Reciprocating mass effect on inertia of an engine?

[inline6]

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?

 

[ivymike]

It definitely impacts vibration. .but given that the recip energy is returned to the crank at tdc and bdc the average effect should be small. There will be a bit more friction loss with heavier recip mass due to higher forces at interfaces. Obviously the "rotating" part of the recip assembly will impact acceleration.

 

[SomptingGuy]

You will see a variation of the acceleration through each engine revolution even if it's given a (nominally) constant torque. The shape will depend on number and layout of the cylinders.

- Steve

 

[sreid]

As the crank accelerates the piston velocity must also increase. And although the pistons return energy, the net energy input must be positive if the piston velocity keeps increasing. Or am I wrong?

 

[Tmoose]

Towards the end of this page there is a graph of the alleged output torque variation of a few engine types, also allegedly including inertia as well as the expected torque pulsations from the cylinder size and firing intervals.

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.

 

[inline6]

i would think that the heavier the reciprocating component is in relation to the rotation would produce a non uniform angular acceleration vs crankangle for a constant torque input. with reciprocating mass approaching zero the system would accelerate uniformly as a function of crank angle and with a extremely heavy reciprocating the variation vs crank angle would be large but perhaps the average does not change?

maybe it is time to do a rigid body mechanism analysis in ANSYS or the like

 

[ivymike]

Sreid, what is the piston velocity at the upper and lower reversal points?

 

[Tmoose]

Towards the end of this page there is a graph of the alleged output torque variation of a few engine types, also allegedly including inertia as well as the expected torque pulsations from the cylinder size and firing intervals.

http://www.604tours.com/images/v6_engine_figures/2.jpg

http://www.604tours.com/v6-engine.html

 

[Lou Scannon]

Conservation of energy and conservation of momentum. The reciprocating masses (piston, and rod small end) simply store and return momentum and kinetic energy to the rotating system as they accelerate and decelerate through a rotational cycle.
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

 

[inline6]

i imagine if one was to look at the kinetic energy of the system both rotational and linear components one could come up with an analytical solution for the equivalent rotational inertia and i would imagine that it would vary as a function of crankangle?

 

[SomptingGuy]

"it would vary as a function of crankangle"

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

 

[sreid]

ivymike,

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.

 

[ivymike]

yes, and when it reduces its speed? As the crankshaft accelerates over several revolutions (or several thousand), the pistons will have higher and higher peak velocities, but they'll always give back that energy at BDC and TDC. There will be more energy into and out of them ... but on average their inertia doesn't "consume" energy, it just stores it for a bit and returns it. Hence, as noted several times above, more speed variation with higher mrecip, but no effect on average acceleration.

 

[sreid]

I can see that if the crank is at constant speed the pistons return the same energy they gained in the previous cycle. But if the crank is accelerating so are the pistons and the pistons must be gaining more energy than they are returning each cycle.

 

[inline6]

SomptingGuy, do you not think that under a constant external torque the angular acceleration will vary slightly with crank angle dependent on the piston mass?

 

[SomptingGuy]

Er, yes.

- Steve

 

[inline6]

so this must mean the effective (or equivalent) mass moment of inertia of the system changes as function of crank angle no?

 

[SomptingGuy]

Is there a dead horse around here somewhere?

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

 

[inline6]

sorry, i interpreted an earlier post as disputing this

 

[140Airpower]

hemi said, "...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...". It is as simple as that!

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.