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

[John Mcrain]

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)

 

[hutchphd]

Because it is a crankshaft and the oil is pressurized

 

[John Mcrain]

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.

 

[Baluncore]

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.

 

[John Mcrain]

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?

 

[Lnewqban]

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.

 

[John Mcrain]

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

 

[hutchphd]

The piston/cylinder pressure-induced force is converted to moment by the crank. And vice-versa

 

[Lnewqban]

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.

 

[Baluncore]

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.

 

[John Mcrain]

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?

 

[Baluncore]

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.

 

[hutchphd]

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.

 

[Lnewqban]

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.
Note what happens to helicopters when the tail rotor stops due to some damage or failure.

 

[jack action]

When the pressure increases in the cylinder, the engine block is fixed and the crankshaft is free to rotate. But it doesn't have to be that way.

Engines were made where the crankshaft is fixed and the block is free to rotate. They are called rotary engines:

There was even one company that made an engine where both the crankshaft and the block were rotating in opposite directions:

 

[John Mcrain]

There was even one company that made an engine where both the crankshaft and the block were rotating in opposite directions:

View attachment 321067​

If block and crankshaft both rotate, than what is at rest?

 

[jack action]

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.

 

[Averagesupernova]

If block and crankshaft both rotate, than what is at rest?

Likely a planetary gear or a similar setup.

 

[John Mcrain]

I'm not sure what you mean by that,

How block and crankshaft are conected to airplane? airplane for sure not rotate.

 

[hutchphd]

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 !

 

[jack action]

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.

 

[John Mcrain]

https://www.enginehistory.org/Piston/Before1925/SiemensHalskeSh3/05Sh3.gif

 

[cjl]

Reaction moment is transferd to engine over contact between piston thrust side and cylinder wall?

I'd never thought about it much before, but yeah, this kinda has to be the case. During the power stroke, the rod is not parallel to the cylinder bore, it's angled, and therefore is transferring a significant side load that must be the primary torque acting on the block. You could mitigate this by moving the bore to be more centered over where the rod is during the power stroke rather than having the bore centered over the crank, but then the pressure forces acting upwards on the head would generate a net torque around the crank (since the upwards force vector on the head would not be aligned with the crank, it would be offset and therefore be acting over a moment arm). Modern engines do tend to have a bit of a bore offset, so it's a combination of both (though you'd have to run some calculations to know which factor is larger).

 

[Baluncore]

I'd never thought about it much before, but yeah, this kinda has to be the case.

The side force on the piston is a distraction, not a significant contributor to the usable engine torque that appears on the crankshaft, relative to the engine block.

Imagine you lengthened the connecting rod, the engine torque would remain the same or increase slightly, while the piston side force would be reduced. At the same time, the crank offset is close to perpendicular to the connecting rod near maximum torque, so the reaction to piston side force does not act tangent to the crank rotation.

The engine torque appears from a triangle defined by three pins. The main crank shaft axis, the offset crank pin, and the wrist or gudgeon pin in the piston. The side thrust of the piston is not part of that major triangle, but is countered internally by a tension in the fixed crank offset.

 

[cjl]

The side force on the piston is a distraction, not a significant contributor to the usable engine torque that appears on the crankshaft, relative to the engine block.

No, it's a necessary reaction force to the torque being applied to the crankshaft. The caveat here is that a bore offset means some of the reaction torque comes from the pressure force on the head instead, so depending on how much bore offset you have, this sideloading will contribute more or less to this reaction.

Imagine you lengthened the connecting rod, the engine torque would remain the same or increase slightly, while the piston side force would be reduced. At the same time, the crank offset is close to perpendicular to the connecting rod near maximum torque, so the reaction to piston side force does not act tangent to the crank rotation.

Yes, but the distance from the location of the side force to the center of the crankshaft would be increased, and since this needs to be a reaction *torque*, the side load can go down with the increased moment arm while still providing the same torque.

The engine torque appears from a triangle defined by three pins. The main crank shaft axis, the offset crank pin, and the wrist or gudgeon pin in the piston. The side thrust of the piston is not part of that major triangle, but is countered internally by a tension in the fixed crank offset.

And yet there has to be a torque applied to the engine block by the rotating assembly, and there's really only 2 places that can occur: offset pressure forces on the head relative to the crankshaft load due to offset bores (as you mention), and piston side loading. In a hypothetical engine with zero bore offset, 100% of the torque being applied to the block by the rotating assembly *must* be getting applied. There's no other mechanism, and of course there has to be a reaction torque by basic Newtonian mechanics.

 

[Baluncore]

I am not disputing the presence of piston side force. I am simply saying that it is an insignificant component of the power output from an engine.

And yet there has to be a torque applied to the engine block by the rotating assembly, ...

The useful torque output from the engine assumes the block is a fixed reference, the block does not rotate in the engine mounts. The engine block is firmly attached to the gearbox housing, which is also mounted to the structure. It is the sum of all crankshaft torque that passes via the flywheel, to the gearbox input shaft, that provides propulsion.

I assume a simple geometry, with the axis of the cylinders passing through the axis of the crankshaft main bearings.

There is no piston side force at TDC=0°, or BDC=180°, when the crank arm is in line, parallel with the cylinder axis. Nor at those instants, is there any crank torque generated by either side or axial piston forces.

When the crank arm is at 90° or 270°, and the engine is doing work, there is a side force on the piston, but there is no torque generated by that on the crank. Then, crank torque comes simply from the axial piston pressure, stepped sideways by the rod, and applied to the crank pin. There is only an internal equal and opposite force, trying to change the offset of the solid crank arm, which is a purely radial force, not circumferential to the crank shaft axis, so not generating torque.

The pistons do not rotate, it is the crankshaft that rotates. The piston does not move sideways significantly, so the side force is an internal balanced force. No useful work is being done by the side force of the piston on the wall of the cylinder.

 

[sophiecentaur]

Reaction moment is transferd to engine over contact between piston thrust side and cylinder wall?

I suppose that this lateral force produces a momentary angular impulse which can twist the engine in its mountings in the same way that the prop shaft torque is balanced by the gearbox / engine reaction.

In a multi-cylinder engine, the choice of cylinder angles and firing sequence etc. can minimise the periodic torque from this effect ('balancing')