Is Horse Power an Outdated unit of Power !!?!!
I've never understood why we use the term Horsepower. We don't calculate a bulb's life in Candle Hours or an air plane's thrust in Eagle Power, so why do we calculate an engine's output in Horsepower? In order to understand how outdated and idiotic Horsepower as a unit is, we need to understand two things: first, the history of how horsepower as a unit came to be, and second, the definition and formula to calculate Horsepower. I learned the definition of Horsepower and the formula to calculate work and Horsepower while I was studying engineering at the University of Hertfordshire. In this article, we will analyze the work of James Watt, the inventor of Horsepower, and we will determine whether or not Horsepower is a good means of measuring how much power an automobile has.
To understand this, we need to look into the history of the unit. Until the 18th Century, before machines became popular, the majority of laborious work was done by horses. With the invention of the steam engine, horses slowly started being replaced with machines. But, the steam engines at the time were not that efficient. In 1763, Scottish engineer, James Watt, took this as an opportunity to make a more efficient steam engine. During this era, some people were beginning to warm up to steam engines, whereas the vast majority still believed in horses. Watt started selling his more efficient steam engine to farmers and miners, but he needed a marketing strategy that proved that his steam engine was more efficient and powerful than other steam engines, as well as horses. To market his invention, Watt needed a unit that helped him do a measurement comparison that potential buyers could relate to the power of horses. But, instead of figuring out exactly how much power a horse really produced, he estimated. If he could successfully make this comparison, he could prove his invention was not only more efficient, but more powerful than the current means of producing energy. It didn't matter if his estimation was based on a pony, a weak horse, a race horse, or a super strong work horse. As long as his measurement was close enough to be believable, his strategy would be successful.
Now, before we go more in depth, it's important to understand horsepower and how it is calculated.
So then, what is Horsepower?
Horsepower is a unit of power.
Well, then what is power?
Power = work / time
So, then, what is work ?
Work = force x distance
So,
Power = (force x distance) / time
Coming back to the question, "What is Horsepower?"
As we learned in the above history lesson, Horsepower is a unit of power that was invented by James Watt. He invented Horsepower to market his steam engine.
His sales pitch would be, "This steam engine replaces 2 horses."
The American Journal of Physics 4, 120 (1936) claims to have derived the below photograph from the original James Watt notes. The photo has been digitally reconstructed for the sake of clarity.
It shows a horse mill with a 12ft radius and a horse churning it. The horse applies force to the mill, turns it around, and keeps going round and round. His notes talk about force and all his formulas. It claims that the horse applies 180 foot-pounds of force to turn the mill. But, here is an interesting fact: the journal doesn't explain how he derived the 180 foot-pounds number.
So, let's put his numbers in the formulas.
Power= (180 x 2π x 12 x 2.5)/minute
Power= 33,929 ft pound force/minute
However, in reality, this is the correct calculation: 2.5(2π x 12ft) = 188.5ft
In Watt's World: 2.5(2π x 12ft)= 180ft (60yds)
When we use his napkin math: Power = 32,400 ft lb force/minute.
In an absurd twist, he and his business partner, Matthew Boulton, standardized the figure at 33,000 ft lb force/minute.
That is how we get our calculation of Horsepower today of one Horsepower equaling 33,000 ft lb force/minute, or 550 ft lb force/second.
You might be asking yourself, what does this all actually mean? Well, it means that if there was a horse that was pulling a 550lb object with a pulley attached, it would pull that object 1ft per second.
In the automobile industry, if there is a car with 200 Horsepower, it would pull the same load of 550lbs 200ft/second (or 110,000lbs 1ft/second). That being said, there are other things that need to be taken into consideration in terms of calculating a vehicle's top speed like aerodynamics, inertia by accelerating, rolling resistance, internal friction, etc. We won't be fully diving into all that in this article.
To further illustrate how ludicrous the concept of Horsepower is, we need to cover Metric Horsepower.
So in Society of Automotive Engineers (SAE) units, we are looking at ft lbs force/second.
In metric, it would be m kg force/second or kg force x meters/second.
1 Horsepower = 550 ft lb force/second.
Or, 1 Metric Horsepower (1PS) = 75 kg force m/second
To explain why European, or metric, Horsepower numbers are different to American Horsepower, numbers we have to do some crazy math.
Don't be scared, just take a deep breath. All we are doing is multiplying it with gravity.
1HP = 550 x (lbs)(32.17ft/s²) = 17,694 lb ft²/s³
If we convert this to metric, we would get 745.6 kg m²/s³
Lets do the same calculation with metric numbers.
1PS= 75 x (1kg)(9.807m/s²)
Then it's 735.5 kg m²/s³
As we can see, both the numbers are not the same despite it being the same thing. In fact, 1 Metric Horsepower = 98.6% of American Horsepower. That is why you will see the difference in Horsepower rating figures in America and Europe.
For example, the Ferrari 812 Superfast is 800CV (Metric Horsepower) or 789HP (American Horsepower)
The McLaren 600LT is 600PS (Metric Horsepower) or 592HP (American Horsepower)
So, we learned that Horsepower, as a unit of power, was invented to market the steam engine. Math used in deriving Horsepower was inaccurate and approximate. The unit, if derived with metric values, is inconsistent. The idea of Horsepower is archaic. In today's world, we rarely get to see horses, so no one knows what the power of a horse is in order to compare it to the modern combustion engine. Moreover, in this day and age, where we plan to have commercialized space travel, do we want to rely on a unit with so many inaccuracies?
It's an error in basic math due to rounding up numbers. But, despite the Watt being a more accurate unit of power, the world is just comfortable with using the more barbaric unit of power- Horsepower. So, we will keep using Horsepower as a unit of power until we have combustion engines. If you want to use a more accurate, modern, and precise unit of power, use the Watt. It's not a huge change, neither is it difficult as we have been using Watt to measure power of bulbs and speakers since its inception.
Do let me know if you enjoy these controversial technical articles and if you would like me to keep doing them. If there is a mistake in this article or if you disagree with this article please feel free to comment below.
If you would like more detail here is the link to Horsepower's Wikipedia page. https://en.wikipedia.org/wiki/Horsepower
