Oil In The Diesel Engine

[grifjl]

1.9TDi, 02 model. I check the oil every 6 weeks or so (roughly 1,000 miles). When I check the level cold the level was below the lower line. When I checked the level after driving and then letting rest for an hour (as the book mentions one should) the level was midway up the 'acceptable' area. I'm confused, how does this happen? Surely the oil has had full chance to drain back. And yes, I do check it several times, wiping stick clean each time.

 

While I was pondering this (over three days) my wife mentioned that the display came up and asked for oil to be topped up.

 

I guessed it wouldn't hurt to put in around 300ml, so went through the whole malarky detailed by others re the 'What oil should I use saga.'

 

VW had the oil in stock and knew exactly what to put in. 1litre cost

[Guest The Doctor]

Wont the increased level when warm be due to thermal expansion of the oil?

[grifjl]

do liquids expand when warm? I didn't think they should (at least not by this much).

 

I'm still puzzled, but no recurrence of the oil light flashing, so Hey Ho - Merry Xmas!!!

[Guest SA Intruder]

Liquid does expand! Plenty. (Boyles Law)

 

Thermometers?

 

Coolant expansion bottle?

 

Central heating expansion vessel/header tank?

 

For water, this is the case above 4oC, but near freezing, the opposite occurs. In the 4 - 0oC range, water expands due to the formation of a crystalline structure and the density decreases again. As result, ice is less dense than water.

 

And solids and gases..... coefficient of thermal expansion is different for every material.

 

Etc Etc

[seatkid]

While I was pondering this (over three days) my wife mentioned that the display came up and asked for oil to be topped up.

 

Isnt it an oil pressure indicator and not a oil level indicator? :wub:

 

Are you sure car was level each time you checked i.e. not on a sloping drive? And was the stick pushed home firmly each time. At last MoT the tester just dropped the stick in - wot a plonker. :lol: Oil doesnt expand much with temperature

[Guest SA Intruder]

Level Indicator on later models.

 

"Oil" expands by approx 6% volume from 10C to 100C (running temp)

[seatkid]

Coolant expansion bottle?

 

Central heating expansion vessel/header tank?

 

Etc Etc

In these cases most of the "expansion" is due to the vessels the liquids are i.e. engine blocks, radiators etc. I think you will find if you boil a pint of water, you'll still only have a pint and a bit. For water between .015% per deg C and 0.07% per deg C (varies between 15 deg - 90 deg)

[seatkid]

Anyway the difference beween grifjl's cold and one hour rested measurement should be minimal on the dipstick.

[Guest SA Intruder]

I think you'll find radiators, pipes and the like expand with temperature. They don't contract!

 

Next time you check the cold coolant level in your expansion bottle, measure how much more liquid there appears to be when hot.....because it has expanded.

 

Assume that coefficient of expansion for water (I know - coolant) is 0.000207, and there is 7L in the coolant circuit, and for arguments sake we assume that the coolant is raised from 0C to 100C (not unreasonable at this time of year!)

 

The volume will increase by a little over 2%, or just over 0.14 Litre.

 

In practice, it would be slightly less, because the coolant is not subject to quite the same temp differential.

 

This is irrepespective of the expansion of anything else - this is just the coolant.

[seatkid]

I think you'll find radiators, pipes and the like expand with temperature. They don't contract!

 

Next time you check the cold coolant level in your expansion bottle, measure how much more liquid there appears to be when hot.....because it has expanded.

 

Assume that coefficient of expansion for water (I know - coolant) is 0.000207, and there is 7L in the coolant circuit, and for arguments sake we assume that the coolant is raised from 0C to 100C (not unreasonable at this time of year!)

 

The volume will increase by 14.49%, or just over 1 Litre.

 

In practice, it would be slightly less, because the coolant is not subject to quite the same temp differential.

 

This is irrepespective of the expansion of anything else - this is just the coolant.

Ahem 0.000207 x 100 x 7L = 0.1449 Litres!

[Guest SA Intruder]

Indeed - well spotted.

 

Corrected - I knew I should have used a slide rule.

 

The ethylene-glycol coolant mix (50/50) has a higher coeff of expansion, which means that coolant should expand slightly more....

 

Unless my brain fails as well...

[seatkid]

I think in the case of an engine block, the piston wall expands much more than the outer wall of the water jacket, yes the block expands but the waterways actually contract. Anyway as Monty Python said "thats my theory ....

[Ivor_E_Tower]

My twopennies'worth:

I think if you boil a pint of water you'll have a large volume of steam and no water (?)

Also, the coolant in a car's cooling system is under pressure to stop it from boiling at "only" 100 deg C (or whatever the addition of coolant may alter the boiling point to).

Merry Christmas ! This is getting too technical for the festive season!

[Guest SA Intruder]

I think you're right Mr M,

 

seasons greets

[suzuki91]

with all this boiling water why not make a cup of tea? :wub:

save it going to waste.

[seatkid]

Forget the water, bring out the wine. Merry Christmas everybody.

God Bless

[Guest The Doctor]

I agree with all of SA Intruders comments except

 

Liquid does expand! Plenty. (Boyles Law)

 

Boyles Law relates volume and pressure of gases not liquids.

[Guest Turbo]

What you have to be careful of is ofcourse, when you change the oil and filter, remember to fill the engine to just below the maximum mark on the dipstick, then go for a drive and recheck after it`s cooled down after the given time. You shouldn`t be far away from the maximum mark then.

[Guest Turbo]

[Guest SA Intruder]

The Doc is right about Boyles Law....

 

It's been so long since I did "real" Physics (about 30 years)

 

Memories.....

[seatkid]

It's been so long since I did "real" Physics (about 30 years)

Me too!

 

If the water goes up by approx 2 cm (from min to max mark) in the expansion bottle (16cm dia approx) this equates to roughly 3.14 x 8 x 8 x 2 cc = 400 ml

 

But does it actually go up much less than this, so I reckon that SA intruders 140ml calc is about right and so liquid expansion is responsible for the majority of changes in visual reading.

 

must join the old codgers club....or certainly the Victor Meldrew society

 

As far as the mysterious oil readings go, something must have been wrong with the procedure...or is there some gizmo (electric pump) that can pump the oil when cold (I do hear strange electrical wizzing and humming under the bonnet at times when the engine is off but the ignition is on)

[grifjl]

Nothing wrong with what I'm doing. Several of us stood around and tried this. Always on the same driveway, not sloped.

 

As I say, given up with it!!! Seems OK now and due for a service in February, where they will overfill anyway!!!

[seatkid]

Nothing wrong with what I'm doing. Several of us stood around and tried this. Always on the same driveway, not sloped.

 

As I say, given up with it!!! Seems OK now and due for a service in February, where they will overfill anyway!!!

You didnt say, but I assume the engine was turned off for at least ten minutes prior to measurement in each case? (it takes ages to drain back when cold)

 

Yes! another reason to DIY - dealers always seem to overfill despite all the stickers on the engine :rolleyes:

[Guest T120]

Whilst we are talking about oil levels, expansion, coeficients, etc & the price of oil? Turbo who said

[seatkid]

who said