Engine/Transmission (1998.5 - 2002) 18 wheeler vs CTD mileage

[Sageair]

As of late I have been hauling cows with a 2001 Peterbilt. This truck has a 475 H. P Cat 3406 . My question is if the Pete will get 8 m. p. g with an empty bull wagon and 6 m. p. g loaded to 80,000 lbs,why cant our CTD's get 8 times that mileage. We are generally at least one tenth the weight and probably no more than 25% of the areodynamic drag. Our engines are 5. 9 liters vs the 14. 7 of the Cat. I realize the Cat turns alot slower so there is some gain in frictional loss inside the engine but there must be more to it than that. Any thoughts?

 

[2ndgen2500]

more gears, efficient engines, lower revs... i really truly dont know the answer but i know for a fact there are tons on here that do.

 

[Newf Limo]

My Kenworth regularly pulls 100,000lbs and gets 6. 2 with a 430hp c-12 Cat. I think the torque it develops is considerable though, and to do that it burns the fuel. Looks like I need to turn up the CTD and get my efficiency up a bit!

 

[lil red cummins]

that is a darn good question



got me thinking here. my coworkers chevy blazer with a 350 gas motor gets around 10mpg. that really is bad!!!



it definately has something to do with the cat. they make a darn nice (and apparently efficient) engine. the few friends i have, that drive big rigs, are normally in the 4+ mpg range loaded.



the mileage is obviously not linear with the amount of weight hauled.



got me scratching my head on this one. cant wait to hear from some others.



jim

 

[Newf Limo]

Don't get me wrong...

if I step on that Kat's tail, she'll scream right along, but I can definately get worse fuel mileage!

 

[Ramafid]

The Mack CH600's that I drive have a 728 ci engine that makes 355 HP. I usually pull 60-70k gross and get about 6. 5 mpg give or take a little. Based on those numbers we should be getting lots better fuel mileage. On the flip side a diesel Jetta weighs about 3000 pounds and gets 48 mpg with a 90 HP engine. I don't think there is a linear relationship.

 

[dresslered]

I'm sure that emissions has something to do with it.

 

[Hohn]

Diesel engines have a distinct advantage in larger applications. In fact, the larger you make the engine, the more efficiently it will operate.



It's no coincidence that the largest diesel in the world (posted on here a couple times) is ALSO the world's most efficient, at least when you take about power per fuel burned (BSFC- brake specific fuel consumption).



Gernerally, a larger engine is more thermally efficient. That is, the ratio of Volume to Surface area improves (higher) as the cylinders get bigger. When you have less surface area for a given displacement, there's less metal area to bleed off heat energy from combustion, so more of it is put to work pushing the pistons.



It's interesting to note that while thermal efficiency goes up as an engine gets bigger (total displacement), it's also more efficient thermally to have that displacement divided among more cylinders. IOW, the 5. 9L dodge v-8 gas engine has a slight advantage in volume/surface area ratio over the 5. 9L I-6 Cummins.



I have a spreadsheet I made that shows how the ratio of surface area to volume changes as you change things like cylinders for a given displacement, ratio of bore to stroke, etc. It was interesting to see how the numbers played out.



Email me a Justin. Hohn@warren. af. mil if you want the Excel 2000 file.



Justin

 

[JohnE]

Originally posted by Hohn

...

Gernerally (sic), a larger engine is more thermally efficient.

...

That's true, but you also have many other factors to view.

Any engine that operates below rated torque or horsepower will not generate the same bsfc numbers.



The efficiency of internal combustion really drops after 5,000 hp compared to gas turbines.



The example of the Jetta with a 90 hp engine is a good one. A smaller rated engine sized for the load operates much higher in the thermal efficiency range than our Cummins.



Once you get into 8 liter and above range, most diesel engines are mostly designed for maximum torque (maximum thermal efficiency) than for peak hp. A 475 hp Bombed B series is just breaking 1,000 ft-lbs of torque. The same 475 in a CAT, Mack, DD, or Cummins is in the 1600+ range.



Jump to the 5. 9l gas engine. A gas engine gets best bsfc at WOT. Why? A gasoline engine has to deal with a HUGE parasidic loss from the pumping loss induced by the throttle body. Basically, the engine has to use energy to pump (suck) in air past a restriction.



The diesel advantage over gasoline is from 2 factors. It always operates at WOT thus no pumping loss and diesel fuel has 20% more BTU (energy) than gasoline per gallon.

 

[Coalsmoke]

To the top few posts:



You are basing your numbers on a linear scale, (looks like a straight line graph) not an exponential one (more of a curve). With the linear scale and the two engines cited, you could 'theoretically' create an engine that has infinite milage using no energy (fuel). All this is what Mr. Issac newton worked so hard to disprove, and we now follow a general rule of thumb called the "Conservation of Energy" which helps remind us that like under the table deals, if the theory seems to good to be true, then it almost certainly is). Now, all you have to do is find a way to "store"the Pete's energy perfuel used in little "energy bottles" and plug it into your cummins, and you will be getting the Pete's energy (per fuel used) in your cummins, which infact would give us that 8 times the milage you were asking about. Except for a whole bunch of other stupid rules that start to really complicate things, it sounds like an exciting impossibility.