What is Torque?
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:
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.
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
Common Unitsof Torque
SI:Newton meter (Nm)
1 Nm = 0.737 lb-ft
1 lb-in = 0.113 Nm
1 lb-ft = 1.356 Nm
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.
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.
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.
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.
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