Intercooling turbo compressor directly to boost flow

[obanion]

A compressor moves a certain VOLUME of air. Even though most compressor maps list airflow by mass (lbs/min), it's actually a matter of VOLUME (CFM), that has been translated to mass by assuming a certain temperature and pressure (~25C and 1 atmosphere).

So, what this means is that a compressor that maxes out at say 55lb/min airflow on it's compressor map, should be able to flow about 10% more (60 lb/min) if the inlet temperature was 0C (0C air is about 10% denser than 25C air).

Right?

I imagine another big part of the limitation of the compressor is the amount of force of stacking up the air and compressing it. Although the ENGINE benefits from a intercooler, the compressor really doesn't care. It's still working hard, and making hot temps. These hot temperatures which result naturally from compression, work to expand the air, while the compressor works to contract it.

So, what if you could "intercool" the compressor itself? Add a mist of methanol/water to the inlet, which would vaporize and cool the air as it passes through the compressor? I'm still unsure on this, but wouldn't that mean a even bigger jump in the efficiency and upper limit of a given compressor, seeing as how it has to work against the air far less?

My guess here is that a compressor works on relative volume in vs out. When you see a compressor map, and one section says say 75% efficient at 60lb/min and a 3 pressure ratio. What REALLY matters, is how much volume is going in, vs going out. You get a 3x reduction in volume through compression, and gain 30% volume through heat expansion (25C in, 150C out). That's a mass boost of 2.1:1.

Now, IF the out temp of the compression process was reduced to say 40C through methanol vaporization, look what happens:

Pressure ratio of 3:1 remains
Temperature gain now only expands the air by 5%.

3*.95=2.85

So, if I'm not missing anything, the same compressor, operating at the same 3:1 pressure ratio, will be able to flow 2.85/2.1=35% more air if the air only heats up to 40C during the process instead of 150C.

Combine the gains of pre-compressor cooling to freezing temps, and the integrated cooling (actually heat gain reduction) during compression, and a compressor rated to only flow 55lb/min could flow: 55*1.1*1.35=81.675lb/min.

 

[ebola]

some second war aeromotive system had this cooling systems, but they reported fast compressor wear due to fuel droplets attack.It could be a reason why its not popular? For roots fuel draw seals rotors, but..those spool ten times slower than turbo...

 

[SBBlue]

What you are describing is a "inlet fogging" system, which is used quite commonly in power generation gas turbines.

And it does work pretty much as you suggest, although I didn't go through your numbers to see how they add up.

 

[obanion]

Wow!!!! Cool, thanks for the confirmation. I suspect it may not be quite as well as I calculated, but at least half of that effect will be more than enough for my highest goals.

It's kind of a deal with the devil isn't it? Increased compressor wear, but superb usage results. I guess I wouldn't mind changing compressor wheels every 10k miles too much, considering I'll do that about every 5 years with this car, and a standard GT37 compressoor is only about $75 anyway.

 

[obanion]

After a little searching, another good term to search under is "wet compression." Pretty much as I described it. If I can make 650RWHP through a compressor rated to max out at 55lb/min, I think that would prove the theroy.

 

[tbuelna]

obanion:

It is common practice in Champ Car racing to inject methanol into the compressor inlet. Intercooling is not permitted by the rules, so injecting methanol fuel (with its high latent heat value) into the compressor inlet provides a significant reduction in inlet manifold air temperatures.

Terry

 

[Andy330hp]

So to tie this in with a few other threads, as I am getting a little confused: If somebody was going to setup a turbo system, and space constraints prevented the use of an intercooler, would it be better to use water injection before or after the compressor? Does this answer change if methanol is used? Do either of these answers change if an intercooler can be used? Let's keep away from OEM, assume a weekend racer/street car, person is smart enough to keep fluid levels high, etc. Goal is maximum power, though if one can make almost as much power and consume a lot less water/alcohol I'd be interested to know.

 

[turbododge]

I have been injecting water/isopropyl alcohol on the turbine blades of my twin turbo 340 Mopar for 15 years now. This kind of system was very popular in the past, as it was much more difficult then to inject into the boost stream because of the high pressure. The blade erosion issue has always come up in discussion, but the only ones I have seen that had problems were either on stationary units that ran the injection constantly, instead of only under boost like in a street car, or if they used methanol in the injected liquid. Methanol is much more corrosive than isopropyl. My turbos have over 40K miles on them with no signs of erosion at all. I use about 1 gallon of water/isopropyl mix per 1000 miles. My system is also variable with speed and boost, so I don't overload the airstream with too much liquid at any time. It also atomizes better if you aim a single spray nozzle directly at the shaft of the turbine, the liquid usually is totally vaporized by the time it is 1/4" from the nut on my setup.

I can run with or without the water very easily, and if I am holding a boost level and turn on the water, you feel a very positive increase in accelleration. I am also running an intercooler, so this is beyond the intercooling affects.

If you do a good job of setting up the system, I would not be afraid to inject ahead of the turbo, as I have seen very good results that way.

 

[obanion]

Thanks for that info turbododge.

Couple things, you say you inject into the turbine, you mean the compressor right?

Secondly, I'm not sure methanol would make a difference. Any wear on the compressor occurs due to mechanical corrosion, ie the collosion of the liquid droplets with the fast moving compressor blades. There is no time or contact for chemical corrosion to occur, which IMO takes hours to days of contact to cause issues. If anything methanols lower viscosity would reduce compressor wear slightly vs water or isopropyl.

 

[turbododge]

Yes definitely on the compressor wheel, poor phraseology on my part.

