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If the "choke line" is the point at which the boundary layer next to the vanes breaks down, then water injection will surely make the compressor more efficient.
The water droplets will make the air denser. Denser fluids have less vorticity. Vorticity is responsible for turbulence and the breakdown of the boundary layer. Therefore denser air makes the turbo more efficient by reducing vorticity and thus stabilizing the boundary layer of air. Makes sense, right? Adrian~ |
Logging
If you have the means to log a couple data streams it should be pretty simple to prove.
On the WRX, you have a MAF (Mass Air Flow) meter that reads directly in grams/second ( if you have the proper logging software) it is a 0-5 volt output so even a peak hold volt meter could be used. Just set the boost to a known value and log the mass air flow at a known rpm. Then turn on the WI and adjust until you hit the same boost setting at the same rpm and compare the new MAF numbers. Larry |
So, if choke occurs later, the compression could continue at higher wheel speed. The speed of sound increases in denser mediums, the factor of which I do not know.
I'm not familiar enough with vorticity other than I can see that disrupting of either the boundary layer or added sonic turbulance would lower compression by preventing laminar flow, and add heat to that air (turbulance) without compression. For the layman, is that what you are saying Adrian? Choke occurs when the small diameter (inlet) of the impeller reaches sonic speeds. You think that the added density with water injection will slow the wheel to under Mach1, yes? Or, that the speed of sound will increase, permitting extension (more wheel speed) of compression until choke? |
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One of the mechanisms for generating vorticity in compressable flows (like air) is the baroclinic generation term. "The baroclinic mechanism for vorticity generation is frequently invoked to explain the existence and direction of off-shore and on-shore breezes and the generation of vorticity in compressible flows. The baroclinic generation mechanism will be non-zero whenever the density and pressure gradients are not aligned." Taken from Navier-Stokes.net Since the evaporation of water is easiest in low pressure regions, vorticity would inherantly be absorbed. This would be because any time the pressure gradients were un-alligned by a low pressure region, some water would evaporate there and bring the pressure up to the same level as the higher pressure region. Which would make the pressure/density gradients uniform and thus reduce vorticity. Worth a read: http://www.navier-stokes.net/nsvcr.htm Does that make sense? In lower pressure regions the water would evaporate and raise the pressure to equal the other regions more or less. Which would stabilize the flow in the compressor and allow it to flow more easily, and with less heat generated from vorticity. That would also stabilize the boundary layer. Adrian~ |
Its clear that you guys have a far better understanding of this than I do! Your comments have stirred up a couple of thoughts though.
Gut feeling :roll: is that with WI just upstream of the compressor relatively little evaporation will occur outside the compressor, most evaporation would occur as the air is heated and shredded inside the compressor. Since the heat will be generated during compression, and the compression occurs progressively, and there will be some latency between the air being heated and the resulting water evaporation, it may be that water evaporation also occurs relatively late within the compression process. I can sort of imagine a plot of temperature against time for a typical molecule passing through the compressor, and seeing the temperature rise quickly for the dry case and slower for the wet case IYSWIM. If so, I expect the impact on the compressor entry conditions would be relatively small. The second thought is that a centrefugal compressor is highly sensitive to density. As well as the 'each shovel holds twice as much air' analogy, it has been pointed out that the denser air will generate a higher outlet pressure for the same RPM or the same pressure at lower RPM. Does this reduced RPM (and presumably reduced air friction drag on the tubine) mean that the compressor will also (a) be more efficient and (b) spool up more quickly (in terms of flow and pressure ratio, even if it may be slower in rotor rpm terms)? There are too many unknowns to speculate, but I would be extremely interested to see what difference it does make to have WI upstream rather than downstream. |
From Stealth316:
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I also like the sentence about how the choke area is almost never noted on a compressor map (the map usually stops at 60% efficiency). That's great for us, we have some room to spare. I wrote to a "turbomachinary" professor in England. His take is as we have noted, the gain in work with "wet compression" over and above cooling the air after compression (intercooling) is that the more air is compressed with the same amount of work energy applied--it's the shovels of air analogy. To recover, the same mass of compressed air without water injection, your need to turn the compressor more to make up for the loss in efficiency. He also said there is no combustion penalty up to 5% water:air mass. Unfortunately, he did not give much of a molecular take on things in fluid dynamics terms. |
what a great read.
