Engine test engineer getting a bit overwhelmed by vibration analysis
Engine test engineer getting a bit overwhelmed by vibration analysis
(OP)
Hello everyone,
A little bit of background: I'm a recent grad (mechie BS) with a decent amount of engine experience for my age and position. Formula SAE powertrain lead, and did a combustion engine research project. I'm familiar with a lot of engine basics (I've read a lot of tech papers, textbooks like Blair and Heywood, mostly self taught). Now I'm out in industry and doing engine testing on a prototype engine. Unconventional design, it's an opposed cylinder/opposed piston 2 stroke diesel. We're a small company and I wear a lot of hats but essentially my boss and I are some of the only folks in the company with any real world experience (a lot of theoretical/simulation guys) so we kind of act as a voice of reason.
Since it's a prototype engine we experience a lot of failures. I'm competent at a lot of mechanical failure analysis; cycle fatigue, excessive loading, heat and friction, etc. But I'm feeling a bit lost when it comes to vibrational failures. I'm using an NI Zonicbook and eZ-TOMAS software to monitor accelerometers on each head and the block, I wrote a LabVIEW Program to perform torsional vibration monitoring on the crank encoder, and of course we have both a high speed combustion analysis system and a low speed acquisition system. But I just haven't quite been able to reconcile all that to perform the kind of conclusive analysis I would like to.
I've been reading some textbooks and looking for tech papers. I found Harris' Shock and Vibration to be a good text, from the small amount of it I've read so far. I just don't quite know how to really unite the theory with my data. How do I know that a component is going into resonance? How does resonance propagate through an engine? Say the front accessory mounting plate is vibrating at the modal failure frequency of my conrod, will that vibration necessarily propagate through the engine block to the conrod? More generally, how do I trace the sources of various frequencies of my vibration in the externally mounted accelerometers to specific engine components? If I see a higher order vibration at 2-3x the amplitude of 1st order (engine speed), what does that tell me about the source of that vibration frequency?
I know these are really vague questions, if anyone has any experience with this I'd really appreciate even pointing me in the right direction. Also if the only way to get familiar with this is to really read a textbook or two cover to cover and/or take a class then I would definitely look into that.
A little bit of background: I'm a recent grad (mechie BS) with a decent amount of engine experience for my age and position. Formula SAE powertrain lead, and did a combustion engine research project. I'm familiar with a lot of engine basics (I've read a lot of tech papers, textbooks like Blair and Heywood, mostly self taught). Now I'm out in industry and doing engine testing on a prototype engine. Unconventional design, it's an opposed cylinder/opposed piston 2 stroke diesel. We're a small company and I wear a lot of hats but essentially my boss and I are some of the only folks in the company with any real world experience (a lot of theoretical/simulation guys) so we kind of act as a voice of reason.
Since it's a prototype engine we experience a lot of failures. I'm competent at a lot of mechanical failure analysis; cycle fatigue, excessive loading, heat and friction, etc. But I'm feeling a bit lost when it comes to vibrational failures. I'm using an NI Zonicbook and eZ-TOMAS software to monitor accelerometers on each head and the block, I wrote a LabVIEW Program to perform torsional vibration monitoring on the crank encoder, and of course we have both a high speed combustion analysis system and a low speed acquisition system. But I just haven't quite been able to reconcile all that to perform the kind of conclusive analysis I would like to.
I've been reading some textbooks and looking for tech papers. I found Harris' Shock and Vibration to be a good text, from the small amount of it I've read so far. I just don't quite know how to really unite the theory with my data. How do I know that a component is going into resonance? How does resonance propagate through an engine? Say the front accessory mounting plate is vibrating at the modal failure frequency of my conrod, will that vibration necessarily propagate through the engine block to the conrod? More generally, how do I trace the sources of various frequencies of my vibration in the externally mounted accelerometers to specific engine components? If I see a higher order vibration at 2-3x the amplitude of 1st order (engine speed), what does that tell me about the source of that vibration frequency?
I know these are really vague questions, if anyone has any experience with this I'd really appreciate even pointing me in the right direction. Also if the only way to get familiar with this is to really read a textbook or two cover to cover and/or take a class then I would definitely look into that.





