# Thread: Engine torque power curve

1. ## Engine torque power curve

power_torque.gif
Ic engine torque power curves

Q. If the rpm at which the max torque occurs is 4500 rpm, is it safe to assume that the rpm at which max power occurs is twice i.e 9000 rpm?

Further, is it usual for automotive engineers to set the upper and lower rev limits at which gear-down and gear-up occur to 4500rpm and 9000 rpm(i.e max torque and max power rpms) respectively? i

2. "If the rpm at which the max torque occurs is 4500 rpm, is it safe to assume that the rpm at which max power occurs is twice i.e 9000 rpm?"

No, it is not safe to assume that at all.

"is it usual for automotive engineers to set the upper and lower rev limits at which gear-down and gear-up occur to 4500rpm and 9000 rpm(i.e max torque and max power rpms) respectively?"

No, it is not usual. There are a lot of factors that weigh in when determining shift points. Are you trying to achieve optimum fuel mileage? Optimum acceleration? Smooth acceleration?

3. Originally Posted by CCR5600Design

"is it usual for automotive engineers to set the upper and lower rev limits at which gear-down and gear-up occur to 4500rpm and 9000 rpm(i.e max torque and max power rpms) respectively?"

No, it is not usual. There are a lot of factors that weigh in when determining shift points. Are you trying to achieve optimum fuel mileage? Optimum acceleration? Smooth acceleration?
If the shift points are on either side of the max. torque point, what would you classify this as? optimum fuel mileage? OR Optimum acceleration? OR Smooth acceleration?

Normally I would use the shift points centred at the maximum power, but in this particular numerical problem, I've only been given the value of the maximum torque and the rpm at which it occurs.(If you would like to have a look at the problem question:http://www.physicsforums.com/showthread.php?t=701019.)

My understanding of the function of gearboxes is that it makes sure the engine is operating in a constant power range for most of the engine speeds.

4. My understanding of the function of gearboxes is that it makes sure the engine is operating in a constant power range for most of the engine speeds.
Operating at constant power is hardly achievable in most engine/gear applications at the different vehicle velocities.

You're exploring the age old question of power (HP) vs torque. Ultimately, gear shift points are best matched to the desired engine application, engine operating parameters in terms of available (design) torque and power.

For example, a truck that is intended to haul large loads will require large amounts of torque however the delivered power (HP) or rate of RPM change and velocity is not as important.

5. But, why do most of the gear design textbooks have gear shift rpms designed around the max power point? My understanding is that such gear designs are meant for maximum acceleration(correct me if I'm wrong pls.).

And gear designs where the gear shift rpms are designed around the max. torque would probably give better fuel consumption?(I guess so because the bsfc is highest near the max. torque points).

Below I try to visually explain by what I mean by the gear shift rpms omega_1 and omega_2.

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