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Power Torque Curve Explained

Power and Torque are two critical elements of any car's specifications. They both impact a car's performance. They therefore are very closely interlinked. Power Torque curves help understanding a cars performance and its behavior over an rpm range more clearly.

Power Torque Or P/T Curve

For simple questions like; Is a car which delivers peak power of 105 PS @5250 Rpm better than another car delivering 123PS @6300 Rpm Or a car delivering 158Nm @4200 Rpm better than car delivering 153 Nm @3800 Rpm

Questions which at a glance should have a simple answer, like; the one with higher power or torque delivery at lower RPM is a better. But unfortunately this may not be true many a times. It’s to solve such mysteries that one need Power Torque Curve.

Power and Torque are the figures that reveals the capacity of a car’s heart, in terms of work the car can perform and rotating force that will be available to its wheels. Just going through the spec-list and then reaching the conclusion after doing some simple addition subtraction mathematics may at times mislead and make you buy a new car that may not live up to your expectations. Power and torque figures are important, but then understanding them beyond just numbers would be a great help. All this Gyan, simply to bring alive the necessity of understating the P/T Curves.

POWER & TORQUE

Power in simple terms can be defined as rate of doing work i.e Power = Work/Time.

In units, it is usually expressed either in PS or bhp. Power essentially means the capability of the cars heart to do work efficiently (in this case its capability to move a car) in relation to time taken to complete it. This means more work done by power plant, more powerful it is; for example engine performing 100 units of work in a minute is better than its counterpart having an output of 80 units of work in same time constraint.

On the other hand, torque or rotational force is calculated by multiplying force and distance. i.e Torque = Force X Distance.

Unit of Torque is Newton-meter (Nm) or Kgm. In case of a cars engine, torque is calculated by multiplying the force applied by piston rod on the crank by length of crank arm. This clearly highlights the role of crank length in generating the torque figures. Longer crank arm length, greater will be amount of torque generated.

POWER TORQUE (P/T) CURVE

                 



Power and Torque for a car are inseparable; therefore
they need to be studied together. P/T curve are plotting of Power and Torque figure on two vertical axis with RPM plotted on X axis. Both these figures for a car are always delivered at a certain engine RPM, therefore a co-relation with RPM is necessary. Some inferences from the P/T curves for Renault and Etios (Please fell free to comment).

Power output of the engine increases with the increase in the rpm regardless of torque i.e. more fast the engine is spinning, more power it will make. This also gives us a fair idea about the spinning level the engine can achieve. For example power plant capable of making 100 PS of power @ 7000 rpm is having far better revving capacity than the engine produces the same power @ 4500 rpm. This also explains why petrol engine are more rev-happy in nature because they make their peak power at higher rpm than their diesel brothers. For example Swift petrol makes its peak power @6000 rpm while its diesel cousin delivers its peak power @4000 rpm.

Torque curve, on the other hand, is related more to the drivability aspect as it is the rotational force available to the wheels. Flatter the torque curve more pleasurable the drive. Driveable in relation to torque means a lesser need for shifting gears as most of the time the car is equipped with enough power to carry on the operation. A car with flat torque curve will provide the required ensure it’s mobility. From the figures above and basis what we have just explained,  Etios in relation to the Renault would offer higher drive pleasure because of a comparatively falter torque curve.
So while deciding on which new car to buy, it would be prudent to take into consideration one’s daily drive composition (i.e quantum of congested city traffic drive vs Highway drive). For car buyers commuting mostly in the city it would be better if they buy a car with a flatter torque curve vs car buyers driving mostly on highways where a torque curve with sharp peaks would be more ideal as it facilitate overtaking on the highways. Car with torque curve which are not very flat will be tiresome as would have to change gears too often in the congested city traffic.

Another important criterion to select the vehicle is to search for the power plant having broad power band i.e. having power and torque figures equal in numbers over an Rpm range. More similarity in both the figures, more wide will be the power band and hence such a car will be that much more convenient to drive. 

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