why do diesels run so long compared to gassers

[Bhooper]

Well basically the subject line is my question. What is it about diesel engines that allows them to stack up so many miles compared to gas engines? I have asked several people but they do not know any more than I do about it. I tried the search but didnt come up with anything. I would think the high compression of the diesel engine would mean closer tolerances meaning faster wear. Any thoughts? thanks

 

[Shovelhead]

I would think that less moving parts, plus a low working RPM (less that 1/2 of a gasser) would contribute to longevity.



Other opinions?

 

[Deezul1]

Low RPM and built super strong to deal with bearing loading from extra high compression and torque loads.

 

[Snow man]

Heavy duty construction,Industrial strength engine. The Cummins is both. They are built for hard HD hard usage,and extremely overkill for our application.

 

[Cummins Cowboy]

well the engine is running on oil for fuel, prolly better than running 2 stroke oil plus it still has regular lubrication supply. the engine is way more beefed up internally. Another thing is there is less things to go wrong. There is no spark or timing system to through fits either. With most gas engines in cars, i don't think that most acutally wear out it is the other things around the engine that go bad

 

[Joe G.]

From what I have read about our engines, they were designed extra strong to survive lousy fuel in 3rd world countries. Joint venture of Case and Cummins. Designed for hard use by people who may not have the resources to maintain them properly. So they are just loafing in our trucks even when we think we are pulling or hauling heavy. Not even close to the work in an F650 dump truck working construction. Or in a commercial fishing boat.

 

[Bhooper]

thanx for the replies

All sounds good to me. . The fuel is kind of oily like Cowboy said so maybe it is lubricating a bit as it is burning, never heard that one. Also didnt think about the rpms being so low, even though I knew it. This is my first Diesel and only it had it for a few months. It had 115000 when I bought it and I knew from this site that is not a lot for this motor. Now I am up to 122 and I start thinking man thats a lot of miles, but I know it isnt really. Sure do like this truck though! Cant believe the guy sold it at such a bargain. . Any more ideas?

 

[Hohn]

A few thoughts on the Legendary Cummins durability

There are a whole bunch of reasons why a diesel runs so long compared to a gasser. There are other reasons why a Cummins is even better than a regular diesel. I am going to repeat some things others have said just to keep my thoughts orderly.



Why diesels last so long:

1) lower RPM range. Engine stress increases with the square of rpm, so doubling rpm from 2K to 4K means FOUR times as much engine stress.

2) They HAVE to be built stronger. Remember when GM tried to put diesels in passcars that were just converted gassers? All kinds of probs-- they just weren't tough enough. For the most part, diesels are designed to operate under conditions that would melt a gasser. 16:1 compression or higher? Onlya few race motors are built to that level in gasserland, and they don't have a turbo increasing cylinder pressure even more!

3) yes, diesel oil burning has a slight lubricating effect (as far as I know) but not much. It definitely helps upper cylinder lubrication

4) diesel fuel burns slower than gasoline does under pressure. This means that the pistons sees less of a hammer blow-- it's more like a steady push. This is relative to designed load. A diesel sees overall a LOT more pressure on the piston. This means that overall the internals have to be much stronger, which means heavier too. That's why diesels rev slower, as if they have a 100 lb flywheel.

5) mechanical simplicity. others have covered this.

6) tougher materials. this is related to #2. You will see extra beefy FORGED steel cranks, tougher piston rings made out of nodular iron and such.

7) A typical diesel has a lot more oil in the crankcase which helps component temperatures among other things. I would think that is would tend to make the whole engine run at a more even temp (less the turbo)



Specifically the Cummins:

1) fewer parts being inline 6

2) 7 bearings for those 6 cylinders, instead of a 8 cylinders on 5 bearings like v8. See the PSD vs CTD thread in the 3 Gen forum for more on this.

3) I am pretty sure that the Cummins NAME alone is worth about an extra 200K miles



Overall, it's just STRONGER. . sorry I can't really explain it better.



Feel free to correct any erroneous info. I fancy myself a gearhead extraordinaire, but I am pretty new to diesels, and have been wrong once or twice before



Hohn

 

[Joe G.]

Bhooper,



At 122K you can take the break in oil out now. Seriously, you are just getting started if you are trying to wear it out. My Green Monster has 206K on it now. I still have good oil pressure. Runs as good or better than it did new. Oil consumption is about like it was at 30K or so (messy blow by tube). I change oil every 5K and the dip stick still shows that there has been no observable oil use. I'm thinking the chrome kit should be good for an extra 200K on it.

 

[phoekema]

another idea

Joe G - Not only will the chrome give you more useful miles, but since the air will now pass over the engine easier, you will probably double your fuel mileage too!!! Sounds like a J C Whitney advertisement... ... ... .

Seriously, though, there is one other thing that may contribute to the diesel longevity. During the compression stroke in a gas engine, there is a mixture of fuel (in this case, gasoline) and air. The gas is almost an 'anti-lubricant', as enough of it will actually wash the oil film from the cylinder walls. This is not true of diesel, the fuel is not introduced until the piston is at the top (or very very close to it) of the stroke, plus diesel fuel is in fact a lubricant in and of itself.

