Oil & Fuel Diesel Anticavition Additives Explanation

[awlchu]

....Something I read about diesel specific antifreeze, I'm having a hard time accepting this explanation of cavitation but obviously it happens, or does it?

from www.ford-trucks.com

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Excuse me if I seem a little dense today, alot going on but I have a question. How exactly could moving water a little faster through the water jackets reduce cavitation. If I am not mistaken, cavitation is caused by the imense compression of Diesel motors. This "banging" if you will is translated to the water causing air bubbles.

A definition that I found on the web pretty much explains what I thought to be the case.



The bubble tries to collapse on its self. This is called imploding, the opposite of exploding. The bubble is trying to collapse from all sides, but if the bubble is laying against a piece of metal such as the impeller or volute it cannot collapse from that side, so the fluid comes in from the opposite side at this high velocity proceeded by a shock wave that can cause all kinds of damage. There is a very characteristic round shape to the liquid as it bangs against the metal creating the impression that the metal was hit with a "ball peen hammer".

This damage would normally occur at right angles to the metal, but experience shows that the high velocity liquid seems to come at the metal from a variety of angles. This can be explained by the fact that dirt particles get stuck on the surface of the bubble and are held there by the surface tension of the fluid. Since the dirt particle has weakened the surface tension of the bubble it becomes the weakest part and the section where the collapse will probably take place.



This banging on your cylinder wall will eventually lead to failure so there is an anti-cavitation property to the antifreeze. I have seen cylinders that have failed from cavitation and it looks like someone beat a hole in the side of them with a hammer. I do not disagree that lowering the surface tention of water makes it wetter, in fact that was a question my science teacher asked in high school, (the answer was add soap) I just don't understand how moving it faster would help.

 

[DANOCHEESE]

It does exist. Search this site for cavatation www.thedieselstop.com

an additive such as fw-16 is used to prenvent cavatation. Test strips are used to test the antifreeze.

 

[QueBota]

Diesel engines with wet sleeves or liners are susceptible to cavitation. I don't know how many small tractor engines are designed like that.

Dave

 

[DK35vince]

</font><font color="blue" class="small">( ....Something I read about diesel specific antifreeze, I'm having a hard time accepting this explanation of cavitation but obviously it happens, or does it? )</font>

I'd be skeptical too. Except I have seen it happen ( several times) on wet sleeve engines.

 

[besserheimerpha]

Okay, an attempt to explain cavitation. . .

At a given pressure, every liquid has a specific boiling point. For example, at sea level where air pressure is 14.69 psia, the boiling point of water is 212 deg F. When you drive to Denver, the increased elevation reduces the air pressure, and a lower temperature is able to boil (vaporize) the same pot of water (which is why it takes longer to hard boil an egg in Denver than Atlanta). As the air pressure goes down, vaporization temperature goes down.

Bernoulli's Principle tells us that as a fluid's velocity increases, its pressure decreases. So when a fluid gets moving fast enough, the fluid pressure can drop so low that the fluid will vaporize at room temperature. This is called "flashing." Lots of tiny bubbles are formed in the fluid. As the fluid moves down the pipe, it gradually slows down due to pipe friction and viscosity. When it slows down enough, the pressure increases to the point that the vapor bubbles collapse upon themselves. This is cavitation. As was said before, when these bubbles collapse near a pipe wall (or other peice of metal), the resulting fluid jet and shockwave literally eat away the material.

So, by keeping the fluid velocity high, the pressure remains low, and the bubbles never collapse, so there is no cavitation. However there is still flashing (bubbles are still created), and that can also be damaging to the system.

FYI, I am a test engineer for a valve company and we do a lot of tests to determine the effects of cavitation. We have a neat cavitation demo that I might be able to get some video of. If I can I'll try to post a link of it somehow.

 

[awlchu]

Could someone explain a "wet sleeve" engine?

