Why do Diesels last sp long?

Can anyone explain to me why diesel engines in general last so much longer than gas engines? I've noticed that the compression ration in diesels is twice as high as gas engines - what does this number mean and why is it so high in diesel engines?

[dwight hinton]

While I am probably the least knowledgeable on the subject (and accordingly to my wife I am therefore the most likely to first respond), I will give you what little I know.

Diesel engines burn a less volatile fuel and the combustion process on a micro basis is a slow burn ignition. THe piston does not suffer the impact loading to the degree that a gasoline engine would. Gasoline is far more volatile and therefore generates more energy (Horsepower).

As I understand it, there may be some improved lubrication qualities realized in diesel fuels vs. gasoline.

Last but not least, is in light of the increased compression ratios, I understand that there is more "overkill"... icnreased level of safety factor in the design... beefier bottom ends than the basic design might call for used in the diesels.

I am sure there may be other reasons, (merely complexity of ignition timing, its effects when slightly out of spec., etc.) that may also play to the detriment of gasoline engines. I'll leave it to the pros to add the details.

Hope this helps.

[BENZ-LGB]

I used to own a 1984 300SD TurboDiesel (W126 body). Many people have said that the 5 cylinder diesel is the best engine ever made by Mercedes Benz. The principal reason for its logenvity, as suggested by Dwight, is that the engine's high compression requires beefier construction All critical engine components are made "tougher" than in other engines. This is the reason why the gas-to-diesel conversions that GM did in the 1980s were such an utter failure. If you take a gas engine and modify it to diesel, it will fall apart, like most GM engines did. With diesel you gotta start from scratch.

High compression is required to naturally ignite the air-fuel mixture without the use of a spark plug.

I recently purchased a batch of old MERCEDES STAR magazines (the ones from Mercedes Benz of North America, not the ones from the Mercedes Benz Club). There are some very interesting articles dealing with diesel engines. If gas prices keep climbing, you will probably see a renewal in interest in diesel technology.

Actually, if one does regular maintenance to a Mercedes gasoline engine, he should realize several hundred thousand miles on the bottom end. The heads will need valve jobs every 150,000 to 200,000 but the bottom end will just keep on going (just like the bunny )

[rickjordan]

The other reason why the 617 engine runs for so long may be because it's all cast iron. Of course I am referring to the block and head. Whereas later they went with aluminum heads. Why MB got away from such a proven "bullet-proof" design, I don't know. Diesels run cooler and slower. BMW's gas engines are racier compared to MB's gas engines, but which one last longer? How many early 80's BMWs do you see, compared to MB.

[SW]

I think for the MB diesels, the oil cooler helps a lot. Also, the 617.952 engine has oil cooled pistons as well.

[rickjordan]

I think all the 617 engines had oil cooled pistons. My engine is the 617.950 and according to the "oil circuit" diagrams from the manual, my engine has them as well. Also each lobe on the cam has a dedicated oil feed, as well as each cam bearing. Along the lines of "over-engineering", The oil filter housing is a lot more than just that. If everyone can picture their oil filters, the lower portion is used when the oil temp. is below 110 c. Once the temp gets above that the oil is filtered through the finer filter that makes up most of the filter. All this temp. sensitive filtering is done by the oil filter housing. Also it directs oil to the oil cooler, again depending on the oil temp. If anyone has the manual, take a look at it, it's amazing how well though out this seemingly simple device is. All along I thought it just held the filter.

[william rogers]

I hate to say it because I dearly love my two diesel MB
cars. But As a one time pro and now a hobby mechanic I have seen a lot of Japanese cars especially Toyota pick ups with 300 k on their engines......
William Rogers

[bobbyv]

maybe because diesels are also run at lower speeds, with their torque down there in the lower rpms.

i believe the ignition of a diesel fuel charge is more violent than that of a spark ignition engine. It depends on compression for ignition, which in a gas engine is pre-ignition, which causes knocking or pinging. Hence diesels are inherently noisier - they are constantly knocking. But most are overdesigned and last very long.

an other question would be: why are industrial, marine and long-haul transport engines diesel? Is it because of their reliability and relative simplicity of operation? Or maybe their high-torque, low-rpm application?

and where does the charm of a benz diesel come from? Is it the way it gathers steam then just cruises? Is it the smell of diesel fuel and fumes? I'm sure the sound of the engine is right up there in the list too. And of course the knowledge that is dead reliable. Or maybe it just fits the Benz character - solid, mature, reliable motoring, no need to be hurried.

I have never imagined myself buying a diesel car, but after reading all your posts, I am very tempted to buy an 85 300D Turbodiesel (someone stop me!). Preferably a wagon. (someone stop me!) In some shade of brown. (someone stop me!) Then i'll put Euro lamps (someone stop me!) ... But then i saw this Hunter Green turbodiesel wagon (someone stop me!) ...

There is indeed a valid reason that AN EQUALLY CARED FOR AND EQUAL QUALITY diesel engine WILL last longer than it's gas equivalent.

