Mechanics Masterclass: Horsepower, torque and other important words.

Hello again from the vast reaches of the world-wide web. A friend of mine, who is considering buying her first bike, posed a very interesting question: Which is more important, torque or horsepower? This had me thinking of how to properly explain to someone what these concepts mean and how they factor in to how a machine performs. How does one explain this mechanical lingo to someone who isn’t really mechanically inclined?

To answer this in brief, one can always refer to the age-old saying : “Horsepower is how fast you hit the wall, torque is how far you drag the wall with you.” However, this doesn’t really capture the true linking of these concepts. To this extent I have done some research and phrased it as best I could, so that even the most mechanically challenged person should be able to understand. So if you think “fuel goes in the tank, magic and unicorn farts happen, and then the vehicle moves forward”, pull up a chair, grab a notepad, because class is in session.

Before we can get into the nitty-gritty detail, I think it is appropriate to first create a scenario to help explain what I’m about to say. So, create the following image: you have a work horse, capable of pulling a specific load, let’s call him Steve. Steve is connected to a rope that runs through an intricate pulley system that can alter how force is applied by a pull of a lever. The other end of the rope is attached to a weight. Everybody got that? Now let’s dive into the first concept.

Torque

You will not find a dictionary definition here, as these are just words that don’t really explain the effect of how torque applies. To define torque, once again it’s important to create an image: Engines consist of pistons moving up and down, applying force to a crankshaft. It’s the crankshaft that converts the up and down motion of the pistons to rotation. Think of torque as the amount of force the engine turns the crankshaft with. Alternatively think of torque as how strong you would have to be in order to stop that rotation. The higher the torque output of an engine, the stronger the rotational force of the crankshaft. This is usually measured in ft.lb (foot pounds) or n.m (Newton metre) depending on which side of the big blue pond you are on.

To bring Steve back into the picture, Steve’s torque output would determine whether or not Steve can apply enough force to move the weight attached to the rope. This is the force measurement that defines how easily Steve can overcome the static friction of the weight, and overcome inertia. In laymen’s terms: It’s the measurement of how quickly Steve can get the weight moving and at what rate the weight would accelerate.

*Side note for all the two-wheel fans reading this: Torque is also the magical measurement that determines if your bike’s front end would still lift in 3rd gear.

Horsepower

This is where the explanation becomes a little tricky, as horsepower is directly linked to torque output. We all now understand that torque is the force that the crankshaft rotates with. We next have to determine how “good” or “efficiently” or “consistently” that rotational force is delivered.

To go back to Steve. We know that Steve can apply X ft.lb or Y N.m of torque. Now we can create the scenario where Steve can move the weight, weighing 100 kg (220 lbs), a distance of 2.5 foot, in one second. This means that Steve, with his torque output of 550 ft.lbs (745,58 n.m), is able to produce 1 horsepower. In this 1 hp = 550 ft.lbs of torque delivered in 1 second.

Now imagine that Steve has a buddy. Another work horse that we will call Phillip. Phillip agrees to help Steve to pull this 100 kg or 220 lb weight. Together Steve and Phillip pull it a distance of 5 foot, in one second. Keep in mind that Phillip and Steve individually can still only apply torque of 550 ft.lbs. The torque output has remained unchanged, but the horsepower output has doubled to 2 hp.

*Side note: the average horse produces about 1 horsepower.

Thus, horsepower is essentially the product of the torque produced or applied in a specific time. Linking this back to an internal combustion engine, usually increasing the number of cylinders would lead to an increase of horsepower. Alternatively, altering how much torque a piston applies to the crankshaft would also alter how much horsepower is produced. That is why, increasing bore size, and compression ratios leads to higher horsepower, because, effectively, each cylinder is now applying more torque to the system.

So… now back to the question of which is more important. To answer this question: torque is the power that can be applied by the engine. Horsepower is how well the aforementioned torque is delivered. Therefore neither is more important in relation to one another, unless another aspect is thrown into the mix…

Power-to-weight ratio

So we now know that horsepower is the amount of torque that can be applied in a set time, but what does this mean for different types of vehicles. Well, let’s first refer back to 1 hp Steve. In the scenario we know that Steve has to pull a weight of 100 kg, and can do so with 550 ft.lbs of torque. So, to determine the power-to-weight ratio of Steve we do the following: We take the horsepower that Steve can apply and divide that by the weight that needs to be moved, in metric tons. This would look a little something like this:

1                                                           |           –  Steve’s horsepower.
0.1                                                        |           – The 100 kg weight converted to metric tons
=  10 horsepower per ton.

