Chevy and GMC Duramax Diesel Forum banner

61 - 80 of 117 Posts

·
Registered
Joined
·
34 Posts
@JD - Couldn't agree more, that is the exact next thing I wanted to check then I went to sleep instead.

I am not exactly sure how you found that drag is based on V^3, everything I have ever seen is based on V^2.
 

·
Registered
Joined
·
1,236 Posts
@JD - Couldn't agree more, that is the exact next thing I wanted to check then I went to sleep instead.

I am not exactly sure how you found that drag is based on V^3, everything I have ever seen is based on V^2.
The force is proportional to v^2, and since power=force*v, the power loss is proportional to v^3.
 

·
Registered
Joined
·
5,867 Posts
From 'The Engineering Toolbox', because it was faster/simplier than typing out:


Any object moving through a fluid experiences drag - the net force in the direction of flow due to pressure and shear stress forces on the surface of the object.

The drag force can be expressed as:

Fd = cd 1/2 ρ v2 A (1)

where

Fd = drag force (N)

cd = drag coefficient

ρ = density of fluid (1.2 kg/m3 for air at NTP)

v = flow velocity (m/s)

A = characteristic frontal area of the body (m2)

The drag coefficient is a function of several parameters like shape of the body, Reynolds Number for the flow, Froude number, Mach Number and Roughness of the Surface.

The characteristic frontal area - A - depends on the body.



Long time since I did any wind tunnel work.
 

·
Registered
Joined
·
1,236 Posts
From 'The Engineering Toolbox', because it was faster/simplier than typing out:


Any object moving through a fluid experiences drag - the net force in the direction of flow due to pressure and shear stress forces on the surface of the object.

The drag force can be expressed as:

Fd = cd 1/2 ρ v2 A (1)

where

Fd = drag force (N)

cd = drag coefficient

ρ = density of fluid (1.2 kg/m3 for air at NTP)

v = flow velocity (m/s)

A = characteristic frontal area of the body (m2)

The drag coefficient is a function of several parameters like shape of the body, Reynolds Number for the flow, Froude number, Mach Number and Roughness of the Surface.

The characteristic frontal area - A - depends on the body.



Long time since I did any wind tunnel work.
True enough, but that's only the force due to drag. To compute the power loss due to this force we must recognize that power=force*velocity. The drag force scales with v^2 as you showed, and to compute the power loss we simply multiply by an additional factor of v. v^2 * v = v^3, and therefore, power loss due to drag scales with v^3.

Let's check the units. From Wikipedia, the drag equation is



[Note: the term 'u' represents the velocity of the body relative to the fluid (air); this corresponds to the quantity I've been referring to as 'v']

rho has units of kg / m^3
u has units of m / s
C_D is unitless
A has units of m^2

(kg / m^3) * (m / s)^2 * m^2 = kg * m / s^2 = newton, a unit of force

Multiplying by a factor of u (units of m / s, as above) to get power, we have

kg * (m / s^2) * (m / s) = kg * m^2 / s^3 = watt, a unit of power

See the field "SI base units" on both Wikipedia entries for the units and you will find that my calculations are correct.
 

·
Registered
Joined
·
105 Posts
You Guys with all your math are giving me a headache. All I know is that I traded in a stock '11 LML 3500 Dually for an '18 L5P 3500 Dually and pulled the same trailer, 19K worth the same route from Florida to Western New York, which includes the Fancy Gap area and the L5P did a much better job than the LML. So for me, it was a good upgrade.
The OP did admit that his LMM was at least partly deleted which may account for the difference.
 

