Cavitation

[Willie.]

I've been thinking about oil a bit recently, and have been pondering the problems us K-Series owners often suffer, namely; surge and cavitation.

 

Surge has been well discussed, and various options investigated/tried...but cavitation not so much.

 

What I am talking about is what often manifests itself as the tappets thrashing, or the engine sounding very ticky. This happens when the car is being driven hard. If you remove the dipstick, you can sometimes even see bubbles. (This is not an overfilled engine, so not whipping).

 

Caterham supply the Apollo Anti-Cavitation tank, and some people fit oil coolers and so on, but this is merely is removing the symptons and not the problem. Hence I would like to know exactly what the problem is. What causes cavitation?

 

My understanding of cavitation is the following... The boiling point of a fluid is determined by atmospheric/ambient temperature. If you were at the top of Everest, water would boil at less than 100C. Low pressure reduces the boiling point. When the fluid boils, it produces vapour bubbles. These then burst when they hit solid surfaces, and can sometimes damage the surface (Pitting and so on).

 

If we are getting cavitation at high RPM, what is causing the low pressure? I say high rpm, as it is only when the engine is being revved high that this happens. I think boat props also can suffer cavitation when revving high. Could the cavitation be caused by the oil's inability to keep up with the oil pump? As the central "star" moves on, the oil behind it can't keep up and there is an area of low pressure. Presumeably, this low pressure becomes lower the higher the pump is rotating.

 

So, is cavitation caused by the oil not being able to be "drawn" behind the pump quick enough, the pressure then becoming so low the oil boils, and then the vapour bubbles make the tappets thrash and we get grumpy?

 

If the above is correct, would a thinner oil be able to keep up with the pump better?

 

On a related theme, does the viscosity of the oil, suggested by the viscosity number (eg, 5w50) affect its lubrication properties? Will a thin oil lubricate just as well as a thick oil, the only difference being the lower pressure?

 

Any thoughts or input welcome.

 

Willie

[Martin Erhard]

Cavitation is bubbles forming in a liquid. If the pick up pipe in the sump is starved of oil during hard cornering, like sucking on a straw at the bottom of a drink, it gulps up air along with the oil. The removal of these bubbles is I think what most people understand the function of the Apollo to be. The phenomenon you describe akin to propellor cavitation is different again and it would be interesting to hear an expert opinion. When doing an oil change I am amazed at the propensity of the stuff to generate and retain small air bubbles within itself just by being poured out of a can, let alone being forced through small galleries etc.

 

 

Edited by - Martin Erhard on 30 Apr 2008 23:14:14

[Peter Carmichael]

The widely used name of "anti-cavitation tank" is a complete misnomer. Its only meanignful job is de-aeration.

 

The only problem present in the installation is "aeration" so a de-aeration solution is appropriate. No such measures are required with a dry sump setup using the same pressure pump. QED.

 

Edited by - Peter Carmichael on 30 Apr 2008 23:58:26

[Willie.]

Surely any dry sump system includes an external tank, which will de-aerate the oil to some extent, regardless of wether it has swirl deisgn, de-aeration plates or nothing.

 

As far as cavitation is concerned, a drysump using the standard pump/pickup is equal to a wetsump with an apollo. Neither stop cavitation, they just remove air.

 

What creates this air? I'm not convinced that it is purely a result of surge. It could be the way the oil is flung about, or it could be cavitation behind the pump.

 

As the oil pump just moves volume, any resistance on the OUT side causes high pressure, and any resistance on the IN side causes low pressure. As the oil pump is above the sump, it has no head to help oil flow in, and there will be a fair bit of resistance from pickup to inlet of pump.

 

I intend on trying a thinner oil to see if it helps.

 

Willie

[oilyhands]

Windage will result in oil pulverisation and increased oil/air volume.

 

Oily

[Adrian Williams]

Willie as Oily has just pointed out it's windage. The sump is too shallow and the crank sits in the oil which causes the problem. Thinner oil wont cure it a dry sump will or an apollo will help treat the symptoms but not the cause.

 

Adrian

[simonbell]

We've been telling him he needs a dry sump since just before he blew up his engine on a track day............he won't listen.........the youth of today

 

Simon Bell - Caterham 7 Duratec R

I`ve seen the future.....and it`s powered by duratec Check out the website here

[simonbell]

Or a proper engine....but that's a different debate 😬

 

Simon Bell - Caterham 7 Duratec R

I`ve seen the future.....and it`s powered by duratec Check out the website here

[Willie.]

Windage even with the windage plate and the big baffle/gasket in place?