The byproducts of combustion in a gasoline engine is carbon. Carbon is a mild abrasive. The byproducts of combustion in a diesel is basically a dirty, sooty, OIL, not abrasive.

There is one other reason, in a diesel engine, you typically do not burn valves and other ills that happen to a gas engine upon leaning out for some reason.

Sure, you can say something like "my Uncle's, second cousins Toyota went farther than my brother-in-laws Benz diesel". But this is no scientific way of comparing a gas to a diesel. These two cars were driven and maintained by two completely different individuals with two completely different attitudes.

Have a great day,

[RunningTooHot]

Larry, could you please explain why the carbon produced during the combustion of diesel fuel (soot) is any less abrasive than that produced by a gasoline engine? Is it a different form of carbon? (Such as graphite, which we all know is not exactly an abrasive.) Or perhaps it is due to particulate size? Just curious.

Also, FYI, you stated that “in a diesel engine, you typically do not burn valves and other ills that happen to a gas engine upon leaning out for some reason”.

The reason is that leaning out the mix in a gasoline engine increases the combustion temperature significantly, but only up to the point of a misfire condition. A diesel is inherently running lean (except under full load?) due to the lack of throttling. The lack of throttling is also a source of increased efficiency due to the lack of pumping losses. This is why there is so much interest in gasoline direct injection. The unfortunate part of GDI *is* it’s lean running condition which produces too much NOx.

Thanks.

[dwight hinton]

An additional point on the 5 cylinder turbo diesels.

(1) The engine was originally designed to run turbines on remote stations in germany where they were seldom monitored. They ran non stop reportedly for years. It was not until much later that the engine was realized to be so absurdly reliable, that it was decided to be used in the mercedes automobile line.

(2) Mercedes has often referred to the 84 300DT as "The Million Mile Car".

AS for diesel drivers being slower, the imput from others on this site is to the contrary.
In my personal experience, my diesel spent most of its life ranging from 80 to 108 mph often on 4-10 hour trips at a time. When I reluctantly sold it at 265,000 miles to a close friend to upgrade myself to an S Class, it was still running like new and would get 20 mpg at 108mph. On the rare trips with my wife, it would get 28mpg at 65 (no honey I was not dozing off, I am just suffering from Attention Deficit Hyderactivity Disorder (ADHD) and am currently lacking sufficient visual stimulation at these reduced speeds). I generally check the condition of my 300DT by top speed. In its life, (outside of times when it needed something...& top end usually would tell me that) it stayed in the 104 to 108 mph range, which I consider to be an acceptable deviation on flat level ground for an extended time period.

While I realize there are 450's, 380's, and even maybe a 560 with 300 to 500K miles, they do not appear to be as prevalent as the 300D's which are common in my area with 300 to 500K miles. My 1984300DT @ 265K miles was a "youngin" in the neighborhood, a mere child.

At last contact, my friend has some 325K on the motor, and still no one has been into the motor (top or bottom) at all.
(However, The AC evaporator core is another story...ouch!)
Good luck to all. Wish I still had my 300DT, but I least I get to see her periodically.

Dwight



[Edited by dwight hinton on 06-29-2001 at 03:51 PM]

[Kebowers]

For most urban driving (stop and go, mix of short and long trips), a gasoline engine will last longer than a diesel simplt because it will have less cylinder wall and piston ring wear caused by unburned fuel. This assumes both are properly maintained with oil changes and quality fuel.

However, for continuous high load operation or long day running like taxi cabs and over the road trucks, diesels will last longer.

At higher loads, gasoline engines will have higher cylinder and ring wear than diesels because of higher temperatures.

Start-stop short trip driving is very hard on diesels.

KEBowers,

Please explain how unburned diesel fuel, which is nothing but oil, causes premature wear.

All other elements being equal, a diesel will outlast a gasoline engine under any driving condition.


RunningTooHot,

In spite of the fact that soot is present, the byproducts of combustion in a diesel are basically oily. If you are using universal grade oil in a diesel, as you should, the soot dispersal additives prevent the soot from having any detrimental effect other than making a really nasty stain on anything it touches.

Most everything in a diesel is backwards from a gas engine. More fuel versus air causes increased combustion chamber heat in a diesel. In a gas engine increasing air vs. fuel increases combustion chamber heat, and it certainly can increase it to a point where it does much more damage than merely causing a misfire. A lean condition in a gas engine can completely destroy it quickly with holes in pistons and burnt valves.


Again, if you are comparing under like driving conditions and maintenance discipline, the comparable diesel will outlast a gas engine EVERY TIME.

Have a great day,

[ke6dcj]

Someone had mentioned the low RPMs, and I believe that along with the closer tolerances of a low RPM engine plus long-wearing materials will make a well-designed diesel engine last a long time.

Check out the following links on engine life:

http://www.metricmechanic.com/pg78.htm
http://www.metricmechanic.com/pg79a.htm
http://www.metricmechanic.com/pg80.htm

Enjoy,

:-) neil