If the engine forces are balanced like in a inline-6, why does the engine twist?
In: Engineering
Umm.. basic physics. If the part that spins accelerates its spinning (from a stop, or just picking up speed), then something – in this case, the rest of the engine – exerted that force upon it… and so since every action has an equal and opposite reaction, the main spinning part will apply a reverse spin force on the rest of the engine. That’s your twisting force. Big semi trucks starting from a stop really show this as the whole cabin seems to want to twist itself off its wheels.
Balanced forces in an engine are more intended to keep the engine from vibrating while spinning at a constant speed, since it contains parts that move up and down, not quite in sync with each other… as opposed to an inline 4 which, since it’s a 4-stroke engine, would have 2 cylinders moving up and 2 moving down simultaneously and in sync at all times. That’s much more naturally balanced. 6 cylinders aren’t so nicely staggered.
Conservation of angular momentum. Inorder to spin something (the flywheel if the car is in neutral) a force must be applied to accelerate it, whilst the force is applied by the pistons going up and down the conservation of ANGULAR momentum requires an equal and opposite rotation force be applied to the engine structure when accelerating. This is transfered through rubber engine mounts to the car chassis but the rubber has some give to it to let it smooth bumps so the engine twists a bit in its mounts
Big metal block has big rotating part inside. Big rotating part rotates REALLY fast. So fast and with soo much force it wants to rotate the big metal block around it too. The result is that the engine twists between the crankshaft which is spinning, and engine mounts which are stationary. The twist is more or less noticeable depending on the mounts used. Loose mounts allow engine to twist. Stiff mounts will make the entire car chassis twist if the engine is strong enough.
The engine can be balanced, but the drive train only rotates one direction, so it’s always going to twist.
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