The Science Garage: Understanding Horsepower and Torque

When it comes to engines, there are two fundamental concepts that govern their performance: horsepower and torque. But what exactly are they, and how do they relate to each other? In this article, we'll delve into the world of engine science and explore the intricacies of horsepower and torque.

So, what is horsepower? Simply put, horsepower (hp) is a unit of measurement that represents the amount of work an engine can do. It's defined as 33,000 foot-pounds per minute, which is equivalent to the energy required to propel a 330-pound horse for one mile. In practical terms, horsepower measures how much power an engine produces.

Now, let's talk about torque. Torque (or rotational force) is a measure of the twisting force that an engine produces when its crankshaft rotates. It's measured in foot-pounds (ft-lbs) and represents the amount of work an engine can do at any given speed. Think of torque as the "muscle" behind an engine, allowing it to accelerate and propel a vehicle forward.

So, how do we convert torque to horsepower? Well, it's actually quite simple. To calculate horsepower from torque, you multiply the torque by the RPM (revolutions per minute) and divide it by 5,252. This is because 1 foot-pound of force at high RPMs can be equivalent to a significant amount of power.

But why does this conversion factor exist? It all comes down to the way engines work. When an engine's crankshaft rotates, it produces torque, which is then transmitted to the wheels through the transmission and drivetrain. The more torque an engine produces, the faster it can accelerate. However, if the engine's RPMs are too high, it can't produce enough horsepower to sustain that acceleration.

The relationship between horsepower and torque is critical in understanding how engines perform. An engine with high torque at low RPMs can provide excellent low-end grunt, making it ideal for towing or hauling heavy loads. On the other hand, an engine with high horsepower at high RPMs can provide exceptional acceleration and responsiveness, making it perfect for racing or high-performance driving.

To illustrate this concept, let's consider a hypothetical example. Suppose we have two engines: one with high torque output at low RPMs (e.g., 200 ft-lbs) and another with high horsepower output at high RPMs (e.g., 500 hp). The first engine would excel in towing or hauling heavy loads, while the second engine would shine on the track, accelerating quickly from 0-60 mph.

So, why do engineers design engines to optimize either torque or horsepower? It's all about trade-offs. Engines can't produce both high torque and high horsepower simultaneously, so designers must choose one over the other. For example, a diesel engine might prioritize torque output at low RPMs for improved towing capacity, but sacrifice some horsepower at higher RPMs.

In conclusion, understanding horsepower and torque is crucial for anyone interested in engines or performance driving. By grasping these fundamental concepts, you'll be better equipped to appreciate the intricacies of engine design and performance. Whether you're a car enthusiast or just curious about the inner workings of an engine, this knowledge will give you a deeper appreciation for the science behind horsepower and torque.

