Thoughts on diesel longevity

[rf33]

I was changing the oil on my 5520 today, and I got to thinking: Why does a 4 cylinder diesel engine take 9 quarts of oil, while most gas 4 cylinder engines only take 5. Got to thinking some more, and wondered why my 8 cylinder diesel in my truck takes 15 quarts while my 8 cylinder gas engine in my SUV takes only 6 quarts? I wonder if anyone has any thoughts as to why diesels seem to have so much larger an oil capacity. I thought perhaps the diesel engines we generally use dont last longer simply because they are over-engineered relative to common gasoline engines?

I tried to think of any fundamental reason for the increased longevity of a diesel, and I could not really come up with any. The lack of the spark plug / distributor system on a diesel is the only fundamental difference which I could come up with, and I think that the chances of that being the determining factor on the longevity seems unlikely at best.

Does anyone out there know why a diesel needs more oil than a comparably sized gas engine?

 

[njrqs]

Not sure but possibly something to do with the fact that diesel oil gets contaminated very quickly by the fuel and combustion by-products.
'
That's why even new diesel oil goes black almost as soon as you start the engine.

Now don't ask me why these by-products are worse in a diesel.

Another thing - My 2.8 litre diesel Hilux takes about 6 litres of oil, Only about 1 litre oil more than my 4.0 litres Falcon petrol engine.

Cheers

 

[Ozarker]

Diesels last longer because they are built stronger to withstand the higher compression ratios required of a compression fired engine and they produce their normal working torque at lower RPM's than their gasoline counterparts.

This can best be seem in a comparison between gas and diesel generators. A 5kw gas generator runs at 3600 rpm while a 5kw diesel generator runs at 1800 rpm to produce the same power. The diesel generator costs about twice as much as the gas one but will last about 5 times longer. Assuming that both are routinely serviced IAW manufactures specification's.

They need larger oil capacities because oil also acts as a coolant and there is more heat to move in a diesel engine.

 

[Rick Jay]

I'll give it a shot, but I'm an electrical engineer, not a mechanic, so keep that in mind.

A diesel engine has a longer piston stroke than a gasoline engine, hence the connecting rod and crankshaft must accommodate this extra travel. It would seem like the crankshaft would then be a larger diameter, making the crankcase larger, thus holding (requiring) more oil. I believe the extra piston travel and crankshaft diameter are what gives the diesel engine it's higher torque at a lower rpm than it's gasoline equivalent.

Just a guess, does this sound reasonable to anyone else?

~Rick

 

[RoyJackson]

I know the truck diesels carry a lot of oil.
My 670 only holds about 3 quarts...of course, it's only ~54 cu. in.

Diesels run a lot cooler (because of the lower RPM requirements) then gasoline engines...that helps longevity.
The main and rod bearings are thicker then those on gasoline engines... due to the higher pressures resulting from the higher compression ratio. The whole engine is more robust, in fact.
Now, as far as greater longevity for one of our tractors...I think that maybe somewhat debatable. Since a gasoline engine may well go over 100,000 miles (and routinely do), how many hours is that? Estimates for the typical compact tract run 3000 to 5000 hours before overhaul. What would that be equivalent to if used as a car or truck? Those are rhetorical questions...not expecting any answers.

Of course, the bigger trucks (Kenworths, Peterbilts, etc) and commercial equipment (Catepillar dozers, etc.)routinely go well in excess of 500,000 miles (or tens of thousands of hours) before an overhaul. Those engines cost as much as some of the smaller compact tractors do, however. I believe some of the pick up truck diesels may also see well up to 300,000-500,000 miles before overhauls (body would probably be gone by then though)

 

[bgott]

The oil is also a coolant, the more oil there is the longer it can stay in the pan and dissipate heat. Also, most diesels have a high volume high pressure oil system. When the engine is cold and the oil is thick it would be possible to pump the oil up into the engine faster than it was capable of running back to the pan and you could effectively run out of oil. Oil starvation on bearings operating under the pressures in a diesel engine ain't a pretty thought.

