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Q: Loss of efficiency (power) due to inertia in a reciprocating internal combustion ( No Answer,   3 Comments )
Subject: Loss of efficiency (power) due to inertia in a reciprocating internal combustion
Category: Science > Technology
Asked by: curtisj-ga
List Price: $50.00
Posted: 07 Apr 2006 12:14 PDT
Expires: 07 May 2006 12:14 PDT
Question ID: 716546
We are looking for the same information that you provided to
derrell-ga on 04 Feb 2003.  "What are the efficiency loses in an
internal combustion engine due to changes in inertia from the pistons
reciprocating motion?"  However, the links that you provided come
up blank. Also, is it possible to put me in touch with "derrell-ga" to
see if he ever found sufficient information to answer his question as
we believe we are asking the same quesion for the same reason.
Thank you

Request for Question Clarification by richard-ga on 09 Apr 2006 07:44 PDT

Star Researcher Pinkfreud-ga was kind enough to contact me after
noticing that it was I who answered derrell-ga's question in 2003.

I will see if I can locate the same or similar sources for you.  As to
derrell-ga, the only way to communicate will be if derrell-ga notices
your question here and chooses to join the discussion.

Google Answers Researcher

Request for Question Clarification by richard-ga on 09 Apr 2006 21:37 PDT
Can you tell me whether any of the papers listed here are useful to you?

Request for Question Clarification by hedgie-ga on 10 Apr 2006 00:55 PDT
It may be more effective to either
a) ask a new question 
  (and refer to the related question by URL, like this)


b)To 'search around' blank links of that old answer,
like this:

You take 'blank' (broken) link, e.g. this:!tr_97-2.pdf

You go up, to

You select 'technical reports' under resources and get a list of reports:

2004: Advanced Empirical Modeling Software Provides Early Warning of
Gas Compression Machinery Failure
2005: Advanced Reciprocating Compression Technology (ARCT) 
1999: Advances in Crankshaft Reliability Methods

You select relevant reports, e.g.

Advanced Reciprocating Compression Technology (ARCT) 

Danny M. Deffenbaugh, Ralph Harris, Robert McKee & Klaus
Brun?Southwest Research Institute

The objective of this research and development project is to develop
the next generation of Advanced Reciprocating Compression Technology
(ARCT) to enhance the efficiency, reliability, and integrity of
pipeline operations through improved compression.


 Whichever method you choose, a) or b)  you  would need to amend your question.

There is no answer at this time.

Subject: Re: Loss of efficiency (power) due to inertia in a reciprocating internal combus
From: vtmemo-ga on 08 Apr 2006 13:57 PDT
Well, part of the issue is due to the conversion of linear
reciprocation versus rotational motion.  The crankshaft is run by the
firing of pistons, which move in a linear (one dimensional) path of
motion.  No matter how many stroke-cycles (2 or 4) you have, you will
be moving something in a linear path.  This piston (and the armature)
have mass.  If you have mass, and you change its direction simply back
and forth (and you aren't rotating it), you're going to lose energy.

Let's say that the crankshaft assembly is turning, which has a mass
and rotational frequency.  The fact that this assembly is rotating
provides us with a value for "kinetic energy due to rotational
inertia" - if all other factors are left unchanged, this should remain
constant when the engine is up to speed.  In other words, if we assume
the engine is up-to-speed when we start observing it, this energy
shouldn't really be changing except due to friction.  The rest of this
comment assumes that you have an "ideal" combustion cycle, and you
don't have to worry about heat loss due to entropy.

With each combustion cycle, the piston moves forward and backward. 
The basic number-crunching here will assume you have a 4-stroke
If you don't know what a 4-stroke ICE works, check out , which also does a great
job of visually showing you the 4 parts of the engine cycle:
Intake (Piston moves 1 chamber length)
Compression (Piston reverses direction and moves 1 chamber length)
Combustion (Piston reverses direction and moves 1 chamber length)
Exhaust (Piston reverses direction and moves 1 chamber length)

Let's solve it symbolically, shall we?

You have a piston of mass M, and your crankshaft moving with a
rotational speed, (Omega).  We know that for every revolution, the
piston has to reverse direction twice, and travel a length L.  If the
piston reaches a linear speed, V, between reversals, then it must
carry a kinetic energy of (1/2)*M*V^2.  Because the piston stops and
reverses direction, this energy has to go to Zero twice per crankshaft
rotation.  Let's also say that your rod-bearing assembly has a radius
of R.  Therefore, the maximum kinetic energy of your piston head will
be (1/2)*M*(Omega*R)^2  (this is an approximation; the linear maximum
speed of your piston head will vary depending on the individual specs
of your engine).

This kinetic energy is created and dissipated twice per rotation.  So
now you have Ke=[M*Omega*R^2] each time the crankshaft rotates.  You
also have a frequency, which is determined by 2*pi*omega assuming your
Omega is in rad/sec.  So now, you will have a frequency in
revolutions-per-second, so I would multiply that kinetic energy
loss-per-cycle by your cycles-per-second.

This final result should yield an answer in energy-per-second, more
commonly described as "power".  This power efficiency will factor in
to your losses due to internal inertia and kinetic mechanical losses,
which occur due to the breakdown between linear and non-linear axes of

Please let me know if this comment/answer helped you.
- vtmemo
Subject: Re: Loss of efficiency (power) due to inertia in a reciprocating internal combustion
From: hedgie-ga on 13 Apr 2006 23:31 PDT
comment by vtmemo-ga makes an assumption that

  " ..This kinetic energy is created and dissipated twice per rotation..."

 which is not generally valid.
  Only small part of linear energy (motion of the piston) is
 dissipated (converted to heat). Most is converted to rotational motion. 

So, I would not recommend relying on those formulas 
to estimate the efficiency of the engine.
Subject: Re: Loss of efficiency (power) due to inertia in a reciprocating internal combus
From: vtmemo-ga on 15 Apr 2006 20:16 PDT
That's probably correct to the extent that the rotational energy is
equal to the mass fixed at a radius, (r), rotating about the center at
(omega).  What's the formula for that, Ke = I*omega^2?

The mass of the piston head may factor into the I term somehow.  If
instead you look at it as a loss due to acceleration, you could get
the ramp-up energy loss (the energy basically "absorbed" into the
kinetic rotational energy of the system).  You're going to want a
modeling / matrix editor program to do that, since hand-calculations
will get really messy, really quick.

Sorry for the initial confusion; hedgie-ga is probably correct in
assuming that this loss is not a per-cycle loss.

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