thanks to all the posters. ken |
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Do you think a temperature probe is a good addition? |
Well, if the shoveling more are per turn is correct, I guess you would get a good power increase due to turbine does not need so much pressure to supply same air mass/pressure, less backpressure on motor will increase power output too.
this all sounds good, let's see the results, I would try on my car but I have twin turbo v6, so would need more hardware ie atleast 1 more hsv, but really 2 would be the one to make sure misting is o.k., - turbos are on oppersite sides of engine bay. also, would injecting straight arfter air cleaner/maf sensor, not just before compressor be even more benaficial, ie longer time to cool the air down makeing it even more dense before it enters the compressor ie shoveling even more air per turn, or is the most benerfit from cooling the compression stage. maybe that could be tested also. Cheers Ryan |
temp probes
A have a friend that has a small number of dual probe temperature gauges that can display delta T between the probes or either probe temp.
I will be buying one from him soon. I think its the best way to confirm the WI is running (ie if no temp drop, means back off the boost). They are a bit expensive but if anyone wants to try one, let me know and I can get you contact info. Larry |
I think a potentially easier idea (for a mass produced unit) would be two PTC resistors (change resistance based on temp, artificially heated, used in GM MAF sensors) ...
Put on a few inches before the WI, and one a few inches after ... when the resistance values are equal a light could come on, and the ECU could be informed that no WI was taking so that it would use a more conservative map. Adrian~ |
Yep
I agree for a mass produced unit something like that would be the best. I need to get the gauge to do some testing.
The only issue I see with temp sensor safety systems is the natural hysteresis time delay in reaction. Suppose you had a logic circuit that gave a 1 ( WI okay) when the delta T > 20 deg F between the upstream and down stream sensor. It would give a 0 ( WI alarm) when the delta T was <= 20 deg F. Set that up so if you sense power to the spray relay, and get a logic 1 then use the high boost map. If power to the spray relay and get logic 0 drop to a safe map and set a Check W I light. possible situations: A --- first use of WI in some period of time. both sensors stabilized at IAT (intake air temp). The down stream sensor would begin to cool very rapidly but there would be a small but finite delay in its temperature drop. So you would want a small wait interval following start of injection before the CWI light circuit checked for delta T between the sensors. B --- WI in use, sensors stabilized at max delta T between up stream and down stream. How often does the CWI logic check the temps ( every 1/2 second ??) C --- Sudden total failure of WI, down stream sensor stabilized at max delta T when WI fails. I see two issues. First if the sensor surface is well wetted with WI fluid, there will be a small period where its temperature will hold low due to evaporation of this fluid film and any fluid that drips from the nozzle ( ie partially blocked nozzle or surface wetting of interior of intake path). Second, if flow is not completely blocked but only impaired you would still see cooling but the amount would drop. Looking at above there would need to be some tests to determine the ideal spacing between the temp sensors to get best detection of WI failure with minimum time delay, and minimum surface wetting of sensor surface to reduce detection errors. Another possibility would be to use an IR diode detector pair that looks across the intake path down stream of the spray nozzle. There should be a very significant reduction / scattering of the IR light off the mist. If you have one detector in direct line with the emitter its sensed output should drop dramatically when the mist plume passed between the detector and emitter. If you also placed a second detector at right angles, or even in line on the same side as the emitter beam it would only see scatter if the mist plume was present due to back scatter off the mist dropplets. You might also be able to detect the presence of the mist plume with a capacitance system that would detect the change in the capacitance between two plates on opposite sides of the intake path. Or a similar inductance system that detected the change in inductance of a coil surrounding the intake path when the plume was present. A third possiblity would be a charge transfer sensor. Place a voltage potential on the injection nozzle, and a collector screen down stream in the mist plume. You should get some small electrical current transfer in the mist dropplets from the nozzle to the collector screen. This charge transfer should vary directly in proportion to the mass volume of spray dropplets that impact the collector screen. I think a combination of a 2 or 3 way detector composed of the delta T photo optical back scatter or intake air capacitance/inductance , and charge transfere detectors would be the most fool proof. It would just require some testing to see the range in detection values and possible false alarm potential for each system. Okay Saabtuner and I have discribed 5-6 workable detection concepts, all you experimental electronics types, --- go build some prototypes and see if any of these work well enough to use. :twisted: Larry |
Richard
Richard:
Perhaps, the last couple posts belong over in the WI safety discussion, or I could double post it over there if you prefer, because there is some value to seeing these discussions in context as they were developed. Just a thought --- Saabtuner -- what do you think? Larry |
I agree. Some of the posts should be moved as they are more safety related than pre-comp related.
