# Why flywheels are needed in automobiles?

• Automotive
Why do we need a flywheel in an automobile?
Most people say that an engine produces power only in one stroke.so a flywheel is used to absorb a part of that energy during the power stroke and help the engine to get through the other three strokes.But why cant that energy be stored in the mass of the vehicle itself during the power stroke( in the form of linear kinetic energy)?

FactChecker
Gold Member
Why do we need a flywheel in an automobile?
Most people say that an engine produces power only in one stroke.so a flywheel is used to absorb a part of that energy during the power stroke and help the engine to get through the other three strokes.But why cant that energy be stored in the mass of the vehicle itself during the power stroke( in the form of linear kinetic energy)?
When an engine is starting up, there is not enough power to move the vehicle. There is barely enough to keep the pistons moving with all their back-and-forth motion and friction.

When an engine is starting up, there is not enough power to move the vehicle. There is barely enough to keep the pistons moving with all their back-and-forth motion and friction.
Also, even when the vehicle already moving the energy goes through all the gears and such to reach the wheels. Without a flywheel close to the engine the energy required to keep the engine moving would take the whole route twice. Apart from the double loss also there would be a considerable load on the mechanical parts due the frequent change in direction of forces.

Mohankpvk and FactChecker
When an engine is starting up, there is not enough power to move the vehicle. There is barely enough to keep the pistons moving with all their back-and-forth motion and friction.
So while starting, if you add a fly wheel, the engine should spend energy to move the vehicle and to rotate the flywheel (against its mass moment of inertia).So the load just gets added up i.e. flywheel will make things worse.

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FactChecker
Gold Member
So while starting, if you add a fly wheel, the engine should spend energy to move the vehicle and to rotate the flywheel against its mass moment of inertia.So the load just gets added up i.e. flywheel will make things worse.
The engine is not started in gear with the clutch engaged. So the vehicle does not move and is not relevant at that point.

Mohankpvk
anorlunda
Staff Emeritus
Apart from the double loss also there would be a considerable load on the mechanical parts due the frequent change in direction of forces.

In addition to that, there would be terrible vibrations of the whole car. Every mechanical linkage has a bit of deadband. When you reverse directions of energy travel, all the deadbands add together into a big CLANK.

Relevant experiment - put your car in first gear and try to start it up in this state...
Don't push it long, otherwise it'll kill the car.

Ps.: most cars won't even allow you to do this I guess... Maybe you should try to find an oldtimer for this?

Mohankpvk
Also, even when the vehicle already moving the energy goes through all the gears and such to reach the wheels. Without a flywheel close to the engine the energy required to keep the engine moving would take the whole route twice. Apart from the double loss also there would be a considerable load on the mechanical parts due the frequent change in direction of forces.

The engine is not started in gear with the clutch engaged. So the vehicle does not move and is not relevant at that point.
So the purpose of a flywheel is to support the engine during starting and during intermittent temporary short halts along the journey(clutch plates disengaged and brakes applied.now the engine will completely depend on the fly wheel) and as the other guy stated, to reduce losses.
Please suggest me some good books for mechanics.

In addition to that, there would be terrible vibrations of the whole car. Every mechanical linkage has a bit of deadband. When you reverse directions of energy travel, all the deadbands add together into a big CLANK.

FactChecker
Gold Member
You can expect to find a lot of references explaining how it is done, but not much about why it is not done other ways (like why not use the vehicle momentum rather than a flywheel). For questions like your original one, you may need to ask them specifically on a forum.

Mohankpvk
We have a forum for that question: https://www.physicsforums.com/forums/science-and-math-textbooks.21/ But you need to provide more information about yourself. What are you studying. What level?

anorlunda
Staff Emeritus

It is not separate. It is a sub-forum of PF. Just click on that link, no extra login required. Look at our main page to see all the sub fortune.

https://www.physicsforums.com

Mohankpvk
CWatters
Homework Helper
Gold Member
Look up "backlash".

Mohankpvk
FWIW, there was a recent suggestion to replace the flywheel by an electric motor, synchronise its power pulses to the gaps between a 4-stroke's firing.
https://newatlas.com/electromagnetic-flywheel-engine/55254/

Could this replace both starter motor and alternator ? It would seem most suited to small engines, perhaps in a hybrid.

Nik-note: I'm not saying it is a good idea or otherwise, just some-one is thinking outside the box...

anorlunda
Staff Emeritus
I recall news stories from the early 1990s about a device that would serve as "starter motor"+alternator+"backup AC generator for the home". With modern electronic controls, it might even do something with the transients between firings as you mention. It sounded like a wonderful idea, but I never saw one come to market.

there was a recent suggestion to replace the flywheel by an electric motor
But inertia works both ways; the heavier the flywheel, the harder it is to accelerate as well as decelerate, so the engine might keep happily rolling through the three non-power strokes, but it's also going to struggle to pick up speed freely when you're on the gas.
The point the author is missing is that the flywheel is just as important for the power stroke as for the other ones. Nobody needs all that BANG through the whole mechanics all the way to the wheels in one go. The flywheel smoothing out that too.
If one needs fast response then just add some electronics and SW for extra fuel during accelerating - or just use a hybrid engine.

tried using a lightweight plastic plumbing part to hold the rotating magnets, and the entire thing ended in a "spectacular explosion."
Guess the author would not be able to keep an DIY topic open here with that attitude...

anorlunda
FWIW, there was a recent suggestion to replace the flywheel by an electric motor, synchronise its power pulses to the gaps between a 4-stroke's firing.
https://newatlas.com/electromagnetic-flywheel-engine/55254/

Could this replace both starter motor and alternator ? It would seem most suited to small engines, perhaps in a hybrid.

Nik-note: I'm not saying it is a good idea or otherwise, just some-one is thinking outside the box...

Not read the article properly, but it's not really outside the box anymore. ;)
It's effectively a P1 hybrid architecture.

Can replace the starter, but not the alternator. As you keep the high voltage and low voltage systems separate.

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Nik_2213
cjl

The point the author is missing is that the flywheel is just as important for the power stroke as for the other ones. Nobody needs all that BANG through the whole mechanics all the way to the wheels in one go. The flywheel smoothing out that too.
If one needs fast response then just add some electronics and SW for extra fuel during accelerating - or just use a hybrid engine.
The article is correct - for applications where you want fast response and rapid changes in RPM, a heavy flywheel is a detriment. Racing vehicles and high performance cars frequently use lighter flywheels for this exact reason. In addition, a lightweight flywheel can improve vehicle performance in low gears - a surprising amount of engine horsepower can be soaked up just accelerating the flywheel in first or second gear when trying to accelerate quickly, resulting in less available power to accelerate the rest of the vehicle and therefore slower acceleration.

anorlunda
Staff Emeritus
There are rotating devices that provide what they call "synthetic inertia". Synchroverter is another name https://en.wikipedia.org/wiki/Synchronverter

Conceptually, similar ideas could be used in a vehicle engine, provided that there is also a source/sink for energy in electrical form. But before someone works on developing that, there must be a motivation. Suppose for the sake of argument that we could make a variable inertia flywheel. What advantages would that offer?

If there are no advantages, there is no reason to research it.

cjl
It would allow the engine to idle smoothly and not be jerky and awkward at low RPM and throttle settings while still allowing very rapid changes in engine speed at high RPM and high throttle.