Why Diesel???

[jcmseven]

I should mention the head to head test I mentioned comes from all our favorite: Consumer Reports. The other test I have seen is on Pickup Truck.com. It has the gasoline v. diesel testing (minus the Tundra). I should note that my 2008 250 has a 13,000 pound towing capacity, as does my friend's GMC--both though with the caveat of a "weight distributing hitch!!" The Tundra, which looks miniscle in size next to my truck, has a 10,800 pound towing capacity, but makes no mention of weight distributing hitch requirements. In looking at the hitch on that truck in person, it does not seem to be as heavy as the one on either the GMC or the Ford, but it still is--I think--considered a class V hitch. One question I have not heard about yet is how these gassers are performing so well, even though on paper, they should be getting blown away. I do not understand this fact. Using a simplistic example, I traded a gas XUV Gator for the same thing in diesel. They are rated as only 1.6 hp and 3 lb.ft. torque different yet the diesel feels as though it has twice the power. It runs circles around the older gas one I had. It seems as though an equivalent powered diesel would run circles around any gas engine, yet even with much more torque both peak and usable, the diesels have a difficult time dispatching of the gas engines. One poster mentioned about hills. In the PickupTruck.com article they did hills, and the gas V10 Ford was really (uncomfortably) close to the diesels in performance there also. Go figure.

JohnM

 

[cp1969]

It's not hard to understand. Horsepower does work, not torque. If you have a 350 hp diesel engine and run it at the rpm that produces 350 hp, it will do exactly the same amount of work that a 350 hp gas engine will do running at the rpm that it produces 350 hp.

It's when we get away from these spots on the power curve where the comparisons begin to differ.

For example, let's say we have a 350 hp diesel that produces that horsepower at 2626 rpm. And we have a 350 hp gas engine that makes 350 hp at 5252 rpm. If you work the hp equation backwards you find that the diesel is making 700 ft-lb of torque and the gas engine is making 350. But they're both making 350 hp and if you have the transmission that will keep them at that peak horsepower rpm, they will perform identically (except for fuel economy).

It is in the portion of the power curve below these settings where the diesel kicks the gasser's rear. Many (most?) diesels are underrated, meaning they limit the amount of fuel given to the engine so that it does not exceed a maximum torque rating. If you graph the torque output of these fuel-limited engines, you find there is a plateau of constant torque from a very low rpm up to or nearly up to the rated max rpm of the engine. So for the case where both engines are operating at 1/2 rated rpm (1313 for the diesel; 2626 for the gasser) the diesel will be operating in a range in which it has as much torque as it did at full rpm, meaning it is now producing 175 hp. No gas engine that I am aware of has this characteristic, i.e., constant torque over a very broad rpm range because gas engines are air limited, not fuel limited. Let's be generous and say that the gas engine lost a mere 25% of it's torque. The gas engine is now producing 131 hp. The diesel produces much more horsepower at low rpms.

It is easy to notice the difference between 131 and 175 hp, a 33% increase.

Sorry for the math lesson, but remember, Professor Flink says, "Elementary chaos theory tells us that all robots will eventually run amok."

 

[DavesTractor]

This is a great thread. So many points. I have two Dodge diesels. One is an older 1997 model and we will retire it soon. The other is a 2006 dually 6-speed. With our heavy gooseneck flatbed, we are at nearly 15k empty and get 15-16 mpg at 58mph on the flats. With an 8k lb skidsteer on the deck (just a recent example) I got 12 mpg on the return trip. So we are at 23k lbs, cruising at 12 mpg with the engine seemingly not working at all. Pull this load in the mountains or at higher speeds and fuel mileage will drop considerably, but the truck feels like it has a lot of reserve. As we do this sort of thing all the time, this is the type of truck we need.

The 1997 Dodge 3/4 t automatic does local deliveries with a bumper pull. We use it for smaller loads, more local deliveries, etc. And we use it to run to town, do a service run, etc. I have been thinking lately that with the vast improvements in the new Chevy gas engines, that we would probably be smart to replace this Dodge diesel with a 3/4 ton gas rig.

Diesel engines partly became so popular as the gas engines were real slugs for a while. They have come a long ways.

Good discusion.

 

[ezslim]

Three little words: Trade In Value

I have always sold my well-cared-for (Cummins) diesel trucks with a little warranty remaining for a price at or above the cost of the diesel option compared to the KBB for a comparable gas job. Given that folks want a well cared for work or family truck that they know will last another 100k, I've never had trouble selling for a price that made it easy to buy another new one. In essence, after resale, the diesel option was free compared to a gas-job (three Dodge Cummins Turbo Diesels sold at approx 85,000 miles). Meanwhile, I got to enjoy real towing torque, 5-10 mpg better, and the melodic whine of a Holset turbo. If you buy well (see Melton Sales, OK), you can sometimes get as good a price on a diesel unit as some dealers will offer on a basic gas job. Not likely I'll go back to gas unless something really big happens in powerplant development. Warning: My advice is worth exactly what you paid for it!

