In the fourth installment of this column, I published an article titled "Why Measure Torque?". This time, I will discuss horsepower, one of the factors used when comparing the power performance of automobiles and other vehicles, explaining what horsepower is in the first place, and how it relates to torque.
Torque is
Let's quickly review the content of the previous column.
Torque is the force that causes an object to rotate around its axis. For example, it is the force used when tightening a bolt with a torque wrench, or the driving force of an engine needed to move a car forward or backward.
Since the enactment of the new Measurement Law in 1993, the unit of torque has been N·m (Newton-meter). As this unit shows, torque is the product of force (force acting in the rotational direction) and length (length from the axis of rotation to the point of application). Therefore, if the force acting in the rotational direction is the same, the longer the length from the axis of rotation to the point of application, the greater the torque that can be obtained. Taking a bicycle as an example, the longer the length of the crank connecting the pedal and the axis of rotation, the less force is required to produce the same torque.
Torque is the force that causes an object to rotate, and its unit is N·m.
What is horsepower?
Horsepower, as the name suggests, is the amount of work a horse can do (the force with which one horse can pull a load).
This unit represents [something]. In an experiment conducted by James Watt, the inventor of the steam locomotive,
A horse pulled a load of 175 pounds (approximately 80 kg) and moved it 188 feet (approximately 57 m) in one minute.
The amount of work done by a horse, converted to a unit of time, was defined as "1 horsepower." In other words, horsepower and
Work is the amount of distance an object of a certain weight can travel in a unit of time.
It's a unit of rate.
Furthermore, horsepower has long been a commonly used term, as it is listed in the specifications of automobile engines, as shown in Table 1.
It has been used for this purpose. Recently, with the promulgation of the new Measurement Law, the notation has been standardized from horsepower (ps) to watts (W) for display purposes.
Table 1. Sample Engine Specifications Table
The relationship between horsepower and watts (W) is as follows:
1 Horsepower (ps) = 735.5 (W) = 0.7355 (kW)
"Horsepower" was the name of a unit conventionally used to measure power output (work rate). Currently, it is expressed in watts (W).
Relationship between torque and power output (kW, ps)
As mentioned earlier, while torque is a "force," output is a "power." Output is calculated by multiplying torque by rotational speed, and is expressed by the following formula:
Output (kW) = Torque (N・m) × Rotational speed (r/min) × 2π ÷ 60 ÷ 1,000 (Equation 1)
To use a car as an example, the torque of a car is the force that rotates the tires, and its magnitude corresponds to the force that makes it want to move forward or to the force that makes it want to go uphill. Increasing the torque makes it easier to start moving quickly and allows it to climb steep hills more smoothly. In other words, torque can be said to be an indicator of instantaneous power and rotational force. If you think of the torque generated by a car engine as the force you apply to pedal a bicycle, the greater the force you apply (torque), the greater the acceleration the bicycle will achieve. This way of thinking might make it easier to understand.
On the other hand, output is an indicator that shows how far a specified load (weight, etc.) can be transported within a certain amount of time.
Yes. It may seem similar to torque, but the two are different. Torque does not have the concept of unit time, but output represents power, so the concept of unit time is necessary.
To use a car as an example, the output of a car is the amount of work done by multiplying the torque (the force that rotates the tires, etc.) by the engine's rotational speed.
Similar to the explanation of torque, if we consider this in terms of the force used to pedal a bicycle, then the force applied to the pedals is weak.
Even without pedaling hard, you can achieve the same output (power) as when you pedal hard by downshifting (using a lighter gear) and increasing the tire rotation speed. Thinking about it this way, it should be relatively easy to visualize, shouldn't it?
In this article, we discussed torque, horsepower, and output, terms often used in car engine specifications. We hope this article has helped you understand what horsepower is and the relationship between torque and output.
The torque-related measuring instruments we handle are as follows:
Please take a look if you are interested.
(Excerpt from the email newsletter issued on June 15, 2022)
