this is watt I found out…
Like everybody else with a broadband internet connection, I spend an embarrassing amount of time watching YouTube videos. Shame on me.
The only redeeming factor of my worthless habit is that the videos I like to watch are usually about stuff I like; history, science, engineering, planes, cars, astronomy, space, computers, well you get the idea. I am a geek.
One of the channels I enjoy is called Engineering Explained; the channel’s host is a smart guy called Jason Fenske, a mechanical engineering graduate from North Carolina State University.
He spends usually 7 to 10 minutes explaining, in a fast and somewhat quirky way, interesting topics that usually are not known to the average video addict. He is funny in a curious way like he did not want to be funny at all. But he is.
In one of his videos, he explains what a horsepower is and how such a curious measurement unit was created. I had asked myself the same question, but I was too lazy to find out. So, I watched the video and learned a couple of interesting things that I will share. If you do not like to read, I am including a link to Jason’s video at the end.
Every car enthusiast is familiar with the amount of power his favorite cars’ engines produce as well as their 0 to 100 km/h (0 to 60 mph) times. But what in the name of Carl Benz is a horsepower?
A horsepower is a power measurement unit created by James Watt, the same Scottish engineer and inventor that, in 1776, dramatically improved the design of the steam engine, obtaining fame but not success. His finances were constantly in bad shape until his death.
The steam engine was created by another Scotsman inventor called Thomas Newcomen, who in 1712 improved an original device made in 1698 by an English mechanical designer called Thomas Savery. Savery invented a steam-powered pump, Newcomen improved it by including a piston, and Watt finally added a separated condenser, improved the piston’s seals, and added a planetary gear system to convert the piston’s linear motion into rotary motion, making the steam-powered engine a fully practical device and therefore starting the Industrial Revolution in one single step.
James Watt required a way to express the equivalent power produced by his steam engine to an average XVIII century entrepreneur. Back then, people were familiar with horses, and horses were used as power-generating devices in coaches, farms, mines, and mills.
So, James Watt made the following calculations: a horse could turn a mill 144 times per hour. The mill’s wheel had a radius of 12 feet, so the horse could cover a distance of 12 x π x 2 feet in one hour. Since 144 turns per hour equals 2,4 turns per minute, Watt just required to measure the force that a horse could make while pulling the mill to know the amount of energy generated.
Since dynamometers were still a thing from the future, Watt “estimated” that a typical horse could exert a pulling force of 180 pounds force. A pound-force is defined as the gravitational force that our Earth exerts on a mass of one avoirdupois pound placed on the surface of the planet.
And what on Newton’s name is an avoirdupois pound? It is a long story, but for clarity, let us settle with the current definition: an avoirdupois pound is exactly 0.45359237 kilograms.
So, Power=Work/time, Work= Force x distance, therefore:
P=W/t=F x d/t= 10 lbf x 2,4 x π x 2 x 12 feet/1 min=32752 ft.lbf/min.
And this is James Watt’s definition of a horsepower, although for practical purposes he rounded the value to 33000 ft. lbf/min. Many engineers disputed Watt’s calculations, but his estimate managed to survive all defies.
In 1789, the French revolution brought to the world great ideals such as Liberté, Égalité, and Fraternité, and a few great ideas such as the metric measurement system. In this system, each of the fundamental dimensions of nature is expressed by a single base unit of measure and its corresponding powers of ten. It eventually became the International Measurement System (IMS), greatly simplifying the measurement of all things’ weight, mass, dimensions, etc. coherently. The IMS was adopted by the whole planet, except for Liberia, Myanmar, and the United States, a group of countries that stubbornly preferred the aging Imperial Measurement System created by the British Empire even until today.
The XIX century’s Industrial Revolution was led by England, so the horsepower became the preferred way to measure power through that century, but eventually collided with the rational benefits of the metric system and a settlement had to be reached. And the settlement was this:
33000 ft.lbf/min = 550 ft.lbf/s (since 1 min= 60 seconds)
= 550 × 0.3048 × 0.45359237 m⋅kgf/s (since 1 ft ≡ 0.3048 m and 1 lb ≡ 0.45359237 kg)
= 76.0402249 kgf⋅m/s
= 76.0402249 × 9.80665 kg⋅m2/s3 since g = 9.80665 m/s2
= 745.699 W since 1 W (Watt) ≡ 1 J/s = 1 N⋅m/s = 1 (kg⋅m/s2)⋅(m/s)
To maintain everybody’s sanity, a relationship of 1 hp = 746 W was adopted.
If you read a German engineering book you will find physical formulas based on scientific calculations. If you read an American engineering book, it will be plagued by empirical formulas and rules of thumb.
Germans hate estimates, so the German DIN (Deutsches Institut für Normung e.V.) standards bureau defined the metric horsepower as the power generated by a horse pulling a 75 kg weight over one meter in one second or:
75 kg × 9.80665 m/s2 × 1 m / 1 s = 75 kgf⋅m/s = 1 PS
PS stands for “Pferdestärke” or steam horsepower in German, or “cheval-vapeur” (CV) in French. It is also known as the metric horsepower.
There is a small difference between an Imperial horsepower and a metric horsepower:
1 PS = 735.49875 W
1 hp= 745.699 W
And that difference was the cause of why until today, the same car engine was advertised producing different horsepower values depending on whether it was sold in Europe or the US.
But in 1972, with the advent of the EEC (Economic European Community), standardization based on the International Measurement System made the PS obsolete and replaced it by the watt, making the use of the kW the default unit for power measurement.
If you look at a car’s European characteristics, you will find that the engine’s power is shown in kilowatts and sometimes, depending upon the country, the equivalent in PS or CV.
With the ongoing electric and hybrid car revolution now in place, the use of the horsepower looks increasingly like a thing from the past. We use watts to measure the power used by lamps, electronic devices, computers, air conditioners, and kitchen appliances. So why not in cars? Nostalgy perhaps.
Electric cars seem to be destined to become mobility appliances instead of personal status symbols in the near future, as their manufacturers keep on adding automation to the driving experience.
In fact, the driving experience will become a synonym of being comfortably seated doing something useful while traveling in an autonomous, battery-powered electric vehicle. And their power will be measured in watts.
Our descendants will forget the horsepower and will wonder what an electric motor’s power output had to do with the amount of work per time a horse could do. I hope they also wonder why we stubbornly kept on using fossil fuels for so long.
Jason Fenske’s Engineering Explained YouTube’s channel episode on the horsepower’s origins can be watched here: https://youtu.be/gC2-JKO0c2I
Try not to overdose on his content, there are other things in life better than cars.