If injecting air late in the compression cycle works, why wouldn't poor atomization in the intake tract work too, given that those large droplets would still vaporize in the compression cycle?

 

Agreed, but be careful here about all-or-nothing reasoning. The pulsing will be dampened by the tubing, sure. But I doubt it will end up being completely damped to the point where it is an average, drastically reduced pressure. Even when I had a single, much larger nozzle on a much longer run of tubing, it was clear that the mist at the nozzle was still pulsing strongly in response to the pump cycling on and off. Anyway, testing will tell. At the worst, there could be some minimum duty cycle below which atomization is not OK, and so I just set the onset duty cycle above that value. Do I really need to flow down to 1.532% duty cycle? Nope. Starting with 20% would be fine, and it is still much better than an on-off system.

No the valve is run at a set frequency with PWM. It is not synced to the inlet pulse. I doubt this is significant, especially since the duty cycle of the spray is essentially 100%. It is the same deal as with batch fired ignition. Batch firing works fine and almost exactly as well as sequential at high injector duty cycles (max power) because the injector is open for almost the entire cycle of the cylinder anyway. Plus the open plenum in the upper will act as a mist accumulator and further negate the need for inlet pulse sync. Strictly speaking I see your point, but I think it is a negligible problem.

This is true regardless if it is a single or multi nozzle system. Flow will be proportional to the HSV cycle, that is the whole point. The HSV is not intended to shut the nozzle flow off completely between pulses. It is just a PWM flow controller that can operate at a high enough frequency to be useful for this application.

All these points are not unique to a multi nozzle setup.

 

Empirical data shows that better atomization controls detonation. In fact development of good port fuel injection is one of the reasons compression ratios could be raised again in the late 80s-early 90s, after plummeting in the 70s. All else being equal, the finer atomization in the cylinder made the engine less detonation prone, and compression ratios could be raised.

Also, things are happening very quickly on the compression stroke. To illustrate by extremes, think of it this way. If all of the droplets were incredibly atomized to the point where they were molecules, then evaporation is instant (it already happened). If atomization was horrible, and you ended up with one 'big' water drop in the cylinder, then it is at least possible that there is not enough time during the compression stroke to completely evaporate that drop and distribute it evenly around the cylinder.

Speaking of distribution, that is important too. Smaller, finely atomized fluids are going to distribute more evenly around the cylinder.

Anyway, this is all academic. It has been shown to be important like Joe pointed out, and with years and years of good test data (the ultimate indicator), that atomization is important. I am not sure why I am even bothering to spend this much time writing about it honestly

 

Answer:
Ok, moving on.


Yeah, I'd not want to pin it down too hard, but definitely one contributing factor (along with better mixture control, more stable and accurate ignition control, refinements in combustion chamber design, etc.)


As much as I hate to disagree with you Jason (you seem like the kind of guy who does his homework, so to speak,) I just don't like the "big droplets are just as good as small droplets" theory. I have seen no data at all to support this observation, I have seen considerable anecdotal evidence to refute it, and it seems highly counterintuitive to me. Large droplets are more likely to come out of suspension as they impact the walls of the intake manifold, ports, valves, etc. Yeah, they will eventually vaporize, but that's not what we want- we want finely atomized water present inside the combustion chamber during the compression cycle in the 50-60° leading up to the ignition event. If the water has already vaporized by that point, it's not being used to its fullest potential.


In the '80s, the OEMs went from big carburetors on crappy manifolds to TBI fuel injectors on crappy manifolds.

In the '90s, the OEMs moved the injectors out of the manifold and into the head, vastly reducing the proclivity of the mixture to become inhomogeneous and virtually eliminating the potential for uneven distribution from one cylinder to the next.

In the '10s, the OEMs are moving the injectors directly into the combustion chamber.

The OEMs may be onto something here.


There are loads and loads of good scientific material on this subject, both contemporary and historical. The paper I linked to is just one of many that I have. I'd been meaning for quite some time to do a sort of thesis on the subject, though to be honest I sort of lost my motivation when I found out that CA puts OBD-II cars on the rollers. (It's a very long story.)

I still intend to do this... At some point.




Because that is the Way of the Internet.

 

I don't understand why you are putting all this effort into safety when you have no idea how your gutted manifokd flows air. You could easily be lean in a cylinder.

 

If you mean uneven air charge, maybe.

Some have gone before me with the gutted manifold and as far as I know they are not having lean cylinder issues. The only debate seemed to be whether or not the flow gains were worthwhile. Of course, they could have a problem and not know it. I think the only way to know for sure would be to run individual runner EGT gauges right at the exhaust ports, right? I think it would be too hot for individual O2 sensors. Either that or do an old-school WOT ignition/fuel cut on the dyno (with no WI) and compare the spark plugs.

At any rate, I think the open plenum into individual runner design is pretty similar to many other OEM designs. It is also similar to several of the successful one-off manifolds I have seen here, albeit they have shorter runners. Plus I would think the larger plenum volume would tend to even out the distribution, not make it worse? The air should stagnate quite a bit in the plenum and help even out the pressure gradient across the volume. Maybe there could be some resonant effects...

 

Oh yeah +1 to what Joe said above. Not trying to be difficult, just saying the same thing Joe did. Improved atomization has been shown to improve combustion and reduce detonation/knock.

 

Yes, you'd only know with individual EGT sensors. Just because other people have done it and aren't noticing problems doesn't mean that there a) aren't problems and b) won't be problems down the road. They are also probably running less boost and boosting much mroe infrequently than you plan to. That means problems could occur much more rapidly.

