ratio at 50psi (lots of heat).

2. Calculated flow of 1GPM at 100LBM air (1000HP or so), peak usage. Hardly unworkable. Avg .5GPM when in use.

3. Good point. Will definetly have a fuel and ignition shutdown, tied to loss of injection pressure OR excess of 160-170F intake temp.

 

[hotrodwrx]

The higher BMEP as I understand it, is mostly a function of the higher specific heat of the added water, with a small contribution due to higher volumetric effeciency putting more moles of combustion products in the cylinder. It takes a larger energy loss to cool the cylinder contents as the power stroke progresses. As a result the pressure stays higher longer -- higher average pressure even though the peak is lower.

For a common example of the effect of humidity in air, look a the behavior of the atmosphere with change of altitude. The dry adiabatic lapse rate is about 5.5 deg F / 1000 ft altitude change, and the wet adiabatic lapse rate for dry air is about 5.5 deg F / 1000 ft altitude. Using that as an example for the effects inside the cylinder, the temperature change with volume change will follow the same pattern. As the volume of the cylinder increases during the power stroke the average temperature of the combustion gases would drop at a lower rate in a WI engine than it would for the same piston movement in a non-WI engine due to the higher water content.

As far as your issue with very high boost. Look into the Reno unlimited air race folks. They run about 50 psi boost, and 160 octane fuel in those engines. They inject about 0.5 lb MW50 (50/50 water methanol) for each 1.0 lb of fuel.

If they are running with no intercooler if I recall correctly they just about double that injection rate.

Even at those high injection rates they are no where near hydraulic lock on the cylinders. The specific volume of the fuel and water if it was all condensed out is very small in comparison to the combustion chamber volume.

Larry

 

[obanion]

I think the octane numbers are greatly exaggerated when used in airplane terms. The fuel I'm fairly certain they use, is VP AIRRACE fuel:

http://www.vpracingfuels.com/vp_10_techdata.html

120 octane, the highest they make, but not 160 octane, which I don't even think is possible by automotive testing standards of octane.

I did find a article mentioning a race plane running 75psi with two stage supercharging. Wow....lots of boost.

They do in fact run MW50, the article called it ADI (Anti-Detonant Injection).

I have discovered I can add a liquid/air intercooler to this project for very little extra cost, by building it into a intake manifold. So the plumbing difficulty and cost aren't much of a factor anymore.

Would I be better off intercooling, then port injecting MW50, or no intercooling, and tons of water? I don't mind needing some extra boost for the same power with just water, if it means the peak pressures in the cylinder stay low. Can I actually inject a 1:1 ratio of fuel:water to a say 80F air charge, and not drown the motor? Not hydro-lock, just where it can't maintain good ignition.

 

[obanion]

How about this idea?

Note: 2 stage turbocharging, one large turbo, blowing into smaller turbo. Target 2:1 ratio for each, final 4:1 ratio (43.5psi gauge pressure).

I use water injection into the large turbo. This will be vaporized during compression in that turbo, and keep outlet temps down to 100-120F. No water injection into second turbo. Then a big water/air unit build into the intake manifold. The core will pose a 4.5" flow depth, with a 4.9"x24" cross section to the air. This is like a Spearco 2-171 core x 2.5 the cross section. I'm estimating a 90% efficiency rate at full power. Perhaps a touch more, as the entire manifold will be cold, as it's all aluminum, and all one piece when finished, and will use a phenolic spacer at the cylinder head, and by completely coated.

The water that was vaporized by stage 1, will be recondensed by the intercooler/manifold. A water:air ratio of ~25:1 injected at the first turbo, will remain as full vapor at 100-120F no problem. But once cooled by the intercooler down to 40-50F (assuming ice water), the water cannot remain as vapor, and must exist mostly as liquid. So I'll have dense (cold) air, which automatically is very dry, and will continue to have the benefits of water injection, as a liquid for cylinder compression and combustion stages.

 

[hotrodwrx]

Oops just noticed I typoed my post above -- should be corrected to read:
The dry adiabatic lapse rate is about 5.5 deg F / 1000 ft altitude change, and the wet adiabatic lapse rate is about 3 deg F / 1000 ft altitude.

Obanion:

I think the octane numbers are greatly exaggerated when used in airplane terms. The fuel I'm fairly certain they use, is VP AIRRACE fuel:

http://www.vpracingfuels.com/vp_10_techdata.html

120 octane, the highest they make, but not 160 octane, which I don't even think is possible by automotive testing standards of octane.

Obanion:
Your right, I was just using the commonly quoted round numbers for that fuels octane number. As you say, the automotive fuel octane scale is undefined above 120. If you'll note that ref is listing the motor octane number that is always the lowest of the RON/MON numbers used in automotive applications. Most people (especially advertisers) use the higher RON number in casual references to fuel octane, and specifically qualify it in more technical discussion. In aviation fuels they use a "performance number" that more or less, matches the octane number in automotive gasoline at octane's below 100 and above 100 they extrapolate a effective octane or anti knock index "performance number" for both lean and rich mixtures.

That was originally based on the amount of TEL needed to achieve the same fuel performance with regard to detonation resistance. This allowed them to effectively stretch the octane concept into detonation resistance numbers that could not be duplicated with the formal octane test process. Folks in the auto racing world, usually accept the rich performance number as being essentially the same as automotive octane for conversational purposes.

The specific number I've seen for that fuel is 157 max power rich performance number and VP specifically states it is the highest octane fuel they make in some of their older literature.

http://www.rocketindustries.com.au/onlinecat/vpfuel/LEADED_FUELS.htm

http://www.smithtex.com/tech/vp_brochure.pdf

Larry

 

[FMOSRacing]

Water injection prior to the turbo could prove fairly troublesome to the compressor impeller.

 

[TTmotors]

The blade on a 2 inch compressor is moving at 62,831 feet per minute along its perimeter. acceleration impacts at that speed cant be pleasent if the droplets are any size at all. Think after compressor water injection would be the safer shot.

 

[turbododge]

There was quite a discussion on this on an earlier thread, you may be able to find it. I have been spraying on my compressor blades for years with no detriment. Spraying afterward is nice, but much more difficult, and does create some of its own problems.

 

[turbododge]

The thread is down a few pages and named "Intercooling turbo compressor directly to boost flow". Dated 1/8/04.

 

[TTmotors]

I have no actaul experience with spraying/misting/squirting much of anything into a compressor assembly. If i were going to do it, Id just be careful to make sure its atomized reallllly well before it hits the assembly. Anything thats quite a bit heavier then the air will not spin in with the air mass around it and will end up hitting the blade well below the blades speed,dulling or altering its shape.

As long as you have a good system atmozied well it would seem like it would work ok because the water would be mixed in with the air and as it approched the compressor it would enter the vortex and come right upto speed with the blades either lightly impacting it or held in suspension by the air around it as its compressed against the fin and ejected. Using a super soaker xj5000 would obviously cause some serious issues though.

 

[patprimmer]

I think the trick is to hit the centre of the end of the shaft, then it spirals out across the blades with no significant impact were speeds are high

Regards
pat

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