NOt to chase that rabbit, but I don't think anyone's saying EGR increases pumping losses-- just lowers peak combustion temps and pressure.

Good post about the enthalpy action. It *is* the fluid medium expanding that drives the turbine. My comment about temps is just that with low temps, you won't have much drive energy.

It's somewhat misleading to distinguish between pressure and temp in this situation, anyway. EGTs aren't JUST an indicator of combustion-- but also of drive pressure. This is because Boyle's law tells us that temp and pressure are related, and that as one goes up, so does the other (and vice versa).

Let's do a little mental exercise (crap, another workout!). Let's say that gas flow through the engine is minimal, and we are just idling. The combination of low intake pressure (no boost) and low rpm means the gas flow in lb/min through the engine is minimal. Under these conditions, the minimal quantity of fuel injected means that the high expansion ratio of the engine will sufficiently expand the charge to where EGT is very cool.

Because the amount of combustion energy is low AND the mass of gas is low as well, the turbine poses essentially zero restriction-- meaning that it is doing no useful work, and that TIT and TOT are essentially the same as well.

But as we add fuel, we find the peak combustion temps go up dramatically. This means that the same expansion ratio of the power stroke cannot completely expand the combustion gasses-- meaning that both temp AND pressure are higher in the cylinder when the exhaust valve opens. This drives the turbine much harder. Also, since actual gas flow is now much higher, the turbine poses proportionally more restriction. Hence, the turbine is now extracting more energy, and producing more work.

Eventually, the residual cylinder pressure and temp are high enough the the turbine is driven hard enough to power the compressor to the point where it pumps more air than the engine consumes-- and we will see the first hints of boost pressure. As more air is introduced, it will support the combustion of more fuel, allowing the turbine to be driven harder, etc etc and we're into the snowball effect.

An easy way to increase TIP would just be to open the exhaust valve earlier-- the hot gasses will take the path of least resistance. Whether or not that is pushing the piston down or driving the turbine depends on the relative restriction posed by each.


So we COULD think of it this way: if TIP is increased, the combustion pressure will apply more force to the piston because the gasses will evacuate at a slower rate, and this mimic the effect of later exhaust valve opening (not exactly, but similar).

So maybe an increase in TIP would actually improve BSFC by a modest amount, in that more of the charge would go to pushing down the piston and less potential energy going out the exhaust valve.

How exactly are you proposing to increase TIP while maintaining "all else equal"? I know injecting water into the intake manifold would have this effect as the water is vaporized in the manifold because it expands and provides more drive energy to the turbine.

Justin

 

Water injected into the charge air:

displaces an insignificant amount of oxygen.

has a small latent-heat-of-evaporation cooling effect (most of the water doesn't change phase until subjected to high compression & combustion-induced cylinder temperatures).

dampens the initial fuel/air combustion reaction, reducing multiple points of ignition/flame fronts and pressure rise prior to piston TDC.

is dissociated by high combustion temps into hydrogen & hydroxl ions, which act as catalysts in the latter stages of combustion with the reaction of CO+O>CO2 - thus accelerating the final (and most powerful, IIRC) phase of combusting hydrocarbon fuel into carbon dioxide and water.

generates steam during the power stroke, which even while cooling the gaseous mixture provides a net increase in cylinder pressure.

reduces carbon build-up in the combustion chamber.

Here's a good summary for my logic:

Take one pound of air at an atmospheric pressure of 14.7 psi and a temperature of 1000 deg F. Mix this with one pound of steam at a pressure of 14.7 psi and a temperature of 212 deg F. What do you get?

Clearly heat will be transferred from the hot air to the steam, and the temperature of the steam will obviously be equal to the temperature of the hot air. If you work out the problem with the steam tables and ideal gas tables, you will discover that the resulting mixture has a temperature of about 490 deg F and a pressure of about 15.3 psi.
-SBBlue

The benefits of water injection are well-known and documented; I'm just approaching the How, Where, When & Why from a novel yet soundly-principled point of view/design.

 

OK please change my last paragraph to read:

How exactly are you proposing to increase TIP while maintaining "all else equal"? I know injecting water into the EXHAUST manifold would have this effect as the water is vaporized in the manifold because it expands and provides more drive energy to the turbine.

DOH!

Sorry for the the subltle error

From a sheer TIP standpoint, would injecting water in the ex manifold be more beneficial than in the intake?

Your thoughts?

 

Bingo!

Welcome to the party.

Even though water's latent heat uses a portion of the exhaust's available thermal energy, the phase-change expansion (~1700X ) nets an actual increase in drive pressure -albeit at cooler EGTs ... remember the exhaust mixture now has a higher specific heat.

I won't post any numbers yet, as I have only indirect access to gas turbine simulation SW through my pointy-headed engineering buds, but so far the calculations show the theory to be sound, and additionally it is partially supported by solid, anecdotal evidence from the annals of competitive motorsports. During the off-season I will be publishing results of this portion of our Techno-Twins project.

 

TIP-wise, you probably know the answer to your question.

It's helpful to breakdown the combustion process from air filter to exhaust tip, in order to identify areas of potential improvement.