RE: Engine test engineer getting a bit overwhelmed by vibration analysis
It is
How do I know that a component is going into resonance? How does resonance propagate through an engine?
Modal analysis
Say the front accessory mounting plate is vibrating at the modal failure frequency of my conrod, will that vibration necessarily propagate through the engine block to the conrod?
Yes, possibly although that particular scenario seems unlikely to cause problems.
More generally, how do I trace the sources of various frequencies of my vibration in the externally mounted accelerometers to specific engine components?
You can't, directly.
If I see a higher order vibration at 2-3x the amplitude of 1st order (engine speed), what does that tell me about the source of that vibration frequency?
1st order shouldn't be present unless it is a single cylinder engine, that means you haven't balanced the engine. You should see strong spikes at firing frequency and multiples, and any inertial forces.
So, what you need to do is wander along to ISVR at Southhamption univeristy and give them a contract or go on their courses.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
There is a chapter dedicated to different crank layouts, and how to analyze their vibration.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
I'm in the US so that course probably is unfeasible for me. But I'll look into other seminars in the area.
Hemi,
Unfortunately I'm not involved in design. If I were I wouldn't be nearly so worried about vibration, it's a very unique and experimental engine layout that hasn't lent itself to smooth operation. At least at this level of optimization.
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
So both pistons do reach TDC at the same time and fire simultaneously?
If so that would be a nice surge of torque once per rev.
Maybe something like this -
http://gasturbinespower.asmedigitalcollection.asme...
It would be load sensitive I'd think.
Do you have info about the component failures to share? They might provide clues if viBraTioN is behind the problem.
In the 50s Both Coventry Climax and Studebaker underestimated a designed-in geometrical stress concentration (in the crankshaft and con rods respectively) of new engines. The failures resulted during fairly normal engine operation.
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
Yeah you've got the right idea. 4 pistons, 2 cylinders. The cylinders don't fire simultaneously but the pistons do move essentially symmetrically (hit TDC at the same time, slightly different strokes).
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
After you have read a bit more about vibration you may wish to check out the opposed piston engines in L.J.K. Setright's 'Some Unusual Engines' to see if there is something to be learned from them.
As far as a study of torsional vibration goes 'Torsional Vibration' by the old B.I.C.E.R.A. group was recently re-printed (Cambridge U.)
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
Bill can both lend insight to your analysis and educate you further. He is the type of engineer that will provide a highly beneficial interaction with a sharp young engineer interested in learning. Get your company to sign an NDA and get his help.
(By the way, Bill does not know that I am writing this response, and I assure you that I get no kick back from recommending him. But I have had a very good learning experience working with him.)
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
Ehudson, I'm amazed that you've been able to come up with this many relevant texts to an unusual engine design. I'll look into all of them.
Pontiacjack, I have but unfortunately my company doesn't have an SAE subscription. I will likely ask them to buy a handful of the more relevant ones for me. I lived for SAE papers while I was doing FSAE though, it's always my first stop on a project like this.
Turbomotor, thanks for the referral. I'll look into him and see if my company wants to reach out.
Thanks again!
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
If the engine is an OP 2T with 4 pistons, that would mean it has a pair of 2 throw cranks. But if it is a 4 cyl OP engine, then it would likely use a pair of 4 throw cranks. The number of throws on each crank, their angular spacing, the combustion pressure forces produced in the engine, the structural properties of the crankshaft, etc. can all have a huge effect on the torsional vibration situation in the crankshaft. A short, stiff 2 throw crank would usually present less torsional vibration problems than a longer, less stiff 4 throw crank. However, a 2T OP diesel engine presents some unique problems due to the gear drive typically used to synchronize the opposing crankshafts.
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
Sorry, I'm trying to walk a line between not divulging company IP and being clear.
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
But as you note, the long slender tension rods do present a unique situation with regards to structural vibration modes.
Out of curiosity, have you found that these long, slender rods are always loaded in tension throughout a full cycle, and at every operating condition?
RE: Engine test engineer getting a bit overwhelmed by vibration analysis
I have to say, despite the learning curve problems like this really get me going. I love a new challenge like this and after working through it I just feel that much more competent as an engineer. Another 'tool' in my mental toolbox, as it were.