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Mart 19-05-2004 07:39 PM

Injecting prior to turbo comp' impellers
 
Hi there,

I've recently installed a charge-cooler system to my car, running ERL's 1s water injection system. I used to have an fmic with the water jet located on the outlet end-tank, which worked well, but because of the short pipe-run with the C.C (approx 8" from turbo to C.C inlet, approx 12" from C.C outlet to plenum), I'm undecided where to locate the jet now?

As I don't really want to insert the jet on the C.C outlet pipe, as I think this would be too near to the plenum (agree/disagree?), and it isn't really possible to have the jet on the C.C inlet pipe/turbo compressor outlet, I was wondering if it would be ok to have the jet/mist firing into the turbo compressor intake air-stream; ie, between the air-filter & compressor housing.

Do you think this idea is feasible? Has it been tried & tested before? (with good results?)

If so, which jet size would be a good starting point, and at what boost threshold level should the jet/pump be activating?

Apologies for all the questions/long-winded post.

Just for reference, the turbo being used is of the T28 variety running @ 25psi, and I have 0.4, 0.5, 0.6 and 0.7mm water jets to hand.

Thanks.

Mart

Charged Performance 22-05-2004 03:46 PM

Depending on your plenum and intake runner lengths 8-12" may be sufficient to get a good uniform water injection/air charge mixture.

There are a few posts around about injecting before the turbo compressor/impeller. Historically it has not been good for the blades. Some think improved materials along with improved jet atomization may have negated some of the prior experience. I think in general though you will not find many people willing to risk injecting in front of their turbo.

bill Shurvinton 10-07-2004 10:41 PM

Sorry to reawaken this, but I just want to put the record straight on this.

Pre-compressor injection offers a host of benefits. In a nutshell, injecting water (and ideally a few other miscible fluids of high specific heat capacity) pushes the compression from adiabatic to near isothermal. This is way more efficient (up to 30%). So you can reach the same boost for considerably less exhaust flow.

Yes atomisation is critical, but the technology is there.

DuMaurier 7 18-07-2004 01:05 AM

I have been spraying into the turbo for more than a year WITHOUT any problems . I am using an 0.9 nozzle and a system pressure of 100 psi , this produses a spray with very fine atomisation.

JohnA 18-07-2004 11:03 AM

Quote:

Originally Posted by bill Shurvinton
... In a nutshell, injecting water (and ideally a few other miscible fluids of high specific heat capacity) pushes the compression from adiabatic to near isothermal. ....

Can you expand more on this mate?
I find it interesting... :)

bill Shurvinton 21-07-2004 12:13 AM

Quickly. Normal compression is adiabatic, which to a first order means that as the gas is compressed, it gets hot. Now this heat is one of the biggest problems with forced induction, for 2 reasons. Firstly you have to get rid of the heat, and secondly you need to take power out the turbine shaft to perform the heating.( heat is work and work is heat).

Now with the right level of water injection, the heat is removed before it builds up, pushing the compression closer to isothermal (not all the way, but closer). In round terms this is about 30% more efficient (less exhaust gas required for the same boost, or more boost at the same exhaust flow).

problem 1 is that water on its own, whilst having a very high latent heat of vapourisation, doesn't have that good a saturation partial pressure. You can only put so much in before the air is at 100%RH. Above this the water will not cool the air until you compress it in the engine. However, if you add a second fluid, say methanol, Dalton proved with his law of partial pressures that the methanol doesn't know that the air is saturated with water vapour and goes on to vapourise as well. Add a 3rd, such as acetone (all available chemicals) and you get a 3rd tranch of cooling.

The net result is that you may be able to cool the air to slightly below ambient with the right mix. So your compressor becomes more efficient, you can throw away the intercooler, increasing your flow, and in some cases significantly improve the flow from turbo to inlet.

If you leave the intercooler in, you are generally warming the air back up again, so you have to take the plunge and remove it to gain the best benefit.

