ossiclating vibrations



I noticed something today while waiting at a stop light:

the engine harmonicly vibrates (as cars do) but it was a rise and fade... how to explain this... you could feel vibes for a second then he next second there would go away, then come back ect ect... im sure some of you who are engeneers or have take advanced mathatics know what i mean by an am modluation type of effect...

its not something I am overly concerned with, I am just wondering what would cause this sort of vibration.
I claim to know very little about it, but there are a few things I learned from Ford Australia on the changes they made to Falcon cranks.

But firstly, the answer to your question is that noise is transfered like a long tuning fork, and different vibes from different parts can cancel out certain vibrations. This is why things aren't linear.

Secondly, noise may emanate from components in different wave-lengths, and part of the noise level change is related to the Dopler Effect, which alteres the amount of noise the listener hears depending on the rate of change in acceleration. So if your car is changing speeds, the engine noise travels to you at varing rates! Nissan Motor Company has done bulk Noise Laboratory work on there cars, and has even developed a system whereby they can "cloak" the noise by cancelling it out. All they do is copy the noise, and broadcast an amplified rendition back into the engine bay. Amazing stuff!

There's more info here, which I've gleaned from many Aussie car and SAE-style papers on Australian six cylinder engines. Sorry it rambles, but it is what it is, man!

Aussie I6 Crank Experience:-

1. Firstly, the six cylinder in-line engine is supposed to be perfectly balanced. The firing impulses are 120 crank degrees, and they are theoretically perfectly balanced. But in practice, the I6 cranks made from 1959 until 1995 (in three areas, USA till 1983, Aussie till 1992, Argentina till 1995) didn't have full counterweights. This causes some small imbalance at various parts of the rev range, and also lowers the critical torsional vibration level to as low as 5400 rpm on a US 250 to higher than 8600 rpm on a little four bearing 144. The more counterweight you can fit, the greater the mass of the crank, and this rasies the dampening of the shaft at high RPM.

2. The other aspect improving stiffness and smoothness is the overlap between main bearings and crank pin bearing surfaces. Little 144 have almost 1 inch of overlap, while a 250 has only 0.310 inches, from memory. This is why smaller sixes are often smoother on a case for case basis than the bigger sixes. Having an extra set of bearings like most 65's also helps. The use of high carbon steel rather than cast nodular iron steel is another way of improving smoothness.

3. In Aussie OHC Falcons, the loss of the crankcase mounted cam allowed engineers to taylor a new crankshaft which had more mass and counter weight area to balance out the vibes. First up, the 3.9 OHCversions were
just OHV cranks with silght changes, but after Jaguar was aquired, Ford Aussie copied the 3.6/4.0 AJ-6 crank counterweighing after two major redesigns. The latest DOHC 4.0 I6 crank is fully counter weighed and is very strong, with an extra 1400 rpm on the last OHV 4.1 redline (5900 rpm verses 4500 rpm). Because todays Aussie sixes are build using basically the same tooling the first 144 cube 1959 Falcon XK used!. The latest Aussie 4.0 OHC is still basically a US 250 style engine/crank, in the same way Gran Papies axe is the same one his forfather used in the war of Independance!

4. The trans to block linkage and the pressed steel oil pan are critical vibration zones. Since the mid 90's, Aussie sixes have used plastic sumps, and laterly, cast alloy sumps solidly buttressed to the transmission in more than just the typical 6 bolt postions. Noise lab work again. Customer focus caused the change. Adding more mass or using materials which don't oscilate as much under vibration was the key