Does a diesel last longer than a gasser? - PeachParts Mercedes-Benz Forum

87300D · #1 06-14-2003, 03:14 AM

I posted this response last month to a very long thread and never got a response so I wanted to through it out again. The discussion was why diesel last longer than gasser. Many said they do not. Below is my reply to them. I am interested in hearing other thoughts.

Well I wasted time looking on the internet for torque info when I had it in my 124 Service Manual. Shows you how smart I am. Here is the HP/rpm Torque lb-ft/rpm for the engines listed.
Gas
130.940 6cyl 158/5800 162/4600 M.Y. 1990
103.983 6cyl 177/5700 188/4400
119.975 8cyl 268/5700 295/3900
119.974 8cyl 322/5700 354/3900
104.980 6cyl 217/6400 195/4600 4 valves per cylinder

Diesel
602.962 5cyl 120/4600 164/2400
603.960 6cyl 143/4600 195/2400

In most gas engines the hp is higher than the torque but you can see how much I know about Mercedes gassers. In all except one the torque is higher than hp. But back to my earlier statement about torque in most gassers, they do not reach max torque until significantly higher rpm than the deisels, as in the case above. You are never going to use the max gas torque numbers from above unless you are going somewhere fast. The only 6cyl from above that has the torque of my 603.960 is the suped-up 104.980 but notice that it does not reach the max torque until 4600 rpm. What that means is the 104.980 is going to get there a hell of alot quicker than I would but I could drive around town and on the highway at normal speeds at significantly lower rpm. That means less engine wear. This is one of the reasons a well built diesel should outlast a well built gas engine.

Reason number 2) Diesel is a light oil and gas is a solvent. Until we get DI gas engines, the intake valves are exposed to a solvent. This is why diesel intake valves should far outlast gas valves. The same case can be made for the cylinder walls as well. Onto the exhaust valves, yes that black smoke coming out of diesels does still have some lubication properties until at least we get low sulfer fuel.

Reason number 3) Diesel engines are built heaver. Sure you need it with a compression ratio of 22 to 1, but the engine will handle other stresses better. We are talking well built engines here. I know everyone has a crappy diesel story but this goes the same as gas engines as well.

There may be other reasons but I think these are the big three.

The Warden · #2 06-14-2003, 05:03 AM

Quote:

Originally posted by 87300D
Reason number 3) Diesel engines are built heaver. Sure you need it with a compression ratio of 22 to 1, but the engine will handle other stresses better. We are talking well built engines here. I know everyone has a crappy diesel story but this goes the same as gas engines as well.

You hit the nail straight on the head there! IMHO that's the single biggest reason that diesels will outlast gas engines. In addition, diesels have fewer things to go wrong. Diesels lack an ignition system (glowplugs don't count

), and their fuel system is only somewhat more complicated than a carbureted gas engine (injector pump internals aside, but the only engines that I know of that have injector pump problems are the IH 6.9l's and 7.3l IDI's

), and IMHO easier than an EFI engine.

Looking at it from a somewhat different tack, it has been my observation that most American built gas engines will go about 100,000 miles before some major maintenance is needed (rings go bad, fuel sysem issues, lubrication issues, etc). They may still run, but they won't be "right" per se. American diesels, on the other hand, can go more like 300,000 miles before reaching that point. The diesel that's in my pickup has over 200,000 miles on it, and I've relaced the injector pump, but have otherwise had no engine issues (I've had problems with the truck overall, but this was mostly due to it sitting in the desert unused for a few years, and stuff dried out). The engine's never been torn into, the head gaskets are original, and she'll fire on the first crank. Most Ford gas-powered trucks that I've seen on the road of this genre are much the worse for wear; theyre typically either not firing on all cylinders, or are putting out smoke. Are there exceptions? Of course there are, just like there are diesels that haven't gone very long (improperly maintained, have a GM logo anywhere on 'em

etc). The Cummins 6BT that Dodge uses is capable of going past 1 million miles! I'd love to see one of Dodge's gas engines pull that off.

