Lost Energy from a Reciprocating Piston

As the piston of an internal combustion engine moves up and down inside the cylinder, it goes from zero-max-zero speed. In a race engine, the maximum speed may reach up to 100 mph. So the question is how much energy is expended every time the piston decelerates to zero speed when it hits bottom dead center (BDC), and the top dead center (TDC.)
An Engineering Professor once told me that there is no (heat) energy loss because the flywheel absorbs the energy and recovers it after the expansion stroke. While that may be true at low engine speed, I believe tremendous amount of energy will be lost at higher speed. This lost energy amount will be in the form of heat but is defined by the basic kinetic energy equation K.E. = 1/2 mv
².
I believe this is a basic physics question, but I also believe it deserves an in-depth understanding because there is not a single article covering why there is so much energy loss in the piston, rings, and crankshaft. The flywheel as the answer will also need to be officially revised if it is not...for the benefit of all.

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Your engineering professor is essentially correct. As long as the engine speeds you are talking about are well within the normal design parameters it would qualify as “low speed” for this purpose.

If this were a big source of energy loss then rotary engines would be more efficient than standard piston engines. But they are in fact less efficient.

Consider the piston moving downwards toward BDC. Why is it decelerating? Follow the forces.

Welcome to PF.

JAlda333 said:

While that may be true at low engine speed, I believe tremendous amount of energy will be lost at higher speed.

You are mistaken in that belief.

An engine is balanced. There are counterweights on the crankshaft that balance the weight of each piston and connecting rod. In multicylinder engines, when one piston is accelerating, another is decelerating. The total kinetic energy is shared by torque transfer, through the crankshaft.

Air is heated during the compression stroke, some of that heat is lost to the cylinder wall, which then heats the coolant, outside that cylinder, in the block. Heat is also lost to the cylinder wall during the power stroke. The waste heat is removed from the coolant at the radiator, where it heats the air.

Dale, yes the flywheel can store and use the energy at low speed (idling) wherein the engine can still be observed to have unstable speed. When the rpm goes up and the rpm becomes stable, the flywheel ceases to have any effect and no longer necessary.

Would it help if we base our discussion on a theoretical single cylinder engine setup so we'll have numbers to do the calculations?

thanks.

gmax137 said:

Consider the piston moving downwards toward BDC. Why is it decelerating? Follow the forces.

The piston is decelerating because of the crankshaft. If you visualize the piston as a bullet, would a small-radius crankshaft be able to stop it without generating heat? If the radius is large, then the answer would be yes... but that is not the case. Most of the piston's kinetic energy will be expended as heat, just like a bullet hitting a solid surface.

JAlda333 said:

Would it help if we base our discussion on a theoretical single cylinder engine setup so we'll have numbers to do the calculations?

What is there to discuss? You already were aware of the answer before you asked the question. You already received correct information from the engineering professor. And I already gave a clear contradiction to your claim. And @Baluncore explained the physics.

What else is there to discuss? You have made a mistake and been corrected by multiple experts. Do you expect further discussion to change your mind when neither consensus among experts nor physics explanations could?

JAlda333 said:

Most of the piston's kinetic energy will be expended as heat, just like a bullet hitting a solid surface.

Again, this is simply false. If it were true then rotary engines would be more efficient than reciprocating engines, and the reverse is true.

JAlda333 said:

Dale, yes the flywheel can store and use the energy at low speed (idling) wherein the engine can still be observed to have unstable speed.

You're misunderstanding a couple of things:

1. The issue of the piston's acceleration is not about the flywheel it's about the crankshaft being balanced. It's what @Baluncore said.

2. The flywheel is there to ride-through inconsistent power and compression, not to provide balance or power for accelerating/decelerating the pistons.

The crankshaft counterbalance will only reduce the engine's vibration, but will not prevent the lost energy from the piston's deceleration. This is no different from the counter rotating balancer (Mitsubishi silent shaft), It effectively cancels out the engine vibration, but it reduces the engine's power output (in order to drive the shafts.)