·
Registered
Joined
·
1,236 Posts
You Guys with all your math are giving me a headache.
Muscles often experience atrophy when not used. ;)

All I know is that I traded in a stock '11 LML 3500 Dually for an '18 L5P 3500 Dually and pulled the same trailer, 19K worth the same route from Florida to Western New York, which includes the Fancy Gap area and the L5P did a much better job than the LML. So for me, it was a good upgrade.
The OP did admit that his LMM was at least partly deleted which may account for the difference.
I agree. I think that the amount of variability in wind conditions, the modifications to the LMM, the fact that the new dually is probably ~1500 lbs heavier than the old SRW, and the possibility of the reported numbers being slightly off (1-2 mph) are more than adequate to explain the difference. If the performance is still unacceptable, I suspect the OP will need to upgrade to a tow vehicle with 10 tires instead of just six.
 

·
Registered
Joined
·
5,867 Posts
True enough, but that's only the force due to drag. To compute the power loss due to this force we must recognize that power=force*velocity. The drag force scales with v^2 as you showed, and to compute the power loss we simply multiply by an additional factor of v. v^2 * v = v^3, and therefore, power loss due to drag scales with v^3.

Let's check the units. From Wikipedia, the drag equation is



[Note: the term 'u' represents the velocity of the body relative to the fluid (air); this corresponds to the quantity I've been referring to as 'v']

rho has units of kg / m^3
u has units of m / s
C_D is unitless
A has units of m^2

(kg / m^3) * (m / s)^2 * m^2 = kg * m / s^2 = newton, a unit of force

Multiplying by a factor of u (units of m / s, as above) to get power, we have

kg * (m / s^2) * (m / s) = kg * m^2 / s^3 = watt, a unit of power

See the field "SI base units" on both Wikipedia entries for the units and you will find that my calculations are correct.



All of which is just force measured over time and distance (or revolutions).


I think you two actually agree, but are arguing from opposite ends and having trouble finding the midpoint and wording (if that makes any sense).
 

·
Registered
Joined
·
32 Posts
Partially joking, but maybe it needs some propane or an O2 boost. A little LOX in the back and altitude problems decrease.
 

·
Registered
Joined
·
61 Posts
25,000 lbs. seems like it would be something big. So what was the wind like that day? I've been on some roads and couldn't figure out why I was dragging an anchor. Finally saw a flag, the knew why.
 

·
Registered
Joined
·
52 Posts
Pulled 28k on my l5p about 3000 miles of the 6000 miles on the odometer. Impressive to say the least. Ezlynk hooked up last night to monitor how this sucker runs. Waiting for deletes as DEF fluid needs refilled every 3000miles so far

Sent from my SM-G930T using Tapatalk
 

·
Registered
Joined
·
280 Posts
I'm waiting---

For those equations to explain why an 18 wheeler has 1800 ft lbs and only 600 HP. Must be because they are going down the hill all of the time.
 

·
Registered
Joined
·
489 Posts
If diesel prices ever go high enough relative to gasoline, LP, or natural gas then you MAY see an 18 wheeler with the same HP but much lower torque.

The combination of diesel producing a slower burn relative to gasoline and the desire to provide large truck engines with a very long operational life results in a large displacement engine design that produces rated horsepower at a fairly low speed. Because HP, torque, and RPM are mathematically related delivering high horsepower at a relatively low RPM requires that torque also be high.

When gasoline power was still popular for semis, the torque and HP specs would have looked strange to the car owner of the same era. For example the largest displacement Red Diamond series International Harvester gas engine, the RD-501, had ratings that varied over its long production life but typical was the spec of 215 [email protected],000 RPM and 451 lb.ft. @ 1,400 RPM. You can bet when pulling a grade that it was running near its governed RPM of 2,700-3,000 RPM depending upon year/application. The torque curve provides a useful characterization of the "personality" of an engine and is a major consideration when designing the rest of the powertrain but its sustained horsepower rating determines how much work it can actually do.

The most efficient internal combustion engine designs tend to cluster around long stroke, low RPM designs and this once again drives the outcome to a design that will have a very large torque number relative to HP.
 