One other way to look at it is to look at the trucking industry. You won't see a gasoline-powered big truck out there anymore. Why? You could build a gas engine that can put out that kinda power. But it won't get anywhere near the fuel mileage that a diesel will get, and won't go to 750K miles before needing an in-frame, either.

Just some thoughts...I'm a firm believer in diesel power, as you can see.

Take a look at the "Why are diesels better?" link in my signature...that goes to a paper I wrote a few years ago about the advantages of diesels over gas engines. Thought you'd be interested.

FWIW, I have most of the maintenace log from my 300D, and unless some major engine problems happened between 196K and 223K miles, the engine's never needed any serious work other than preventative maintenance, and other than some blowby, runs like new. To me, that says a lot, especially at 240K miles.

MS Fowler · #3 06-14-2003, 07:08 AM

Here's a few more...
The fuel is a lubricant, not a solvent. The injection pump thus runs in lubricant, inside and out.
Engine speeds are slower.
The fuel burns slower, but with more total energy than gasoline, thus the loads on the pistons are generated over a longer period of time. As opposed to a gasser where the force of the gasoline burn, is more like hitting the piston with a hammer.
Unless, I am really mistakes, the actual temperature of the combustion process is lower, thus all components in the combustion chamber, (pistons and valves) are subjected to lower heat stress.
Most of these were mentioned in a "Road Test " tear-down article of a Peugeot 504 diesel.

#4 06-14-2003, 09:15 AM

In the days of carburetors diesels did INDEED outlast gas engines with the exceptions that are always experienced.

The BIGGEST reason for the delta was that the carburetor was dripping raw fuel down the cylinder walls duriing cold start ups. This acted as a solvent stripping the oil film.

The intake valves do not last longer because of fuel going over them. In a gas engine things occasionally happen that cause a lean condition that will burn valves. This is not an issue in a diesel.

Yes the diesel fuel is nothing but light oil. The byproducts of diesel combustion is an oily soot rather than abrasive carbon as in a gas engine. This not because diesel is an oil and gas is a solvent, rather the carbon by product being the abrasive.

Now that we have fuel injection gas engines, I think that an equal quality, equally well cared for gas engine will last about as long as its diesel counterpart.

My $0.02,

73MB280SEL · #5 06-14-2003, 09:40 AM

I agree with Larry, what kills Gas engines is the cylinder fuel washdown. That used to be an everyday event with carbureted gas engines due to the lack of good control of the fuel to air ratio. That in turn is due to a primitive fuel system (carburetor) that is unable to cope with all of the various environmental factors it experiences (cold temp/wide open throttle, ect).

Remember, a gas engine must run pretty darn close to 14.7:1 air to fuel ratio. A Diesel runs all over the place, but never gets near stoichometery.

With today's computer controlled fuel injection systems, the air to fuel ratio is controlled more exactly. The only time you get into an excessively rich situation is when a sensor fails (like the temp sensor). In this case, the check engine light should come on.

One other thing: You normally worry about EXHAUST valves and burning, not intake valves. They are the ones exposed to the high temps. Diesels will have a lower exhaust temperature than an equivalent gas engine. That is due to the higher compression ratio and is a natural result of the Diesel being more thermodynamically efficient. If you extract more work from the burning fuel, you have less heat left in the exhaust. This makes it easier on the exhaust valves and turbo.

That being said, MB's hardly ever burn exhaust valves anyway. They are typically sodium filled (at least on the gas engines).

BTW: There are plenty of examples of well cared for gas engines that have high mileage on them. One of the Technicians that works for us has a Ford pickup he bought with 35k miles on it and now has 350k on it. He drives from radar site to radar site and really racks up the miles. Another guy, Mr. Jordan, bought a new '76 Chevy stepside pickup with an iron headed Chevy small block in it. He lives in Norman, OK and commutes to OKC. He has 500k on his ORIGINAL, NEVER OPENED engine. Of course, he cares for his pickup meticuously. The ONLY nonscheduled service he has ever needed is R-12 to R-134 service.