Another example: consider a perfectly balanced typical flywheel. It would take only a small amount of energy to spin it, and will continue on for some time. However, if the flywheel is divided into two pieces and each supported by its own bearings, it will still behave as a balanced rotor but the bearings will be subject to centrifugal forces that will generate heat.

Dale said:

Your engineering professor is essentially correct. As long as the engine speeds you are talking about are well within the normal design parameters it would qualify as “low speed” for this purpose.

If this were a big source of energy loss then rotary engines would be more efficient than standard piston engines. But they are in fact less efficient.

Our present day rotary engine is the Wankel wherein the rotor/piston rotates while at the same time reciprocating up and down. It suffers from premature seal failure, so maybe the rotor friction is excessive which drags the efficiency down?
The Gnome engine of 1920's was also a rotary. I believe it lost out to the more compact inline engine for its size as it can be conveniently fitted in the plane's nose. The Gnome's lack of speed control as it continue to "freewheel" at high speed was also problem for the pilot during landing. These are of course info available in the Internet which the veracity I am not sure of. However, as the IC engine was still at it's infancy during that time, may be efficiency was not their prime concern? Are you aware of any data from a recent test of this engine type?

Baluncore said:

Welcome to PF.You are mistaken in that belief.

An engine is balanced. There are counterweights on the crankshaft that balance the weight of each piston and connecting rod. In multicylinder engines, when one piston is accelerating, another is decelerating. The total kinetic energy is shared by torque transfer, through the crankshaft.

Air is heated during the compression stroke, some of that heat is lost to the cylinder wall, which then heats the coolant, outside that cylinder, in the block. Heat is also lost to the cylinder wall during the power stroke. The waste heat is removed from the coolant at the radiator, where it heats the air.

Thank you for the welcome. To all, please excuse my previous posted replies as I failed to use the "insert quote" function. They now seemed out of order. Also, while I sincerely appreciate everyone's insight, the thread's goal is to have a conclusive answer (preferably supported by calculations) that we can hopefully all agree on without doubt.

Now as to a balanced multi-cylinder engine, the energy lost from one reciprocating piston cannot be recovered by another piston that is 180 deg out of phase (for inline cylinders.) The conrod's upper and lower bearings, including the respectively shared crankshaft bearings for each cylinder are to remain subject to the same reciprocating piston load. If the components of a rotor are not moving together supported by a common bearing, then each component will incur energy losses on its own.

JAlda333 said:

Now as to a balanced multi-cylinder engine, the energy lost from one reciprocating piston cannot be recovered by another piston that is 180 deg out of phase (for inline cylinders.)

The balance of an engine, comes from the 180° phase components.
The energy-circulation in the engine, comes from 90° phase components.

When a piston comes to a halt, and then reverses direction, it does so, by following the peak of a sinewave. The force and acceleration are limited, and constant, while it is changing direction. That is NOT like a bullet hitting a target.
Have you been introduced to calculus? Look at the slope of a cosine wave and its relationship to a sine wave.

There is an oil wedge between the bearing surfaces. That prevents contact between the metal surfaces and reduces friction. The oil is heated by shear in the fluid, but the oil is then cooled and recirculated.

All bearing areas must be large enough to handle the forces through the oil film.
The crankshaft must be strong enough to counter the forces between cylinders.
The connecting rods must be strong enough to pull and push the pistons.

The airflow and fuel are limited. The RPM is limited. If any component of a well-designed engine was reduced, it would fail while running at high speed or power.

You should not argue that "if it increased speed beyond the design envelope", it would fail. That is abuse of the machinery and engineering.

JAlda333 said:

Our present day rotary engine is the Wankel wherein the rotor/piston rotates while at the same time reciprocating up and down.

Yes, the Wankel is what I was talking about. The point is that the rotors never stop, meaning their KE stays fairly constant. So if your theory were correct that a standard piston’s frequent stopping represents a “tremendous amount of energy” loss for the engine then an engine which avoids this “tremendous” loss would be inherently more efficient. It isn’t.

JAlda333 said:

maybe the rotor friction is excessive which drags the efficiency down?