The problem with the sensor being wetted is moot, it is heated to 220C above ambient to prevent this already. The amount of current needed to maintain that temperature determines the mass-flow through the system. Even if it were wetted, so too would the rest of your induction be. Therefore the WI effect would taper off slowly. Which is exactly what was seen in that dissertation I posted from Linkoping on WI. Since these are resistors, and very accurate ones at that, all you really need to do is this: 1. Have the sensors active at all times, but passive. 2. Have a boost pressure solenoid that activates the safety circuit just as your WI comes (at a fixed boost pressure) on and deactivates as it goes off. 3. Have the first PTC resistor (RH) heated to 220C above ambient. (Ambient temp is measured by a third special PTC resistor (RC)) The amount of voltage required to keep that sensor at that temperature differential will depend on the mass flow of the air across the (RH) sensor. 4. Have a third resistor post-WI "wet" (RH) which is heated to 220C above the temp of the special resistor (RC), and if the value is equal, or near to, your "dry" PTC (RH) resistor the power to your boost solenoid should be cut (thus limiting you to base boost, which should be knock free) ... 5. If the voltage required to maintain the second "wet" (RH) PTC resistor increases significantly while the safety system is active (step 2), which means the power required to keep it hot has dropped, power should be cut to the boost solenoid. The level of drop which causes this should be adjustable becuase the WI may not produce a 100% constant cooling effect. Make sense? Should be relatively simple to setup with the right tools and electronics knowledge. The parts could be scavengened from any GM Mass AirFlow sensor. (Each contains two PTC (RH) resistors and another PTC (RC) resistor.) Pricing is in the $200 though. If your car is injecting pre-turbo and already has a MAF sensor, you'd just need a second MAF sensor. The value of the second one should just be significantly higher. Adrian~ |
Thanks to everyone for your input in this informative and idea inspiring thread!
I am wondering how colder ambient air temps would affect the evaporation of pre-compressor water/ETOH droplets. Hotrod and I live in Colorado and can experience cold winters. Larry, have you had any experience with your pre-comp injection in the winter time? I just wonder if the water mixture would have more of a chance of not evaporating fully and possibly damaging compressor blades. Another thought relating to the idea of increased air mass and O2 content with pre-comp WI. Some posts here have suggested that the increased air mass due to colder/denser air would not have more O2 content because more air is not actually entering the system through the intake. Wouldn't the O2 content of the air be increased with the addition of WI prior to the turbo because H2O and methanol themselves contain O2? I thought this was one reason for using methanol injected after the turbo because it has a high O2 content (in addition to it's knock suppression qualities). What happens at the molecular level to the O2 in the water and/or methanol at the time of evaporation? -Craig |
Winter WI
I usually shut off my WI when temps get below about 55 deg F. I simply don't need it for knock supression at those temps with my current set up, and it makes the mixture too cold -- leads to compressor surge problems on my setup in cold temps.
