Injecting prior to turbo comp' impellers

[Richard L]

You are correct on the MAF location in general but some aftermarket bolt-on systems do have the turbo before the MAF.

I think we shall see slowMX5's results to make this discussion more meaningful.

[b_boy]

I'll be tuning soon as well and post numbers. We will see what we get. Maybe Hotrod can be presuaded to throw a thermocouple in his set up and run plus/minus WI. He should see definitely see a drop in temp, but will he see an increase in boost is the question.

It's a tough experiment to compare because of detonation.

[hotrod]

I don't have the resources to do the thermocouple test, but I know the result. Several folks have run similar tests and the temp drop is on the order of 30 - 40 deg F for common WI injection rates. For straight water injection with no alcohol the temp drop will be about 20 -30 deg F, depending on the relative humidity.

As far as the boost, I already know the answer to that question, My electronic boost controller has a peak hold feature that tells me the maximum boost achieved on a given run. I had to turn down my boost controller to avoid over boosting after I installed the pre-compressor injection.

The mass flow through a centrifugal compressor varies directly with the absolute temperature of the inlet air. Lower the inlet temperature, mass flow goes up. Its the nature of the beast.

Larry

[ryan]

HI,

Arch my brain cell herts, na just kidding love this thread,

I was thinking that when you inject water post compressor, this cools the intake temp,

but does this cool it enough to reduce the volume (less heat less volume) when the injected water mass/volume is included,

if so are you not getting the same effect , ie compressor pumpping more air but is hot, then its cooled back down and dropped psi (less volume same mass+water mass) to the same mass/volume as the preturbo injection goes?. or fairly close to it, ie both will have water taking air space, and have lower temp-more air mass.


just a thought, sorry if this sounds dumb to you, but mayby has some relivance to this topic as far as how much more air will be moved with pre-compressor injection vs post compressor injection.

although preturbo would make for good mixing/evap of injected liquad.

and the lower temp in the compressor exit would make for less volume(same mass), so there would be less pressure for the compressor to fight at the same mass flow so less heating and more efficenecy , hmm O.K. I'll stop typing now.

Cheers
Ryan

[Greenv8s]

I've been watching this thread with interest, thanks for such an informative discussion guys.

I don't have any practical experience to contribute, but a comment by Hotrod has resolved the main issue that had been puzzling me.

I couldn't see why it made any difference whether the air was allowed to heat up in the compressor and then cooled by downstream injection, or whether upstream injection was used to prevent the air from heating up as much in the first place. At first glance it should not make much difference since a similar amount of water is evaporated (OK maybe *slightly* more evaporation in the upstream case due to the turbulence inside the compressor) so the final charge temperature should be similar. I don't think anybody has suggested any reason why the compressor efficiency would be improved by upstream water injection, and I can't think of any.

However, Hotrod has pointed out that the compressor works more effectively when it has denser air going through it. By reducing the air temperature rise inside the compressure, the upstream injection increases the air density inside the compressor and therefore increases the effectiveness of the compressor. (I think the distinction between effectiveness and efficiency is important in this case.) This means the same compressor will produce a higher boost pressure and greater mass air flow with upstream WI than with downstream WI, which I think was the original contention. My mistake was to assume this resulted from increased efficiency; I don't think it does.

Does this make any sense, or am I still missing the point?

[hotrod]

That's a good summary of the situation.

It also has a secondary impact that is not often discussed. If you increase the effeciency of the compressor, it takes less power from the turbine side to do the same work. The result of that, is the turbo spools faster and at a give boost pressure it will produce less exhaust gas back pressure.

What you have is a situation were several small, and complementary incremental improvements in the performance of the entire system make a noticable difference.

On my car, since the stock TD04L-13G turbo on a WRX is undersized by the factory, to get good midrange response. Added to that since I live and race at 5800 ft altitude, the effects of operating at high altitude made the workload for the turbocharger even greater.

The seat of the pants improvement in turbo spool and improved midrange power and torque were very noticable.