So that already puts an upper limit on the “tremendous” amount of energy loss you think exists. It cannot be larger than the additional Wankel engine rotor friction vs a standard engine. If it were larger then the Wankel engine would have a benefit.

JAlda333 said:

Are you aware of any data from a recent test of this engine type?

Wankel rotary engines are used in the Mazda MX-30 currently.

I think that you need to recognize that there is no “tremendous” amount of energy loss through the mechanism you mention. It simply cannot be there or a rotary engine would have a corresponding “tremendous” advantage which is not the case.

JAlda333 said:

the thread's goal is to have a conclusive answer (preferably supported by calculations) that we can hopefully all agree on without doubt

So what sort of calculations would you accept as conclusive and would leave you without doubt that you were mistaken?

Yes, it would be much appreciated if you could support the flywheel-balanced piston energy balance with a calculus equation.

Just be aware that the issue on hand is the transfer of energy from one component to the next. The bearing design will only determine if it can survive the load or not. The transferred energy to be accounted for is the piston's deceleration from its maximum velocity as defined by 1/2mv².
Also, I never implied test of engine beyond its design limit to quantify the energy lost from the piston's reciprocating action. However, the lost energy increases with the square of the speed so it becomes significant at higher speed (see brake specific fuel consumption map of the most efficient IC engines.)

Thanks.

JAlda333 said:

So the question is how much energy is expended every time the piston decelerates to zero speed when it hits bottom dead center (BDC), and the top dead center (TDC.)

This may be the root of your misunderstanding: decelerating to zero implies that the decelerating object loses kinetic energy but does not necessarily imply an expenditure of energy (for a straightforward example of the difference, consider a weight/spring harmonic oscillator). Something, in this case mostly the con rod, is applying a force to the piston and by Newton’s third law the piston is applying an equal force on the rod. As long as that force is accelerating something else in the engine there is no “expenditure” of energy, just a transfer to somewhere else. As for what that something else is, it very much depends on the design of the motor, but the general principle has been posted above:

gmax137 said:

Consider the piston moving downwards toward BDC. Why is it decelerating? Follow the forces.

Another misunderstanding is

JAlda333 said:

yes the flywheel can store and use the energy at low speed (idling) wherein the engine can still be observed to have unstable speed. When the rpm goes up and the rpm becomes stable, the flywheel ceases to have any effect and no longer necessary.

This is incorrect. The speed never becomes exactly stable; the flywheel is always exchanging energy with the reciprocating components (and the valve springs, which also contribute a varying load as the cams rotate) by slightly varying its speed. However the energy stored in the flywheel goes as the square of the engine speed while the energy being exchanged is (roughly) linear in the engine speed so as the rpm increases the speed fluctuations become smaller; at sufficiently high speeds the rpms are for all practical purposes constant. The flywheel is only “no longer necessary” in the sense that the rotating mass of the crankshaft may be all the flywheel we need.

JAlda333 said:

The bearing design will only determine if it can survive the load or not.

The load is limited by the slope of a sine wave, which has a maximum near zero.
The bearing was designed, by an engineer, to be sufficient.

Simple Harmonic Motion involves the cyclic exchange of energy between two forms in quadrature, (at 90°).

Circulation involves SHM in two dimensions.
Is "circulating energy" a new concept for you ?

JAlda333 said:

it would be much appreciated if you could support the flywheel-balanced piston energy balance with a calculus equation.

Just be aware that the issue on hand is the transfer of energy from one component to the next

And if we produce such an equation you would then accept that as a conclusive answer that would leave you without doubt, even if it contradicts your hypothesis?

I hesitate to actually provide an equation, because I am reasonably sure that the only consequence is to be asked for another one, but:

The energy the OP claims is lost is given by (55 hp)(rpm/3000)², for reasonable values - 4 cylinders, 10 cm stroke, and 1 kg pistons.

The Honda Fit produces 117 hp at 6600 RPM. The above equation suggests it can't even get to 6600 rpm.

No, as said before, the energy that was in the pistons' kinetic energ=y does not go entirely into heat. Some of it is transferred to other pistons, and some of it goes to moving the car.

