# Torque and speed of I.C engines

We are all aware of the mathematical meaning of functions.

for example a function like y=f(x) means that the value of y is dependent on the value of x.

if x changes so does y, implying that x is an independent variable, whereas y is dependent.

So can the same relation be established between the speed and torque of an I.C engine ? at different speeds, the engine torque varies, so would it be correct to say that engine torque is a function of engine speed.

besides the torque v.s speed curve of an I.C engine is also bound to have an equation.

## Replies

• Ramani Aswath
IC engine behaves differently from motors.
bioramani
IC engine behaves differently from motors.
• mechky
The torque increases with increasing rpm, reaches its maximum value(at a certain rpm) and then begins to fall as the rpm further increases.

We have discussed this matter earlier - #-Link-Snipped-#
• zaveri
mechky
The torque increases with increasing rpm, reaches its maximum value(at a certain rpm) and then begins to fall as the rpm further increases.

We have discussed this matter earlier - #-Link-Snipped-#
i know that, but what i want to know is whether speed and torque are independent of each other or not ?
• Ramani Aswath
i know that, but what i want to know is whether speed and torque are independent of each other or not ?
Please see the relevant graph in the above.
• Sanoj Rajan
Speaking in general, speed and torque are inversely proportional to each other.
When speed increases, torque decreases. When speed decreases, torque increases.
Provided that the power is same.
• Sanoj Rajan
mechky
The torque increases with increasing rpm, reaches its maximum value(at a certain rpm) and then begins to fall as the rpm further increases.

We have discussed this matter earlier - #-Link-Snipped-#
As you said, that torque increases with rpm, reaches its max., and then falls as the rpm further increases.

This is because, from the starting f the engine, till it attains max torque, the power is continuously increasing. But after this point, the power remains almost constant. But the speed increases, because of the induction of more fuel into the combustion chamber.
But as i said earlier, if the power is same, speed and torque is inversely proportional. As a result of which, the torque further decreases.
• mechky
Sanoj Rajan
As you said, that torque increases with rpm, reaches its max., and then falls as the rpm further increases.

This is because, from the starting f the engine, till it attains max torque, the power is continuously increasing. But after this point, the power remains almost constant. But the speed increases, because of the induction of more fuel into the combustion chamber.
But as i said earlier, if the power is same, speed and torque is inversely proportional. As a result of which, the torque further decreases.
The torque decreases after a certain rpm because of two reasons - decreased filling ability of engine and increased internal friction losses at high rpm.
• Sanoj Rajan
mechky
The torque decreases after a certain rpm because of two reasons - decreased filling ability of engine and increased internal friction losses at high rpm.
No!
That ain't the reason!
The above said 2 reasons decrease the power, and not the torque!
The exact reason for this is mentioned in my previous replies.
• mechky
Sanoj Rajan
No!
That ain't the reason!
The above said 2 reasons decrease the power, and not the torque!
The exact reason for this is mentioned in my previous replies.
Absolutely not. You're getting confused between the two terms. Refer to the power and torque vs rpm curves of an engine below.

Following points are worth noting from this curve:
1. Torque and Power both increases with rpm up to a maximum point and then decreases.
2. The point of maximum Torque is different from the point of maximum Power, i.e we get maximum torque at relatively lower rpm and maximum power at higher rpm. This is true for every engine.
I'm explaining the reason for this below:
• The torque curve depends on how well the cylinders are filled with fresh mixture, how well it burns that mixture and losses in that process. The maximum torque will be the point at which the engine is having maximum volumetric efficiency. After a certain rpm the filling ability of cylinders or the aspiration of engine falls down. Also the internal frictional losses also increase with increasing rpm. So, the net result is decreasing torque.
• The power of an engine is related to its torque by the formula : Power = (Torque X RPM) / 5252. So, as rpm increases, torque increases initially and the power also increases. But after certain rpm the torque starts to decrease and rpm keeps on increasing. But the decrease in torque is much less as compared to increase in rpm, so the net effect is increasing power. But at certain rpm, the internal frictional losses become too much significant and the power starts to decrease.
• Sanoj Rajan
Can you please send the source of this graph.
• mechky
Sanoj Rajan
Can you please send the source of this graph.
Look at previous post by ramani sir.

You are reading an archived discussion.

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