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General Automotive General automotive discussion. This is intended to be a discussion about other not VW and Diesel cars you may have or interested in.

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Old March 25th, 2006, 17:39   #1
bjwhite41
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Default Torque? What does this mean...technically?

Ok, I understand this concept a little bit, however I was hoping someone could explain it in more detail in everymans' terms so I can understand it even more.

Here's what I need explained. Can someone explain to me what '207 lb-ft. torque at 1800 RPM' means in a simple, somewhat techincal manner. How is this number caculated; what is this number telling me? So many people yip yap about HP and Torque, however, few can explain exactly what they are in simple terms.

I've noticed the expertise on this site and have a strong feeling that someone will be able to explain this in simple terms for my simple mind.

Appreciate the help,

Brian
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Old March 25th, 2006, 18:54   #2
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Horsepower is torque at a certain RPM.

So, if you have lots of low-end torque, you have lots of horsepower at low RPM.

Usually, when horsepower numbers are given for an engine, they're given at PEAK horsepower. In the case of a 52 horsepower 1.6NA, it has peak power at 4800 RPMs. Using the "5252 rule" (horsepower = (torque * RPMs) / 5252), one can determine that the engine is producing 47.5 foot-pounds of torque at that RPM level.

At the peak torque level, 2000 RPMs, my engine produces 71.5 foot-pounds of torque. Doing the math, it's producing 27.2 horsepower at 2000 RPMs. More low-end torque means more low-end horsepower.

Now, how does this all tie together?

A higher revving engine with the same amount of torque at a certain RPM level will have more horsepower at that RPM level. However, usually, high-revving engines have little torque, and rev high to compensate. This means that useful power is only at high RPM levels.
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Old March 25th, 2006, 23:03   #3
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In other words, drive the crap out of your diesel motor before it reaches 2600 RPM, and you not only use less fuel, you beat the pants off the hybrid in the lane next to you...
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Old March 25th, 2006, 23:39   #4
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I know someone will correct me if I'm wrong but I seem to recall reading that originally internal combustion engines were all measured by foot-pounds of torque, meaning what the engine's power equivelant is compared to taking a 1-foot bar and turning the engine manually. Therefore a 200 ft/lb motor is producing the equivelant power to taking a 1-foot bar and cranking with 200 lbs of pressure.

The reason people started referring to horsepower was because the average person couldn't relate to ft/lbs of torque when looking at an internal combustion driven vehicle. So a comparison was made to how many "horse" power a vehicle had, comparing that vehicle to a horse-drawn cart. Later, a formula was developed to standardize the comparison of horsepower to torque.

Of course this could all be a steaming pile of horse puckey and I could be talking out my a$$ because I'm just going by what I recall.
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Old March 27th, 2006, 09:16   #5
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Quote:
Originally Posted by MrMopar
Torque is force applied, horsepower is a unit of work - which is force measured over time.
Most of the rest is fine, but the above isn't quite right. Torque is a force -- what is required to accelerate a mass, like moving a piston. It has units of mass x acceleration, such as Newton-meters or foot pounds. If you exert that force over a distance, you do work, which has units of energy, such as Joules. If you turn your crankshaft you do work. Turn the crank once in a minute or turn it once in a second and you do the same amount of work, but clearly it's easier to do the former than the latter -- the latter requires more power, which is energy per time (not force per time). To convert between metric and 'Merkin, a factor of 3/4 gets you close: multiply Nm by 3/4 to get ft lbs (it's actually 0.7376), multiply kW by 4/3 to get hp (it's actually 1.341). That is, 100 Nm is 73.8 ft lbs, and 100 hp is 74.6 kW.

To get from point A to point B requires doing work. To do it quickly requires power. In a race, power is clearly key. In a piston engine, torque determines where in the RPM band that power is made.
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Old March 27th, 2006, 09:31   #6
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Torque is a measure of force; horsepower is a measure of how fast you are accomplishing that work. Apply a pound of weight to a 1 foot wrench, and it will be 1 ft-lb of force; but if it doesn't spin, no work is being done, and the result is zero hp.

