Reaction moment on I.C. engine when we increase RPM

  • #1
John Mcrain
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Every time when you increase RPM on longitudinal placed internal combustion engine, car shake to side in oposite direction of crankshaft rotation.
This is reaction moment for change in RPM of crankshaft/flywheel.

How this reaction moment transfer to engine block/car if crankshaft is separated from block with oil film?
(In the case or electric motor is clear, electromagnetic force between rotor and stator transfer this reaction moment to motor hausing)

 
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  • #2
hutchphd
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Because it is a crankshaft and the oil is pressurized
 
  • #3
John Mcrain
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Because it is a crankshaft and the oil is pressurized
So what ?
Oil do not get moment to crankshaft, piston does.
Oil is here only to reduce friciton.
 
  • #4
Baluncore
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Because gas pressure in the cylinders, push the piston and cylinder head apart, and the power stroke is always in the same direction.
The forces associated with the change in the angular momentum of the engine, are transferred through the engine mounts to the body.
 
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  • #5
John Mcrain
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Because gas pressure in the cylinders, push the piston and cylinder head apart, and the power stroke is always in the same direction.
The forces associated with the change in the angular momentum of the engine, are transferred through the engine mounts to the body.
Reaction moment is transferd to engine over contact between piston thrust side and cylinder wall?
 
  • #6
Lnewqban
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You could imagine that in order for the crankshaft to accelerate angularly (to increase rpm’s rapidly), it needs to push the block and the rest of the car in the opposite angular direction.

Construction workers sometimes suffer wrist fracture when drilling concrete and unexpectedly the bit hits a steel bar inside it: same principle.
 
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  • #7
John Mcrain
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You could imagine that in order for the crankshaft to accelerate angularly (to increase rpm’s rapidly), it needs to push the block and the rest of the car in the opposite angular direction.
Yes I know that reaction moment must exist(Newton law), but I want to know how this moment is transferd, at which points exactly..
Construction workers sometimes suffer wrist fracture when drilling concrete and unexpectedly the bit hits a steel bar inside it: same principle.
Reaction moment is here transfered from rotor to stator, through el.magnetic force
 
  • #8
hutchphd
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The piston/cylinder pressure-induced force is converted to moment by the crank. And vice-versa
 
  • #9
Lnewqban
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Yes I know that reaction moment must exist(Newton law), but I want to know how this moment is transferd, at which points exactly..
As explained above, the only “solid” bridge of connection is the exploding gas (repidly increasing pressure) inside the cylinders.
Those are internal forces pushing the same amount in both directions.

You could only avoid that reaction by using an external force to speed the crankshaft up (hand crank, hill or people pushing the manual-transmission-car, etc.), or using a counter-rotation shaft that cancel the reaction.
 
  • #10
Baluncore
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Reaction moment is transferd to engine over contact between piston thrust side and cylinder wall?
The sides of the cylinder wall are not important. The area of the cylinder head (while the valves are closed), is opposed by the area of the piston head. The piston pushes on the offset crank through the connecting rod.
 
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  • #11
John Mcrain
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The sides of the cylinder wall are not important. The area of the cylinder head (while the valves are closed), is opposed by the area of the piston head. The piston pushes on the offset crank through the connecting rod.
Yes that make sense.

So earth stop engine/car to rotate around itself.
If put engine is space, engine will rotate with slower RPM around itslef than crankshaft, because it has higher moment of inertia then crankshaft?
It will rotate only when RPM is changed?
 
  • #12
Baluncore
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If put engine is space, engine will rotate with slower RPM around itslef than crankshaft, because it has higher moment of inertia then crankshaft?
When disconnected in a free space, the size of the moment of inertia is not really important. If the crank begins to rotate one way, the block must rotate the other way to conserve angular momentum.

It will rotate only when RPM is changed?
Yes, except for friction in main bearings, clutch drag, and the cooling fans in the viscous air.
 
  • #13
hutchphd
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No. The rates of rotation will change when engine accelerates. When it decelerates then rates will return to status quo ante. This is how the space station changes orientation using internal flywheels with motors attached.
 
  • #14
Lnewqban
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Yes that make sense.

So earth stop engine/car to rotate around itself.
If put engine is space, engine will rotate with slower RPM around itslef than crankshaft, because it has higher moment of inertia then crankshaft?
It will rotate only when RPM is changed?

Don’t need to go that high. :smile:
Note what happens to helicopters when the tail rotor stops due to some damage or failure.

helicopter-crash.gif
 
  • #15
jack action
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  • #17
jack action
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If block and crankshaft both rotate, than what is at rest?
I'm not sure what you mean by that, but the point is that the engine block rotates about the crankshaft as much as the crankshaft rotates about the engine block. It is just a different point of view and one is not better than the other.
 
  • #18
Averagesupernova
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If block and crankshaft both rotate, than what is at rest?
Likely a planetary gear or a similar setup.
 
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  • #19
John Mcrain
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I'm not sure what you mean by that,
How block and crankshaft are conected to airplane? airplane for sure not rotate.
 
  • #20
hutchphd
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How block and crankshaft are conected to airplane? airplane for sure not rotate.
If it is four-cycle then airplane could be bolted to the camshaft! What a goofy idea !
 
  • #21
jack action
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How block and crankshaft are conected to airplane? airplane for sure not rotate.
Like @Averagesupernova said, with a planetary gearset. If you fix the carrier of the satellite gears, the ring and sun gears will turn in opposite directions. In the following image, the carrier (gray) would be connected to the airplane, the sun gear (yellow) to the crankshaft, and the ring gear (blue) to the engine block.

SoulfulFastBluejay.webp
 
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