Diesel/Gasoline MPG???

[J&EINPA]

I am sure someone can explain this to me. If a diesel engine has more torque,(grunt), why is it when towing at full capacity load a diesel will drop in MPG by 50% over driving solo and a gasser will only drop 20% to 25%? I understand that solo a diesel gets much better mileage,I just don't understand the mechanics of why the high percentage drop, towing.
Thanks, John

[madmatt]

the operating systems of the two are completely different beasts. to put it as simple as possible ( not that I think your dumb, it's just a complicated system) Most all diesels I know of are designed to use and allow the max. amount of fuel at a designated "lug" point which is where the Max torque is needed and achieved, whereas a gasser basicly just regulates it's fuel to keep up w/ it's mixture needs, rich or lean. More engine speed = more fuel needed, less engine speed = less fuel needed. It's all much more complicated then I went into but I hope this gives you a general Idea, explaning the hole process would be a lenghty and time consuming post.

[markrinker]

Where did you get your statistics?

Everything Matt said, plus some other over-simplified observations, based on my use of Chevy's current 6.0L Vortech gas and 6.6L Duramax diesels for towing and transportation:

- Both have similar displacement;
- Both run about ~2K rpm at 'cruising' speeds;
- Both push the same trucks around well when the trucks are empty.

- Gas injection system maintains a nearly constant 14:1 air to fuel ratio throughout the powerband for optimal combustion of 87 octane gasoline at a static 9.5:1 compression ratio.

- Diesel injection system delivers a widely variable fuel to air ratio, based on power demand. Turbocharging creates a variable compression ratio (obviously the mechanical compression ratio is static, but the cylinder pressure is dynamic based on boost pressure...)at the same time, based mostly on engine load. Result: A much more dynamic, but narrower RPM band engine, well suited for moderate speed towing or fast empty miles - but not both!

The 6.0L gasser, by comparison, really isn't any good at either duty. The ROI gap of diesel vs. gasoline in light duty trucks widens as fuel prices increase.

My observation: 2005 6.6L Duramax average towing mileage is only 30% less than its average empty driving mileage. (~20mpg drops to ~14mpg.)

My observation: 2003 6.0L Vortech average towing mileage is similarly 30% less than its average empty driving mileage. (~12mpg drops to ~8.5mpg.)

Same trailer, same loads, same driver, same ground speeds. Been studying it for four years trying to make some money 'hot-shot' hauling as fuel prices have doubled (rates have not...) and diesel has swapped places with regular gasoline as the 'more expensive' fuel.

[ 10-10-2005, 07:33 AM: Message edited by: Mark Rinker ]

[rat4go]

Things to consider when comparing gas motor to Diesel fuel consumption---

Diesels have no throttle blade, so the amount of work that the engine has to do to 'pump' air on the intake stroke at light throttle is much less with a diesel than a gas motor. At heavy enough load, the gas motor is running close to WOT, thus similar intake air restriction that the diesel engine has to work against on the intake stroke, so at that point, it's just a matter of fueling. Gas motors at heavy load tend to run richer than 'perfect' 14.7:1 to keep stuff cooler, so they loose some economy that way. I'm less familiar with how a diesel determines air/fuel ratio at high load, but I'd guess that they don't run as rich as the gas motors as it'd result in black smoke out the exhaust.

Another advantage for diesels: diesel motors have higher compression ratio. Physics dictates higher compression ratio means higher effeciency (all other things being equal).

I think I also read somewhere that diesel fuel contains more 'energy' per gallon than gasoline, so you need a bit less to do the same work, but I can't find info to support this right now.

For what it's worth, I had a 6.0L V8 4L80 equiped 1500 HD with a 3.73 axle. It got 14 on a good day empty where similar conditions with my '03 DMax give me closer to 20 mpg. Towing my 7K TT, things dropped to about 8mpg with the gas motor and I'm usually near 12 mpg with the Dmax and that same trailer. In very round numbers, the diesel is 50% better than the gas motor in both conditions.

Another tidbit - I've keep track of the fuel costs really closely, and also do (via a spreadsheet) a 'what if I still had the gas motor truck' calculation. In short, the diesel has saved me about $2000 over 50k miles of driving over the past 30 months. In round numbers, this covers the difference in monthly payment.

Hope this helps!

Rich

[J&EINPA]

Thanks all for the reply and information, very helpful.

Did everyone see the srory about the first plant to produce diesel fuel from waste coal? The plant should be online in 2008, in PA. Lets hope that it works out well. They expect to be able to produce diesel fuel at a break even cost of $1.00 per gallon.

[Black95TD]

FYI Just ran across the figures again. 125,000 btu/gallon gasoline, 139,000btu/gallon #2 diesel and 135,000btu/gallon Kero.