 

I thought the crank swung significantly above the oil level.

 

I'm going to drop the sump soon anyway, so will have a look then.

 

Simon, proper engines have the exhaust on the passengers side

[mav]

>

 

Listen to what you are being told in response to your question.

 

A seach of the archives will also reveal a number of indepth discussions on this subject!

 

 

[timbo]

 

ohhhhhh nnoooooo...... He's not ranting on here too. 😬

 

BUSA POWWWER

[susser]

Sorry but have to disaggree with Martin Erhard;

Gulping air is Ventiation.

 

As for Cavitation; If the oil pump caused Cavitation at the suction, due to the low pressure-vapour pressure bit, the cavities would collapse within the pump itself, as soon as the pressure rose above the Vapour Pressure.

The result of Cavitation on a Progressive Cavity type of pump is A, Damage to the pump and B Reduction in pump delivery volume.

I wouldn't think that the Cavititation that must occur in our pumps would have a noticeable effect, as it is likely to be so small.

 

What we have with our K series is Ventilaion and Aeration, I believe.

[Martin Erhard]

Cavitation = 'the formation of bubbles, or of a vacuum in a liquid' OED

[Willie.]

Gentlemen,

 

Having spent a few days pondering this, I've just got my head around it...and no longer think that the tappet problem is a result of cavitation.

 

Cavitation is the formation of vapour bubbles due to low pressure allowing the fluid to boil (Or almost boil), thus becoming vapour/gas. However, as soon as this vapour bubble is faced with high pressure it returns to its liquid state...the energy while doing so causing damage to nearby solid walls/pump parts.

 

Cavitation will only happen at the pump, and the vapour bubbles will be long gone by tappet stage.

 

Air bubbles are also gas, but it is impossible for air to become a liquid above approx -190degC, and so the air in the oil will not implode/become liquid, but be carried along by the oil unless removed.

 

This air could be ingested by numerous ways; surge, windage and so on. I'm now going to see if I can discover which.

 

Thanks for all the replies which helped me get my head around this.

 

Willie

 

 

 

Edited by - Opposite Lock on 3 May 2008 00:48:48

[Jason Plato]

what is whipped cream full of ?

 

what are the tappets full of ?

 

what is the difference between a wet sump K and a dry sump K ? ........ sump pan holding oil......

 

crank smashes into oil in the sump, air is held within the oil, oil travels around the system faster than the air can escape from the oil , air builds up until an acoustic indicator is heard - that being the tappets whcih are no longer just full of oil alone ....

 

the crank bearings suffer their own torture but you dont get much of an imediate acoustic indication

 

despite everyones contribution 😬 you appear to have convinced yourself otherwise 😶‍🌫️ 😬

 

here is C7 TOP

Taffia rear gunner

 

[Willie.]

Dave,

 

Sorry, maybe didn't make myself clear. I totally agree with you about air in the oil being the problem...its the source of this air that I couldn't decide on.

 

I thought it was maybe the oil pump cavitating (Oil boiling due to low pressure) making the "air" in the oil, but it's not...its actually normal air bubbles as you say.

 

Now I just have to fit some form of de-aeration, or find the cause of it and stop it.

 

Willie

[Johnty Lyons]

Oh Willie it's so well ive been in Blighty at a family funeral and missed all the chat on Se7ens list and here or as Mav says I've a had to slap ya. But well done for getting it in end Now with your ingenuity lets see how quick you can make a DS system out of old oil pumps a Copper heating boiler and some old tractor hydraulic pipes all for less than 50 quid Go on you know you want to. 😶‍🌫️

 

---

jj

N.I. L7C AR 🙆🏻

Membership No.3927.

240BHP 1900cc K Series 40th Anniversary

 

[Willie.]

JJ, don't jest...engine is out and on the stand for that purpose.

 

Maybe not a dry sump system, haven't decided yet. Currently have all sorts of bizarre ideas.

 

Anyone know what pressure a powersteering pump works at?

 

Willie

[Mick Day]

Anyone know what pressure a powersteering pump works at?

Just to rub salt in the wounds a pump does not provide pressure, it just provides flow.

 

In simple terms Pressure = Flow x peripheral resistance.

 

The flow is the output from the pump & the peripheral resistance is provided by your bearings.

 

However in a more pedantic equation it would be Pressure = Force/Area

 

[Johnty Lyons]

Argggggggggghhhhh.

 

Mick don't start him off again Pleasssssssssseeeeeeeeeeeeee...............

 

---

jj

N.I. L7C AR 🙆🏻

Membership No.3927.

240BHP 1900cc K Series 40th Anniversary