combustion noise and mechanical noise 2

[shipvibration]

are there any analytical methods to find the difference between combustion noise and mechanical noise in a diesel engine? under normal running what are the factors that indicate the noise emnating is from either of the sources?
any parameters(frequency etc.,)?
any ideas?

 

[GregLocock]

In practice:

Run it at no load at a given speed. All the noise is mechanical. Run it at full load at the same speed. The difference between the two spectra is the combustion noise.

We actually motor engines over with an electric motor to get a true no-load reading.

Analytically: Do you mean by analysing the spectra from a running engine, or are you trying to build a model of the noise radiated by an engine? In a diesel engine you could probably ignore the mechanical contribution as combustion dominates. In a 4 cylinder in line petrol engine then the second order inertial forces dominate.



Cheers

Greg Locock

 

[shipvibration]

thanx for the response.
some doubts...when the engine is running(say a big marine diesel of @15000BHP, can we differentiate by ears the combustion noise?
self had an experience in which a sharp noise(charactristic of mechanical fouling)in an engine of such size turned out to be from an airlocked piston Cooling Water side.
my question is: can we put any kind of analytical instruments(fitted or otherwise) and differentiate the noises?if so what are the values going to be like?
thanx.

 

[GregLocock]

I think someone familiar with the engine in question could probably work out which is combustion noise and which is mechanical. Since your ears are somewhat similar to microphones you ought to be able to identify which is which on a spectrum.

However I am 90% sure that the differences found would be specific to an engine type. Cheers

Greg Locock

 

[MachineryWatch]

Greg,

I'm working on an automotive engine derivative natural gas burning emergency generator project. I'm assuming the discussion so far was primarily centered on airborne noise. When you say the difference in this noise (no-load versus load) is primarily due to combustion energy, would you also say this is true of the structureborne energy? I understand that some of the vibration would have it's sources at the generator end and be related to electromagnetic forces. Unit is 4.2 litre V6 coupled to 80 KW generator at 3600 RPM.

Skip Hartman

http://www.machinerywatch.com

 

[GregLocock]

yes, I was thinking of airborne noise, on automotive diesels.

Structureborne noise should follow the same pattern, what is your engine - SI or CI? What's the V angle? does it have balancer shafts? it sounds as though the engine is not working very hard.

If it is an SI 90 degree V6 without balancers then almost all the structureborne noise will be inertial, from the engine. You can separate out some of the generator's contribution by considering which frequencies are generated by which mechanism but I don't have much experience with noise from generators, other than balance and fan problems.

Cheers

Greg Locock

 

[shipvibration]

that is interesting!
airborne and structure borne noises: are there any definite decibel level values for such noises?
e.g., turbocharger noise, combustion noise, camshaft/poppet valve noises, crosshead slamming noises etc.,
any help on this?
thanx.

 

[gunnarhole]

Gentlemen,

Do you think it would be possible to examine the differences between synchronously averaged spectrum data vs. "free run" averaged data and to attribute these differences to the combustion process?

Regards,

Gunnar

 

[GregLocock]

<Gunnar> Nice try, unfortunately many of the mechanical noises, eg piston slap, valve opening, are phase locked to the rotation, as is the combustion noise.


<shipvibration> No, not generally. On a given installation you could try a condition monitoring approach and see if some harmonics grow with time. Frankly the way we do it is build ten engines, listen to them, measure any objectionable noises, and then tear the bad ones down to find the cause (or not, as the case may be). Probably not a very helpful approach for the marine environment!




Cheers

Greg Locock

 

[MachineryWatch]

Greg,

I am unfamiliar with the terms SI and CI. The engine has no balance shafts. We have a 1.5X RPM frequency on the generator bearing which grows in amplitude significantly as you add load. I believe this is a harmonic of the 0.5X RPM frequency I see on the engine and believe it is almost purely combustion related. The reason I think it is higher on the generator than any engine locations is that the generator rotor has a calculated critical of 87Hz. This is very close to 90 Hz (1.5X RPM) and the actual critical may be slightly higher than calculated. I think we are getting some resonant amplification on the generator because of this.

Skip Hartman

http://www.machinerywatch.com

 

[GregLocock]

SI=spark ignition, CI=compression ignition=diesel

V6s often create 1.5 order as a rotating couple (ie the centre of the engine is motionless, each end of the crank orbits). This is inertial in origin, ie invariant with load.

This is very difficult to eliminate without balancer shafts.

Half order on engines is primarily associated with cylinder to cylinder variations in imep, although it can also be seen due to crankshaft flexure, or block flexure (usually crank). Obviously this varies with load, but may be worst when lightly loaded, because the cyl to cyl mixture varies more. With the right resonances around this might excite 1.5 order as well. Another source of 1.5 order is the valve gear.

If this is beginning to sound horribly complex, yes it is. Cheers

Greg Locock

 

[MachineryWatch]

Greg,

This is an SI natural gas fueled engine. Since the 1.5X is normally invariant with load what do you think would lead to the fairly steady progression upward in vibration amplitude as we ratcheted up the load in 10% Full Rated Load steps from 0 - 100%? It seemed to be most prominent in the horizontal measurement plane.

Best regards,

Skip Hartman

http://www.machinerywatch.com

 

[GregLocock]

Assuming the engine is functioning correctly,

Unequal length runners

Crankshaft torsionals

camshaft torsionals

cylinder to cylinder mixture variation




Cheers

Greg Locock

 

[MachineryWatch]

Greg,

I used a photocell sensor on the generator fan with 14 blades as a speed reference for my data acquisition. I also briefly routed this signal to a spectral analysis channel and captured a 0 - 1000 Hz FFT. There were 90 Hz sidebands around the 840 Hz blade pass frequency. Torsional?

Skip Hartman

http://www.machinerywatch.com

 

[GregLocock]

Looks likely, doesn't it. We use almost exactly that setup for tuning torsional vibration dampers. (ie a toothed wheel and an inductive pickup)

On the other hand the frequency is too low for crankshaft resonance but it could be block bending if the generator is directly mounted to the block, but I'd run with combustion variation coming through as torsionals.

Can you do a quick impact test to get the vertical bending mode of the block? that should eliminate that.

Cheers

Greg Locock

 

[MachineryWatch]

The block and generator are bolted together becoming a single rigid unit. There are 2 mounting feet on the engine and 2 mounting feet on the generator. The unit sits on isolators.

The block/generator bending mode sounds likely at the 1X RPM frequency. I did a phase analysis (poor man's Operating Deflection Shape) at 1X RPM and found that at the extreme ends of the machine in the vertical direction the vibration was &quot;in phase&quot; and the amplitudes were the highest. The more central (aft engine block, forward generator) measurement positions were much lower in amplitude and phase was &quot;out of phase&quot;.

My instrumentation did not allow me to do this analysis for 1.5X RPM (the instrumentation manufacturer is working on this). I guess if there is a vertical plane bending mode of the block/generator at 60 Hz the unit could be a little stiffer in the horizontal plane and there could be another bending mode at 90 Hz (1.5X RPM).

Unfortunately, I can't just walk out and do a quick impact test. I did a few impacts on components when I was on site, but did not try to excite the whole engine/generator unit. I may get another shot at doing so.

Skip Hartman

http://www.machinerywatch.com