Turbo Efficiency Discussion

[AleroB888]

TGP37,

Water injection does not effect the compression ratio (as defined by the inlet area vs the outlet area). If anything it increases the compression ratio slightly (though I seriously doubt it) as the total volume of compressible gas is decreased.

The volumetric efficiency would decrease at the same time, since the water in the charge reduces the effective (combustion chamber + cylinder) volume.

As far as taking GM's research with a grain of salt, that also applies to manufacturer's research when they are trying to sell you a product.

So many words, so little time.........

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[AleroB888]

The ONLY reason you put anything in the water is to prevent freezing. In fact some people use blue washer fluid as it is water with alcohol and some blue coloring in it. Of course it has the ability to leave behind the blue coloring and clog jets etc....

In practice pure alcohol as a method to cool the intake quickly is better, alcohol evaporates faster than water. Water is also added to make it a non-flammable mixture for safety in some installations.

[TGP37]

In practice pure alcohol as a method to cool the intake quickly is better, alcohol evaporates faster than water. Water is also added to make it a non-flammable mixture for safety in some installations.

What would qualify as safe when dealing with pure alcohol? Noting it is more effective, less can be used to achieve the same result versus that with water.

Only issue I see is having a BOV exhaust the alcohol vapor into the atmosphere. But if a recirculation bypass was used instead, redirecting the alcohol soaked aircharge back through the turbo compressor, then it should be safe. As well as cooling the compressor with the recirculated alcohol vapor which could be convenient immediately after boosted runs.

[NateD4]

TGP,

I think you are mixing a few things up. The methanol has a combustion redox reaction, which essentially produces CO2, H2O and as mentioned some CO. However, this is simply normal combustion for Methanol (and ethanol). You do generate some steam pressure from the combustion of the alcohol (just as you would for gasoline).

If we talk pure alcohol engines there is a reason racers use methanol. It has a lower boiling point (requires less energy to boil), but compared to gasoline has a higher vaporization temperature and energy. It also has a lower heat content so the effect is you need more alcohol to carry the same stored energy into the combustion chamber compared to gasoline. With more fuel comes more energy required for vaporization and thus more cooling effect.

Additionally I'm sure the chemical kinetics of the alcohol molecule are more stable than that of gasoline which further helps to reduce detonation.

When you use a 50/50 water/alcohol mix I'd consider the portion of alcohol to be fuel, not anti detonation fluid. SO if you were to do any math on it you'd effectively be doing a three species, three phase redox calculation as partial fractions of the total fuel/air mixture. So you'd have the gasoline and air, the alcohol and air and the water. I'm not quite sure how the equilibrium would balance temperature and internal energy wise between them, but overall you could probably assume an average for high level estimation purposes......

[NateD4]

Alero,

You are right... conservation of mass upholds. you can only squeeze so much matter in the space available. So when you add water to the intake tract (or fuel) you displace some of the volume of air with water and change the local gas (state) properties. By itself this will probably negatively impact VE, however with the cooling effect able to contract the gas volume it probably enhances VE as any inter cooling process does. It really is a ratio of cooling vs volume though... there is some break even temperature difference...

[Joseph Upson]

Tech talk can become overwhelming and hard to follow in the absence of pictures. I'm working on getting some realtime results regarding efficiency as it relates to boost and back pressure right now. A simple tap into the exhaust to measure back pressure trends.

The liquid to air intercooler will have to wait until funding arrives for it due to more pressing expenses that accompanied the reassembly of the forged motor. In the mean time I'll be collecting data to compare when it is installed.

[TGP37]

Tech talk can become overwhelming and hard to follow in the absence of pictures. I'm working on getting some realtime results regarding efficiency as it relates to boost and back pressure right now. A simple tap into the exhaust to measure back pressure trends.

The liquid to air intercooler will have to wait until funding arrives for it due to more pressing expenses that accompanied the reassembly of the forged motor. In the mean time I'll be collecting data to compare when it is installed.

That is fantastic, awesome. I planned the same thing, but the mini-project is on hold for now. I even used a coil of brake line to prevent the sensor from heating up too much.

I used steel line.



I have been investigating the headers impact on turbo efficiency. It is amazing how drastic a difference there is between NA and turbo. Even differences between turbo and supercharged...

[Joseph Upson]

The car is limited to 3 psi of boost and so far under load exhaust back pressure is gradually maxing out at about 10 psi by 5000 rpm. That has more to do with the exhaust system than the turbo. I can blip the throttle for an immediate ~8psi pressure increase but it drops off to 0 with sustained rpm below 4000. I believe the brief increase is due to the time it takes the turbine wheel to spool up to exhaust flow speed and the actual pressure build up under load due to the exhaust system itself.

I have two turbo packs welded together, the 3" pipe out of the turbo joins in between them terminating into the 4" diameter short pipe that joins them. They have about a 2.25" inlet and are baffled although they look like common glasspacks. There are two 1.75" offset pipes internally and that is where the pressure build up is likely occuring. Unfortunately the 5" diameter case version still used 1.75" offset tubes instead of moving up to 2" internal tubes so I decided on the 4" case.

The car sounds great and the drone is gone which is what I was after from the start. I'll be adding a cutout at the center joint to allow open exhaust flow. The car is amazingly fast even as the rpm and back pressure starts to climb and I'm curious about whether the backpressure is responsible for the apparent city driving fuel economy improvement. It doesn't guzzle even when running it hard but I suspect that's a combination of the car feeling as fast as before on nearly half as much boost. Once it's turned up to 7 psi things may change.

[NateD4]

Any dyno numbers available? I'm interested in BSFC numbers and general HP numbers. I'm working on a similar build. Though I'm de-stroking... curious how much boost you were able to add before detonating.

[Joseph Upson]

Any dyno numbers available? I'm interested in BSFC numbers and general HP numbers. I'm working on a similar build. Though I'm de-stroking... curious how much boost you were able to add before detonating.

The stock motor did 15 psi on premium without excessive timing retard, the HO motor is still on restrictions for a nice an easy approach to the intended boost level. I don't want to have to build this thing again over a preventable accident. The most I've run so far is a tad over 3 psi and so far that's been pretty impressive compared to the stock motor on 15 psi mostly because it has a top end. I just have to get back home and add a free flow exhaust option to see how much more it has in it at the current boost level. I hope to get it to the track or dyno next month. So far it runs great.