inertia mass 1

[al1]

To settle a debate of inertia mass, I would like some input.

The debate is that there is so much emphasis on light weight reciprocating and rotating mass for throttle respond and/or racing etc. that in application like constant RPM and constant load, the weight of these masses has minimal effect on power output. I think it’s understood about the rotating mass not making any real difference; the real question is about the reciprocating mass. And since it’s at a constant RPM you don’t have to accelerate it, the assumption is that the energy to accelerate the mass from TDC is returned at BDC regardless of the mass. Your opinions are appreciated.
al1

 

[ivymike]

Excluding friction effects, the reciprocating components' kinetic energy is recovered by the crank at TDC and BDC, if that was your question.

There are other reasons to keep reciprocating mass low.

 

[al1]

Thanks ivymike,
Yes that was the question, but could you give me the other reasons to keep the mass low.
al1

 

[ivymike]

ease of balancing, lower engine mass, reduced vibration, reduced noise, higher overspeed capability, lower bearing loads, longer conrod life (Al rods), etc... depends on the application.

 

[Fabrico]

The only thing not making any real difference is the idea of constant RPM. There is no such thing for an internal cumbustion engine. The basic rules still apply.

 

[160385]

Let say that we have 2 engines with evrything the same but the rotating assembly is made of aluminium in one of them and is 15lbs lighter than in the other engine,would the engine with the AL make more power??

 

[patprimmer]

160385

Did you read all the posts in the thread. ivymike has already answered the question.

Power is measured at steady RPM and as previously indicated lowering rotating mass will make no difference and reducing reciprocating mass will make little if any difference.

Apparent power difference can be substantial in an accelerating engine as some power is lost in accelerating the moving engine parts.

Regards

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[JackFu]

ptprimmer is right.

 

[JackFu]

When I perform vehicle fuel consumption and performance simulation, the engine mass of inertia should be determined.
I don't know how to calculate or measure this parameter accurately, may anybody give me some advises?

Regards

-Fu

 

[ivymike]

are you looking for effective polar moment of inertia of the rotating assembly, referred to the crankshaft?

 

[JackFu]

It's engine's moment of inertia, I am not sure whether it's effective polar moment of inertia of the rotating assembly exactly. Somebody just told me it's about 0.1-0.2 kg*m^2 for light duty car.

Regards

-Fu

 

[ivymike]

to calculate it, you would need to know the PMOI of each of the rotating components. It is common to add (the mass of the rod below the big end plus 1/2 the mass of the recip assembly) times (crank radius squared) to the rotational inertia of the crank/flywheel/damper assembly. The cam and driven accessories are sometimes neglected, but can also be included. Be sure to account for the drive ratio of each, and not just sum the inertias (if you're not sure how, double-check by calculating the rotational kinetic energy of each vs rpm - the effective inertia should be proportional).

CAD models can be used for many components to determine PMOI. In the absence of CAD models, you can measure PMOI using a torsional pendulum (for the rotating comps anyway).

 

[JackFu]

ivymike

Thank you very much for your help. I am working on it.

Regards

-Fu