·
Registered
Joined
·
1,236 Posts
I'm waiting---

For those equations to explain why an 18 wheeler has 1800 ft lbs and only 600 HP. Must be because they are going down the hill all of the time.
I provided a clear explanation of the relevant physics, taking time to make sure that everything I said was accurate and backed up with evidence. Incidentally, another poster did the same and reached the exact same conclusion. You have yet to refer to a single equation or offer a shred of proof for your assertion that there's some fundamental relationship between maximum engine torque and the ability of a vehicle to ascend a hill of a given grade with a given load at a given speed. Man up and back up your claim with some math and physics already.

And, if you must know, the high torque in large truck engines is not by design. It is merely a byproduct of a low-RPM engine (lower RPMs are associated with greater thermodynamic efficiency, i.e., fuel economy) because power=torque*RPM, so for a lower RPM the torque must be higher to achieve the same power.
 

·
Registered
Joined
·
489 Posts
I am not sure whether the project went off the rails or is just taking quite some time to come to market but 6 years ago Allison and Dana announced they would be bringing a CVT to the medium and heavy duty market by the end of the decade so they have a couple of years left. An efficient CVT that could stand up to the power transmission requirements of this market could be a game changer. In its simplest form the engine could be run at its most efficient RPM for steady state cruising over a wide range of desired speed with a second nearly as efficient RPM point that is slightly up the power curve to allow high efficiency during normal acceleration in stop and go driving. The same transmission would allow the engine to run at its horsepower peak when full power is needed avoiding the drop in performance as the engine moves in and out of its peak power band while accelerating through the gears or holding speed on an increasing slope at altitude where even a forced induction engine begins to suffer some power loss.

Here is one of the original articles from 6 years ago that provides the basics: https://www.overdriveonline.com/allison-to-produce-gearless-transmissions/

Hopefully a variant is coming one day to the "heavy light" truck market.
 

·
Premium Member
Joined
·
46 Posts

·
Registered
Joined
·
280 Posts
I provided a clear explanation of the relevant physics, taking time to make sure that everything I said was accurate and backed up with evidence. Incidentally, another poster did the same and reached the exact same conclusion. You have yet to refer to a single equation or offer a shred of proof for your assertion that there's some fundamental relationship between maximum engine torque and the ability of a vehicle to ascend a hill of a given grade with a given load at a given speed. Man up and back up your claim with some math and physics already.

And, if you must know, the high torque in large truck engines is not by design. It is merely a byproduct of a low-RPM engine (lower RPMs are associated with greater thermodynamic efficiency, i.e., fuel economy) because power=torque*RPM, so for a lower RPM the torque must be higher to achieve the same power.
JD, I have an BS and and MS and know enough that the equations you cite are good over the conditions they have been derived from. I also know that it is folly to try and re-invent the equations to prove the data that is already in front of your face unless it absolutely has to be done.

Are you going to tell me you drop rocks off buildings and they accelerate constantly before they hit the ground or do they hit terminal velocity? I suppose you read the 2nd chapter in your book but maybe you forgot it. I believe Galileo proceeded Newton or at least they were contemporaries but Newton wasn't going to take his F=ma equation to Galileo and say well the rocks didn't hit the ground at the same time. Instead Newton said well my equation holds perfectly in a vacuum just too bad Newton didn't have a vacuum container along side the Leaning Tower of Pisa. Sound familiar???

Your understanding of diesels, (probably all engines) is also sorely lacking. The diesel engine is designed to better manage the properties of the fuel as WQ stated above and low RPM torque isn't just a happen stance of math. Your wrong there just like you were wrong earlier in this thread that the trans provide power management and not torque management. Essentially an engine isbuilt around the properties of the fuel as are all engines. It isn't some happen stance it a a product of a good design matching the mechanical specs of the engine to the properties of the fuel for a desired goal. Get your foot out of your mouth!!!. Pushing something up a hill requires more force or torque. Period end of story. You can take the kids in your neighbor hood on bicycles and tell them to pedal as fast as they can to make it up the hill. No doubt some clever kid will stand up on his pedals and push for all he is worth and leave the other kids and you behind.

