Reciprocating mass effect on inertia of an engine?
Reciprocating mass effect on inertia of an engine?
(OP)
does reducing the reciprocating mass lower the inertia of the crankshaft system (i.e. crankshaft, flywheel, pulleys, piston, rods, pins etc etc)? or does the fact the piston both accelerates and decelerates negate this over a single full rotation?
the rod obviously does more than complicated motion than the piston which is purely reciprocating so what about it? i know people often split the rod into a rotational part and reciprocating part.
if i did an experiment and attached an electric motor to the flywheel of an engine in isolation and applied a fixed torque to accelerate the system from rest would reducing either of the piston or rod mass change the angular acceleration of the system?
the rod obviously does more than complicated motion than the piston which is purely reciprocating so what about it? i know people often split the rod into a rotational part and reciprocating part.
if i did an experiment and attached an electric motor to the flywheel of an engine in isolation and applied a fixed torque to accelerate the system from rest would reducing either of the piston or rod mass change the angular acceleration of the system?





RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
- Steve
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
Every month there is a tech column called TDC in Cycle World by Kevin Cameron. I think within the last year or so there was a discussion of changes some manufacturers have made (and sometimes back again) with crankpin orientation and the effects noted by riders with ethereal riding skills. Differences in inertia torque variation were mentioned as significant I believe.
In some literature one of the Phils associated with Vincent Motorcycles is referenced as selecting a cylinder firing interval that requires less flywheel inertia for smooth running.
RE: Reciprocating mass effect on inertia of an engine?
maybe it is time to do a rigid body mechanism analysis in ANSYS or the like
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
http://www.604tours.com/images/v6_engine_figures/2...
http://www.604tours.com/v6-engine.html
RE: Reciprocating mass effect on inertia of an engine?
So changing the mass of the reciprocating parts has exactly the same effect, from energy and momentum point of view, as changing the rotational inertia of the rotating parts (e.g. crank and rod big end).
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
The distribution of kinetic energy will vary with crank angle. It gets passed back and forth among the components via the complex forces that act between them. Mechanical/dynamic analysis of reciprocating/rotating machinery is a big and interesting subject, even when simplified back to idealised multi-body analysis.
- Steve
RE: Reciprocating mass effect on inertia of an engine?
The piston speed at TDC and BDC is zero. My point was that anything that is increasing in speed is getting it's energy from somewhere.
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
- Steve
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
The effective rotational inertia as seen at the crank is a function of the crank angle. Some people directly model it that way. Some model the system as a load of connected bodies, each with their own masses and inertias. The end result is the same. The effective centre of mass of the engine may move up and down too, even if the pistons seem balanced to the naked eye (one up for each one down).
- Steve
RE: Reciprocating mass effect on inertia of an engine?
RE: Reciprocating mass effect on inertia of an engine?
The piston's and reciprocating part of the rod's motion are harmonic to the first order. They move along a single dimension of a two-dimensional rotation. Their motion also contains higher order single dimension components of rotational motion.