I am not up on the viscosity/vapor pressure etc with methanol vs isopropyl, but I know I have seen methanol systems corrode the compressor wheel, but never an isopropyl setup do the same. Downstream corrosion is even more of a problem with with methanol, and I have even seen corroded throttle shafts stick in the carb base. Intercoolers will also sometimes condense out a small amount of water or alcohol in cold weather and the methanol will also go work on the intercooler. I would guess that the compressor gets wet enough with the methanol someplace along the line, either at cold start, cold idle or such. The corrosion I have seen covered more of the depth of the blade than impingement damage which tended to be very close to the entrance edges of the blades.

The other thing that goes with this is that all the other systems that had problems with corrosion also were on/off, non variable, systems, which can heavily overload the compressor at low rpms, cool air conditions. I have literally seen liquid drip from the compressor housing hose connections after a run.

I am sure there are lots of folks that have successfully run methanol in their setups, but for me the isopropyl makes it much easier and more reliable, with very little downside.

 

[patprimmer]

Even if the methanol does not corode more during engine operation, if the compressor turbine blades are wet with water methanol mixture, when the engine is stationary for an extended time, they will corrode.

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[overrange]

I use about 1 gallon of water/isopropyl mix per 1000 miles.
Assume "x" miles under boost with injection. For x = 1 or 2, if you calculate the heat of vaporization versus air intake mass, this has virtually no effect. A better cold air intake system would do more IMHO.

overrev

Dave at USI

 

[turbododge]

Overrange: We drive the car a lot, but spend a very, very small percent of the time at throttle positions that would turn on the water. If we do a 500 mile "cruise" through the countryside, it is very possible that we will not use any water at all. My system does not come on until 8 psi of boost, (out of 14psi maximum),and since the system matches water flow to boost and rpm, water flow at that point is quite low. I can tell you that a 340 CID V8 will accellerate very quickly at 7 psi, and it is easy to stay within that range by controlling the throttle. To use the full 14 psi on the street requires a very good road in a remote area as the tires can go loose at any time, in any of the first 3 of 5 gears, depending on the traction. Even if the water does come on, it is only on for 10 or so seconds per use (maximum) before you are way over 100 mph. If I am testing and tuning, I have used a much as a quart in a day.

Although I did not mention it earlier, as the question I was addressing was the feasibility of injecting into the compressor, is that my system is not primarily used for inlet charge cooling, as I have an intercooler for that. My system is to allow me to run decent boost, without detonating, on 92 octane pump gas. Without the water I will get detonation at 10 to 12 psi, depending on the gas and weather conditions. It takes much less water to reduce detonation (as long as you have an intercooler to cool the air) than it does to cool the air enough to make power. That said, if I run up to 10 psi with the water turned off, and then turn it on, you can feel the car pull harder, so I am either gaining from cooling of the air further, or because of a better combustion process in the cyliders. I don't know what it is, but you sure can tell if the water is on or off.

The car also has a very good cold air system on it that takes air in through the boxed wheelwells that a fed from spoiler ducts, with only 6" of ducting before the turbos, after the large K & N filters.

 

[obanion]

Some clarification of my usage intentions:

1. Only operate under boost, no spray during starting, idle, cruise, etc.

2. Be fully progressive. More than a straight ratio of methanol:air, the ratio will start lean, then get richer at higher boost, to counteract the higher charge air temperature increasing at higher pressure ratios. I'd calculate a perhaps 20:1 air:methanol ratio at activation, going to 10:1 at full boost. I have the delivery and fuel computer to do this. The idea is to add only enough methanol to achieve a static turbo out temperature, and not leave extra methanol to stream along as liquid. Since the primary fuel I will be using is propane, I'd like to preserve a totally vapor fuel charge.

 

[Teeps]

methanol is corrosive, so most metals with get EATEN by methanol. Which exact metals or if All I'm not sure.
Also, on several occassions I have seen turbo housings GLOWING ORANGE from heat after a high end run. I'm not sure I'd feel comfortable injecting a flamable liquid with air, into something that hot, mind you, before the engine air inlet, < HEH. You'd be ingesting pre-ignited mixture into the engine, resulting in SUPER HOT chamber temps, severe detonation, and No doubt engine damage.
Water injection does help, but it is no replacement for an intercooler/aftercooler. It depends how hot your induction is running in the first place, as to how effective water will really be. On an eaton roots blower, at about 8psi it only buys you about 2 degree's of timing, heh.
Having said that, if you do actually raise the efficiency of the compressor by cooling down air for/and aft, your going to be running more air into the engine at given shaft speeds, so depending on what induction systems your running adjustmens maybe be needed with your tune, unless you left a lage enough margin in your A/f ratio's. I don't know if your running EFI, if so, best solution is a blow through MAF (or other method) between the turbo and the engine, measuring actuall air flow regardless of compressor efficiency.

Good Luck!

 

[overrange]

Another of my 2 cents. Have owned a '87 Buick Turbo that I modified for drags and a '92 SSEi with SC that was street only (3.8L). The Tu had annoying lag but about twice the power of the SC. With intercooler I was able to run 21 PSI first gear then fold back to 18 PSI. ATR mod kit brought the HP up to about 380. The SC with no intercooler ran only ~7.5 PSI. Turbo itself went 115K miles until it was sold. SC had Eaton blower self destruct, lock engine and turn belts into linguine at about 75K miles.

If I were to build another, it would be Tu, have a larger intercooler, and no water injection. Anything you inject that is not air or fuel reduces the charge density.

Back in the late 70's we ran a 327 with dual turbos with a carb; fuel air mix thru the turbos. Ran high 10's. Bad mixture distribution problems and kept cracking heads at the siamesed ports. Much better with today's LS1 type heads where ports are equally spaced and tempting to brew up another.

Dave at USI