# Engine Friction

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I want to find out how to exactly calculate engine braking and friction.

I kind of know that it takes in RPM, friction torque and 2 coefficients and returns torque. However, I don't know the equation which calculates it all. What is the equation?

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I don't think it's one empirical equation. It's a result of applied mathmatics. Most simulation softwares I see adapt to the design of the engine, rather than use one single equation which would only work for one specific engine design.

But dont take my word om it.

Edited by Tangletail

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I calculated it as a standard quadratic formula based on the engine rpm:

frictionTorque = c0 + c1 * w + c2 * w^2

where w is the angular velocity of the engine. The friction curve can then be configured via coefficients c0, c1, and c2. The friction torque is applied in the opposite direction as the engine angular velocity (it opposes to the movement).

The engine friction is the dashed red line in this picture:

[attachment=31258:2016-03-25_225855.png]

The other lines are: blue -> engine power, dotted yellow -> combustion torque, green -> final engine torque.

The final engine torque (green) is the sum of the combustion torque (dotted orange) and the friction torque (dashed red).

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The w is

w = rpm / 3600 * 2 * pi


right?

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No, the correct conversion is:

w = (rpm / 60) * 2 * pi

leaves w in radians per second.

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OK! In the full version of image you've got 3 constants:

26, 0.012 and 0.015. I forgot their names. However, which one goes in which slot?

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If you mean the full version of the image here, then those values correspond to c0, c1 and c2 respectively.

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Yes, it's that function:

http://fooplot.com/plot/scg2hqx71y

I've just added a minus ("-") sign so it appears negative, and zoomed the graph to match the bounds used by the engine:

Vertical (Y) is torque in Nm

Horizontal (X) is angular velocity in rad/s (750 rads/s = 7162 rpms)

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Oh, OK! So it's rad/s on x. Now I get it;)

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