The oxygen in the water is not available for combustion so you can ignore it. The methanol does reduce the oxygen demand slightly as it consumes less oxygen as it burns than an equivalent weight of gasoline. The colder intake air mass flow does increase the amount of oxygen available to burn simply due to its increased density, but the percent oxygen in the intake air is essentially unchanged from normal atmospheric 21% O2. You can ignore the very small dilution effect of the extra water vapor. For example if you have dry air (0% RH) at sea level pressure, 59.9 deg F and 29.9 in/hg air pressure and call that 100% air density, if you bring it up to 100% humidity, the density is now 99.35%. The effect on engine power from humidity change is quite small. If you take an engine with intake air temp of 102 deg F, 0% RH, and absolute air pressure of 29.9 in/hg and call that 100% engine power. If you changed only the relative humidity of the intake air the engine should make 92% of its rated power. The dyno correcton factor for that humidy change would be 1.087. Okay you lost 8.7% power, but if you now look at the temperature drop effect of Water injection on power output, and drop the intake air temp to 62 deg F. you get your power back. This would be due to the cooling effect of the water evaporation in the intake tract, you now get an engine power output of 102.2%. So you have a net gain in power of 2.2% in spite of the dilution effects of the water thanks to its intake cooling. Obviously in the real world you'd never start at 0% humidity, but on the same token most of us get extra evaporative cooling from the methanol in the WI mix. The example does show that even in a worst case condition you get more back than you loose. This is totally ignoring the advantages you get on a boosted engine with increased detonation threshold and being able to crank up the boost. If you want to play with some numbers and read a fascinating web page on atmospheric pressure effects on engine power check out: http://wahiduddin.net/calc/calc_hp.htm Follow the links he has a TON of information on his web site. In fact we probably should invite him to participate in this discussion. I'm going to send him an email and see if he'd like to join in on this thread. Larry |
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So the questions are: - where to install the pre-compressor nozzle? - how much water to map to the pre-compressor nozzle (and when to switch water on)? - what combinations of injection should I try (post IC water off, pre-compressor on etc)? - anything else? I have full data logging ability as well, so checking for improved spoolup during say a 5th gear low rpm roll on can be done as well, and I should be able to supply other data that may be of interest. So guidance and suggestions will be appreciated. My car can be seen at the sites below to help with deciding where to site the pre-compressor nozzle (how about pushing the 4mm ID nylon line through the rubber end of the filter? (doesn't leave holes behind in my intake piping!). BTW I'm injecting a 20/80 methanol/water mix Steve. |
I am surprised that there is so far no offers to your questions.
A long thread on theory and the some practical answers could be just a few posts ahead. I guess you will have to go alone and place the jet in front of the turbo. I would try a 0.5mm jet first (pre compressor) and plot the result against a 0.5mm jet (post compressor) and post IC with the same jet. The logged data can be compared. What do you think? |
Suggestions
Sorry for the delay getting back to the board, just started a new job but on 12 hour midnight shifts so --------- Argggg --- its friday.
Lets see: - where to install the pre-compressor nozzle? My nozzle is positioned about 20 inches so if you can I'd suggest about 0.5 meters for your setup. - how much water to map to the pre-compressor nozzle (and when to switch water on)? Start with about 2% of max air flow mass, that will give you near the ideal 3% or so at peak torque rpms. I'd turn it on about 50% - 70% of max boost. If you turn it on too early it can send the compressor into surge, If it operates close to the surge line at mid range rpms just as it comes on boost. Low air flow, but fairly high boost, places the knee of the air flow plot at its closest approach to the surge line on many turbos. I'm getting into surge on my new turbo and may need to install an rpm window switch so the pre-compressor WI can't come on until I get over a certain rpm. Summit racing has a cheap rpm switch that I'm looking at. - what combinations of injection should I try (post IC water off, pre-compressor on etc)? For documentation I'd like to see the Pre-turbo only values, then numbers for both pre-turbo and post IC. I suspect you will want to delay turn on of the post IC injection until you reach the boost level that begins to stress the Intercooler and increase manifold air temps. - anything else? Yes, a couple passes with the pre-compressor injection on with pure water, and the common 50/50 mix would be nice if you can. That would give 3 reference points so we could guess-ti-mate what other injection mixes would give. I'd throw out a pre-test guess of around 25 - 30 deg F drop on water alone and around 40-50 deg F drop on 50/50 mix. Looking forward to your numbers. Oh almost forgot, please record ambient temp and humidity conditions if you can. Larry |
Does anyone agree that having two identical jets, one before and one after the impeller should give a very good indication of how the two work.
