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:
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.
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.
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.