Diesel is a slower burning fuel meaning it needs higher temperatures which call for a higher compression ratio. That higher compression ratio gives you a longer moment arm around the crank which gets you a higher torque. The explosion of the mixture also provides greater push to the piston. If I wanted an F1 engine to spin at 14,000 RPM I wouldn't get a higher compression engine. In fact it would likely have a larger bore then stroke to produce HP. A long stroke engine would just rip it self apart trying to run over 10K RPM. Yet with all the HP that F1 car has I doubt it can pull 80k lbs up the Rockies. Here is another one, why don't I put diesel in an F1 engine??? Find that one in the Wikipedia equations. Once again those engines are purpose built around the fuel they are burning to produce an appropriate result whether that is to produce more torque quicker or to run higher HP for a longer duration.

If you could get that then you could answer any of my question about why the HP torque curves are different in say a diesel vs a gas engine. In the example I asked you to prove, the answer is that the efficiency of the slow burning fuel as been reduced at higher RPM and is now producing less torque but while it is spinning higher it is still producing more HP. That is the answer and it is in the fuel and the long compression stroke not in your equations.

Why have dirt bikes gone to 4-stroke engines vs 2-stroke engines? Could it be that the lower down torque they have helps them pull hills or pull out of corners quicker then a 2-stroke giving them a better chance of winning? Surely the 2-strokes revving to 10K RPM have more HP so why don't they always win??? Why did my 4-stroke 600cc snowmobile out pull a 2-stroke 1000cc sled on short bursts or coming out of corners. The 2-stroke could get me across a frozen lake faster but not any twisty trails with turns as I constantly out accelerate in every single turn due to more torque at lower RPM.

Your equations haven't answered anything and they won't because they don't consider all the variables of the "physical limitations" of the variety of conditions we speak of!!

Now speaking of manning up!!!! You have just been schooled.

For homework your going to tell us why we have adopted 4 cylinder engines in cars versus 2 stroke engines. After all 2 strokes and spin higher and result in more HP don't they.
 

·
Registered
Joined
·
241 Posts
“Why have dirt bikes gone to 4-stroke engines vs 2-stroke engines? Could it be that the lower down torque they have helps them pull hills or pull out of corners quicker then a 2-stroke giving them a better chance of winning? Surely the 2-strokes revving to 10K RPM have more HP so why don't they always win???”

Then why do 450cc 4 strokes race in the 250cc 2 stroke classes if they are so much better all things equal? The only reason there aren’t as many 2 stroke dirt bikes around is the EPA putting their foot down on manufactures about off road toy’s exhausts.


“Why did my 4-stroke 600cc snowmobile out pull a 2-stroke 1000cc sled on short bursts or coming out of corners. The 2-stroke could get me across a frozen lake faster but not any twisty trails with turns as I constantly out accelerate in every single turn due to more torque at lower RPM.”


If the 1000cc? 2 stroke sled was running right and geared properly, no 600 4 stroke is gonna touch it. I only ride an 800 RMK, but I can guarantee you that it stands on the tail with very little throttle input and rips off from everything but the modded turbo 4 strokes.


“For homework your going to tell us why we have adopted 4 cylinder engines in cars versus 2 stroke engines. After all 2 strokes and spin higher and result in more HP don't they.”


4 strokes burn much cleaner, last longer and are more efficient. Look at an 8v92TA that is used by the military, over 1900ft/lbs at 1200 rpm, you don’t necessarily have to spin a 2 stroke high.
 

·
Registered
Joined
·
1,144 Posts
:popcorn brain fight!
 

·
Registered
Joined
·
1,236 Posts
JD, I have an BS and and MS and know enough that the equations you cite are good over the conditions they have been derived from. I also know that it is folly to try and re-invent the equations to prove the data that is already in front of your face unless it absolutely has to be done.

