Diesel knock

[jens]

removed

[Quinnf]

Jens,

There's knocks, and then there's knocks.  They don't all sound the same and they all have different causes.

A 'roid can knock from a number of causes:

1.  Loose or ill-fitting gib keys can cause the flywheel to rock back and forth on the crankshaft, taking up hard against the sides of the gib key with each power stroke and compression stroke.  Sounds just like rod knock.  My engine had it.  It took me almost a year to track it down to poorly fitting gib keys and keyways in the crankshaft that didn't match the width of the keyways in the flywheels (Ashwamegh engine).  Brass shims solved my problem.

2.  Rod knock.  Loose or worn big end bearings are the usual cause.  Shouldn't be any problem in a new engine with 0.002" to 0.005" or thereabouts clearance.

3.  Timing gear backlash.  Probably the biggest source of mechanical noise in these engines, and the most difficult to fix.  This is an ongoing subject of discussion, and about the only way it can be fixed is by fitting an offset idler gear spindle.  Dave (Xyzer) seems to have pretty well nailed down that solution.  A number of folks have carefully measured the backlash in the timing gear train and found it to be excessive.  But it varies engine-to-engine.  Some engines absolutely rattle (my Ashwamegh 6/1 does), while others like the Beta Test engine were about right out of the crate.  .

4.  [EDIT: Detonation] Pre-ignition.  Can have a few causes: 

a.  Depending on where your engine is injecting its fuel charge, you might have detonation pre-ignition.  [I'll admit this is more applicable to gas engines than diesel, but I thought it was funny, so I stand by it by virtue of artistic license]  Sort of like that embarrassing male problem that all of us have heard sometimes happens to other guys, but never to ourselves, if the charge is injected before when the conditions are not optimal, the results can be disappointment.  And if left untreated, engine damage can occur. 

b.  Pre-ignition Detonation can also be caused by burning fuel with the wrong cetane rating.  Rather than burning, which is a smooth, relatively slow and controlled process, the fuel charge can detonate, literally exploding in a chaotic blast.  Try burning white gas (lantern fuel) in your motorcycle and you'll hear what it sounds like (don't ask me how I know!).

c.  Pre-ignition Detonation can also be caused by an overheated engine.  I mean overheated to the point of near destruction (don't ask me how I know!).


So to say simply that your engine is making a knocking sound doesn't tell anyone very much because the causes can be so diverse.

Hope this helps.

[RAB makes a very valid point about the distinction between pre-ignition and detonation, which I'm aware of, but got sidetracked in writing the above.  They are both bad things for an engine, and indicate that the engine needs attention.  And, yes, 0.005" is too much slop on a new engine but you might find that kind of clearance in an old engine, especially one run on heavier lube oil to keep it quiet.  And, yes, IDI engines are quieter and have a different sound than a DI engine, but I thought we're all talking of IDI Listeroids here.]

Quinn

[oliver90owner]

Quinnf,

You failed to mention the difference in sound from a DI and IDI engine.
IDI has always ben regarded as a quieter engine than a DI.  It was one reason why early diesel cars were IDI. Remember though that IDI is somewhat less efficient (generallY) than a DI equivalent because of the greater heat loss.

Some of your comments like pre-ignition refer to spark ignition engines only, not a diesel operating as a diesel because the fuel can't prignite inside the injector!  Lets compromise and speak of detonation rather than 'pre-ignition' if it is caused by some other reason such as incorrect fuel grade.  I know, I'm picky - but there may be some readers out there who are not so aware of the differences!

Referring to new engines and 5 thou rod clearance - I would not accept that.  An old engine maybe.  Lister recommended 'no more than 3 thou.'  5 thou with a modern thin walled shell bearing will spell imminent disaster.

I think Jens was referring to normal running noises not the 'death knell' of an engine just before a catastrophic failure.

Mine is IDI and reasonably quiet.  It's a proper Lister.  And it's water cooled; Air cooled engines are notoriously louder - even the proper Listers.

Regards, RAB

[cschuerm]

I would also be greatly interested to hear what others have done to quiet intake noise.  That's the last area to get some attention on my generator trailer.  I have plans to experiment with a remoted air box.  Think I'll first try running aircraft SCEET flex hose from the engine to the box.  May wrap it in insulation depending on how much noise comes from the hose itself.  Probably a thicker hose (radiator) would work better, but would be harder to work with.
Who has a good solution

Chris

[diesel guy]

Jens,

You can tell where the knock is coming from. Put your head down low and listen if the knock comes from there, it is most likely is a crank/rod bearing clearance problem. If it is in the top end, it is most likely too advanced timing. If you are at 18 degrees as stated earlier, the diesel  knock should be subdued, especially since you have IDI heads on your 20/2 engine.  As others have accurately mentioned, there are other sources of the engine noise as well, but in my option, I think these are the two to start with.

As far as your F-350 goes, earlier engines were IDI and later versions have DI heads. I have a new Excursion with a Power Stroke and it is very quite, I had 6.5 diesels in Suburbans and they sounded like they were going to blow up when started at low temps.

Diesel Guy

[sanchez]

Hi guys,

I am the owner of the lovson that jens saw at the bananas garden, My engine is direct injection (no changeover valve), the little knock that you can ear is normal under high load, as the mine is. can you post a audio-video footage

[Quinnf]

Jens,

Keep in mind that the various videos that you can find on U-tube and whatnot are for the most part shot with simple hand-held digital video cameras with camera-mounted microphones.  Depending on how the AGC (automatic gain control) is set up in the camera's audio circuitry, the sound is unlikely to be true to life.  I can tell you that my engine doesn't sound like anything I've heard on the web. 

Thanks to RAB for his comments.  I don't disagree with anything he said.  My point in all of that was just to emphasize that engine noises can have many different causes.  One additional possibility that came to mind after I wrote that is that a crack in a flywheel hub can loosen the fit of the gib key in its keyway, causing a knock to be heard on power and compression strokes.  There has only been one instance of that reported here, however, and it occurred after something like 3000 hours of running.  In that case the sudden change in engine sound alerted the owner, who shut the engine down immediately and didn't discover the cause until the next morning.

Quinn

[ronmar]

loosen the fit of the gib key in its keyway, causing a knock to be heard on power and compression strokes.  Quinn

I had this happen after instaling wheels, on the generator belt drive side.  Flywheel was fine, I just hadn't fit the key properly during my testing and it worked loose.  This was another reason I added 2"' locking collars (inside and out) at each flywheel.  The knock it made sounded very similar to any other diesel knock I have heard as it was most noticeable on the power stroke when standing on the starting side and looking down on the engine,  The location was readilly apparent as comming from the shaft/wheel when I squatted down and listened to it from different places around the engine.

Ron 

[Quinnf]

Ron,

When that happened to me, I heard the sound distinctly coming from the crankcase.  I think the sound was telegraphing along the crankshaft and resonating somehow in the crankcase like the body of an acoustic guitar.  So I thought it had to be a rod bearing.  Plastigauge said 0.002" all around, so I kept looking and eventually figured it out after measuring the gib key and keyways and noting the difference.

Quinn