# Effects of inertia forces and friction forces on a piston in engine

## Main Question or Discussion Point

Hello,
I am curious regarding the forces other than the pressure created by combustion process in IC engine.Whether inertia and friction affect worth considering or not, I want to know magnitude and nature of these forces and their effects On efficiency and performance.Thank you...

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SteamKing
Staff Emeritus
Homework Helper
Hello,
I am curious regarding the forces other than the pressure created by combustion process in IC engine.Whether inertia and friction affect worth considering or not, I want to know magnitude and nature of these forces and their effects On efficiency and performance.Thank you...
Absolutely, friction and inertia are worth considering. That's why pistons are made out of aluminum rather than cast iron, as they were many years ago: the reduction in weight of aluminum pistons over iron ones means less energy is required to rotate the moving parts of the engine.

Friction is always the enemy where moving parts are concerned. Friction causes heat build-up and wear on the rubbing surfaces. It's why moving parts are lubricated to reduce friction and the energy it takes to keep an engine turning.

In the US, where the fuel economy of new cars is regulated, much work has gone into the design of engines over the last quarter century or so to reduce the friction not only in new engine designs, but also friction in things like wheel bearings and tires (primarily in the flexing of the sidewalls of the tire, which is why radial construction is preferred).

AlephZero
Homework Helper
Make an estimate the acceleration of the pistons, based on the stroke and the engine RPM, assuming simple harmonic motion. That isn't accurate because it ignores the details of the motion of the connecting rod, but it will give the right order of magnitude.

You should find that a piston with a mass of one pound has a "weight" of a few tons when the engine is running.

jack action
Gold Member
Inertia is a problem when considering acceleration only. If you need to change rpm quickly, inertia is an enemy. Furthermore, the weight of the pistons that need to be decelerated and accelerated throughout its up-and-down motion creates stresses in the parts which limits the maximum rpm the engine can reach. Power-wise, it is not a problem and it is even a desired feature. Flywheels are voluntarily installed on engines to recuperate part of the energy produced during the power stroke and send it back to the output during the other strokes - when the engine tends to decelerate - such that the rpm is more stable.

Friction affects greatly the power output and cannot be ignore. It was found to be proportional to the mean piston speed.

256bits
Gold Member
The wiki has a essay on engine balance, which comes from dynamic and static considerations.
http://en.wikipedia.org/wiki/Engine_balance
It might be what you are looking for, and it is not as easy as it looks to balance an engine.

Thank you all

Piston velocity and acceleration

This attachment may be useful, its the piston velocity and acceleration at any crank position, also makes a good excel excercise :

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Inertia is a problem when considering acceleration only. If you need to change rpm quickly, inertia is an enemy. Furthermore, the weight of the pistons that need to be decelerated and accelerated throughout its up-and-down motion creates stresses in the parts which limits the maximum rpm the engine can reach.
Jack, I think you are mistaken here. Your own statement has an internal contradiction. You say that "Inertia is not a problem..." but then you point out that the weight of the components being accelerated contributes to internal stress. The latter is a problem.

But it is actually worse than that. The inertia of the moving components contributes a major, nonsteady, torque which serves to drive torsional vibration in the entire engine and drive train.

Component inertia is a major factor in engine design, particularly as it affects torsional vibrations and shaking forces.

jack action
Gold Member
Jack, I think you are mistaken here. Your own statement has an internal contradiction. You say that "Inertia is not a problem..." but then you point out that the weight of the components being accelerated contributes to internal stress. The latter is a problem.

But it is actually worse than that. The inertia of the moving components contributes a major, nonsteady, torque which serves to drive torsional vibration in the entire engine and drive train.

Component inertia is a major factor in engine design, particularly as it affects torsional vibrations and shaking forces.
From the excerpt you quoted from me, all I could find was «Inertia is a problem [...]» and «[...] inertia is an enemy.»

But if you would have quoted more, you would have found the following complete sentence (context is important) «Power-wise, it is not a problem and it is even a desired feature.» The sentence following explains why.

Adding a flywheel (i.e. adding inertia) is also a possible solution to the torsional vibration problem you've stated (because vibrations are all about acceleration, which lead to the rpm instability I was talking about).

Sorry, Jack, but you are still off the mark a bit.

Flywheels can store energy and give power smoothing for the engine as a whole. They also affect the internal torsional vibratory motion, but they never completely eliminate this motion.

Torsional vibration (with or without a flywheel) is excited primarily by (1) the periodic firing torque pulse due to combustion, and (2) the variable effective inertia of each cylinder. If the inertia variation could be eliminated (which it can only be done with massless a slider-crank), then that component of the excitation would be eliminated. Thus it is the inertia of the slider-crank assembly that is the cause of one excitation. No amount of context can change that.

There is no rpm instability (rpm is usually taken to refer to average angular velocity of the crank), nor is there an instability in the instantaneous crank speed. There is always a fluctuation in the in the instantaneous crank speed; it is unavoidable in a slider-crank IC engine.

jack action
Gold Member
Dr.D,

I really don't understand what your are trying to do. You repeat exactly what I say and then put other words in my mouth.

