Source of diesel noise at idle?

[RdTxTd]

In one of the links that tom mac provided in the Honda Clean Diesel thread. Honda says the reason diesels are noisy at idle is because of piston slap. I don't doubt Honda knows their stuff. I always thought it was combustion noise and it was addressed by pilot injection.

If you follow this link... http://www.honda.co.uk/change/ ...click on the image of the motor in the middle, then click a couple of times on the Change-It button, you'll see what I mean.

Dave

[DmaxMaverick]

I don't buy into that "piston slap" theory. If that were the case, we'd see a much higher instance of elongated bore diameters. More like that of the "lesser piston slapping" gassers, which is quite common at overhaul, at much less miles. The "Diesel noise" is the violent explosion occuring when the fuel combusts, and resonates through the engine. Pilot injection reduces the noise by creating a more gradual combustion. If it were piston slap, then why would deep skirting have such an effect on the noise? Deep skirting helps absorb the shock wave, not piston slap. Their definintion of "piston slap" must have lost something in the translation.

[Robyn]

I agree
If it were piston slap the skirts would be in the pan before the day was done.
More closely related to detonation.
As our GM diesels get miles on them the noise abates some. This is due to the gradual reduction of "POP" pressure in the injector nozzles from wear.
The finer the spray mist and this being due to high pressure the noisier the engine at idle.
Compare the 6.2 and the 6.5TD and there is quite a lot of sound difference between the two.
I think the boys at Honda are using the wrong term here.

[vaceros]

hey if the honda dudes are into ionic recirculation of oil and volumetric efficient flow who cares about the resonance or knock factor. can't we just ring a bell and get our wings. i can remember the cvcc and its pre-combustion chamber. just resently they had a big tree fall on that pretty yellow one in a tv commercial/\. why can't he also use a constant velocity trans to keep the load going. it makes a little more sense possibly than a vtec system. less resonance still. must be the lawyers,guns,and money. not to mention the oil droplet. the kind in miliken and copper, or whatever thermal breakdown you require. oh yeah now i can here the faintest noise through the walls. and one more thing, some balancced ergonomics would be nice. i thought my 1978 bus was bad. its kind of like here passenger, you drive. hmm that might be a hint to try a flat four to harness most of the benefits of burning diesel fuel.especially in a sports style car. 50\50 weight mid engine two small transmissions from front rear output shafts. forward facing driver passenger rear facing rear seat passengers. four wheel steering to increase grip. flat torque throughout range of motor for best control of vehicle.

[tommac95]

I agree that the animated piece has it wrong about the diesel noise.
Dr Lee described it as "structural vibrations" -- the block flexing as the pressure wave spread throughout. It is possible that Honda adjusted the cylinder geometry to minimize the noise , possibly through sophisticated techniques.

I was fascinated/impressed that the same engineer who devised the Vtec invented the on-board-from-exhaust ammonia supply. This is impressive innovation ; hopefully the fellow will be compensated.