Once the data are logged, it will be nice to compare the results. |
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If the water amount injected is equal, it should give a good comparison of the difference in effect. If the same jet-size is used, it will still be useful in determining proper jet-size before and after the impeller. Adrian~ |
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This is perfectly true, the pressure must be the same for both jets at the point of injection. The water pressure can easily be adjusted between 2-10bar on the 2c/2d. The post-turbo jet will run at a pressure higher, equal to the boost pressure. |
An example of Bert Waite using pre-compressor water injection to increase the power of his turbines engine.
http://www.aquamist.co.uk/forum/gallery/bert/1.jpg http://www.aquamist.co.uk/forum/gallery/bert/2.jpg http://www.aquamist.co.uk/forum/gallery/bert/3.jpg He has also added this on his email to me: I finished the Turbine project that I was working on. We went from 1,400 HP to 1,800 HP with the water injection system. The addition of the water to the saturation point not only increased the turbine compressor efficiency, but allowed us to add additional fuel due to temperature management. THANKS FOR ALL YOUR HELP! He has been using water injection for many years and here are a few of his past and present projects: In the Lotus application, I injected water before the turbo and after the intercooler. Injecting before the turbo or any supercharger/compressor , increases the efficiency of the compressor by increasing the density of the air. This one one of the main advantages in the turbine application. http://www.aquamist.co.uk/forum/gallery/bert/4.jpg http://www.aquamist.co.uk/forum/gallery/bert/5.jpg I am working on a new project now that I am back on my feet. It is a boat like the turbine project. It is powered by two 6.6 liter small block Chevy engines with Whipple super chargers. It is intercooled and running about 1 bar of boost. The engines make 850 HP on a mix of 100 LL av gas and 110 octane racing fuel. The boat is a 8.5 meter cat and should run about 241 km/hr. I would like to run the boat on straight 100 octane av gas with the addition of a water injection system. According to my calculations, each engine would need between 300 ml and 500 ml of water per minute or a total of up to 1,000 ml. Does that sound correct to you? At that rate, I am considering using the Shurflo 8030-813-239 pump which is capable of 9.5 bars at 3.3 l/m. http://www.aquamist.co.uk/forum/gallery/bert/8.jpg |
So it is agreed that tow identical jet but alter the water line pressure for each jet so the back pressure will be the same for both jets.
Anyone else think there are other factors that we need to consider before slowMX5 starts his test? |
Installed mine this evening :D
I started yesterday, prepared the jet adaptor fitting: http://home.pchome.com.tw/personal/r...ptoronHose.JPG The adaptor is fitted on a piece of 2mm alu. I hammered the alu board into shape & then drilled/tapped for the adaptor. Some silicon adhesive (gasket maker) was used to fix the assembly onto the hose. ( That was really a mess to work with ) ( And the hose was drilled previously of course ) Additional nylon ties secure the whole thing one step furthur. I hope the fitting can be trouble-free. I did hesitate a lot about the jet location. For better access, it would be too close to my hot wire MAF. I was afraid, on such location, I'd experiece the hiccup as Hodrod did. Avoiding that, I'd have to place the jet just a few inches ahead of the compressor. That's just what I did. On the picture above, the air flow is from left to right. The branch facing down in the middle is the outlet of recirc BOV ( which is 1" ID FYR ). The elbow on the right leads to compressor. You can see the jet is quite close to that. The following picture shows it better: http://home.pchome.com.tw/personal/r...JetonElbow.JPG It's about 6" to the end of the elbow, another 2" to the compressor blades. I took 0.5mm jet on the pre-comp injection, 0.6 for pre-throttle. ( I ran 0.7 on pre-throttle only previously ) Since I have no check valves on hand right now, so I can just T the two jets directly. I know there would be some cross-flow & siphon, but I can't do anything about it. So... I arranged bigger jet on the pressured side, hopefully it could offset the cross flow somewhat. (I may be totally wrong & just fool myself) Yet another thing that's not so optimum is that the pre-comp jet is quite far from HSV, about 3ft. Anyway, it works! Lukily I didn't experience the WOT bog down as Gelf. Yet not so lucky as Hodrod to get a rise in boost. Car just pulls harder on boost, especially in the midrange. The performance gain is easily sensible with a quiter turbo hiss. This is really a very effective & worthy mod to me. Two thumbs up. Now I'm just waiting for Richard's mail of my check valves & other accessories to make the whole thing complete. Thanks every contributor on this thread :wink: |
I have been following this thread with great interest. I am trying to develop a WI process for the duramax TDiesel with 30 lbs of boost. I have had initial reports that HP increases with it are not as dramatic as gas, especially the "tuned" trucks. I'm not sure why this would be, maybe someone could explain.