Apply the same 1lb of force, and make it spin at say 1000 rpm, you will get:

torque * rpm / 5252 = 1 * 1000 / 5252 = 0.19hp.

Now, if you apply the same force but double the speed, you will double the horsepower.
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Old March 28th, 2006, 17:09   #7
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Quote:
Originally Posted by supton
Torque is a measure of force; horsepower is a measure of how fast you are accomplishing that work. Apply a pound of weight to a 1 foot wrench, and it will be 1 ft-lb of force; but if it doesn't spin, no work is being done, and the result is zero hp.

Apply the same 1lb of force, and make it spin at say 1000 rpm, you will get:

torque * rpm / 5252 = 1 * 1000 / 5252 = 0.19hp.

Now, if you apply the same force but double the speed, you will double the horsepower.
Torque is technically a Force times a perpendicular distance (lb-ft or ft-lbs). Mathematically, it's a vector cross-product, yielding a vector quantity for torque. A vector is a quanity that has a magnitude and a direction. Whenever we refer to the amount of torque our engines have at a particular RPM, we're referring to the magnitude of the vector quanity at that RPM. We normally don't bother specifying the vector's direction simply because that's already known, i.e., the direction the engine turns and therefore the wheels turn. So when we speak of torque, we're referring to the magnitude only, even though torque is technically a vector quantity in mathematical terms.

So think of torque simply as a "twisting force" applied to an object, like the "twist" applied on a bolt when you apply a force to the end of a wrench to crank the bolt down. The torque's magnitude is simply the force times the perpendicular distance. If I put 10 lbs of force perpendicular to the end of a 1 ft wrench, I'm applying 10 lb-ft of torque ("twist") to the bolt I'm cranking down. If I use a 2 ft long wrench and apply the same 10 lbs of force perpendicular to the end of the 2 ft wrench, I'm applying 20 lb-ft of torque to the bolt. Consequently, I could also apply 20 lb-ft of torque to the bolt by applying 20 lbs of force to the end of the 1 ft long wrench. Torque is technically a force times a perpendicular distance! In terms of torque from an engine and given that there's no "distance" specified, it's equivalent to cranking the engine's load manually using a wrench and applying a force (lbs) on the end of the wrench times the distance away (ft., i.e., length of wrench) from the load being cranked.

And sort of like Supton said, think of HP as a rate in which you are making that torque. You can see it in the units of HP, ft-lbs/sec, while torque is simply lb-ft or ft-lbs. I sometimes like to think of HP simply as a rate of torque being made.

When people wonder how my TDI can be so driveable with its stock performance of "only 90HP", I explain how "people buy horsepower but drive torque" and it's really the 155 ft-lbs of torque that's actually moving the car, not the 90HP. I tell them to not be fooled by the seemingly "low" HP rating because they will always seem lower because of a diesel's lower RPM range when compared to a high-revving gasser. Back when my Golf TDI was bone stock, friends of mine who rode with me were amazed at how driveable the car was, even though it "only had 90HP". Now that it's been chipped and injectored, they're blown away by the weapons-grade torque.
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Old March 29th, 2006, 17:15   #8
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Quote:
Originally Posted by n1das
We normally don't bother specifying the vector's direction simply because that's already known, i.e., the direction the engine turns and therefore the wheels turn.
Actually, the direction is at right angles to both the direction the engine turns and to the moment arm. That is, in line with the crankshaft.

Quote:
So think of torque simply as a "twisting force" applied to an object, like the "twist" applied on a bolt when you apply a force to the end of a wrench to crank the bolt down.
Yep.