Its good, very very good.

Bill

JohnA 18-08-2004 06:31 PM

Bill,

your theory sounds very exciting (to me, at least)

So to recoup, you're saying that injecting before the compressor actually increases the efficiency of the compressor - leading to less heat in the charge as it leaves the compressor.

You're also saying that injecting a water/methanol mix at that stage would be far better than just pure water, right?

I always thought that such a setup would decrease the intercooler's efficiency, compensating most of the benefits. You agree with this, to the point where you see the intercooler as a thermal liability

So what sort of injection volumes are we talking here?

hotrod 18-08-2004 10:18 PM

spray point
 
Yes spraying before the compressor impeller has its place.

As stated above, it improves the effeciency of the compressor. Many people tend ot look at WI from an engine centric point of view. If they cannot infer a direct benefit to the engine they assume it is a bad idea.

You are working with a complex system of mechanical devices that interact with each other in many ways. Even though on first blush injecting infront of the compressor or between the compressor and the intercooler might appear to be less effecient you need to account for ALL the interactions. In many cases we simply don't have enough information to predict the results so frequently experimentation will give you better data in a matter of minutes, than all the incomplete computer simulations you can afford.

Injection in front of the compressor accomplishes several things. A turbocharger is a constant pressure variable volume "DYNAMIC" compressor.

A turbocharger only knows 2 important properties of the gas it is compressing. The density of the gas at the compressor inlet and the pressure ratio it is operating at, which is determined by the rotor rpm and the gas density. If you increase the pressure or reduce the temperature at the inlet you will modify both of those parameters. In both cases (increased inlet pressure, or lower inlet temperature) you increase the apparent density of the gas passing through the compressor. At a given rotor rpm with a given gas density you will flow a very specific volume of gas and it will be compressed to a specific pressure ratio on exit. That is what the compressor map is based on. If you change the inlet conditions (gas density) you in effect slide the compressor map left and right. This is the "corrected flow" of the turbocharger.

By injecting water/alcohol ahead of the compressor two things happen. You cool the inlet air substantially, this in effect moves your true operating point to the left on the compressor map. (in most cases for max performance this is a good thing, although on some turbocharger conditions it can cause compressor surge.)

You also change the pressure temperature profile inside the compressor wheel itself. You probably actually change the shape of the compressor map. As the gas moves outward and is compressed, heat that would have gone into heat and increased pressure is absorbed by the WI mist and so the compressor has less work to do since it is no longer fighting this temperature driven pressure increase, it can achieve more mass flow at that pressure ratio. The cooling should also modify the speed of sound in the gas and the mach number of the compressor blade tips should also change. This should change the choke flow characteristics of the compressor but I don't have the information to comment in detail on that.

Net result is, you increase the mass flow through the compressor --- in effect you make it act like it is bigger than under normal conditions.

During WWII this was the way ADI (anti detonation injection --- the common term in the aircraft world for WI ) was set up on military aircraft in most cases. The water and the fuel was injected into the eye of the centrifugal supercharger.

Errosion of the compressor blades is not a problem if steps are taken to ensure the mist is very fine when it arrives at the compressor inlet so that it follows the airflow and does not imping on the blades with a high differential speed.

The ideal is to get drop size down as close to 10 microns as possible but due to the brief periods of use and intermittent nature of most WI systems, in reality you can live with larger drop sizes in real world systems.

If you have ever ridden a bicycle or motor cycle in a rain storm you know how sharp the impact of a large water dropplet can be, but a fog or drizzle will not cause the same painful experience because the droplets are small enough they are strongly influenced by the direction of flow of the air stream around you and impact with much less velocity and obviously also have lower momentum.

For people in hot dry climates that lose a lot of turbocharger performance in hot weather, pre-turbo injection should be looked at.

