If you drive a car, you probably know that your engine produces rotations per minute (“RPM”) and “torque.” But what are their differences, and how are they related? Essentially, both describe the engine’s output in some capacity. While torque is a measured output, RPM is calculated. Rotations per minute describe the speed at which a motor spins. Torque is a measure of the twisting force generated by the engine, which is usually accomplished through a shaft. Power is a measure of how quickly a strenuous task is accomplished, or how quickly one can produce work in a given period of time. Understanding either, however, first requires understanding work and energy.
Work and Energy
The term “work” refers to a force moving across a set distance. Using a car as an example, if your car stalls and you try to push it but nothing happens, you have done no work. Alternatively, if you push the car and it does move, you’ve created work. Work is defined in a formula as pounds of force times feet of distance. Energy is similar but different. No movement is required with energy. Energy is the capacity of an individual (or object) to do work.
Knowing the Facts
There are three fundamental rules to keep in mind when comparing the differences:
- Power (the rate of work) depends on torque and RPM
- Torque and RPM are measured quantities of engine output
- Power is calculated by the equation (HP = Torque x RPM/5252)
Understanding the relationship between the two factors also requires knowing a little about an engine’s parts. The engine generates power using a rotating shaft that exerts a certain amount of torque on a load at a given RPM. The amount of power that the engine generates varies based on the RPM.
What is Torque?
This term refers to the load (of power) on an engine at a set rotation per minute. This gives a basis from which to create a definition. The total amount of power that the engine produces differs depending on rotations per minute. Essentially, the measurement translates to a force around a certain point. The force is then applied to a radius. Ultimately, this results in a final measurement expressed as “pound-foot.” The unit of work, contrarily, is expressed as “foot-pound.” Torque is a vector that usually acts in a certain direction. Therefore, it is expressed in newton-meters as well. Torque can also be described as “static,” which means that no direction is involved in its measurement. However, static torque is also expressed in pounds-feet.
Knowing how engines generate power is a good first step in understanding how your car works. However, it’s also important to measure power output. Fortunately, skilled mechanics can determine a vehicle’s engine output using specialized tools. These tools are called Eddy current dynamometers. They have a magnetic field that transfers torque from the crankshaft to a lever arm that bears against a load cell, or a static force gauge. Another type of dynamometer, called a water brake, can be used as well. This equipment uses a spinning set of pump vanes and a static set to transfer power from a lever arm to load cell.
Your Car’s Engine
Even though these two measurements apply to engines across the board, there are many variations in power output and capacity among vehicle engines on the market. The total power capacity of its engine determines many aspects of a car’s performance, including how fast it can travel, how much it can tow, and how quickly it accelerates. Although the numbers differ, all vehicles have something called an “idle-to-redline” range, which is a measure of how quickly the car accelerates from a standstill.