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Torque

from howstuffworks.com

Torque is a force that tends to rotate or turn things. You generate a torque any time you apply a force using a wrench. Tightening the lug nuts on your wheels is a good example. When you use a wrench, you apply a force to the handle. This force creates a torque on the lug nut, which tends to turn the lug nut.

English units of torque are pound-inches or pound-feet; the SI unit is the Newton-meter. Notice that the torque units contain a distance and a force. To calculate the torque, you just multiply the force by the distance from the center. In the case of the lug nuts, if the wrench is a foot long, and you put 200 pounds of force on it, you are generating 200 pound-feet of torque. If you use a 2-foot wrench, you only need to put 100 pounds of force on it to generate the same torque.

A car engine creates torque and uses it to spin the crankshaft. This torque is created exactly the same way: A force is applied at a distance. Let's take a close look at some of the engine parts:

http://static.howstuffworks.com/flash/fpte-engine.swf
Figure 2. How torque is generated in one cylinder
of a four-stroke engine

The combustion of gas in the cylinder creates pressure against the piston. That pressure creates a force on the piston, which pushes it down. The force is transmitted from the piston to the connecting rod, and from the connecting rod into the crankshaft. In Figure 2, notice that the point where the connecting rod attaches to the crank shaft is some distance from the center of the shaft. The horizontal distance changes as the crankshaft spins, so the torque also changes, since torque equals force multiplied by distance.

You might be wondering why only the horizontal distance is important in determining the torque in this engine. You can see in Figure 2 that when the piston is at the top of its stroke, the connecting rod points straight down at the center of the crankshaft. No torque is generated in this position, because only the force that acts on the lever in a direction perpendicular to the lever generates a torque.


Torque Comparison

If you have ever tried to loosen really tight lug nuts on your car, you know a good way to make a lot of torque is to position the wrench so that it is horizontal, and then stand on the end of the wrench -- this way you are applying all of your weight at a distance equal to the length of the wrench. If you were to position the wrench with the handle pointing straight up, and then stand on the top of the handle (assuming you could keep your balance), you would have no chance of loosening the lug nut. You might as well stand directly on the lug nut.

http://static.howstuffworks.com/flash/fpte-curve.swf
Figure 3. A simulated dynamometer test of two different engines
Click here for the large version.

Figure 3 shows the maximum torque and power generated by two different engines. One engine is a turbocharged Caterpillar C-12 diesel truck engine. This engine weighs about 2,000 pounds, and has a displacement of 732 cubic inches (12 liters). The other engine is a highly modified Ford Mustang Cobra engine, with a displacement of 280 cubic inches (4.6 liters); it has an added supercharger and weighs about 400 pounds. They both produce a maximum of about 430 horsepower (hp), but only one of these engines is suitable for pulling a heavy truck. The reason lies partly in the power/torque curve shown above.

When the animation pauses, you can see that the Caterpillar engine produces 1,650 lb-ft of torque at 1200 rpm, which is 377 hp. At 5,600 rpm, the Mustang engine also makes 377 hp, but it only makes 354 lb-ft of torque. If you have read the article on gear ratios, you might be thinking of a way to help the Mustang engine produce the same 1,650 lb-ft of torque. If you put a gear reduction of 4.66:1 on the Mustang engine, the output speed would be (5,600/4.66 rpm) 1,200 rpm, and the torque would be (4.66 * 354 lb-ft) 1,650 lb-ft -- exactly the same as the big Caterpillar engine.

Now you might be wondering, why don't big trucks use small gas engines instead of big diesel engines? In the scenario above, the big Caterpillar engine is loafing along at 1,200 rpm, nice and slow, producing 377 horsepower. Meanwhile, the small gas engine is screaming along at 5,600 rpm. The small gas engine is not going to last very long at that speed and power output. The big truck engine is designed to last years, and to drive hundreds of thousands of miles each year it lasts.

http://www.howstuffworks.com/fpte.htm/printable
for longer text with pics/animations

anyone knows how to get thos swf's to work?
 
Also, torque gets you up to speed, and horsepower is what keeps you going.
 
From a physics/dynamics point of view, the explanation given by howstuffworks is kinda misleading.

Torque is a couple moment, not a force. It also refers to the ability or the resistivity of being able to turn something. For example, a car that produces 300 ft-lbs of torque is able to produce a torque, whereas a bolt that's been torqued to 100 ft-lbs is capable of resisting up to 100 ft-lbs of torque.

Also, Torque only acts along the major axis of an object. I wish I had a scanner to scan in pretty pictures to show you the physics, but I also realize that I'm going past the technical and into the engineering relm.
 
Horsepower determins how fast you hit the wall, torque is what keeps you going

Torque is the ability to go fast without actually having to change gears. EG: A Monaro and a WRX STI. They both cost the same however the Monaro has a big naturally aspirated 6.0L where-as the WRX has a small 2.0L turbo. The Monaro has more torque because it has higher power bands and you don't really need to change gear that often. The WRX...well you have to change gear ALOT I hear.

Hope that helps :)

Torque can be a bitch to define at the best of times.
 
It's best to think of torque and power in terms of the curves, IMO, since each one changes as the engine revs change. You usually only hear about peak power and torque, which is really only half the picture. Also, the acceleration in each gear is related directly to the torque curve of an engine, but you'll accelerate the fastest when you keep power at a maximum. I made a post explaining this in detail a while back:

I said:
Torque is not that important to how fast a car can accelerate. I'll explain:

The torque curve determines the acceleration in gear, period. Your in gear acceleration is determined solely by the shape of the torque curve. Your maximum acceleration in any particular gear is at the RPM where you make the most torque.