In any event, going to use it to reduce EGT's that are quite high, and to aid tow vehicles that overheat (seems the CAC is imparting too much heat to the rad). Considering 2 stages. Stage 1-towing-15 psi-2 nozzles-post turbo pre-CAC. Stage 2-performance-throttle switched at the floor-2 additional nozzles, 1 pre turbo-1 intake Michael |
WI on a diesel
You can probably get some additional good feed back in the diesel forum as well as on this thread.
Just need some back ground info. I'm not familiar with the acronym CAC I assume your refering to an after cooler ? Most of the conventional wisdom is that injecting between the turbo and the intercooler/after cooler is less effective than injecting after the last stage of intercooling/after cooling. Are you using a pure water injection or a mixture of water and alcohol? Water and methanol mixtures are supposed to produce some very impressive performance gains and lower EGT's. The bad news is the bump in power is addictive and too much methanol can push the engine to the point of blowing head gaskets due to high cylinder pressures, but the increase in torque is supposed to be very impressive. The pre-turbo injection is most useful when your maxing out the airflow capacity of the turbo compressor at high rpm. Right now my system is set up as pre-turbo only, and triggered with a pressure switch, but I plan on changing over to a system that is a bit more complex. I'm going to add an rpm switch in line with the pressure switch so the pre-compressor injection only comes on after the engine gets to about 70% of max rpm. My experience recently also leads me to consider adding a temp sensor in the intake tract so that once the intake air temp drops below about 70 deg F. the pre-compressor injection is completely disabled. Larry |
CAC, charge air cooler, IC. The stock Garrett turbo has 25 psi stock, 30 when chiped. A few people have played around with WI only getting 15-30 HP, 5% increases, from the 300-400 that are coming from the stocker, with 1200-1400EGT's.
That is using WW fluid, with meth. Michael |
thanks
Thanks I figured it was and intercooler, but couldn't figure out the usage. Unfortunately each automotive enthusiast group develops their own common usage acronyms and some times the alphabet soup is hard to translate.
Second question, is your CAC an air to air system postioned infront of the radiator? If so then the heat transfer to the radiator would be due to the CAC pre-heating the air going through the radiator. Two options come to mind, first under the heading of keeping it simple, you could also look at direct water spray on the outside of the CAC or on the radiator or both, rather than injection in the air stream between the turbo and the CAC. The other way to take heat load off the cooling system is to add an oil cooler. The oil moves quite a bit of engine heat. On the WRX in stock configuration it uses an oil cooler that passes heat to the engine coolant. Folks have had good luck adding an air cooled oil cooler and passing that heat directly to the air rather than the radiator through the coolant. When you refer to "gas" are you talking about propane injection? The pre-compressor injection will only increase the max air flow by a few percent, so your 5% gain seems about right in that regard. As I understand the effects on diesels you need to add some fuel to make full use of the air that you are already flowing so if you upped your fuel flow and then held the EGT temps in check with WI you should get more power than 5% Larry |
I must apologise for being slow to get my testing done - but a combination of a dead laptop (couldn't data log) and poor weather here in SW England has slowed me down. Doing 3rd gear WOT power runs on damp/wet roads isn't possible (it's dangerous and wheel spin will mess up the power/torque calcs) - so I am waiting on completely dry roads - or at least for my test stretch of road to be dry.
Hopefully this will happen by next weekend and I'll get the tests people have asked for done. I'll either post details of each test with torque and power figures or post complete comparison power/torque graphs overlaid on a website. PuntoRex's findings are encouraging. |
Re: thanks
Quote:
I guess I am not clear why water-meth solution added pre turbo, vs post IC, results in less power added. |
Pre-compressor injection
Pre-compressor injection is a special case situation.