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WEBVTTKind: captionsLanguage: en- Drag Racing!NASCAR!Rally!Racing!Awesome cars make awesome power.You know how we compare em; with stats.But what's it really mean?We're about to find out.We're talking about horsepower and torque.(trap music)The way Tony talks aboutit, torque is a grunt,(grunts)and horsepower is screaming.Torque and horsepower areboth ways of measuring force.Torque measures twisting force,and horsepower measures work,and work is force over time.So let's start with torque.(grunts)You've probably heard of foot pounds.Pound is a measure of force.If you weigh 180 pounds,that's because your mass,what you're made of isexerting 180 pounds of forceon the earth.A foot pound is a pound of force amplifiedby a foot of leverage.This is Jesse's toolbox,and this is a weird bolt in a board.This is a wrench.(soft piano plays)If I use a foot-long wrench,and apply one pound of force at that bolt,I'm applying one foot poundof torque on the bolt.If I used this two foot bolt buster,and apply one pound of force at this end,I'm applying two foot pounds of torque.If I apply two poundsof force at one foot,it's force on the bolt is thesame as one pound of forcefrom two feet.Two foot pounds, or pound feet.It's the same unit because it's math.The distance from the point of measurementamplifies the force through the lever arm.I wonder where it all goes.(yells)Engine torque measuresthe amount of this forcethat an engine can produce.Let's get inside there.The pistons drive this crank shaft.You can see that where it attachesturns around the center axis.Just like the wrenchesturned around the bolt.Each explosion in thecylinder pushes the pistondown with force.That force is exerted on the crank pin,and transferred to theshaft to get it spinning.Torque is determined by two factors.The amount of force on the crank pin,which comes from the piston,and the distance of thatforce from the center axis,or throw, which varies by crank shaft.Make sense?If the throw stays the same,we produce more force from the piston.Maybe with more displacementwe can increase the torque.If the force from thepiston stays the same,we can increase the distance of the pinfrom the crank shaft centerto increase the torque.There's millions ofcombinations and variationsthat we can tweak to adjust the forceexerted on the crank shaft.From the number of cylinders,to the shape of the engine,even to the type of fuel we use,which we can get back to in a minute.Horsepower is a measurement of work,and work is force over time.James Watt lived from 1739 to 1819and he loved doing math.Steam engines were changing everythingduring the Industrial Revolution,and Watt wanted to givepeople a way to measureall of the work that was getting done.Watt was watching work ponies,which are not tiny horses.Ponies are their own thing.You can look it up.Fact: he noticed that apony took about a minuteto raise 220 pounds of coalout of a hundred foot well,and being a scientisthe increased that numberby 50 percent because a horseis about 50 percent biggerthan a pony.Whatever. It's a unit.As long as it's consistent,it can be whatever he wanted.He figured that one horsecould do 33,000 foot poundsof work every minute.A horse, exerting one horse power,can raise 330 pounds ofcoal 100 feet in a minute,or 33 pounds 1,000 feet in a minute,or a 1,000 pounds ofcoal 33 feet in a minute.That's work.That's horsepower.The amount of force.Foot pounds.Torque produced over time is horsepower.Because torque is how muchforce an engine produces,horsepower is how quickly itcan produce how much force.We can dive into this bylooking at how we measurecar horsepower on a dynamometer.You can either put the car on a rollerthat'll measure the forceproduced at the wheels,or if you attach shaft to an engineyou can measure the drivingthe shaft and it's speed.Start your car.You put it in neutral and floor it.Well the engine wouldrun so fast it'd explode.A dynamometer finds aload to the floored engineand measured the speed it moves.You can hook an engineto the dyno, floor it,and use the dynamometer to applyenough of a load to the engineto keep it at, say 7,000 RPM.You record how much loadthe engine can handlethen you apply additional loadto knock the enginespeed down to 6,500 RPMand record the load there,then even more to getit to 6,000 and so on.You can do the samething starting at 500 RPMand then work your way up.What the dynamometer isactually measuring is torque,and because of math, toconvert torque to horsepoweryou just multiply the torque by the RPMand divide it by 5,252.That's horsepower.Why 5,252?Well I'm glad you asked.You remember the horse?from before?With the coal.Okay. That horse moved 330pounds 100 feet in a minute.So 33,000 foot poundsof work in one minuteis one horsepower.Now for my dumb brainLet's use a hundred pounds,330 feet in one minute,because that's also one horse power.We need to show 330 feetin one minute as revs,so let's think of the wheel.The distance traveled on one revolutionwould be two pi r, or sixpoint two eight three one six.330 feet in revolutions is 52.52.Starting to look familiar?A hundred foot pounds at52.52 RPM is one horsepower,therefore one foot pound at 5,252 RPMis also one horsepower.So you take the torque,multiply it by the revsand divide by 5,252 andyou found horsepower.(yells)(breathes briskly)When you plot horsepowerversus the revs per minute for the engine,you get a curve like this one.An engine has peak torque whereit generates the most force.It has peak horsepower whereit produces the most torquemost quickly.When you're trying to accelerate quicklyyou wanna try to keep the engineclose to it's maximumhorsepower point on the curve.That's why you downshift to accelerate.By downshifting you getcloser to the peak horsepoweron the curve.You wanna launch your car from a stop?You rev engine right at peak torquethen you release the clutchto dump maximum powerto the tires.Let's take a minute totalk about why dieselmakes more torque but not much horsepower.Think about what torque is.Remember the long wrench?Well diesel has a muchhigher compression ratio.Which means that the head ofthe piston can travel furtherin the cylinder.That means the distance fromthe center of the crank shaftcan be greater.That means more torquebut because the throw isgreater it's got more distanceto travel to get through a revolutionso it can't move as quicklyand because horsepoweris work done quicklydiesel engines can do a lot of workbut not as quickly as as a gasoline enginewhen it's cranking at high revs.That's horsepower versus torque.Hit that subscribe button.It means a lot.Thanks to Audible forsponsoring this video.Start a 30 day trial and yourfirst Audible book is free.Learn more at audible.com/sciencegarage.Audiobooks are a greatsidekick for summer activitieslike hiking, or running,or biking, or cruisingwith the top down.Listening is a better wayto binge content you lovewhile doing the things you love.Audible's got almost any book you want.Right now I'm listening to Horatio's Driveby Dayton Duncan and Ken Burns.It's about HoratioNelson who made the firstUS cross-country drive in 1903.You can get through tons of bookshands and eyes free whileyou do just about anything.Audible lets you switchseamlessly between devicespicking up exactly where you left off.Hop out of the car.Start it on your phone.Get into the house; 'HeyAlexa, read my book.'Wherever you are you canget tons of books readwhile you are doing anything.Audible members get a credit every monthgood for any audiobook inthe store regardless of priceand unused credits rollover into the next month.You don't like your audiobook?Well you can exchangeit no questions asked.Plus your books are yours to keep.With Audible you can goback and re-listen any timeeven if you cancel your membership.Well I don't know why you'd want to.Start a 30 day trial anddownload your first audiobook.That's any book you want for free.Go to Audible dot com slash science garageor text science garage to 500-500.You got that?Go to audible.com/sciencegarageor text Science Garage tofive hundred five hundred.You can do it with Audiobooks.The more people subscribe to Donutthe more cool stuff we get to do for you.I love reading your comments.Let me know what you wanna see.Please like the show page.You like horsepower and torquecheck out James driving the Dodge Demon,or check out this wheelhouse on F1.Follow me on Instagram at @Bidsbarto.Follow Donut at @Donutmedia.Don't tell my wife that's Jesse's toolbox.She thinks I can fix things.