Flywheels can store energy and give power smoothing for the engine as a whole.
This is exactly what I've said in my first post. Nothing more.

They also affect the internal torsional vibratory motion, but they never completely eliminate this motion.
I never said they eliminated the motion. I said it is «a possible solution to the torsional vibration»; Just like you mentioned that they affect this motion. (I supposed you mean like me, by reducing their impact.)

There is no rpm instability (rpm is usually taken to refer to average angular velocity of the crank), nor is there an instability in the instantaneous crank speed. There is always a fluctuation in the in the instantaneous crank speed; it is unavoidable in a slider-crank IC engine.
I'm not sure what are your definitions of rpm, angular velocity and instantaneous crank speed, as it all sound pretty much the same to me. rpm is a type of unit for angular velocity and if someone used the term crank speed, I would think he or she is referring to the angular velocity of the crankshaft.

I'm not sure what your definition of instability and fluctuation are either but - when referring to angular velocity - it sounds pretty much the same also.

And how can you say there is an average value, if there is no instability ?

Furthermore, the only fluctuation I know that is «unavoidable in a slider-crank IC engine» is the piston speed, not the crank speed. In all kinematic studies I've seen about piston-crank assemblies, the angular velocity is assumed to be constant, hence there are no fluctuations on that point of view.

Where there are fluctuations is in the torque delivery, which is higher during the power stroke than in any other stroke. Because of that, the angular velocity tends to increase during that stroke and it tends to decelerate during the other strokes. To remedy that, you use a flywheel which takes some of the energy to reduce the acceleration during the power stroke and then gives it back during the other strokes to reduce the deceleration. This reduces the fluctuation, hence my original choice of words «more stable rpm» (or angular velocity if one wants to be exact).

Now, you can re-read my original statement and click on the link Flywheels to see that what I've said - and only what I've said, with the words I've used - makes perfect sense and is agreement with the general consensus in the scientific community:

Flywheels are voluntarily installed on engines to recuperate part of the energy produced during the power stroke and send it back to the output during the other strokes - when the engine tends to decelerate - such that the rpm is more stable.

Uh, oh.... scientific consensus ... has anybody seen Algore?

"If it is consensus, it is not science." -- Michael Crichton

Without attempting to address all of your misunderstandings, I will focus on three of them.

I'm not sure what are your definitions of rpm, angular velocity and instantaneous crank speed, as it all sound pretty much the same to me. rpm is a type of unit for angular velocity and if someone used the term crank speed, I would think he or she is referring to the angular velocity of the crankshaft.
The term "crank angle" is usually understood to refer to the angle turned by the crank from a fixed reference line. That is the way I use this term. The "crank speed" and "crank angular velocity" are used interchangeably (ignoring the vector nature of an angular velocity) to refer to the time derivative of the crank angle.

Now, while it is narrowly correct to say that rpm is simply the same thing as crank speed, it is not common industrial usage to do so. Rather, for a machine operating at constant nominal speed, rpm is usually taken to refer to the time average value of the crank speed, expressed in units of revolutions per minute, rather than the radians per second commonly used for crank speed. The words, "per minute" suggest a longer measuring interval, although this is only understood, not stated.

The term "instability" is understood in mathematics to refer to a solution that either (1) diverges continually from a fixed value, or (2) never converges to a fixed value, although it may oscillate around a fixed value. Thus, the solutions of x'' - k*x = 0 exhibit an instability (exponential growth), and the solutions of x'' + k*x also are also unstable (oscillation around a fixed point). To get a stable solution (no instability), we need to consider a system such as x'' + c*x' + k*x = 0 for c>0, k>0 that has a damped (stable) solution.

The speed fluctuations that occur in an operating IC engine are not necessarily periodic, but rather ever combustion event is unique and hence produces a different motion. Thus speed can fluctuate (assume many different values near a fixed value), without necessarily oscillating like an unstable motion.

Furthermore, the only fluctuation I know that is «unavoidable in a slider-crank IC engine» is the piston speed, not the crank speed. In all kinematic studies I've seen about piston-crank assemblies, the angular velocity is assumed to be constant, hence there are no fluctuations on that point of view.
It is easy, and common, in a mathematical analysis of the kinematics of a slider-crank to assume that the crank speed is constant. But this is an assumption; the real, operating engine knows nothing at all about that assumption, and could hardly care less. It is unfortunate that this is the only sort of analysis that you have seen. I regularly publish technical papers where this assumption is not made; I published two through ASME last month (I would give you the citations, but I don't think PF would allow that). I have been doing torsional vibration consulting work for various engine companies for 40 years now, and the assumption of constant instantaneous crank speed is never made, although constant average speed is routinely assumed.

I think I'm finished on this thread. It is getting really tedious.

jack action
Gold Member
I think I'm finished on this thread.
Thank you.

I published two through ASME last month (I would give you the citations, but I don't think PF would allow that). I have been doing torsional vibration consulting work for various engine companies for 40 years now
Yeah. But how much can you bench?

Ranger Mike