JAlda333 said:

it deserves an in-depth understanding because there is not a single article covering why there is so much energy loss in the piston, rings, and crankshaft.

Actually, there is such an article. It breaks down the total friction of an engine into the various causes. It's a good read because there are causes of friction that are not obvious. Get a copy of SAE paper 640807 titled Effect of Design Variables on Friction and Economy. And it's still available: https://www.sae.org/publications/technical-papers/content/640807/. That link also points to some newer papers on the same subject. One of many graphs in the 640807 paper:

Nugatory said:

. . . so as the rpm increases the speed fluctuations become smaller; at sufficiently high speeds the rpms are for all practical purposes constant.

Did you mean. . . at sufficiently high rpms the speed fluctuations are for all practical purposes constant?

OCR said:

Did you mean. . . at sufficiently high rpms the speed fluctuations are for all practical purposes constant?

the speed in rpms fluctuates, but the fluctuations are small compared with the speed so the speed can be considered not to be fluctuating.

Nugatory said:

the speed in rpms fluctuates, but the fluctuations are small compared with the speed so the speed can be considered not to be fluctuating.

OK . . . carry on. .

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Nugatory said:

This may be the root of your misunderstanding: decelerating to zero implies that the decelerating object loses kinetic energy but does not necessarily imply an expenditure of energy (for a straightforward example of the difference, consider a weight/spring harmonic oscillator). Something, in this case mostly the con rod, is applying a force to the piston and by Newton’s third law the piston is applying an equal force on the rod. As long as that force is accelerating something else in the engine there is no “expenditure” of energy, just a transfer to somewhere else. As for what that something else is, it very much depends on the design of the motor, but the general principle has been posted above:Another misunderstanding isThis is incorrect. The speed never becomes exactly stable; the flywheel is always exchanging energy with the reciprocating components (and the valve springs, which also contribute a varying load as the cams rotate) by slightly varying its speed. However the energy stored in the flywheel goes as the square of the engine speed while the energy being exchanged is (roughly) linear in the engine speed so as the rpm increases the speed fluctuations become smaller; at sufficiently high speeds the rpms are for all practical purposes constant. The flywheel is only “no longer necessary” in the sense that the rotating mass of the crankshaft may be all the flywheel we need.

Please correct me if I am wrong again based on your above answer: the flywheel only releases energy when it decelerates. This is entirely in reference to just speed fluctuation.

If we can agree on the above, then we should also be able to agree that the flywheel's diminishing effect with engine speed increase (rpm) cannot possibly absorb the piston's geometrically increasing energy loss.

On the possibility of some engine component's "spring effect", I can agree on it too except there is none that is built-in to the engine by design (correct me if I'm wrong.) Thus I would surmise that spring effect, while possible, cannot be significant.

To the others, my sincere apology for not being fast enough on my replies. Though I intend to answer all. Thank you for joining the discussion.

jrmichler said:

Actually, there is such an article. It breaks down the total friction of an engine into the various causes. It's a good read because there are causes of friction that are not obvious. Get a copy of SAE paper 640807 titled Effect of Design Variables on Friction and Economy. And it's still available: https://www.sae.org/publications/technical-papers/content/640807/. That link also points to some newer papers on the same subject. One of many graphs in the 640807 paper:
View attachment 337836

There's a lot of articles and test data available, but I am not able to find any on the piston's kinetic energy that positively identifies it as one of the losses. Here's one more from MIT's Opencourseware Lec. 19:

JAlda333 said:

I am not able to find any on the piston's kinetic energy that positively identifies it as one of the losses.

That's because you are looking for something that does not exist, as was explained in Posts 2, 3, 4, 7, 8, 12, 13, 15, 16, and 18. Yes, the piston speeds up and slows down. The energy to do that comes from and goes into the flywheel. The energy is not converted to heat, it just transfers back and forth between the piston and flywheel as explained in the quote from @Nugatory below:

Nugatory said:

However the energy stored in the flywheel goes as the square of the engine speed while the energy being exchanged is (roughly) linear in the engine speed so as the rpm increases the speed fluctuations become smaller; at sufficiently high speeds the rpms are for all practical purposes constant.