In regard to the effects on the compressor mass flow, maximum results appear to occur with mist flow of about 3% - 10% of the air mass, so you will likely need to inject additional WI near the throttle body to reach maximum detonation suppression and best power.

Larry

JohnA 19-08-2004 09:19 AM

Re: spray point
 
Quote:

Originally Posted by hotrod
...By injecting water/alcohol ahead of the compressor two things happen. You cool the inlet air substantially

If you mean that the ambient temp air flow is affected, I wouldn't expect that to be much. Say ambient is 20C, how much lower can it get by injecting mist of water that's 30C+ (the bottle is in the engine bay!). I suspect that after the pump the water is even warmer.
Quote:

this in effect moves your true operating point to the left on the compressor map. (in most cases for max performance this is a good thing, although on some turbocharger conditions it can cause compressor surge.)
I would have thought that this 'efficiency gain' is the effect of water droplets inside the compressor blades, as they try to squeeze the air.
Am I right?
Quote:

.You also change the pressure temperature profile inside the compressor wheel itself. You probably actually change the shape of the compressor map.
This one actually
Quote:

... it can achieve more mass flow at that pressure ratio. The cooling should also modify the speed of sound in the gas and the mach number of the compressor blade tips should also change.
fascinating :!: ...

hotrod 19-08-2004 03:54 PM

cooling
 
Quote:


If you mean that the ambient temp air flow is affected, I wouldn't expect that to be much. Say ambient is 20C, how much lower can it get by injecting mist of water that's 30C+ (the bottle is in the engine bay!). I suspect that after the pump the water is even warmer.
Quote:

Actually it can be very substantial. The cooling is by evaporation, and typically will exceed 10 - 20 deg C . The initial temp of the water is of little consequence. The cooling due to evaporation (latent heat of evaporation) is very much larger than the latent heat of the liquid water.
As you can see below the evaporation of a gram of water will absorb 542 x the heat energy required to lower that same quantity of liquid water one degree in temperature. That means that even if the water were nearly at boiling temps when injected, evaporative cooling would reduce the air flow temp below ambient temperatures long before all the water changed to vapor.

Specific heat capacity of liquid water - 4.187 kJ/kgK
Latent heat of evaporation - 2,270 kJ/kg

Since the evaporation of the water and the alcohol will essentially stop when the air becomes saturated (ie. 100% humidity) there is a practical limit to the cooling or around 20-30 deg C for alcohol water mixes, and in real systems you seldom get much more than about 15-20 deg C.

For every 5 deg C you drop the inlet air temp you will increase mass flow by about 1% due to density increases.


Quote:

I would have thought that this 'efficiency gain' is the effect of water droplets inside the compressor blades, as they try to squeeze the air.
Am I right?

Only in a very crude sense. Your thinking in a mechanical piston pushes on air sort of way, but what happens inside the compressor impeller occurs at a molecular level.

Think of it this way If you could freeze frame time, and stop what was happening inside the impeller while its spinning at 120,000 rpm. Each impeller passage between adjacent pairs of compressor blades contains a wedge shaped parcel of air. When spinning at 120,000 rpm the air is subject to huge centrifugal forces as it moves away from the hub of the impeller and toward the rim of the compressor. The trapped air would like very much to be slung out of the impeller but like a crowd at a stadium after a match it simply cannot all get out as fast as it would like. As a result it stacks up (compresses) as it gets near the exit. In this process a lot of internal friction occurs. The air near the tips of the compressor might be moveing near the speed of sound at maximum flow, this heating makes the air try to expand. This increases the pressure which fights the outward movement of the air. Eventually a balance is achieved between the centrifugal forces trying to throw the air out of the impeller and the pressure build up due to the compression and the pressure build up due to the heating. The addition of the water mist removes a very large fraction of the pressure gain due to heating. As a result more air can exit the impeller over a given period of time, and more of the pressure gain is real compression rather than waste heat. The net result is a more isothermic compression which is always more effecient than an adiabatic compression.

Larry


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