Everything You Need to Know About Horsepower
We try to answer your every horsepower-related questionby Tudor Rus, on LISTEN 06:44
We all use the term horsepower, we hear and read it whenever people are talking about cars, but what exactly is horsepower and how it came into usage? The answer to that question and much more await you below.
Horsepower - a short history
Let’s start with the beginning here. The term was coined all the way back in the 1700s, when Scottish engineer James Watt - otherwise famous for his work on steam engines and getting them to deliver better performance - wanted a way to refer to just how much power a coal-lifting mine horse could deliver.
While further researching the topic, Watt concluded that a mine pony could do, on average, 22,000 foot-pounds of work in a minute. He then increased the number by 50 percent, and so he coined the measurement of horsepower: 33,000 foot-pounds of work in one minute.
OK, but what exactly is horsepower?
There’s a saying that goes like this: horsepower is how fast you hit the wall, and torque is how far you take the wall with you.
Well, that’s not entirely accurate as Engineering Explained’s Jason Fenske showed in one of his videos.
Ask Encyclopaedia Britannica, however, and they’ll tell you that horsepower is the rate at which work is done. Remember James Watt? One horsepower is 33,000 foot-pounds of work per minute. In other words, horsepower reflects the power necessary to lift a mass of 33,000 over one foot in one minute.
Horsepower also has an electric equivalent, as in one horsepower equals 764 watts. On top of that, there’s also the metric horsepower - another unit for power - which equals 32,549 foot-pounds per minute or 0.9863 horsepower (German and some European manufacturers use it when giving out engine power - it’s that ‘PS’ you sometimes stumble upon when reading car literature).
To put things into (mundane) perspective a bit more, Popular Science gives some neat real-life examples of horsepower and its, well, power. For example, the average cyclist can develop one horsepower during “a momentary dash of a flat-out sprint.” What’s more, one horsepower is enough to power a “standard drip coffee maker.”
It also helps to get your grip with the following terms and what they mean:
Work - the amount of force generated over a distance, measured using units such as the foot-pound, kilowatt-hour, or BTU (British Thermal Unit).
Energy - a mechanism’s capacity to do work by various means - mechanical, thermal, or electrical.
Power - rate of work, in other words, how much work is done divided by the time needed to do it.
What’s the difference between horsepower and torque, then?
Essentially, torque is a force applied at a distance.
Think of when you’re trying to use a wrench to tighten a bolt. You push down the wrench, thus applying a force, which is applied to the wrench’s arm (which has a certain length) - so the force generated by your muscles and the wrench itself creates torque on whatever you’re tightening. So, torque equals force multiplied by distance. With that in mind, you can increase torque two ways: by either increasing the applied force or by bumping up the distance. This is why it’s easier to break loose really tight bolts with a longer ratchet or "breaker bar." The more leverage — distance — you have the more torque is created.
The same principle applies inside an engine. There, the force is provided by the combustion (the air-fuel mix igniting inside the cylinder), which in turn presses on the piston which in turn moves the crankshaft. So you’ve got your force and your distance (the length of the piston arm), therefore torque is generated at the crankshaft.
Horsepower, on the other hand, is force multiplied by distance multiplied by rpm. We know force multiplied by distance is torque, so horsepower is torque multiplied by rpm. Which brings us to the next question.
How can you measure engine horsepower?
The obvious answer is a dynamometer, also known as a dyno. However, get this: the dyno actually measures an engine’s torque, not its horsepower. So to get a horsepower reading, you just multiply the torque by the rpm and divided it by 5252. Why that number? Good question. The video above gives you a neat explanation courtesy of Donut Media (PS: you might want to skip to 5:20, by the way).
Why diesel engines make more torque than horsepower?
It’s all about compression ratio. Diesels have a greater compression ratio than gasoline engines. What does that mean?
The piston can travel further in the cylinder, thus the distance to the center of the crankshaft is bigger
- remember how the longer wrench generated more torque? It’s the same principle.
However, the piston also has to travel over a lot more distance do get through a revolution, which means it can’t move as quickly as one from a gasoline engine. Now, remember how horsepower is work done quickly? Well, diesels can do a lot of work alright, but they just can’t do it as swiftly as a gas mill, so while they have high levels of torque, horsepower is on the short side.
That’s why your large truck can haul a lot of goods, or a bus has enough grunt to transport a lot of people, but it can’t go as fast as a gasoline-powered regular car.
What does horsepower mean?
Horsepower is the rate at which work is done. As described above, one horsepower equals 33,000 pound-feet of work in a minute. Horsepower is also torque multiplied by time, i.e. how much work can be done over a given time frame.
Does one horsepower really equal the power of one horse?
No. The term ‘horsepower’ stems from James Watt’s usage of mine horses in his endeavour to determine how much work could one such pony do in a minute.
What is the average car horsepower?
Nothing is set in stone and as we all know, horsepower varies a lot from car to car and from segment to segment - you’ve got your econoboxes and at the other end of the spectrum your supercars and hypercars. However, in a typical mainstream American car, you can expect, on average, between 180 and 200 horsepower.