I'm seeking a precise description of engine torque.

We talk about torque in terms of the twisting force that the piston exerts on the crankshaft with a radius equivalent of the rod throw.

Assume these two scenarios:

Engine 1: 6 cylinder, 2.5L diesel making 480 Nm torque

Engine 2: 4 cylinder, 2.0L diesel making 390 Nm torque

Cylinder volume on eng 1 is 2.5 / 6 = 0.416L

Cylinder volume on eng 2 is 2.0 / 4 = 0.5L

So its apparent that the cylindrical volume on the smaller displacement engine is larger.

How come the 6-cylinder engine produces more torque, instead of only smoothening out the performance due do less crank rotation per firing event?

I'm thinking that torque is measured with two crank rotations in mind.

Am I correct on this assumption, and if not - how is engine torque described precisely?

  • 1
    $\begingroup$ The torque is that which can be constantly applied to a brake on a heavy-ish flywheel. Historically, a strap brake was hung over the flywheel of the test load and weights were added. Thus "brake hp" was born. The torque specs today still reflect this original standard and are average values over the entire cycle. $\endgroup$
    – Phil Sweet
    Sep 21 '18 at 19:37
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    $\begingroup$ Note that 480/2.5 = 192 Nm/L, 390/2.0 = 195 Nm/L. So on a displacement basis, the motors are very similar. Since the reported torque is the average, it is additive with respect to additional cylinders. The peak instantaneous torques can be much higher. $\endgroup$
    – Phil Sweet
    Sep 21 '18 at 19:44
  • $\begingroup$ The “radius equivalent of the rod throw” is equal to zero twice of course. $\endgroup$
    – Solar Mike
    Sep 21 '18 at 21:13
  • $\begingroup$ @PhilSweet 'average' is the keyword I was looking for. So if I understood you correctly - more cylinders means more firing events that results in more consistent and higher averages of torque? So when you're saying the peak instantaneous torque can be much higher, is it then correct to deduct than in the moment of ignition, the torque will be very high but quickly wear off until the next firing event - making the reported torque land at an average? $\endgroup$
    – Erik
    Sep 21 '18 at 21:49
  • 1
    $\begingroup$ Max torque is when the crank and the connecting rod are at a 90 degree angle, but when they are in line there is no torque applied and this happens at TDC and BDC... $\endgroup$
    – Solar Mike
    Sep 21 '18 at 21:55

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