Does piston weight create a flywheel effect?

[Hemi Joel]

I always figured that the heavier a set of pistons is, the more power it takes to accelerate them. But I didn't think that they have a flywheel effect when decelerating. Since they reciprocate, I think they absorb power all the time. Someone on another forum says that in a multicylinder engine, the pistons create a flywheel effect, and the heavier the piston the more the flywheel affect. So what do you guys think?

[AlanCasida]

I think you need to lay off the Aquavit, don't you know! Just kidding.  Seriously though I never really thought about it that way before either.

[allrightmike]

Taking the thought experiment out to the absurd, imagine an engine with 50lb. pistons. It would decelerate at a very slow rate, that is if they were still attached to the rods! Or would it? The weight of the pistons may be canceling each other out since we are dealing with reciprocating forces as opposed to rotational forces. Gee, I guess I don't know after-all.

[chris401]

A heavier piston takes a heavy crank. The crank and big ends will have some inertia but in my mind I don't see how anything above the big end would.

[WConley]

The piston itself doesn't store any momentum.  During an up or down stroke, the piston has to be accelerated at the beginning, but then gives all of that momentum back when it decelerates at the end.

As mentioned above, the crank needs heavier counterweights with heaver pistons.  That raises your effective flywheel mass.

[cammerfe]

But isn't it true that the diameter of the flywheel is a significant component of 'the flywheel effect'? Since the diameter of the crankshaft counterweights is much less than the diameter of the flywheel, the crank contribution of the overall inertia is much smaller than it would be otherwise, no? (I'm going back to High School physics for this.)

KS

[WConley]

Ken - Of course any mass you stick way out on the edge of the flywheel is going to have more effect.  (The momentum is proportional to mass x diameter.)  The crank still has a lot of mass, and if you increase it you'll notice the effect on engine acceleration.

[cleandan]

I always figured that the heavier a set of pistons is, the more power it takes to accelerate them. But I didn't think that they have a flywheel effect when decelerating. Since they reciprocate, I think they absorb power all the time. Someone on another forum says that in a multicylinder engine, the pistons create a flywheel effect, and the heavier the piston the more the flywheel affect. So what do you guys think?

Hemi Joel, lets look at the "flywheel effect" first...but not indepth as that gets...well, boring.
A flywheel is first stationary, only having mass. When a force is applied to put the flywheel in motion the flywheel absorbs and stores that energy.

The flywheel stores this energy as potential energy to be used when the external force is removed.

The mass and diameter, as well as the external energy put into the flywheel, all come into play in the final energy output the flywheel can provide.

The flywheel works because it is rotating in a single direction, thus maintaining the input energy efficiently, to be used in times of input energy pauses, such as between power cycles in the engine.

Watch any old steam type tractor...the really big ones show this best, specially a "hit and miss" firing style. With these you get a fire, or power, stroke once every 3-10 seconds, but they are erratic so building flywheel energy takes some time.
These big engine use HUGE, HEAVY flywheels to absorb and store the engines power because if left to just the engine things would be terribly erratic and rough.

Anyway, watch one of these power up. The first couple minutes, as the engine fires and the flywheel gains speed and stored energy, they shake and will literally walk along if not held in place due to the energy being built in the giant flywheel.

Once the flywheel is up to speed and has its maximum stored energy for the rpm limits set by the governors, things smooth out considerable and the machine now takes on a neat rhythm of chuffing erratically and keeping the flywheel in motion.

If you overspeed the flywheel the stored energy becomes too much for the material to contain and it flies apart as it releases the stored energy.

None of that includes any portion of stopping the flywheel, changing rotational direction, and starting again because each time you stop the flywheel you would give up all stored energy to begin anew.

Same with a piston.
Yes, a piston has mass and speed, thus it has force (F=MxA) Force=mass time acceleration
But the piston is held in a singular plane as it moves up and down in the cylinder so it is not rotating in a flywheel manner.
The piston also comes to a complete stop at both the top and bottom of its stroke so it does not have a flywheel effect.

Heavier pistons take more energy to move, as well as more energy to stop and change direction.

The crankshaft, on the other hand, is rotating in a single direction, it does have mass and acceleration, which equals a stored amount of force, which can be called the flywheel effect...no matter how slight that effect may be in relation to the overall forces of the running engine.