If Delo will go 400,000 miles in a pick-up diesel, what engine mechanical features are different in a gas engine to prevent it from reaching the same milage?
Thanks,
Dale
Thanks,
Dale
diesel vs. gasoline
- Thread starter djroil
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Diesel fuel has a much higher lubricity value compared with gasoline.
One is basically an oil ( diesel ) and the other is basically a solvent ( gas ).
When your fuel is a lubricant, it really plays a major factor in how long your engine will last.
I just sold my 91 Jetta TD with 580,000 kms on it. Still ran great, and started instantly.
One is basically an oil ( diesel ) and the other is basically a solvent ( gas ).
When your fuel is a lubricant, it really plays a major factor in how long your engine will last.
I just sold my 91 Jetta TD with 580,000 kms on it. Still ran great, and started instantly.
Thanks,
Dale
Dale
It is the problem of basic design itself, gasoline spark ignited engine have different load characteristic, gasoline engines run hotter with way less thermal efficiency than diesel, since diesels don't have to rev hard to make power, they are usually designed with extra strong low end and therefore all these factors plus some more make diesels last longer.
The higher compression, I think my Cummins is something like 17:1, and the typical inline 6 configuration makes for a heavy but stiff block with a massive crankshaft and lots of bearings. My engine weighs over 900 lbs. The sump in the 5.9 liter holds 3 gallons while the 8 liter engines hold 10 gallons. The diesels often produce lower horsepower for a given displacement than a gas engine buts lots more torque, as they typically operate at lower rpms and are tuned for different purposes. As an example the current high output 5.9 liter Cummins only produces 325 hp but 600 lb/ft of torque at 1600 rpm. The 11 liter ISM is offered in 280 hp to 410 hp ratings, very low for the displacement, but torque ranges from 1150 to 1550 lb/ft :^)
I hope to get 500k miles, and am currently getting 17 to 18 mpg in town and 19 to 20 mpg on the highway, in a 3/4 ton crew cab pickup.
I hope to get 500k miles, and am currently getting 17 to 18 mpg in town and 19 to 20 mpg on the highway, in a 3/4 ton crew cab pickup.
Another important factor is that most diesels in this country are commercial vehicles that pile on miles and get reasonably well maintained. They also have something like 30% fewer moving parts. In addition, the higher compression means manufacturers build them stouter. Mt 2000 VW TDI engine would surely go 300,000 miles, but I'll get sick of it and trade it in waaaayyy before that happens.
I have seen MB Om 616/617 easily cross over 500,000 miles with heavy use and basic maintenance, same goes for HINO commercial diesel engines as well as Toyota 2C engines, the lubricity factor of diesel fuel indeed helps in extending the life of average well designed diesel engines, however, take the early Olds gasoline converted diesels or some Detroit Diesel passenger car lemons and you also have some real nightmares.
The diesel is designed from the ground up to be a heavy duty engine. It is used in applications with high hours of run time. As pointed out the fuel has greater lubricity and doesn't wash down the bores. EGT tends to be lower all things being equal.
That said gas engines can live very long lives. Many of today's gas engines could run 400K miles given the right operating conditions (few cold starts, lots of steady highway miles). EFI has gone a long way to fix the cold start wear issues (bore wash).
That said gas engines can live very long lives. Many of today's gas engines could run 400K miles given the right operating conditions (few cold starts, lots of steady highway miles). EFI has gone a long way to fix the cold start wear issues (bore wash).
Lack of fuel lubricity was a very significant factor on older gas engines with auto chokes. Due to the crude calibration of the chokes, engines would frequently run richer than needed during the first few minutes after a cold start. The distribution in the intake manifolds was frequently poor, leading to heavy fuel dilution into some cylinders. This washed the bores of oil and led to heavy wear.
This led to tapered cylinder wear with a large ridge at the top of piston ring travel. I've seen this taper exceed .030" on the old iron V8 engines.
