You might have seen the engine torque curve of diesel or petrol engine in your vehicle manual. Did you really understand it?
Torque, Horse power (HP) and RPM
The twisting force applied to a shaft or wheel kind of object is (not so technically) called as torque. You try to loosen a bolt by means of a spanner means you are applying torque irrespective of the fact whether the bolt has loosen or not. Most common unit of torque is Lbf-ft or lb-ft.
HP is a type of unit of power. 550 Lbf-ft works done per second is termed as 1 Horse power.
RPM is revolution of crankshaft (or anything for that matter) per minute.
You can calculate HP from the torque and RPM from the relation between torque and horsepower (HP):
HP= (Torque x RPM)/5250
You can measure torque and RPM and you have to calculate the Horsepower (HP) from them and not vice versa.
A typical engine performance curve is shown below:
Torque curve (shown in Dark red color) is the graph plotted between torque and RPM of the engine. Normally it also plotted along with the HP curve of the engine (shown in green).
- You must observe that as the RPM increases, both the curve become steeper. This is because, as the RPM increases the cylinder pushes more frequently which results better torque and better HP.
- After reaching the peak, the torque curve starts declining but Hp curve still goes up, why? This is because, as RPM reaches very high engine couldn’t breathe that fast because the valves and the air supply and return pipes become insufficient for meeting the air demand of the cylinder at that elevated RPM. So torque start decreasing, but since RPM is very high, so the product of RPM and torque remain higher for some more time so HP curve is still steep.
- The point where both the curve intersects is the 5250 RPM point.