low inertia engines

[C M Smith <cmsmith@xxxxxxxxxxxxxxx>]

This would be a relative term but primarily refers to rotational inertia. 
This is proportional to both the mass of the rotating components of the 
engine but also the torque the mass would exert when the rate of rotation 
was accelerated. Thus short stroke engines will have less rotational 
inertia than long strokers. The flywheel weight makes a difference but not 
as much as people think.

The long stroke engine has long throw cranks on the crankshaft, placing the 
mass further from the center of rotation thus increasing the inertia of a 
given size of crankshaft. Short stroke cranks can also be made lighter due 
to efficiencies in metal use. Very short stroke engines have overlapping 
crank and main journals allowing the crank to be lighter with similar 
stiffness. Also, the bending moment on the cranks themselves, as well as 
the crank as a whole will be less for short stroke engines allowing less 
metal to be used for the same structural strength. The connecting rods are 
usually longer on the long stroke engine. The pistons will be larger, and 
they are not part of the rotational inertia, but will contribute inertia, 
even though they move in balanced synchronicity. Pistons are aluminum 
though and cranks and con rods are usually steel so the net gain is in 
favour of short strokers.

Finally, a V engine of a given displacement (including flat variants) will 
have less inertia than an inline of similar displacement simply due to the 
shorter, stronger crank layout.

Love that short stroke sound and acceleration, the 2.5 V6 is particularly 
attractive in that regard, with the 3.0 liter being substantially slower to 
rev up, longer stroke eh?

Cheers

Michael


Michael Smith
White 1991 164L
Original owner
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