This has nothing to do with 0-60 times however.

Your maximum acceleration at any given speed is determined entirely by your horsepower curve. That is, you accelerate the fastest when you keep the RPMs at the horsepower peak. That's why high hp cars accelerate fast, regardless of the torque.

The reason this is true is because of something called torque multiplication. let's say your engine makes 200lb-ft of torque at 2000RPM, and only 100lb-ft of torque at 8000RPM. The torque is only half that at 2000RPM, but the revs are 4 times higher, so it's twice as much hp.

Let's say you're going a certain speed in a certain gear, and the engine is rotating at 2000RPM. The gear is a 1:1 ratio, so the wheels are also rotating 2000RPM and there is 200lb-ft of torque at the wheels. If instead you downshifted to a gear with a 4:1 ratio, the engine would be going 8000RPM while the wheels still go at 2000RPM. Only now with the gear ratio, you have 100lb-ft x 4 = 400lb-ft at the wheels, twice as much as when you were doing 2000RPM, which reflects the fact that the engine makes twice as much power at 8000RPM than at 2000RPM.

And that's why power determines acceleration. :)

I was paraphrasing what I had read on this website.

The reason you hear people say things like "torque gets you going, power keeps you there" or "torque is for acceleration, horsepower is for top speed" is that typically, the discussion has been between engines that took different approaches to make power.

Usually it's a high-revving, high-performance European or Japanese DOHC motor versus a larger, lower-revving, high-performance American pushrod or SOHC motor. Torque is usually directly related to engine size, so larger engines, in general, make more torque than smaller ones. However, the smaller engines can make the same amount of power since:

power = torque x RPM

They make up their lack of torque with higher rev-limits. Now another thing to keep in mind is that the smaller engines tend to be more highly tuned than their larger American competitors, which raises peak horsepower, but narrows the powerband at the same time. What I mean is that they sacrifice torque at lower RPMs to concentrate the torque at higher RPMs, where it will make more power. This increases the maximum horsepower the engine can make, but it also narrows the powerband, which is the RPM range in which the engine makes the most power. Essentially, the engine makes more power, but for a shorter amount of time. Sometimes, people call an engine like this "peaky."

Your typical American engine, on the other hand, is not tuned as highly, so the powerband is wider. The power isn't as high as it could be, but it's made up for by more displacement. The reason people say "there's no replacement for displacement" is because it's a way to make more power without narrowing the powerband. The advantage being that, given the same peak power, the engine with the widest powerband will give you the best acceleration.

The reason why a wide powerband is useful comes down to gearing. The whole purpose of having multiple gears is to keep the engine in the powerband for as long as possible. I'll give an example. Suppose we have two engines: one with a wide powerband and one with a narrow one. Even if their peak power is the same, as soon as you move from the peak, the engine with the wider powerband will be making more power. Visually, it looks like this (ignore the actual numbers):

A Narrow Powerband (the power is the upper curve):
dyno.jpg


A Wide Powerband (the power is the red curve):
v8_curve_large.jpg


As you can see from the graph above, the first engine has a big jump in power, but it's only between 10k and 11k RPM. The second engine makes its power over a much larger region, around 8.5k to 10.5k, and it approaches the peak power much more gradually than the first motor. Even if the first motor had a higher peak power figure, the second motor would be making more power most of the time. You can make up for a narrow powerband somewhat by adding more gears to the gearbox, but this increases cost and lowers reliability (the only manufacturers I know of that make 7-speed and higher transmissions are BMW, Mercedes and Lexus, and none of them are "true" manuals with a clutch). Also, more gears means more time wasted changing gears, so there's a limit.




In the end, there are a few things you can tell about an engine just by looking at the peak torque and power. Mostly, comparing the two tells you about the character of the engine. If the engine has a small torque figure compared to it's power, it revs high, and is probably peaky (this is mostly due to manufacturer traditions and is not a strict rule. High revving engines with wide powerbands do exist). Another engine with the same amount of power but a lot more torque won't rev as high, but will probably have a wider powerband. A lot of this last paragraph boils down to personal preference. Some people like high-revving engines with narrow powerbands because they like the sound of an engine at high RPM, and they feel it gives the engine "soul." Some people prefer to have a wider powerband because it feels more useable, and is better on the motorway. I hope I was able to help. ;)

EDIT: I think I got a little carried away here... :p
 
^nice post!

can you name an engine with high revs and still wide powerband? i'd be interested in one of those :D
 
one of the widest powerbands i can think of is the Engines in the volvo R cars. their peak torque figure is stable from 1750-5250 rpm. Thats on a 2.5l I5 turbo. something like 297ft-lb

Here's the power and torque graphs
V70Rpowergraph.jpg


Its clearly no huge V8, but a nice wide powerband.
 
Very good post Chaos!! People tend to show too much interest in numbers concerning peak power and often forget the other factors you were talking about that play an important role.

Bone: For example the BMW S54 engine is both high-revving while still retaining a rather broad powerband.
 
When I explain this concept to non-enthusiasts I use the following example:

An elephant has a lot of torque but relatively low horsepower--like a truck or bus.
A cheetah has a lot of horsepower but relatively low torque--like a Ferrari.

And most people are satisfied with this explanation. What do you guys think?
 
Z Draci said:
When I explain this concept to non-enthusiasts I use the following example:

An elephant has a lot of torque but relatively low horsepower--like a truck or bus.
A cheetah has a lot of horsepower but relatively low torque--like a Ferrari.

And most people are satisfied with this explanation. What do you guys think?

i think its rather good ;) :thumbsup:
 
I once used a weightlifter and a sprint runner as examples... :lol:
 
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