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#1
Old 02-28-2004, 12:24 PM
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Why do diesel engines last longer than gasoline engines?

A buddy of mine recently bought a new truck with a diesel engine. He said that, unlike a gasoline engine, a diesel can easily go 500,000 miles without a major overhaul.

I nodded in concurrence, as Iíve heard this all my life. But is it true? If it is true, why does a diesel have such a longer lifespan than a gasoline engine? I thought the only major difference was that the fuel-air mixture in a diesel engine combusts due to pressure (thus no spark plug is needed), while a gasoline engine must rely on spark plugs. If this is the case, why would this attribute make the diesel engine last so much longer?
#2
Old 02-28-2004, 12:39 PM
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Quote:
Originally Posted by Crafter_Man
A buddy of mine recently bought a new truck with a diesel engine. He said that, unlike a gasoline engine, a diesel can easily go 500,000 miles without a major overhaul.

I nodded in concurrence, as Iíve heard this all my life. But is it true? If it is true, why does a diesel have such a longer lifespan than a gasoline engine? I thought the only major difference was that the fuel-air mixture in a diesel engine combusts due to pressure (thus no spark plug is needed), while a gasoline engine must rely on spark plugs. If this is the case, why would this attribute make the diesel engine last so much longer?
Just my $0.02, the greater pressures needed to make the diesel-fuel misxture combust means the engine has to be made tougher too and that has a knock on effect on reliability. Just a guess
#3
Old 02-28-2004, 01:36 PM
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One reason is that the diesel engine is generally a lower revving unit than the petrol engine, fewer firing cycles, fewer rotations of the major moving parts.

This alone would make a big differance, the valves will certainly last longer, and the cam and timing mechanisms will last longer, as will the piston rings.

I'd expect the gear train has very differant ratios and this would also mean fewer rotations of the major components.
The higher compression ratios, along with the higher loads generated by the combustion process will also mean that bearing surfaces need to be manufactured to take this load, either by using larger bearings, or by using better materials.
#4
Old 02-28-2004, 02:45 PM
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If he has a Dodge RAM with the cummins diesel, remember that that particular engine is designed for medium-duty trucks... it's probably engineered around operating a 24-foot straight truck, dump truck, or construction equipment.
Compare the redline on his engine to the redline on a gas V8. His redline's probably 2000-3000 RPM lower.
#5
Old 02-28-2004, 03:01 PM
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One thing that comes to mind is gasoline explodes during the ignition spark while diesil is more a sustained burn as it is injected during the combustion cycle. Add to that the fuels are different to take advantage of the 2 cycles and it would seem that galoline would be much harsher to an engine.
#6
Old 02-28-2004, 03:01 PM
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Engineering 101: You can design any device of any kind to be cheap OR to last a long time. The current state of the art in whichever technology is relevant determines the terms of the tradoff. But the tradeoff is always there.

New car buyers want cheap cars now and don't really care about out-year maintenance costs. Adding say $1000 to the price of the car to make the engine last 500,000 miles would not make the customers happy; they'd rather spend the $1000 on a better stereo or heated seats or a bigger car altogether. Even in luxury cars, folks care about price, or at least they care about having lots of features which amounts to the same thing, and super-long-life engines are not a feature they value. So cars don't have long-life engines.

Conversely, long haul trucks are run hard, 250K miles a year, often for 10 or 20 years before being retired. And their owners are businesses who have some idea of total lifetime costs of ownership. For them, paying extra up front for longevity and reliability makes sense.

There is very little inherent in Diesel vs spark-ignition engine design that guarantees one will always outlast the other. And, with proper care, a gently-driven spark-ignition car engine could also go 500K miles between overhauls. It'd use up a bunch of accessory-drive belts, alternators and waterpumps along the way, but the basic engine may well last the 500K.

