BertP
Edmonton, Alberta, Canada
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Joined: 11/07/2003
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Already been done. It is difficult to find, but I do remember reading a couple of times about the DMax and 8.1 going head to head in quarter mile pulls with (I believe) 12,000 lb trailers.
Here is the text that Pete sent to me:
[i][blue]If you want something to play with regarding these, I know that www.macktrucks.com has spec sheets for all our engines and transmissions.
First a basic one;
MPH = 60 x RPM/ M x Ra x Rt
where M = tire revs/mile (a truck reference is 504 revs/mile for 11R22.5 tires). Ra and Rt are the axle ratio and transmission ratio, respectively.
So, for a specific vehicle speed and a set gearing, you'll get hard rpm options.
You can also rearrange the formula to get the impact of final drive changes or determine optimum gearing, etc.
Road-Load Power (Pr) is next. This is the power in hp required at the wheels to move the vehicle at a certain speed. The base formula is:
Pr = [CrWv+0.0025CdAvSv2]Sv/375
Wv = Vehicle weight (GVW or GCW) in pounds.
Sv = Vehicle speed in mph.
Av = Frontal area in ft2 = width x (height - 0.75).
Cr = Coefficient of rolling resistance:
Pneumatic car tires on:
Pavement = 0.015
Concrete or asphalt = 0.013
Gravel = 0.02
Tarmacadam = 0.025
Unpaved road = 0.05
Field = 0.1 .... 0.35
Pneumatic truck tires on concrete = 0.006 .... 0.01
Cd = Coefficient of drag:
Cars = 0.3 - 0.5
Most trucks and tractor/trailers = 0.7
Doubles, triples, and flatbeds with loads = 0.77
Car haulers = 1.00
That's the power to move the vehicle on a flat surface. For every percent grade, add/subtract:
Wv Sv/37,500
Now to convert that to net engine power required at the flywheel, divide Pr by 0.85 for automatic transmissions or 0.95 for manuals to account for efficiency losses in the drivetrain.
Then simply look at the power curve for the rpm you need. If you are under the curve, you're at part load with power and acceleration to spare. If you're over it, you're slowing down until power required meets power available.
So, for reference, it takes about 200 hp to haul an 80,000 lb tractor/trailer rig down the road at 50 mph. It takes an additional 110 horses to pull it up a 1% grade. That's why Over The Road (OTR) trucks love big 500-600 hp engines. A trash truck at 300 hp may have way more power than it needs since they only go about 15-20 miles a day, even up and down the hills of San Francisco.
A lot of people automatically equate rpm with fuel consumption also. That's not always true. If I'm at full load in overdrive and I kick down into 3rd, the part load operation may make up for the higher engine speed. That's what I've found a lot with my Expedition, particularly in the mountains around my house.
This is also helpful when looking at gearing. A nice wide, flat power band is ideal in a normal engine. When you shift, your power requirements remain the same, so ideally you shift from the top of the power band to the bottom of the power band which keeps you accelerating smoothly. If the spacing between ratios is too big, you could shift to a point where you don't have the power you need and the engine will lug.
In a F1 car, however, you're going for high, peaky horsepower so the power band is narrow at 18,000 rpm. So they use 7 or 8 speed trannys with very close gearing to give them the rpm range that keeps them right at the peak. That's why they usually have to be pushed out of the pits- no low speed performance. It's also why mere mortals probably can't even get the things to move.
And why the different truck transmissions? Well, a dump truck will generally run at lower speeds so they won't be requiring as much power. An 18-speed tranny might give the driver a bunch of gears he won't ever get too and they might make his arm fall off from all the shifting. An OTR truck, however, will spend most of the time in top cruise gear and all that gearing will keep him right in the fuel economy sweet spot. Different strokes for different needs.
The general rule of thumb is that a good powertrain will have at least a 0.5% gradeability (enough power to tackle at least a 0.5% grade) at your highest cruise rpm and at least 1.5% gradeability at the peak torque rpm. The more the merrier.
For reference, the "gold standard" of grade tests is Baker Grade in California (I15 between Barstow and Vegas) which is 6-7% in 120° F ambients. El Cajone pass outside of LA at 8% is also referred to on occasion.