I don’t really know whether this is good or bad, seeing as Steve is a horse, and one would never really need to determine the power-to-weight ratio of a horse. However we now know how to determine the power-to-weight ratio. Which means that we can now use this math in a different scenario. Let’s say that we have a 150 hp engine that we are now going to use in 3 different types of vehicles. A bike, weighing around 190 kg, with the engine installed and without a rider. An empty car, with the engine installed weighing around 1200 kg. Lastly a truck, without a trailer, with the engine installed, weighing about 4500 kg. Keep in mind that the exact same engine would be fitted in each of the vehicles. Now the math looks a little something like this;

Bike

150                                                | – the horsepower from the engine.
0.19                                               | – the 190 kg weight converted to metric tons.
= 789 horsepower per ton.

This would be a ratio in the supercar territory, and which explains why most modern sports bikes accelerate like a bat out of hell. A vehicle with this type of power-to-weight ratio would definitely feel very aggressive and sporty on acceleration.

Car

150                                                | – the horsepower from the engine.
1.2                                                 | – the 1200 kg weight converted to metric tons.
= 125 horsepower per ton.

This is pretty much what one can expect from the family sedan. It would be able to adequately transport you from A to B, but it wouldn’t be an experience to write home about.

Truck

150                                                | – the horsepower from the engine.
4.5                                                 | – the 4500 kg weight converted to metric tons.
= 33 horsepower per ton.

This is very poor. This semi-truck would barely be able to move itself about, let alone pull a loaded trailer. Normally a vehicle such as this would have much, much more horsepower, however this was only done to illustrate that different vehicle weights also factor in on how a vehicle performs.

With all the above in mind it becomes a little more clear. Horsepower is the definite defining characteristic when it comes to how a vehicle can and will perform, when the weight of that vehicle is also brought into consideration. In essence, the power-to-weight ratio of a vehicle will determine how well the vehicle copes with acceleration and an all together aggressive feel.

Bonus concept: Power curve

For those of you who have now scratched your heads and thought : “how is this power available from idle” or “whats the purpose of a gearbox then”. This section is especially for you.

This is the reason why Steve’s rope was connected to a pulley system that can alter force with the pull of a lever. Think of Steve as being the engine of that system, and to make Steve’s job as easy as possible one sometimes need to trade torque for speed, or vice versa. This is exactly the job of the gearbox.

08_triumph675.jpg

Power curve of the 2008 Triumph Street Triple 675. View source here.

View the power curve above. As one can see the torque output of the engine is very consistent throughout the RPM range. However, at around 2500 RPM the engine barely produces 20 hp. This means at low revolutions, the engine is not very efficiently applying torque to the system. The maximum of 92 hp is only achieved at the peak end of the rev range. This means that if an engine would only have a direct drive, one would only have 20 hp to pull away with. This is would prove to be very difficult.

This is where our hero, the gearbox, steps in. This is a mechanism containing different ratios (usually 5 or 6) between the speed of the input (crankshaft) and output (wheels) shafts. These different ratios allow the driver to trade speed for torque (the 1st gear end of the spectrum), where the crankshaft could be turning around 2 and a half times faster than the wheels. Thus giving the vehicle more torque to overcome inertia. In essence the driver would be able to use close to the maximum horsepower to get the vehicle moving from a standstill, without much strain on the engine.

Or vice versa, trading torque for speed (5th or 6th gear end of the spectrum) where the crankshaft could be completing about half a rotation for every rotation of the wheel. Here the driver trades torque for speed. In essence having the wheels rotate faster than the crankshaft. This either leads to an increase in top speed, or alternatively a more fuel-economic lower speed. However, attempting to accelerate quickly would put a lot of strain on the engine, hence the intuitive gearing down before accelerating.

It is now, with all this new-found knowledge, that I must send you into the world to experience the science for yourself. In that sense I wish you a safe ride and very enjoyable journeys.

Advertisements

Share your thoughts

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s