# Understanding torque curve

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.

References:

## 8 thoughts on “Understanding torque curve”

1. Hamidreza says:

Hi,
I am not sure to well understanding the relationship between torque and the cylinder pushes force.Dear Expert,why you consider torque as cylinder pushes,please?

2. MechGuru says:

Because torque = force x distance between the application point of the force and axis of rotation. In case of an IC engine Torque= cylinder push x distance from crankshaft axis to journal axis
So, you can see as the cylinder push (force) will increase,the torque will also increase

3. Hamidreza says:

Thank you for your kind support.Could I have a sketch on which you show the distance and force.I seem unable to understand your comment.I am not sure to well understanding about cylinder push and distance from crankshaft axis and journal axis.

4. MechGuru says:
5. Hamidreza says:

Hi,Thank you for your attachment.However,it is not mentioned about crankshaft axis and journal axis and cylinder push.Dear Expert,please let me have a sketch for my better understanding including journal axis,crankshaft axis and cylinder push.Br

6. MechGuru says:

Crankshaft axis is the centreline of the crankshaft with respect to which it rotates.
Journal axis is the centreline of the journal with respect to which the connecting rod rotates.
Cylinder push is the force exerted by the cylinders because of the combustion.

7. satonix says:

Hej Hamidreza you should be more polight.

8. Ted cramer on the ketch Ribit says:

Hello Survo

I have a marine diesel engine(Westerbeke W 80) installed with a prop that I can vary the pitch at any time(Hundastat),and thereby vary the load on the motor.I would like to be able to pitch the prop for max fuel efficiency at whatever boat speed is necessary.I have installed an exhaust temp gauge but can not get much information on it’s use.