And the Diesel will also use up a few accessories over the 500Kmile. For a big-rig, not many since those parts are also built for longevity, but the small Diesels in pickup trucks & such often use the same accessories as their spark-ignition cousins, with corresponding reliability/ longevity.
#7
Old 02-28-2004, 05:11 PM
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Good point LSL guy, but also I think a factor is most people only expect a car to last 5 years at a low to 10 years at a high end. At the avg of 15,000 mi/yr, we're talking about a lifetime of 150,000 miles, which with maintance most cars can do.

You also mention trucks lasting maybe 10 years or so, so the time frame is the same, just the truck is driven more miles, so I think designing for the expected lifespan has something to do with it, just like the mars rover and dust collecting on the solar panels.
#8
Old 02-29-2004, 09:51 AM
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kanicbird,

Exactly. Each machine is designed for the economic life its original buyers demand of it.

With the increasing use of new-car leasing and the attendant churn in cars, I wouldn't be surprised to see cars starting to be designed more for a 5 year life than 10.
#9
Old 02-29-2004, 11:01 AM
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Quote:
Originally Posted by LSLGuy
kanicbird,

Exactly. Each machine is designed for the economic life its original buyers demand of it.

With the increasing use of new-car leasing and the attendant churn in cars, I wouldn't be surprised to see cars starting to be designed more for a 5 year life than 10.
I would be. Toyota and Honda are designing their cars to last 500 thousand km, and other manufacturers are scrambling to catch up. It's not that the buyers/leasors of new cars care how long they last, but that they want higher resale value. And people who buy 3 year old cars do care how long they'll last. Cars don't lose their value nearly as fast if they have a longer expected lifespan.
#10
Old 02-29-2004, 06:05 PM
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Quote:
Originally Posted by LSLGuy
With the increasing use of new-car leasing and the attendant churn in cars, I wouldn't be surprised to see cars starting to be designed more for a 5 year life than 10.
While churn is a big problem, it's mostly because of increased American quality. Heck, I want to make jokes about the Focus, but it's actually a good car. That leaves me with the Cavalier, but I think it's discontinued now. Oh well.

I happen to work for one of the big two-and-a-half; in many respects we're still playing catchup to the Japanese automakers. Luckily we own parts of Japanese makers, so can take advantage of their engineering and practices in upcoming launches. The biggest competetive disadvantage, though, is the increased labor costs and inefficiencies that our US-based hourly labor pool causes versus the US-based foreign manufacturers. Hence dollar-for-dollar, I think any US-produced car will always cost more for the same quality. People pay a lot of attention to price, though, so we have to make compromises. Hence the big exodus to Mexico and Asia for manufacturing operations. Being US based, I'd like to think my job will be here for many years.
#11
Old 02-29-2004, 09:15 PM
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I've got to say that the cost of maintenance items on diesels is much higher than on gas burners. While an oil change on a pickup with a gas engine is around $30.00 the diesel engine can cost up to $80, and everything has to be changed more often on the diesel. And if something does go wrong it can really cost to fix it if it is out of warranty. I've seen bills for over ten thousand dollars to replace an engine on one of these.
#12
Old 02-29-2004, 11:11 PM
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Another point that I'm surprised no one has mentioned yet is lubrication. Diesel is a much heavier oil, and a much better lubricant, than gasoline.

A gasoline engine is a battleground between lube oil coating relevant surfaces to keep them slippery, and gasoline washing them down. As anyone who's been caught with no kero or degreaser can tell you (and while this is certainly not adviseable) you can clean oily party squeaky clean with gasoline. Squeaky clean is the opposite of well lubricated.

Obviously diesel is not as good a lubricant as lube oil, but it is a hellava lot better than gas. So a diesel engine is better lubricated.
#13
Old 03-01-2004, 12:46 AM
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Most of the 15 to 20 year old Mercedes Benzes I see are diesels.

In fact, I dont recall seeing a MB with a running gas engine made before 1980!
#14
Old 03-01-2004, 12:56 AM
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Quote:
Originally Posted by LSLGuy

Conversely, long haul trucks are run hard, 250K miles a year,.
total nitpick, but I drove long-haul for 6 years, and have to correct this.