 

[rf33]

Thanks for the replies guys, I guess I was really just thinking outloud, and I appreciate the info. I really had not thought so much about the oil acting to dissipate the heat. I know that the higher compression ratios require beefier connecting rods, heads, and cylinder walls, so it stands to reason it would take a little more oil. Thanks for all the explanations to this engine dummy. Maybe some day I will get it./w3tcompact/icons/smile.gif

 

[ekrampitzjr]

Diesels have greater longevity just for the reasons stated. The blocks, pistons, and connecting rods are heavier. There is another important factor, however, that is commonly overlooked, which is that diesel fuel acts to a certain degree as a lubricant, which would generally be beneficial to an engine that can use it. After all, some people call the stuff "diesel oil". Recently, lubricity in diesel fuel has become an issue as refiners have had to reformulate diesel fuel to meet new environmental regulations by removing sulfur, which helped provide lubricity. Gasoline acts pretty much as a solvent and has essentially no lubricity.

The reason for greater oil capacity in diesels is not just because the oil gets dirty quickly, but also because a beefed-up block is going to have a certain amount of extra capacity from extra height, etc., in general, though this isn't true for all diesel engines. As one of the posts above mentioned, diesels typically have a longer stroke for more torque, which also affects block size.

The oil gets dirty quickly because the higher compression in a diesel more readily forces gases and soot past the piston rings and into the oil, even in a new or freshly rebuilt engine. This can't be avoided.

In turn, there are technical reasons why all diesels generate soot that I won't get into, but that's why diesel manufacturers are having a hard time eliminating "particulate matter" entirely from diesel emissions. It's in the nature of the beast. This issue is endemic to diesels, and little can be done about it without major external modifications.

All that said, diesels are among the most efficient engines going. They are far better in a tractor than a typical gasoline engine would be.

 

[rf33]

I actually have also learned that the diesel engine in my truck (a Ford Powerstroke) uses engine oil in the fuel injection process too. Not exactly sure the inner workings of this, but there appear to be two seperate oil systems drawing from this big oil pan. One high pressure used for the injectors, and a low pressure system used for lubrication. I have been doing a little homework on this engine lately, as it up and died on me going down the highway last week. Stopping a 7000 lb truck from 75 MPH with no power steering and no power brakes can be pretty interesting/w3tcompact/icons/frown.gif. Turns out it was a fairly common problem among Ford Powerstrokes called a cam position sensor. Who knew?

 

[dmccarty]

rf33,

I hope that you have been reading up on the PowerStrokes on
<A target="_blank" HREF=http://www.ford-diesel.com>http://www.ford-diesel.com</A>. If not go there fast! /w3tcompact/icons/smile.gif

One of the first things I did after buying the truck was to go to
an International Dealer and buy the CAM sensor. I'm going to
have to pay for it if it fails anyway and just having it in the truck
is cheap insurance if it should go. It seems like they have a
high failure rate.

I know how hard it is to stop and steer my F350. My road
ends on the top of a hill. At the bottom of the hill I have a gate.
Sooo, instead of starting up the truck to run 1000 feet down the
hill, I park the truck on certain "magic" spot. If I'm lucky I can
put in the clutch and let the truck coast down the hill to the gate.
I don't touch the breaks until I really have to since the seems
to be a limited amount of "boost" left in the system. Same with
steering. /w3tcompact/icons/smile.gif

BTW. I just got my oil analysis back on the second oil change.
Looks real good so far on the engine. Very little wear. The report
said it should be a good wearing engine. I think the engine is
still breaking in even though I'm over 8300 miles. The milage
is still improving. Last fill up was 19 mpg which was mainly
city and some rural drivng. The last week, about 150 miles,
the overhead is showing 20.0 mpg with the same driving...
Pretty danged good for a 7,700 pound truck! Very happy so far.

Later...
Dan