Quote:
And sort of like Supton said, think of HP as a rate in which you are making that torque. You can see it in the units of HP, ft-lbs/sec, while torque is simply lb-ft or ft-lbs. I sometimes like to think of HP simply as a rate of torque being made.
OK, the confusing thing here is that the ft-lbs in HP being ft-lbs/sec is NOT the same as the ft-lbs in torque. In the power definition, the ft-lbs is a force (lbs) exerted over a linear distance (ft). In the torque definition, it's a force (lbs) exerted at the end of a lever arm (ft). This is easier to see in metric: a torque of 1 N m exerted around a full circle does 2 pi J of work. Or just go back to high school physics:

Force: that required to accelerate/change direction/deform mass. Has units of mass x acceleration (think F = ma). In rotational systems this is torque. In metric the unit is the Newton, in USian, hmm, I don't think there is a standard USian for force except for rotational systems, where it's the ft-lb.

Work: what you do when you exert a force over a distance (linear) or angle (rotational). Work = force x distance (linear) or torque x angular displacement (rotational). In metric the unit is the Joule, in USian it could be anything from BTU to ft-lb (not the same as torque), so long as it doesn't make sense. Has units of mass x (velocity)^2 or energy (think E = mc^2)

Power: rate at which you do work, that is, energy/time. In metric this is the Watt = 1 J/sec, in USian it's typically horsepower.

And just to clarify(?) things further, the pound (lb) is legally a unit of mass, unless you're talking in engineering or scientific circles where it's a unit of force, and where the corresponding unit of mass is the slug. Great, huh?
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Old March 29th, 2006, 19:36   #9
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There was an excellent discussion on this topic 3-5 weeks ago. Try a search you know, the third word from the right in the blue band above?
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Old March 30th, 2006, 08:56   #10
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I always thought torque was rotational force ...

Check out wikipedia: http://en.wikipedia.org/wiki/Torque
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Old March 30th, 2006, 12:32   #11
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To add to the confusion, torque is a twisting force.

All of this theory is well and good, but how do we relate this to an actual engine? The question still remains, WHY? Why does the diesel engine produce more torque?
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Old March 30th, 2006, 13:35   #12
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It does more pressure-volume work for a given amount of injected fuel versus a gasoline engine, meaning, the hot expanding gasses push harder on the piston, because there is more heat and gas (CO2 + water vapor)produced for a given level of fuel combusted versus a gasoline engine. Partially this is due to the greater volumetric density of energy diesel has versus gasoline (5-7% advantage here.) Partially this is because of the higher compression ratio raises net efficiency. And partially it is because a diesel engine has little pumping losses (doesn't have to pull in intake air past a huge restrictor like a gasoline engine does), so more of the pushing effort goes to push the piston down instead of sucking air in the other (intake cycle) pistons), which is where the gasoline engine wastes energy.

Also, that hot, expanding gas is acting on the piston starting very close to TDC, versus a gasoline engine which must advance to BTDC to work properly, wasting some of the gas expansion by pushing on a still rising piston.
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Old March 30th, 2006, 13:49   #13
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So please correct me if i'm wrong. Since the diesel engine is injecting fuel as the piston is moving down, more force is pushing on the piston, which in turn, pushes on the crankshaft at an angle. This would cause the massive torque numbers we love so dearly. I'm sure direct injection gasoline engines work the same way, but because as mentioned above, diesel is more efficient when it ignites and expands, we get more torque out of the engine. And the torque is at a low rpm because this is where the rate of expansion is most effective.... Or am i off in left field on this one?
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Old March 30th, 2006, 16:01   #14
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That's pretty much it as I understand it.

Direct-injection gasoline engines have much higher torque values than their corresponding non-direct injected gas brothers.
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Old April 3rd, 2006, 13:24   #15
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Quote:
Originally Posted by R-Sixxer
. And the torque is at a low rpm because this is where the rate of expansion is most effective....
. . . . .because the diesel has a much higher compression ratio in the first place, which gives rise to higher efficiencies, and a greate rate of expansion, thus greater forces on that piston, equals greater torque, and more power at low rpm.

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