Now, could I have done the same with torque? Yep, but why? Particularly when everything in my business is measured in units per horsepower hour. Want to know how much fuel a truck will use in 150,000 miles? Take bsfc at 50% load, 50% rpm at an average truck speed of 50 mph and run the numbers out. That approximation will be close enough for friends.
Now there's one place that power doesn't work. Since you have to have motion to have power, you need something else to evaluate standing starts. That's Startability (S) given by:
S = (T@800)RaRtM/10.7Wv
where T@800 is the torque at clutch engagement.
That number should be at least:
14 for turnpike
16 for general on-highway
25 for moderate on/off highway
30 for severe on/off highway
And, again, the formula can be rearranged to give you the required gearing or torque for a desired startability.
There's another approach given by the National Truck Equipment Association (NTEA).
For startability, they calculate 2 gradeabilities.
The first is given by:
GA = (T x E x G x 1,200/R x Wv) - RR
where T = engine's peak net torque
E = Efficiency (assumed to be 85%)
G = Total ratio (Ra x Rt)
R = Loaded tire radius
RR = Road resistance in grade percent; 1.2 for average main highway, 1.5 for gravel
This gives you the grade that the truck will climb after starting. The second just subtracts 10 from the first. That's the grade on which the truck will start.
Then there's 2 formulas for top truck speed. The first shows the top speed that the truck is geared for, neglecting load and power:
MPH = RPM x R x 0.00594/G
The second neglects gearing:
MPH = 37,500 x HP x E/(% grade + RR) x Wv
where HP = Net horsepower at preferred RPM.
Calculate both top speed formulas and use the lowest number.
The NTEA approach can be useful if you know a lot about your operating conditions. Both approaches will give you similar analysis numbers.[/blue][/i]
As you can see, none of this is a diesel vs gasser issue. It is talking about power. How you produce that power is irrelevant as long as it is produced.
Bert
Note: Due to invalid formatting, all formatting has been ignored.
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Rogerg
WESTERN N.Y. U.S.A.
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Joined: 01/23/2004
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Wow! That is alot of math. It will take me a while to go through it all. In actual practice I can understand what you are saying. I have talked to many people driving or hauling the same amount of weight,22,000#-26,000# GVW or GCW, and as one guy said,"It doesn,t matter what engine I have, I get 8-9 mpg." No one in our groupe pulls away from the other. Makes you wonder if diesels are worth the money.
Roger
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Lord-Dogbert
Fontana, Ca.
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Joined: 09/17/2004
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Why would one wish to race along the freeway with his friends with 10-15,000 pounds of trailer, reckless and/or stupid comes to mind.
You think cutting off a guy in a caravan makes friends at the park just so that you can prove that your TV is superior some how.
The biggest problems I experience are the drivers in cars doing 45 up hill as I try to keep my momentum at 60-63. I hate to see the instant economy drop to Gallons Per Mile as I pass and pick up speed again.
There's more to life than MPG
2007.5 Chevy 3500HD crew cab dually with LMM 6.6l Diesel and Allison Trans. Integrated brake controller, 60 gallon aux tank in the bed.
2007 38' Skyline TrailRider Toy Hauler with 2 A/C's, 5.5kw Onan generator, dual slides.
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Rogerg
WESTERN N.Y. U.S.A.
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Joined: 01/23/2004
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Sounds like someone has a pricker stuck in their butt. Nobody was talking about racing. I was confirming what Bert said. Sounds like your the one racing to pass someone. I wouldn't be trying to pass someone if I had the wife and kids in the truck. That is just plain stupid. Maybe if you are trying to keep momentum going up a hill you don't have enough truck for the trailer.
Roger
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Sam Ryan
Sunnyvale, TX via Wyoming + Omaha, Nebraska
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Joined: 09/08/2004
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Diesel. . . .'nuff said!