Larry

[b_boy]

Studying the thermodynamics is dizzying. There are so many variables, I just cannot predict the outcome of measurement. Evidence from multiple sources makes me believe that the turbo will actually compress more air, but proving this with heat in the mix will be difficult.

I see two ways to measure an effect. One is to tune to knock with only WI at the throttle body, and then turn the water on pre-turbo and tune again to see if more power can be had. This only establishes that more power can be had. Perhaps injection of water immediately post-turbo would have the same effect? I'm not willing to do that control experiment.

The second way is to measure pressure and temperature post turbo with and without WI. We might hope to rely on the ideal gas law to compute a gain over and above mere evaporative cooling. Since on a mass basis we are only adding a small percentage of water, the molar contribution to the gas's mass flow should be minimal when computing pressure, as well, any effect on the R value should be mininal as well.

Using PV=nRT, we will be solving for n, number of moles, or an increase in the mass of the air compressed. We will have to adjust any pressure reading by compensating for the compressed gas temperature post-turbo. If there is a difference of more than a few percent at near redline RPM we should see it. Any ability to increase power without knock will be only confirmational.

I'm buying a hypodermic thermocouple to stick into the hose that runs to the intercooler. I don't know where the stock pressure sensor for boost is, preferable before the intercooler. (Do you know hotrod? I'm in a WRX STi.)

[hotrod]

Sounds good to me.

The MAP sensor on the WRX is on the throttle body passanger side right above the throttle shaft. It has a three pin connector and is held in place with 2 bolts.

Larry

[Greenv8s]

As far as I understand it, the theory is that upstream water injection enables the compressor to achieve greater mass air flow (by reducing the temperature rise and loss of density that occurs inside the compressor).

I don't think it is possible to prove this theory by comparing with/without water injection, because we already know that water can be beneficial wherever it is injected.

The only way I can see to prove the theory is see whether we get more mass air flow using WI immediately upstream of the compressor or with the same amount of WI immediately downstream of the compressor. By concentrating on mass air flow at the compressor I think we can avoid complications from other changes in the tune. To be conclusive, we would need to ensure that a similar amount evaporates in both cases. I think it might be possible to achieve this by using only a small amount of WI so that we get total evaporation in both cases, does this seem reasonable? Since mass air flow is the thing we are looking for, it would make sense to measure that directly if possible, rather than trying to infer it from changes in temperature and pressure or detonation, net bhp etc.

[b_boy]

I think we agree. I'm talking about computing the number of moles (mass of air) and you are talking about mass flow, but we are both talking about mass.

In the end we are asking: does the compressor carry more air particles if cooling occurs during compression, than if cooling occurs after compression?

I believe the answer is yes, and here is my lastest thought experiment. It's gets right to the heart of the matter: number of air molecules compressed per rotation.

Imagine this case. The ambient air is -20C, when compressed this air, with each turn of the compressor wheel, contains more air molecules (cold is denser) than if the air were at 20C (less dense few air molecules/unit area).

Likewise, if during compaction, temperature (i.e. kinetic energy) is kept constant, the air being compacted will be more dense, and hence again with each turn of the compressor more air molecules are compacted.

The work that we are interested in is the work of compression, which is all about the pressure ratio given by the centrifugal forces than are acting on the air. Imagine that each time the impeller blade swings by is analogous to a shovel full of air. If each shovel full wieghs twice as much, then you end up with twice as much air shoveled per unit time. Similarly, with a pressure ratio of 2 (isothermic) each swoop of the impeller results in a compression of twice as much gas.

If without water the turbo is compressing air at a PR of 2, and we could completely suppress all heat during compression, we will have gained back all of the adiabatic heat loss and all of the compressor's inefficiency. The percent gain will be directly related to the PR. The larger the PR, the more gain can be had for each compression event--just like the shovels of air, 4X density air is more than 2X air.

As has been said before, the gain could be huge (40%+) if all the heat is absorbed by a small amount of water. To tie this back to yesterday's discussion. We would need a much larger turbo to compress say 40% more air, that was then cooled by WI post injection. It's all in the air density per rotation of the compressor--more dense air (cooler) leads to more air molecules compressed per rotation of the impeller.