 

[QueBota]

Wet-sleeve vs. parent-bore - Parent-bore engines have cylinders bored directly into the engine block. In contrast, wet sleeve engines have cylinder bores fitted with replaceable sleeves surrounded by engine coolant (hence the term "wet-sleeve"). Better cylinder heat dissipation allows a wet-sleeve engine to last longer. And at overhaul, cylinder sleeves can be replaced and fitted with new pistons restoring factory specifications - without costly engine removal and cylinder reboring needed for parent-bore engines. Which is best depends on usage patterns and total expected mileage. Generally, the higher initial cost of a wet-sleeve engine pays dividends in repair cost, overhaul cost, and resale/auction value.

Dave

 

[awlchu]

thanks!

 

[TMcD_in_MI]

</font><font color="blue" class="small">( Okay, an attempt to explain cavitation. . .
...
As was said before, when these bubbles collapse near a pipe wall (or other peice of metal), the resulting fluid jet and shockwave literally eat away the material.
...
)</font>

Thank you besserheimerpha, for a great explanation. Hope you can answer a couple of questions for me.

First, just out of idle curiosity, what kind of fluid velocity are we talking about that will reduce the pressure of the fluid enough to boil at normal coolant temperatures?

Second, is there any kind of nucleation effect that will trigger vapor bubbles to collapse, like there can be to form them in the first place? By that I mean, will a rough spot in the engine casting or pipe have any effect on the rate of bubble collapse as the bubble-laden coolant stream passes over it?

Thanks

Tom

 

[Slamfire]

I once worked in a Hydro electric plant, where one of the old bronze Francis turbines was mounted on a dolly for show and tell with local school kids. The bronze looked like swiss cheese, just full of holes. They had been replaced with stainless steel which hardly showed any wear. During start up you could hear the cavitation, it sounded like someone threw a handfull of gravel in the scroll case.

 

[besserheimerpha]

</font><font color="blue" class="small">( Thank you besserheimerpha, for a great explanation. Hope you can answer a couple of questions for me.

First, just out of idle curiosity, what kind of fluid velocity are we talking about that will reduce the pressure of the fluid enough to boil at normal coolant temperatures? )</font>

Well, in a particular test I ran, cavitation started with a fluid velocity of ~77 fps, and had become damaging around ~150 fps. Cavitation starts small with just a few bubbles, and the very low percentage of bubbles that contact the pipe/valve internals keeps damage to a minimum. The real problem occurs when cavitation is present throughout the entire cross section of the flow, when lots of bubbles are making contact with the metal parts. That's when the "gravel in the pipe" starts, and yeah it turns everything to swiss cheese really fast.

</font><font color="blue" class="small">( Second, is there any kind of nucleation effect that will trigger vapor bubbles to collapse, like there can be to form them in the first place? By that I mean, will a rough spot in the engine casting or pipe have any effect on the rate of bubble collapse as the bubble-laden coolant stream passes over it? )</font>

I would assume yes, but it is so difficult to predict cavitation especially on such a small scale. For our valve applications, we just test every product. Sometimes a seemingly insignificant change to the product can change the flow enough to really screw things up. As the fluid flows over small imperfections in the pipe/valve, it could cause enough change in pressure to form or collapse the bubbles. I should note, however, that flashing (bubbles form but don't collapse) is also bad for the fluid system. The ultimate goal is to not have any bubbles form. When the bubbles appear, the fluid expands as it goes from liquid to gas, and to maintain a constant mass flow rate the velocity has to skyrocket. Anyway, if the fluid is flashing there isn't much you can due to prevent it from cavitating other than keep the velocity high or vent to a tank or atmosphere. Eventually the roughness on the inside of the pipe will slow the fluid down enough that it would cavitate.

Cavitation is also very dependant on the fluid properties. In our lab we have water at 60°F. The addition of coolant alone would make our data not apply to this situation.

 

[besserheimerpha]

Just thought I'd throw this out for reference:

77 fps = 52.5 mph, 150 fps = 102.3 mph

That test was on a 3" valve, ~67% open. At 77 fps the flow rate was 750 gpm, and at 150 fps the flow rate was 1350 gpm.

 

[Soundguy]

Ditto what blas... said.. it's a vapor pressure issue.

Soundguy