I understand that a pressurized plenum will flow differently than one on an NA engine but you are still taking pretty big risks IMO. There are some threads here with computer modeled flow (I forget the name of the program) and manifolds that look pretty darn good flow like crap in the simulators. I don't know how much work mazda put into making their OEM manifolds flow evenly, but they probably at least sorta tried right?

Nyways, instead of putting a have-at-ye-with-a-dremel- manifold on your nice shiny built engine I would take the time/money to find a properly flow bench tested manifold.

 

All good points. However I think it would be prudent to have THIS manifold flow tested before deciding it is inadequate (and dropping big bucks and/or big time on a custom-fabricated, fully analyzed and tested manifold). I still suspect this might be overthinking things a bit. Do you know of example of where someone did something similar to this and damaged an engine from flow imbalance?

Anyway, let's get back to the WI discussion.

 

For the failsafes, could you use some kind of (NC) switched one-way valve instead of flow meters?

I'm not current on low flow rate meters, but for the kind of flow we're talking about I don't know how quickly they'd react and let you know a nozzle has taken a dump.

I'm thinking of a switch that is wired off the pump signal wire, that grounds a big warning light if both: A.) pump is on and B.) check valve is closed. You could use these instead of the regular check valves (which you need anyhow).

Just throwing that out there...open to riducule. I had drawn a diagram of it, but can't upload at work.

 

Hang on, that's for fuel. Does the paper you posted (I haven't read it yet, it's huge), show that to be true for water injection?

 

I think Joe's paper was in regards to water injection? Joe I think you linked to the wrong paper BTW

At any rate, from what I remember from a discussion I had with an R&D Gasoline Engine Engineer buddy of mine, the same thing applies to WI as well. It has to do with propagation speed of the flame front and evenness of the combustion. The WI well suspended in the charge effectively cools the oncoming flame front and thus slows down the flame front propagation speed, and the better suspended it is, the better/faster it absorbs energy from the flame front. I think it is due to the increase in surface area with the finer droplets.

BTW, IIRC, I do not think the WI droplets are all evaporated by the compression stroke. Neither is the fuel for that matter. Either it is not hot enough, or the increase in pressure lowers the evap temp, or there is not enough time for evaporation, or a combo of these.

When we start talking about really detailed gas engine combustion physics/behavior though, I am getting out of my area of Engineering expertise. My buddy knows a lot more than I do since it is HIS job. I pick his brain for info he can share with me (non-proprietary) on a regular basis.

Anyway, go find a reputable paper/test that says it is not true. Until then, let's assume atomization is important. Please.

 

I think I am following you here. So by check valve, you are talking about a on-off switch that is triggered by flow? If it was adjustable and repeatable that could work nicely for an on-off system.

I am also not aware of any small, low cost, accurate, low flow meters that could work for this, at least not last time I looked (albeit briefly). There are of course fantastic flow meters out there that will measure low flow rates with ridiculous accuracy, but they are not cheap. You can dig around here to see what I mean.
Flow and Level

 

I find it believable that finer atomization is better, but I want to question it.
Seeing that there are so many papers on WI, there might be something that actually tests it.

And read what I wrote about HSV + long hoses = reduced pressure at nozzle, and atomization goes out the window.

 

I don't see how thats possible. The restriction that increases pressure is the nozzle, not the valve.

 

The valve pulses on and off. Between the valve and the nozzles is rubbery expandable hose. If that hose has enough compliance, the hose will absorb pressure pulses, and at the nozzles, all you have is some average, lower pressure isntead of the pulsed pressure you want in order to get pulsed flow.

Note how Joe mentioned that when he turns off the flow, it takes a second for the flow at the nozzle to fade away.

The way to prevent this is to have some short, rigid piping from the HSV to the nozzles.

Try it - pull one nozzle out and run the HSV at say 50% duty cycle at say 5 Hz and observe the flow.
Come back and tell me who's yo daddy!

 

Assuming Aquamist uses the same polypropylene(polyethylene?poly-something) hose that everyone else uses I doubt it expands outward much at all. It is rated to something like 450psi working pressure, and over 1000 burst. It is pretty rigid, it just has good bending properties so that it doesn't kink. The reason water continues to come out is because the water in the hose after the valve will drain out from suction and/or gravity, pressure equalizing to atmospheric... So granted, you will have lower pressure after the valve shuts off. When using a true HSV that cycles at something like 200Hz though, I highly doubt that the pressure will drop considerably between cycles. In the end though even Aq recommends keeping the length of hose between the valve and nozzle as short as possible.

 

I already read it, and responded. Go back and read THAT. Simply put, no, and even if it is, which is is not, so what? And, it is not unique to my setup. And I am not running the valve at only 5 Hz so that is pointless. I might as well try your experiment at 0.5 Hz, in which case I agree, yes, it will have low flow at some point.

 

So I had my R&D gas engine Engineer buddy take a look at the manifold and he does not see any obvious problems with the gutting that would cause a flow imbalance between cylinders.

 

Good news on the manifold. I'd just go for it, despite the naysayers. I forget what you're using for knock protection...consider trying Jeff's new device?

On the one-way(check) valve switch...you've got the right idea, I've just yet to find one in 12V. Tried Grainger and McMaster...need to look around more. If we find one I think I'm going to order 20 of them as they could be applied to ANY setup. You'd just need 4, and a lot of wiring, for yours.