Both diesel engines and modern FI gas engines are far better in this regard.
[ September 17, 2004, 02:36 PM: Message edited by: tpi ]
This led to tapered cylinder wear with a large ridge at the top of piston ring travel. I've seen this taper exceed .030" on the old iron V8 engines.
Both diesel engines and modern FI gas engines are far better in this regard.
[ September 17, 2004, 02:36 PM: Message edited by: tpi ]
Malfunctioning injectors in an otherwise good diesel engine can result in very high exhaust gas temperatures, leading to holed pistons, burned valves, etc.
Fuel lubricity is a factor, but how big of one ? If it's the main reason that diesels last longer than gas engines then gas engines can never last as long as diesels (not) and turning a gas engine into a diesel will make it last as long as other diesels (not).
Fuel lubricity is a factor, but how big of one ? If it's the main reason that diesels last longer than gas engines then gas engines can never last as long as diesels (not) and turning a gas engine into a diesel will make it last as long as other diesels (not).
http://www.machinedesign.com/BDE/mechanical/bdemech5/bdemech5_1.html
Diesels generally become competitive with gasoline engines above 30 hp, and they tend to be a more logical alternative as power demand increases. They are normally chosen because of their operating economy and greater durability. In general, a diesel costs about 2 1/2 times as much as a gasoline engine, but on the average a diesel lasts about 2 1/2 times longer. Besides being more costly than gasoline engines, diesels also tend to produce more noise and vibration. Diesels also run over a narrow rpm range and generally require considerable shifting when used in vehicles without torque converters. They require fuel injection, which contributes to their higher cost.
Diesels have acquired their reputation for being rugged heavy engines primarily because they are built to withstand the high firing forces and high cylinder pressures that are a consequence of the high compression needed to produce spontaneous ignition.
Diesels generally become competitive with gasoline engines above 30 hp, and they tend to be a more logical alternative as power demand increases. They are normally chosen because of their operating economy and greater durability. In general, a diesel costs about 2 1/2 times as much as a gasoline engine, but on the average a diesel lasts about 2 1/2 times longer. Besides being more costly than gasoline engines, diesels also tend to produce more noise and vibration. Diesels also run over a narrow rpm range and generally require considerable shifting when used in vehicles without torque converters. They require fuel injection, which contributes to their higher cost.
Diesels have acquired their reputation for being rugged heavy engines primarily because they are built to withstand the high firing forces and high cylinder pressures that are a consequence of the high compression needed to produce spontaneous ignition.
The main reason is the diesel engine is designed for heavy duty use and to be run long hours. The other factors are significant but not overwhelming. EGT on a diesel will be lower, things being equal due to greater airflow through engine and more of the energy turned to power (less to heat).
Several heavy duty diesel engines are sold in a natural gas fired version. They have spark plugs. Everything I've heard they serve long hours turning agricultural pumps, etc.
Both engine types can be designed for extremely long life. Diesels are spec for heavy duty applications because they are more efficient.
Edit: Another plus for the diesel engine contributing to their use in large powerplants: The cylinders can be bigger without the threat of detonation. In a gas engine a very large cylinder fills with premixed fuel/air. Combustion starts and pressures rise. The increasing pressure will force detonation in unburned pockets of fuel/air.
Diesel engines with their compression ignition don't suffer from this problem. Look at some of the pistons from diesel powerplants, you won't see similar sizes in gasoline power
[ September 17, 2004, 08:34 PM: Message edited by: tpi ]
I don't think that fuel lubricity is a big factor, although I guess that's what GM hoped for when they managed to turn off an entire generation of motorists to diesel with some horrid engines a couple of decades ago. Their flagship diesel until the Isuzu came out was mediocre in durability, and so underpowered that if you towed you avoided places with mountains. Some gas engines have had a repuation for durability, like some of the Mercedes, Volvo, Toyota, the Dodge slant six, etc., engines, and the ones that last seem to be stoutly built and a bit underpowered.
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