Sholin

jfikentscher · #6 06-14-2003, 10:29 AM

I'm not sure of the answer to this question, but I had a 1987 190E 2.6 that I bought with 96,000 miles on it. I ran it to 450,000 before trading it in on the diesel. Of course I took care of it, 3000 mile oil changes etc.

In the end it was using a lot of oil. About a quart every 400 miles. The exhaust system was so clogged that the engine couldn't breathe, so I took it to "Johnny Muffler". The small pre-mufflers on the 2.6 engine were so clogged that even a hammer and chisel would not break up the clog.

He said that when an engine can't breathe correctly it will tend to suck oil into the combustion chamber therefore using alot of oil. After he fixed it it ran ok but I traded it in anyway (two days later). I wasn't sure his oil story was on the level.

Well I thought 450,000 was alot on a gas engine and if I waited to sell it I would have more problems.

I love the diesel. Now havw about 206500 on it and looking for 500000 more.

Joe.

psfred · #7 06-14-2003, 11:37 AM

Gas engines are lower compression, so there is less pressure on the bearings on the upstroke. Many manufactures make them less sturdy (along with the notion of disposable engines -- 100,000 and they've been thrown away anyway thinking), but the controlled explosion of the gasoline air mixture generated considerably more heat and pressure than a diesel burn -- diesel fuel is injected for a rather long period, up to most of the power stroke at full load, so the pressure and temp are lower, but maintained longer.

This long burn is the source of the higher torque, since torque in a piston engine is pressure applied to a piston top. The drawback is that the engine must be built considerably heavier to withstand the high compression stroke stress, particularly the crank journals and head. Along with that, the long burn actaully causes the piston crown to get hotter because the temperature of the burning charge stays higher longer than in a gas engine (unless you have detonation in the gas engine, that will melt the piston!). Turbo diesel always have oil cooled pistons -- there is an oil jet in the crankcase that sprays oil up into the piston, and the piston these days usually is a diecasting, with a rather intricate oil channel that runs the oil around the piston several times on the way out to transfer the heat to the cooling system rather than to the oil pan.

A gasoline engine can be made to last just as long as a diesel -- MB engines are famous for lasting forever -- at the cost of more weight and size. The mains on an M117 are actually slightly smaller diameter than a Chevy 350, but are almost twice as wide. I've seen 350s run 300,000 miles and more, but they are the exception rather than the rule. It's fairly rare to see an M117 with a bad lower end -- most go 500,000 or more in normal use without excessive wear.

Peter

Ken300D · #8 06-14-2003, 02:39 PM

I'm a little fearful of the all-aluminum engines that are showing up on some products these days. Just having an aluminum head can be something of a drawback too.

But, the maintenance regimen of the owner is a major variable.

I've been lucky enough (and perhaps through frequent oil changes too

) to have never required any major engine service on anything I've owned. That includes one lucky 2.6-l GM engine with 350k miles.

It used to be routine to require a valve job in the 80-100k mile range on a gasolene engine. Somehow the designs have changed and that is not required as often now.

But you can still get a "lemon" design. We can sure expose plenty of them, even in the Mercedes lineup.

All said, just look around you. What 20-year-plus cars do you still see on the road as daily drivers? W123 diesels. I don't ever see any of the Ford Mustangs from the 1980's like I once owned. The square boxed end style that followed the Mustang II (which I also owned - and you don't see either.)

Ken300D

psfred · #9 06-14-2003, 03:32 PM

The all aluminum engines usually last longer than cast iron (with the notable exception of the Chevy Vega engine!) because the cylinder walls are coated with a silcate material. Looks like opalescent glass, and is MUCH harder than the rings. No wear at all unless seriously overheated, in which case the coating will fail.

