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  #1  
Old 07-19-2005, 03:48 PM
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Cavitation possible from air bubbles in WVO?

The one thing that worries me about running on veggie is that sometimes I notice some small bubbles in the line. I've read that air bubbles in the line can cause cavitation to the IP which is something that can ruin a pump. I have a spare tank that I use to supply WVO to the engine. Due to the type of tank it is, it draws the fuel from the top rather than the bottom like on the Mercedes' fuel tank. I've thought about tapping a new outlet at the bottom to draw the fuel like the car's tank but the pump seems to do well enough as it is. I've also thought it advantageous to run it this way as it would avoid drawing any crud from the bottom of the tank that may settle. I haven't found any leaks in the suction line so I'm thinking the bubbles are formed when the tank gets shook from driving.
Anyone with thoughts on this?
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Old 07-19-2005, 04:20 PM
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Cavitation occurs when the velocity of a fluid causes the fluid's pressure to drop below its vapor pressure. Since the IP greatly INCREASES pressure in its function to deliver fuel to the injectors, I wouldn't worry about cavitation.
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  #3  
Old 07-19-2005, 04:59 PM
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Quote:
Originally Posted by Pete Burton
Cavitation occurs when the velocity of a fluid causes the fluid's pressure to drop below its vapor pressure. Since the IP greatly INCREASES pressure in its function to deliver fuel to the injectors, I wouldn't worry about cavitation.
Cavitation is a genuine issue in diesel injection pumps but not because of bubbles in the feed stream. Free water in the fuel causes cavitation in the piston and delivery valve assemblies of the IP and ultimately causes it to fail.

I'll find that info in the Bosch IP book and post it.
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Old 07-19-2005, 05:01 PM
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I have stated my opinion on the infopop forum, but for consistency I will state it here as well. I agree with Pete in that cavitation is probably one of the lesser worries you have when running WVO. But, everybody needs something to worry about.

The bigger question in this is where are your air bubbles coming from? Do you have a vacuum gauge in the system any where? It is possible you have a loose fitting somewhere leaking a bit of air, but it is also likely you are pulling a massive amount of vacuum on the system and that is where your bubbles are coming from. In either case, bubbles in the system aren't "normal" and should be corrected.
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Old 07-19-2005, 05:13 PM
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Quote:
Originally Posted by R Leo
Cavitation is a genuine issue in diesel injection pumps but not because of bubbles in the feed stream. Free water in the fuel causes cavitation in the piston and delivery valve assemblies of the IP and ultimately causes it to fail.

I'll find that info in the Bosch IP book and post it.
I don't disagree with this, as it addresses pressure changes on liquid (I should have said liquid, not fluid). Still, I'd certainly like to see what the Bosch IP book has to say.
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Old 07-19-2005, 11:32 PM
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I was incorrect about free water being the cause of cavitation; Now, I don't have any idea where I picked that up at. The Bosch book's statement on cavitation leads me to believe that it is caused by the design of galleries and passages in the pump.

Here is a scan of the page from the Bosch book discussing cavitation in IPs (caution, file is 1.1 mb).

Cavitation in diesel injection pumps.
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Old 07-20-2005, 09:34 AM
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Quote:
Originally Posted by R Leo
I was incorrect about free water being the cause of cavitation; Now, I don't have any idea where I picked that up at. The Bosch book's statement on cavitation leads me to believe that it is caused by the design of galleries and passages in the pump.

Here is a scan of the page from the Bosch book discussing cavitation in IPs (caution, file is 1.1 mb).

Cavitation in diesel injection pumps.
I think free water does cause this effect. Something about pressure differences between what is going on with the water(steam?) and the oil around it.
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Old 07-20-2005, 09:38 AM
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LOT more info here at Maui Green Infopop
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The Glow Plug Wait: This waiting period is a moment of silence to pay honor to Rudolph Diesel. The longer you own your diesel the more honor you will give him". by SD Blue

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Old 07-20-2005, 10:27 AM
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Quote:
Originally Posted by coachgeo
I think free water does cause this effect. Something about pressure differences between what is going on with the water(steam?) and the oil around it.
Yes, the vapor pressure of water is higher than diesel, so "free water" will cavitate in conditions where diesel will not. Another reason to keep your fuel dry.
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Old 07-20-2005, 11:05 AM
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I must be getting a leak somewhere in my suction line, though I haven't found where it could be. Fortunately I don't have too many connections but the problem is I have my hose clamps as tight as they'll go before slipping. I've thought about changing over to fuel injection clamps but I'm not sure they'll be any better.
Lately I have had some stumbling running on my WVO that goes away when switching back to diesel so there is a leak somewhere.
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Old 07-20-2005, 11:28 AM
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Hose clamps can be a PITA. This is one instance where cheaper is definitely not better. The barbed fittings can make a big difference as well. Fittings such as Parker Push-Loc or similar are much easier to seal than the run of the mill hardware store variety. Not that one can't get by with cheap fittings and hose clamps, but a few dollars more for the better varieties can save a lot of headache.
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Old 07-20-2005, 03:28 PM
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I do have barbed fittings as well. I'll check out the push-locs.
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  #13  
Old 07-20-2005, 03:56 PM
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The Push-Lok (sorry I spelled it wrong before) fittings are barb fittings, just higher quality barb fittings. They are advertised as not needing hose clamps, but I usually put clamps on them when used on a vehicle.
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Old 07-20-2005, 08:01 PM
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Are any of those barb fittings screwed into something and barb on the other side? If so, you might try plumbers goo on these treads ? Not tape, this will eventually leak air. You have to use the goo/paste.

Might try some of it on your barb before installling hose?
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Old 07-20-2005, 08:45 PM
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Yes I do have some threaded type of fittings. I used teflon tape on the fittings. The strange thing that started happening today was I would get a sputtering problem on hard accleration with WVO. It was like not enough fuel was getting to the pump. I thought that the filters are plugged but after switching back to diesel the problem goes away. I don't think the oil is to blame since I didn't have any trouble yesterday and it runs fine under normal acceleration.
Puzzled.
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