A Matter of Harmonics?

“We can’t visualize vibrations, but we tend to experience the effects of vibration, kind of like electricity,” Linda said. The shop had a high-tech vibration analyzer, embedded with a tri-axial accelerometer and laser tachometer.

“I get into vibration diagnostics because we see plenty of souped-up Duramax trucks down here,” She said.

I knew what she meant. Combustion puts such a forceful load on the crankshaft, it actually elastically deforms each time combustion occurs; fortunately, it fully recovers. An 8-cylinder deforms at least four times per rotation.

“Combustion puts a huge amount of force on each crank web, twisting the metal ahead of itself,” she said.

“Right,” I said, “and then it snaps back and creates harmonic frequencies that travel back and forth through the crankshaft. Normally, the harmonics are controlled so they have negligible effect on the main bearings because they work within a defined range of harmonics,” I said.

“Yes, but I’ve seen modified Duramax-equipped trucks where the harmonics affect the bearings and other parts big-time,” Linda said.

She was right because as the frequencies got closer to the natural frequencies of the components, wear increases tremendously due to uncontrolled oscillations.

“So many times the elastomeric damper can’t handle the modifications,” she said.

It wasn’t the case with the 6.2l engine we were working on. I studied the vibration. With the tri-axis sensor in the analyzer we were using (Fluke), my measuring point was in the seat track. This was a rough idle in drive we were dealing with.

Linda and Jeff explained that it was hard to duplicate the vibration. “Most rough idle in drive concerns are temperature dependent, and can be difficult to re-create at times,” I said. With A/C On, I performed several short heavy launch events from stop to stop to bring the temperatures back up while trying to re-create the rough idle vibration in drive with the meter’s sensor placed on inboard seat track in horizontal and vertical axis. It was picking up an E1.5 disturbance.

E1 is a first order engine vibration, simply engine speed, expressed in Hz (E1 x 60 = RPM). E1 vibrations can be accredited to parts related to engine speed such as flywheel and pulleys. High vibration levels detected at E0.5 can be related to half engine speed components, such as camshafts and associated auxiliaries. Usually, E2, a second order engine vibration happens at twice engine speed and will be the highest level of vibration (for a 4-cylinder engine) given we have two combustion events for every revolution of the crankshaft (two shocks applied to the crank), generating a characteristic high E2.  E2 vibration levels can be attributed to combustion events or components rotating at twice engine speed. On an 8 cylinder engine, E4 would be highest.  Ours was an E1.5, and with the vibration duplicated, at the inboard forward seat track location, it measured approximately 6.5 mg. In other words, we were looking at units of g (acceleration due to gravity)—specifically, milli-g. 
“I know,” Linda said, “I stress how important math is when it comes to diagnosis.”
Linda did not overlook the basic inspection, which was great. And she didn’t mind when I checked everything myself.  Engine oil and coolant level were confirmed correct; whilst hoses, brackets, harness routing and engine mounts were all checked for security, interference or fouling with the chassis.

 

 She did all these things but understood that I had to do them myself—for my own peace of mind. 
The shop tried to resettle the engine mounts but it didn’t affect the vibration. It has been my experience that oftentimes, an idle vibration caused by ground-out within an engine mount can turned off/on with a single bolt location – shimming that bolt location may greatly cut back the vibration. In some cases, a combination of bolt location shimming may be needed.
We managed to get it down to 2.6 mg, using mild steel spacer washers, and then road-tested the vehicle. The customer was satisfied.
Linda wasn’t aware of GM PI documents, and I showed the one we worked off of.

“I learned some things from you today, RJ. Thank you so much.”