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Aquaduct
Winchester, VA USA
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Joined: 05/30/2004
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Quote:Wow! That is alot of math. It will take me a while to go through it all. In actual practice I can understand what you are saying. I have talked to many people driving or hauling the same amount of weight,22,000#-26,000# GVW or GCW, and as one guy said,"It doesn,t matter what engine I have, I get 8-9 mpg." No one in our groupe pulls away from the other. Makes you wonder if diesels are worth the money.
Roger
Don't get either Bert or I wrong, diesel has one indisputable advantage, fuel economy. A diesel will get 20-40% better mileage on the road and use 50% less fuel when idling. Combine that with cheaper fuel most of the time and that's pretty significant.
On the flip side, they are going to be heavier and more expensive. But, if you tow a lot, there is a definite economic incentive for diesel. That's why commercial rigs use them.
And, unfortunately, the subscripts and superscripts that I used didn't translate to this format at all. The Sv2 in the road load formula should be Vehicle Speed (S subscript v) squared. If some things confuse you, I'll be happy to clarify (I'm sure Bert will too).
And just remember, power and torque are 2 faces to the same coin, mathematically, so comparisons based on one will carry over to the other.
For instance, I was reading a review of a motorcycle I like. The writer commented that the extreme acceleration was due to a torque curve that started at something like 100 lb/ft of torque and stayed flat to 1800 rpm. Good low end torque.
True enough, but look at what happens to power. At those engine rpms, the power shoots up from 15 hp to 34 hp in 1000 rpms (I think peak power comes on at 6000 or 7000 rpm). This is for an 800 lb. bike with negigible rolling resistance and miniscule air resistance. The power required to move the bike is very small. Either way you look at it, torque or horses, it's a rocket.
Again, neither Bert nor I claim diesel or gas have absolute advantage. There are pluses and minuses and to each his own, depending on your needs and preferences.
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Rogerg
WESTERN N.Y. U.S.A.
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Joined: 01/23/2004
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The fuel economy has me questioning if the diesel is worth the money. I average 8-9 mpg with the same gcw as guys with the gas motors. With the newer gas motors having the same life span as the diesels,is it really worth the money to buy diesel. I own two diesel trucks and frankly I don't like the cost of the maintenance. 137,000 miles on a 1999 and 65,000 miles on a 2001. The motors run great,the trucks do the job,but you still have the wear and tear on the rest of the truck.
Roger
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Sam Ryan
Sunnyvale, TX via Wyoming + Omaha, Nebraska
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Joined: 09/08/2004
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QUOTE
Gas engines have the same lifespan of diesel engines?????
High maintenance costs on a diesel engine?????
(Which solar system is this guy living in????? LOL!)
________________________________________________________________________________
If gas engines have the same lifespan of diesel engines, then why is there no deduction for miles on a diesel engined motorhome, versus a gas motorhome???
If diesel engines have higher maintenace costs than gas engines, why is it, that I almost never out of pocket on engine repairs, with EVERY diesel engine I've ever owned? All with hundreds of thousands of miles on them when sold.
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AZDodge4Sand
Cave Creek, AZ, USA
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Joined: 10/04/2004
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BertP and Aquaduct - I was trying not to get too technical, and I have to agree with what you said. I wonder how many people are actually going to read all of what was written if it's beyond them, or if they just don't care. I certainly do :-) That's the nerd in me saying woohoo!
All of what has been said comes down to measuring the rear wheel hp and torque applied by the vehicle in question. Engine torque and hp are only relevant if the drive ratios are equalized.
I still believe that with todays typical TV, when running in OD or 1:1 ratios with equal ratio rear ends and tire sizes, the diesel has an advantage. Notice how I had to narrow everything down so much. As soon as the gasser down shifts, everything changes - including the amount of fuel gushing into that gasser!
I don't think anyone here can argue that TODAY's diesels have better efficiency and in the long haul (no pun intended) offer a better solution as a TV.
Chuck in AZ- It's not hot here, hehehe.
2005 Silverado Crew Cab 1500 5.3L
2004.5 Dodge CTD - gone but not forgotten
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brainpause
Nashville, TN
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Joined: 07/12/2002
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my two cents:
I love that sound of a diesel starting up and running!
(Of course a properly piped gasser sounds great too!)
Larry
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