No visible wear at 300,000 miles on the one I saw apart. The bearings are the same, so is the crank, should last forever.

Peter

jcd · #10 06-14-2003, 05:42 PM

It is my feeling that Mercedes diesels, and probably other Mercedes, thrive on maintenance. You maintain them and they last, you maintain a diesel, it lasts "forever".

IMHO, Mercedes diesels are like Zippo lighters. Replace the wick, flint and keep it full of fluid and they last forever. "Most" other cars are like Bic lighters, buy them, use them, throw them away.

As most of you know I love my 300D. The inherant reliability of a diesel combined with the great engineering of this car make is a joy to drive for a LONG time.

My $.02

JCD

rwthomas1 · #11 06-14-2003, 09:45 PM

I have to agree with most of what has been posted here, especially Larrys post. There are a couple of things more I would add: Most diesels are commercial vehicles. As capital investments they are often treated to far more rigorous and regular maintainence than the average car. Lets remember a Peterbuilt costs well over 100K and doesn't make any money if its broken. Passenger car diesels attract a certain type of person. Look around this board. Fanatics all of us. Regardless of income we do what it takes to care for these vehicles. Again a far cry from the average Camry driver. With regular maintainence I see no reason why almost ANY gasser on the market wouldn't go 300K. This is not the reality of the disposable society we now live in. I have seen far too many gassers in commercial service with 200K+ and many at 300k to think that modern gasser technology is in any way inferior. Its just the way most cars are treated that kills them. Ever wonder why most neighborhoods have 20K-40K in vehicles parked in the driveway and the garage is jammed with crap that isn't worth more than $100? A car is the second most expensive thing most people with ever buy yet they are often the most abused and mistreated. Its silly really to watch people throw money away by not maintaining their vehicles. RT

BlueBabyBenz · #12 06-18-2003, 12:45 PM

My two cents....

Sometimes when people see I drive a Mercedes Diesel, I need to apply what I refer to as my Average Guy-to-Mercedes-Guy dictionary to translate their comments.

Example 1

Average Guy- Oh. I see you have a diesel. I hear they go forever.

Translation- There must be some reason you are driving an old car that is slow and stinks when you go up hills.

Example 2

Average Guy- I hear diesels have a lot of torque.

Translation- My car is a lot faster than your diesel, but I think its a different kind of speed. ( I love that implication. I get that one from a buddy who has a 16V.)

My point is this. Although we can intellectually justify any purchase we make somehow, in the end it only matters that we have a gut satisfaction with what we drive, wear, eat, etc. If we don't, we should do our best to change your life so that we do have satisfaction.

Isn't that what life is about?

Brad123D · #13 06-18-2003, 06:19 PM

A 1968 Mercedes 220D owned by Edward Donaldson went 902,000 on it's ORIGINAL DIESEL engine. Irv Gordon's 2 million mile Volvo P-1800 had it's gas engine rebuilt at 675,000. Both of these are standing world records as far as I know. The Volvo-most total miles on a non-commercial vehicle. The DIESEL Benz- Most miles without major engine service.

CarolinaMBZ · #14 06-18-2003, 06:44 PM

Scanned some of the posts and didn't see this, but my understanding of one of the primary reasons for Diesel longevity vs. sparkers is that a when a gasser begins to burn oil from wear, the plugs foul and in addition to operational problems, starting becomes very difficult. With a Diesel at the same wear point, it functions fine with crankcase oil simply serving as an auxillary fuel source. At least as long as it is replenished as part of "refueling" and you have enough compression to get it running you can go forever.

73MB280SEL · #15 06-18-2003, 08:10 PM

I'll agree with that too. A diesel has to be really worn to not run pretty well. I've owned a few gassers that burned so much oil you could pretty much do a continuous oil change (i.e. burn 6 quarts by the time the change was due), but they didn't run very well.

If you can get it started, a Diesel should run well enough to get around.

Sholin