In a perfect world the ideal place to inject WI water/methanol mix would be after the CAC and as far as practical before the intake manifold. That gives you time for the spray to mix well with the intake, and you get the additional cooling of the air charge from evaporation . That additional reduction in charge air temp, increases the VE of the engine (ie you actually get more air in on each intake stroke for the same manifold pressure) On the other hand, pre-compressor injection is a trade off. By injecting ahead of the compressor it increases the mass flow through the compressor so it is very helpful on engines that have undersized compressors. The down side of pre-compressor injection, is that it reduces the effeciency of the CAC slightly. An intercoolers heat dissipation rate is controlled by two things, the mass flow of cooling air through the intercooler, and the average temperature differential between the charge air and the cooling air. Cooling varies as a log function of the temp difference. If you inject the WI mist prior to the compressor you end up with a cooler discharge from the turbocharger for a given boost condition. That means a smaller temperature differential between the charge air and the cooling air flow --- result, your manifold air temp can actually be slightly higher with pre-compressor injection only, than it would be if you injected the same amount of fluid in the post CAC location. If you have an adequate sized turbocharger compressor using pre-compressor injection is not as productive as the same injection rate post CAC. On a car with a significantly undersized turbocharger, the increase in mass flow through the turbocharger out weighs the slight reduction in cooling from the intercooler. Hope that makes sense? That is why with my new turbo I will be limiting the pre-compressor injection only to high temp, high flow situations, as at moderate boost and engine rpm the turbo has plenty of flow. Larry |
I completely understand your explanation, nicely done. As a Chemical Engineer, I have a good working knowledge of the thermo dynamic.
Now the balancing act for my unique application: How to design one WI system that serves 2 purposes. Cooling enhancement for hi-load towing focusing on lowering CAC and egt temps, plus a second (stage 2)performance aspect (has to be fun afterall :lol: ). Question: Can you make a guess at how much flow to dedicate to pre-CAC, post turbo misting to cool that charge 100 degrees (350 to 250) on a 30% RH day? I assume it would be the same flow, whether pre-turbo or post-turbo? It's a 6.6 l motor (diesel). Diesel posts seem to suggest 400 cfm moving at full tilt, but not sure on this. Using your rationalle, I don't want to overfeed this stage, just kill the fire a bit. Save the big water for the 2nd performance stage. Unless you have another idea. |
quick ball park computation
Well I used a 3 and a 4 gal/hr nozzle on my pre-compressor WI setup on a turbo that was rated with a max flow of 360 CFM. The 4 gal/hr was a bit too much, and the 3 seemed to be about right. Since I was pushing that turbo very hard a lot of that water went to cool the compressor discharge to sane levels.
In your case you have: 6.6 L displacement @ 30 psi, lets assume 4000 rpm --- engine needs about -- 6.6 L = 403 CID x 2000/1728 = 466 CFM, at 30 psi equals a pressure ratio of `approx 2:1 . Your engine should need about 932 CFM at 4000 rpm or 64.3 lb/min of air flow. (this is air flow into the compressor inlet at atmospheric pressure). Not sure what the VE for a diesel engine is so this is at 100% VE, for a gasonline engine you'd mulitply by about .85 to get a real world air flow. If we do a quick on the napkin computation of the specific heat of that air flow we get: 64.3 lb/min x .024 btu/lb x 100 deg F delta T = 154.3 BTU/min deg F Latent heat of water = 970 BTU/lb deg F at atmospheric pressure. (it will be a little lower at high pressure but this is close enough for a ball park computation). To absorbe that heat flow you, would need to evaporate about 0.16 lb of water /min or 73 ml/min of water. That would be just over a 1 gal/hr water flow, or about the same as an Aquamist 0.4 mm nozzle at 40 psi line pressure. Not trying to be condescending by working it out, but just wanted to show where the numbers were coming from, so everyone could follow the reasoning. Larry |
I;m not sure if this was disccussed but can i run straight nethanol and still inject Pre-turbo?
Will it have any effect on the turbo or the inlet temperature? |
yes sort of
Yes you could inject pure methanol pre-compressor, but you would get more cooling effect with a water methanol mix.
Larry |
Larry, excellent work on explaining the math. How about another?
How much water does it take to bring 900cfm from 10% humidity to 100% humidity? What is the resultant temp if ambient is 120 degrees, assuming a 70% efficient, 30 psi turbo. |
humidity and air at high pressure
I can't give you an exact number as I lost a reference book I used to have on compressed air systems. If you need an exact answer you might try calling a company that specializes in industrial compressed air systems. They have to work out the water weight in the compressed air to design, drying systems so the user does not end up getting wet compressed air at their tools.
