Engine oil in diesel engines

[rallen]

My brother in law has 2 diesels and he said that when you put new oil in it is the normal very light brown colour, but after 100 miles it becomes black like tar. I checked his oil dip stick and have not seen blacker oil before in my life.

Is there something diesel engines do to the oils?

 

[paulcallender]

My brother in law has 2 diesels and he said that when you put new oil in it is the normal very light brown colour, but after 100 miles it becomes black like tar. I checked his oil dip stick and have not seen blacker oil before in my life.

Is there something diesel engines do to the oils?

You mean apart from making it black? I can assure you this is quite normal and nothing to worry about. I don't know the exact details or the chemistry or physics going on to explain exactly why; no doubt someone else on the forum could provide this (oilman??) perhaps.

 

[fourstroke]

The oil is just doing its job. It is designed to attract all the gunk in your engine so that when you drain your oil all the nasty carbon deposits and stuff are stuck to your oil and not the inside of your engine. You might need to run an engine flush through your engine before the next change to make sure all the nasties are removed

 

[rallen]

But what can be in the engine that makes it go so black? Does the diesel burn black and somehow mixes with the oil through the piston rings?

 

[paulcallender]

Maybe its the little soot elves, that live inside the engine?

 

[steve kane]

I think you will find its the Elves alright,,,,that and the fact that Diesel does leave a very sooty deposit, you can see this if you take a peek at the exhaust tailpipe, my guess would be that being a very high compresion engine there is also a tendancy for there to be a little more leakage past the rings and therefore the oil is made dirty quicker than would be the case with a petrol engine, of course once the engine is a few years old its very dirty inside anyhow and the fresh oil is made dirty by this as soon as its put in, overall thats why (I think this is still the case?) diesel oil changes are at shorter service intervals. Steve.

 

[mjtray]

My 300D would have black oil within a couple of hundred miles of changing it....its down to the higher levels of detergent in diesel oil, and the soot that is produced from burning diesel.

 

[BaintonBeemer]

all replies correct but add:

if an indirect injection engine, the jet coming out of the pre-combustion chamber (in the cylinder-head) tends to blow past the crown of the piston and past the rings on the opposite side. Indeed, with indirect injection engines there can be substantial contamination of the engine oil with unburnt diesel fuel which is another reason for changing it regularly.

On a direct-injection engine (as older more commercial engines were, and common-rail engines are) assymetric pressure is not put on the piston rings, so there is less blow-bye.

Don't worry about the colour, but keep changing the oil!
bb
This is one reason why indirect-injection engines have a shorter service intervals than (even older) direct injection ones.