Water injected prior to the combustion chamber (notice I didn't specify where ) provides benefits that otherwise can't be realized.

Post-combustion water treatment has beneficial effects that wouldn't be possible with intake spraying (except perhaps in extreme applications).

Even though our focus is on practical applications to improve turbocharger performance, the underlying goal is to more efficiently manage the engine's thermal environment - that translates to improved BSFC, which shows up in MpG gains and increased power production.

 

Now this is very very interesting!!

So by injecting water post combustion chamber your going to drive the turbo with more energy, but this energy is kind of free from the point that its not coming from the engine? So could you use this to spool a larger turbo that you normally couldn't without lots more fueling?

Aaron

 

Yes & No...

Actively recovering waste heat is only one part of the triad supporting our Techno-Twins project.

 

But you are going to get some of the benefits of water injection, and you will get them without increasing your peak cylinder pressure? Which your HG will thank you.

You will also reduce your egt's but in a different way right? The turbo will be driven with more energy so it will provide more air, than normal for the fuel provided, which will cool the cylinder more. Right?

 

Now - I didn't say that!
Pre-combustion chamber water injection reduces peak cylinder temperature - but not necessarily pressure, although it certainly produces higher net cylinder pressure throughout the power stroke.

EGTs will be reduced, but once again - that's a side effect, not the system's goal. Of course, lower EGT would allow more fueling, in addition to the extra fueling made possible by the extra boost...

 

So is there any benefit as to where you inject the water in the exhaust? Would you want 6 injectors as close to the head as possible? Or would one or two in the collector before the turbine housing?

 

Keep in mind that the expansion of the water vapor goes "both ways" so while you'll drive the turbo harder, you WILL have an increase in drive pressure as well.

The benefit of the water injection is the MASS increase which drives the turbine harder-- not so much the pressure increase.

jh

 

Which is exactly why I was thinking you would use this type of system in conjunction with a larger turbo. In order to keep your drive pressures down.

 

I recall an industrial parallel involving saturated steam. Water droplets in the fluid stream will cause premature erosion of the impeller blades. I don't know if our impellers are resistant to this, and a shade tree mechanic has no way of knowing if all the water has flashed to steam before it reaches the turbo. One could wait until the engine is hot before turning on the water, but it's still an unknown even if the water injectors are as near to the head as possible.

 

That's true - liquid phase water is undesirable from the turbine's point of view!

Obviously, a successful design would preclude that possibility...

BTW Justin - the water's mass effect isn't as large as you think, although it is instrumental in increasing the exhaust gas's specific heat. The majority of the drive pressure increase comes from phase-change expansion.

I.E. A CTD at max BHP might exhaust 5000lbs/hr A/F mixture, and water could be 100lbs/hr on the high end...

 

I know that sooner of later this discussion could result in the viable developement of a hydrogen fussion type engine.

But......

I believe, from more than one source, that increasing boost over a critical threshold value can increase ET's and lower Hp numbers. I realize that ET's or dyno's may or may not apply to your discussion. Still are a measurable bench mark.

But...

I have pondered this as well a time of two for myself.

And gotten nowhere....

In a perfect isentropic (no change in net energy) compression / de-compression process what would higher or lower boost matter as long as the combustion was occuring under stoich acceptable conditions. More excess air simply lowers the EGT. From that side of the coin the pumping losses, although larger, would cancel and the engine would simply make more Hp to offset.

Still....

It is not a perfect isentropic process so there has to be some losses there. These losses would only seem to increase the leaner the mixture became over pure stoich.

But....

Not so simple, because the specific heat rate of the engine would seem to drop. As a result the thermal efficientcy would drop. This would mean that with the same fuel rate there is less Hp production. Because the combustion temperature drops? That would leave less available energy for the turbocharger as well as thrust on the piston during the power stroke.

And....

At the end of the day the engine must be able to "live" under the conditions to sustain a combustion process. This boundary is given simply by the physical metalugy properties.

And yet.........

The drive pressure ratios must remain below 1.0 other wise pumping losses become a still larger factor.

Also....

In the end, I have decided that I don't know. Just throwing some stuff out there, good discussion as always....

I ponder this curve often.

These are my home grown drawings of a "Perfect Diesel Cycle". The Heat Input is proportional to the amount of fuel injected. The length of this line represents the duration of combustion (10-15 degrees?). For a Balanced Heat Condition, IE no net change in temperature, the Heat Input and Heat Output (rejection) values must be equal as shown by lines of the same length.

Diesel Cycle with the area inside the curve relative to shaft Hp.


Explanation of the cycle. The orange is gross Hp, the green is net Hp and the blue is pumping (loss) Hp.


Raise the boost. Hp goes up as indicated by the larger area inside of the upper (30 psi boost) curve. Where EGT is a function of the length of the power stroke line. The shorter the line the higher the EGT.


Raise the fuel. Notice the very short power stroke line and imagine aluminum solder running down over the crank.


Raise the drive pressure, not sure about this one. I have trouble connecting the dots. Just a hunch on my part.


Jim