- Oct 18, 2003
- Saline Michigan
Engine mounts in both cars and airplanes are sized to give 1st order resonance of the block moving in torsion below idle speed, so it is isolating during operation. Usually, you want to put the resonance between firing order at cranking and firing order at idle, so the engine quickly passes through resonance during start accel and after fuel cut-off, but does not run there even while cranking. You see this when you shut down many piston airplane engines - the shudder as it slows below idle.Cars (and trucks) have another card up their sleeve - soft engine mounts. This means that the engine block can effectively "rotate" about the crankshaft to some extent, which helps in the "soft spring" approach. And some modern mounts are pretty trick, fluid damped units, not the rubber blocks of old.
ps I have seen failed engine mounts in rally cars which I would swear have died due to overheating, not overload.
Engine mounts don't really help much with torsional vibration of rotating internals. During firing pulses, the engine block is accelerating one way while the crank accelerates the other way. The acceleration of the internals is much bigger than of the block/heads/accessories. The pressures happen way too fast and the block accels are way too low for it to influence the crank accels much.
If somehow they get into extended operation near resonance, all mounts will heat up and cook if the temps get in range for the mount materials.