Dale said:

And if we produce such an equation you would then accept that as a conclusive answer that would leave you without doubt, even if it contradicts your hypothesis?

I posted the topic for discussion. My goal is not to prove anyone wrong but to exchange ideas/views. If we do not openly share our thoughts, then there will be no discussion. Hopefully, we can all arrive on one conclusion. And if we still cannot agree, I will definitely not be the judge... but perhaps the readers may be able to. Thanks again.

This isn't a "view" like "I hate peanut butter". It's not a matter of opinion, it's a matter of fact. As James McNeil Whistler said (when he wasn't making ugly paintings of his mother) "Two plus two continue to make four, despite the whine of the amateur for three or the cry of the critic for five."

JAlda333 said:

I posted the topic for discussion. My goal is not to prove anyone wrong but to exchange ideas/views. If we do not openly share our thoughts, then there will be no discussion. Hopefully, we can all arrive on one conclusion. And if we still cannot agree, I will definitely not be the judge... but perhaps the readers may be able to. Thanks again.

Yes, that is pretty much what I expected. So let me summarize how the rest of this thread will go.

We will continue to provide accurate physics and engineering information, as we have already done. You will continue to disbelieve it, as you have already done. We will continue to provide references and experimental results, as we have already done. You will continue to disbelieve it, as you have already done. We will continue to provide mathematical analyses, as we have already done. You will continue to disbelieve it, as you have already done.

I hope to be proven wrong.

JAlda333 said:

If we can agree on the above, then we should also be able to agree that the flywheel's diminishing effect with engine speed increase (rpm) cannot possibly absorb the piston's geometrically increasing energy loss.

No, it doesn't increase geometrically. The piston's speed at any given point in it's stroke is directly proportional to the angular velocity, ie, rpm.

EDIT here take a look at this
https://en.wikipedia.org/wiki/Piston_motion_equations

Vanadium 50 said:

I hesitate to actually provide an equation, because I am reasonably sure that the only consequence is to be asked for another one, but:

The energy the OP claims is lost is given by (55 hp)(rpm/3000)², for reasonable values - 4 cylinders, 10 cm stroke, and 1 kg pistons.

The Honda Fit produces 117 hp at 6600 RPM. The above equation suggests it can't even get to 6600 rpm.

No, as said before, the energy that was in the pistons' kinetic energ=y does not go entirely into heat. Some of it is transferred to other pistons, and some of it goes to moving the car.

Is your calc result 55hp for a 4cyl with 10cm stroke, 1kg piston each (4kg. total), and
running at 3000rpm? My calc's result is just 16.5hp using the parameters.

Also it does not seem correct to conclude that the engine would not be able to reach 6600rpm if the loss comes out higher than the engine's actual 117bhp output. It would simply mean that the engine is becoming more inefficient. Additionally, an engine with a stroke of 10cm running at 6600rpm would mean the piston travelling at 77mph. At that speed for a stock engine would be pushing it to its limit.

Consider a 4 stroke engine, with the piston at TDC on the intake stroke. The piston speed starts at zero and accelerates to its maximum speed. How do you think that happens?

JAlda333 said:

I posted the topic for discussion. My goal is not to prove anyone wrong but to exchange ideas/views. If we do not openly share our thoughts, then there will be no discussion. Hopefully, we can all arrive on one conclusion. And if we still cannot agree, I will definitely not be the judge... but perhaps the readers may be able to. Thanks again.

The issue here is that reciprocating internal combustion engine technology is extremely mature, having well over a century of technological progress and billions of production examples. This area is very well understood and there is an objectively right or wrong answer here. It's not up for a debate or an 'exchange of ideas' in the sense that you can just claim something and have an inherent right to have people seriously consider your idea. If you're going to propose something and then disagree when people correct you, please have a valid reference supporting you.

If you can find a reference supporting your claim that there is significant energy loss due to acceleration/deceleration of the pistons, feel free to contact myself or another mentor and we can discuss reopening the thread. Until then, thread locked.