I found one reference that gives some ball park values on a chart of lb water in lb air at different temps and pressures. Airs ability to hold water vapor changes significantly at higher pressures so it is not a simple relationship. As you can see below as pressure goes up, the amount of water vapor the air can carry at a given temp goes down. Also as the air temp goes down so does the water capacity. Bottom line if you saturate the air going into the compressor, as it leaves it will be quite dry, while it is still hot, but after being cooled in the CAC it will become super-saturated as the temp approaches the compressor inlet temp. In other words if you had a transparent intercooler you'd probably see a dense fog going through the throttle body. A secondary conclusion from the above fog formation on cooling just came to me ! The temperature where your charge air becomes supersaturated and forms a water fog will be a hard limit to the minimum temperature the intercooler can reduce the charge air temp to. Once water fog begins to form, the latent heat of condensation of the water vapor released as the charge cools will exactly balance the heat taken out by the intercooler. This is likely the reason pre-compressor injection can appear to reduce the effeciency of the intercooler. 900 CFM would be about 63 lbs / min air flow at 1 atm From the chart in the Chemicals Engineers handbook (Perry and Chilton) 5th editon, Fig 3-4 Air at: 1 atmosphere and 100 deg F holds 0.04 lb water / lb air 2 atmosphere and 100 deg F holds 0.03 lb water / lb air 3 atmosphere and 100 deg F holds 0.015 lb water / lb air 1 atmosphere and 125 deg F = 0.09 lb water /lb air 2 atmosphere and 125 deg F = 0.045 lb water /lb air 3 atmosphere and 125 deg F = 0.03 lb water /lb air 1 atmosphere and 200 deg F holds > 1 lb water / lb air 2 atmosphere and 200 deg F holds 0.43 lb water / lb air 3 atmoshpere and 200 deg F holds 0.23 lb water / lb air Assuming your intake air is completely dry: If you take a small turbo like I have on the WRX which has a max flow of 35 lb / min, and the inlet temp is 100 deg F, and the exit temp from the intercooler is 125 deg F (at 2 bar boost = 3 bar absolute pressure), that 35 lb of air could hold a max of about (.03 x 35) or 1.05 lb or water per minute. That is just over 475 cc/min of liquid water completely evaporated. If your intercooler is heat soaked, then that same air flow at 200 deg F and 3 bar absolute would hold almost 8 lbs of water per minute or nearly 3654 cc / min of liquid water completely evaporated. We know that the pre- compressor injection drastically cools the intake charge, so if the inlet air is cooled to 50 deg F by the water evaporation before it hits the compressor, that cool air can only hold a max of about 0.008 lb water / lb of air, or .28 lb of water or 127 cc/min will be able to evaporate before it arrives at the compressor. (that is equal to about .8% of mass air flow). If the air leaves the compressor at 200 deg F and 3 bar absolute pressure then on exit it could hold a max of 0.23 lb water / lb air. That would equal 8.05 lb water / lb air (3655 cc/min). If your injecting at 3% of air flow then you injected only 1.05 lb water, so the relative humidity of the air leaving the compressor will be less than 13%. Now you cool it down to 125 deg as it goes through the intercooler and you have that same 1.05 lb water but it is traveling in an air stream that is now is only capable of holding about 0.03 lb water / lb air, or 1.05 lb water --- surprise surprise your at 100% humidity as the air leaves the intercooler. ( I didn't plan this to come out at exactly saturation but I'll take the lucky break ;) ) For your example of 900 CFM you have 63 lbs/min air flow in. If you get 30 psi boost ( 3 bar absolute) and it exits the CAC (intercooler) at 125 deg F then at 100 % RH it could hold 0.03 x 63 = 1.89 lb water / min or 858 cc/min at saturation. Someone check my numbers but I think that is all correct! Larry |
This is so funny, I was browsing threads (after posting earlier today) and came across this. Said to myself, this is the response to the question I posed earlier. It took 5 minutes to figure out that this was the same thread!
Now I have to go back and read it better. Thanks Larry. |
Yes, the hot air really holds a LOT more water. Then it stands to reason that, if misted efficiently, more total water can be evaporated post-IC only, vs pre-turbo plus post-IC??
Does the turbo performance enhancement of pre-turbo misting make it better performing anyway? |
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