Are you going to tell me you drop rocks off buildings and they accelerate constantly before they hit the ground or do they hit terminal velocity? I suppose you read the 2nd chapter in your book but maybe you forgot it. I believe Galileo proceeded Newton or at least they were contemporaries but Newton wasn't going to take his F=ma equation to Galileo and say well the rocks didn't hit the ground at the same time. Instead Newton said well my equation holds perfectly in a vacuum just too bad Newton didn't have a vacuum container along side the Leaning Tower of Pisa. Sound familiar???

Your understanding of diesels, (probably all engines) is also sorely lacking. The diesel engine is designed to better manage the properties of the fuel as WQ stated above and low RPM torque isn't just a happen stance of math. Your wrong there just like you were wrong earlier in this thread that the trans provide power management and not torque management. Essentially an engine isbuilt around the properties of the fuel as are all engines. It isn't some happen stance it a a product of a good design matching the mechanical specs of the engine to the properties of the fuel for a desired goal. Get your foot out of your mouth!!!. Pushing something up a hill requires more force or torque. Period end of story. You can take the kids in your neighbor hood on bicycles and tell them to pedal as fast as they can to make it up the hill. No doubt some clever kid will stand up on his pedals and push for all he is worth and leave the other kids and you behind.

Diesel is a slower burning fuel meaning it needs higher temperatures which call for a higher compression ratio. That higher compression ratio gives you a longer moment arm around the crank which gets you a higher torque. The explosion of the mixture also provides greater push to the piston. If I wanted an F1 engine to spin at 14,000 RPM I wouldn't get a higher compression engine. In fact it would likely have a larger bore then stroke to produce HP. A long stroke engine would just rip it self apart trying to run over 10K RPM. Yet with all the HP that F1 car has I doubt it can pull 80k lbs up the Rockies. Here is another one, why don't I put diesel in an F1 engine??? Find that one in the Wikipedia equations. Once again those engines are purpose built around the fuel they are burning to produce an appropriate result whether that is to produce more torque quicker or to run higher HP for a longer duration.

If you could get that then you could answer any of my question about why the HP torque curves are different in say a diesel vs a gas engine. In the example I asked you to prove, the answer is that the efficiency of the slow burning fuel as been reduced at higher RPM and is now producing less torque but while it is spinning higher it is still producing more HP. That is the answer and it is in the fuel and the long compression stroke not in your equations.

Why have dirt bikes gone to 4-stroke engines vs 2-stroke engines? Could it be that the lower down torque they have helps them pull hills or pull out of corners quicker then a 2-stroke giving them a better chance of winning? Surely the 2-strokes revving to 10K RPM have more HP so why don't they always win??? Why did my 4-stroke 600cc snowmobile out pull a 2-stroke 1000cc sled on short bursts or coming out of corners. The 2-stroke could get me across a frozen lake faster but not any twisty trails with turns as I constantly out accelerate in every single turn due to more torque at lower RPM.

Your equations haven't answered anything and they won't because they don't consider all the variables of the "physical limitations" of the variety of conditions we speak of!!

Now speaking of manning up!!!! You have just been schooled.

For homework your going to tell us why we have adopted 4 cylinder engines in cars versus 2 stroke engines. After all 2 strokes and spin higher and result in more HP don't they.
Let me know when you have some mathematics to support your anecdotes.

I have deliberately avoided citing my credentials because I don't think they're especially relevant, although if anyone cares I will provide them. I challenge anyone (you included) to point out any fault in my calculations (i.e., using math, not words). The fact that you have been unable to do speaks volumes about the strength of your claims.
 

·
Registered
Joined
·
69 Posts
This thread has gone to hell.

I wish we had a moderator to take the math war somewhere else. I just want to know what is wrong with this poor guys truck.
 
61 - 80 of 117 Posts
Top