 

[R.E. Miller]

If you want to see why diesels last longer then gassers, take two six cylinders apart similar in displacment and application and compare. Last week at work, we in-framed an 855. I think the mileage was 1,200,000. Original everything, if it weren't for the camshaft wearing out, it would have run longer. Bearings were really spent, but not bad for a million spins. The one thing that makes me wonder about the B is that it only has one cam bushing... I guess that just means less to spin he he he



Russell

 

[Big Best]

A stock diesels internal heat is 45% cooler than a gasser also.

 

[DiTrani]

Lower RPM

While most diesels are slow-turning, some of the benefit of this is offset by the long stroke that most diesels run (Cummins 5. 9 included). The real problem with high rpm running is piston velocity, which increases with stroke. Actually, the _real_ problem is how quickly the pistons are forced to accelerate and decelerate at the top and bottom of their strokes. Long-stroke engines operate at higher stress at a given rpm than short-stroke engines.



Now, because diesels seems to be more free to be big and heavy (per displacement vs. gas), they can (and I think do) employ longer con rods (which necessitate the engine being taller), allowing for lower peak piston acceleration. Longer rods also allow a long-stroke engine to have the force of the piston transmitted on a more straight line to the crank, which decreases stress on the piston and it's wrist pin. So the diesel wins back some of the benefit of lower rpm despite it's long stroke.



From a design point of view, when the total weight of the vehicle you're powering is large and your power/weight ratio is low, as in an OTR tractor/trailer combo, adding extra weight to the engine isn't nearly as important as in a typical gas-powered vehicle (car, light-truck), where the engine weight is a larger percentage of the total weight, and therefore a big factor in performance.



So I think most diesels are just built beefier than most gassers since they're built to haul heavy or do hard work. And I think there is something to be said for the lubricity of the fuel on the cylinder walls adding to ring life.



At least this is my impression from countless hours reading the SAE journal back in college.

 

[RonaldLarson]

The Cummins..

is the only real diesel in pickups, and it has several great features which enhance it's life. Ever see a connecting rod from a 5. 9? Clessie Cummins must have been a physisist. The rod is designed with the bearing journal turned 45 or so degrees around the crank journal. Make a fist with your hand, then form an opening with your fingers. That's what a typical rod looks like. Now, turn your wrist to the left. That is what our rods look like. What it does, is to take most of the pressure from the power stroke and the exhaust stroke and place it on the strongest part of the rod, instead of onto the rod bolts.

The coolant system in the Cummins is also clever. The coolant flow is directed so that it does not favor any part of the engine, but instead, maximizes even flow to all parts of the engine. Coolant between cylinders is becomming a thing of the past in gas engines, for space saving reasons. This causes the cylinders to wear egg shaped, and cool un-evenly.

There is no such thing as a stupid question!

Ron

 

[drees1]

Diesel Vs Gas

Some more observations.



Gasoline is easier to build cheap Hp at the expense of efficiency. This is why most personal vehicles are gas and most commercial are diesel. Anything that eats a lot of fuel in business has a requirement for efficiency.



Diesels are more fuel efficient for several reasons. There is no theroretical limit to turbo charging, where on gas detonation is a problem at some point on the boost gage regardless of octane used. The diesel cycle is theroretically more effiecient, but in practice is actually close to gas. The diesel does not pull a vacuum in the manifold because it does not need to regulate air to keep an air fuel ratio with in a burnable range, like gas. Gas engines waste hp pumping a vacuum at all conditions except wide open throttle. That is why big gassers don't get much worse mileage towing. Bad mileage empty from being a big vacuum pump, bad mileage towing from inefficiency.



The knock sound of the diesel is generated by the ignition delay and huge pressure spike in the cylinder. All components must be structurally built to handle that pressure spike. As a benefit, they are overbilt for most of the 720 degrees the engine rotates each cycle. Modern diesels are changing, and advanced combustion technology with pilot injection is creating softer combustion (for emissions). It is possible to design a lighter engine around the new combustion systems. The 6. 0 power stroke and VW TDI may be the start of that.



It is entirely possible to build a high mileage durable gasoline engine, but, so far, the extra expense to do so is not tolerated by the consumer. Remember, most original owners have traded before 100,000 miles so if its worn out, why should the OEM care? They are not in the market to make good used vehicles.



GM proved (several times) that a gas engine conversion to diesel cannot be done in a lightweight design. Volkswagen proved that to do it right, the peak firing loads must be kept in line with what the structure could handle. That resulted in a Rabbitt diesel with 48 - 52 Hp dog, that would go over 300,000 miles. The VW shares the block and crank with the gas engine.



So, it boils down to building a structure that can handle the combustion pressure loads, (and getting longevity as a result) for diesel or building the cheapest thing that will make Hp ( a gasser)... A combination of physics and Economics.



Doug Rees