Power and torque – these are the two key output parameters which typically determine the broad performance of an automobile. Now while everyone talks about power and torque, there are quite a few amongst us who don’t fully understand the differences between the two values. What exactly is power, and how is it different from torque? Which of the two is more desirable? Which aspects of a vehicle’s parameter do power and torque affect? There is a whole array of questions associated with the two values, and in this article, we will try and answer. We’ll also try and explain as to how the two quantities affect a vehicle’s performance from a layman’s perspective.
In order to understand power and torque in detail, we’ll first need to understand energy. Energy is the capability to do work. This energy can be expended in the form of heat or mechanical energy, or can be contained within an object as potential energy. When energy is expended, work is done. The unit of energy is Joule. For example, if you are pulling out water from a well, you are using your body’s muscular energy in doing work, which is pulling out the water. Energy and work are essentially same entities, and are represented by Joules as their SI unit.
Now, power is the rate at which energy is expended, or work is done. So, in order pull the water bucket out of the well faster, you’ll need to expend energy at a faster rate. A faster rate of energy consumption would result in a faster rate of work done. And it’s this rate of consumption of energy, which is represented as power.
In technical terms, power, as the rate of doing work is represented by Joules per second, which in turn is represented by a single unit termed as Watt. The rate of consumption of energy, or doing work in automotive terms is also represented by horsepower, which is defined as the power needed to move 33,000 pounds of mass to one foot and in one second. For perspective, 1 mechanical horsepower is equal to 745.7 watts.
So that’s power, or the rate of doing work. It is a scalar quantity, which basically means that it doesn’t have multiple values associated with its measurement. Power is calculated only in terms of magnitude, and doesn’t have any direction associated with it. Direction brings us to another quantity, termed as torque, which is a vector quantity, and has both magnitude and direction. Don’t fret though; we’ll break it down in simple terms for you.
To cut right through the jargon, torque is a measure of the force that rotates an object about its axis, fulcrum or pivot. Turning that door knob, pushing-open a door, and using a spanner to unscrew a nut are all examples of you using torque. Now unlike power, torque also has direction, and it’s termed as a vector quantity unlike power, which is a scalar quantity.
To elaborate this, if you try to apply force on a spanner as a tangent to the circle that it’s making with the axis of the bolt, you’d be able to turn it most efficiently. If you try to partially pull the spanner outwards or push it inwards while turning it, you’ll waste your force and won’t be able to unscrew the nut very efficiently. This means that the effectiveness of the applied torque is dependent on the direction in which it’s applied – and that’s what you call a vector quantity for a layman.
Unlike power, which is represented by Joules per Second, or Watt, Torque is represented by Newton Meters in SI system, or by foot-pound in the British Imperial System. Mathematically, torque can be written as T = F * r * sin (θ) where r is the distance from the pivot point to the point where force is applied and F is the force applied, whereas θ is the angle between r and f.
Forget those complex formulae though, they are here only to let you know that an angle is also involved, and the effect of the force applied at a lever will also depend on which direction you applied the force in.
In pure automotive engine terms, when the piston moves, after charge detonation, torque is the turning force that pushes down the piston, turns the crankshaft and rotates the flywheel. On the other hand, horsepower is the torque, multiplied by the engine rpm, or the rate of work done. So torque, in essence is the sheer force produced when the air-fuel mixture combusts, and how fast, or at what frequency (rpm) you can produce that torque is the power.
Now to put all of that jargon aside, and understand the effects of power and torque on a machine’s performance, let’s create two imaginary motorcycles with exactly the same weight, size and appearance. Of these two bikes, let’s assume one bike has 50 horsepower and 200 Nm of Torque. The other bike, for assumption’s sake, has 100 horsepower and 100 Nm of torque. This means that the first bike has half the horsepower, but twice the torque of the second bike.
Now, the second bike with 100 horsepower should be able to accelerate faster and should be able to achieve a higher top speed owing to its higher power. In the simplest terms, since power is the rate of work done, it should be able to move the motorcycle at a faster rate. However, the second bike won’t have the amount of turning force required to carry heavy loads. Load the second bike up with two heavily built men, with weighty luggage, and the performance of this motorcycle will be adversely affected.
On the other hand, the first bike, with a high torque figure, will not sprint as quickly as the second bike and will have a lower top speed as well. With twice the amount of turning force, however, even with a lot of rider load and luggage on it, its overall performance will still relatively be much less affected.
All things being equal, the machine with higher horsepower will accelerate faster, and have a better top speed. It would, however, not be able to lug heavy loads. The one with higher torque, however, will be relatively lazy, but will be able to lug heavy loads without any fuss.
The above facts are the reason why sports bikes, meant to accelerate hard and gain higher top speeds have lightweight high-revving engines which propel them to high speeds very quickly. They are kept light to ensure that their performance is not adversely affected by weight. On the other hand, big cruiser bikes are all about torque, and while they don’t accelerate too hard and are built very heavy, they can cruise effortlessly at decent speeds all day long irrespective of the weight you load them up with. This is also the reason why the big load-lugging trucks and lorries don’t have the power to match up even to even a sports car, but boast torque which goes into thousands of Newton meters.
Finally, torque can be manipulated using gears. For example, to loosen a tight nut, you can always use a spanner with a longer handle. However, once the nut is loosened, a longer spanner wouldn’t be too handy to turn the nut very quickly. So again, if it’s about the rate of work done, horsepower wins. However, for the sheer load of work that needs to be carried out irrespective of the rate, torque is King.
We hope this detailed explanation of differences between power and torque went some way in dispelling your confusion about the two quantities. If you think you’re now clear about what the two terms mean, you can probably help your friends understand them better. You can also share this article with your friends who would be interested in the concept.
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