There are several sources:

Combustion noise (or knock).  This is caused by the sudden combustion of fuel that has already evaporated when combustion starts.  This is the traditional clatter that's worse at startup (low temperatures - long ignition delay).  Can be reduced with pilot injection.

Pump noise.  As you mentioned.  Less of an issue with common rail.  Used to be really bad with in-line pumps.

Gear rattle.  Big diesels with big pumps require super-strength front-end drives.  These gears often rattle.  Not a problem with the more modern diesels that have their pumps driven off the rear.

you can also, on occasion, hear the rattling of "loose-fitting" pistons in the cylinders, if you're used to what engines of a family should sound like and have a relatively quiet environment.  Most people can't tell the difference, and it's really tough to discern with an accelerometer, but OEM engine mechanics most certainly can identify such pistons with remarkable accuracy (auditory ID of nonconforming parts statistically demonstrated with high confidence).

SomptinGuy, Thanks for the reply but I am still confussed. Is it combustion noise or injector noise.

ivymike, Boy did you hit my hot button. I am a vibration technician that collects and analyzes data from various industries. They are primarily motors, fans and gearboxes. I would really like to learn more about vibration characteristics of engines. Where can I find such info? What frequency would identify piston Knock? Piston resonance? 2 times turning speed?

Quote:

Is it combustion noise or injector noise.

Could be either.  You need to do tests that eliminate one or the other to dtermine.

As ivymike points out, piston-slap is also a contributor.  But how to isolate that?  Maybe you just retard so much that combustion noise goes away?

SomptingGuy- Can you elaberate on, "This is caused by the sudden combustion of fuel that has already evaporated when combustion starts", more so I fully understand it? What is evaporated? Is it not a mass of atamized fuel that explodes when compression creates heat enough to ignite?

Most of the "diesel clatter" noise is a direct result of the rapid change in cylinder pressure at the start of combustion.  This is affected by many engine & injection system atributes.  Indirect injection systems limit the pressure rise by throttling of the combustion gasses through the pre-chamber throat.  Advanced direct injection engines use pilot injection to start combustion off more slowly.  Injection timing, rate of injection, compression ratio, turbo boost, etc all effect how much diesel clatter one hears.  If the piston is a loose fit that only make the problem worse because the pressure spike slams the piston skirt against the cylinder wall.  Additional valve and gear train noise make it difficult to tell exactly where it is all coming from.  Go to a fuel injection specialist and listen to an injection system on a test stand by itself and you can hear what the pump and injectors sound like without an engine.  Most systems are pretty noisy but you won't hear the "diesel clatter".

steve383:

In a Diesel engine, the fuel injection period is approx. 22 degrees of crankshaft. This corresponds to approx. 35 milliseconds (msec) for a low speed engine of two strokes, and approx. 9 msec for a medium speed engine of four strokes.

Since the start of the injection, the time required by the atomized fuel to evaporate and then auto ignite, is known as "ignition delay". Typical "ignition delays" are up to 10 msec in a medium speed engine and up to 20 msec in a low speed engine.

Detonation may occur if delay due to fuel quality is large.

For the purpose of vibration analysis, you can visit the ISO web site and take a look, among others, to the International Standard ISO 8528.

 

steve383:

Check out the latter slides in this presentation.  The amount of fuel that burns pre-mixed determines the level of clatter.

http://me.queensu.ca/courses/MECH435/6.%20Combustion%20in%20IC%20Engines.ppt

A few years ago i had the pleasant job of rebuilding a tractor engine that was both petrol and diesel.  International maybe.

Was rather interesting how it was built. one side of the engine was the manifolds, with carby and ignition system, other side was injector pump. Inline 4.
there were an extra set of valves for each cylinder that were open to expose the sparkplugs and reduce compression for petrol running. Once the engine was warmed up 2 large levers were moved, one to engage the injector pump and the other to close the previously mentioned valves. The change in engine noise was from nice quiet petrol purr, to clattery diesel!
From this it answered a question much like yours i had been wondering about for years. The ignition of the diesel fuel certainly makes a lot of noise!!

(disclamer, this was not a modern engine by any means)
Ken

Diesel fuel does not evaporate in the cylinder, as does gasoline in a gasoline engine.  It is injected as a liquid under high pressure (in the order of 2500 psi) and is atomized into a spray pattern of small droplets.  The heat of compression is hot enough to ignite the atomized fuel droplets.

Regards
Dave

Quote:

Diesel fuel does not evaporate in the cylinder, as does gasoline in a gasoline engine.

Uh?  Yes it does.  That's precisely where the knock comes from.  And most gasoline engines are port-injected, with most of the evaporation going on outside the cylinder.

Cessna,  add another 0.  Over 30,000 psi is not unusual anymore.

Kenre:  You are correct.  The International MD had that engine.

There was an SAE paper from '85 or so that detailed noise reduction in diesels via EGR.  Slowing down the combustion event definately had an effect on the noise. The presenter, had a tape recording which he played at the conference.

Another factor is cetane rating of the fuel:  generally,  higher cetane fuels will run quieter due to (presumably) the shorter delay time and therefore less opportunity for fuel evaporation and the resulting uncontrolled combustion.

And not all direct injection diesels are equally noisy.  Listen to a current-model Cummins ISB engine idling sometime,  and you'll think you're listening to an SI engine (well, almost).  The comparable Cat engine however...

One virtually useless peice of info for the forum..  In the USA, Diesel fuel is about 5-8 cetane points lower on average than in the EU.  Aside from the fact that more of our refinery output here in the US is for gasoline, the yield threshold leaves less available desirable crude product to be refined to higher cetane No 2 fuel.  

Regardless of injection strategy and other theory, lower cetane fuels are slower burning but a source from an additive mfgr suggested that available additives such as magnesium, etc, effectively level the playing field and reduce diesel clatter by decreasing combustion delay on newer common rail diesels.  The best diesels with true variable rate shaping via piezo or magnetostrictive actuation (no moving parts aka solenoid) are perceptively as quiet as gasoline engines under moderate load.