Diesel Noise
Diesel Noise
(OP)
Can someone explain where the clatter noise on a diesel engines comes from? I always thought it was from the injection pump(hydraulic noise). I believe the newer trucks have electronic injection and the noise is still there.
RE: Diesel Noise
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.
RE: Diesel Noise
RE: Diesel 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?
RE: Diesel 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?
RE: Diesel Noise
RE: Diesel Noise
RE: Diesel Noise
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.
RE: Diesel Noise
Check out the latter slides in this presentation. The amount of fuel that burns pre-mixed determines the level of clatter.
http:/
RE: Diesel Noise
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
RE: Diesel Noise
Regards
Dave
RE: Diesel Noise
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.
RE: Diesel Noise
RE: Diesel Noise
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.
RE: Diesel Noise
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...
RE: Diesel Noise
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.