 

[firedog]

It's not hard to understand. Horsepower does work, not torque. If you have a 350 hp diesel engine and run it at the rpm that produces 350 hp, it will do exactly the same amount of work that a 350 hp gas engine will do running at the rpm that it produces 350 hp.

Well that's the first time I've ever heard that. Most of the time it's "HP sells cars, Torque moves mountains".
I've always looked at both as mere equations representing different aspects of the same power your trying to achieve. However, looking at the curves for both the diesel & gas.
cummins: 1998 - 2000 ISB engine specs
magnum: http://dodgeram.org/tech/gas/specs/figures/97-59L_hpx.jpg

The pulling power of my truck matches the torque curve not the HP curve. The torque starts to drop off @ 2700 where the HP peaks. This is the same point where my power drops off. At 1400 the torque peaks and the HP is a mere 100, yet the power I have is constant throughout this range just like the torque curve.

But like many of the previous poster have stated. Who wants to pull a load with their engine screaming at 5252 rpm.

 

[unreconstructed]

You guys make me feel bad about that V-10 gasser I just bought. Edmond's showed the diesel at 6,800 retail and about 5,900 for the so called dealer's cost. Guess I'll be happy with my little gasser and the 5,900 bucks.

 

[Z-Michigan]

If you graph the torque output of these fuel-limited engines, you find there is a plateau of constant torque from a very low rpm up to or nearly up to the rated max rpm of the engine. So for the case where both engines are operating at 1/2 rated rpm (1313 for the diesel; 2626 for the gasser) the diesel will be operating in a range in which it has as much torque as it did at full rpm, meaning it is now producing 175 hp. No gas engine that I am aware of has this characteristic, i.e., constant torque over a very broad rpm range because gas engines are air limited, not fuel limited.

Actually, lots and lots of gasoline engines do have this characteristic; virtually all are turbocharged, and a lot of them are 1.5-2.5L inline 4's in smaller sporty cars. Look at SAAB I-4 engines for instance, a lot of them have flat torque from 2000rpm or less up to over 5000rpm. The cause of the flat torque is the turbocharger fixing all the breathing issues that for a naturally aspirated engine can only really be optimized at one rpm.

Of course I have never seen a turbocharged gasoline engine in a truck or tractor. Why do I think that is? Probably because once you have the expense of a turbo, the expense of making the engine a diesel is very little more and results in better economy. Secondarily, turbo gas engines generally require higher octane fuel to resist detonation, and that gets pricey. No point making a class 8 truck that takes premium unleaded and only gets 2mpg while burning it.

 

[jcmseven]

To all:

Thus far this seems to be a really good post. I, for one, have learned a great deal and am feeling a little better about owning my diesel truck and less bad about the money I paid for it. Specifically to Bradley, I have only one instance to report, but my V10 gas engineed truck (2001) model which I traded on my (2002) 7.3L diesel actually brought percentage-wise exactly the same is expected from a diesel truck. It was low mileage and in great shape but the dealer gave me top trade in dollar and still sold it within three days himself. I doubt he took a loss. This surprised me, but I was curious to see that at least in this case, the gasser held comparable value.

John M

 

[RollingsFarms]

You guys make me feel bad about that V-10 gasser I just bought. Edmond's showed the diesel at 6,800 retail and about 5,900 for the so called dealer's cost. Guess I'll be happy with my little gasser and the 5,900 bucks.

don't feel bad about that V-10 my friend. that is one heckuva motor! i'm still scratching my head as to why GM quit with the 8.1 option and now only offer the 6.0 as an alternitive to the Duramax. a farmer that i'm good friends with has a f-350 with that V-10 in it and he wouldn't trade it for the world. loves the thing! i'm not against a diesel motor, i'd love to have one, but i also don't want to have to live on the street just to afford to drive one.haha i make it just fine with my '04 Z71, works as hard as i need it to. maybe when i buy again i'll look for a good used diesel who knows. this is a very interesting discussion though.

 

[firedog]

There is an advantage for diesel in the way we buy it, but not at the gram level. Grams are a unit of mass (or weight) and diesel and gasoline have about the same amount of energy per unit weight (about 120,000 BTU/lb). If you'd have said 'gallon' instead of gram, you'd be correct. The advantage is we buy fuel by volume (gallon) but burn it by weight (pound) which accounts for about 14% of their 30% advantage diesels offer over gas engines in fuel economy.

Right you are....my bad....meant ml. (I must have typing mislexia)