The only way you're getting 250k a year on a class 9 rig (the driving part of 18-wheelers for those not in the know) is if you have a tandem driving team running the full 20 hours a day, 7 out of 8 days. The US allows for 10 hours total a day, and ca cumulative of 70 hours for any consecutive 8 days.

I broke several NHTSA and FHA regs during my third year, driving the shit out of my truck, but still only came in at 130k. From all I've seen I'd guess the ave to be 80-100k per year.

Anyway, back to the engines. Thanks for the time
#15
Old 03-01-2004, 12:58 AM
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Quote:
Originally Posted by duffer
The US allows for 10 hours total a day, and ca cumulative of 70 hours for any consecutive 8 days.
damn, that's for a single driver. Teams obviously can twice that amount.

Anyway.....
#16
Old 03-01-2004, 02:04 AM
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One thing is that you're comparing passenger car gas engines and long-haul on-road diesels like used in tractor-trailers.

I'd guess the main reason that the latter get much higher mileage is due more to the difference in mileage/types of miles typically put on the vehicles.

Passenger cars tend to get more stop-start traffic, FAR more cold starts, and far more short trips where the engine doesn't get warmed up.

Long-haul trucks on the other hand, tend to get far fewer cold starts and go at relatively constant speeds for longer periods of time.

This is my guess as to the differences in longevity.

Now if someone can compare say... Mercedes-Benz sedans with diesel engines to Mercedes-Benzes with gas engines, then we have something interesting.
#17
Old 03-01-2004, 03:19 AM
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great point bump. How many have seen "highway miles" in an ad?
#18
Old 03-01-2004, 10:21 PM
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Duffer,

Thanks for the clarification on long-haul mileage. I wonder when you were driving and for whom?

My experience with long-haul is limited to talking to my brother who did it for 6 months ending just recently. The company he drove for demanded the full 10 hours of on-the-road time every day and demanded they average 65 mph, for 650 miles per workday, and 26 days a month. That's solo, not team. Hence my math leading to around 250K miles/yr.

How did they really do it? Drive 18 hours and sleep 3 or 4, day in and day out, then fake the logbooks to appear to comply with the law. The sleep was usually in 2 or 3 90-minute naps while waiting for loading and unloading.

And his employer was widely regarded as one of the better ones in the industry.

The current state of the trucking industry cries out for automated enforcement directed at the employers, not the drivers. The drivers are required to break the law to retain their jobs and the employers get away with it every day.
#19
Old 03-02-2004, 12:11 AM
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Fewer parts = less to break

A few educated guesses and one WAG for you.

Diesels have fewer components to coordinate and maintain that internal combustion engines (ICEs). Fewer parts means theres less there to actually fail. No spark plugs, means no distributor/controller, no coils, less drain on the electrical system. Only recently have diesels been made compliant with the emissions standards of ICE cars, no catalytic, no EGR, no emissions crap.

The injectors are also in the heads and not the intake manifold. I find this significant because its one less hole in a part that should be streamlined. The heads are already full of holes and passages, one more wont matter much.

Diesels are also more thermally efficient. To some extent, the combustion chamber of a diesel should be kept hot to help the fuel detonate and increase the pressure of the incoming air. A hotter running engine won't need as much of a cooling system as a motor that needs to be kept cooler.

Heres the WAG. Diesel engines also get more miles per gallon of fuel, IIRC. That means the engine is making better use of what it has.
#20
Old 03-02-2004, 12:40 AM
dqa dqa is offline
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Diesel owner here (VW).

My understanding is that the longevity of diesels stems mainly from two things:
1. Lubricity of diesel fuel
2. Lower running temps (my thermostat is normally at 190 F)

Also, I believe the sparked gasoline explosion is a bit more violent than the compression diesel explosion, which puts more stress on things.
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